56 MODEL AVIATION
The additional light weight of the E-flite
gyro made no difference in the flying
characteristics of these ultra-light models.
The E-flite G110 Micro Heading Lock gyro, shown attached to the side of a typical flat
foamie, can be placed anywhere on an airframe as long as the yaw axis of the gyro is
parallel to a line that is perpendicular to the airplane’s top view.
The fleet of smaller airplanes used in flight tests. The two that received the author’s “Most Improved” awards are the molded
foam Gee Bee (with “NR2100” on the right wing) and the Sig Rascal above its right wingtip.
Photos by the author
attempts, you succeed at a wobbly takeoff (which is probably
cross-runway into the wind) and finally get a good flight going.
After all, the model does fly well.
One might wonder if all flights are going to start this way. The
answer is no, but achieving consistent takeoffs and landings
usually requires intensive practice with that aircraft or plain luck.
Don’t worry; there is another way.
The previous scenario is autobiographical. I have had many of
those kinds of takeoffs with some of my Scale and sport models,
and it seemed to happen all the time with a recently acquired
Messerschmitt Bf 109.
What my squirrely airplanes have had in common is a
conventional landing gear setup (sometimes called a tail-dragger).
There is usually a small wheel or skid at the tail end of the
aircraft.
A degree in aeronautical engineering from Purdue University
helped me understand the math behind the reasons why a
conventional landing gear setup often results in a wild takeoff, but
that is unnecessary. The following description is adequate.
When an aircraft is moving at a moderate speed down the
runway, one wheel is suddenly bumped or stopped by a small
pebble or chunk of grass. The CG tries to keep going forward but
1. Remember to turn off the HH mode after the
model is a wingspan or two above the ground. If you
leave the HH mode on and the airplane starts into its
first turn away from the pit area, the rudder will go hard
over to try to stop the fuselage from turning. Apply
rudder input in the direction of the turn to counter the
effect.
2. Don’t touch the rudder stick at any time during
takeoff. Doing so will offset the angle at which the
aircraft wants to track down the runway. Cut power and
redo the takeoff.
3. When in doubt about what is happening, cut power
to the model and think things through.
4. You are allowed to use the ailerons, elevator, and
power plant in a normal manner. MA
—Ben Lanterman
Gyro Rules to Remember
12sig2.QXD_00MSTRPG.QXD 10/21/10 10:54 AM Page 56
can’t.
Forces from the propeller (see the sidebar about
propeller woes) are there, and the stopped wheel causes the CG
to deflect a bit to the side. Then the airplane rotates even more
around the restrained wheel.
Unless a tail wheel, a tail skid, or the vertical tail has enough
side force to stop the rotation, the fuselage will continue to go
around, to the point at which the CG will try to go along the
original line of travel, even when the fuselage is sideways.
Since the model is moving slowly, the vertical tail won’t
contribute enough aerodynamic stabilizing force to stop the
turning. If the tail wheel is off of the ground, the situation is
worse.
A tricycle landing gear setup doesn’t suffer from this problem.
The CG in front of the main wheels exerts a stabilizing force, and
the nose wheel guides the fuselage.
The worst thing an aircraft with tricycle gear typically does is
go crooked when a gust of side wind hits it. However, such an
airplane can have takeoff problems if the nose gear is set up to be
too touchy.
If conditions around the model were smooth all the time and
the tail wheel worked effectively against a side force, we wouldn’t
have a big problem. But any number of small things can trigger
the divergence to the unstable yaw condition.
However, the biggest factor for most of us is the massive
gusting side wind that seems to come up when we find the rare
spare hour or two hour to fly our models.
The Gyro: A Google Internet search of the problem led me to the
RC Universe forum, where I found a thread concerning the use of
gyros in models. Because these devices are mandatory in RC
helicopters, there have been terrific advances in both affordability
and capability. The consensus on the thread was that using a gyro
on the yaw axis of an airplane could help.
Instructions that come with gyros do not recommend them for
airplanes. I generally recommend following the manufacturer’s
directions, but using a gyro heading-hold (HH) mode for the
specific purpose of aiding a takeoff seems to work well.
The pilot must remember to turn off the gyro when the aircraft
December 2010 57
Above: The intermediate-size models used in
testing. All were reasonably good during takeoff,
but adding the gyro removed the need for
constant pilot corrections in a crosswind.
has obtained an altitude of 3-4 feet. At that time the flier takes
control of the model.
Luckily for me, and probably also you, I learned that I can
use an all-solid-state gyro without knowing a thing about what is
inside the little box. I think of the device as a tiny person who
senses the airplane’s yawing motions during takeoff and
instantly gives the rudder command needed to stop them.
The gyro works exactly the way I try to—but a heck of a lot
faster and more accurately than I can.
To use such a unit you must first hook it up correctly—and
that is relatively easy. A modern gyro has two modes: rate
damping (RD) and HH. The RD mode will diminish oscillations
in yaw, but it won’t necessarily keep the aircraft on the runway.
The HH mode is what we will use for the takeoff problem.
Springy landing gear on the
Gemini biplane makes
takeof fs interesting. As
shown, the gyro is set to
HH mode and the fuselage
is yawed, illustrating
that the rudder is
commanded to try to
stop the yaw.
Below: The big Junkers Ju 52 (at the
bottom) has right-handed propellers; the JR
G500T Ring Gyro offered good results. The
Bf 109 (at lower left) is a ground-handling
nightmare but can easily make long or short
takeoffs with the gyro onboard.
12sig2.QXD_00MSTRPG.QXD 10/21/10 10:58 AM Page 57
58 MODEL AVIATION
We want our model to go straight down the runway with no
corrective input from us.
When the gyro is hooked up and working in the HH mode, you
can grab the vertical tail and manually yaw the airplane’s body 10°.
The result is that the rudder will correct and hold that correction until
the original heading is required (that centerline thingy).
The amount of rudder that the gyro gives is proportional to the
yaw that the model develops as it goes down the runway. As soon as
the main wheels clear the ground, the gyro commands are neither
needed nor wanted; by then the vertical tail has more authority to
offer adequate directional stability.
I usually turn off the gyro after the airplane has gained a few feet
of altitude. The entire takeoff is performed absent of rudder input.
I set two criteria for a gyro: it had to be reasonably inexpensive
and work with older analog servos as well as newer digital servos.
That was so I could cheaply retrofit some of my older models.
You should consider the total investment in the aircraft, compare
it with the cost of the gyro, and determine whether or not making the
modification is worth it. The cost is worth it for me, because now my
takeoffs look professional and I have reduced the chance of breaking
a favorite airplane.
The two most expensive gyros I tested were an older JR G500T
Ring Gyro (I believe it was $200 at the time of purchase) and a JR
G770 3D Gyro ($190 from Horizon Hobby). The least expensive unit
was the E-flite G110 Micro Heading Lock Gyro ($65 from Horizon
Hobby).
These products worked well with both analog and digital servos. I
moved the JR gyros in and out of various airplanes, but I have
purchased many of the E-flite gyros.
Other makes of HH gyros probably work as well, but I couldn’t
afford to test them all. Those I tried have worked well for this series
of tests, and they remain in the models shown in the photos.
The Test Setup: I used a JR 12X transmitter and all varieties of
Spektrum receivers. The only requirement for those was that they had
a fifth channel available for remote gyro control.
I conducted all of the flights from a slick asphalt surface. That is
the worst case from a ground-looping standpoint, because it makes it
easier for the tail wheel to slide sideways.
To ensure that my test results were not flukes—limited to one
airplane or gyro combination—I used the three gyro types in 20
The big IMAA-legal Great Planes Super Stearman sometimes
made fine takeoffs but could be difficult to keep on the runway with
a side wind. The JR G770 3D Gyro gave it a totally different
personality on takeoff.
Put your hand in a pail of water and stir it until you
have a spinning mass of water. Now stop your hand and
hold it flat to block the spinning water. You can feel a
large force. This is essentially what happens when the
spiral propwash (SP) hits the vertical tail of a model.
Spiral means “in a rotating motion”
and propwash is the air that the propeller
blows toward the rear. So we have a
spiraling mass of air moving to the rear of the
airplane: a small tornado.
With the power plant in front, the tail in back, and
the power plant turning clockwise as viewed from the
back, the SP has an opportunity to hit the fuselage.
With no vertical tail, no side force is generated
when the SP hits the fuselage.
When we stick a vertical tail on top of the fuselage in
that mass of rotating air, we get a force on the left side
of the vertical tail. The result of that force on our
aircraft is a yawing moment that wants to force the nose
of the model to the left.
Factors that will result in a higher SP are more
power, bigger propeller, and high rpm. Factors that
make the SP more effective are a bigger vertical tail and
a slow-moving airplane.
The result is that we need to input right rudder
immediately upon power application for takeoff. There
is little we can do about the effect of SP; it is simply a
negative part of a model configuration that is otherwise
is great.
The propeller produces P-factor. A propeller moving
through the air with no angle relative to the air has zero Pfactor.
If the axis of the power plant is tilted upward, such
as in a tail-dragging airplane with conventional landing
gear running along the ground, we develop P-factor.
The up-going propeller blade (on the left side of the
aircraft) has a lower relative angle of attack than the
down-going propeller blade (on the right side of the
aircraft). This means that the right side of the propeller
“disk” produces more forward thrust than the left side.
The result is a left-yawing moment on the model.
A higher-power motor or engine, a bigger propeller,
and a higher angle of attack of the airplane give us a
larger P-factor, and we need more right rudder to
counter it.
Gyroscopic Effects: As the aircraft accelerates down
the runway, the propeller blast on the horizontal tail lifts
the tail wheel off of the ground, leaving the model free
to pivot in yaw on the two main wheels.
As the propeller blast lifts the tail, it forces the
rotating propeller (which is now a big gyroscope) to
change its pitch angle (because it is attached to the
motor shaft). Also known as “gyroscopic procession,”
that effect further increases the need for corrective
rudder input.
The gyroscopic effect applies a yawing moment to
the aircraft that tends to swing the nose to the left. The
magnitude of the gyroscopic torque depends on the mass
of the propeller, the rpm of the propeller, and the pitch
rate of the fuselage.
Heavy propellers plus high rpm plus rapid pitch
rates equals large gyroscopic effects and major leftyawing
tendencies. A large-scale World War I model
with a big propeller that starts out at a hefty angle with
respect to the ground will have a much larger
gyroscopic effect. MA
—Ben Lanterman
Propeller Woes
12sig2.QXD_00MSTRPG.QXD 10/21/10 11:02 AM Page 58
airplanes of all sizes, weights, and landing
gear configurations.
The models, shown in the photos, range
from a light foam indoor aerobatic model
(flown outside in the wind), to the large
Junkers Ju 52 (spanning 94 inches with a
15-pound flying weight), to the huge Great
Planes Super Stearman.
The gyro is mounted in the aircraft so
that the yaw axis of the device (see
instructions) is parallel to the yaw axis of
the model. The yaw axis of the airplane is
a line that goes through its CG and is
perpendicular to the model when it is held
level. The gyro does not have to be
positioned on the aircraft CG.
However, it does need to be mounted
so that vibration from the airplane engine
or motor will not shake it apart. I
recommend using Velcro to attach the
gyro to a light-plywood base. Glue that
base to a piece of thin foam, and use
Velcro to attach the foam piece to the
model. This provides double insulation
from power plant vibration and allows the
gyro to be easily moved if necessary.
The gyro has two plugs that go to the
receiver: one goes into the rudder channel
and the other goes into Channel 5. The
auxiliary channel switch is used to change
the gyro from its HH mode to its RD mode
(or off altogether). The rudder servo plugs
directly into the gyro.
Following the instructions, the HH
limit (gain) is set high (approximately
100%) and the RD gain is set extremely
low (roughly 10%). Because the low RD
gain effectively turns off the gyro, the
transmitter auxiliary switch is used to turn
the HH mode on and off. You will have to
determine what works using your radio
and gyro instructions.
Set the stick trim sliders to neutral and
rudder subtrim values (if available) to
zero. Use rudder pushrod adjustments to
mechanically zero the rudder position.
Make sure that the rudder operates freely
and correctly.
To make initial settings for the gyro
gain, turn on the transmitter with the
auxiliary switch set to the gyro RD mode
position. Turn on the receiver and gyro,
letting the model remain still for 15-20
seconds; this will let the gyro initialize.
Then switch on the gyro HH mode.
You might see the rudder drift slightly
or drive to the extremes of throw. Switch
back to the RD mode and use the rudder
stick trim sliders to adjust the rudder drift
direction to be opposite of the observed
drift. Then switch back to HH and observe
the rudder action.
Repeat this process until you get a nice
zero-drift setting in HH mode. Switch
back to the RD mode and check to make
sure that the mechanical setup is still
giving you a zero rudder position, and
adjust the mechanical zero if necessary.
Now you must determine the direction
of the gyro correction. That is easy to do.
In HH mode, if you yaw the airplane to
the left, the rudder should be driven to the
right as it responds to the yaw. If the
rudder deflection is going the wrong way,
use the small switch on the side of the
gyro to reverse it.
If the rudder-angle change is equal to
or more than the applied yaw angle, the
aircraft is in a good starting place. An
exact angular response can be fine-tuned
later, but I have found that it is
noncritical.
Flight Testing: In preflight setups, the
only practical difference in the gyros I
have worked with is that the more
expensive JR versions are stable with
respect to keeping settings from day to
day. I haven’t had to adjust anything since
the first setup. The E-flite gyro is much
more inexpensive, and with that
apparently comes a small amount of
positional instability from day to day.
The practical meaning of that to me, as
a pilot, is that before each flight I need to
check to ensure that the rudder holds
neutral position when the gyro is switched
to HH. If the rudder starts moving slowly
when it is set to the HH mode, it can be
adjusted with a click or two of rudder
trim.
I have made the HH drift check a part
of my normal preflight checklist, so it is
not a big problem. I like the E-flite gyro
so well that I have 18 airplanes with one
in each.
My initial taxi-only trials involved
using small park flyers in a corner of an
empty parking lot. On a day that had wind
speeds gusting up to 18 mph, I made the
taxi runs so that a full crosswind was
available.
I gave each airplane a bit of power and
allowed it to run approximately 150 feet
with the tail always on the ground. I did that
with the gyro on and off.
It was interesting to watch the rudder
work to counter the wind gusts. A model
that normally would have been blown all
over the parking lot was following straight
lines.
I moved the next tests out to the
Boeing’s Phantom Flyers field in Saint
Charles, Missouri. The club has a good
asphalt runway with a white stripe down the
center (which always seemed to be jeering
at me before).
During the corn-growing season, when
the wind is from the north, a terrific rotor is
caused by the wind spilling over the corn. It
hits the runway area as a rotating
crosswind.
We flew aircraft after aircraft with
consistent results, regardless of wind
direction or airplane configuration,
exercising such variables as P-factor, spiral
propwash, gyroscopic effects, tire spacing,
etc. Both slow and fast takeoffs were made
to try to mess up the “gyro on” takeoffs,
and none did.
The device in HH mode consistently
guided the models to make good takeoffs.
Some airplanes took off straight but with
evident oscillations, some were straight but
moved sideways a bit as the wind blew
them, and other takeoffs were straight. The
results have been uniformly positive with
the HH gyros I have tested.
A great thing about using a gyro is that
if the nose gear is set incorrectly, the tail
wheel is bent, or a landing gear part gets
bent on a bad landing to the point where
the aircraft would normally veer off to the
side during takeoff, the device can still
adjust the takeoff. The model will be
directed to take off straight down the
runway, although the body might have an
apparent yaw angle.
The only exceptions to airplanes going
reasonably straight down the runway were
light models in a hard side wind. The
fuselage would point down the runway
and try to be parallel to the centerline, but
the hard side wind would blow the whole
airplane sideways. But it was better than
the ground loop that normally took place.
Conclusion: The modern RC gyro with
HH capability is a great tool to use to help
almost any model configuration attain
better takeoffs and landing approaches.
All airplanes I tested certainly improved;
ground-looping monsters were turned into
well-mannered aircraft.
The Bf 109 now makes reasonably
fine takeoffs, with no pilot input needed.
A video showing some of my
comparisons between having the HH gyro
off and on, including in the
Messerschmitt, is shown on YouTube. I
made sure that the side winds were
extraordinarily high, to ensure that the
benefits of the HH gyros were exercised
to the max. MA
Edition: Model Aviation - 2010/12
Page Numbers: 55,56,57,58,60,62
Edition: Model Aviation - 2010/12
Page Numbers: 55,56,57,58,60,62
56 MODEL AVIATION
The additional light weight of the E-flite
gyro made no difference in the flying
characteristics of these ultra-light models.
The E-flite G110 Micro Heading Lock gyro, shown attached to the side of a typical flat
foamie, can be placed anywhere on an airframe as long as the yaw axis of the gyro is
parallel to a line that is perpendicular to the airplane’s top view.
The fleet of smaller airplanes used in flight tests. The two that received the author’s “Most Improved” awards are the molded
foam Gee Bee (with “NR2100” on the right wing) and the Sig Rascal above its right wingtip.
Photos by the author
attempts, you succeed at a wobbly takeoff (which is probably
cross-runway into the wind) and finally get a good flight going.
After all, the model does fly well.
One might wonder if all flights are going to start this way. The
answer is no, but achieving consistent takeoffs and landings
usually requires intensive practice with that aircraft or plain luck.
Don’t worry; there is another way.
The previous scenario is autobiographical. I have had many of
those kinds of takeoffs with some of my Scale and sport models,
and it seemed to happen all the time with a recently acquired
Messerschmitt Bf 109.
What my squirrely airplanes have had in common is a
conventional landing gear setup (sometimes called a tail-dragger).
There is usually a small wheel or skid at the tail end of the
aircraft.
A degree in aeronautical engineering from Purdue University
helped me understand the math behind the reasons why a
conventional landing gear setup often results in a wild takeoff, but
that is unnecessary. The following description is adequate.
When an aircraft is moving at a moderate speed down the
runway, one wheel is suddenly bumped or stopped by a small
pebble or chunk of grass. The CG tries to keep going forward but
1. Remember to turn off the HH mode after the
model is a wingspan or two above the ground. If you
leave the HH mode on and the airplane starts into its
first turn away from the pit area, the rudder will go hard
over to try to stop the fuselage from turning. Apply
rudder input in the direction of the turn to counter the
effect.
2. Don’t touch the rudder stick at any time during
takeoff. Doing so will offset the angle at which the
aircraft wants to track down the runway. Cut power and
redo the takeoff.
3. When in doubt about what is happening, cut power
to the model and think things through.
4. You are allowed to use the ailerons, elevator, and
power plant in a normal manner. MA
—Ben Lanterman
Gyro Rules to Remember
12sig2.QXD_00MSTRPG.QXD 10/21/10 10:54 AM Page 56
can’t.
Forces from the propeller (see the sidebar about
propeller woes) are there, and the stopped wheel causes the CG
to deflect a bit to the side. Then the airplane rotates even more
around the restrained wheel.
Unless a tail wheel, a tail skid, or the vertical tail has enough
side force to stop the rotation, the fuselage will continue to go
around, to the point at which the CG will try to go along the
original line of travel, even when the fuselage is sideways.
Since the model is moving slowly, the vertical tail won’t
contribute enough aerodynamic stabilizing force to stop the
turning. If the tail wheel is off of the ground, the situation is
worse.
A tricycle landing gear setup doesn’t suffer from this problem.
The CG in front of the main wheels exerts a stabilizing force, and
the nose wheel guides the fuselage.
The worst thing an aircraft with tricycle gear typically does is
go crooked when a gust of side wind hits it. However, such an
airplane can have takeoff problems if the nose gear is set up to be
too touchy.
If conditions around the model were smooth all the time and
the tail wheel worked effectively against a side force, we wouldn’t
have a big problem. But any number of small things can trigger
the divergence to the unstable yaw condition.
However, the biggest factor for most of us is the massive
gusting side wind that seems to come up when we find the rare
spare hour or two hour to fly our models.
The Gyro: A Google Internet search of the problem led me to the
RC Universe forum, where I found a thread concerning the use of
gyros in models. Because these devices are mandatory in RC
helicopters, there have been terrific advances in both affordability
and capability. The consensus on the thread was that using a gyro
on the yaw axis of an airplane could help.
Instructions that come with gyros do not recommend them for
airplanes. I generally recommend following the manufacturer’s
directions, but using a gyro heading-hold (HH) mode for the
specific purpose of aiding a takeoff seems to work well.
The pilot must remember to turn off the gyro when the aircraft
December 2010 57
Above: The intermediate-size models used in
testing. All were reasonably good during takeoff,
but adding the gyro removed the need for
constant pilot corrections in a crosswind.
has obtained an altitude of 3-4 feet. At that time the flier takes
control of the model.
Luckily for me, and probably also you, I learned that I can
use an all-solid-state gyro without knowing a thing about what is
inside the little box. I think of the device as a tiny person who
senses the airplane’s yawing motions during takeoff and
instantly gives the rudder command needed to stop them.
The gyro works exactly the way I try to—but a heck of a lot
faster and more accurately than I can.
To use such a unit you must first hook it up correctly—and
that is relatively easy. A modern gyro has two modes: rate
damping (RD) and HH. The RD mode will diminish oscillations
in yaw, but it won’t necessarily keep the aircraft on the runway.
The HH mode is what we will use for the takeoff problem.
Springy landing gear on the
Gemini biplane makes
takeof fs interesting. As
shown, the gyro is set to
HH mode and the fuselage
is yawed, illustrating
that the rudder is
commanded to try to
stop the yaw.
Below: The big Junkers Ju 52 (at the
bottom) has right-handed propellers; the JR
G500T Ring Gyro offered good results. The
Bf 109 (at lower left) is a ground-handling
nightmare but can easily make long or short
takeoffs with the gyro onboard.
12sig2.QXD_00MSTRPG.QXD 10/21/10 10:58 AM Page 57
58 MODEL AVIATION
We want our model to go straight down the runway with no
corrective input from us.
When the gyro is hooked up and working in the HH mode, you
can grab the vertical tail and manually yaw the airplane’s body 10°.
The result is that the rudder will correct and hold that correction until
the original heading is required (that centerline thingy).
The amount of rudder that the gyro gives is proportional to the
yaw that the model develops as it goes down the runway. As soon as
the main wheels clear the ground, the gyro commands are neither
needed nor wanted; by then the vertical tail has more authority to
offer adequate directional stability.
I usually turn off the gyro after the airplane has gained a few feet
of altitude. The entire takeoff is performed absent of rudder input.
I set two criteria for a gyro: it had to be reasonably inexpensive
and work with older analog servos as well as newer digital servos.
That was so I could cheaply retrofit some of my older models.
You should consider the total investment in the aircraft, compare
it with the cost of the gyro, and determine whether or not making the
modification is worth it. The cost is worth it for me, because now my
takeoffs look professional and I have reduced the chance of breaking
a favorite airplane.
The two most expensive gyros I tested were an older JR G500T
Ring Gyro (I believe it was $200 at the time of purchase) and a JR
G770 3D Gyro ($190 from Horizon Hobby). The least expensive unit
was the E-flite G110 Micro Heading Lock Gyro ($65 from Horizon
Hobby).
These products worked well with both analog and digital servos. I
moved the JR gyros in and out of various airplanes, but I have
purchased many of the E-flite gyros.
Other makes of HH gyros probably work as well, but I couldn’t
afford to test them all. Those I tried have worked well for this series
of tests, and they remain in the models shown in the photos.
The Test Setup: I used a JR 12X transmitter and all varieties of
Spektrum receivers. The only requirement for those was that they had
a fifth channel available for remote gyro control.
I conducted all of the flights from a slick asphalt surface. That is
the worst case from a ground-looping standpoint, because it makes it
easier for the tail wheel to slide sideways.
To ensure that my test results were not flukes—limited to one
airplane or gyro combination—I used the three gyro types in 20
The big IMAA-legal Great Planes Super Stearman sometimes
made fine takeoffs but could be difficult to keep on the runway with
a side wind. The JR G770 3D Gyro gave it a totally different
personality on takeoff.
Put your hand in a pail of water and stir it until you
have a spinning mass of water. Now stop your hand and
hold it flat to block the spinning water. You can feel a
large force. This is essentially what happens when the
spiral propwash (SP) hits the vertical tail of a model.
Spiral means “in a rotating motion”
and propwash is the air that the propeller
blows toward the rear. So we have a
spiraling mass of air moving to the rear of the
airplane: a small tornado.
With the power plant in front, the tail in back, and
the power plant turning clockwise as viewed from the
back, the SP has an opportunity to hit the fuselage.
With no vertical tail, no side force is generated
when the SP hits the fuselage.
When we stick a vertical tail on top of the fuselage in
that mass of rotating air, we get a force on the left side
of the vertical tail. The result of that force on our
aircraft is a yawing moment that wants to force the nose
of the model to the left.
Factors that will result in a higher SP are more
power, bigger propeller, and high rpm. Factors that
make the SP more effective are a bigger vertical tail and
a slow-moving airplane.
The result is that we need to input right rudder
immediately upon power application for takeoff. There
is little we can do about the effect of SP; it is simply a
negative part of a model configuration that is otherwise
is great.
The propeller produces P-factor. A propeller moving
through the air with no angle relative to the air has zero Pfactor.
If the axis of the power plant is tilted upward, such
as in a tail-dragging airplane with conventional landing
gear running along the ground, we develop P-factor.
The up-going propeller blade (on the left side of the
aircraft) has a lower relative angle of attack than the
down-going propeller blade (on the right side of the
aircraft). This means that the right side of the propeller
“disk” produces more forward thrust than the left side.
The result is a left-yawing moment on the model.
A higher-power motor or engine, a bigger propeller,
and a higher angle of attack of the airplane give us a
larger P-factor, and we need more right rudder to
counter it.
Gyroscopic Effects: As the aircraft accelerates down
the runway, the propeller blast on the horizontal tail lifts
the tail wheel off of the ground, leaving the model free
to pivot in yaw on the two main wheels.
As the propeller blast lifts the tail, it forces the
rotating propeller (which is now a big gyroscope) to
change its pitch angle (because it is attached to the
motor shaft). Also known as “gyroscopic procession,”
that effect further increases the need for corrective
rudder input.
The gyroscopic effect applies a yawing moment to
the aircraft that tends to swing the nose to the left. The
magnitude of the gyroscopic torque depends on the mass
of the propeller, the rpm of the propeller, and the pitch
rate of the fuselage.
Heavy propellers plus high rpm plus rapid pitch
rates equals large gyroscopic effects and major leftyawing
tendencies. A large-scale World War I model
with a big propeller that starts out at a hefty angle with
respect to the ground will have a much larger
gyroscopic effect. MA
—Ben Lanterman
Propeller Woes
12sig2.QXD_00MSTRPG.QXD 10/21/10 11:02 AM Page 58
airplanes of all sizes, weights, and landing
gear configurations.
The models, shown in the photos, range
from a light foam indoor aerobatic model
(flown outside in the wind), to the large
Junkers Ju 52 (spanning 94 inches with a
15-pound flying weight), to the huge Great
Planes Super Stearman.
The gyro is mounted in the aircraft so
that the yaw axis of the device (see
instructions) is parallel to the yaw axis of
the model. The yaw axis of the airplane is
a line that goes through its CG and is
perpendicular to the model when it is held
level. The gyro does not have to be
positioned on the aircraft CG.
However, it does need to be mounted
so that vibration from the airplane engine
or motor will not shake it apart. I
recommend using Velcro to attach the
gyro to a light-plywood base. Glue that
base to a piece of thin foam, and use
Velcro to attach the foam piece to the
model. This provides double insulation
from power plant vibration and allows the
gyro to be easily moved if necessary.
The gyro has two plugs that go to the
receiver: one goes into the rudder channel
and the other goes into Channel 5. The
auxiliary channel switch is used to change
the gyro from its HH mode to its RD mode
(or off altogether). The rudder servo plugs
directly into the gyro.
Following the instructions, the HH
limit (gain) is set high (approximately
100%) and the RD gain is set extremely
low (roughly 10%). Because the low RD
gain effectively turns off the gyro, the
transmitter auxiliary switch is used to turn
the HH mode on and off. You will have to
determine what works using your radio
and gyro instructions.
Set the stick trim sliders to neutral and
rudder subtrim values (if available) to
zero. Use rudder pushrod adjustments to
mechanically zero the rudder position.
Make sure that the rudder operates freely
and correctly.
To make initial settings for the gyro
gain, turn on the transmitter with the
auxiliary switch set to the gyro RD mode
position. Turn on the receiver and gyro,
letting the model remain still for 15-20
seconds; this will let the gyro initialize.
Then switch on the gyro HH mode.
You might see the rudder drift slightly
or drive to the extremes of throw. Switch
back to the RD mode and use the rudder
stick trim sliders to adjust the rudder drift
direction to be opposite of the observed
drift. Then switch back to HH and observe
the rudder action.
Repeat this process until you get a nice
zero-drift setting in HH mode. Switch
back to the RD mode and check to make
sure that the mechanical setup is still
giving you a zero rudder position, and
adjust the mechanical zero if necessary.
Now you must determine the direction
of the gyro correction. That is easy to do.
In HH mode, if you yaw the airplane to
the left, the rudder should be driven to the
right as it responds to the yaw. If the
rudder deflection is going the wrong way,
use the small switch on the side of the
gyro to reverse it.
If the rudder-angle change is equal to
or more than the applied yaw angle, the
aircraft is in a good starting place. An
exact angular response can be fine-tuned
later, but I have found that it is
noncritical.
Flight Testing: In preflight setups, the
only practical difference in the gyros I
have worked with is that the more
expensive JR versions are stable with
respect to keeping settings from day to
day. I haven’t had to adjust anything since
the first setup. The E-flite gyro is much
more inexpensive, and with that
apparently comes a small amount of
positional instability from day to day.
The practical meaning of that to me, as
a pilot, is that before each flight I need to
check to ensure that the rudder holds
neutral position when the gyro is switched
to HH. If the rudder starts moving slowly
when it is set to the HH mode, it can be
adjusted with a click or two of rudder
trim.
I have made the HH drift check a part
of my normal preflight checklist, so it is
not a big problem. I like the E-flite gyro
so well that I have 18 airplanes with one
in each.
My initial taxi-only trials involved
using small park flyers in a corner of an
empty parking lot. On a day that had wind
speeds gusting up to 18 mph, I made the
taxi runs so that a full crosswind was
available.
I gave each airplane a bit of power and
allowed it to run approximately 150 feet
with the tail always on the ground. I did that
with the gyro on and off.
It was interesting to watch the rudder
work to counter the wind gusts. A model
that normally would have been blown all
over the parking lot was following straight
lines.
I moved the next tests out to the
Boeing’s Phantom Flyers field in Saint
Charles, Missouri. The club has a good
asphalt runway with a white stripe down the
center (which always seemed to be jeering
at me before).
During the corn-growing season, when
the wind is from the north, a terrific rotor is
caused by the wind spilling over the corn. It
hits the runway area as a rotating
crosswind.
We flew aircraft after aircraft with
consistent results, regardless of wind
direction or airplane configuration,
exercising such variables as P-factor, spiral
propwash, gyroscopic effects, tire spacing,
etc. Both slow and fast takeoffs were made
to try to mess up the “gyro on” takeoffs,
and none did.
The device in HH mode consistently
guided the models to make good takeoffs.
Some airplanes took off straight but with
evident oscillations, some were straight but
moved sideways a bit as the wind blew
them, and other takeoffs were straight. The
results have been uniformly positive with
the HH gyros I have tested.
A great thing about using a gyro is that
if the nose gear is set incorrectly, the tail
wheel is bent, or a landing gear part gets
bent on a bad landing to the point where
the aircraft would normally veer off to the
side during takeoff, the device can still
adjust the takeoff. The model will be
directed to take off straight down the
runway, although the body might have an
apparent yaw angle.
The only exceptions to airplanes going
reasonably straight down the runway were
light models in a hard side wind. The
fuselage would point down the runway
and try to be parallel to the centerline, but
the hard side wind would blow the whole
airplane sideways. But it was better than
the ground loop that normally took place.
Conclusion: The modern RC gyro with
HH capability is a great tool to use to help
almost any model configuration attain
better takeoffs and landing approaches.
All airplanes I tested certainly improved;
ground-looping monsters were turned into
well-mannered aircraft.
The Bf 109 now makes reasonably
fine takeoffs, with no pilot input needed.
A video showing some of my
comparisons between having the HH gyro
off and on, including in the
Messerschmitt, is shown on YouTube. I
made sure that the side winds were
extraordinarily high, to ensure that the
benefits of the HH gyros were exercised
to the max. MA
Edition: Model Aviation - 2010/12
Page Numbers: 55,56,57,58,60,62
56 MODEL AVIATION
The additional light weight of the E-flite
gyro made no difference in the flying
characteristics of these ultra-light models.
The E-flite G110 Micro Heading Lock gyro, shown attached to the side of a typical flat
foamie, can be placed anywhere on an airframe as long as the yaw axis of the gyro is
parallel to a line that is perpendicular to the airplane’s top view.
The fleet of smaller airplanes used in flight tests. The two that received the author’s “Most Improved” awards are the molded
foam Gee Bee (with “NR2100” on the right wing) and the Sig Rascal above its right wingtip.
Photos by the author
attempts, you succeed at a wobbly takeoff (which is probably
cross-runway into the wind) and finally get a good flight going.
After all, the model does fly well.
One might wonder if all flights are going to start this way. The
answer is no, but achieving consistent takeoffs and landings
usually requires intensive practice with that aircraft or plain luck.
Don’t worry; there is another way.
The previous scenario is autobiographical. I have had many of
those kinds of takeoffs with some of my Scale and sport models,
and it seemed to happen all the time with a recently acquired
Messerschmitt Bf 109.
What my squirrely airplanes have had in common is a
conventional landing gear setup (sometimes called a tail-dragger).
There is usually a small wheel or skid at the tail end of the
aircraft.
A degree in aeronautical engineering from Purdue University
helped me understand the math behind the reasons why a
conventional landing gear setup often results in a wild takeoff, but
that is unnecessary. The following description is adequate.
When an aircraft is moving at a moderate speed down the
runway, one wheel is suddenly bumped or stopped by a small
pebble or chunk of grass. The CG tries to keep going forward but
1. Remember to turn off the HH mode after the
model is a wingspan or two above the ground. If you
leave the HH mode on and the airplane starts into its
first turn away from the pit area, the rudder will go hard
over to try to stop the fuselage from turning. Apply
rudder input in the direction of the turn to counter the
effect.
2. Don’t touch the rudder stick at any time during
takeoff. Doing so will offset the angle at which the
aircraft wants to track down the runway. Cut power and
redo the takeoff.
3. When in doubt about what is happening, cut power
to the model and think things through.
4. You are allowed to use the ailerons, elevator, and
power plant in a normal manner. MA
—Ben Lanterman
Gyro Rules to Remember
12sig2.QXD_00MSTRPG.QXD 10/21/10 10:54 AM Page 56
can’t.
Forces from the propeller (see the sidebar about
propeller woes) are there, and the stopped wheel causes the CG
to deflect a bit to the side. Then the airplane rotates even more
around the restrained wheel.
Unless a tail wheel, a tail skid, or the vertical tail has enough
side force to stop the rotation, the fuselage will continue to go
around, to the point at which the CG will try to go along the
original line of travel, even when the fuselage is sideways.
Since the model is moving slowly, the vertical tail won’t
contribute enough aerodynamic stabilizing force to stop the
turning. If the tail wheel is off of the ground, the situation is
worse.
A tricycle landing gear setup doesn’t suffer from this problem.
The CG in front of the main wheels exerts a stabilizing force, and
the nose wheel guides the fuselage.
The worst thing an aircraft with tricycle gear typically does is
go crooked when a gust of side wind hits it. However, such an
airplane can have takeoff problems if the nose gear is set up to be
too touchy.
If conditions around the model were smooth all the time and
the tail wheel worked effectively against a side force, we wouldn’t
have a big problem. But any number of small things can trigger
the divergence to the unstable yaw condition.
However, the biggest factor for most of us is the massive
gusting side wind that seems to come up when we find the rare
spare hour or two hour to fly our models.
The Gyro: A Google Internet search of the problem led me to the
RC Universe forum, where I found a thread concerning the use of
gyros in models. Because these devices are mandatory in RC
helicopters, there have been terrific advances in both affordability
and capability. The consensus on the thread was that using a gyro
on the yaw axis of an airplane could help.
Instructions that come with gyros do not recommend them for
airplanes. I generally recommend following the manufacturer’s
directions, but using a gyro heading-hold (HH) mode for the
specific purpose of aiding a takeoff seems to work well.
The pilot must remember to turn off the gyro when the aircraft
December 2010 57
Above: The intermediate-size models used in
testing. All were reasonably good during takeoff,
but adding the gyro removed the need for
constant pilot corrections in a crosswind.
has obtained an altitude of 3-4 feet. At that time the flier takes
control of the model.
Luckily for me, and probably also you, I learned that I can
use an all-solid-state gyro without knowing a thing about what is
inside the little box. I think of the device as a tiny person who
senses the airplane’s yawing motions during takeoff and
instantly gives the rudder command needed to stop them.
The gyro works exactly the way I try to—but a heck of a lot
faster and more accurately than I can.
To use such a unit you must first hook it up correctly—and
that is relatively easy. A modern gyro has two modes: rate
damping (RD) and HH. The RD mode will diminish oscillations
in yaw, but it won’t necessarily keep the aircraft on the runway.
The HH mode is what we will use for the takeoff problem.
Springy landing gear on the
Gemini biplane makes
takeof fs interesting. As
shown, the gyro is set to
HH mode and the fuselage
is yawed, illustrating
that the rudder is
commanded to try to
stop the yaw.
Below: The big Junkers Ju 52 (at the
bottom) has right-handed propellers; the JR
G500T Ring Gyro offered good results. The
Bf 109 (at lower left) is a ground-handling
nightmare but can easily make long or short
takeoffs with the gyro onboard.
12sig2.QXD_00MSTRPG.QXD 10/21/10 10:58 AM Page 57
58 MODEL AVIATION
We want our model to go straight down the runway with no
corrective input from us.
When the gyro is hooked up and working in the HH mode, you
can grab the vertical tail and manually yaw the airplane’s body 10°.
The result is that the rudder will correct and hold that correction until
the original heading is required (that centerline thingy).
The amount of rudder that the gyro gives is proportional to the
yaw that the model develops as it goes down the runway. As soon as
the main wheels clear the ground, the gyro commands are neither
needed nor wanted; by then the vertical tail has more authority to
offer adequate directional stability.
I usually turn off the gyro after the airplane has gained a few feet
of altitude. The entire takeoff is performed absent of rudder input.
I set two criteria for a gyro: it had to be reasonably inexpensive
and work with older analog servos as well as newer digital servos.
That was so I could cheaply retrofit some of my older models.
You should consider the total investment in the aircraft, compare
it with the cost of the gyro, and determine whether or not making the
modification is worth it. The cost is worth it for me, because now my
takeoffs look professional and I have reduced the chance of breaking
a favorite airplane.
The two most expensive gyros I tested were an older JR G500T
Ring Gyro (I believe it was $200 at the time of purchase) and a JR
G770 3D Gyro ($190 from Horizon Hobby). The least expensive unit
was the E-flite G110 Micro Heading Lock Gyro ($65 from Horizon
Hobby).
These products worked well with both analog and digital servos. I
moved the JR gyros in and out of various airplanes, but I have
purchased many of the E-flite gyros.
Other makes of HH gyros probably work as well, but I couldn’t
afford to test them all. Those I tried have worked well for this series
of tests, and they remain in the models shown in the photos.
The Test Setup: I used a JR 12X transmitter and all varieties of
Spektrum receivers. The only requirement for those was that they had
a fifth channel available for remote gyro control.
I conducted all of the flights from a slick asphalt surface. That is
the worst case from a ground-looping standpoint, because it makes it
easier for the tail wheel to slide sideways.
To ensure that my test results were not flukes—limited to one
airplane or gyro combination—I used the three gyro types in 20
The big IMAA-legal Great Planes Super Stearman sometimes
made fine takeoffs but could be difficult to keep on the runway with
a side wind. The JR G770 3D Gyro gave it a totally different
personality on takeoff.
Put your hand in a pail of water and stir it until you
have a spinning mass of water. Now stop your hand and
hold it flat to block the spinning water. You can feel a
large force. This is essentially what happens when the
spiral propwash (SP) hits the vertical tail of a model.
Spiral means “in a rotating motion”
and propwash is the air that the propeller
blows toward the rear. So we have a
spiraling mass of air moving to the rear of the
airplane: a small tornado.
With the power plant in front, the tail in back, and
the power plant turning clockwise as viewed from the
back, the SP has an opportunity to hit the fuselage.
With no vertical tail, no side force is generated
when the SP hits the fuselage.
When we stick a vertical tail on top of the fuselage in
that mass of rotating air, we get a force on the left side
of the vertical tail. The result of that force on our
aircraft is a yawing moment that wants to force the nose
of the model to the left.
Factors that will result in a higher SP are more
power, bigger propeller, and high rpm. Factors that
make the SP more effective are a bigger vertical tail and
a slow-moving airplane.
The result is that we need to input right rudder
immediately upon power application for takeoff. There
is little we can do about the effect of SP; it is simply a
negative part of a model configuration that is otherwise
is great.
The propeller produces P-factor. A propeller moving
through the air with no angle relative to the air has zero Pfactor.
If the axis of the power plant is tilted upward, such
as in a tail-dragging airplane with conventional landing
gear running along the ground, we develop P-factor.
The up-going propeller blade (on the left side of the
aircraft) has a lower relative angle of attack than the
down-going propeller blade (on the right side of the
aircraft). This means that the right side of the propeller
“disk” produces more forward thrust than the left side.
The result is a left-yawing moment on the model.
A higher-power motor or engine, a bigger propeller,
and a higher angle of attack of the airplane give us a
larger P-factor, and we need more right rudder to
counter it.
Gyroscopic Effects: As the aircraft accelerates down
the runway, the propeller blast on the horizontal tail lifts
the tail wheel off of the ground, leaving the model free
to pivot in yaw on the two main wheels.
As the propeller blast lifts the tail, it forces the
rotating propeller (which is now a big gyroscope) to
change its pitch angle (because it is attached to the
motor shaft). Also known as “gyroscopic procession,”
that effect further increases the need for corrective
rudder input.
The gyroscopic effect applies a yawing moment to
the aircraft that tends to swing the nose to the left. The
magnitude of the gyroscopic torque depends on the mass
of the propeller, the rpm of the propeller, and the pitch
rate of the fuselage.
Heavy propellers plus high rpm plus rapid pitch
rates equals large gyroscopic effects and major leftyawing
tendencies. A large-scale World War I model
with a big propeller that starts out at a hefty angle with
respect to the ground will have a much larger
gyroscopic effect. MA
—Ben Lanterman
Propeller Woes
12sig2.QXD_00MSTRPG.QXD 10/21/10 11:02 AM Page 58
airplanes of all sizes, weights, and landing
gear configurations.
The models, shown in the photos, range
from a light foam indoor aerobatic model
(flown outside in the wind), to the large
Junkers Ju 52 (spanning 94 inches with a
15-pound flying weight), to the huge Great
Planes Super Stearman.
The gyro is mounted in the aircraft so
that the yaw axis of the device (see
instructions) is parallel to the yaw axis of
the model. The yaw axis of the airplane is
a line that goes through its CG and is
perpendicular to the model when it is held
level. The gyro does not have to be
positioned on the aircraft CG.
However, it does need to be mounted
so that vibration from the airplane engine
or motor will not shake it apart. I
recommend using Velcro to attach the
gyro to a light-plywood base. Glue that
base to a piece of thin foam, and use
Velcro to attach the foam piece to the
model. This provides double insulation
from power plant vibration and allows the
gyro to be easily moved if necessary.
The gyro has two plugs that go to the
receiver: one goes into the rudder channel
and the other goes into Channel 5. The
auxiliary channel switch is used to change
the gyro from its HH mode to its RD mode
(or off altogether). The rudder servo plugs
directly into the gyro.
Following the instructions, the HH
limit (gain) is set high (approximately
100%) and the RD gain is set extremely
low (roughly 10%). Because the low RD
gain effectively turns off the gyro, the
transmitter auxiliary switch is used to turn
the HH mode on and off. You will have to
determine what works using your radio
and gyro instructions.
Set the stick trim sliders to neutral and
rudder subtrim values (if available) to
zero. Use rudder pushrod adjustments to
mechanically zero the rudder position.
Make sure that the rudder operates freely
and correctly.
To make initial settings for the gyro
gain, turn on the transmitter with the
auxiliary switch set to the gyro RD mode
position. Turn on the receiver and gyro,
letting the model remain still for 15-20
seconds; this will let the gyro initialize.
Then switch on the gyro HH mode.
You might see the rudder drift slightly
or drive to the extremes of throw. Switch
back to the RD mode and use the rudder
stick trim sliders to adjust the rudder drift
direction to be opposite of the observed
drift. Then switch back to HH and observe
the rudder action.
Repeat this process until you get a nice
zero-drift setting in HH mode. Switch
back to the RD mode and check to make
sure that the mechanical setup is still
giving you a zero rudder position, and
adjust the mechanical zero if necessary.
Now you must determine the direction
of the gyro correction. That is easy to do.
In HH mode, if you yaw the airplane to
the left, the rudder should be driven to the
right as it responds to the yaw. If the
rudder deflection is going the wrong way,
use the small switch on the side of the
gyro to reverse it.
If the rudder-angle change is equal to
or more than the applied yaw angle, the
aircraft is in a good starting place. An
exact angular response can be fine-tuned
later, but I have found that it is
noncritical.
Flight Testing: In preflight setups, the
only practical difference in the gyros I
have worked with is that the more
expensive JR versions are stable with
respect to keeping settings from day to
day. I haven’t had to adjust anything since
the first setup. The E-flite gyro is much
more inexpensive, and with that
apparently comes a small amount of
positional instability from day to day.
The practical meaning of that to me, as
a pilot, is that before each flight I need to
check to ensure that the rudder holds
neutral position when the gyro is switched
to HH. If the rudder starts moving slowly
when it is set to the HH mode, it can be
adjusted with a click or two of rudder
trim.
I have made the HH drift check a part
of my normal preflight checklist, so it is
not a big problem. I like the E-flite gyro
so well that I have 18 airplanes with one
in each.
My initial taxi-only trials involved
using small park flyers in a corner of an
empty parking lot. On a day that had wind
speeds gusting up to 18 mph, I made the
taxi runs so that a full crosswind was
available.
I gave each airplane a bit of power and
allowed it to run approximately 150 feet
with the tail always on the ground. I did that
with the gyro on and off.
It was interesting to watch the rudder
work to counter the wind gusts. A model
that normally would have been blown all
over the parking lot was following straight
lines.
I moved the next tests out to the
Boeing’s Phantom Flyers field in Saint
Charles, Missouri. The club has a good
asphalt runway with a white stripe down the
center (which always seemed to be jeering
at me before).
During the corn-growing season, when
the wind is from the north, a terrific rotor is
caused by the wind spilling over the corn. It
hits the runway area as a rotating
crosswind.
We flew aircraft after aircraft with
consistent results, regardless of wind
direction or airplane configuration,
exercising such variables as P-factor, spiral
propwash, gyroscopic effects, tire spacing,
etc. Both slow and fast takeoffs were made
to try to mess up the “gyro on” takeoffs,
and none did.
The device in HH mode consistently
guided the models to make good takeoffs.
Some airplanes took off straight but with
evident oscillations, some were straight but
moved sideways a bit as the wind blew
them, and other takeoffs were straight. The
results have been uniformly positive with
the HH gyros I have tested.
A great thing about using a gyro is that
if the nose gear is set incorrectly, the tail
wheel is bent, or a landing gear part gets
bent on a bad landing to the point where
the aircraft would normally veer off to the
side during takeoff, the device can still
adjust the takeoff. The model will be
directed to take off straight down the
runway, although the body might have an
apparent yaw angle.
The only exceptions to airplanes going
reasonably straight down the runway were
light models in a hard side wind. The
fuselage would point down the runway
and try to be parallel to the centerline, but
the hard side wind would blow the whole
airplane sideways. But it was better than
the ground loop that normally took place.
Conclusion: The modern RC gyro with
HH capability is a great tool to use to help
almost any model configuration attain
better takeoffs and landing approaches.
All airplanes I tested certainly improved;
ground-looping monsters were turned into
well-mannered aircraft.
The Bf 109 now makes reasonably
fine takeoffs, with no pilot input needed.
A video showing some of my
comparisons between having the HH gyro
off and on, including in the
Messerschmitt, is shown on YouTube. I
made sure that the side winds were
extraordinarily high, to ensure that the
benefits of the HH gyros were exercised
to the max. MA
Edition: Model Aviation - 2010/12
Page Numbers: 55,56,57,58,60,62
56 MODEL AVIATION
The additional light weight of the E-flite
gyro made no difference in the flying
characteristics of these ultra-light models.
The E-flite G110 Micro Heading Lock gyro, shown attached to the side of a typical flat
foamie, can be placed anywhere on an airframe as long as the yaw axis of the gyro is
parallel to a line that is perpendicular to the airplane’s top view.
The fleet of smaller airplanes used in flight tests. The two that received the author’s “Most Improved” awards are the molded
foam Gee Bee (with “NR2100” on the right wing) and the Sig Rascal above its right wingtip.
Photos by the author
attempts, you succeed at a wobbly takeoff (which is probably
cross-runway into the wind) and finally get a good flight going.
After all, the model does fly well.
One might wonder if all flights are going to start this way. The
answer is no, but achieving consistent takeoffs and landings
usually requires intensive practice with that aircraft or plain luck.
Don’t worry; there is another way.
The previous scenario is autobiographical. I have had many of
those kinds of takeoffs with some of my Scale and sport models,
and it seemed to happen all the time with a recently acquired
Messerschmitt Bf 109.
What my squirrely airplanes have had in common is a
conventional landing gear setup (sometimes called a tail-dragger).
There is usually a small wheel or skid at the tail end of the
aircraft.
A degree in aeronautical engineering from Purdue University
helped me understand the math behind the reasons why a
conventional landing gear setup often results in a wild takeoff, but
that is unnecessary. The following description is adequate.
When an aircraft is moving at a moderate speed down the
runway, one wheel is suddenly bumped or stopped by a small
pebble or chunk of grass. The CG tries to keep going forward but
1. Remember to turn off the HH mode after the
model is a wingspan or two above the ground. If you
leave the HH mode on and the airplane starts into its
first turn away from the pit area, the rudder will go hard
over to try to stop the fuselage from turning. Apply
rudder input in the direction of the turn to counter the
effect.
2. Don’t touch the rudder stick at any time during
takeoff. Doing so will offset the angle at which the
aircraft wants to track down the runway. Cut power and
redo the takeoff.
3. When in doubt about what is happening, cut power
to the model and think things through.
4. You are allowed to use the ailerons, elevator, and
power plant in a normal manner. MA
—Ben Lanterman
Gyro Rules to Remember
12sig2.QXD_00MSTRPG.QXD 10/21/10 10:54 AM Page 56
can’t.
Forces from the propeller (see the sidebar about
propeller woes) are there, and the stopped wheel causes the CG
to deflect a bit to the side. Then the airplane rotates even more
around the restrained wheel.
Unless a tail wheel, a tail skid, or the vertical tail has enough
side force to stop the rotation, the fuselage will continue to go
around, to the point at which the CG will try to go along the
original line of travel, even when the fuselage is sideways.
Since the model is moving slowly, the vertical tail won’t
contribute enough aerodynamic stabilizing force to stop the
turning. If the tail wheel is off of the ground, the situation is
worse.
A tricycle landing gear setup doesn’t suffer from this problem.
The CG in front of the main wheels exerts a stabilizing force, and
the nose wheel guides the fuselage.
The worst thing an aircraft with tricycle gear typically does is
go crooked when a gust of side wind hits it. However, such an
airplane can have takeoff problems if the nose gear is set up to be
too touchy.
If conditions around the model were smooth all the time and
the tail wheel worked effectively against a side force, we wouldn’t
have a big problem. But any number of small things can trigger
the divergence to the unstable yaw condition.
However, the biggest factor for most of us is the massive
gusting side wind that seems to come up when we find the rare
spare hour or two hour to fly our models.
The Gyro: A Google Internet search of the problem led me to the
RC Universe forum, where I found a thread concerning the use of
gyros in models. Because these devices are mandatory in RC
helicopters, there have been terrific advances in both affordability
and capability. The consensus on the thread was that using a gyro
on the yaw axis of an airplane could help.
Instructions that come with gyros do not recommend them for
airplanes. I generally recommend following the manufacturer’s
directions, but using a gyro heading-hold (HH) mode for the
specific purpose of aiding a takeoff seems to work well.
The pilot must remember to turn off the gyro when the aircraft
December 2010 57
Above: The intermediate-size models used in
testing. All were reasonably good during takeoff,
but adding the gyro removed the need for
constant pilot corrections in a crosswind.
has obtained an altitude of 3-4 feet. At that time the flier takes
control of the model.
Luckily for me, and probably also you, I learned that I can
use an all-solid-state gyro without knowing a thing about what is
inside the little box. I think of the device as a tiny person who
senses the airplane’s yawing motions during takeoff and
instantly gives the rudder command needed to stop them.
The gyro works exactly the way I try to—but a heck of a lot
faster and more accurately than I can.
To use such a unit you must first hook it up correctly—and
that is relatively easy. A modern gyro has two modes: rate
damping (RD) and HH. The RD mode will diminish oscillations
in yaw, but it won’t necessarily keep the aircraft on the runway.
The HH mode is what we will use for the takeoff problem.
Springy landing gear on the
Gemini biplane makes
takeof fs interesting. As
shown, the gyro is set to
HH mode and the fuselage
is yawed, illustrating
that the rudder is
commanded to try to
stop the yaw.
Below: The big Junkers Ju 52 (at the
bottom) has right-handed propellers; the JR
G500T Ring Gyro offered good results. The
Bf 109 (at lower left) is a ground-handling
nightmare but can easily make long or short
takeoffs with the gyro onboard.
12sig2.QXD_00MSTRPG.QXD 10/21/10 10:58 AM Page 57
58 MODEL AVIATION
We want our model to go straight down the runway with no
corrective input from us.
When the gyro is hooked up and working in the HH mode, you
can grab the vertical tail and manually yaw the airplane’s body 10°.
The result is that the rudder will correct and hold that correction until
the original heading is required (that centerline thingy).
The amount of rudder that the gyro gives is proportional to the
yaw that the model develops as it goes down the runway. As soon as
the main wheels clear the ground, the gyro commands are neither
needed nor wanted; by then the vertical tail has more authority to
offer adequate directional stability.
I usually turn off the gyro after the airplane has gained a few feet
of altitude. The entire takeoff is performed absent of rudder input.
I set two criteria for a gyro: it had to be reasonably inexpensive
and work with older analog servos as well as newer digital servos.
That was so I could cheaply retrofit some of my older models.
You should consider the total investment in the aircraft, compare
it with the cost of the gyro, and determine whether or not making the
modification is worth it. The cost is worth it for me, because now my
takeoffs look professional and I have reduced the chance of breaking
a favorite airplane.
The two most expensive gyros I tested were an older JR G500T
Ring Gyro (I believe it was $200 at the time of purchase) and a JR
G770 3D Gyro ($190 from Horizon Hobby). The least expensive unit
was the E-flite G110 Micro Heading Lock Gyro ($65 from Horizon
Hobby).
These products worked well with both analog and digital servos. I
moved the JR gyros in and out of various airplanes, but I have
purchased many of the E-flite gyros.
Other makes of HH gyros probably work as well, but I couldn’t
afford to test them all. Those I tried have worked well for this series
of tests, and they remain in the models shown in the photos.
The Test Setup: I used a JR 12X transmitter and all varieties of
Spektrum receivers. The only requirement for those was that they had
a fifth channel available for remote gyro control.
I conducted all of the flights from a slick asphalt surface. That is
the worst case from a ground-looping standpoint, because it makes it
easier for the tail wheel to slide sideways.
To ensure that my test results were not flukes—limited to one
airplane or gyro combination—I used the three gyro types in 20
The big IMAA-legal Great Planes Super Stearman sometimes
made fine takeoffs but could be difficult to keep on the runway with
a side wind. The JR G770 3D Gyro gave it a totally different
personality on takeoff.
Put your hand in a pail of water and stir it until you
have a spinning mass of water. Now stop your hand and
hold it flat to block the spinning water. You can feel a
large force. This is essentially what happens when the
spiral propwash (SP) hits the vertical tail of a model.
Spiral means “in a rotating motion”
and propwash is the air that the propeller
blows toward the rear. So we have a
spiraling mass of air moving to the rear of the
airplane: a small tornado.
With the power plant in front, the tail in back, and
the power plant turning clockwise as viewed from the
back, the SP has an opportunity to hit the fuselage.
With no vertical tail, no side force is generated
when the SP hits the fuselage.
When we stick a vertical tail on top of the fuselage in
that mass of rotating air, we get a force on the left side
of the vertical tail. The result of that force on our
aircraft is a yawing moment that wants to force the nose
of the model to the left.
Factors that will result in a higher SP are more
power, bigger propeller, and high rpm. Factors that
make the SP more effective are a bigger vertical tail and
a slow-moving airplane.
The result is that we need to input right rudder
immediately upon power application for takeoff. There
is little we can do about the effect of SP; it is simply a
negative part of a model configuration that is otherwise
is great.
The propeller produces P-factor. A propeller moving
through the air with no angle relative to the air has zero Pfactor.
If the axis of the power plant is tilted upward, such
as in a tail-dragging airplane with conventional landing
gear running along the ground, we develop P-factor.
The up-going propeller blade (on the left side of the
aircraft) has a lower relative angle of attack than the
down-going propeller blade (on the right side of the
aircraft). This means that the right side of the propeller
“disk” produces more forward thrust than the left side.
The result is a left-yawing moment on the model.
A higher-power motor or engine, a bigger propeller,
and a higher angle of attack of the airplane give us a
larger P-factor, and we need more right rudder to
counter it.
Gyroscopic Effects: As the aircraft accelerates down
the runway, the propeller blast on the horizontal tail lifts
the tail wheel off of the ground, leaving the model free
to pivot in yaw on the two main wheels.
As the propeller blast lifts the tail, it forces the
rotating propeller (which is now a big gyroscope) to
change its pitch angle (because it is attached to the
motor shaft). Also known as “gyroscopic procession,”
that effect further increases the need for corrective
rudder input.
The gyroscopic effect applies a yawing moment to
the aircraft that tends to swing the nose to the left. The
magnitude of the gyroscopic torque depends on the mass
of the propeller, the rpm of the propeller, and the pitch
rate of the fuselage.
Heavy propellers plus high rpm plus rapid pitch
rates equals large gyroscopic effects and major leftyawing
tendencies. A large-scale World War I model
with a big propeller that starts out at a hefty angle with
respect to the ground will have a much larger
gyroscopic effect. MA
—Ben Lanterman
Propeller Woes
12sig2.QXD_00MSTRPG.QXD 10/21/10 11:02 AM Page 58
airplanes of all sizes, weights, and landing
gear configurations.
The models, shown in the photos, range
from a light foam indoor aerobatic model
(flown outside in the wind), to the large
Junkers Ju 52 (spanning 94 inches with a
15-pound flying weight), to the huge Great
Planes Super Stearman.
The gyro is mounted in the aircraft so
that the yaw axis of the device (see
instructions) is parallel to the yaw axis of
the model. The yaw axis of the airplane is
a line that goes through its CG and is
perpendicular to the model when it is held
level. The gyro does not have to be
positioned on the aircraft CG.
However, it does need to be mounted
so that vibration from the airplane engine
or motor will not shake it apart. I
recommend using Velcro to attach the
gyro to a light-plywood base. Glue that
base to a piece of thin foam, and use
Velcro to attach the foam piece to the
model. This provides double insulation
from power plant vibration and allows the
gyro to be easily moved if necessary.
The gyro has two plugs that go to the
receiver: one goes into the rudder channel
and the other goes into Channel 5. The
auxiliary channel switch is used to change
the gyro from its HH mode to its RD mode
(or off altogether). The rudder servo plugs
directly into the gyro.
Following the instructions, the HH
limit (gain) is set high (approximately
100%) and the RD gain is set extremely
low (roughly 10%). Because the low RD
gain effectively turns off the gyro, the
transmitter auxiliary switch is used to turn
the HH mode on and off. You will have to
determine what works using your radio
and gyro instructions.
Set the stick trim sliders to neutral and
rudder subtrim values (if available) to
zero. Use rudder pushrod adjustments to
mechanically zero the rudder position.
Make sure that the rudder operates freely
and correctly.
To make initial settings for the gyro
gain, turn on the transmitter with the
auxiliary switch set to the gyro RD mode
position. Turn on the receiver and gyro,
letting the model remain still for 15-20
seconds; this will let the gyro initialize.
Then switch on the gyro HH mode.
You might see the rudder drift slightly
or drive to the extremes of throw. Switch
back to the RD mode and use the rudder
stick trim sliders to adjust the rudder drift
direction to be opposite of the observed
drift. Then switch back to HH and observe
the rudder action.
Repeat this process until you get a nice
zero-drift setting in HH mode. Switch
back to the RD mode and check to make
sure that the mechanical setup is still
giving you a zero rudder position, and
adjust the mechanical zero if necessary.
Now you must determine the direction
of the gyro correction. That is easy to do.
In HH mode, if you yaw the airplane to
the left, the rudder should be driven to the
right as it responds to the yaw. If the
rudder deflection is going the wrong way,
use the small switch on the side of the
gyro to reverse it.
If the rudder-angle change is equal to
or more than the applied yaw angle, the
aircraft is in a good starting place. An
exact angular response can be fine-tuned
later, but I have found that it is
noncritical.
Flight Testing: In preflight setups, the
only practical difference in the gyros I
have worked with is that the more
expensive JR versions are stable with
respect to keeping settings from day to
day. I haven’t had to adjust anything since
the first setup. The E-flite gyro is much
more inexpensive, and with that
apparently comes a small amount of
positional instability from day to day.
The practical meaning of that to me, as
a pilot, is that before each flight I need to
check to ensure that the rudder holds
neutral position when the gyro is switched
to HH. If the rudder starts moving slowly
when it is set to the HH mode, it can be
adjusted with a click or two of rudder
trim.
I have made the HH drift check a part
of my normal preflight checklist, so it is
not a big problem. I like the E-flite gyro
so well that I have 18 airplanes with one
in each.
My initial taxi-only trials involved
using small park flyers in a corner of an
empty parking lot. On a day that had wind
speeds gusting up to 18 mph, I made the
taxi runs so that a full crosswind was
available.
I gave each airplane a bit of power and
allowed it to run approximately 150 feet
with the tail always on the ground. I did that
with the gyro on and off.
It was interesting to watch the rudder
work to counter the wind gusts. A model
that normally would have been blown all
over the parking lot was following straight
lines.
I moved the next tests out to the
Boeing’s Phantom Flyers field in Saint
Charles, Missouri. The club has a good
asphalt runway with a white stripe down the
center (which always seemed to be jeering
at me before).
During the corn-growing season, when
the wind is from the north, a terrific rotor is
caused by the wind spilling over the corn. It
hits the runway area as a rotating
crosswind.
We flew aircraft after aircraft with
consistent results, regardless of wind
direction or airplane configuration,
exercising such variables as P-factor, spiral
propwash, gyroscopic effects, tire spacing,
etc. Both slow and fast takeoffs were made
to try to mess up the “gyro on” takeoffs,
and none did.
The device in HH mode consistently
guided the models to make good takeoffs.
Some airplanes took off straight but with
evident oscillations, some were straight but
moved sideways a bit as the wind blew
them, and other takeoffs were straight. The
results have been uniformly positive with
the HH gyros I have tested.
A great thing about using a gyro is that
if the nose gear is set incorrectly, the tail
wheel is bent, or a landing gear part gets
bent on a bad landing to the point where
the aircraft would normally veer off to the
side during takeoff, the device can still
adjust the takeoff. The model will be
directed to take off straight down the
runway, although the body might have an
apparent yaw angle.
The only exceptions to airplanes going
reasonably straight down the runway were
light models in a hard side wind. The
fuselage would point down the runway
and try to be parallel to the centerline, but
the hard side wind would blow the whole
airplane sideways. But it was better than
the ground loop that normally took place.
Conclusion: The modern RC gyro with
HH capability is a great tool to use to help
almost any model configuration attain
better takeoffs and landing approaches.
All airplanes I tested certainly improved;
ground-looping monsters were turned into
well-mannered aircraft.
The Bf 109 now makes reasonably
fine takeoffs, with no pilot input needed.
A video showing some of my
comparisons between having the HH gyro
off and on, including in the
Messerschmitt, is shown on YouTube. I
made sure that the side winds were
extraordinarily high, to ensure that the
benefits of the HH gyros were exercised
to the max. MA
Edition: Model Aviation - 2010/12
Page Numbers: 55,56,57,58,60,62
56 MODEL AVIATION
The additional light weight of the E-flite
gyro made no difference in the flying
characteristics of these ultra-light models.
The E-flite G110 Micro Heading Lock gyro, shown attached to the side of a typical flat
foamie, can be placed anywhere on an airframe as long as the yaw axis of the gyro is
parallel to a line that is perpendicular to the airplane’s top view.
The fleet of smaller airplanes used in flight tests. The two that received the author’s “Most Improved” awards are the molded
foam Gee Bee (with “NR2100” on the right wing) and the Sig Rascal above its right wingtip.
Photos by the author
attempts, you succeed at a wobbly takeoff (which is probably
cross-runway into the wind) and finally get a good flight going.
After all, the model does fly well.
One might wonder if all flights are going to start this way. The
answer is no, but achieving consistent takeoffs and landings
usually requires intensive practice with that aircraft or plain luck.
Don’t worry; there is another way.
The previous scenario is autobiographical. I have had many of
those kinds of takeoffs with some of my Scale and sport models,
and it seemed to happen all the time with a recently acquired
Messerschmitt Bf 109.
What my squirrely airplanes have had in common is a
conventional landing gear setup (sometimes called a tail-dragger).
There is usually a small wheel or skid at the tail end of the
aircraft.
A degree in aeronautical engineering from Purdue University
helped me understand the math behind the reasons why a
conventional landing gear setup often results in a wild takeoff, but
that is unnecessary. The following description is adequate.
When an aircraft is moving at a moderate speed down the
runway, one wheel is suddenly bumped or stopped by a small
pebble or chunk of grass. The CG tries to keep going forward but
1. Remember to turn off the HH mode after the
model is a wingspan or two above the ground. If you
leave the HH mode on and the airplane starts into its
first turn away from the pit area, the rudder will go hard
over to try to stop the fuselage from turning. Apply
rudder input in the direction of the turn to counter the
effect.
2. Don’t touch the rudder stick at any time during
takeoff. Doing so will offset the angle at which the
aircraft wants to track down the runway. Cut power and
redo the takeoff.
3. When in doubt about what is happening, cut power
to the model and think things through.
4. You are allowed to use the ailerons, elevator, and
power plant in a normal manner. MA
—Ben Lanterman
Gyro Rules to Remember
12sig2.QXD_00MSTRPG.QXD 10/21/10 10:54 AM Page 56
can’t.
Forces from the propeller (see the sidebar about
propeller woes) are there, and the stopped wheel causes the CG
to deflect a bit to the side. Then the airplane rotates even more
around the restrained wheel.
Unless a tail wheel, a tail skid, or the vertical tail has enough
side force to stop the rotation, the fuselage will continue to go
around, to the point at which the CG will try to go along the
original line of travel, even when the fuselage is sideways.
Since the model is moving slowly, the vertical tail won’t
contribute enough aerodynamic stabilizing force to stop the
turning. If the tail wheel is off of the ground, the situation is
worse.
A tricycle landing gear setup doesn’t suffer from this problem.
The CG in front of the main wheels exerts a stabilizing force, and
the nose wheel guides the fuselage.
The worst thing an aircraft with tricycle gear typically does is
go crooked when a gust of side wind hits it. However, such an
airplane can have takeoff problems if the nose gear is set up to be
too touchy.
If conditions around the model were smooth all the time and
the tail wheel worked effectively against a side force, we wouldn’t
have a big problem. But any number of small things can trigger
the divergence to the unstable yaw condition.
However, the biggest factor for most of us is the massive
gusting side wind that seems to come up when we find the rare
spare hour or two hour to fly our models.
The Gyro: A Google Internet search of the problem led me to the
RC Universe forum, where I found a thread concerning the use of
gyros in models. Because these devices are mandatory in RC
helicopters, there have been terrific advances in both affordability
and capability. The consensus on the thread was that using a gyro
on the yaw axis of an airplane could help.
Instructions that come with gyros do not recommend them for
airplanes. I generally recommend following the manufacturer’s
directions, but using a gyro heading-hold (HH) mode for the
specific purpose of aiding a takeoff seems to work well.
The pilot must remember to turn off the gyro when the aircraft
December 2010 57
Above: The intermediate-size models used in
testing. All were reasonably good during takeoff,
but adding the gyro removed the need for
constant pilot corrections in a crosswind.
has obtained an altitude of 3-4 feet. At that time the flier takes
control of the model.
Luckily for me, and probably also you, I learned that I can
use an all-solid-state gyro without knowing a thing about what is
inside the little box. I think of the device as a tiny person who
senses the airplane’s yawing motions during takeoff and
instantly gives the rudder command needed to stop them.
The gyro works exactly the way I try to—but a heck of a lot
faster and more accurately than I can.
To use such a unit you must first hook it up correctly—and
that is relatively easy. A modern gyro has two modes: rate
damping (RD) and HH. The RD mode will diminish oscillations
in yaw, but it won’t necessarily keep the aircraft on the runway.
The HH mode is what we will use for the takeoff problem.
Springy landing gear on the
Gemini biplane makes
takeof fs interesting. As
shown, the gyro is set to
HH mode and the fuselage
is yawed, illustrating
that the rudder is
commanded to try to
stop the yaw.
Below: The big Junkers Ju 52 (at the
bottom) has right-handed propellers; the JR
G500T Ring Gyro offered good results. The
Bf 109 (at lower left) is a ground-handling
nightmare but can easily make long or short
takeoffs with the gyro onboard.
12sig2.QXD_00MSTRPG.QXD 10/21/10 10:58 AM Page 57
58 MODEL AVIATION
We want our model to go straight down the runway with no
corrective input from us.
When the gyro is hooked up and working in the HH mode, you
can grab the vertical tail and manually yaw the airplane’s body 10°.
The result is that the rudder will correct and hold that correction until
the original heading is required (that centerline thingy).
The amount of rudder that the gyro gives is proportional to the
yaw that the model develops as it goes down the runway. As soon as
the main wheels clear the ground, the gyro commands are neither
needed nor wanted; by then the vertical tail has more authority to
offer adequate directional stability.
I usually turn off the gyro after the airplane has gained a few feet
of altitude. The entire takeoff is performed absent of rudder input.
I set two criteria for a gyro: it had to be reasonably inexpensive
and work with older analog servos as well as newer digital servos.
That was so I could cheaply retrofit some of my older models.
You should consider the total investment in the aircraft, compare
it with the cost of the gyro, and determine whether or not making the
modification is worth it. The cost is worth it for me, because now my
takeoffs look professional and I have reduced the chance of breaking
a favorite airplane.
The two most expensive gyros I tested were an older JR G500T
Ring Gyro (I believe it was $200 at the time of purchase) and a JR
G770 3D Gyro ($190 from Horizon Hobby). The least expensive unit
was the E-flite G110 Micro Heading Lock Gyro ($65 from Horizon
Hobby).
These products worked well with both analog and digital servos. I
moved the JR gyros in and out of various airplanes, but I have
purchased many of the E-flite gyros.
Other makes of HH gyros probably work as well, but I couldn’t
afford to test them all. Those I tried have worked well for this series
of tests, and they remain in the models shown in the photos.
The Test Setup: I used a JR 12X transmitter and all varieties of
Spektrum receivers. The only requirement for those was that they had
a fifth channel available for remote gyro control.
I conducted all of the flights from a slick asphalt surface. That is
the worst case from a ground-looping standpoint, because it makes it
easier for the tail wheel to slide sideways.
To ensure that my test results were not flukes—limited to one
airplane or gyro combination—I used the three gyro types in 20
The big IMAA-legal Great Planes Super Stearman sometimes
made fine takeoffs but could be difficult to keep on the runway with
a side wind. The JR G770 3D Gyro gave it a totally different
personality on takeoff.
Put your hand in a pail of water and stir it until you
have a spinning mass of water. Now stop your hand and
hold it flat to block the spinning water. You can feel a
large force. This is essentially what happens when the
spiral propwash (SP) hits the vertical tail of a model.
Spiral means “in a rotating motion”
and propwash is the air that the propeller
blows toward the rear. So we have a
spiraling mass of air moving to the rear of the
airplane: a small tornado.
With the power plant in front, the tail in back, and
the power plant turning clockwise as viewed from the
back, the SP has an opportunity to hit the fuselage.
With no vertical tail, no side force is generated
when the SP hits the fuselage.
When we stick a vertical tail on top of the fuselage in
that mass of rotating air, we get a force on the left side
of the vertical tail. The result of that force on our
aircraft is a yawing moment that wants to force the nose
of the model to the left.
Factors that will result in a higher SP are more
power, bigger propeller, and high rpm. Factors that
make the SP more effective are a bigger vertical tail and
a slow-moving airplane.
The result is that we need to input right rudder
immediately upon power application for takeoff. There
is little we can do about the effect of SP; it is simply a
negative part of a model configuration that is otherwise
is great.
The propeller produces P-factor. A propeller moving
through the air with no angle relative to the air has zero Pfactor.
If the axis of the power plant is tilted upward, such
as in a tail-dragging airplane with conventional landing
gear running along the ground, we develop P-factor.
The up-going propeller blade (on the left side of the
aircraft) has a lower relative angle of attack than the
down-going propeller blade (on the right side of the
aircraft). This means that the right side of the propeller
“disk” produces more forward thrust than the left side.
The result is a left-yawing moment on the model.
A higher-power motor or engine, a bigger propeller,
and a higher angle of attack of the airplane give us a
larger P-factor, and we need more right rudder to
counter it.
Gyroscopic Effects: As the aircraft accelerates down
the runway, the propeller blast on the horizontal tail lifts
the tail wheel off of the ground, leaving the model free
to pivot in yaw on the two main wheels.
As the propeller blast lifts the tail, it forces the
rotating propeller (which is now a big gyroscope) to
change its pitch angle (because it is attached to the
motor shaft). Also known as “gyroscopic procession,”
that effect further increases the need for corrective
rudder input.
The gyroscopic effect applies a yawing moment to
the aircraft that tends to swing the nose to the left. The
magnitude of the gyroscopic torque depends on the mass
of the propeller, the rpm of the propeller, and the pitch
rate of the fuselage.
Heavy propellers plus high rpm plus rapid pitch
rates equals large gyroscopic effects and major leftyawing
tendencies. A large-scale World War I model
with a big propeller that starts out at a hefty angle with
respect to the ground will have a much larger
gyroscopic effect. MA
—Ben Lanterman
Propeller Woes
12sig2.QXD_00MSTRPG.QXD 10/21/10 11:02 AM Page 58
airplanes of all sizes, weights, and landing
gear configurations.
The models, shown in the photos, range
from a light foam indoor aerobatic model
(flown outside in the wind), to the large
Junkers Ju 52 (spanning 94 inches with a
15-pound flying weight), to the huge Great
Planes Super Stearman.
The gyro is mounted in the aircraft so
that the yaw axis of the device (see
instructions) is parallel to the yaw axis of
the model. The yaw axis of the airplane is
a line that goes through its CG and is
perpendicular to the model when it is held
level. The gyro does not have to be
positioned on the aircraft CG.
However, it does need to be mounted
so that vibration from the airplane engine
or motor will not shake it apart. I
recommend using Velcro to attach the
gyro to a light-plywood base. Glue that
base to a piece of thin foam, and use
Velcro to attach the foam piece to the
model. This provides double insulation
from power plant vibration and allows the
gyro to be easily moved if necessary.
The gyro has two plugs that go to the
receiver: one goes into the rudder channel
and the other goes into Channel 5. The
auxiliary channel switch is used to change
the gyro from its HH mode to its RD mode
(or off altogether). The rudder servo plugs
directly into the gyro.
Following the instructions, the HH
limit (gain) is set high (approximately
100%) and the RD gain is set extremely
low (roughly 10%). Because the low RD
gain effectively turns off the gyro, the
transmitter auxiliary switch is used to turn
the HH mode on and off. You will have to
determine what works using your radio
and gyro instructions.
Set the stick trim sliders to neutral and
rudder subtrim values (if available) to
zero. Use rudder pushrod adjustments to
mechanically zero the rudder position.
Make sure that the rudder operates freely
and correctly.
To make initial settings for the gyro
gain, turn on the transmitter with the
auxiliary switch set to the gyro RD mode
position. Turn on the receiver and gyro,
letting the model remain still for 15-20
seconds; this will let the gyro initialize.
Then switch on the gyro HH mode.
You might see the rudder drift slightly
or drive to the extremes of throw. Switch
back to the RD mode and use the rudder
stick trim sliders to adjust the rudder drift
direction to be opposite of the observed
drift. Then switch back to HH and observe
the rudder action.
Repeat this process until you get a nice
zero-drift setting in HH mode. Switch
back to the RD mode and check to make
sure that the mechanical setup is still
giving you a zero rudder position, and
adjust the mechanical zero if necessary.
Now you must determine the direction
of the gyro correction. That is easy to do.
In HH mode, if you yaw the airplane to
the left, the rudder should be driven to the
right as it responds to the yaw. If the
rudder deflection is going the wrong way,
use the small switch on the side of the
gyro to reverse it.
If the rudder-angle change is equal to
or more than the applied yaw angle, the
aircraft is in a good starting place. An
exact angular response can be fine-tuned
later, but I have found that it is
noncritical.
Flight Testing: In preflight setups, the
only practical difference in the gyros I
have worked with is that the more
expensive JR versions are stable with
respect to keeping settings from day to
day. I haven’t had to adjust anything since
the first setup. The E-flite gyro is much
more inexpensive, and with that
apparently comes a small amount of
positional instability from day to day.
The practical meaning of that to me, as
a pilot, is that before each flight I need to
check to ensure that the rudder holds
neutral position when the gyro is switched
to HH. If the rudder starts moving slowly
when it is set to the HH mode, it can be
adjusted with a click or two of rudder
trim.
I have made the HH drift check a part
of my normal preflight checklist, so it is
not a big problem. I like the E-flite gyro
so well that I have 18 airplanes with one
in each.
My initial taxi-only trials involved
using small park flyers in a corner of an
empty parking lot. On a day that had wind
speeds gusting up to 18 mph, I made the
taxi runs so that a full crosswind was
available.
I gave each airplane a bit of power and
allowed it to run approximately 150 feet
with the tail always on the ground. I did that
with the gyro on and off.
It was interesting to watch the rudder
work to counter the wind gusts. A model
that normally would have been blown all
over the parking lot was following straight
lines.
I moved the next tests out to the
Boeing’s Phantom Flyers field in Saint
Charles, Missouri. The club has a good
asphalt runway with a white stripe down the
center (which always seemed to be jeering
at me before).
During the corn-growing season, when
the wind is from the north, a terrific rotor is
caused by the wind spilling over the corn. It
hits the runway area as a rotating
crosswind.
We flew aircraft after aircraft with
consistent results, regardless of wind
direction or airplane configuration,
exercising such variables as P-factor, spiral
propwash, gyroscopic effects, tire spacing,
etc. Both slow and fast takeoffs were made
to try to mess up the “gyro on” takeoffs,
and none did.
The device in HH mode consistently
guided the models to make good takeoffs.
Some airplanes took off straight but with
evident oscillations, some were straight but
moved sideways a bit as the wind blew
them, and other takeoffs were straight. The
results have been uniformly positive with
the HH gyros I have tested.
A great thing about using a gyro is that
if the nose gear is set incorrectly, the tail
wheel is bent, or a landing gear part gets
bent on a bad landing to the point where
the aircraft would normally veer off to the
side during takeoff, the device can still
adjust the takeoff. The model will be
directed to take off straight down the
runway, although the body might have an
apparent yaw angle.
The only exceptions to airplanes going
reasonably straight down the runway were
light models in a hard side wind. The
fuselage would point down the runway
and try to be parallel to the centerline, but
the hard side wind would blow the whole
airplane sideways. But it was better than
the ground loop that normally took place.
Conclusion: The modern RC gyro with
HH capability is a great tool to use to help
almost any model configuration attain
better takeoffs and landing approaches.
All airplanes I tested certainly improved;
ground-looping monsters were turned into
well-mannered aircraft.
The Bf 109 now makes reasonably
fine takeoffs, with no pilot input needed.
A video showing some of my
comparisons between having the HH gyro
off and on, including in the
Messerschmitt, is shown on YouTube. I
made sure that the side winds were
extraordinarily high, to ensure that the
benefits of the HH gyros were exercised
to the max. MA
Edition: Model Aviation - 2010/12
Page Numbers: 55,56,57,58,60,62
56 MODEL AVIATION
The additional light weight of the E-flite
gyro made no difference in the flying
characteristics of these ultra-light models.
The E-flite G110 Micro Heading Lock gyro, shown attached to the side of a typical flat
foamie, can be placed anywhere on an airframe as long as the yaw axis of the gyro is
parallel to a line that is perpendicular to the airplane’s top view.
The fleet of smaller airplanes used in flight tests. The two that received the author’s “Most Improved” awards are the molded
foam Gee Bee (with “NR2100” on the right wing) and the Sig Rascal above its right wingtip.
Photos by the author
attempts, you succeed at a wobbly takeoff (which is probably
cross-runway into the wind) and finally get a good flight going.
After all, the model does fly well.
One might wonder if all flights are going to start this way. The
answer is no, but achieving consistent takeoffs and landings
usually requires intensive practice with that aircraft or plain luck.
Don’t worry; there is another way.
The previous scenario is autobiographical. I have had many of
those kinds of takeoffs with some of my Scale and sport models,
and it seemed to happen all the time with a recently acquired
Messerschmitt Bf 109.
What my squirrely airplanes have had in common is a
conventional landing gear setup (sometimes called a tail-dragger).
There is usually a small wheel or skid at the tail end of the
aircraft.
A degree in aeronautical engineering from Purdue University
helped me understand the math behind the reasons why a
conventional landing gear setup often results in a wild takeoff, but
that is unnecessary. The following description is adequate.
When an aircraft is moving at a moderate speed down the
runway, one wheel is suddenly bumped or stopped by a small
pebble or chunk of grass. The CG tries to keep going forward but
1. Remember to turn off the HH mode after the
model is a wingspan or two above the ground. If you
leave the HH mode on and the airplane starts into its
first turn away from the pit area, the rudder will go hard
over to try to stop the fuselage from turning. Apply
rudder input in the direction of the turn to counter the
effect.
2. Don’t touch the rudder stick at any time during
takeoff. Doing so will offset the angle at which the
aircraft wants to track down the runway. Cut power and
redo the takeoff.
3. When in doubt about what is happening, cut power
to the model and think things through.
4. You are allowed to use the ailerons, elevator, and
power plant in a normal manner. MA
—Ben Lanterman
Gyro Rules to Remember
12sig2.QXD_00MSTRPG.QXD 10/21/10 10:54 AM Page 56
can’t.
Forces from the propeller (see the sidebar about
propeller woes) are there, and the stopped wheel causes the CG
to deflect a bit to the side. Then the airplane rotates even more
around the restrained wheel.
Unless a tail wheel, a tail skid, or the vertical tail has enough
side force to stop the rotation, the fuselage will continue to go
around, to the point at which the CG will try to go along the
original line of travel, even when the fuselage is sideways.
Since the model is moving slowly, the vertical tail won’t
contribute enough aerodynamic stabilizing force to stop the
turning. If the tail wheel is off of the ground, the situation is
worse.
A tricycle landing gear setup doesn’t suffer from this problem.
The CG in front of the main wheels exerts a stabilizing force, and
the nose wheel guides the fuselage.
The worst thing an aircraft with tricycle gear typically does is
go crooked when a gust of side wind hits it. However, such an
airplane can have takeoff problems if the nose gear is set up to be
too touchy.
If conditions around the model were smooth all the time and
the tail wheel worked effectively against a side force, we wouldn’t
have a big problem. But any number of small things can trigger
the divergence to the unstable yaw condition.
However, the biggest factor for most of us is the massive
gusting side wind that seems to come up when we find the rare
spare hour or two hour to fly our models.
The Gyro: A Google Internet search of the problem led me to the
RC Universe forum, where I found a thread concerning the use of
gyros in models. Because these devices are mandatory in RC
helicopters, there have been terrific advances in both affordability
and capability. The consensus on the thread was that using a gyro
on the yaw axis of an airplane could help.
Instructions that come with gyros do not recommend them for
airplanes. I generally recommend following the manufacturer’s
directions, but using a gyro heading-hold (HH) mode for the
specific purpose of aiding a takeoff seems to work well.
The pilot must remember to turn off the gyro when the aircraft
December 2010 57
Above: The intermediate-size models used in
testing. All were reasonably good during takeoff,
but adding the gyro removed the need for
constant pilot corrections in a crosswind.
has obtained an altitude of 3-4 feet. At that time the flier takes
control of the model.
Luckily for me, and probably also you, I learned that I can
use an all-solid-state gyro without knowing a thing about what is
inside the little box. I think of the device as a tiny person who
senses the airplane’s yawing motions during takeoff and
instantly gives the rudder command needed to stop them.
The gyro works exactly the way I try to—but a heck of a lot
faster and more accurately than I can.
To use such a unit you must first hook it up correctly—and
that is relatively easy. A modern gyro has two modes: rate
damping (RD) and HH. The RD mode will diminish oscillations
in yaw, but it won’t necessarily keep the aircraft on the runway.
The HH mode is what we will use for the takeoff problem.
Springy landing gear on the
Gemini biplane makes
takeof fs interesting. As
shown, the gyro is set to
HH mode and the fuselage
is yawed, illustrating
that the rudder is
commanded to try to
stop the yaw.
Below: The big Junkers Ju 52 (at the
bottom) has right-handed propellers; the JR
G500T Ring Gyro offered good results. The
Bf 109 (at lower left) is a ground-handling
nightmare but can easily make long or short
takeoffs with the gyro onboard.
12sig2.QXD_00MSTRPG.QXD 10/21/10 10:58 AM Page 57
58 MODEL AVIATION
We want our model to go straight down the runway with no
corrective input from us.
When the gyro is hooked up and working in the HH mode, you
can grab the vertical tail and manually yaw the airplane’s body 10°.
The result is that the rudder will correct and hold that correction until
the original heading is required (that centerline thingy).
The amount of rudder that the gyro gives is proportional to the
yaw that the model develops as it goes down the runway. As soon as
the main wheels clear the ground, the gyro commands are neither
needed nor wanted; by then the vertical tail has more authority to
offer adequate directional stability.
I usually turn off the gyro after the airplane has gained a few feet
of altitude. The entire takeoff is performed absent of rudder input.
I set two criteria for a gyro: it had to be reasonably inexpensive
and work with older analog servos as well as newer digital servos.
That was so I could cheaply retrofit some of my older models.
You should consider the total investment in the aircraft, compare
it with the cost of the gyro, and determine whether or not making the
modification is worth it. The cost is worth it for me, because now my
takeoffs look professional and I have reduced the chance of breaking
a favorite airplane.
The two most expensive gyros I tested were an older JR G500T
Ring Gyro (I believe it was $200 at the time of purchase) and a JR
G770 3D Gyro ($190 from Horizon Hobby). The least expensive unit
was the E-flite G110 Micro Heading Lock Gyro ($65 from Horizon
Hobby).
These products worked well with both analog and digital servos. I
moved the JR gyros in and out of various airplanes, but I have
purchased many of the E-flite gyros.
Other makes of HH gyros probably work as well, but I couldn’t
afford to test them all. Those I tried have worked well for this series
of tests, and they remain in the models shown in the photos.
The Test Setup: I used a JR 12X transmitter and all varieties of
Spektrum receivers. The only requirement for those was that they had
a fifth channel available for remote gyro control.
I conducted all of the flights from a slick asphalt surface. That is
the worst case from a ground-looping standpoint, because it makes it
easier for the tail wheel to slide sideways.
To ensure that my test results were not flukes—limited to one
airplane or gyro combination—I used the three gyro types in 20
The big IMAA-legal Great Planes Super Stearman sometimes
made fine takeoffs but could be difficult to keep on the runway with
a side wind. The JR G770 3D Gyro gave it a totally different
personality on takeoff.
Put your hand in a pail of water and stir it until you
have a spinning mass of water. Now stop your hand and
hold it flat to block the spinning water. You can feel a
large force. This is essentially what happens when the
spiral propwash (SP) hits the vertical tail of a model.
Spiral means “in a rotating motion”
and propwash is the air that the propeller
blows toward the rear. So we have a
spiraling mass of air moving to the rear of the
airplane: a small tornado.
With the power plant in front, the tail in back, and
the power plant turning clockwise as viewed from the
back, the SP has an opportunity to hit the fuselage.
With no vertical tail, no side force is generated
when the SP hits the fuselage.
When we stick a vertical tail on top of the fuselage in
that mass of rotating air, we get a force on the left side
of the vertical tail. The result of that force on our
aircraft is a yawing moment that wants to force the nose
of the model to the left.
Factors that will result in a higher SP are more
power, bigger propeller, and high rpm. Factors that
make the SP more effective are a bigger vertical tail and
a slow-moving airplane.
The result is that we need to input right rudder
immediately upon power application for takeoff. There
is little we can do about the effect of SP; it is simply a
negative part of a model configuration that is otherwise
is great.
The propeller produces P-factor. A propeller moving
through the air with no angle relative to the air has zero Pfactor.
If the axis of the power plant is tilted upward, such
as in a tail-dragging airplane with conventional landing
gear running along the ground, we develop P-factor.
The up-going propeller blade (on the left side of the
aircraft) has a lower relative angle of attack than the
down-going propeller blade (on the right side of the
aircraft). This means that the right side of the propeller
“disk” produces more forward thrust than the left side.
The result is a left-yawing moment on the model.
A higher-power motor or engine, a bigger propeller,
and a higher angle of attack of the airplane give us a
larger P-factor, and we need more right rudder to
counter it.
Gyroscopic Effects: As the aircraft accelerates down
the runway, the propeller blast on the horizontal tail lifts
the tail wheel off of the ground, leaving the model free
to pivot in yaw on the two main wheels.
As the propeller blast lifts the tail, it forces the
rotating propeller (which is now a big gyroscope) to
change its pitch angle (because it is attached to the
motor shaft). Also known as “gyroscopic procession,”
that effect further increases the need for corrective
rudder input.
The gyroscopic effect applies a yawing moment to
the aircraft that tends to swing the nose to the left. The
magnitude of the gyroscopic torque depends on the mass
of the propeller, the rpm of the propeller, and the pitch
rate of the fuselage.
Heavy propellers plus high rpm plus rapid pitch
rates equals large gyroscopic effects and major leftyawing
tendencies. A large-scale World War I model
with a big propeller that starts out at a hefty angle with
respect to the ground will have a much larger
gyroscopic effect. MA
—Ben Lanterman
Propeller Woes
12sig2.QXD_00MSTRPG.QXD 10/21/10 11:02 AM Page 58
airplanes of all sizes, weights, and landing
gear configurations.
The models, shown in the photos, range
from a light foam indoor aerobatic model
(flown outside in the wind), to the large
Junkers Ju 52 (spanning 94 inches with a
15-pound flying weight), to the huge Great
Planes Super Stearman.
The gyro is mounted in the aircraft so
that the yaw axis of the device (see
instructions) is parallel to the yaw axis of
the model. The yaw axis of the airplane is
a line that goes through its CG and is
perpendicular to the model when it is held
level. The gyro does not have to be
positioned on the aircraft CG.
However, it does need to be mounted
so that vibration from the airplane engine
or motor will not shake it apart. I
recommend using Velcro to attach the
gyro to a light-plywood base. Glue that
base to a piece of thin foam, and use
Velcro to attach the foam piece to the
model. This provides double insulation
from power plant vibration and allows the
gyro to be easily moved if necessary.
The gyro has two plugs that go to the
receiver: one goes into the rudder channel
and the other goes into Channel 5. The
auxiliary channel switch is used to change
the gyro from its HH mode to its RD mode
(or off altogether). The rudder servo plugs
directly into the gyro.
Following the instructions, the HH
limit (gain) is set high (approximately
100%) and the RD gain is set extremely
low (roughly 10%). Because the low RD
gain effectively turns off the gyro, the
transmitter auxiliary switch is used to turn
the HH mode on and off. You will have to
determine what works using your radio
and gyro instructions.
Set the stick trim sliders to neutral and
rudder subtrim values (if available) to
zero. Use rudder pushrod adjustments to
mechanically zero the rudder position.
Make sure that the rudder operates freely
and correctly.
To make initial settings for the gyro
gain, turn on the transmitter with the
auxiliary switch set to the gyro RD mode
position. Turn on the receiver and gyro,
letting the model remain still for 15-20
seconds; this will let the gyro initialize.
Then switch on the gyro HH mode.
You might see the rudder drift slightly
or drive to the extremes of throw. Switch
back to the RD mode and use the rudder
stick trim sliders to adjust the rudder drift
direction to be opposite of the observed
drift. Then switch back to HH and observe
the rudder action.
Repeat this process until you get a nice
zero-drift setting in HH mode. Switch
back to the RD mode and check to make
sure that the mechanical setup is still
giving you a zero rudder position, and
adjust the mechanical zero if necessary.
Now you must determine the direction
of the gyro correction. That is easy to do.
In HH mode, if you yaw the airplane to
the left, the rudder should be driven to the
right as it responds to the yaw. If the
rudder deflection is going the wrong way,
use the small switch on the side of the
gyro to reverse it.
If the rudder-angle change is equal to
or more than the applied yaw angle, the
aircraft is in a good starting place. An
exact angular response can be fine-tuned
later, but I have found that it is
noncritical.
Flight Testing: In preflight setups, the
only practical difference in the gyros I
have worked with is that the more
expensive JR versions are stable with
respect to keeping settings from day to
day. I haven’t had to adjust anything since
the first setup. The E-flite gyro is much
more inexpensive, and with that
apparently comes a small amount of
positional instability from day to day.
The practical meaning of that to me, as
a pilot, is that before each flight I need to
check to ensure that the rudder holds
neutral position when the gyro is switched
to HH. If the rudder starts moving slowly
when it is set to the HH mode, it can be
adjusted with a click or two of rudder
trim.
I have made the HH drift check a part
of my normal preflight checklist, so it is
not a big problem. I like the E-flite gyro
so well that I have 18 airplanes with one
in each.
My initial taxi-only trials involved
using small park flyers in a corner of an
empty parking lot. On a day that had wind
speeds gusting up to 18 mph, I made the
taxi runs so that a full crosswind was
available.
I gave each airplane a bit of power and
allowed it to run approximately 150 feet
with the tail always on the ground. I did that
with the gyro on and off.
It was interesting to watch the rudder
work to counter the wind gusts. A model
that normally would have been blown all
over the parking lot was following straight
lines.
I moved the next tests out to the
Boeing’s Phantom Flyers field in Saint
Charles, Missouri. The club has a good
asphalt runway with a white stripe down the
center (which always seemed to be jeering
at me before).
During the corn-growing season, when
the wind is from the north, a terrific rotor is
caused by the wind spilling over the corn. It
hits the runway area as a rotating
crosswind.
We flew aircraft after aircraft with
consistent results, regardless of wind
direction or airplane configuration,
exercising such variables as P-factor, spiral
propwash, gyroscopic effects, tire spacing,
etc. Both slow and fast takeoffs were made
to try to mess up the “gyro on” takeoffs,
and none did.
The device in HH mode consistently
guided the models to make good takeoffs.
Some airplanes took off straight but with
evident oscillations, some were straight but
moved sideways a bit as the wind blew
them, and other takeoffs were straight. The
results have been uniformly positive with
the HH gyros I have tested.
A great thing about using a gyro is that
if the nose gear is set incorrectly, the tail
wheel is bent, or a landing gear part gets
bent on a bad landing to the point where
the aircraft would normally veer off to the
side during takeoff, the device can still
adjust the takeoff. The model will be
directed to take off straight down the
runway, although the body might have an
apparent yaw angle.
The only exceptions to airplanes going
reasonably straight down the runway were
light models in a hard side wind. The
fuselage would point down the runway
and try to be parallel to the centerline, but
the hard side wind would blow the whole
airplane sideways. But it was better than
the ground loop that normally took place.
Conclusion: The modern RC gyro with
HH capability is a great tool to use to help
almost any model configuration attain
better takeoffs and landing approaches.
All airplanes I tested certainly improved;
ground-looping monsters were turned into
well-mannered aircraft.
The Bf 109 now makes reasonably
fine takeoffs, with no pilot input needed.
A video showing some of my
comparisons between having the HH gyro
off and on, including in the
Messerschmitt, is shown on YouTube. I
made sure that the side winds were
extraordinarily high, to ensure that the
benefits of the HH gyros were exercised
to the max. MA
