Trimming
August 2006 55
by Dean Pappas
Part 2 From the Ground Up
The simplest way to measure the right-thrust angle is to measure the distance from
the propeller tip to the tail post on both sides. Either use that trigonometry you forgot
or remember that with a 12-inch propeller, 3/16-inch difference equals 1°.
PICKING UP Where We Left Off: In the
first installment of this “Trimming From
the Ground Up” series we dealt with the
subject of pitch trim. As it turns out, it’s a
whole lot more than just moving the
transmitter trim lever a few clicks or beeps
until the model flies without climbing or
diving.
Airplane trimming is similar to setting
up a race car: even when a crew chief says
the “race car was fast right out of the
trailer,” he really means that the team was
able to go through the entire list of setup
checks quickly. That usually means few or
no adjustments were necessary, but it
doesn’t mean every little thing wasn’t
checked anyway.
As it turns out, there aren’t all that
many things to check when test-flying a
new airplane, but if one of the tests
described here shows a problem, you are
working harder than you need to when
flying your aircraft. If your model is
intended for the all-important one of
training mission, that’s a bad thing. If you
are a more advanced flier, you are simply
missing out on flying and looking better
than you already do.
The mission of this series is to describe
the tests and corrective actions, in a
systematized way, to help you make your
airplane fly better. None of it is any great
effort, and you don’t have to attack it all at
once. Your model’s pitch behavior can be
investigated separately from something
such as the unfortunate tendency the
airplane has to turn left immediately after
takeoff.
Engine Right Thrust: I’ve already
written about the adjustment of
downthrust, so now it’s time to discuss
right thrust.
Some years ago I attended a Scale
Masters Qualifier meet. The airplanes sure
were beautiful; there just aren’t enough
people who build like that! One of the
competitors was flying a Cessna L-19 Bird
Dog: a slow-flying, military, forward-aircontrol-
type airplane. The same aircraft, in
civilian livery, has been used as a trainer
and glider tow airplane for decades.
We were taking off from right to left
that day, and every one of the pilot’s
takeoffs veered left, over the flightline—
and the pits—and the parking area! Many
of you have seen this one, right? No, left!
It’s particularly tough for a student to deal
with, and it’s a serious safety hazard as
well. This is what happens when the right
thrust is not correctly set up.
The full-scale pilots deal with this
situation differently—emphasizing the
proper application of right rudder to
counteract “torque” on takeoff. Although
that is an excellent skill to develop, the fix
for most aeromodels is to put the proper
amount of right thrust into the engine. This
minimizes the rudder corrections that are
necessary during takeoff.
This is important for the beginner pilot,
who is still learning to take off. Should we
let the beginner give everyone in the pits a
haircut until he or she learns to use his or
her rudder hand simultaneously and
independently of the right hand? I think
not. Actually most model-airplane fliers
never really master accurate and
independent control with their rudder
hand, but it is a worthy goal.
Before I describe how to put right
thrust into the model and how to test for
whether or not the airplane has the correct
amount of right thrust, I need to discuss
the nature of the right-thrust trimming
adjustment. Right thrust is a compromise
because it is used to counteract an
airspeed-dependent problem. As it turns
out, it is usually a good compromise.
The Application: At low and part throttle
the effect of right thrust is minimal. Here’s
where the compromise comes in. We set
the right thrust to straighten out a fullpower
takeoff climb and accept the small,
unwanted influence it has at cruise.
In the glide the right thrust has
practically no effect, so it’s no problem.
Assuming that the airplane is trimmed to
glide in a straight line in calm air, the job
of right thrust is to preserve that straight
flight path under full power.
That’s all there is to it. Typically, the
right-thrust adjustment determines how
straight the airplane climbs after takeoff.
Good landing-gear setup will reduce the
steering workload until the student gets a
model in the air. The landing-gear
discussion comes later.
Let’s adjust the right thrust. As you
read a bit ago, the requirement is for the
model to go straight in the glide with the
engine idling and at full throttle in a
Right-Thrust Measure
56 MODEL AVIATION
This tail-wheel assembly has no caster so that side loads can apply no twist to the rudder.
The pivot location, ahead of the rudder hinge line, and the connection of the tiller a short
distance behind it reduce the tail-wheel throw to roughly half the rudder throw.
If the tire contact patch is not directly in line with the axis of steering rotation, the
impact of landing will try to twist the nose-wheel sideways.
Tail Wheel Caster
Steering Axis Offset
An Alternate
Down-Thrust Check for
Advanced Sport Models
There is another way to
check the downthrust, for those
of you flying sport airplanes that
are not intended to be as stable
as trainers. This applies to most
designs with semisymmetrical
and symmetrical airfoils.
Set up a hands-off level pass
on the far edge of the runway,
approximately 50 feet up, so you
can see the airplane very well.
As you fly along nice and
straight, with the model trimmed
“hands off,” suddenly pull the
power to idle.
For a second or two the
airplane will still be zipping along
at cruise speed. The downthrust
is taken out of the balance
equation for just that second or
so—and the aerodynamic trim
predominates. The difference (if
any) is the effect of the
downthrust.
If, as you pull the throttle
back, the nose twitches up and
then the model slows into a
glide, you have too much
downthrust.
If the nose abruptly drops a
tiny bit and the airplane instantly
assumes a fast, nose-down glide
when the throttle is pulled, you
need more downthrust. That’s
because your elevator trim has
been fighting down against the
engine-induced climb all along.
If the downthrust is correct,
the airplane continues straight
for a second or two and
gradually fades into the glide
angle.
It is also possible to see the
effect of wrong downthrust
when the power is applied
rather than pulled back. In cases
where much more downthrust is
needed, you may see the model
abruptly nose-up when the
power is suddenly fire-walled for
a go-around on landing. This is
bad news—especially with a
heavy Scale model that does not
forgive a nose-too-high climb. MA
—Dean Pappas
Drawings and photos by the author except as noted
August 2006 57
A good ground stance has the wing sitting at a small but positive
angle of attack: somewhere between 0° and 3° positive.
A nose-down stance, like a bloodhound, forces the model to
accelerate beyond the necessary airspeed for flight and then
leap into the air, resulting in a steep climbout.
Nose-up stances cause two problems: overly sensitive steering
caused by “wheelbarrowing” and premature liftoff, leading to
departure stalls.
The proper ratio between rudder throw and nose-wheel throw
is usually had when the rudder pushrod is connected to the
outside of the rudder-servo wheel/arm and the nose-gear
pushrod is connected to the innermost hole on the servo wheel
(above) and the outermost hole of the nose-gear arm (below).
takeoff climb. For now let’s assume the airplane glides without
turning since it was trimmed for straight and level flight at cruise
power. (In the next installment we will discuss making the model
fly straight at all airspeeds.)
What we really need to do is adjust the engine right thrust so it
adds the right amount of correction for “engine torque” during
climb. Engine torque makes the airplane turn left. The word
“torque” is a misnomer, but it is a convenient catchall.
The Right-Thrust Test: To start with, let’s make sure the model is
trimmed to fly nice and straight at cruise power. Next, I like to set
the airplane up so it is pointed straight away from me and headed
either directly into the wind or directly downwind. You don’t
want to do this lined up with a crosswind because the sideward
wind drift hides the turn for which you are looking.
Now that you are lined up, add full throttle and smoothly pull
up into a climb, at the same angle as your steepest post-takeoff
A Little Theory
The primary cause of what we call “torque” is the
spiral airflow that comes off of the propeller. There are
two other sources—the “P” factor and the pure torque
of the engine—but they are small contributors.
If you were to hold a crepe-paper streamer behind the
propeller, you would see that the airflow coming from the
propeller follows a corkscrewlike path in the same
direction as the propeller rotation. This airflow strikes the
left side of the fin and rudder, which is usually above the
thrustline, and, as a result, yaws the airplane left.
As the model accelerates, the pitch of this corkscrew
pattern gets straighter and straighter. Because of this, the
torque effect is greatest at low speed and high throttle,
less at cruise speed and power, and gone at idle. MA
—Dean Pappas
climb. We typically climb into the wind,
but doing this downwind also works, and it
allows you to pick the direction so you
don’t have to fly over the pits or the safety
line at the field.
You don’t want to climb so steeply that
the model is stalling, but you do want to
climb as steeply as your horsepower will
permit. The airplane will lose airspeed
during the climb, and it may become more
easily influenced as the flying surfaces lose
some of their control power. In all
likelihood the model will start to turn.
If the airplane deviates to the left, you
will have to add more right thrust. On the
next flight retrim for straight and level
flight (probably just a click of rudder) and
repeat the test until the model climbs
straight.
If the airplane has too much right-thrust,
it will deviate to the right in the climb. That
doesn’t happen often.
If the right thrust is close to correct, and
if there is enough wind to make the model
bounce around, you may have to repeat the
test a couple times to be sure of which
direction the airplane is turning. That
usually means you are getting close.
It is best to adjust the right-thrust angle
one degree at a time and repeat the process.
Most airplanes have, or at least need, 2°-3°
of right-thrust, although a rare few need
much more.
Right-Thrust Measurement: The easiest
way I have found to determine the rightthrust
angle is to measure the distance from
each propeller tip to the tail post. With a
12-inch propeller the difference between
the two measurements will be 3/16 inch for
every degree of right-thrust. Three degrees
of right-thrust works out to 9/16 inch
difference between the two measurements
from the tail post. With a 16-inch propeller
this ratio works out to 1/4 inch per degree.
You might have to readjust the right
thrust a time or two, but if you start with it
adjusted as the kit recommends, you should
have to make only a fine adjustment or
two. Many kits and ARFs may not make
how much right thrust is recommended
entirely clear, but if you can’t find anything
on the plans or in the instructions, start
with 21/2° or so.
Landing Gear: An airplane that rolls
straight and responds predictably to
steering input, especially on takeoff, will
be easier to fly. If you want to look like a
hero at the flying field, die-straight takeoffs
and smooth landings that roll to a straight
stop will help.
On the other hand, if you really crave
attention, zigzagging across the runway
will have everyone watching you—as they
run for cover! That’s not how you want to
be noticed, so we will devote some
attention to describing good landing-gear
setup. Most trainers are designed with
tricycle landing gear, so I will cover
models with that kind first, followed by
tail-draggers.
A few problems can afflict a tricyclegeared
airplane. The most common is the
use of a nose-wheel steering linkage that has
way too much throw.
The model does not need to be able to
turn within its own wingspan; the minimum
turning radius should be roughly 15 feet
with full rudder control applied. That
probably works out to only 5° of turn at the
nose wheel.
This is accomplished by connecting the
linkage to the innermost hole of the servo
arm and the outermost hole on the nose-strut
steering arm. It is sometimes helpful to drill
a new hole in the servo arm that is as close
as possible to the center post. Too much
steering throw not only makes it difficult to
steer straight at speed, but it can overload
the rudder servo and prematurely age or
damage it.
The next problem is an overly flexible
steering linkage. You need positive control,
and a springy linkage does not offer that. If
the steering linkage has too much give in it,
the nose-wheel may even twist sideways at
touchdown (impact?). This makes the
airplane “curtsy” in the middle of the
runway and can even tear out the firewall if
repeated often enough.
Some fliers will tell you that a springy
linkage can save the servo, but the best way
to do that is to give the servo maximum
mechanical advantage, as described in theDid you ever try to make a gentle turn
while running with a fully laden
wheelbarrow? It tried to tip, didn’t it? The
same is true with a tricycle-geared
airplane if it is running up on only the
nose wheel.
The ideal attitude is with the wing
chord line (or flat bottom) within a few
degrees of level with the ground. A wellset-
up trainer will lift off with just a tiny
touch of up-elevator when the airspeed is
right. For trainers with flat-bottomed
wings, this stance will lift off by itself
when the airplane is going fast enough.
The last tricycle-gear problem is the
fore and aft location of the main gear. If
the main gear is placed too far aft, the
airplane has a great deal of weight on the
nose wheel. This also makes the highspeed
steering more sensitive and requires
lots of up-elevator input to break ground.
Try pushing down on the stabilizer to lift
the nose wheel, to get a feel for how
much force the up-elevator control has to
make.
Again, this can lead to an overly steep
departure after an excessively long
takeoff roll. It also causes the airplane to
“slap” onto the ground during landing;
that can add to the wear and tear on the
nose gear.
The ideal location for the main gear
makes the nose wheel very light when the
fuel tank is empty. Either bend or shim
the main gear so that the wheels move
forward. The model should almost sit on
its tail when the tank is empty.
There is one problem that afflicts taildraggers
and tricycle-geared models:
overly springy landing gear. Sometimes
the kit comes with wire landing gear that
is too springy for the airplane’s weight.
That can make bounce-free landings
difficult; anything less than a grease job is
turned into a roller-coaster ride.
The solutions to this problem range
from wire and rubber-band
reinforcements to replacing the gear with
a beefier aluminum unit.
So Why Are You Dragging Your Tail
Around? Tail-draggers have different
versions of the same problems as tricyclegear
airplanes, with one interesting
difference: the fore-and-aft location of the
main gear.
If the main gear is mounted too far aft,
the airplane tends to nose-over easily.
That’s embarrassing at the very least.
What is not as often appreciated is that
if the mains are mounted too far forward,
you get that high-speed wheelbarrow
problem I previously discussed. The
airplane will be difficult to keep straightbe reduced. This is actually easy to do. For
those of you using the “two springs”-type
steering linkage, all you need to do is hook
up to the inner end of the rudder horns and
the outer end of the tail-wheel horns.
If you used the “tiller arm”-type
linkage, where a single piece of wire runs
along the bottom of the rudder and is
attached with some kind of clip, it’s a bit
tougher to do this unless you are still
assembling the airplane; then it is easy.
All you need to do is move the tailwheel
pivot forward and find a location
for the clip on the bottom of the rudder
where the steering throw is reduced. This
is simple and offers positive steering
control.
Takeoff, Climbout, and the CG: Let’s
cover what happens on takeoff when the
airplane is nose-heavy. A severely noseheavy
model will require lots of upelevator
to lift the nose wheel and break
ground. The problem could also be
landing-gear position, the ground stance,
or the CG.
The last two are easy to eliminate, but
you need the information you gathered in
the air to tell whether to move the landing
gear or not. If the CG is in the right spot,
holding a constant climb angle is easier. If
the airplane is nose-heavy, you will find
yourself needing a quick elevator
adjustment a split second after liftoff.
Let’s look at the other, more urgent
side of the problem. On takeoff, tailheaviness
often shows itself as climbouts
that quickly become too steep, even when
they did not start out that way. If you find
yourself chasing the elevator in a pilotinduced
oscillation (PIO), you’ve probably
got a tail-heavy airplane.
Tail-heavy airplanes tend to snap roll
too, and that is usually how they get
turned back into their component parts.
Try moving the CG forward temporarily,
and see if it’s easier to fly a smooth
departure climb.
Pitch Trim Revisited: Now that the airplane
is departing nicely, it is time for Part 2 of the
“From the Ground Up” series on basic
trim to depart as well. I’ll land back here
next month and wrap things up. MA
Dean Pappas
Edition: Model Aviation - 2006/08
Page Numbers: 55,56,57,58,60,62
Edition: Model Aviation - 2006/08
Page Numbers: 55,56,57,58,60,62
Trimming
August 2006 55
by Dean Pappas
Part 2 From the Ground Up
The simplest way to measure the right-thrust angle is to measure the distance from
the propeller tip to the tail post on both sides. Either use that trigonometry you forgot
or remember that with a 12-inch propeller, 3/16-inch difference equals 1°.
PICKING UP Where We Left Off: In the
first installment of this “Trimming From
the Ground Up” series we dealt with the
subject of pitch trim. As it turns out, it’s a
whole lot more than just moving the
transmitter trim lever a few clicks or beeps
until the model flies without climbing or
diving.
Airplane trimming is similar to setting
up a race car: even when a crew chief says
the “race car was fast right out of the
trailer,” he really means that the team was
able to go through the entire list of setup
checks quickly. That usually means few or
no adjustments were necessary, but it
doesn’t mean every little thing wasn’t
checked anyway.
As it turns out, there aren’t all that
many things to check when test-flying a
new airplane, but if one of the tests
described here shows a problem, you are
working harder than you need to when
flying your aircraft. If your model is
intended for the all-important one of
training mission, that’s a bad thing. If you
are a more advanced flier, you are simply
missing out on flying and looking better
than you already do.
The mission of this series is to describe
the tests and corrective actions, in a
systematized way, to help you make your
airplane fly better. None of it is any great
effort, and you don’t have to attack it all at
once. Your model’s pitch behavior can be
investigated separately from something
such as the unfortunate tendency the
airplane has to turn left immediately after
takeoff.
Engine Right Thrust: I’ve already
written about the adjustment of
downthrust, so now it’s time to discuss
right thrust.
Some years ago I attended a Scale
Masters Qualifier meet. The airplanes sure
were beautiful; there just aren’t enough
people who build like that! One of the
competitors was flying a Cessna L-19 Bird
Dog: a slow-flying, military, forward-aircontrol-
type airplane. The same aircraft, in
civilian livery, has been used as a trainer
and glider tow airplane for decades.
We were taking off from right to left
that day, and every one of the pilot’s
takeoffs veered left, over the flightline—
and the pits—and the parking area! Many
of you have seen this one, right? No, left!
It’s particularly tough for a student to deal
with, and it’s a serious safety hazard as
well. This is what happens when the right
thrust is not correctly set up.
The full-scale pilots deal with this
situation differently—emphasizing the
proper application of right rudder to
counteract “torque” on takeoff. Although
that is an excellent skill to develop, the fix
for most aeromodels is to put the proper
amount of right thrust into the engine. This
minimizes the rudder corrections that are
necessary during takeoff.
This is important for the beginner pilot,
who is still learning to take off. Should we
let the beginner give everyone in the pits a
haircut until he or she learns to use his or
her rudder hand simultaneously and
independently of the right hand? I think
not. Actually most model-airplane fliers
never really master accurate and
independent control with their rudder
hand, but it is a worthy goal.
Before I describe how to put right
thrust into the model and how to test for
whether or not the airplane has the correct
amount of right thrust, I need to discuss
the nature of the right-thrust trimming
adjustment. Right thrust is a compromise
because it is used to counteract an
airspeed-dependent problem. As it turns
out, it is usually a good compromise.
The Application: At low and part throttle
the effect of right thrust is minimal. Here’s
where the compromise comes in. We set
the right thrust to straighten out a fullpower
takeoff climb and accept the small,
unwanted influence it has at cruise.
In the glide the right thrust has
practically no effect, so it’s no problem.
Assuming that the airplane is trimmed to
glide in a straight line in calm air, the job
of right thrust is to preserve that straight
flight path under full power.
That’s all there is to it. Typically, the
right-thrust adjustment determines how
straight the airplane climbs after takeoff.
Good landing-gear setup will reduce the
steering workload until the student gets a
model in the air. The landing-gear
discussion comes later.
Let’s adjust the right thrust. As you
read a bit ago, the requirement is for the
model to go straight in the glide with the
engine idling and at full throttle in a
Right-Thrust Measure
56 MODEL AVIATION
This tail-wheel assembly has no caster so that side loads can apply no twist to the rudder.
The pivot location, ahead of the rudder hinge line, and the connection of the tiller a short
distance behind it reduce the tail-wheel throw to roughly half the rudder throw.
If the tire contact patch is not directly in line with the axis of steering rotation, the
impact of landing will try to twist the nose-wheel sideways.
Tail Wheel Caster
Steering Axis Offset
An Alternate
Down-Thrust Check for
Advanced Sport Models
There is another way to
check the downthrust, for those
of you flying sport airplanes that
are not intended to be as stable
as trainers. This applies to most
designs with semisymmetrical
and symmetrical airfoils.
Set up a hands-off level pass
on the far edge of the runway,
approximately 50 feet up, so you
can see the airplane very well.
As you fly along nice and
straight, with the model trimmed
“hands off,” suddenly pull the
power to idle.
For a second or two the
airplane will still be zipping along
at cruise speed. The downthrust
is taken out of the balance
equation for just that second or
so—and the aerodynamic trim
predominates. The difference (if
any) is the effect of the
downthrust.
If, as you pull the throttle
back, the nose twitches up and
then the model slows into a
glide, you have too much
downthrust.
If the nose abruptly drops a
tiny bit and the airplane instantly
assumes a fast, nose-down glide
when the throttle is pulled, you
need more downthrust. That’s
because your elevator trim has
been fighting down against the
engine-induced climb all along.
If the downthrust is correct,
the airplane continues straight
for a second or two and
gradually fades into the glide
angle.
It is also possible to see the
effect of wrong downthrust
when the power is applied
rather than pulled back. In cases
where much more downthrust is
needed, you may see the model
abruptly nose-up when the
power is suddenly fire-walled for
a go-around on landing. This is
bad news—especially with a
heavy Scale model that does not
forgive a nose-too-high climb. MA
—Dean Pappas
Drawings and photos by the author except as noted
August 2006 57
A good ground stance has the wing sitting at a small but positive
angle of attack: somewhere between 0° and 3° positive.
A nose-down stance, like a bloodhound, forces the model to
accelerate beyond the necessary airspeed for flight and then
leap into the air, resulting in a steep climbout.
Nose-up stances cause two problems: overly sensitive steering
caused by “wheelbarrowing” and premature liftoff, leading to
departure stalls.
The proper ratio between rudder throw and nose-wheel throw
is usually had when the rudder pushrod is connected to the
outside of the rudder-servo wheel/arm and the nose-gear
pushrod is connected to the innermost hole on the servo wheel
(above) and the outermost hole of the nose-gear arm (below).
takeoff climb. For now let’s assume the airplane glides without
turning since it was trimmed for straight and level flight at cruise
power. (In the next installment we will discuss making the model
fly straight at all airspeeds.)
What we really need to do is adjust the engine right thrust so it
adds the right amount of correction for “engine torque” during
climb. Engine torque makes the airplane turn left. The word
“torque” is a misnomer, but it is a convenient catchall.
The Right-Thrust Test: To start with, let’s make sure the model is
trimmed to fly nice and straight at cruise power. Next, I like to set
the airplane up so it is pointed straight away from me and headed
either directly into the wind or directly downwind. You don’t
want to do this lined up with a crosswind because the sideward
wind drift hides the turn for which you are looking.
Now that you are lined up, add full throttle and smoothly pull
up into a climb, at the same angle as your steepest post-takeoff
A Little Theory
The primary cause of what we call “torque” is the
spiral airflow that comes off of the propeller. There are
two other sources—the “P” factor and the pure torque
of the engine—but they are small contributors.
If you were to hold a crepe-paper streamer behind the
propeller, you would see that the airflow coming from the
propeller follows a corkscrewlike path in the same
direction as the propeller rotation. This airflow strikes the
left side of the fin and rudder, which is usually above the
thrustline, and, as a result, yaws the airplane left.
As the model accelerates, the pitch of this corkscrew
pattern gets straighter and straighter. Because of this, the
torque effect is greatest at low speed and high throttle,
less at cruise speed and power, and gone at idle. MA
—Dean Pappas
climb. We typically climb into the wind,
but doing this downwind also works, and it
allows you to pick the direction so you
don’t have to fly over the pits or the safety
line at the field.
You don’t want to climb so steeply that
the model is stalling, but you do want to
climb as steeply as your horsepower will
permit. The airplane will lose airspeed
during the climb, and it may become more
easily influenced as the flying surfaces lose
some of their control power. In all
likelihood the model will start to turn.
If the airplane deviates to the left, you
will have to add more right thrust. On the
next flight retrim for straight and level
flight (probably just a click of rudder) and
repeat the test until the model climbs
straight.
If the airplane has too much right-thrust,
it will deviate to the right in the climb. That
doesn’t happen often.
If the right thrust is close to correct, and
if there is enough wind to make the model
bounce around, you may have to repeat the
test a couple times to be sure of which
direction the airplane is turning. That
usually means you are getting close.
It is best to adjust the right-thrust angle
one degree at a time and repeat the process.
Most airplanes have, or at least need, 2°-3°
of right-thrust, although a rare few need
much more.
Right-Thrust Measurement: The easiest
way I have found to determine the rightthrust
angle is to measure the distance from
each propeller tip to the tail post. With a
12-inch propeller the difference between
the two measurements will be 3/16 inch for
every degree of right-thrust. Three degrees
of right-thrust works out to 9/16 inch
difference between the two measurements
from the tail post. With a 16-inch propeller
this ratio works out to 1/4 inch per degree.
You might have to readjust the right
thrust a time or two, but if you start with it
adjusted as the kit recommends, you should
have to make only a fine adjustment or
two. Many kits and ARFs may not make
how much right thrust is recommended
entirely clear, but if you can’t find anything
on the plans or in the instructions, start
with 21/2° or so.
Landing Gear: An airplane that rolls
straight and responds predictably to
steering input, especially on takeoff, will
be easier to fly. If you want to look like a
hero at the flying field, die-straight takeoffs
and smooth landings that roll to a straight
stop will help.
On the other hand, if you really crave
attention, zigzagging across the runway
will have everyone watching you—as they
run for cover! That’s not how you want to
be noticed, so we will devote some
attention to describing good landing-gear
setup. Most trainers are designed with
tricycle landing gear, so I will cover
models with that kind first, followed by
tail-draggers.
A few problems can afflict a tricyclegeared
airplane. The most common is the
use of a nose-wheel steering linkage that has
way too much throw.
The model does not need to be able to
turn within its own wingspan; the minimum
turning radius should be roughly 15 feet
with full rudder control applied. That
probably works out to only 5° of turn at the
nose wheel.
This is accomplished by connecting the
linkage to the innermost hole of the servo
arm and the outermost hole on the nose-strut
steering arm. It is sometimes helpful to drill
a new hole in the servo arm that is as close
as possible to the center post. Too much
steering throw not only makes it difficult to
steer straight at speed, but it can overload
the rudder servo and prematurely age or
damage it.
The next problem is an overly flexible
steering linkage. You need positive control,
and a springy linkage does not offer that. If
the steering linkage has too much give in it,
the nose-wheel may even twist sideways at
touchdown (impact?). This makes the
airplane “curtsy” in the middle of the
runway and can even tear out the firewall if
repeated often enough.
Some fliers will tell you that a springy
linkage can save the servo, but the best way
to do that is to give the servo maximum
mechanical advantage, as described in theDid you ever try to make a gentle turn
while running with a fully laden
wheelbarrow? It tried to tip, didn’t it? The
same is true with a tricycle-geared
airplane if it is running up on only the
nose wheel.
The ideal attitude is with the wing
chord line (or flat bottom) within a few
degrees of level with the ground. A wellset-
up trainer will lift off with just a tiny
touch of up-elevator when the airspeed is
right. For trainers with flat-bottomed
wings, this stance will lift off by itself
when the airplane is going fast enough.
The last tricycle-gear problem is the
fore and aft location of the main gear. If
the main gear is placed too far aft, the
airplane has a great deal of weight on the
nose wheel. This also makes the highspeed
steering more sensitive and requires
lots of up-elevator input to break ground.
Try pushing down on the stabilizer to lift
the nose wheel, to get a feel for how
much force the up-elevator control has to
make.
Again, this can lead to an overly steep
departure after an excessively long
takeoff roll. It also causes the airplane to
“slap” onto the ground during landing;
that can add to the wear and tear on the
nose gear.
The ideal location for the main gear
makes the nose wheel very light when the
fuel tank is empty. Either bend or shim
the main gear so that the wheels move
forward. The model should almost sit on
its tail when the tank is empty.
There is one problem that afflicts taildraggers
and tricycle-geared models:
overly springy landing gear. Sometimes
the kit comes with wire landing gear that
is too springy for the airplane’s weight.
That can make bounce-free landings
difficult; anything less than a grease job is
turned into a roller-coaster ride.
The solutions to this problem range
from wire and rubber-band
reinforcements to replacing the gear with
a beefier aluminum unit.
So Why Are You Dragging Your Tail
Around? Tail-draggers have different
versions of the same problems as tricyclegear
airplanes, with one interesting
difference: the fore-and-aft location of the
main gear.
If the main gear is mounted too far aft,
the airplane tends to nose-over easily.
That’s embarrassing at the very least.
What is not as often appreciated is that
if the mains are mounted too far forward,
you get that high-speed wheelbarrow
problem I previously discussed. The
airplane will be difficult to keep straightbe reduced. This is actually easy to do. For
those of you using the “two springs”-type
steering linkage, all you need to do is hook
up to the inner end of the rudder horns and
the outer end of the tail-wheel horns.
If you used the “tiller arm”-type
linkage, where a single piece of wire runs
along the bottom of the rudder and is
attached with some kind of clip, it’s a bit
tougher to do this unless you are still
assembling the airplane; then it is easy.
All you need to do is move the tailwheel
pivot forward and find a location
for the clip on the bottom of the rudder
where the steering throw is reduced. This
is simple and offers positive steering
control.
Takeoff, Climbout, and the CG: Let’s
cover what happens on takeoff when the
airplane is nose-heavy. A severely noseheavy
model will require lots of upelevator
to lift the nose wheel and break
ground. The problem could also be
landing-gear position, the ground stance,
or the CG.
The last two are easy to eliminate, but
you need the information you gathered in
the air to tell whether to move the landing
gear or not. If the CG is in the right spot,
holding a constant climb angle is easier. If
the airplane is nose-heavy, you will find
yourself needing a quick elevator
adjustment a split second after liftoff.
Let’s look at the other, more urgent
side of the problem. On takeoff, tailheaviness
often shows itself as climbouts
that quickly become too steep, even when
they did not start out that way. If you find
yourself chasing the elevator in a pilotinduced
oscillation (PIO), you’ve probably
got a tail-heavy airplane.
Tail-heavy airplanes tend to snap roll
too, and that is usually how they get
turned back into their component parts.
Try moving the CG forward temporarily,
and see if it’s easier to fly a smooth
departure climb.
Pitch Trim Revisited: Now that the airplane
is departing nicely, it is time for Part 2 of the
“From the Ground Up” series on basic
trim to depart as well. I’ll land back here
next month and wrap things up. MA
Dean Pappas
Edition: Model Aviation - 2006/08
Page Numbers: 55,56,57,58,60,62
Trimming
August 2006 55
by Dean Pappas
Part 2 From the Ground Up
The simplest way to measure the right-thrust angle is to measure the distance from
the propeller tip to the tail post on both sides. Either use that trigonometry you forgot
or remember that with a 12-inch propeller, 3/16-inch difference equals 1°.
PICKING UP Where We Left Off: In the
first installment of this “Trimming From
the Ground Up” series we dealt with the
subject of pitch trim. As it turns out, it’s a
whole lot more than just moving the
transmitter trim lever a few clicks or beeps
until the model flies without climbing or
diving.
Airplane trimming is similar to setting
up a race car: even when a crew chief says
the “race car was fast right out of the
trailer,” he really means that the team was
able to go through the entire list of setup
checks quickly. That usually means few or
no adjustments were necessary, but it
doesn’t mean every little thing wasn’t
checked anyway.
As it turns out, there aren’t all that
many things to check when test-flying a
new airplane, but if one of the tests
described here shows a problem, you are
working harder than you need to when
flying your aircraft. If your model is
intended for the all-important one of
training mission, that’s a bad thing. If you
are a more advanced flier, you are simply
missing out on flying and looking better
than you already do.
The mission of this series is to describe
the tests and corrective actions, in a
systematized way, to help you make your
airplane fly better. None of it is any great
effort, and you don’t have to attack it all at
once. Your model’s pitch behavior can be
investigated separately from something
such as the unfortunate tendency the
airplane has to turn left immediately after
takeoff.
Engine Right Thrust: I’ve already
written about the adjustment of
downthrust, so now it’s time to discuss
right thrust.
Some years ago I attended a Scale
Masters Qualifier meet. The airplanes sure
were beautiful; there just aren’t enough
people who build like that! One of the
competitors was flying a Cessna L-19 Bird
Dog: a slow-flying, military, forward-aircontrol-
type airplane. The same aircraft, in
civilian livery, has been used as a trainer
and glider tow airplane for decades.
We were taking off from right to left
that day, and every one of the pilot’s
takeoffs veered left, over the flightline—
and the pits—and the parking area! Many
of you have seen this one, right? No, left!
It’s particularly tough for a student to deal
with, and it’s a serious safety hazard as
well. This is what happens when the right
thrust is not correctly set up.
The full-scale pilots deal with this
situation differently—emphasizing the
proper application of right rudder to
counteract “torque” on takeoff. Although
that is an excellent skill to develop, the fix
for most aeromodels is to put the proper
amount of right thrust into the engine. This
minimizes the rudder corrections that are
necessary during takeoff.
This is important for the beginner pilot,
who is still learning to take off. Should we
let the beginner give everyone in the pits a
haircut until he or she learns to use his or
her rudder hand simultaneously and
independently of the right hand? I think
not. Actually most model-airplane fliers
never really master accurate and
independent control with their rudder
hand, but it is a worthy goal.
Before I describe how to put right
thrust into the model and how to test for
whether or not the airplane has the correct
amount of right thrust, I need to discuss
the nature of the right-thrust trimming
adjustment. Right thrust is a compromise
because it is used to counteract an
airspeed-dependent problem. As it turns
out, it is usually a good compromise.
The Application: At low and part throttle
the effect of right thrust is minimal. Here’s
where the compromise comes in. We set
the right thrust to straighten out a fullpower
takeoff climb and accept the small,
unwanted influence it has at cruise.
In the glide the right thrust has
practically no effect, so it’s no problem.
Assuming that the airplane is trimmed to
glide in a straight line in calm air, the job
of right thrust is to preserve that straight
flight path under full power.
That’s all there is to it. Typically, the
right-thrust adjustment determines how
straight the airplane climbs after takeoff.
Good landing-gear setup will reduce the
steering workload until the student gets a
model in the air. The landing-gear
discussion comes later.
Let’s adjust the right thrust. As you
read a bit ago, the requirement is for the
model to go straight in the glide with the
engine idling and at full throttle in a
Right-Thrust Measure
56 MODEL AVIATION
This tail-wheel assembly has no caster so that side loads can apply no twist to the rudder.
The pivot location, ahead of the rudder hinge line, and the connection of the tiller a short
distance behind it reduce the tail-wheel throw to roughly half the rudder throw.
If the tire contact patch is not directly in line with the axis of steering rotation, the
impact of landing will try to twist the nose-wheel sideways.
Tail Wheel Caster
Steering Axis Offset
An Alternate
Down-Thrust Check for
Advanced Sport Models
There is another way to
check the downthrust, for those
of you flying sport airplanes that
are not intended to be as stable
as trainers. This applies to most
designs with semisymmetrical
and symmetrical airfoils.
Set up a hands-off level pass
on the far edge of the runway,
approximately 50 feet up, so you
can see the airplane very well.
As you fly along nice and
straight, with the model trimmed
“hands off,” suddenly pull the
power to idle.
For a second or two the
airplane will still be zipping along
at cruise speed. The downthrust
is taken out of the balance
equation for just that second or
so—and the aerodynamic trim
predominates. The difference (if
any) is the effect of the
downthrust.
If, as you pull the throttle
back, the nose twitches up and
then the model slows into a
glide, you have too much
downthrust.
If the nose abruptly drops a
tiny bit and the airplane instantly
assumes a fast, nose-down glide
when the throttle is pulled, you
need more downthrust. That’s
because your elevator trim has
been fighting down against the
engine-induced climb all along.
If the downthrust is correct,
the airplane continues straight
for a second or two and
gradually fades into the glide
angle.
It is also possible to see the
effect of wrong downthrust
when the power is applied
rather than pulled back. In cases
where much more downthrust is
needed, you may see the model
abruptly nose-up when the
power is suddenly fire-walled for
a go-around on landing. This is
bad news—especially with a
heavy Scale model that does not
forgive a nose-too-high climb. MA
—Dean Pappas
Drawings and photos by the author except as noted
August 2006 57
A good ground stance has the wing sitting at a small but positive
angle of attack: somewhere between 0° and 3° positive.
A nose-down stance, like a bloodhound, forces the model to
accelerate beyond the necessary airspeed for flight and then
leap into the air, resulting in a steep climbout.
Nose-up stances cause two problems: overly sensitive steering
caused by “wheelbarrowing” and premature liftoff, leading to
departure stalls.
The proper ratio between rudder throw and nose-wheel throw
is usually had when the rudder pushrod is connected to the
outside of the rudder-servo wheel/arm and the nose-gear
pushrod is connected to the innermost hole on the servo wheel
(above) and the outermost hole of the nose-gear arm (below).
takeoff climb. For now let’s assume the airplane glides without
turning since it was trimmed for straight and level flight at cruise
power. (In the next installment we will discuss making the model
fly straight at all airspeeds.)
What we really need to do is adjust the engine right thrust so it
adds the right amount of correction for “engine torque” during
climb. Engine torque makes the airplane turn left. The word
“torque” is a misnomer, but it is a convenient catchall.
The Right-Thrust Test: To start with, let’s make sure the model is
trimmed to fly nice and straight at cruise power. Next, I like to set
the airplane up so it is pointed straight away from me and headed
either directly into the wind or directly downwind. You don’t
want to do this lined up with a crosswind because the sideward
wind drift hides the turn for which you are looking.
Now that you are lined up, add full throttle and smoothly pull
up into a climb, at the same angle as your steepest post-takeoff
A Little Theory
The primary cause of what we call “torque” is the
spiral airflow that comes off of the propeller. There are
two other sources—the “P” factor and the pure torque
of the engine—but they are small contributors.
If you were to hold a crepe-paper streamer behind the
propeller, you would see that the airflow coming from the
propeller follows a corkscrewlike path in the same
direction as the propeller rotation. This airflow strikes the
left side of the fin and rudder, which is usually above the
thrustline, and, as a result, yaws the airplane left.
As the model accelerates, the pitch of this corkscrew
pattern gets straighter and straighter. Because of this, the
torque effect is greatest at low speed and high throttle,
less at cruise speed and power, and gone at idle. MA
—Dean Pappas
climb. We typically climb into the wind,
but doing this downwind also works, and it
allows you to pick the direction so you
don’t have to fly over the pits or the safety
line at the field.
You don’t want to climb so steeply that
the model is stalling, but you do want to
climb as steeply as your horsepower will
permit. The airplane will lose airspeed
during the climb, and it may become more
easily influenced as the flying surfaces lose
some of their control power. In all
likelihood the model will start to turn.
If the airplane deviates to the left, you
will have to add more right thrust. On the
next flight retrim for straight and level
flight (probably just a click of rudder) and
repeat the test until the model climbs
straight.
If the airplane has too much right-thrust,
it will deviate to the right in the climb. That
doesn’t happen often.
If the right thrust is close to correct, and
if there is enough wind to make the model
bounce around, you may have to repeat the
test a couple times to be sure of which
direction the airplane is turning. That
usually means you are getting close.
It is best to adjust the right-thrust angle
one degree at a time and repeat the process.
Most airplanes have, or at least need, 2°-3°
of right-thrust, although a rare few need
much more.
Right-Thrust Measurement: The easiest
way I have found to determine the rightthrust
angle is to measure the distance from
each propeller tip to the tail post. With a
12-inch propeller the difference between
the two measurements will be 3/16 inch for
every degree of right-thrust. Three degrees
of right-thrust works out to 9/16 inch
difference between the two measurements
from the tail post. With a 16-inch propeller
this ratio works out to 1/4 inch per degree.
You might have to readjust the right
thrust a time or two, but if you start with it
adjusted as the kit recommends, you should
have to make only a fine adjustment or
two. Many kits and ARFs may not make
how much right thrust is recommended
entirely clear, but if you can’t find anything
on the plans or in the instructions, start
with 21/2° or so.
Landing Gear: An airplane that rolls
straight and responds predictably to
steering input, especially on takeoff, will
be easier to fly. If you want to look like a
hero at the flying field, die-straight takeoffs
and smooth landings that roll to a straight
stop will help.
On the other hand, if you really crave
attention, zigzagging across the runway
will have everyone watching you—as they
run for cover! That’s not how you want to
be noticed, so we will devote some
attention to describing good landing-gear
setup. Most trainers are designed with
tricycle landing gear, so I will cover
models with that kind first, followed by
tail-draggers.
A few problems can afflict a tricyclegeared
airplane. The most common is the
use of a nose-wheel steering linkage that has
way too much throw.
The model does not need to be able to
turn within its own wingspan; the minimum
turning radius should be roughly 15 feet
with full rudder control applied. That
probably works out to only 5° of turn at the
nose wheel.
This is accomplished by connecting the
linkage to the innermost hole of the servo
arm and the outermost hole on the nose-strut
steering arm. It is sometimes helpful to drill
a new hole in the servo arm that is as close
as possible to the center post. Too much
steering throw not only makes it difficult to
steer straight at speed, but it can overload
the rudder servo and prematurely age or
damage it.
The next problem is an overly flexible
steering linkage. You need positive control,
and a springy linkage does not offer that. If
the steering linkage has too much give in it,
the nose-wheel may even twist sideways at
touchdown (impact?). This makes the
airplane “curtsy” in the middle of the
runway and can even tear out the firewall if
repeated often enough.
Some fliers will tell you that a springy
linkage can save the servo, but the best way
to do that is to give the servo maximum
mechanical advantage, as described in theDid you ever try to make a gentle turn
while running with a fully laden
wheelbarrow? It tried to tip, didn’t it? The
same is true with a tricycle-geared
airplane if it is running up on only the
nose wheel.
The ideal attitude is with the wing
chord line (or flat bottom) within a few
degrees of level with the ground. A wellset-
up trainer will lift off with just a tiny
touch of up-elevator when the airspeed is
right. For trainers with flat-bottomed
wings, this stance will lift off by itself
when the airplane is going fast enough.
The last tricycle-gear problem is the
fore and aft location of the main gear. If
the main gear is placed too far aft, the
airplane has a great deal of weight on the
nose wheel. This also makes the highspeed
steering more sensitive and requires
lots of up-elevator input to break ground.
Try pushing down on the stabilizer to lift
the nose wheel, to get a feel for how
much force the up-elevator control has to
make.
Again, this can lead to an overly steep
departure after an excessively long
takeoff roll. It also causes the airplane to
“slap” onto the ground during landing;
that can add to the wear and tear on the
nose gear.
The ideal location for the main gear
makes the nose wheel very light when the
fuel tank is empty. Either bend or shim
the main gear so that the wheels move
forward. The model should almost sit on
its tail when the tank is empty.
There is one problem that afflicts taildraggers
and tricycle-geared models:
overly springy landing gear. Sometimes
the kit comes with wire landing gear that
is too springy for the airplane’s weight.
That can make bounce-free landings
difficult; anything less than a grease job is
turned into a roller-coaster ride.
The solutions to this problem range
from wire and rubber-band
reinforcements to replacing the gear with
a beefier aluminum unit.
So Why Are You Dragging Your Tail
Around? Tail-draggers have different
versions of the same problems as tricyclegear
airplanes, with one interesting
difference: the fore-and-aft location of the
main gear.
If the main gear is mounted too far aft,
the airplane tends to nose-over easily.
That’s embarrassing at the very least.
What is not as often appreciated is that
if the mains are mounted too far forward,
you get that high-speed wheelbarrow
problem I previously discussed. The
airplane will be difficult to keep straightbe reduced. This is actually easy to do. For
those of you using the “two springs”-type
steering linkage, all you need to do is hook
up to the inner end of the rudder horns and
the outer end of the tail-wheel horns.
If you used the “tiller arm”-type
linkage, where a single piece of wire runs
along the bottom of the rudder and is
attached with some kind of clip, it’s a bit
tougher to do this unless you are still
assembling the airplane; then it is easy.
All you need to do is move the tailwheel
pivot forward and find a location
for the clip on the bottom of the rudder
where the steering throw is reduced. This
is simple and offers positive steering
control.
Takeoff, Climbout, and the CG: Let’s
cover what happens on takeoff when the
airplane is nose-heavy. A severely noseheavy
model will require lots of upelevator
to lift the nose wheel and break
ground. The problem could also be
landing-gear position, the ground stance,
or the CG.
The last two are easy to eliminate, but
you need the information you gathered in
the air to tell whether to move the landing
gear or not. If the CG is in the right spot,
holding a constant climb angle is easier. If
the airplane is nose-heavy, you will find
yourself needing a quick elevator
adjustment a split second after liftoff.
Let’s look at the other, more urgent
side of the problem. On takeoff, tailheaviness
often shows itself as climbouts
that quickly become too steep, even when
they did not start out that way. If you find
yourself chasing the elevator in a pilotinduced
oscillation (PIO), you’ve probably
got a tail-heavy airplane.
Tail-heavy airplanes tend to snap roll
too, and that is usually how they get
turned back into their component parts.
Try moving the CG forward temporarily,
and see if it’s easier to fly a smooth
departure climb.
Pitch Trim Revisited: Now that the airplane
is departing nicely, it is time for Part 2 of the
“From the Ground Up” series on basic
trim to depart as well. I’ll land back here
next month and wrap things up. MA
Dean Pappas
Edition: Model Aviation - 2006/08
Page Numbers: 55,56,57,58,60,62
Trimming
August 2006 55
by Dean Pappas
Part 2 From the Ground Up
The simplest way to measure the right-thrust angle is to measure the distance from
the propeller tip to the tail post on both sides. Either use that trigonometry you forgot
or remember that with a 12-inch propeller, 3/16-inch difference equals 1°.
PICKING UP Where We Left Off: In the
first installment of this “Trimming From
the Ground Up” series we dealt with the
subject of pitch trim. As it turns out, it’s a
whole lot more than just moving the
transmitter trim lever a few clicks or beeps
until the model flies without climbing or
diving.
Airplane trimming is similar to setting
up a race car: even when a crew chief says
the “race car was fast right out of the
trailer,” he really means that the team was
able to go through the entire list of setup
checks quickly. That usually means few or
no adjustments were necessary, but it
doesn’t mean every little thing wasn’t
checked anyway.
As it turns out, there aren’t all that
many things to check when test-flying a
new airplane, but if one of the tests
described here shows a problem, you are
working harder than you need to when
flying your aircraft. If your model is
intended for the all-important one of
training mission, that’s a bad thing. If you
are a more advanced flier, you are simply
missing out on flying and looking better
than you already do.
The mission of this series is to describe
the tests and corrective actions, in a
systematized way, to help you make your
airplane fly better. None of it is any great
effort, and you don’t have to attack it all at
once. Your model’s pitch behavior can be
investigated separately from something
such as the unfortunate tendency the
airplane has to turn left immediately after
takeoff.
Engine Right Thrust: I’ve already
written about the adjustment of
downthrust, so now it’s time to discuss
right thrust.
Some years ago I attended a Scale
Masters Qualifier meet. The airplanes sure
were beautiful; there just aren’t enough
people who build like that! One of the
competitors was flying a Cessna L-19 Bird
Dog: a slow-flying, military, forward-aircontrol-
type airplane. The same aircraft, in
civilian livery, has been used as a trainer
and glider tow airplane for decades.
We were taking off from right to left
that day, and every one of the pilot’s
takeoffs veered left, over the flightline—
and the pits—and the parking area! Many
of you have seen this one, right? No, left!
It’s particularly tough for a student to deal
with, and it’s a serious safety hazard as
well. This is what happens when the right
thrust is not correctly set up.
The full-scale pilots deal with this
situation differently—emphasizing the
proper application of right rudder to
counteract “torque” on takeoff. Although
that is an excellent skill to develop, the fix
for most aeromodels is to put the proper
amount of right thrust into the engine. This
minimizes the rudder corrections that are
necessary during takeoff.
This is important for the beginner pilot,
who is still learning to take off. Should we
let the beginner give everyone in the pits a
haircut until he or she learns to use his or
her rudder hand simultaneously and
independently of the right hand? I think
not. Actually most model-airplane fliers
never really master accurate and
independent control with their rudder
hand, but it is a worthy goal.
Before I describe how to put right
thrust into the model and how to test for
whether or not the airplane has the correct
amount of right thrust, I need to discuss
the nature of the right-thrust trimming
adjustment. Right thrust is a compromise
because it is used to counteract an
airspeed-dependent problem. As it turns
out, it is usually a good compromise.
The Application: At low and part throttle
the effect of right thrust is minimal. Here’s
where the compromise comes in. We set
the right thrust to straighten out a fullpower
takeoff climb and accept the small,
unwanted influence it has at cruise.
In the glide the right thrust has
practically no effect, so it’s no problem.
Assuming that the airplane is trimmed to
glide in a straight line in calm air, the job
of right thrust is to preserve that straight
flight path under full power.
That’s all there is to it. Typically, the
right-thrust adjustment determines how
straight the airplane climbs after takeoff.
Good landing-gear setup will reduce the
steering workload until the student gets a
model in the air. The landing-gear
discussion comes later.
Let’s adjust the right thrust. As you
read a bit ago, the requirement is for the
model to go straight in the glide with the
engine idling and at full throttle in a
Right-Thrust Measure
56 MODEL AVIATION
This tail-wheel assembly has no caster so that side loads can apply no twist to the rudder.
The pivot location, ahead of the rudder hinge line, and the connection of the tiller a short
distance behind it reduce the tail-wheel throw to roughly half the rudder throw.
If the tire contact patch is not directly in line with the axis of steering rotation, the
impact of landing will try to twist the nose-wheel sideways.
Tail Wheel Caster
Steering Axis Offset
An Alternate
Down-Thrust Check for
Advanced Sport Models
There is another way to
check the downthrust, for those
of you flying sport airplanes that
are not intended to be as stable
as trainers. This applies to most
designs with semisymmetrical
and symmetrical airfoils.
Set up a hands-off level pass
on the far edge of the runway,
approximately 50 feet up, so you
can see the airplane very well.
As you fly along nice and
straight, with the model trimmed
“hands off,” suddenly pull the
power to idle.
For a second or two the
airplane will still be zipping along
at cruise speed. The downthrust
is taken out of the balance
equation for just that second or
so—and the aerodynamic trim
predominates. The difference (if
any) is the effect of the
downthrust.
If, as you pull the throttle
back, the nose twitches up and
then the model slows into a
glide, you have too much
downthrust.
If the nose abruptly drops a
tiny bit and the airplane instantly
assumes a fast, nose-down glide
when the throttle is pulled, you
need more downthrust. That’s
because your elevator trim has
been fighting down against the
engine-induced climb all along.
If the downthrust is correct,
the airplane continues straight
for a second or two and
gradually fades into the glide
angle.
It is also possible to see the
effect of wrong downthrust
when the power is applied
rather than pulled back. In cases
where much more downthrust is
needed, you may see the model
abruptly nose-up when the
power is suddenly fire-walled for
a go-around on landing. This is
bad news—especially with a
heavy Scale model that does not
forgive a nose-too-high climb. MA
—Dean Pappas
Drawings and photos by the author except as noted
August 2006 57
A good ground stance has the wing sitting at a small but positive
angle of attack: somewhere between 0° and 3° positive.
A nose-down stance, like a bloodhound, forces the model to
accelerate beyond the necessary airspeed for flight and then
leap into the air, resulting in a steep climbout.
Nose-up stances cause two problems: overly sensitive steering
caused by “wheelbarrowing” and premature liftoff, leading to
departure stalls.
The proper ratio between rudder throw and nose-wheel throw
is usually had when the rudder pushrod is connected to the
outside of the rudder-servo wheel/arm and the nose-gear
pushrod is connected to the innermost hole on the servo wheel
(above) and the outermost hole of the nose-gear arm (below).
takeoff climb. For now let’s assume the airplane glides without
turning since it was trimmed for straight and level flight at cruise
power. (In the next installment we will discuss making the model
fly straight at all airspeeds.)
What we really need to do is adjust the engine right thrust so it
adds the right amount of correction for “engine torque” during
climb. Engine torque makes the airplane turn left. The word
“torque” is a misnomer, but it is a convenient catchall.
The Right-Thrust Test: To start with, let’s make sure the model is
trimmed to fly nice and straight at cruise power. Next, I like to set
the airplane up so it is pointed straight away from me and headed
either directly into the wind or directly downwind. You don’t
want to do this lined up with a crosswind because the sideward
wind drift hides the turn for which you are looking.
Now that you are lined up, add full throttle and smoothly pull
up into a climb, at the same angle as your steepest post-takeoff
A Little Theory
The primary cause of what we call “torque” is the
spiral airflow that comes off of the propeller. There are
two other sources—the “P” factor and the pure torque
of the engine—but they are small contributors.
If you were to hold a crepe-paper streamer behind the
propeller, you would see that the airflow coming from the
propeller follows a corkscrewlike path in the same
direction as the propeller rotation. This airflow strikes the
left side of the fin and rudder, which is usually above the
thrustline, and, as a result, yaws the airplane left.
As the model accelerates, the pitch of this corkscrew
pattern gets straighter and straighter. Because of this, the
torque effect is greatest at low speed and high throttle,
less at cruise speed and power, and gone at idle. MA
—Dean Pappas
climb. We typically climb into the wind,
but doing this downwind also works, and it
allows you to pick the direction so you
don’t have to fly over the pits or the safety
line at the field.
You don’t want to climb so steeply that
the model is stalling, but you do want to
climb as steeply as your horsepower will
permit. The airplane will lose airspeed
during the climb, and it may become more
easily influenced as the flying surfaces lose
some of their control power. In all
likelihood the model will start to turn.
If the airplane deviates to the left, you
will have to add more right thrust. On the
next flight retrim for straight and level
flight (probably just a click of rudder) and
repeat the test until the model climbs
straight.
If the airplane has too much right-thrust,
it will deviate to the right in the climb. That
doesn’t happen often.
If the right thrust is close to correct, and
if there is enough wind to make the model
bounce around, you may have to repeat the
test a couple times to be sure of which
direction the airplane is turning. That
usually means you are getting close.
It is best to adjust the right-thrust angle
one degree at a time and repeat the process.
Most airplanes have, or at least need, 2°-3°
of right-thrust, although a rare few need
much more.
Right-Thrust Measurement: The easiest
way I have found to determine the rightthrust
angle is to measure the distance from
each propeller tip to the tail post. With a
12-inch propeller the difference between
the two measurements will be 3/16 inch for
every degree of right-thrust. Three degrees
of right-thrust works out to 9/16 inch
difference between the two measurements
from the tail post. With a 16-inch propeller
this ratio works out to 1/4 inch per degree.
You might have to readjust the right
thrust a time or two, but if you start with it
adjusted as the kit recommends, you should
have to make only a fine adjustment or
two. Many kits and ARFs may not make
how much right thrust is recommended
entirely clear, but if you can’t find anything
on the plans or in the instructions, start
with 21/2° or so.
Landing Gear: An airplane that rolls
straight and responds predictably to
steering input, especially on takeoff, will
be easier to fly. If you want to look like a
hero at the flying field, die-straight takeoffs
and smooth landings that roll to a straight
stop will help.
On the other hand, if you really crave
attention, zigzagging across the runway
will have everyone watching you—as they
run for cover! That’s not how you want to
be noticed, so we will devote some
attention to describing good landing-gear
setup. Most trainers are designed with
tricycle landing gear, so I will cover
models with that kind first, followed by
tail-draggers.
A few problems can afflict a tricyclegeared
airplane. The most common is the
use of a nose-wheel steering linkage that has
way too much throw.
The model does not need to be able to
turn within its own wingspan; the minimum
turning radius should be roughly 15 feet
with full rudder control applied. That
probably works out to only 5° of turn at the
nose wheel.
This is accomplished by connecting the
linkage to the innermost hole of the servo
arm and the outermost hole on the nose-strut
steering arm. It is sometimes helpful to drill
a new hole in the servo arm that is as close
as possible to the center post. Too much
steering throw not only makes it difficult to
steer straight at speed, but it can overload
the rudder servo and prematurely age or
damage it.
The next problem is an overly flexible
steering linkage. You need positive control,
and a springy linkage does not offer that. If
the steering linkage has too much give in it,
the nose-wheel may even twist sideways at
touchdown (impact?). This makes the
airplane “curtsy” in the middle of the
runway and can even tear out the firewall if
repeated often enough.
Some fliers will tell you that a springy
linkage can save the servo, but the best way
to do that is to give the servo maximum
mechanical advantage, as described in theDid you ever try to make a gentle turn
while running with a fully laden
wheelbarrow? It tried to tip, didn’t it? The
same is true with a tricycle-geared
airplane if it is running up on only the
nose wheel.
The ideal attitude is with the wing
chord line (or flat bottom) within a few
degrees of level with the ground. A wellset-
up trainer will lift off with just a tiny
touch of up-elevator when the airspeed is
right. For trainers with flat-bottomed
wings, this stance will lift off by itself
when the airplane is going fast enough.
The last tricycle-gear problem is the
fore and aft location of the main gear. If
the main gear is placed too far aft, the
airplane has a great deal of weight on the
nose wheel. This also makes the highspeed
steering more sensitive and requires
lots of up-elevator input to break ground.
Try pushing down on the stabilizer to lift
the nose wheel, to get a feel for how
much force the up-elevator control has to
make.
Again, this can lead to an overly steep
departure after an excessively long
takeoff roll. It also causes the airplane to
“slap” onto the ground during landing;
that can add to the wear and tear on the
nose gear.
The ideal location for the main gear
makes the nose wheel very light when the
fuel tank is empty. Either bend or shim
the main gear so that the wheels move
forward. The model should almost sit on
its tail when the tank is empty.
There is one problem that afflicts taildraggers
and tricycle-geared models:
overly springy landing gear. Sometimes
the kit comes with wire landing gear that
is too springy for the airplane’s weight.
That can make bounce-free landings
difficult; anything less than a grease job is
turned into a roller-coaster ride.
The solutions to this problem range
from wire and rubber-band
reinforcements to replacing the gear with
a beefier aluminum unit.
So Why Are You Dragging Your Tail
Around? Tail-draggers have different
versions of the same problems as tricyclegear
airplanes, with one interesting
difference: the fore-and-aft location of the
main gear.
If the main gear is mounted too far aft,
the airplane tends to nose-over easily.
That’s embarrassing at the very least.
What is not as often appreciated is that
if the mains are mounted too far forward,
you get that high-speed wheelbarrow
problem I previously discussed. The
airplane will be difficult to keep straightbe reduced. This is actually easy to do. For
those of you using the “two springs”-type
steering linkage, all you need to do is hook
up to the inner end of the rudder horns and
the outer end of the tail-wheel horns.
If you used the “tiller arm”-type
linkage, where a single piece of wire runs
along the bottom of the rudder and is
attached with some kind of clip, it’s a bit
tougher to do this unless you are still
assembling the airplane; then it is easy.
All you need to do is move the tailwheel
pivot forward and find a location
for the clip on the bottom of the rudder
where the steering throw is reduced. This
is simple and offers positive steering
control.
Takeoff, Climbout, and the CG: Let’s
cover what happens on takeoff when the
airplane is nose-heavy. A severely noseheavy
model will require lots of upelevator
to lift the nose wheel and break
ground. The problem could also be
landing-gear position, the ground stance,
or the CG.
The last two are easy to eliminate, but
you need the information you gathered in
the air to tell whether to move the landing
gear or not. If the CG is in the right spot,
holding a constant climb angle is easier. If
the airplane is nose-heavy, you will find
yourself needing a quick elevator
adjustment a split second after liftoff.
Let’s look at the other, more urgent
side of the problem. On takeoff, tailheaviness
often shows itself as climbouts
that quickly become too steep, even when
they did not start out that way. If you find
yourself chasing the elevator in a pilotinduced
oscillation (PIO), you’ve probably
got a tail-heavy airplane.
Tail-heavy airplanes tend to snap roll
too, and that is usually how they get
turned back into their component parts.
Try moving the CG forward temporarily,
and see if it’s easier to fly a smooth
departure climb.
Pitch Trim Revisited: Now that the airplane
is departing nicely, it is time for Part 2 of the
“From the Ground Up” series on basic
trim to depart as well. I’ll land back here
next month and wrap things up. MA
Dean Pappas
Edition: Model Aviation - 2006/08
Page Numbers: 55,56,57,58,60,62
Trimming
August 2006 55
by Dean Pappas
Part 2 From the Ground Up
The simplest way to measure the right-thrust angle is to measure the distance from
the propeller tip to the tail post on both sides. Either use that trigonometry you forgot
or remember that with a 12-inch propeller, 3/16-inch difference equals 1°.
PICKING UP Where We Left Off: In the
first installment of this “Trimming From
the Ground Up” series we dealt with the
subject of pitch trim. As it turns out, it’s a
whole lot more than just moving the
transmitter trim lever a few clicks or beeps
until the model flies without climbing or
diving.
Airplane trimming is similar to setting
up a race car: even when a crew chief says
the “race car was fast right out of the
trailer,” he really means that the team was
able to go through the entire list of setup
checks quickly. That usually means few or
no adjustments were necessary, but it
doesn’t mean every little thing wasn’t
checked anyway.
As it turns out, there aren’t all that
many things to check when test-flying a
new airplane, but if one of the tests
described here shows a problem, you are
working harder than you need to when
flying your aircraft. If your model is
intended for the all-important one of
training mission, that’s a bad thing. If you
are a more advanced flier, you are simply
missing out on flying and looking better
than you already do.
The mission of this series is to describe
the tests and corrective actions, in a
systematized way, to help you make your
airplane fly better. None of it is any great
effort, and you don’t have to attack it all at
once. Your model’s pitch behavior can be
investigated separately from something
such as the unfortunate tendency the
airplane has to turn left immediately after
takeoff.
Engine Right Thrust: I’ve already
written about the adjustment of
downthrust, so now it’s time to discuss
right thrust.
Some years ago I attended a Scale
Masters Qualifier meet. The airplanes sure
were beautiful; there just aren’t enough
people who build like that! One of the
competitors was flying a Cessna L-19 Bird
Dog: a slow-flying, military, forward-aircontrol-
type airplane. The same aircraft, in
civilian livery, has been used as a trainer
and glider tow airplane for decades.
We were taking off from right to left
that day, and every one of the pilot’s
takeoffs veered left, over the flightline—
and the pits—and the parking area! Many
of you have seen this one, right? No, left!
It’s particularly tough for a student to deal
with, and it’s a serious safety hazard as
well. This is what happens when the right
thrust is not correctly set up.
The full-scale pilots deal with this
situation differently—emphasizing the
proper application of right rudder to
counteract “torque” on takeoff. Although
that is an excellent skill to develop, the fix
for most aeromodels is to put the proper
amount of right thrust into the engine. This
minimizes the rudder corrections that are
necessary during takeoff.
This is important for the beginner pilot,
who is still learning to take off. Should we
let the beginner give everyone in the pits a
haircut until he or she learns to use his or
her rudder hand simultaneously and
independently of the right hand? I think
not. Actually most model-airplane fliers
never really master accurate and
independent control with their rudder
hand, but it is a worthy goal.
Before I describe how to put right
thrust into the model and how to test for
whether or not the airplane has the correct
amount of right thrust, I need to discuss
the nature of the right-thrust trimming
adjustment. Right thrust is a compromise
because it is used to counteract an
airspeed-dependent problem. As it turns
out, it is usually a good compromise.
The Application: At low and part throttle
the effect of right thrust is minimal. Here’s
where the compromise comes in. We set
the right thrust to straighten out a fullpower
takeoff climb and accept the small,
unwanted influence it has at cruise.
In the glide the right thrust has
practically no effect, so it’s no problem.
Assuming that the airplane is trimmed to
glide in a straight line in calm air, the job
of right thrust is to preserve that straight
flight path under full power.
That’s all there is to it. Typically, the
right-thrust adjustment determines how
straight the airplane climbs after takeoff.
Good landing-gear setup will reduce the
steering workload until the student gets a
model in the air. The landing-gear
discussion comes later.
Let’s adjust the right thrust. As you
read a bit ago, the requirement is for the
model to go straight in the glide with the
engine idling and at full throttle in a
Right-Thrust Measure
56 MODEL AVIATION
This tail-wheel assembly has no caster so that side loads can apply no twist to the rudder.
The pivot location, ahead of the rudder hinge line, and the connection of the tiller a short
distance behind it reduce the tail-wheel throw to roughly half the rudder throw.
If the tire contact patch is not directly in line with the axis of steering rotation, the
impact of landing will try to twist the nose-wheel sideways.
Tail Wheel Caster
Steering Axis Offset
An Alternate
Down-Thrust Check for
Advanced Sport Models
There is another way to
check the downthrust, for those
of you flying sport airplanes that
are not intended to be as stable
as trainers. This applies to most
designs with semisymmetrical
and symmetrical airfoils.
Set up a hands-off level pass
on the far edge of the runway,
approximately 50 feet up, so you
can see the airplane very well.
As you fly along nice and
straight, with the model trimmed
“hands off,” suddenly pull the
power to idle.
For a second or two the
airplane will still be zipping along
at cruise speed. The downthrust
is taken out of the balance
equation for just that second or
so—and the aerodynamic trim
predominates. The difference (if
any) is the effect of the
downthrust.
If, as you pull the throttle
back, the nose twitches up and
then the model slows into a
glide, you have too much
downthrust.
If the nose abruptly drops a
tiny bit and the airplane instantly
assumes a fast, nose-down glide
when the throttle is pulled, you
need more downthrust. That’s
because your elevator trim has
been fighting down against the
engine-induced climb all along.
If the downthrust is correct,
the airplane continues straight
for a second or two and
gradually fades into the glide
angle.
It is also possible to see the
effect of wrong downthrust
when the power is applied
rather than pulled back. In cases
where much more downthrust is
needed, you may see the model
abruptly nose-up when the
power is suddenly fire-walled for
a go-around on landing. This is
bad news—especially with a
heavy Scale model that does not
forgive a nose-too-high climb. MA
—Dean Pappas
Drawings and photos by the author except as noted
August 2006 57
A good ground stance has the wing sitting at a small but positive
angle of attack: somewhere between 0° and 3° positive.
A nose-down stance, like a bloodhound, forces the model to
accelerate beyond the necessary airspeed for flight and then
leap into the air, resulting in a steep climbout.
Nose-up stances cause two problems: overly sensitive steering
caused by “wheelbarrowing” and premature liftoff, leading to
departure stalls.
The proper ratio between rudder throw and nose-wheel throw
is usually had when the rudder pushrod is connected to the
outside of the rudder-servo wheel/arm and the nose-gear
pushrod is connected to the innermost hole on the servo wheel
(above) and the outermost hole of the nose-gear arm (below).
takeoff climb. For now let’s assume the airplane glides without
turning since it was trimmed for straight and level flight at cruise
power. (In the next installment we will discuss making the model
fly straight at all airspeeds.)
What we really need to do is adjust the engine right thrust so it
adds the right amount of correction for “engine torque” during
climb. Engine torque makes the airplane turn left. The word
“torque” is a misnomer, but it is a convenient catchall.
The Right-Thrust Test: To start with, let’s make sure the model is
trimmed to fly nice and straight at cruise power. Next, I like to set
the airplane up so it is pointed straight away from me and headed
either directly into the wind or directly downwind. You don’t
want to do this lined up with a crosswind because the sideward
wind drift hides the turn for which you are looking.
Now that you are lined up, add full throttle and smoothly pull
up into a climb, at the same angle as your steepest post-takeoff
A Little Theory
The primary cause of what we call “torque” is the
spiral airflow that comes off of the propeller. There are
two other sources—the “P” factor and the pure torque
of the engine—but they are small contributors.
If you were to hold a crepe-paper streamer behind the
propeller, you would see that the airflow coming from the
propeller follows a corkscrewlike path in the same
direction as the propeller rotation. This airflow strikes the
left side of the fin and rudder, which is usually above the
thrustline, and, as a result, yaws the airplane left.
As the model accelerates, the pitch of this corkscrew
pattern gets straighter and straighter. Because of this, the
torque effect is greatest at low speed and high throttle,
less at cruise speed and power, and gone at idle. MA
—Dean Pappas
climb. We typically climb into the wind,
but doing this downwind also works, and it
allows you to pick the direction so you
don’t have to fly over the pits or the safety
line at the field.
You don’t want to climb so steeply that
the model is stalling, but you do want to
climb as steeply as your horsepower will
permit. The airplane will lose airspeed
during the climb, and it may become more
easily influenced as the flying surfaces lose
some of their control power. In all
likelihood the model will start to turn.
If the airplane deviates to the left, you
will have to add more right thrust. On the
next flight retrim for straight and level
flight (probably just a click of rudder) and
repeat the test until the model climbs
straight.
If the airplane has too much right-thrust,
it will deviate to the right in the climb. That
doesn’t happen often.
If the right thrust is close to correct, and
if there is enough wind to make the model
bounce around, you may have to repeat the
test a couple times to be sure of which
direction the airplane is turning. That
usually means you are getting close.
It is best to adjust the right-thrust angle
one degree at a time and repeat the process.
Most airplanes have, or at least need, 2°-3°
of right-thrust, although a rare few need
much more.
Right-Thrust Measurement: The easiest
way I have found to determine the rightthrust
angle is to measure the distance from
each propeller tip to the tail post. With a
12-inch propeller the difference between
the two measurements will be 3/16 inch for
every degree of right-thrust. Three degrees
of right-thrust works out to 9/16 inch
difference between the two measurements
from the tail post. With a 16-inch propeller
this ratio works out to 1/4 inch per degree.
You might have to readjust the right
thrust a time or two, but if you start with it
adjusted as the kit recommends, you should
have to make only a fine adjustment or
two. Many kits and ARFs may not make
how much right thrust is recommended
entirely clear, but if you can’t find anything
on the plans or in the instructions, start
with 21/2° or so.
Landing Gear: An airplane that rolls
straight and responds predictably to
steering input, especially on takeoff, will
be easier to fly. If you want to look like a
hero at the flying field, die-straight takeoffs
and smooth landings that roll to a straight
stop will help.
On the other hand, if you really crave
attention, zigzagging across the runway
will have everyone watching you—as they
run for cover! That’s not how you want to
be noticed, so we will devote some
attention to describing good landing-gear
setup. Most trainers are designed with
tricycle landing gear, so I will cover
models with that kind first, followed by
tail-draggers.
A few problems can afflict a tricyclegeared
airplane. The most common is the
use of a nose-wheel steering linkage that has
way too much throw.
The model does not need to be able to
turn within its own wingspan; the minimum
turning radius should be roughly 15 feet
with full rudder control applied. That
probably works out to only 5° of turn at the
nose wheel.
This is accomplished by connecting the
linkage to the innermost hole of the servo
arm and the outermost hole on the nose-strut
steering arm. It is sometimes helpful to drill
a new hole in the servo arm that is as close
as possible to the center post. Too much
steering throw not only makes it difficult to
steer straight at speed, but it can overload
the rudder servo and prematurely age or
damage it.
The next problem is an overly flexible
steering linkage. You need positive control,
and a springy linkage does not offer that. If
the steering linkage has too much give in it,
the nose-wheel may even twist sideways at
touchdown (impact?). This makes the
airplane “curtsy” in the middle of the
runway and can even tear out the firewall if
repeated often enough.
Some fliers will tell you that a springy
linkage can save the servo, but the best way
to do that is to give the servo maximum
mechanical advantage, as described in theDid you ever try to make a gentle turn
while running with a fully laden
wheelbarrow? It tried to tip, didn’t it? The
same is true with a tricycle-geared
airplane if it is running up on only the
nose wheel.
The ideal attitude is with the wing
chord line (or flat bottom) within a few
degrees of level with the ground. A wellset-
up trainer will lift off with just a tiny
touch of up-elevator when the airspeed is
right. For trainers with flat-bottomed
wings, this stance will lift off by itself
when the airplane is going fast enough.
The last tricycle-gear problem is the
fore and aft location of the main gear. If
the main gear is placed too far aft, the
airplane has a great deal of weight on the
nose wheel. This also makes the highspeed
steering more sensitive and requires
lots of up-elevator input to break ground.
Try pushing down on the stabilizer to lift
the nose wheel, to get a feel for how
much force the up-elevator control has to
make.
Again, this can lead to an overly steep
departure after an excessively long
takeoff roll. It also causes the airplane to
“slap” onto the ground during landing;
that can add to the wear and tear on the
nose gear.
The ideal location for the main gear
makes the nose wheel very light when the
fuel tank is empty. Either bend or shim
the main gear so that the wheels move
forward. The model should almost sit on
its tail when the tank is empty.
There is one problem that afflicts taildraggers
and tricycle-geared models:
overly springy landing gear. Sometimes
the kit comes with wire landing gear that
is too springy for the airplane’s weight.
That can make bounce-free landings
difficult; anything less than a grease job is
turned into a roller-coaster ride.
The solutions to this problem range
from wire and rubber-band
reinforcements to replacing the gear with
a beefier aluminum unit.
So Why Are You Dragging Your Tail
Around? Tail-draggers have different
versions of the same problems as tricyclegear
airplanes, with one interesting
difference: the fore-and-aft location of the
main gear.
If the main gear is mounted too far aft,
the airplane tends to nose-over easily.
That’s embarrassing at the very least.
What is not as often appreciated is that
if the mains are mounted too far forward,
you get that high-speed wheelbarrow
problem I previously discussed. The
airplane will be difficult to keep straightbe reduced. This is actually easy to do. For
those of you using the “two springs”-type
steering linkage, all you need to do is hook
up to the inner end of the rudder horns and
the outer end of the tail-wheel horns.
If you used the “tiller arm”-type
linkage, where a single piece of wire runs
along the bottom of the rudder and is
attached with some kind of clip, it’s a bit
tougher to do this unless you are still
assembling the airplane; then it is easy.
All you need to do is move the tailwheel
pivot forward and find a location
for the clip on the bottom of the rudder
where the steering throw is reduced. This
is simple and offers positive steering
control.
Takeoff, Climbout, and the CG: Let’s
cover what happens on takeoff when the
airplane is nose-heavy. A severely noseheavy
model will require lots of upelevator
to lift the nose wheel and break
ground. The problem could also be
landing-gear position, the ground stance,
or the CG.
The last two are easy to eliminate, but
you need the information you gathered in
the air to tell whether to move the landing
gear or not. If the CG is in the right spot,
holding a constant climb angle is easier. If
the airplane is nose-heavy, you will find
yourself needing a quick elevator
adjustment a split second after liftoff.
Let’s look at the other, more urgent
side of the problem. On takeoff, tailheaviness
often shows itself as climbouts
that quickly become too steep, even when
they did not start out that way. If you find
yourself chasing the elevator in a pilotinduced
oscillation (PIO), you’ve probably
got a tail-heavy airplane.
Tail-heavy airplanes tend to snap roll
too, and that is usually how they get
turned back into their component parts.
Try moving the CG forward temporarily,
and see if it’s easier to fly a smooth
departure climb.
Pitch Trim Revisited: Now that the airplane
is departing nicely, it is time for Part 2 of the
“From the Ground Up” series on basic
trim to depart as well. I’ll land back here
next month and wrap things up. MA
Dean Pappas
Edition: Model Aviation - 2006/08
Page Numbers: 55,56,57,58,60,62
Trimming
August 2006 55
by Dean Pappas
Part 2 From the Ground Up
The simplest way to measure the right-thrust angle is to measure the distance from
the propeller tip to the tail post on both sides. Either use that trigonometry you forgot
or remember that with a 12-inch propeller, 3/16-inch difference equals 1°.
PICKING UP Where We Left Off: In the
first installment of this “Trimming From
the Ground Up” series we dealt with the
subject of pitch trim. As it turns out, it’s a
whole lot more than just moving the
transmitter trim lever a few clicks or beeps
until the model flies without climbing or
diving.
Airplane trimming is similar to setting
up a race car: even when a crew chief says
the “race car was fast right out of the
trailer,” he really means that the team was
able to go through the entire list of setup
checks quickly. That usually means few or
no adjustments were necessary, but it
doesn’t mean every little thing wasn’t
checked anyway.
As it turns out, there aren’t all that
many things to check when test-flying a
new airplane, but if one of the tests
described here shows a problem, you are
working harder than you need to when
flying your aircraft. If your model is
intended for the all-important one of
training mission, that’s a bad thing. If you
are a more advanced flier, you are simply
missing out on flying and looking better
than you already do.
The mission of this series is to describe
the tests and corrective actions, in a
systematized way, to help you make your
airplane fly better. None of it is any great
effort, and you don’t have to attack it all at
once. Your model’s pitch behavior can be
investigated separately from something
such as the unfortunate tendency the
airplane has to turn left immediately after
takeoff.
Engine Right Thrust: I’ve already
written about the adjustment of
downthrust, so now it’s time to discuss
right thrust.
Some years ago I attended a Scale
Masters Qualifier meet. The airplanes sure
were beautiful; there just aren’t enough
people who build like that! One of the
competitors was flying a Cessna L-19 Bird
Dog: a slow-flying, military, forward-aircontrol-
type airplane. The same aircraft, in
civilian livery, has been used as a trainer
and glider tow airplane for decades.
We were taking off from right to left
that day, and every one of the pilot’s
takeoffs veered left, over the flightline—
and the pits—and the parking area! Many
of you have seen this one, right? No, left!
It’s particularly tough for a student to deal
with, and it’s a serious safety hazard as
well. This is what happens when the right
thrust is not correctly set up.
The full-scale pilots deal with this
situation differently—emphasizing the
proper application of right rudder to
counteract “torque” on takeoff. Although
that is an excellent skill to develop, the fix
for most aeromodels is to put the proper
amount of right thrust into the engine. This
minimizes the rudder corrections that are
necessary during takeoff.
This is important for the beginner pilot,
who is still learning to take off. Should we
let the beginner give everyone in the pits a
haircut until he or she learns to use his or
her rudder hand simultaneously and
independently of the right hand? I think
not. Actually most model-airplane fliers
never really master accurate and
independent control with their rudder
hand, but it is a worthy goal.
Before I describe how to put right
thrust into the model and how to test for
whether or not the airplane has the correct
amount of right thrust, I need to discuss
the nature of the right-thrust trimming
adjustment. Right thrust is a compromise
because it is used to counteract an
airspeed-dependent problem. As it turns
out, it is usually a good compromise.
The Application: At low and part throttle
the effect of right thrust is minimal. Here’s
where the compromise comes in. We set
the right thrust to straighten out a fullpower
takeoff climb and accept the small,
unwanted influence it has at cruise.
In the glide the right thrust has
practically no effect, so it’s no problem.
Assuming that the airplane is trimmed to
glide in a straight line in calm air, the job
of right thrust is to preserve that straight
flight path under full power.
That’s all there is to it. Typically, the
right-thrust adjustment determines how
straight the airplane climbs after takeoff.
Good landing-gear setup will reduce the
steering workload until the student gets a
model in the air. The landing-gear
discussion comes later.
Let’s adjust the right thrust. As you
read a bit ago, the requirement is for the
model to go straight in the glide with the
engine idling and at full throttle in a
Right-Thrust Measure
56 MODEL AVIATION
This tail-wheel assembly has no caster so that side loads can apply no twist to the rudder.
The pivot location, ahead of the rudder hinge line, and the connection of the tiller a short
distance behind it reduce the tail-wheel throw to roughly half the rudder throw.
If the tire contact patch is not directly in line with the axis of steering rotation, the
impact of landing will try to twist the nose-wheel sideways.
Tail Wheel Caster
Steering Axis Offset
An Alternate
Down-Thrust Check for
Advanced Sport Models
There is another way to
check the downthrust, for those
of you flying sport airplanes that
are not intended to be as stable
as trainers. This applies to most
designs with semisymmetrical
and symmetrical airfoils.
Set up a hands-off level pass
on the far edge of the runway,
approximately 50 feet up, so you
can see the airplane very well.
As you fly along nice and
straight, with the model trimmed
“hands off,” suddenly pull the
power to idle.
For a second or two the
airplane will still be zipping along
at cruise speed. The downthrust
is taken out of the balance
equation for just that second or
so—and the aerodynamic trim
predominates. The difference (if
any) is the effect of the
downthrust.
If, as you pull the throttle
back, the nose twitches up and
then the model slows into a
glide, you have too much
downthrust.
If the nose abruptly drops a
tiny bit and the airplane instantly
assumes a fast, nose-down glide
when the throttle is pulled, you
need more downthrust. That’s
because your elevator trim has
been fighting down against the
engine-induced climb all along.
If the downthrust is correct,
the airplane continues straight
for a second or two and
gradually fades into the glide
angle.
It is also possible to see the
effect of wrong downthrust
when the power is applied
rather than pulled back. In cases
where much more downthrust is
needed, you may see the model
abruptly nose-up when the
power is suddenly fire-walled for
a go-around on landing. This is
bad news—especially with a
heavy Scale model that does not
forgive a nose-too-high climb. MA
—Dean Pappas
Drawings and photos by the author except as noted
August 2006 57
A good ground stance has the wing sitting at a small but positive
angle of attack: somewhere between 0° and 3° positive.
A nose-down stance, like a bloodhound, forces the model to
accelerate beyond the necessary airspeed for flight and then
leap into the air, resulting in a steep climbout.
Nose-up stances cause two problems: overly sensitive steering
caused by “wheelbarrowing” and premature liftoff, leading to
departure stalls.
The proper ratio between rudder throw and nose-wheel throw
is usually had when the rudder pushrod is connected to the
outside of the rudder-servo wheel/arm and the nose-gear
pushrod is connected to the innermost hole on the servo wheel
(above) and the outermost hole of the nose-gear arm (below).
takeoff climb. For now let’s assume the airplane glides without
turning since it was trimmed for straight and level flight at cruise
power. (In the next installment we will discuss making the model
fly straight at all airspeeds.)
What we really need to do is adjust the engine right thrust so it
adds the right amount of correction for “engine torque” during
climb. Engine torque makes the airplane turn left. The word
“torque” is a misnomer, but it is a convenient catchall.
The Right-Thrust Test: To start with, let’s make sure the model is
trimmed to fly nice and straight at cruise power. Next, I like to set
the airplane up so it is pointed straight away from me and headed
either directly into the wind or directly downwind. You don’t
want to do this lined up with a crosswind because the sideward
wind drift hides the turn for which you are looking.
Now that you are lined up, add full throttle and smoothly pull
up into a climb, at the same angle as your steepest post-takeoff
A Little Theory
The primary cause of what we call “torque” is the
spiral airflow that comes off of the propeller. There are
two other sources—the “P” factor and the pure torque
of the engine—but they are small contributors.
If you were to hold a crepe-paper streamer behind the
propeller, you would see that the airflow coming from the
propeller follows a corkscrewlike path in the same
direction as the propeller rotation. This airflow strikes the
left side of the fin and rudder, which is usually above the
thrustline, and, as a result, yaws the airplane left.
As the model accelerates, the pitch of this corkscrew
pattern gets straighter and straighter. Because of this, the
torque effect is greatest at low speed and high throttle,
less at cruise speed and power, and gone at idle. MA
—Dean Pappas
climb. We typically climb into the wind,
but doing this downwind also works, and it
allows you to pick the direction so you
don’t have to fly over the pits or the safety
line at the field.
You don’t want to climb so steeply that
the model is stalling, but you do want to
climb as steeply as your horsepower will
permit. The airplane will lose airspeed
during the climb, and it may become more
easily influenced as the flying surfaces lose
some of their control power. In all
likelihood the model will start to turn.
If the airplane deviates to the left, you
will have to add more right thrust. On the
next flight retrim for straight and level
flight (probably just a click of rudder) and
repeat the test until the model climbs
straight.
If the airplane has too much right-thrust,
it will deviate to the right in the climb. That
doesn’t happen often.
If the right thrust is close to correct, and
if there is enough wind to make the model
bounce around, you may have to repeat the
test a couple times to be sure of which
direction the airplane is turning. That
usually means you are getting close.
It is best to adjust the right-thrust angle
one degree at a time and repeat the process.
Most airplanes have, or at least need, 2°-3°
of right-thrust, although a rare few need
much more.
Right-Thrust Measurement: The easiest
way I have found to determine the rightthrust
angle is to measure the distance from
each propeller tip to the tail post. With a
12-inch propeller the difference between
the two measurements will be 3/16 inch for
every degree of right-thrust. Three degrees
of right-thrust works out to 9/16 inch
difference between the two measurements
from the tail post. With a 16-inch propeller
this ratio works out to 1/4 inch per degree.
You might have to readjust the right
thrust a time or two, but if you start with it
adjusted as the kit recommends, you should
have to make only a fine adjustment or
two. Many kits and ARFs may not make
how much right thrust is recommended
entirely clear, but if you can’t find anything
on the plans or in the instructions, start
with 21/2° or so.
Landing Gear: An airplane that rolls
straight and responds predictably to
steering input, especially on takeoff, will
be easier to fly. If you want to look like a
hero at the flying field, die-straight takeoffs
and smooth landings that roll to a straight
stop will help.
On the other hand, if you really crave
attention, zigzagging across the runway
will have everyone watching you—as they
run for cover! That’s not how you want to
be noticed, so we will devote some
attention to describing good landing-gear
setup. Most trainers are designed with
tricycle landing gear, so I will cover
models with that kind first, followed by
tail-draggers.
A few problems can afflict a tricyclegeared
airplane. The most common is the
use of a nose-wheel steering linkage that has
way too much throw.
The model does not need to be able to
turn within its own wingspan; the minimum
turning radius should be roughly 15 feet
with full rudder control applied. That
probably works out to only 5° of turn at the
nose wheel.
This is accomplished by connecting the
linkage to the innermost hole of the servo
arm and the outermost hole on the nose-strut
steering arm. It is sometimes helpful to drill
a new hole in the servo arm that is as close
as possible to the center post. Too much
steering throw not only makes it difficult to
steer straight at speed, but it can overload
the rudder servo and prematurely age or
damage it.
The next problem is an overly flexible
steering linkage. You need positive control,
and a springy linkage does not offer that. If
the steering linkage has too much give in it,
the nose-wheel may even twist sideways at
touchdown (impact?). This makes the
airplane “curtsy” in the middle of the
runway and can even tear out the firewall if
repeated often enough.
Some fliers will tell you that a springy
linkage can save the servo, but the best way
to do that is to give the servo maximum
mechanical advantage, as described in theDid you ever try to make a gentle turn
while running with a fully laden
wheelbarrow? It tried to tip, didn’t it? The
same is true with a tricycle-geared
airplane if it is running up on only the
nose wheel.
The ideal attitude is with the wing
chord line (or flat bottom) within a few
degrees of level with the ground. A wellset-
up trainer will lift off with just a tiny
touch of up-elevator when the airspeed is
right. For trainers with flat-bottomed
wings, this stance will lift off by itself
when the airplane is going fast enough.
The last tricycle-gear problem is the
fore and aft location of the main gear. If
the main gear is placed too far aft, the
airplane has a great deal of weight on the
nose wheel. This also makes the highspeed
steering more sensitive and requires
lots of up-elevator input to break ground.
Try pushing down on the stabilizer to lift
the nose wheel, to get a feel for how
much force the up-elevator control has to
make.
Again, this can lead to an overly steep
departure after an excessively long
takeoff roll. It also causes the airplane to
“slap” onto the ground during landing;
that can add to the wear and tear on the
nose gear.
The ideal location for the main gear
makes the nose wheel very light when the
fuel tank is empty. Either bend or shim
the main gear so that the wheels move
forward. The model should almost sit on
its tail when the tank is empty.
There is one problem that afflicts taildraggers
and tricycle-geared models:
overly springy landing gear. Sometimes
the kit comes with wire landing gear that
is too springy for the airplane’s weight.
That can make bounce-free landings
difficult; anything less than a grease job is
turned into a roller-coaster ride.
The solutions to this problem range
from wire and rubber-band
reinforcements to replacing the gear with
a beefier aluminum unit.
So Why Are You Dragging Your Tail
Around? Tail-draggers have different
versions of the same problems as tricyclegear
airplanes, with one interesting
difference: the fore-and-aft location of the
main gear.
If the main gear is mounted too far aft,
the airplane tends to nose-over easily.
That’s embarrassing at the very least.
What is not as often appreciated is that
if the mains are mounted too far forward,
you get that high-speed wheelbarrow
problem I previously discussed. The
airplane will be difficult to keep straightbe reduced. This is actually easy to do. For
those of you using the “two springs”-type
steering linkage, all you need to do is hook
up to the inner end of the rudder horns and
the outer end of the tail-wheel horns.
If you used the “tiller arm”-type
linkage, where a single piece of wire runs
along the bottom of the rudder and is
attached with some kind of clip, it’s a bit
tougher to do this unless you are still
assembling the airplane; then it is easy.
All you need to do is move the tailwheel
pivot forward and find a location
for the clip on the bottom of the rudder
where the steering throw is reduced. This
is simple and offers positive steering
control.
Takeoff, Climbout, and the CG: Let’s
cover what happens on takeoff when the
airplane is nose-heavy. A severely noseheavy
model will require lots of upelevator
to lift the nose wheel and break
ground. The problem could also be
landing-gear position, the ground stance,
or the CG.
The last two are easy to eliminate, but
you need the information you gathered in
the air to tell whether to move the landing
gear or not. If the CG is in the right spot,
holding a constant climb angle is easier. If
the airplane is nose-heavy, you will find
yourself needing a quick elevator
adjustment a split second after liftoff.
Let’s look at the other, more urgent
side of the problem. On takeoff, tailheaviness
often shows itself as climbouts
that quickly become too steep, even when
they did not start out that way. If you find
yourself chasing the elevator in a pilotinduced
oscillation (PIO), you’ve probably
got a tail-heavy airplane.
Tail-heavy airplanes tend to snap roll
too, and that is usually how they get
turned back into their component parts.
Try moving the CG forward temporarily,
and see if it’s easier to fly a smooth
departure climb.
Pitch Trim Revisited: Now that the airplane
is departing nicely, it is time for Part 2 of the
“From the Ground Up” series on basic
trim to depart as well. I’ll land back here
next month and wrap things up. MA
Dean Pappas
