Finding Your Personal IMAC Setup - 2010/06

June 2010 43
IMAC Setup
by
Baron
Johnson
WHEN IT COMES to RC Scale
Aerobatics—specifically International
Miniature Aerobatic Club (IMAC)
competition—aircraft-and-radio setup is
probably the most significant thing that can
improve your flying, aside from burning a lot
of gas. It is also one of the most personal and
widely varying subjects.
A common theme with successful IMAC
pilots is that each has found the setup that
works best for him or her. Some
arrangements are similar, while others are
drastically different, but individuality is the
key to success.
This is not meant to be a step-by-step
procedure to follow when setting up a model.
If you’re looking for that information, Peter
Goldsmith wrote an excellent guide that was
published in the February and April 2005 RC
Scale Aerobatics columns in MA. They are
reprinted on the IMAC Web site, for
members to read.
This feature will assist you in developing
your personalized setup. To guide people on
how to do that, I usually tell them that they
need to try many different techniques.
To know what methods to test, you
typically have to talk to a few successful
pilots to learn what works for them. Your
ultimate setup will likely not mirror anyone
else’s, but will be a combination of elements
from various sources.
Finding what arrangement works best for
you can be a long process, but it is well worth
it. As you move up through the IMAC
classes, you might find yourself changing
what you thought was your best setup, which
is a natural part of the process.
Some setup elements that work well for
complicated maneuvers in the Unlimited
class may be unnecessary for most pilots in
Sportsman. And as your precision flying
skills improve, you might detect deficiencies
in how you do certain things and become
aware of areas that different setup techniques
can improve.
Don’t think of a setup as something you
establish with a new model and then don’t
alter. I have adjusted many aspects of my
combination after flying the same airplane in
competition for more than a year. As you
gain experience in IMAC competition and
with your aircraft, you’ll make much
slighter adjustments to your arrangement.
Before proceeding, I’ll review the purpose
of aircraft-and-radio setup. The goal of
anything you do with it is to accomplish one
or—I hope—both of the following goals:
reduced pilot workload and consistency.
It is important to keep those goals in
mind any time you adjust your setup. It is
possible to get wrapped up in nitty-gritty
details of trying to make a model fly
perfectly all the time and end up working
against those objectives.
The remainder of this article will cover
some of the fundamental decisions each
Findi n g Y o u r P e r s o n a l
On deck for a round of
Unlimited, Ivan Munninghoff
warms up the DA engine in his
Carden Extra 330. A pilot who is
promptly ready to fly makes a good
impression on judges.
Show
10in yourown way
the judges a
06sig2_00MSTRPG.QXD 4/22/10 12:53 PM Page 43
44 MODEL AVIATION
Photos by the author and MA staff Illustrations by the author
Right: This Quique Somenzini Pitts Python
requires minimal elevator and aileron input
to hold knife-edge when rudder input is
given. It does need some, and programming
that mix helps the pilot. The model doesn’t
have to point straight but must fly a straight
path for the judge to score high.
Right: A Carden Edge
540T pulls out from a
down- l ine vertical .
Almos t al l Scale
Aerobat ics model s
need down-elevator
trim/mix to hold a
straight line downward.
Straight lines must be
precise for the judges
to score as high as
possible.
Below: A popular CG point test is to hold a model,
such as this Hangar 9 Extra 260, inverted and judge the
amount of down-elevator input needed to maintain a heading.
Lots of down pressure can mean that a model is nose-heavy. A
“dip” in heading is a downgrade from the judges.
Two ways to hold the
transmitter. Thumbs only
(above) gives you less access
to the switches. Pinching with
the thumb and foref inger
(below) provides more access.
Do what feels right to you.
Above: The author is poised to pilot his
35% Carden Edge 540T with his father,
Fred, as his caller. The pilot and caller,
who warns the pilot of any safety hazards,
have to work as a team.
Setup Objectives
1. Reduced pilot workload. The less work you have to do, the more you
can focus on other, finer aspects of a maneuver. A pilot has a seemingly
endless list of things to concentrate on while flying precision Aerobatics, so
removing any of them through setup is a huge benefit.
2. Consistency. This refers not only to how often you can properly execute
a maneuver within a single contest or throughout the season, but also to how
consistently can you perform the maneuver if you haven’t practiced in a week
or a month.
Some approaches to setup can produce extremely good maneuvers when
you execute them properly but can have severe consequences if they are not
done perfectly for any number of reasons, such as lack of practice, nerves, or
weather conditions. Envision how many Snap Rolls, Spins, and Hammerheads
you’ll perform throughout the course of a season! MA
—Baron Johnson
06sig2_00MSTRPG.QXD 4/22/10 12:05 PM Page 44
June 2010 45
Why Right Thrust?
Right thrust (or throttle-to-rudder mixing) is required to
compensate for the yawing moment caused by an effect called
“spiral slipstream” (not “torque,” as is often misidentified).
The air being accelerated by the propeller does not travel
rearward in a straight line. Instead, the propeller tends to
slightly rotate air in the direction of the propeller rotation while
accelerating it rearward.
Propwash rotates around the fuselage in what is called the
“spiral slipstream.” This airflow strikes the asymmetrical vertical
tail section and creates a nose-left yawing moment.
This causes the propwash to swirl around the fuselage in a
clockwise direction (as viewed from behind the model) as it
approaches the tail. One common method of visualizing this
effect is noticing the concentration of exhaust or smoke oil
residue on particular parts of the tail section, caused by
spiraling flow.
Why does this spiraling airflow cause the aircraft to yaw left?
The second part of this condition lies with the asymmetry of
most airplanes’ vertical tails. As the spiraling airflow
approaches the tail section, it strikes the vertical fin from the
left, creating a force pushing the tail to the right and, therefore,
a moment yawing the model to the left.
If the vertical fin were symmetrical and had a matching fin
below the fuselage, the spiraling airflow would strike this
surface from the right, thereby canceling out the force and yaw
moment caused by the upper vertical fin. Very few aircraft have
such a symmetrical vertical tail, though, because it introduces
more challenges in design, construction, and operation.
Neither right thrust nor throttle-to-rudder mixing provides
a truly proper compensation for spiral slipstream. The yaw
moment caused by spiral slipstream is a function of not only
throttle setting, but also airspeed.
As the airplane flies faster, the spiral airflow is somewhat
straightened, or stretched in length relative to the fuselage
length. This decreases the yawing moment at a given throttle
setting if the airspeed increases.
The yawing moment is strongest at low airspeed and high
throttle setting and weakest at high airspeed and low throttle
setting. Both compensation methods provide a yaw moment as
a function of throttle setting, but not airspeed, so they can
compensate only for one of the two variables. Without airspeed
feedback, though, they are the best methods we have available.
Is right thrust detrimental when inverted? No. Some people
have this misconception because of the confusion of spiral
slipstream with other propeller forces, namely P-factor.
P-factor depends on angle of attack, so it does reverse the
required rudder compensation when flying inverted (negative
angles of attack). We have no method to compensate for the
yawing moment caused by P-factor except rudder stick.
Now that you understand the causes of the spiral slipstream,
which we compensate for with right thrust or throttle-torudder
mixing, you can see that the effect will act in the same
direction regardless of aircraft orientation.
In any orientation, the propeller rotates the same direction
and the vertical tail is still located above the fuselage, which
creates a left yawing moment. Therefore, right rudder or right
thrust compensates for spiral slipstream in any orientation. MA
—Baron Johnson
Why Down-Line Mix?
The primary factor leading to the common
nose-up pitching of an Aerobatics model on a
vertical down-line is the longitudinal stability. If the
aircraft’s CG is located forward of the neutral point
(NP), the airplane is statically stable in the
longitudinal (pitch) axis. This static stability
indicates that when the model is disturbed in the
pitch axis by an outside source, it will create a
restoring moment, returning the airplane to the
trimmed condition.
A neutrally stable aircraft, which has the CG
located on the NP, would remain at the disturbed
pitch angle. A statically unstable model, which has
the CG located behind the NP, would continue
pitching in the direction of the disturbance.
Longitudinal stability is desired for most
airplanes—including aerobatic types—to behave
predictably and have good handling qualities.
Blue arrow shows required elevator trim to
balance pitching moment caused by longitudinal
stability.
This longitudinal stability comes with one small
downside. To trim the aircraft for level flight, upelevator
is required to balance the rotational
moments.
Why? Keep in mind that airplanes rotate about
their CG. If the center of lift, which coincides with
the NP, is located behind the CG, the lift will create
a nose-down pitching moment. Therefore, upelevator
is required to create a nose-up pitching
moment to balance out the nose-down moment
and make the model fly straight in the pitch axis.
When you hear mention of up-elevator trim,
that doesn’t necessarily mean actual deflection you
can see on the elevator. It could be accomplished
by horizontal tail incidence angle. It could also be
caused naturally by the downwash from the wing
striking the horizontal tail, thus placing the
horizontal tail at a slightly negative (or lower)
angle of attack compared to the wing.
The bottom line is that a nose-up pitching
moment is generated by a downward force from
the tail.
Upthrust or downthrust could also balance a
model’s pitching moments. However, that would
make the aircraft sensitive to
throttle changes, which is
undesirable.
So how does that translate
to a pitching tendency on a
vertical down-line? When the
airplane is flying vertically, the
wings are not generating an
appreciable amount of lift.
This eliminates the nosedown
pitching moment. But
elevator trim is still present,
which creates a nose-up
pitching moment. Mixing a
small amount of downelevator
at idle throttle
essentially removes the upelevator
trim, which is
required for level flight. MA
Nose-up pitching moment on
vertical down-line is caused
by elevator trim.
—Baron Johnson
06sig2_00MSTRPG.QXD 4/22/10 12:17 PM Page 45
pilot must make when developing his or her
setup. There is no right or wrong when it
comes to these decisions; they are based
purely on personal preference, and there are
successful IMAC pilots who use all of these
techniques.
I hope that discussing some of these
decisions will allow you to test various
setup methodologies and form your own
setup, even if you don’t have the
opportunity to regularly talk with successful
IMAC pilots.
Switch Flipping? Using a single condition
(combination of travel, exponential, mixing,
etc.) or flipping switches for various
maneuvers or situations will have a large
impact on most future setup decisions.
The argument for a single condition is
that you always fly a model with the same
response and characteristics. In addition,
you don’t have to worry about switch
positions or performing a maneuver in an
incorrect condition.
The other side of that argument is that
our computer transmitters have incredibly
sophisticated capabilities. Therefore, you
might as well use them to make the aircraft
fly the best and most consistently for
particular types of maneuvers, and not try to
find a one-size-fits-all condition for each.
Both sides of that debate can play into
the two objectives of setup (reduced pilot
workload and consistency). Your decision
might come down to something as simple
as how you hold your transmitter.
If you fly with thumbs only and the rest
of your fingers grasping the transmitter
body, it might be distracting to reach the
switches between maneuvers, much less in
the middle of a maneuver. But if you pinch
the sticks, you might have much easier
access to the switches.
Some common maneuvers that pilots
dedicate conditions to include Snap Rolls,
Spins, Hammerheads, and Rolling Circles.
Hammerhead and Rolling Circle conditions
use a great deal of—if not all—available
rudder travel, while other conditions use
less rudder travel. These amounts vary with
airplane and flier preference.
Some pilots prefer to fly in a Snap Roll
condition most times and then switch before
particular maneuvers. Others prefer to fly in
a condition conducive to Hammerheads,
Rolling Circles, or general flying, and then
change conditions before performing Snap
Rolls.
Dual Rates Vs. Flight Modes: If you have
decided to flip switches in flight, this is the
next issue you must address. Dual (and
triple) rates have been available on many
transmitters for quite sometime. Flight
modes (or conditions) is a newer extension
on the dual rate concept.
Dual rates allow you to change the travel
and exponential (expo) of a specific control
surface with a switch. Flight modes allow
you to put any number of travel and expo
changes on a single switch, along with other
features such as mixes, throttle curves, etc.
Arguments can be made for both
systems. The benefit of using dedicated
dual rate switches is that you can have
many combinations of travels, with each
control surface having two or three
independent rates.
The downside to this is that it mandates
that you keep track of the position of
several switches. Additionally, you might
have to assign additional switches to tasks
such as turning a mix on or off. (More about
that later.)
The benefit of using flight modes is that
you have fewer switches to deal with and
can make all necessary changes with one
action. This can lead to less distraction
while flying and less possibility of being in
the wrong combination of rates.
The downside is that a limited number of
flight modes provides fewer combinations
of rates, compared with having dual or
triple rates on every control surface.
Right Thrust Vs. Throttle-to-Rudder
Mixing: These items compensate for the
yawing effect of “spiral slipstream.” See the
sidebar for a detailed explanation. Both
approaches can create satisfactory results.
By far the most common method among
IMAC pilots is right thrust, or mounting the
engine so that it points slightly to the right.
This is probably because it has been around
for quite awhile (long before computer
radios) and has proven itself to be adequate.
And many models now come from their
manufacturers with approximately correct
right thrust built into the firewalls.
The downside to right thrust is that to
make changes, you usually need to remove
the cowling and add or remove spacers.
The other approach is mounting the
engine straight and creating the required
yaw moment by mixing right rudder with
throttle position. This makes it easier to
mount the power plant and exhaust, and it
minimizes the eyesore of an offset spinner
if the cowling is not shaped for it.
Throttle-to-rudder mix provides greater
flexibility for changing the compensation
to tweak it or to compensate for propeller
changes. Additionally, most high-end
transmitters allow you to do a curve mix, or
nonlinear mix, which can help fine-tune the
compensation throughout the throttle range
(although not throughout the airspeed
range). See sidebar for details.
Some pilots use a third approach: a
combination of right thrust and throttle-torudder
mixing. You can mount the engine
with the manufacturer-specified right thrust
or get it as close as possible and then make
small adjustments with the transmitter.
This methodology captures aspects of both
original approaches but can be timeconsuming.
Mixes: On All the Time? This is a difficult
question that pilots often ask as they add
mixes to their setups. Some fliers turn off
mixing—particularly down-line mixing—
for landing so they don’t get caught offguard
during approach.
In terms of within the sequence, the
question is harder to answer. Some pilots
advocate turning off some or all mixes for
Rolling Circles or Hammerheads, while
many fliers leave all mixes on all the time.
This issue must be addressed on a
model-by-model and mix-by-mix basis.
With numerous airplanes I have had, I
found no reason to turn off mixes for any
maneuver. However, I have had models in
which a mix was detrimental to certain
maneuvers.
I had to fly maneuvers numerous times
in different conditions before I could
conclusively determine a mix as the cause.
I ultimately reduced or eliminated the
mixes in question to improve those
particular maneuvers.
Judging from these experiences, I
recommend planning on leaving mixes on
all the time until you see a reason to do
otherwise. Switching in and out of mixes
adds a considerable workload if it’s
unnecessary.
48 MODEL AVIATION
Down-Line Mixing: Most statically stable
aerobatic aircraft will exhibit a slight noseup
pitching tendency when placed on a
power-off, vertical down-line. See the
sidebar for a detailed explanation.
This is one situation in which most
IMAC pilots use the same setup principle:
down-line mixing. Down-line mixing inputs
a small amount of down-elevator when the
throttle is reduced to idle or near idle
(within a couple of ratchets).
The alternative, which is listed on some
“trim charts,” is to simply move the CG
rearward until the pitch tendency goes
away. This is certainly a way to eliminate
that characteristic, but in doing so you
move the CG to the neutral point, which
eliminates longitudinal static stability.
A neutrally stable airplane will not fly
well for most other precision maneuvers
and will cause more headaches than mere
down-line pitching. It is a small price to pay
for a nice, stable model.
If you’ve never applied a mix such as
this, there are numerous methods to use to
accomplish the same thing with today’s
transmitters. You can use a curve or
nonlinear throttle-to-elevator mix and give
it a value of zero at all points except near
idle. You can also use a traditional throttleto-
elevator mix and employ the throttle
position as the activation switch.
Another technique is to use a
traditional throttle-to-elevator mix and
utilize the offset function to effectively
move the “center” point where the mix
switches from one direction to the other.
Offset it enough so that the mix switches
from one direction to the other at a couple
of ratchets above idle.
Then leave the value activated by high
throttle at zero mix. Use the needed
amount of mix on the other direction that
activates when the throttle is at idle.
This article has been anything but
exhaustive. There are more methods to
cover to address the preceding issues, and
there are countless other issues that can be
resolved with setup. I encourage you to
explore other aspects of setup that you either
learn from others or devise yourself.
I’ve tried to give a jump-start to those
who don’t have access to experienced
IMAC pilots. However, no article can
completely replace talking with other fliers
and having them give you opinions, in
person, about your setup.
If you are interested in competing in
IMAC, find a contest near you and compete.
You will probably learn more and get more
advice—about both setup and flying—
during that short event than you could from
a year of practicing on your own. And you’ll
have a lot more fun! MA
Baron Johnson
[email protected]
Sources:
International Miniature Aerobatic Club
PAT. PENDING www.mini-iac.com

June 2010 43
IMAC Setup
by
Baron
Johnson
WHEN IT COMES to RC Scale
Aerobatics—specifically International
Miniature Aerobatic Club (IMAC)
competition—aircraft-and-radio setup is
probably the most significant thing that can
improve your flying, aside from burning a lot
of gas. It is also one of the most personal and
widely varying subjects.
A common theme with successful IMAC
pilots is that each has found the setup that
works best for him or her. Some
arrangements are similar, while others are
drastically different, but individuality is the
key to success.
This is not meant to be a step-by-step
procedure to follow when setting up a model.
If you’re looking for that information, Peter
Goldsmith wrote an excellent guide that was
published in the February and April 2005 RC
Scale Aerobatics columns in MA. They are
reprinted on the IMAC Web site, for
members to read.
This feature will assist you in developing
your personalized setup. To guide people on
how to do that, I usually tell them that they
need to try many different techniques.
To know what methods to test, you
typically have to talk to a few successful
pilots to learn what works for them. Your
ultimate setup will likely not mirror anyone
else’s, but will be a combination of elements
from various sources.
Finding what arrangement works best for
you can be a long process, but it is well worth
it. As you move up through the IMAC
classes, you might find yourself changing
what you thought was your best setup, which
is a natural part of the process.
Some setup elements that work well for
complicated maneuvers in the Unlimited
class may be unnecessary for most pilots in
Sportsman. And as your precision flying
skills improve, you might detect deficiencies
in how you do certain things and become
aware of areas that different setup techniques
can improve.
Don’t think of a setup as something you
establish with a new model and then don’t
alter. I have adjusted many aspects of my
combination after flying the same airplane in
competition for more than a year. As you
gain experience in IMAC competition and
with your aircraft, you’ll make much
slighter adjustments to your arrangement.
Before proceeding, I’ll review the purpose
of aircraft-and-radio setup. The goal of
anything you do with it is to accomplish one
or—I hope—both of the following goals:
reduced pilot workload and consistency.
It is important to keep those goals in
mind any time you adjust your setup. It is
possible to get wrapped up in nitty-gritty
details of trying to make a model fly
perfectly all the time and end up working
against those objectives.
The remainder of this article will cover
some of the fundamental decisions each
Findi n g Y o u r P e r s o n a l
On deck for a round of
Unlimited, Ivan Munninghoff
warms up the DA engine in his
Carden Extra 330. A pilot who is
promptly ready to fly makes a good
impression on judges.
Show
10in yourown way
the judges a
06sig2_00MSTRPG.QXD 4/22/10 12:53 PM Page 43
44 MODEL AVIATION
Photos by the author and MA staff Illustrations by the author
Right: This Quique Somenzini Pitts Python
requires minimal elevator and aileron input
to hold knife-edge when rudder input is
given. It does need some, and programming
that mix helps the pilot. The model doesn’t
have to point straight but must fly a straight
path for the judge to score high.
Right: A Carden Edge
540T pulls out from a
down- l ine vertical .
Almos t al l Scale
Aerobat ics model s
need down-elevator
trim/mix to hold a
straight line downward.
Straight lines must be
precise for the judges
to score as high as
possible.
Below: A popular CG point test is to hold a model,
such as this Hangar 9 Extra 260, inverted and judge the
amount of down-elevator input needed to maintain a heading.
Lots of down pressure can mean that a model is nose-heavy. A
“dip” in heading is a downgrade from the judges.
Two ways to hold the
transmitter. Thumbs only
(above) gives you less access
to the switches. Pinching with
the thumb and foref inger
(below) provides more access.
Do what feels right to you.
Above: The author is poised to pilot his
35% Carden Edge 540T with his father,
Fred, as his caller. The pilot and caller,
who warns the pilot of any safety hazards,
have to work as a team.
Setup Objectives
1. Reduced pilot workload. The less work you have to do, the more you
can focus on other, finer aspects of a maneuver. A pilot has a seemingly
endless list of things to concentrate on while flying precision Aerobatics, so
removing any of them through setup is a huge benefit.
2. Consistency. This refers not only to how often you can properly execute
a maneuver within a single contest or throughout the season, but also to how
consistently can you perform the maneuver if you haven’t practiced in a week
or a month.
Some approaches to setup can produce extremely good maneuvers when
you execute them properly but can have severe consequences if they are not
done perfectly for any number of reasons, such as lack of practice, nerves, or
weather conditions. Envision how many Snap Rolls, Spins, and Hammerheads
you’ll perform throughout the course of a season! MA
—Baron Johnson
06sig2_00MSTRPG.QXD 4/22/10 12:05 PM Page 44
June 2010 45
Why Right Thrust?
Right thrust (or throttle-to-rudder mixing) is required to
compensate for the yawing moment caused by an effect called
“spiral slipstream” (not “torque,” as is often misidentified).
The air being accelerated by the propeller does not travel
rearward in a straight line. Instead, the propeller tends to
slightly rotate air in the direction of the propeller rotation while
accelerating it rearward.
Propwash rotates around the fuselage in what is called the
“spiral slipstream.” This airflow strikes the asymmetrical vertical
tail section and creates a nose-left yawing moment.
This causes the propwash to swirl around the fuselage in a
clockwise direction (as viewed from behind the model) as it
approaches the tail. One common method of visualizing this
effect is noticing the concentration of exhaust or smoke oil
residue on particular parts of the tail section, caused by
spiraling flow.
Why does this spiraling airflow cause the aircraft to yaw left?
The second part of this condition lies with the asymmetry of
most airplanes’ vertical tails. As the spiraling airflow
approaches the tail section, it strikes the vertical fin from the
left, creating a force pushing the tail to the right and, therefore,
a moment yawing the model to the left.
If the vertical fin were symmetrical and had a matching fin
below the fuselage, the spiraling airflow would strike this
surface from the right, thereby canceling out the force and yaw
moment caused by the upper vertical fin. Very few aircraft have
such a symmetrical vertical tail, though, because it introduces
more challenges in design, construction, and operation.
Neither right thrust nor throttle-to-rudder mixing provides
a truly proper compensation for spiral slipstream. The yaw
moment caused by spiral slipstream is a function of not only
throttle setting, but also airspeed.
As the airplane flies faster, the spiral airflow is somewhat
straightened, or stretched in length relative to the fuselage
length. This decreases the yawing moment at a given throttle
setting if the airspeed increases.
The yawing moment is strongest at low airspeed and high
throttle setting and weakest at high airspeed and low throttle
setting. Both compensation methods provide a yaw moment as
a function of throttle setting, but not airspeed, so they can
compensate only for one of the two variables. Without airspeed
feedback, though, they are the best methods we have available.
Is right thrust detrimental when inverted? No. Some people
have this misconception because of the confusion of spiral
slipstream with other propeller forces, namely P-factor.
P-factor depends on angle of attack, so it does reverse the
required rudder compensation when flying inverted (negative
angles of attack). We have no method to compensate for the
yawing moment caused by P-factor except rudder stick.
Now that you understand the causes of the spiral slipstream,
which we compensate for with right thrust or throttle-torudder
mixing, you can see that the effect will act in the same
direction regardless of aircraft orientation.
In any orientation, the propeller rotates the same direction
and the vertical tail is still located above the fuselage, which
creates a left yawing moment. Therefore, right rudder or right
thrust compensates for spiral slipstream in any orientation. MA
—Baron Johnson
Why Down-Line Mix?
The primary factor leading to the common
nose-up pitching of an Aerobatics model on a
vertical down-line is the longitudinal stability. If the
aircraft’s CG is located forward of the neutral point
(NP), the airplane is statically stable in the
longitudinal (pitch) axis. This static stability
indicates that when the model is disturbed in the
pitch axis by an outside source, it will create a
restoring moment, returning the airplane to the
trimmed condition.
A neutrally stable aircraft, which has the CG
located on the NP, would remain at the disturbed
pitch angle. A statically unstable model, which has
the CG located behind the NP, would continue
pitching in the direction of the disturbance.
Longitudinal stability is desired for most
airplanes—including aerobatic types—to behave
predictably and have good handling qualities.
Blue arrow shows required elevator trim to
balance pitching moment caused by longitudinal
stability.
This longitudinal stability comes with one small
downside. To trim the aircraft for level flight, upelevator
is required to balance the rotational
moments.
Why? Keep in mind that airplanes rotate about
their CG. If the center of lift, which coincides with
the NP, is located behind the CG, the lift will create
a nose-down pitching moment. Therefore, upelevator
is required to create a nose-up pitching
moment to balance out the nose-down moment
and make the model fly straight in the pitch axis.
When you hear mention of up-elevator trim,
that doesn’t necessarily mean actual deflection you
can see on the elevator. It could be accomplished
by horizontal tail incidence angle. It could also be
caused naturally by the downwash from the wing
striking the horizontal tail, thus placing the
horizontal tail at a slightly negative (or lower)
angle of attack compared to the wing.
The bottom line is that a nose-up pitching
moment is generated by a downward force from
the tail.
Upthrust or downthrust could also balance a
model’s pitching moments. However, that would
make the aircraft sensitive to
throttle changes, which is
undesirable.
So how does that translate
to a pitching tendency on a
vertical down-line? When the
airplane is flying vertically, the
wings are not generating an
appreciable amount of lift.
This eliminates the nosedown
pitching moment. But
elevator trim is still present,
which creates a nose-up
pitching moment. Mixing a
small amount of downelevator
at idle throttle
essentially removes the upelevator
trim, which is
required for level flight. MA
Nose-up pitching moment on
vertical down-line is caused
by elevator trim.
—Baron Johnson
06sig2_00MSTRPG.QXD 4/22/10 12:17 PM Page 45
pilot must make when developing his or her
setup. There is no right or wrong when it
comes to these decisions; they are based
purely on personal preference, and there are
successful IMAC pilots who use all of these
techniques.
I hope that discussing some of these
decisions will allow you to test various
setup methodologies and form your own
setup, even if you don’t have the
opportunity to regularly talk with successful
IMAC pilots.
Switch Flipping? Using a single condition
(combination of travel, exponential, mixing,
etc.) or flipping switches for various
maneuvers or situations will have a large
impact on most future setup decisions.
The argument for a single condition is
that you always fly a model with the same
response and characteristics. In addition,
you don’t have to worry about switch
positions or performing a maneuver in an
incorrect condition.
The other side of that argument is that
our computer transmitters have incredibly
sophisticated capabilities. Therefore, you
might as well use them to make the aircraft
fly the best and most consistently for
particular types of maneuvers, and not try to
find a one-size-fits-all condition for each.
Both sides of that debate can play into
the two objectives of setup (reduced pilot
workload and consistency). Your decision
might come down to something as simple
as how you hold your transmitter.
If you fly with thumbs only and the rest
of your fingers grasping the transmitter
body, it might be distracting to reach the
switches between maneuvers, much less in
the middle of a maneuver. But if you pinch
the sticks, you might have much easier
access to the switches.
Some common maneuvers that pilots
dedicate conditions to include Snap Rolls,
Spins, Hammerheads, and Rolling Circles.
Hammerhead and Rolling Circle conditions
use a great deal of—if not all—available
rudder travel, while other conditions use
less rudder travel. These amounts vary with
airplane and flier preference.
Some pilots prefer to fly in a Snap Roll
condition most times and then switch before
particular maneuvers. Others prefer to fly in
a condition conducive to Hammerheads,
Rolling Circles, or general flying, and then
change conditions before performing Snap
Rolls.
Dual Rates Vs. Flight Modes: If you have
decided to flip switches in flight, this is the
next issue you must address. Dual (and
triple) rates have been available on many
transmitters for quite sometime. Flight
modes (or conditions) is a newer extension
on the dual rate concept.
Dual rates allow you to change the travel
and exponential (expo) of a specific control
surface with a switch. Flight modes allow
you to put any number of travel and expo
changes on a single switch, along with other
features such as mixes, throttle curves, etc.
Arguments can be made for both
systems. The benefit of using dedicated
dual rate switches is that you can have
many combinations of travels, with each
control surface having two or three
independent rates.
The downside to this is that it mandates
that you keep track of the position of
several switches. Additionally, you might
have to assign additional switches to tasks
such as turning a mix on or off. (More about
that later.)
The benefit of using flight modes is that
you have fewer switches to deal with and
can make all necessary changes with one
action. This can lead to less distraction
while flying and less possibility of being in
the wrong combination of rates.
The downside is that a limited number of
flight modes provides fewer combinations
of rates, compared with having dual or
triple rates on every control surface.
Right Thrust Vs. Throttle-to-Rudder
Mixing: These items compensate for the
yawing effect of “spiral slipstream.” See the
sidebar for a detailed explanation. Both
approaches can create satisfactory results.
By far the most common method among
IMAC pilots is right thrust, or mounting the
engine so that it points slightly to the right.
This is probably because it has been around
for quite awhile (long before computer
radios) and has proven itself to be adequate.
And many models now come from their
manufacturers with approximately correct
right thrust built into the firewalls.
The downside to right thrust is that to
make changes, you usually need to remove
the cowling and add or remove spacers.
The other approach is mounting the
engine straight and creating the required
yaw moment by mixing right rudder with
throttle position. This makes it easier to
mount the power plant and exhaust, and it
minimizes the eyesore of an offset spinner
if the cowling is not shaped for it.
Throttle-to-rudder mix provides greater
flexibility for changing the compensation
to tweak it or to compensate for propeller
changes. Additionally, most high-end
transmitters allow you to do a curve mix, or
nonlinear mix, which can help fine-tune the
compensation throughout the throttle range
(although not throughout the airspeed
range). See sidebar for details.
Some pilots use a third approach: a
combination of right thrust and throttle-torudder
mixing. You can mount the engine
with the manufacturer-specified right thrust
or get it as close as possible and then make
small adjustments with the transmitter.
This methodology captures aspects of both
original approaches but can be timeconsuming.
Mixes: On All the Time? This is a difficult
question that pilots often ask as they add
mixes to their setups. Some fliers turn off
mixing—particularly down-line mixing—
for landing so they don’t get caught offguard
during approach.
In terms of within the sequence, the
question is harder to answer. Some pilots
advocate turning off some or all mixes for
Rolling Circles or Hammerheads, while
many fliers leave all mixes on all the time.
This issue must be addressed on a
model-by-model and mix-by-mix basis.
With numerous airplanes I have had, I
found no reason to turn off mixes for any
maneuver. However, I have had models in
which a mix was detrimental to certain
maneuvers.
I had to fly maneuvers numerous times
in different conditions before I could
conclusively determine a mix as the cause.
I ultimately reduced or eliminated the
mixes in question to improve those
particular maneuvers.
Judging from these experiences, I
recommend planning on leaving mixes on
all the time until you see a reason to do
otherwise. Switching in and out of mixes
adds a considerable workload if it’s
unnecessary.
48 MODEL AVIATION
Down-Line Mixing: Most statically stable
aerobatic aircraft will exhibit a slight noseup
pitching tendency when placed on a
power-off, vertical down-line. See the
sidebar for a detailed explanation.
This is one situation in which most
IMAC pilots use the same setup principle:
down-line mixing. Down-line mixing inputs
a small amount of down-elevator when the
throttle is reduced to idle or near idle
(within a couple of ratchets).
The alternative, which is listed on some
“trim charts,” is to simply move the CG
rearward until the pitch tendency goes
away. This is certainly a way to eliminate
that characteristic, but in doing so you
move the CG to the neutral point, which
eliminates longitudinal static stability.
A neutrally stable airplane will not fly
well for most other precision maneuvers
and will cause more headaches than mere
down-line pitching. It is a small price to pay
for a nice, stable model.
If you’ve never applied a mix such as
this, there are numerous methods to use to
accomplish the same thing with today’s
transmitters. You can use a curve or
nonlinear throttle-to-elevator mix and give
it a value of zero at all points except near
idle. You can also use a traditional throttleto-
elevator mix and employ the throttle
position as the activation switch.
Another technique is to use a
traditional throttle-to-elevator mix and
utilize the offset function to effectively
move the “center” point where the mix
switches from one direction to the other.
Offset it enough so that the mix switches
from one direction to the other at a couple
of ratchets above idle.
Then leave the value activated by high
throttle at zero mix. Use the needed
amount of mix on the other direction that
activates when the throttle is at idle.
This article has been anything but
exhaustive. There are more methods to
cover to address the preceding issues, and
there are countless other issues that can be
resolved with setup. I encourage you to
explore other aspects of setup that you either
learn from others or devise yourself.
I’ve tried to give a jump-start to those
who don’t have access to experienced
IMAC pilots. However, no article can
completely replace talking with other fliers
and having them give you opinions, in
person, about your setup.
If you are interested in competing in
IMAC, find a contest near you and compete.
You will probably learn more and get more
advice—about both setup and flying—
during that short event than you could from
a year of practicing on your own. And you’ll
have a lot more fun! MA
Baron Johnson
[email protected]
Sources:
International Miniature Aerobatic Club
PAT. PENDING www.mini-iac.com