Aileron-Rudder Mixing Explained

Aileron-Rudder Mixing Explained
Aileron-Rudder Mixing Explained
“Building Good Habits for a Better Future”
By Dave Scott. Instructor, 1st U.S. R/C Flight School
Illustrations by Dave Scott
Drag
Adverse Yaw
GHT SCH
. R/C FLI
1ST U.S
OOL
1ST U.S. R/C FLIGHT SCHOOL
The following article details the use of
programmable Aileron-Rudder mixing
to eliminate “adverse yaw”.
While
primary aimed at eliminating adverse
yaw on a primary trainer, the
descriptions and explanations should
help cement your grasp of a subject
that few in the sport understand, even
through all pilots experience the effects
of adverse yaw every time they fly.
Introduction
Nearly 100% of computer radios
today feature Aileron-Rudder (A/R)
mixing to reduce adverse yaw, i.e., the
inherent opposite yaw or skid that is
especially pronounced during aileron
deflections on flat-bottomed wing
aircraft, such as those used for primary
flight training.
As the name implies, adverse yaw is an
adverse or unfavorable condition that,
among other things, inhibits progress.
Traditionally, struggling and
committing to many hours of practice
before soling has been the assumed
normal burden of the novice pilot until
his or her skills improve. However,
they have unknowingly been fighting
the additional challenge of flying with
adverse yaw as well. Indeed, pilots
have always assumed that the lack of
correlation between their control
inputs/intentions and the response of
the plane to be strictly the need for more
practice, and sometimes wind, when in
fact adverse yaw has been a big reason!
It’s probably safe to say that most of the
people reading this learned to fly by
trial-and-error or at the side of a
recreational flyer/instructor with little
pre-flight preparation. As a result,
most pilots learn to fly by “reacting" to
what the plane does. Consequently,
many pilots naturally think that
getting better at making corrections,
good reflexes, and more stick-time are
the keys to becoming a better flyer.
Thus, rarely does adverse yaw or the
advantages of A/R mixing when
learning to fly ever come up. However,
if you where to objectively compare the
results achieved training with A/R
mixing versus without, you would
discover an immediate improvement in
consistency and therefore the rate of
learning. In fact, as you will soon see,
learning to fly with A/R mixing can be
credited for helping to instill proper
control habits that actually accelerate
future progress.
Adverse Yaw
Space does not permit going into all
the aerodynamics involved during
aileron deflections, so put simply;
adverse yaw is caused by the wing with
the down aileron generating more lift
and therefore more drag than the wing
with the raised aileron (figure 1). The
drag differential causes the airplane to
yaw/skid in the opposite direction that
the ailerons are applied while banking
into turns, making course corrections,
exiting turns, etc.. Pilots therefore have
to hold in the aileron longer to overcome
the adverse skid, thus increasing the
potential for over-controlling, as well as
deal with a lack of consistency caused
by the out-of-sync relationship between
their control inputs and the response of
the plane.
Dave Scott is a champion full-scale
aerobatic competitor and airshow pilot. He
founded 1st U.S. R/C Flight School and has
professionally training more than 1500 R/C
pilots of all skill levels. His groundbreaking
training manuals and articles feature the
accelerated teaching techniques developed
during his 14,000 hours of instruction and
experience setting up and test flying over
More
1000 airplanes at his school.
information about his books and school can
be found at www.rcflightschool.com
A dverse yaw is most pronounced on
high lift flat-bottom wing aircraft and
gets worse at slower airspeeds and/or
when making larger aileron inputs.
(Adverse yaw is so severe on a scale
Piper Cub for example, that when flown
near stall speed it will actually turn left
when right aileron is applied, and viceversa.) Also, since the principle effect
of wind is exaggerating deviations that
would otherwise be minor on calmer
days, adverse yaw creates a whole slew
of problems when trying to fly a trainer
in windy conditions.
Some common approaches to reduce
the effects of adverse yaw in R/C have
been: Flying at higher speeds; making
the trainer less stable and more
maneuverable by lessening wing
dihedral; differential aileron travel
(more up aileron travel than down);
avoiding wind; accepting it as how
trainers fly; and continued reassurance
from club members that the student will
eventually get it with more practice -- all
of which only help to small and varying
degrees.
Aileron-Rudder (A/R) Mixing
The logical solution to counter
adverse yaw is with the surface that
controls yaw, i.e., the rudder (figure 2).
Coordinated rudder deflections along
with and in the same direction as the
ailerons prevent the plane from
skidding in the opposite direction while
banking into and out of turns, making
course corrections, rolling, etc.. Most
importantly, with adverse yaw
eliminated, the airplane response more
closely matches the inputs and
intentions of the pilot!
Right Bank
Axial
Original flight path
Adverse yaw: An inherent
opposite yaw or skid during
aileron deflections that is
especially pronounced on
flat-bottom wing airplanes.
A coordinated rudder deflection in the same direction
as the aileron prevents the nose from yawing in the
opposite direction during aileron deflections, thereby
keeping banks and rolls axial.
Coordinated
Right Bank
Increased drag on
the lifting wing drags
the wing rearward,
causing the nose to
yaw left as the plane
is banking right.
Rudder deflecting in the
same direction with the
ailerons prevents the nose
from skidding to the left.
Lif
t
Be clear, the function of the rudder here
is not to turn the airplane. Rather, the
purpose of the rudder is strictly to
prevent adverse yaw in order to achieve
a precise “axial” bank and roll
response.
1st U.S. R/C Flight School trains its
students on planes setup to
automatically coordinate the rudder
with the aileron through the A/R
mixing function in the radio. Radio
manufacturers have in fact been
providing A/R mixing for the purpose of
countering adverse yaw since the
1980’s, but since most people are
inclined to keep passing down the way
they were taught, it is still not widely
used or even understood in R/C.
Those who learn to fly an honest trainer
set up to more accurately reflect the
control inputs they make are obviously
going to learn proper control earlier.
However, as a bonus, A/R mixing also
expands the aerobatic capabilities of a
primary trainer airplane by helping
aileron rolls remain axial and on
heading throughout. Furthermore, the
improved control achieved with A/R
mixing permits flying in winds that
would normally ground most trainers.
E.g., The main challenge of flying in
wind is that it tends to exaggerate
deviations, however, the positive
control achieved with A/R mixing
makes it possible to more precisely and
promptly correct deviations before the
wind has a chance to exploit them.
Thus, even experienced sport flyers
have good reasons to utilize this setup
on their flat-bottom wing planes.
Of course, when A/R mixing is being
used, pilots still have independent
rudder control on the left stick for lefthand ground steering and maneuvers
requiring independent rudder. In fact,
learning to use independent rudder on
the left stick proves easier after
learning to fly with A/R mixing because
much of the right stick control will have
become routine or automatic thanks to
the consistency achieved with the mix.
Aileron-Rudder Mixing
Setup Rules-of-Thumb
Upon activating A/R mixing, you
need to confirm that the rudder moves
in the same direction as the aileron
(rudder moves toward the up aileron).
The rule-of-thumb on a flat-bottom
wing airplane is to adjust the A/R
mixing percentage so that the degree of
rudder deflection matches the degree of
aileron deflection 1-to-1 (figure 3). At
1st U.S. R/C Flight School we simply
gauge the degree (angle) of aileron
deflection visually, and visually match
an equal degree (angle) of rudder. If for
some reason we are unable to set a
1-to-1 relationship, we’ll get it as close
as we can, knowing from experience
that a few degrees more or less is not
going to make any appreciable
difference. We then check the setup by
flying the airplane directly at or away
from us while banking left and right to
confirm that the banks are axial and
the fuselage stays pointed in the same
direction throughout.
Of course, i f you are hesitant to use A/R
mixing, you can always start with less,
and then keep increasing it until the
bank and roll response is finally axial.
Although, you can be confident that
after applying the 1-to-1 rule-of-thumb
to a flat-bottom wing airplane adverse
yaw will be virtually undetectable:
Banks, corrections, and rolls will be
smooth and axial, and you will feel
more connected to the plane when you
fly. By comparison, adverse yaw is
minimal during aileron deflections on
fully- symmetrical wing airplanes
(except during slow flight), and
therefore fully- symmetrical wing
airplanes require little or no A/R mix .
That means that a semi-symmetrical
wing (in-between flat-bottom and fullysymmetrical) requires approx. half as
much rudder deflection as aileron to
eliminate adverse yaw.
Differential Aileron
Full deflection
Rear view
Fin
top
view
1-to-1 equal degree aileron-rudder
deflection
On a flat-bottom wing airplane,
adjust the A/R mix percentage so
that the degree of rudder
deflection matches the aileron
deflection 1-to-1. On a semisymmetrical wing plane, setup
the A/R mix so that the rudder
deflects half as far as the
ailerons. Using A/R mixing on a
fully-symmetrical wing plane is
optional, and typically no more
than a degree or two when
activated.
A/R Mixing for the Future
Many new flyers eventually go on to
enjoying the “flying on rails” handling
and increased capabilities of
symmetrical wing aerobatic models.
Semi-symmetrical
2-to-1 aileron-rudder deflection
Fully-symmetrical
Minimal aileron-rudder mix
Axial
Symmetrical
If your airplane utilizes 2 aileron
servos, you can program a small
amount of differential aileron travel
(more up aileron deflection than down)
to help further reduce the chances of
adverse yaw occurring, particularly at
slower airspeeds.
W hile differential aileron travel is a
common practice used to reduce
adverse yaw, its effect is slight, and the
only way to fully eliminate adverse yaw
is with simultaneous rudder. Note that
if you did attempt to reduce adverse
exclusively with differential, you would
end up with so much up aileron travel
that the airplane would be unduly
prone to dropping at the start of turns
and rolls. Thus, a little differential is
good, just don’t get carried away.
Right wing
Axial
Flat-bottom
Once again, symmetrical wing
airplanes require little or no A/R
mixing because adverse yaw is
negligible with this type (until slowed).
Those who learn to fly a flat-bottom
wing trainer with A/R mixing will
actually find the transition into
symmetrical wing models easier than
most.
That’s because the control
habits learned flying an A/R mixed
basic trainer are the same techniques
used to fly symmetrical wing airplanes,
since in both instances pilots are flying
Symmetrical wing airplanes exhibit
minimal adverse yaw and remain almost
perfectly axial while banking and rolling
(except when the airspeed is low), and thus
there is little or no need for A/R mixing with
this type.
Flat-bottom wing airplanes exhibit
significant adverse yaw and thus require
significant A/R mixing to achieve an axial
bank/roll response.
In short, learning to fly a flat-bottom wing
trainer utilizing A/R mixing leads to learning
the same control habits used to fly a
symmetrical wing aircraft, thus making the
transition easier because pilots are flying
without adverse yaw in both cases.
without adverse yaw and maintaining a
direct correlation between their inputs
and the response of the plane (figure 4).
Conversely, those who learn to fly with
adverse yaw (un-mixed) will have to retrain their habits when flying an
aerobatic model without much adverse
yaw. Thus , the aim of A/R mixing a
primary trainer is not only to facilitate
learning to fly, but ultimately to
compliment the transition into higher
performance airplanes that require
little or no A/R mixing.
It’s important to note that if you’re
inclined at some point to switch off the
A/R mixing on a flat-bottom wing
airplane, expect to need a lot more
control inputs to overcome the sloppier
responses (something that you do not
want to make a habit of if you also plan
to fly less encumbered aerobatic
models).
Of course, you could
physically coordinate the aileron and
rudder control sticks using 1-to-1
movements to eliminate adverse yaw,
but remember that technique only
applies to flat-bottom wing airplanes
and would not be appropriate when
flying symmetrical wing airplanes.
Conclusion
As stated, m aintaining a direct
correlation between control inputs and
the response of the plane is
instrumental to developing optimum
control habits.
Consider that when
the initial control inputs are applied
correctly, the need for additional
corrections may not even exist. That’s
when a pilot becomes free to think
ahead of the airplane and more
efficiently take on new challenges.
Thus, by r emoving the obstacle of
adverse yaw, A/R mixing proves to be
one of the most effective tools to ensure
that pilots learn proper control from the
start and therefore continue to enjoy
steady advancement and a more
successful future . Happy flying!
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