Trimming Aerobatic Aircraft

Trimming your Aerobatic Model
When we speak of “trimming” your aerobatic model we’re not talking about trimming in the traditional sense of adjusting
the control surfaces to maintain level flight. In this case, we will focus on the small changes to the rigging and control
setup that allow the model to fly as precisely as possible. The general trimming steps include:
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Setting CG
Adjusting thrust line
Adding Aileron differential
Sideslip (Knife-edge) Coupling
The first two on the above list, CG and thrust line, are completed by making physical changes to the rigging. The final
two steps, aileron differential and sideslip mixes, require changes to the model setup in your computer radio. We chose
to highlight the programming procedures for two of the most common and popular mid-high end computer radios, the
Futaba 9CAP and JR 9303. I’ll begin each section with a brief overview of the trim step, followed by specific
programming techniques for each radio individually. If you find the need for additional technical descriptions of any of the
trimming steps, be sure to have your May '07 Fly RC on hand as a reference.
I’ve chosen the new 3D Hobby Shop 47-inch Yak 54 as our example airframe. The Yak is a great flying electric
3D/precision aerobatic park flyer that offers unlimited performance with a 300+ watt brushless outrunner motor. It makes
a great example airframe due to its slight mixing requirements - which are quite similar to those of most precision
aerobatic or 3D models, allowing us to outline the process in a completely realistic and applicable manner.
With that in mind, I have two separate but identically equipped Yaks that we’ll use. One has an FMA M5 receiver that I’ll
match to my Futaba 9C, while the other uses the JR R700 receiver and JR 9303.
Important Safety Note!
Many of these steps will require you to power both the transmitter and airplane to confirm the correct direction of
movement of the controls. There is a high probability that you’ll inadvertently bump the throttle during the programming,
possibly causing an inadvertent motor start, which can be very dangerous! The safest way to do this is with a receiver
pack that bypasses the electronic speed control if you are setting up an electric airplane. A secondary option would be to
remove the propeller from the model during setup and testing.
Aileron Differential
If you think back to the trimming process outlined in the companion print article, by the time you’ve reached the need for
aileron differential you already set both the CG and the thrust line of your model. As a refresher on aileron differential
I’ve included an excerpt from the print article below:
“With the CG and thrust lines set, the next step is to make your model roll axially as if on a string by adjusting aileron
differential. Barring any manufacturer specific direction, I generally recommend symmetrical throws as a good starting
point. Some models, however, just won’t roll axially with symmetrical throws, and aileron differential is a great solution.
I find the best way to test for axial roll characteristics is on vertical down-lines. With the throttle closed, establish a
down-line and begin a constant roll. Note any barreling of the roll. Perfectly trimmed, the model should maintain a
straight vertical down-line while rolling axially as if on a taught string. To correct for a barreled roll, start by reducing
the throw of the down aileron for that roll direction incrementally. I find that small changes in differential - often just a
couple of degrees, are enough. Don’t forget to also test the roll in the opposite rolling direction, as it isn’t uncommon
for the two rolling directions to be slightly different.
Note, I recommend testing for axial rolls only on vertical downlines - not on level lines. The reason for this is that on
vertical down lines there is no “up” or “down.” Any non-axial rolling tendencies noticed on the vertical line relate only to
the rolling characteristics of the model, not the effects of gravity. Also, unlike upright level flight where the wing is at a
slightly positive or negative AOA, vertical lines are close to zero AOA. At zero AOA, we can truly assess whether the
model needs differential to roll axially.”
So the burning question – how do we modify the aileron throws to allow differential? Different radio manufacturers give
you different tools to accomplish this, so we’ll look at two of the more common techniques available.
Before we do, however, I’d like to show you a really cool tool we’ll use to precisely measure control throw – the throw
meter.
Several different versions are available, but the AeroWorks throw meter is one of my favorites. It uses a large clothespin
as its attachment device which may be unacceptable for smaller models, but is ideal for the larger models I prefer.
We’ll use the throw meter for measuring aileron differential, but it is also a great tool for setting dual rates and measuring
control throw in general.
If an airplane displays the need for aileron differential, the most common corrective change is a reduction of the “down’
aileron, i.e. reducing the throw of the throw on each aileron as it travels below the wing when viewed from above the
model. Once you establish the need for differential to make your model roll axially, you need to decide which of the
following two techniques will work with your transmitter setup – electronic differential or end point adjustment. The JR
9303 allows differential while using dual aileron servos as flaperons, so that’s the first technique I’ll cover. The Futaba
9CAP inhibits the differential function during flaperon use, so we’ll focus the end point technique for that radio.
JR 9303 – Aileron Differential Function
The first step in setting up differential with the 9303 is to ensure that you are using the Flaperon mode to control the two
individual aileron servos. Enter the system menu by holding the ENT button as you turn on the radio.
Select the Wing Type menu using the scroll wheel and ENT button.
Scroll to the Wing: Icon and select Flaperon. With the radio now set to
Flaperon mode, each aileron servo uses its individual slot in the receiver
and the differential function is available to individually set the throw of
each aileron.
Exit the System menu by turning the radio off and back on again. Enter the Function menu by pushing the List button.
Scroll to and select the Aileron Differential function.
Notice the two separate “Position” settings – Pos 0 and Pos 1. These relate to the position of the dual rate switches, one
setting for each of the two dual rate positions. Initially the two settings will both indicate a linear mix, i.e. the same travel
up and down. We need to change that to reduce the total travel of the aileron that moves below the wing. Selecting the
first position (Pos 0) with the radio powered and the ailerons fully deflected, change the setting in either direction and
note which aileron moves and in which direction. If the up aileron moves, roll the setting the other direction until the
down aileron begins to reduce its throw towards neutral. I generally start with around 2-3% of differential and adjust as
necessary to fine tune the setting.
In this instance after testing and resetting, I ended up with a differential of 7%, meaning that the aileron travels 7%
further up than down. Also be sure to set both the Pos 0 and Pos 1 settings to the final mix. The photos below illustrate
the magnitude of aileron throw reduction.
Neural aileron with the surface centered and the tail propped up to show zero degrees of throw on the throw meter
30 degrees of throw up aileron.
Approximately 28 degrees of down aileron throw.
With this setting, the Yak rolls perfectly axially on both up and down lines.
Futaba 9CAP – End Point Aileron Differential
As I mentioned earlier, the aileron differential function on the Futaba 9CAP is inhibited when using dual aileron servos in
the flaperon model, which is the mode I recommend for most aerobatic models. The aileron differential function on the
Futaba 9CAP is designed to be used with multi servo sailplane wings. The solution is to set each aileron servos end
point individually. The first step in that sequence is to ensure flaperons are selected. Enter the programming mode by
pushing and holding the Mode/Page button. The screen will change to the Basic/Acro menu.
Pushing the Mode/Page button a second time puts the radio into the Advance/Acro function menu, which is where we’ll
activate the flaperon function, allowing each aileron to be controlled independently.
Highlight and select the Flaperon function.
The flaperon is inhibited initially, so you’ll need to activate it along the top left side of that page. Next, ensure all aileron
rates are set to 100% as shown in the picture above.
With flaperons activated, each aileron servo can be controlled independently by adjusting the end point of each
individual servo direction. Push the END button to return to the Advanced/Acro menu. Push the Model/Page button to
return to the Basic/Acro menu. Highlight and select the End Point Function.
Select the aileron channel. With the radio powered, input full aileron throw and reduce the end point watching which
aileron moves. If the up aileron moves, return its end point to the previous setting and attempt it in the opposite direction.
You will want to reduce the down aileron throw by approximately 2-3% initially. This time I ended up with an 8%
reduction.
Now for the confusing part – all you’ve adjusted to this point is the down aileron throw of one aileron. You need to repeat
this process for the other aileron. Using the Futaba 9CAP, I chose channel 5 for the second aileron servo, so I had test
and set the end point for that servo as well. Note in the photo below that Channel 5 is selected, and although it shows
that channel as the Gear, it is the second aileron servo. Depending upon your particular setup, the second aileron servo
could be assigned any of channels 5, 6 or 7.
Sideslip (KE) Coupling
Most models require some mixing for perfectly clean knife-edge flight and the modern computer radio makes this step as
easy as it can be. There are several different methods available to test for the need for sideslip (KE) coupling mixes
including knife-edge flight and level flight rudder usage. If coupling is apparent in one, it is generally apparent in both.
Again, let's look to the print article to more completely describe that process:
“Sideslip coupling, more commonly known as knife-edge coupling, is the last trim adjustment I make. Some pilots start
making this adjustment right away, but I find both CG and thrust line have an effect on sideslip coupling, so setting
those first, then coming back to sideslip mixing makes the most sense. Any adjustments to the sideslip coupling on
your model are done through mixing functions on your transmitter. In reality, all we are doing is mixing in a certain
amount of elevator or aileron with rudder deflection to compensate for sideslip coupling.
Before we get too far into the discussion I think it’s important to clarify a misnomer – specifically that knife-edge
coupling only occurs during knife-edge flight. In reality, if a model displays knife-edge coupling, the same coupling
exists anytime the model is flown in a sideslip condition. So what’s a sideslip condition? Uncoordinated flight would be
the technical answer to that question, but in layman’s terms, sideslip flight occurs virtually every time you deflect the
rudder in your model with the lone exception of coordinated turns. Next time you’re at the field, take a model with
known knife-edge coupling issues and aggressively deflect the rudder while in level flight. If your model pitches to the
landing gear and rolls with the rudder in knife-edge flight, it will also pitch to the gear and roll with the rudder from level
flight with rudder usage.
The easiest test to determine the need for a sideslip mix is to simply apply rudder from level flight. If the model pitches
or rolls you’ll need to apply a mix with the rudder channel as the master, and the correction as the slave. Most
aerobatic models tend to pitch to the landing gear and roll one direction or the other. I generally start by mixing out the
pitch coupling first, then focus on roll coupling. As a secondary test, fly knife-edge lines both directions and fine tune
your previous adjustments. While I’ll discuss the technical aspects of setting sideslip mixes in a future article, its
important to note that you should never try to make any adjustments to your computer radio while flying – don’t ask, its
been attempted before with less than stellar results. I’ve found better luck drafting a flying buddy for scribe duties
during each flight to accurately record impressions, making adjustments after each flight. In most cases, I can
accurately tune sideslip mixes in less than 10 flights.
One item to note, sideslip coupling is rarely identical from left to right. With one wing high in knife-edge flight, the
model may pitch to the landing gear slightly, and with the other wing high may pitch to the canopy. Simply adjust your
mixes to match your particular model’s needs. Finally, I recommend you set up your sideslip mixes to be active during
all phases of flight. For precision aerobatics, there are very few instances in which the mix would hamper the precision
of your flight, as sideslip coupling applies any time the rudder is used.”
So let’s apply that knowledge towards programming both the JR9303 and Futaba 9CAP. Both radios use programmable
mixes that allow virtually any channel to be mixed with any other channel at either a specified linear ratio or along a
multi-point curve (which allows adding an exponential component to the mix.) Applied to KE coupling, our goal is to mix
small percentages of elevator and aileron with rudder to compensate for the coupling traits of each model. In the case of
the 3D Hobby Shop Yak 54, in perfect knife-edge flight, the Yak pitches slightly to its belly and rolls slightly into the
rudder (left rudder gives left roll.) In a perfect world, we want the model to maintain a perfectly straight KE line with no
additional input from the pilot except rudder to maintain altitude. To achieve this with the Yak, we need to mix in up
elevator to correct for a pitch to the landing gear, and aileron opposite the rudder direction (left rudder/right aileron, and
so on.) Let’s start with the 9303.
Rudder/Elevator Mixing – JR 9303
The 9303 has a total of six programmable mixes in addition to several preset mixes. The first two programmable mixes,
or P-mixes, allow you to adjust a variable rate mix over a total of seven programmable points, which is great because it
allows a slightly non-linear mix if necessary. The remaining four P-mixes are all single ratio mixes that allow linear
percentage based mixing. I prefer to use the first two P-mixes for knife-edge coupling mixes as they allow me more
flexibility if a non-linear mix becomes necessary.
Most computer radios mixing programs use a master/slave protocol, where you first set the “master” control, and then
adjust a percentage of “slave” control that will be mixed in with the master control. In our case, we want the master to be
the rudder, and we want to mix in a percentage of up elevator as the slave to overcome a slight pitch to the landing gear
in knife-edge or sideslip flight. The first step is to enter the Function menu by pressing the LST button with the
transmitter turned on.
Using the scroll wheel highlight and select Prog. Mix 1
The mix will be inhibited initially, and you’ll need to activate it by placing the cursor over the INH icon and selecting it.
With the mix active, the following screen allows you to make modifications to the mix, including master channel, slave
channel, and mixing percentages.
Note the master channel is the first channel listed, and the slave is the second channel listed – both are preset to throttle
by the mixing logic. I recommend you start by setting the correct master and slave channels, which in our case the
master is the rudder, and the slave is the elevator. Then, zero out all of the mix ratios at each programmable point (i.e.
Point- 0, Point- 1, and so on). When you’re done, the page should look like this.
If you choose to switch your mixes on and off, now would be a good time to do so. There is a switch select icon on the
left side of the screen for this purpose. I, however, recommend leaving the mix on at all times. There are very few
instances where this will present even the slightest problem, so the simplicity allowed by eliminating the need for any
additional switching just makes sense.
With the base mix menu set, it is time to adjust the values. In general, most small models require less than 10% mixing
in either the pitch or roll sense. I would recommend starting with a small mix of 2-3%, followed by a test flight and further
adjustments. The final numbers on the Yak were slightly higher than my initial estimate, so after test flying several times,
I ended up with a rudder/elevator mix screen that looks like this.
Looking at that screen, you’ll notice the two mixes aren’t exactly the same. One direction has slightly more mix
percentage than the other, and this is completely normal. It is rare that models need the same mix with rudder in both
directions. Also note that the mix is basically linear, i.e. at full deflection, the mix point is 8, then at 2/3 deflection its 5,
and at 1/3 deflection it is 3 – basically a straight line on the graph. The beauty of the 9303 is that it allows me to precisely
fine-tune the mix to match my airplane perfectly with its multi-point mix curve.
Rudder/Aileron Mixing – JR 9303
I generally use the second P-mix for rudder/aileron mixing and use the exact same technique for rudder/aileron mixing
as I did for rudder/elevator. Start by activating the mix, setting the master and slave channels, and zeroing the mix
values. Your screen should look like this.
On the Yak, I ended up with what I would describe as a 5% linear mix.
Notice that the mix graph looks slightly different than the rudder/elevator graph. For the rudder/elevator mix, we were
using up elevator for both directions of rudder deflections. With the rudder/aileron mix we use aileron opposite the rudder
direction, thus the graph of the rudder/aileron mix shows a line that extends above the baseline on the left, and below
the baseline on the right. This indicates different directions of travel on each side of neutral rudder. Be sure when you’re
setting the model up to check the actual direction of the mix by moving the flight controls and visually confirming the mix
is moving the slave channel correctly.
Rudder/Elevator Mixing - Futaba 9CAP
The Futaba 9CAP uses P-mixes that are similar to the 9303, with a total of seven Prog Mixes (P-mixes). The first five
are a basic simple linear ratio mix, while mixes 6 and 7 each have a 5-point curve similar to the 9303. The procedure to
use the multi-point mix is virtually identical to that used in the 9303 detailed above, so let’s take a look at applying one of
the basic percentage mixes to compensate for coupling in the Yak.
With the transmitter on, enter the Basic/Acro programming mode by holding the Mode/Page button.
The Prog Mixes are located on the Advanced/Acro function page, which is reached by pressing the Mode/Page button a
second time.
Remember, Prog Mixes 1-5 are a basic percentage mix and lack the ability to fine tune the mix through a multi-point mix.
If you anticipate the need for the multi-point mix, I’d recommend using Prog Mixes 6 or 7. In my case, the Yak is
perfectly happy with a basic linear mix, so I chose Prog Mix 1 for the rudder/elevator KE mix. Selecting P-mix 1 brings up
the following page.
Notice that the Preset mix on Prog Mix 1 is inhibited and shows the incorrect master and slave. Both will need to be
adjusted. Using the Select Cursor down button will move the cursor to the second page of Prog mix 1 until you come to
this screen.
On this screen, you can activate the mix, change the master and slave channels, and add a switch selection to turn the
mix on or off if desired. Again, I leave all of my mixes on, so I simply set the switch position to null, which means it
disregards switch position and the mix is on constantly. Here is how the screen should look after setting the Rudder to
the master channel, the Elevator as the slave channel, and turning the mix on.
With the second page of the mix complete, use the cursor button to return to page one of the Prog Mix 1 function to set
the precise values for the mix. In the case of the Yak, I settled on a 6% mix one direction and an 8% mix in the other.
Like before, you’ll want the airplane powered and close-by to confirm that the mix is working correctly. In our case, we
want up elevator mixed with rudder movement in both directions.
Rudder/Aileron Mixing – Futaba 9CAP
The procedure for mixing rudder/aileron with the Futaba 9CAP is nearly identical to mixing rudder/elevator described
above. Knowing that my KE coupling needs are very linear, I again chose to use a simple percentage Prog Mix. I chose
Prog Mix 2. Start by opening up your preferred Prog Mix, setting the master and slave channels, and activating the mix.
One additional step is necessary to ensure both ailerons respond to the mix, and that is to turn the “Link” on. The link
function is used to link both aileron servos to the mix. With the link off, the radio will only mix the primary servo plugged
into the aileron channel (channel 1). With the link activated, both aileron servos will be used when mixing aileron as a
slave. With that complete, your screen should look like this.
Returning to page 1 of Prog Mix 2, set the desired values. Again, with the model powered and close-by, confirm the
correct direction of control movement. In our case, we want aileron deflection opposite the rudder deflection.
Conclusion
From my May 2007 Fly RC article, “I’ve heard many competition pilots describe the trimming process as a never-ending
circle. Each additional change you make can force modifications to previous changes. I find, however, that most sport
aerobatic pilots require only one trip through the steps outlined above to dramatically improve their models precision
aerobatic flying traits. By following the basic order of adjustments outlined above, you will be making the model fly more
precisely on its own.
This means less to think about, allowing you to focus on flying each figure as precisely as possible. Some look down on
electronic mixing. Many others widely accept mixing as reasonable and necessary to achieve the best possible flying
model. I find it to be very helpful and from my perspective, any adjustments you can make to help your model fly precise
lines, roll axially, and maintain clean knife-edge passes is time well spent.”
Setting up your model for precision aerobatics is time well spent. We hope that by combining the information found in
this article with the May 2007 print article you’ll soon be flying more precisely than ever. Till next time, remember,
aerobatics make the world go ‘round.