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Sep 05, 2014, 09:08 AM
F3B and F3K
RetoF3X's Avatar
Thread OP

Flaperon Linkage with maximum mechanical advantage

An important topic, as I see quite regularly suboptimal linkages.

I like to make all my linkages with maximum mechanical advantage, that is, using ALL available servo throw to achieve the desired surface deflections.

I got asked how to do this on a flaperon that has asymmetric deflections (i.e. more down than up).

Here is my procedure how to design a flaperon linkage that uses all servo throw:

1) On your transmitter, make sure that you unlock all available servo throw. On Futaba, that means 140% dual rate, max rate on diff and max endpoints and travel. The aileron servo should now move from around plus to minus 50 degrees. See Figure 1.

2) Mount the servo horn at neutral transmitter trim such that it points vertically down

3) Using a balsa wedge, fix the aileron in the maximum upward deflection position. Since this will be max up, be generous (i.e. if you want 15mm up, fix it at 18mm up). See Figure 2.

4) Bring the servo to the end point corresponding maximum aileron up. The arm should now point 50 degree backwards in a bottom drive system, as shown in Figure 3. For a top drive linkage, the arm has to point forward, see Figure 6.

5) Hook up the linkage to wing and servo and glue in the L wires into the carbon rod, while aileron is fixed and servo is in full deflection for aileron up.

7) Once it is well cured, move the servo on the transmitter to neutral. You will notice that the flaps are already partially deflected downwards (see Figure 4). This is as it should be, consider the following example: If you want 15mm flap travel up and 45mm flap travel down, then the geometric middle of the total travel range is 15mm deflected down. This is where the neutral position of the servo on the transmitter has to be.

8) Using subtrim or Trim mix or Flap menu on your transmitter, adjust the flaps in each flight mode to the desired trim. It takes about 30% to bring the flaps to 0 camber. (Figure 5).

If you follow this steps, you get a linkage that indeed uses all travel range the servo has, i.e. one endpoint of the servo travel is full aileron up, the other is full flap down.

On the Snipe, I use a 6mm Servo horn and the flaperon horn is 10.5mm long, thus yielding an excellent mechanical advantage of 1.75 (ratio surface horn to servo horn). This gives me 18mm up and 42mm down using the outlined procedure. Which corresponds to plenty of angular travel given the small chord size of the Snipe flaperons. On a DLG with larger flaps, the values in mm obviously get larger.
The ratio of surface horn to servo horn, for servos with large angular range like the MKS, can be up to 1.5. I recommend a value somewhere in between 1 and 1.5 (the servo horn should be slightly shorter than the surface horn).

A fantastic tool to simulate the deflections while varying offset and horn lengths can be found here:

Last edited by RetoF3X; Jun 05, 2016 at 07:40 AM.
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Sep 16, 2014, 09:17 AM
Registered User
elkski's Avatar
Where do you set the 8fg butterfly settings to start? They go 0-200
And do you want flaps on throttle to go till it hits mechanical limit or do you save some down flap for aileron input?
Sep 18, 2014, 06:52 PM
F3B and F3K
RetoF3X's Avatar
Thread OP
Hi Elski

I normally do not use the butterfly menu to start. I use just full aileron throws (dual rate and endpoint maxed out) to put the servos to the endpoints. Once I set up the butterfly, I end up around 130% on the 8fg.

I do not use more flap down for aileron when the brakes are fully deployed. Upon aileron input, it only moves up.

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Oct 03, 2014, 12:38 PM
Time for me to Fly...
Mr. Wiz's Avatar
That's interesting, Reto. I'm just mounting some new servos in a Snipe now and I'm going to give this a try.

Thanks for taking the time to post it!

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Mar 27, 2015, 04:46 PM
Charles Martin
BavarianCharles's Avatar
Bump - Reto's method works really well!

Sep 22, 2015, 07:30 PM
Registered User
Hi guys,

Sorry to bring this thread back up. I am a new Taranis user and wondered if maxing out my travel adjust and dual rate was safe on this tx as i read that 100% travel adjust on all other tx's was only 80% on the Taranis. If i max out the travel and DR will i over drive and damage my kst 08 servos?


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