OpenTX - Application Note 16
Pot Control of Aileron Differential
Set up ailerons to have different downward deflection to upward deflection to avoid unwanted yaw. Use pot S1 to control a global variable and experimentally find the best differential value.
When you wish to roll the aircraft, the aileron stick causes one aileron to deflect up and the other to deflect downward. However, when an aileron deflects downward, it causes more drag than when it deflects the same angle up upward. The aircraft not only rolls, but also turns or yaws towards the lowered aileron. To null this effect, the lowered aileron is deflected less than the raised aileron. The amount of difference is called ‘differential’ and it is different for different aircraft. The right amount is found by trial and error, starting with a small amount.
Differential can only be added when the ailerons are controlled by two separate channels. In this example, the left aileron is channel 2 and the right aileron is channel 3.
Both channels will have the same differential value so a global variable GV1 is used to set the differential amount for both channels. While experimenting to determine the best differential setting, pot S1 is used to vary GV1 within safe limits. When the best differential value is found, pot S1 control can be deleted.
We will choose an experimental range of 0 to 30% for the differential.
To control global variable GV1 with pot S1 the following steps are needed.
Scale Pot S1 Range With CH7 and CV3
In the mixer, the spare channel CH7 line uses curve CV3 as follows.
Curve CV3 is as follows.
CH7 now has a range of 0% to 30%.
Use the channel monitor to check that the CH7 value varies from 0 to 30 as pot S1 is turned from minimum to maximum.
Special Function To Make CH7 Control GV1
Special function SF1 continuously copies CH7 value to GV1 as follows.
On the Global Variable screen, enter ‘diff’ as the name of GV1. Also monitor the value of GV1 as pot S1 is turned. It should have a range of 0 to 30 under FM0.
Control Aileron Differential Using GV1
GV1 is placed in the Differential curves of the aileron mixer channels CH2 and CH3 as follows.
*Note that the curve Diff parameter may be GV1 or –GV1 in one of the channels, depending on how the aileron servos move.
This channel monitor screen shows S1 has set CH7 differential value to 30%. With the aileron stick fully to the left, the left aileron is 100% up and the right aileron is only 100 – 30 = 70% down.
This channel monitor screen shows S1 has set CH7 differential value to 20%. With the aileron stick fully to the right, the right aileron is 100% up and the left aileron is only 100 – 20 = 80% down.
Start with pot S1 fully anticlockwise so differential is zero. While flying, quickly pulse the aileron stick to one side so the aircraft rolls to about 10 degrees. Pulse in the other direction to level the wings. Observe how much the tail oscillates sideways and adjust pot S1 to cancel out the oscillation.
When the best differential value is chosen, make a note of its value. The mixer channel CH7 line and the special function SF1 are no longer needed and can be deleted. Ensure GV1 retains its value.