Tutorials : Setting up Spoiler / Elevator Compensation

Mike Shellim   31 December 2001

Many gliders experience a change in pitch trim when spoilers are applied, and most advanced computer radios provide a way to compensate for this by mixing in an elevator correction ('compensation').

 This article shows two ways of doing this on the mc4000. The first method uses a built-in mixer and is simple to set up. The second method is more complex to set up, but allows very easy adjustment on the field when the compensation employs a multi-point curve.

Both methods support simultaneous use of the COMBI mixer.

Solution 1: Using VTAIL+ mixer

The first method uses the VTAIL+ built-in mixer.

VTAIL+ Mixer Overview
Elevator --> |             |
Rudder  -->  |   VTAIL +   |--> V-tail 1
Spoiler -->  |   mixer     |--> V-tail 2
             |             |

Mixer set up

Input Control/Mixer Switch  Curve        Remarks
1 Elevator ON -100/0/100 Adjust movement using Control Travel screen, for Elevator
2 Rudder ON -100/0/100 Adjust movement using Control Travel screen, for Rudder.
3 Spoiler ON x/y/z Set x,y,z to required elevator compensation.

The two Vtail servos are assigned to this mixer. Each servo will 'see' its own set of curves for Elevator, Rudder and Spoiler.

To change the elevator trim compensation, it is necessary to adjust and balance two Spoiler curves (one for each servo).

Solution 2: Using servomix and multimix

Solution 1 uses two curves which must be matched. Making adjustments is easy if you're using 3-point curves, but it becomes awkward if the curves employs more points. 

The second method is more complex to set up, but is easier to adjust on the field since adjustment involves one curve only.

Mixer Setup
                                |              |
Rudder -----------------------> |  "LEFT_VEE"  |
Elevator----------------------> |  Servomix    | --> Left V-tail
            |-------------|     |              |     Servo
            |             |---> |              |
Spoiler --> |"COMPENSATE" |     |--------------|
            | Multimix    |
            |             |     |--------------|
            |             |---> |              | 
            |-------------|     |  "RIGHT_V"   | --> Right V-tail
                                |  Servomix    |     Servo
Rudder -----------------------> |              |
Elevator----------------------> |              |

The COMPENSATE multimix has a single Spoiler input. The relationship between Spoiler movement and elevator compensation is set up in this mixer. 

The  LEFT_V and RIGHT_V servomixs add the Rudder, Elevator and Compensate values and send them to the V-tail servos.

Multimix set up

"COMPENSATE" multimix
Input  Control/Mixer Switch  Curve      Remarks
1 Spoiler ON a/b/c/d/e Use the curve to generate the elevator compensation signal. It can be non-linear as required.
  • The curve can be 3, 5, 7, 9 or 13 point as desired. 
  • Changing the compensation is easy as only a single curve needs to be changed.

Servomix set up

LEFT_V & RIGHT_V servomixes
Input  Control/Mixer Switch  Curve        Remarks
1 Rudder ON -100/0/100  To reduce movement, use the Control Adjust -> Travel screen.
2 Elevator ON -100/0/100 To reduce movement, use the Control Adjust -> Travel screen.
3 COMPENSATE ON -100/0/100 To alter the elevator compensation, adjust the curve in the COMPENSATE mixer.

Vtail servos are assigned LEFT_V and RIGHT_V mixers. Each shares similar setup above.

-100 / 0 / 100 is a "do-nothing" curve. It may be necessary to adjust the exact end points to balance the servos (it may also be necessary to reverse the curve (i..e change it to +100/0/-100)).