Tutorials : Schemp-Hirth Airbrake with Auto-locking and Servo Relief

John McNamara  3 Feb 2002

For scale glider guiders: a macro to ensure that airbrakes are properly closed and locked without the worry of the servo being stalled.

You know the problem with sliding lever airbrakes ("Schemp-Hirth" because this company originally designed them and fitted them to the Minimoa). Because there is some slop in the linkages of all the commercially made units - and I suspect in all home made units too - it is necessary to make the servo travel further to take up the slack. If there is even the slightest "stickiness" in the linkage (There will be, certainly after some use!), we end up with either the servo stalled or the airbrakes not properly locked. You pull some high alpha or high speed manoeuvre and out one pops!

With the P4000 we can drive the brake fully home stalling the servo, wait a second, and then slowly bring the servo back from the stalled position.

Setting up

Menu/screen Task
Assign controls - K Aux*
Assign servos - Spoiler Servomix
Assign Mixers - Spoiler Input 1 = Spoiler (No trim)
Input 2 = Aux*
Input 3 = [See note 1]
Assign switches - Control switch 4 (B or D depending on mode) 1 point = max 93%
Servo Travel/Curve - Spoiler Servo Input 1/ Spoiler - 13 point curve (see note 2 later)
Input 2/ Aux* - set centre point to +/-10% and input switch to C4-D (Mode 1) or C4-B (Mode 2). With control stick in fully closed position control switch is on.
Servo Slow - Spoiler Servo Input 2/Aux* = 500.0 secs, 625.0 secs.

What happens is that when the spoiler control is closed beyond 93% of its travel control switch C4 toggles on. This now switches in Aux* input to +/-10% centre. Whether + or - depends on the value of servo travel to close the brakes. If this is a positive value then the Aux centre should be a negative value, and vice versa. However, since this input is on a slow in this travel direction, the servo has time to reach its full travel position and stay there whilst the slop in the linkage catches up (very scientific explanation!) before the servo slowly creeps back to unload.

NB1: The 3rd mixer input can be used for Elevator. On many modern full size gliders the tailplane volume coefficient (TVC) is only just big enough. At scale model sizes Reynolds number sometimes means that the TVC is insufficient for the model to be stable. In order to fly such a model we need to have a very forward CG position (18% on my DG300) and a tailplane continually loaded and suffering trim drag. This arrangement works but elevator control is very numb especially at flare out. If airbrakes are fully deployed at the flare the resulting event is known commonly as a crash! To ensure that you still have enough control at the flare in these circumstances where you must spoiler off until touchdown with such a model, elevator input into our spoiler mixer allows elevator to close the airbrakes.

NB2: The values that I have suggested are just a starting point and some experimentation may be necessary for individual models.

NB3: A further development of the system is to add a multi point curve to the Aux* input. If most of the points are crowded into the closed end of the servo travel, this will cause further delay to the movement at that end of the travel, making the servo dwell longer in the closed position.

NB4: A still further sophistication is to produce a linear airbrake effect. Airbrakes as set up normally do not give linear braking effect for linear stick input. This is mainly due to the movement required to unlock the brake and the slop in the linkages. Secondly, the airbrake is slightly more effective at higher airflow streams than close to the wing. This means that typically the first 50% of stick movement provides no braking effect. By using a multipoint curve we can arrange the airbrakes to unlock in the first 5-10% of stick movement and just begin to crack. We can also then cause more servo travel to occur just after the brakes have cracked and gradually less, proportional to stick travel, as the brakes are opened. The result is smooth proportional airbrake control that beats crow braking any day of the week!

Copyright © J. McNamara   2002