Fault-finding airbrake servos?

[Jolly Roger]

Being a bear of little brain, I could use some help diagnosing this one:

My Ka6 is set up with a mechanical airbrake in each wing, each airbrake driven by its own servo (Hitec HS82).

The 2 servos are connected to the receiver via a Y lead.

After several hours of trouble-free action, I noticed that when I closed the airbrakes after a long dive from height, the left one closed but the right remained about 1/4 open. After that, neither airbrake responded, and both servos appeared dead.

I assumed I had fried the servos because I'd operated them on 5 cell NiMH receiver packs, delivering 6.5V under load. However, in the workshop...

When only the left airbrake servo is connected it operates perfectly again...
but when the right airbrake servo is connected, both go dead again (as they did in flight).
When the right alone is connected, it is dead too.

So... why does connecting the right servo stop either working?

Thanks for any thoughts.

Rog

[Peter Balcombe]

Roger, As Ian says, the HS82 servo should be ok for operation from either 4.8 or 6v packs according to the specification, but this will probably assume Nicad or NiMh type packs.
Anyway, it definitely looks as if the signal input on the RH servo has gone faulty and is dragging down the other one when both are connected.
Presumably if you connect another servo in place of the RH one then all is ok.
As Ian says, it looks to be a case of replacing the RH servo & all will be well.

[john greenfield]

What receiver are you using ?
I have seen this problem with some "cheap" receivers where the signal height is not sufficient to drive all the servos.

AEB

[Roger-Spragg]

Couple of thoughts.
1. Are both your airbrakes equally "free" to operate i.e. is one loading up? In your long dive there may have been some stress on the mechanics of the airbrake itself.
2. As battery packs age their internal resistance increases. This has the effect of reducing the voltage at the receiver/servo's under load - the more load the lower the voltage. Have you tried it with a "new" battery pack?
Worth checking before you pull all your hair out and it's always better to find the definitive reason than just change things not knowing why it doesn't work.

Roger

[Peter Balcombe]

Roger,
I should have thought that the definitive way to prove if the Right hand servo is ok or if it is a binding linkage stopping it from working, is to temporarily disconnect the brake linkage & run just that servo from a receiver or servo tester.
Even better to remove the servo & try in splendid isolation.
Then if the servo still doesn't work, you need to replace the servo.
It is possible that the previous high load condition overstressed the servo circuitry & caused it to fail in a manner which prevents anything working on that Rx output.

[Roger-Spragg]

Peter
Agree absolutely. I just hoped to empathise the importance of systematically testing all of the components that could possibly have failed in the system and therefore added a couple of possibilities. I'm sure that you too have seen the consequences of optimistically jumping to the wrong conclusion!

Roger

[Jolly Roger]

Heartfelt thanks to all for your responses. Valuable ideas, clearly expressed.

Not had a chance to investigate today but I agree the lead suspect is a short in the wiring on the RH wing, because I've just looked back in my logbook and seen that the original servo on the RH wing only lasted 8 flights. Sorry forgot to mention earlier. So this is middle age.

Other info:

I think both Rx and power supply are in the clear - receiver is a Mpx 9ch IPD synth 35MHz receiver with a clean track record and which I've flown subsequently with no problem in other models. Power supply is dual 2.4Ah 5 cell NiMh batteries, fed into above receiver - both delivering healthy voltage throughout flight (reported by Piccalorio) and with plenty of charge remaining even after the 3 hour flight.

I also think the RH airbrake itself works smoothly but will check it tomorrow. It does fit to a very fine tolerance.

I'll take out the RH servo tomorrow and see if it's died, then test the wiring in sections.

Thanks again chaps. Will report back.

Rog

[Peter Balcombe]

Roger, a further thing to check - even if the servo is now dead:-
Although the RH brake action appears ok on the bench with no air drag load, it may be worth checking to see if the mechanism binds at all with a simulated rearwards pressure (gently push with finger), particularly when going back to retracted state from the open state.

I recall that Cliff Evans had a lot of trouble with several burnt out servos on one wing of his 1/4 scale Olympia, despite the mechanism appearing to be free and the servos working ok when initially tested. The other wing was trouble free.
I think his problem was resolved in the end by fitting a more powerful airbrake servo

[Jolly Roger]

Roger, a further thing to check - even if the servo is now dead:-
Although the RH brake action appears ok on the bench with no air drag load, it may be worth checking to see if the mechanism binds at all with a simulated rearwards pressure (gently push with finger), particularly when going back to retracted state from the open state.

I recall that Cliff Evans had a lot of trouble with several burnt out servos on one wing of his 1/4 scale Olympia, despite the mechanism appearing to be free and the servos working ok when initially tested. The other wing was trouble free.
I think his problem was resolved in the end by fitting a more powerful airbrake servo

Again - useful thanks Peter.

I will do as you say.

Handily, the whole episode was videoed by my fin camera and yes the model was still diving when I tried to retract the airbrakes - this could well have caused the binding. By chance I've just been looking at electric airbrakes on the Pichler site - a possible solution if it turns out the airbrake is the origin of the problem.

Best wishes,
Rog

[Jolly Roger]

Full marks if you suspected a stuck airbrake.

I removed the RH servo (sadly, dead) and after 3 minutes of fiddling the pushrod back and forth, managed to get it stuck at exactly the same position as it did in flight - about 1/4 open.

So I removed first the lower airbrake blade and then the top blade and could still get it to stick, so I knew it wasn't the blades.

Looking inside, it seems the end of the brass slider, driven by the pushrod, catches on the small metal pin that sticks out of the black plastic swinging arm - see photo.

I think that as the airbrake closes, the brass slider is supposed to clear this pin by a couple of millimetres, and then during the last bit of movement, slide over the pin to lock the airbrake shut. Instead the slider pushes horizontally against the pin and even when I pushed hard on the pushrod, nothing was going to budge it. No wonder the servo died. Lucky I had 2 hefty batteries or it could have dragged the whole Rx voltage down.

Anyone else had this problem with a fouled locking pin?

And what's the solution?