Servo failure on my Piper Pawnee 40

[MikeS]

Flying my Piper Pawnee the other day I suffered an elevator servo failure. The kits use's one servo but I had installed two each controlling its own elevator.

Glad I did.

 

Edited By MikeS on 08/08/2012 09:51:53

[Biggles' Elder Brother - Moderator]

Good recovery Mike - you're obviously a man that keeps his head in an emergency! That's the benefit of the two servos - definitely a plus. You can fly without a rudder, you can fly with one aileron - or even none at a push, but no elevator is doomsday time!

BEB

PS Nice flying strip by the way!

[Hogster]

I fly like that without a servo failure

And what a great site to fly from

[Mark Powell 2]

One or two servos is a quandary. You were skiled enough to be ok with just the one working, and I note your comment that it rolled on the landing approach. Yes, ailerons are not so effective at low speeds.

But as I said, it is a quandary whether to install one or two. Installing two of course doubles the chance of a failure, but, as in your case, increases the likelyhood of your getting away with it if one fails.

Some people with large models use two servos driving one pushrod. I think that is silly. It is unlikely the remaining good servo can overcome both the resistance of the failed servo and the aerodynamic loads. So you end up with twice the chance of a failure and no effective remedy. If you do this, you should use three servos.as it is unlikely that two will fail simultaneosly. But, of course, you treble the chances of a servo failure.

I NEVER install separate servos for each flap, if I have flaps. It is simpler to do, but I take the extra trouble to use one servo and a linkage to the flaps. One is unlikely toi recover from the consequences of just one flap deploying on a landing approach. People in 'real' aircraft have been killed when that happens. Again, a separate servo for each flap doubles the chances of a flap servo failing.

Well done!

[MikeS]

When the servo failed I had no idea what was going other than it wanted to climb. I figured something had given way but it caught me off guard.

My first thought was to land but I wanted to know for sure I still had good control of it. Once it was down all of course became clear what had happened.

Its a very nice strip to fly from but does get very windy being open and a strong cross wind seems to be always waiting to catch the unwary pilot.

[Allan Bennett]

Posted by Biggles' Elder Brother - Moderator on 08/08/2012 10:07:24:

... but no elevator is doomsday time! ...

If it's a powered model, you can use the engine to control height if the elevator fails in the neutral position. I had to do that once many years ago and achieved a perfect landing because I normally use engine power, not elevator, to adjust height during landing approaches.

[Paul Marsh]

Quite a few models I fly now use two elevator servos, even when they aren't part of the kit. On the Silence Twister and a HET RC Extra 330l model would have only used one servo. For a 120 size model, I feel more confident that either if one servo fails, still have limited elevator movement to at least make a controlled landing and/or have enough power shared between two "cheaper" servos than trusting one expensive servo to the gods.

[Chris Channon]

Hi Allan, totally agree, years ago i flew a Flair Cub using just throttle and rudder for the whole flight,

Landing was a bit fast but totally safe.

Makes you wonder why we bother with all the other controls !!!!!!!!!

Regards

Chris C

[Paul Marsh]

I had the elevator servo fail on my YT Mustang racer. Was flying it around, then felt strange, but had to land, anyway, as low on fuel. Came to flair, but it didn't and scaped on the tarmac. Found the elevator servo had just failed, luckely with a bit of "up" - enough for level flight and would climb, but of course not quite enough to flair on landing.

No damage - until later on when the wing came off in flight and terminted that.

[Frank Skilbeck]

Posted by Mark Powell 2 on 08/08/2012 10:34:32:

I NEVER install separate servos for each flap, if I have flaps. It is simpler to do, but I take the extra trouble to use one servo and a linkage to the flaps. One is unlikely toi recover from the consequences of just one flap deploying on a landing approach. People in 'real' aircraft have been killed when that happens. Again, a separate servo for each flap doubles the chances of a flap servo failing.

There's always one, on a split flap model using one servo to drive both flaps the linkage on one flap came loose........................

[cymaz]

Yep....Still doesn't explain why the servos failed, or have I missed it?????

You really need to find out and quick, you might not be so lucky next time.

Good effort and well done

[MikeS]

Cymaz.

I have no real idea why the servo failed. Both units are of the same type and model. There is no binding in the control linkages on either side. My rx power is regulated to 5.9v (AR7000 RX).

The servos are rated for 4.8v & 6.0v. I had one die on me in another model a few years back when I was setting it up on the bench it just stopped working. Both units are Futaba S148's.

Strange is both servos showed the same thing the motor had failed. I get readings down the motor wires when moving the tx sticks but nothing from the motor.

My flying stylle is towards scale and on a 46fx(at the time) was no speed demon.

Mike

Edited By MikeS on 28/09/2012 08:51:48

[cymaz]

It might be a good thing if you could as I'm sure you dont want that to happen again. Its never good to lose a plane and you don't know why.............been there got the T-shirt.

Tried the same servos with a different reciever. Check the pins for looseness in the plug body. Any corrosion. Do you have a fail safe? A radio glitch might have done it.

Just trying to think about different reasons but it sounds like you are already checking out things. Keep at it, good luck.

[MikeS]

I'm using the same RX and all connections are tight. Cant say I seen any corrosion I assume you mean on the pins and wires.

Failsafe is setup throught the radio for throttle at idle etc. A radio clitch is possible never can rule that one out.

I use a DX7 and only have AR7000 rx's in my planes. I do use a system log that so far has not shown up anything worring.

At the moment I feel that it is just a duff servo. I do a range check before each flight and do the usual pre-flight checks. So far all good.

I'm planning on going up to a DX8 next month.

[Delta Whiskey]

Posted by Hogster on 08/08/2012 10:19:24:

I fly like that without a servo failure

And what a great site to fly from

The site is Kempton Park Model Flying Club. **LINK**

Well done Mike in getting it down in one piece !

[MikeS]

Thanks Doug.

I'm just to good a pilot. lol

[Martin Harris - Moderator]

Posted by Allan Bennett on 09/08/2012 20:10:19:

Posted by Biggles' Elder Brother - Moderator on 08/08/2012 10:07:24:

... but no elevator is doomsday time! ...

If it's a powered model, you can use the engine to control height if the elevator fails in the neutral position. I had to do that once many years ago and achieved a perfect landing because I normally use engine power, not elevator, to adjust height during landing approaches.

Happened to me when I was test flying a Protech Cougar for a clubmate (not the funfly) and elevator flutter detached the clevis (metal, with fuel tube keeper) from the servo.

Fortunately it settled into reasonably trimmed flight and power changes had just enough effect to give a small measure of pitch control - although I didn't feel it was a good idea to initiate turns with rudder in case the nose dropped significantly. It was quite illuminating to see how large the turn radii were without being able to balance them with elevator but luckily I managed to get round a circuit, line it up and get it safely back on the ground. The landing was a little heavy as the burst of power to raise the nose for the flare didn't... but no damage was done.

...and many years ago, a clubmate at my full size gliding club landed an ASW20 with a disconnected elevator, controlling the approach with the camber changing flaps.  Mind you, he was a serving member of the clergy and may have had his boss on a hotline!

Edited By Martin Harris on 28/09/2012 11:05:14

[cymaz]

Am I right in thinking that throttle controls altitude of model and elevator controls rate of climb and descent

[Martin Harris - Moderator]

Not entirely. In the context of circuit flying with the average model, the elevator primarily controls airspeed (or more correctly angle of attack) and the throttle controls climb and descent rates.

Edited By Martin Harris on 28/09/2012 12:44:57

[cymaz]

your right

[MikeS]

Yep he is right

[Mark Agate]

I have a 2.5m Salto V-tail glider which I flew from the slope earlier this year. Couldn't understand why it seemed to climb when I did a left hand turn and dive when I turned right. When I stopped trying to do co-ordinated turns and just used ailerons it seemed a lot better. Then when I landed I found one of the servos had failed - a good argument for V-tails, and a bit embarassing that I only half noticed the problem!