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Dual Elevator Servo set up.

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Dual Elevator Servo set up.

Old 08-26-2013, 12:44 AM
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Boomerang1
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Default Dual Elevator Servo set up.

Hi gents, I'm after some information on how to set up dual elevator servos.

Not the usual set up, what I'm after is the system where both servos drive the
one pushrod. If one servo fails somewhere near the middle of it's travel the
other servo can still drive the pushrod with about half the elevator travel to get
you back on the ground.

Commercial or home designed, doesn't matter.

Thanks, - John.
Old 08-26-2013, 01:42 AM
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Lifer
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What if the bad servo locks up?
Old 08-26-2013, 03:23 AM
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ahicks
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The only setup I can think of uses one servo to move a second servo mounted in a sliding tray.

They aren't in parallel, so the elevator only gets half travel if one servo quits. As the OP mentions, the problems may start if one of the 2 were to quit too far from neutral.
Old 08-26-2013, 02:01 PM
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Boomerang1
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What if the bad servo locks up?
Provided it doesn't lock up too far from neutral the other
servo will drive the pushrod, albeit with half elevator travel.

The only setup I can think of uses one servo to move a second servo mounted in a sliding tray.
A bit like a mechanical V-tail mixer? - John.
Old 08-26-2013, 02:47 PM
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I am curious about what is the reasoning behind this type of setup? What are you using this for?
If you were to have a control horn or pushrod failure at the control surface you are done. This could likely create a problem that you are trying to avoid.
With the usual 1 servo per elevator half that would not happen.
Perhaps this is for an unusual plane which is why I asked.
Old 08-26-2013, 04:40 PM
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ahicks
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Originally Posted by Boomerang1 View Post
Provided it doesn't lock up too far from neutral the other
servo will drive the pushrod, albeit with half elevator travel.



A bit like a mechanical V-tail mixer?
- John.
Yup! That's exactly what I was thinking of. Curious about your application as well? -Al
Old 08-26-2013, 11:00 PM
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Boomerang1
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It's for a very high quality scale model of the Varga Kachina.

It has quite an obvious one piece elevator with the single horn
enclosed by the tail cone. I would still like to use dual elevator
servos for redundancy but maintain the scale appearance.

Model will be sized to suit an OS 300 twin.

In this situation servos screwed through the fuselage sides with
exposed linkages (like most ARF's) is just not good enough!

John.

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Last edited by Boomerang1; 08-26-2013 at 11:08 PM.
Old 08-27-2013, 01:50 AM
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Something like this? Im planning on using this system on a 1:3 scale PZL Wilga which also have a single elevator with only one pushrod/horn.
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Old 08-27-2013, 03:47 PM
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Thanks Tinus, that's the sort of linkages I was thinking of. - John.
Old 08-30-2013, 11:17 AM
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whats the point?
Old 08-30-2013, 12:35 PM
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What's the point? For one thing, redundancy for a critical flying surface on a rather expensive airplane! I am also an advocate of dual batteries/switches for bigger/more expensive planes as well.
Old 08-30-2013, 01:37 PM
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Originally Posted by Lifer View Post
What's the point? For one thing, redundancy for a critical flying surface on a rather expensive airplane! I am also an advocate of dual batteries/switches for bigger/more expensive planes as well.
I said what the point with one pushrod. two recievers, two switches, two batteries. half the airplane on one reciever the other half on the other reciever. thats the only true redundancy. split elevators and split aelirons.
Old 08-30-2013, 03:04 PM
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How did I know that a simple question would result in a 'discussion'?

whats the point?
Should be pretty obvious.

One pushrod & horn - has to be that way as explained earlier, scale appearance & besides, I've never
had a linkage failure in the 40 years I've been flying.

Two batteries & switches - SOP for large models & jets for me, the only battery related failure I've had was when the fuselage hatch failed on a crappy, German foamy ARF & the battery fell out!

Receiver? One failure since I started flying. I always use top of the line receivers in top of the line models so I'll take my chances on that one.

Dual servos on critical control surfaces? On important models, always. My last failure was an aileron servo, failed & jammed at about half deflection. The other servo & lots of trim got me home safely.

Redundancy is always a two way street, doubling up on equipment theoretically improves reliability but does adding more equipment just mean more items that can fail meaning, in reality reliability falls?

I also fly jets & these guys load every expensive, complicated & heavy device known to man to try to cover every obscure point of failure, the result? They still crash.


I'll stick to backing up what many years of experience has taught me may bring me to grief while still keeping things as simple as possible. John.

PS Did you know that one of the arguments used to convince full size aviation authorities that large, twin engine airliners were suitable for long, over water flights (ETOPS) was that a twin engine airliner is half as likely to have an engine failure as a four engine airliner!
Old 08-30-2013, 04:20 PM
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Xx

Last edited by radfordc; 08-30-2013 at 04:22 PM.
Old 08-31-2013, 07:47 AM
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Looks like you are looking for something like this!

http://www.swbmfg.com/categories/2X-...g-Servo-Trays/
Old 08-31-2013, 08:31 AM
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Though that is a nice device it is really for a pull-pull rudder. He is looking for a single pushrod to the elevator which stays hidden in the rear cone of fuselage I think.
140.00. Ouch.
Old 08-31-2013, 08:47 AM
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I agree with you, But I think this is as close as it is going to get. Yes... OOOUCH!!!
Old 08-31-2013, 09:25 AM
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Since the Op is after redundantcy and not doubling up for power I don't think that will work.
I think though making a complex device to acomplish what he is after will create too many variables for potential failure points and he would be better off with a quality single servo and one pushrod setup. Less things to go wrong.
Unless he could somehow split the elevator and make it not appear split. I don't know much about that plane.
Old 08-31-2013, 11:58 AM
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Yep, the setups from SWB look to gang the servos together for more muscle but offer no redundancy.

Thanks for the suggestions guys, Al's idea with a sliding tray for the second servo looks to be the best
option for me. I think it's the simplest solution & the one I could make the most accurately with the smallest
amount of free play. I'll have a play with the idea on the bench to see how practical it is. - John.
Old 08-31-2013, 12:09 PM
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What they do is if one goes bad it will continue to work. But if you have a servo go out, more than likely there it will crash as it will drain your battery so fast that you are unable to get down fast enough.


Originally Posted by Boomerang1 View Post
Yep, the setups from SWB look to gang the servos together for more muscle but offer no redundancy.

Thanks for the suggestions guys, Al's idea with a sliding tray for the second servo looks to be the best
option for me. I think it's the simplest solution & the one I could make the most accurately with the smallest
amount of free play. I'll have a play with the idea on the bench to see how practical it is. - John.
Old 08-31-2013, 02:06 PM
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radfordc
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You would have to have a dead short in the servo to drain the battery...and the fire would probably make the crash spectacular. I've had several servos fail, but never a dead short...usually the magic smoke escapes and the servo just stops.
Old 08-31-2013, 04:55 PM
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If you want redundancy then split the elevator on centerline right at the base of the skin stiffener, leave the diamond shaped stiffener on one side of the elevator and the other side smooth, precision fit a .020" gap between them and when they are together, it will look like one piece if done correctly. Now mount two servos inside that removable tail cone and drive the elevators independently but together in travel up and down from two control horns inside the fuse on centerline. Now should you have a failure or lockup on one of them, you stand a better chance getting her home. I just don't like gadgets...

Bob

Last edited by sensei; 08-31-2013 at 05:28 PM.
Old 08-31-2013, 06:07 PM
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ahicks
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Originally Posted by gjhinshaw View Post
Looks like you are looking for something like this!

http://www.swbmfg.com/categories/2X-...g-Servo-Trays/
Wow, that linkage is a brain teaser for sure! I think though, it would offer redundancy should one of the servos quit, and it would work on a single pushrod if that pushrod were fastened to the bell crank? If one of those cables were to break though....

That major short scenario can only be eliminated by going entire redundant system, using 2 receivers, and splitting all control functions for each side of the plane equally. Personally, I think that's over the top and would only use that on something much bigger than I ever plan on flying. That's me though... -Al
Old 08-31-2013, 06:14 PM
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actually it was the development of hugh engines that had the power to fly on one engine. a 4 engine has far more safty than a 2 engine. a 747 can remain airborn on 1 and still have the saftey of 3 if 1 fails. the twin is in deep doo doo if the second fan gets very quiet. i would far more like to be in the 747 when i fly over land or sea. most smaller twin crashes on one screw crash due to pilot error because of poor single engine training. even my P&W R-985 went into auto rough when heading out to Block Island and then went back to smooth when over the island,,,,hehe
Old 08-31-2013, 06:25 PM
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to add one thing,, over land i always flew by the concrete composs,,,the longest runways in the world. the interstate highways!!!! seeing nothing but woods under me allways gave me a bad feeling.

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