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Old Oct 07, 2011, 10:32 PM
Augernaut
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United States, KS, Overland Park
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Modifying pushrods on an ARF could use some advice.

Alright I'm putting together a H9 cub 40. I just lost a plane to an elevator servo failure. So I'm after a touch of redundancy here. I want to set the plane up so that the elevator is powered by two servos. I'm trying to think of ways to accomplish this. The servos tray is under the butt of the pilot. it uses a wooden dowel that has 2 metal pushrods on one end to attch to the elevator. My thought is one of two things.

1) assemble pushrod normally. leave the horn 90 degrees from the horn attaching the servo to pushrod intact. mount another servo beside this servo and attash the two servos. This assumes that a second servo could overpower a dead servo and give me enough control to at least pull out of a dive and work it to the runway. Not sure if it works that way.

2) ditch the stock pushrod and make 2 pushrods that lead to 2 servos.

problem here is the pushrods have a ways to go to get to the elevator. and I don't want to rip out all the covering to make supports for it. so what material can be used to run a long pushrod to an elevator and not flex under load?


gonna take some work, but damnit I'm not losing another plane to an elevator servo.
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Old Oct 08, 2011, 04:51 AM
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Jacksonville Fla.
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The only way that I know of is with 2 servos on 2 channels....that is still no guarentee that a bad elevator servo won't cause a crash....as you won't have as much elevator authority if you loose a servo.....but it's better than no elevator at all.....
Instead of ripping your new plane apart why not use good quality servo and ensure there is no binding in the set up
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Old Oct 08, 2011, 06:07 AM
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Wookster,

What type of servo was it that failed, I'd be interested to know as a fellow club member seems to have had more than his fair share of these. I'd also echo Jetmech's comment about good quality servos. Some of the more established makes seem to be expensive for the spec you get but I'm sure this has a lot to do with long-term reliability.

A.
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Old Oct 08, 2011, 08:57 AM
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In large, very expensive planes I can understand redundancy because they can afford the extra weight. But for smaller planes that extra weight has a bigger impact.

Question is.....At what point do you say enough redundancy is enough?? If you are truly worried about plane lose due to equipment failure, you would have to double up on your RX, RX batt, power switch, servos, etc, etc.

What if you install two servos for your elevator and then lose the plane to switch failure? Or RX failure.

I think equipment failure is something we all have to live with. There are so many things that can cause a crash that it is impossible to prevent them 100%.
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Old Oct 08, 2011, 09:16 AM
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You use two pushrods, one for each elevator half. The two elevator halves are not joined in the center, so that they both work independently of each other. Then with two elevator servos one for each half you have some redundancy if a servo fails or something.

Now if you have the room, put the two elevator servos in the rear a little in front of the elevators, and run a short pushrod to each elevator half. Now this method uses mixing in that the elevator channel is mixed with a unused AUX channel, then you can reverse one channel to get the two servos to work in unison correctly. This method is pretty common on the aerobatic planes nowadays.
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Old Oct 08, 2011, 10:06 AM
Augernaut
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The servo that went was the Hitech HS-485. I've lost four of them in the past year. all different functions on the plane. The elevator servo was the one that made it so it didn't come home.
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Old Oct 08, 2011, 12:07 PM
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And just to be clear, the two planes I want to do this to are the H9 cub and H9 Pawnee. split elevators and 80 inch wingspans. The pawnee has digital karbonite ball bearing servos. Don't remember the number off hand. but they were about $30 a piece at the time (7 total) The cub doesn't have any servos yet. Mounting an extra elevator servo looks simple since they both are split elevators. My problem is the long lenght the pushrods have to run to get from the servo to the elevator. I need something that doesn't flex. any ideas on that? I can't let servo failure be a norm, at least not once every 2 months kind of norm. I can't afford to go out and throw money at new airframes and parts every 2 months. especially since it seems as my skills progress the dollar amount of the airframe does as well. I'll throw a little money at reliability to ward off replacement. These damn servos keep making life interesting. 2 throttles a rudder and an elevator servo on the last plane. all 485's that failed. all the same failure. they just go to neutral and stick there. they will use power to keep themselves there, but will not respond to control input unless you phyically force it to move out of center, then it comes back to life, until you let it center, then it stays there again. the rudder was a non issue, the throttles were frustrating, it takes a darn long time to run out of gas at 1/2 throttle. and the elevator decided to leave me while the plane was vertical and climbing. managed to bury it in the runway in order to avoid the flightline.

So thats the skinny on why I am taking these measures. I can bring it in when the servos keep dying at center on everything, except the elevator.
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Old Oct 08, 2011, 02:29 PM
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In all the years I've been flying I have only had 1 confirmed servo failure....and a maybe...I don't use standard servos...instead I tend to go bigger in servos.....servo failure for me are rare indeed....I use 6V or in my Li-Ion battery power planes a voltage regulator at 5.8V.....But I have heard that Hi-Tech 485 has had a problem...not from anyone I know but from these forums.....
I fly HiTech 5645's or larger most of the time
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Old Oct 08, 2011, 04:14 PM
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If the pushrod length is longer than normal, use carbon fibre tubes. They look like the fiberglass arrowshafts (which would work too). But the CF tubes are really pretty stiff with almost no flex to them. You simply sand or carve a wood dowel to fit inside the tube, drill a small hole to match the piano wire ends, then cut a groove for the piano wire end, bend a 90 degree end in the part going into the CF tube, slip the wire into the tube up to where the 90 degree bend fits into the hole you drilled, then slip the wood dowel into place and glue it in. You can hold off bending the 90 degree end part, then you can slide the wires in and out to get the length just right, mark them and make the bends then. You can use a dremel cut off disc to shorten the wire tip where the bend is done. I like to use 5 minute epoxy but super glue works too.

Another method is the same thing, but after you cut the groove in the dowel for the pushrod wire ends, you can drill a hole in the dowel instead of the tube.

Now they do have commercial products available to make the CF pushrod tube ends too.
But it is simple to just make them yourself.

Sig Mfg has a nice article about it here, and you can see what I was talking about in the pic on the fiberglass arrowshaft and how they depict it.
http://www.dynmodel.com/sig/007.htm

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Old Oct 09, 2011, 05:17 AM
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Quote:
Originally Posted by jetmech05 View Post
In all the years I've been flying I have only had 1 confirmed servo failure. I don't use standard servos...instead I tend to go bigger in servos.....servo failure for me are rare indeed....I use 6V or in my Li-Ion battery power planes a voltage regulator at 5.8V.
Definitely go larger than you think you need, especially if you keep having these kind of servo failures. Repeated failure indicates something is wrong - either the product itself, what its used for, or the conditions under which it is being used.

Also ensure your linkages are giving you maximum mechanical advantage at the servo if you havent done that already. Its best to use the full throw of the servo to actuate the amount of throw you want.

Not trying to patronise you, but making sure all bases are covered is never a bad idea.

Cheers - boingk
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Old Oct 09, 2011, 07:14 PM
Augernaut
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United States, KS, Overland Park
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Quote:
Originally Posted by boingk View Post
Definitely go larger than you think you need, especially if you keep having these kind of servo failures. Repeated failure indicates something is wrong - either the product itself, what its used for, or the conditions under which it is being used.

Also ensure your linkages are giving you maximum mechanical advantage at the servo if you havent done that already. Its best to use the full throw of the servo to actuate the amount of throw you want.

Not trying to patronise you, but making sure all bases are covered is never a bad idea.

Cheers - boingk
Understood. I did the same thing. It's just been the one plane that failed with the 485's, but a pinch of caution might help with all my other airframes. I mean we are talking about a $12 servo and a pushrod set to give it redundancy. With it being a cub too I think the weght will be minimal. Dang things have so much lift as is.
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Old Oct 09, 2011, 10:59 PM
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Righteo, no worries.

Weird thing is I'm using the cheapest gear I can find (Hobbyking HK15168/HK15178 servos @ $2.20/$1.80 each) and am not having any problems at all using them in parkflyer style EP builds up to 1kg and 40" wingspan.

Going to gassers, I've gone straight to their HK15148 units. I haven't had any flight time with them yet but based on the experience with the other units they should work alright.

Cheers - boingk
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Old Oct 11, 2011, 10:43 AM
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Quote:
Originally Posted by Wookster View Post
The servo that went was the Hitech HS-485. I've lost four of them in the past year. all different functions on the plane. The elevator servo was the one that made it so it didn't come home.
Is that one of the "karbonite" gear servos? Curious, as I've seen many of the newer "karbonite" servos fail.

In that situation, it sounds like that's a large"ish" plane.. I might look at mounting 2 servos in the tail... one each side, just under the horizontal stab.. then you can simply "Y" harness them to the Rx. The pushrod would be much shorter, which makes for a stiffer/tighter linkage. Thats with whatever servo you end up with.. but in any case the each servo does half the work that one servo did before.

The disadvantage might be balance.. you would probably have to add a neglible bit of weight to compensate for the rear mounted servos, but if I were looking for redundancy, and reliablity, an ounce of weight is a small price to pay..

I've got a few aerobats(Extra's, Edge's, etc..) that are set up just like described above. To get the servos proper rotation, you will need a "servo-reversing" Y-harness...that device will reverse one servo, so you can mount them "mirroring" each other. Horizon has those for about $12..

Its really a decent set-up, and I've never liked long pushrods anyway... too flexy.. they "pull" decently, but its the "push" where they just flex greatly.. just grab any surface and move it that has one hooked up, you'll see alot of play in it, I'd bet on it. So there's a few ideas too... Good luck.
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Old Oct 11, 2011, 06:33 PM
Augernaut
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United States, KS, Overland Park
Joined Jan 2010
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Quote:
Originally Posted by DGrant View Post
Is that one of the "karbonite" gear servos? Curious, as I've seen many of the newer "karbonite" servos fail.

In that situation, it sounds like that's a large"ish" plane.. I might look at mounting 2 servos in the tail... one each side, just under the horizontal stab.. then you can simply "Y" harness them to the Rx. The pushrod would be much shorter, which makes for a stiffer/tighter linkage. Thats with whatever servo you end up with.. but in any case the each servo does half the work that one servo did before.

The disadvantage might be balance.. you would probably have to add a neglible bit of weight to compensate for the rear mounted servos, but if I were looking for redundancy, and reliablity, an ounce of weight is a small price to pay..

I've got a few aerobats(Extra's, Edge's, etc..) that are set up just like described above. To get the servos proper rotation, you will need a "servo-reversing" Y-harness...that device will reverse one servo, so you can mount them "mirroring" each other. Horizon has those for about $12..

Its really a decent set-up, and I've never liked long pushrods anyway... too flexy.. they "pull" decently, but its the "push" where they just flex greatly.. just grab any surface and move it that has one hooked up, you'll see alot of play in it, I'd bet on it. So there's a few ideas too... Good luck.
it was the karbonite. but I will mention i have never lost the gears. it seems to be an electronic failure.
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Last edited by Wookster; Oct 11, 2011 at 06:33 PM. Reason: Ugh, bad day at work. Bandages and typing don't mix.
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