Extreme Speed Prop Planes Discuss the need for speed with fast prop planes (Screamin Demon, Diamond Dust, Shrikes or any REAL sound breakin'''' plane)

Throttle servo "vibration hardening"?

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Old 03-15-2013, 06:59 PM
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Rudeboy
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Default Throttle servo "vibration hardening"?

So the HS-81 I had on the throttle of my Diamond Dust died after about 15 flights. As far as I can tell, the feedback pot couldn't stand the vibes the West 36 is putting out.

Since it fits the same mount, I'm going to try a HS-85 next...I opened up both servo's and the HS-85 appears to built a little bit more solid.

But I was thinking: the reason the servo died is probably because it is resonating in sync with the engine at some point. I mean, I use HS-81 for the cut-off in Q-500 planes, and those engines aren't exactly soft natured either. They hold up fine in that application.

So, as a solution, could it be beneficial for instance, to glue a couple of strips of aluminium to the sides of the servo? The object being to alter the resonance frequency of the entire servo, and thus to shift that frequency away from the frequency band of my engine-airframe combo.

Or am I just grasping at straws here, and should I just tear into the plane and go for a sturdier servo altogether? I hate tearing into new planes!
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Old 03-15-2013, 07:07 PM
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Default RE: Throttle servo

Add a couple of wheel collars to the linkage about 2 inches from the servo. The extra mass moves the frequency response of pushrod below the frequency of the engine. I've used 5 HS-81 servos on all the controls of several Nelson powered quickies (and a couple of Jetts as well). Only lost the servos when mid-air's have destroyed the airplane.
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Old 03-15-2013, 07:46 PM
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Default RE: Throttle servo

Should be using the stiff braided cable..right..?
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Old 03-15-2013, 07:58 PM
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Default RE: Throttle servo

Well I'm using a short piece of Sullivan Goldn'Rod or whatever it's called...but only the inner rod, since the distance covered is so short.
It's about 5 inches or so from the servo to the carb. I doubt this short piece of very light plastic rod is vibrating so frantically it kills the servo...but who will tell.

Maybe a heavier piece of braided steel cable would react differently. It's all a guess to me at this point.
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Old 03-15-2013, 08:51 PM
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Default RE: Throttle servo

I use a couple inch long piece of music wire with a "Z" bend to connect the servo, then switch to cable to a ball link at the throttle/air valve (depending on the engine). The two 1/8" wheel collars connect the cable to the music wire, but the mass of the setup acts like a mechanical filter. I've used this method for 20+ years on everything that I fly.
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Old 03-15-2013, 09:54 PM
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Default RE: Throttle servo

Yes straight metal solid rod in 2-56 or 4-40 is all I use. only had one servo just up and quit. I use HS-65HB most of the time for all throttle jobs even on DLE 35's
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Old 03-15-2013, 10:46 PM
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Default RE: Throttle servo

ORIGINAL: Rudeboy
So the HS-81 I had on the throttle of my Diamond Dust died after about 15 flights. As far as I can tell, the feedback pot couldn't stand the vibes the West 36 is putting out.

Since it fits the same mount, I'm going to try a HS-85 next...I opened up both servo's and the HS-85 appears to built a little bit more solid.

But I was thinking: the reason the servo died is probably because it is resonating in sync with the engine at some point. I mean, I use HS-81 for the cut-off in Q-500 planes, and those engines aren't exactly soft natured either. They hold up fine in that application..........
Rudeboy, as an ex-retailer who did in excess of $800,000 business with Hitec RCD across 6 years, I can verify that your first 3 thoughts on this issue go right to the heart of the problem like a guided missile. Uncanny.

1) It is the feedback pot that gets killed when hs-81's are exposed to vibration.

2) The hs-85's f.b. potentiometer is isolated one shaft further away from vibration coming in through the output shaft than the hs-81's pot. Use them!

3) the reason the 81's held for you on Q-500 cut-offs is because with that linkage vibration only makes it past the output shaft, to the pot when the linkage is under tension (which terminates the vibration).

Soo, long story short, in dealing with customers returning failed servos, I found out right away that while the electric flyers got superb reliability from the hs-81's, the glow flyers experienced an atrocious failure rate. And it was almost always the pot that failed.

If you go to the hs-85's for your vibration intensive applications and you are still killing them, then something truly is wrong with your installation.

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Old 03-16-2013, 07:00 AM
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Default RE: Throttle servo

It is encouraging to hear the HS-85 will probably hold up better than the HS-81! That is good news.

The main problem I think with this plane, is that the throttle servo is basically mounted to the back of the engine mount...since the plane inself IS the engine mount.

If all else fails, I'll probably end up simply moving the servo to another location, to get it more isolated from the vibes.
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Old 03-16-2013, 09:32 AM
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Default RE: Throttle servo

Something to consider about mounting servos. Inbedding them in a puddle of silicone [or Shoe Goop] works very well, especially in fiberglass fuselages. Some local pylon flyers convinced me that this is a good way to go ...at least a good alternative to conventional mounting.
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Old 03-16-2013, 09:51 AM
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Default RE: Throttle servo

I've also used a couple of layers of double sided foam tape in the past, which works well. So that's also an option.
The same thing I used to use on my helis for the gyro...it really stuck good.

I could also use a bladder tank and just rig the carb to full throttle all the time...but on a DD without a fuselage to hold on to, that would be a little dangerous to start. Not to mention that having no way to cut the engine isn't exactly safe for bystanders either.
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Old 03-16-2013, 08:41 PM
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Default RE: Throttle servo

ORIGINAL: Rudeboy

I've also used a couple of layers of double sided foam tape in the past, which works well. So that's also an option.
The same thing I used to use on my helis for the gyro...it really stuck good.

I could also use a bladder tank and just rig the carb to full throttle all the time...but on a DD without a fuselage to hold on to, that would be a little dangerous to start. Not to mention that having no way to cut the engine isn't exactly safe for bystanders either.
Thats it. I give up. You might as well hold it by the pipe like I use to do myself; and while yer at it, take the throttle clean out of the equation. With a big ass venturi like Combative Pig would have insisted upon all along back when he was not pre-occupied with political correctness.

Pig?
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Old 03-16-2013, 09:07 PM
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Default RE: Throttle servo

Big venturi very good.
Engine kill..make it "good".
Full throttle launch...usually good.
Political correctness..?
Always good..!
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Old 03-16-2013, 09:17 PM
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Default RE: Throttle servo

Crap, I suppose I was looking for a fight, and I just did not get what I was after.

Top of the night to you C.P.
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Old 03-17-2013, 06:47 AM
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Default RE: Throttle servo




ORIGINAL: C_Roundy



Thats it. I give up. You might as well hold it by the pipe like I use to do myself;
No can do...my pipe mount is "designed" as a pipe mount...not as a handle... [8D]
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Old 03-17-2013, 07:25 AM
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Default RE: Throttle servo

I have used Goop/silicone mounting and 2 x double side tape mounting before, both seem successful - but not having problems doesn't exactly tell you what works, but having problems tells you what doesn't.

I always glue the double side tape to the airframe, I am never happy with adhesion of the tape to wood or fiberglass. So after I apply the foam tape to the servo, I hit the fuselage with some CA kicker, let that flash off, then wet the tape surface with medium CA before mounting.

I've tried making up linkage schemes using a short piece of fuel tubing as a coupler.. it works okay but I probably could have done a better job and been happier with the effort. Besides I am not sure how much vibration that would actually eliminate.

So how big is the issue of vibes transmitted directly to the servo through the airframe? I'm sure the linkage is the major killer, with soft mounting using tape or goo helping to reduce any potential effects from the former.

Regards airframe vibes with an upright engine in a drumhead airframe like a delta, now this has me wondering if there is any advantage to horizontal versus vertical mounting of the servo.



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Old 03-17-2013, 08:40 AM
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Default RE: Throttle servo

I like them laid horizontal for elevons and for engine kill. It's nice if the servo comes with both style mounts cast into the case so you don't have to build your own hardwood side mount adapters.
You know how a harmonic flywheel [harmonic balancer] is built for engines.......that would be something to experiment with for building the ultimate vibration dampening firewall / engine mount for model planes. I'm not talking about that goofy design [with rubber stand offs] that allow the engine to wobble around at idle, but an aluminum backplate mount that has a rubber ring built in to seperate the inner part of the mount from the outer part that attaches to the plane. I have no clue how they make the automotive style dampeners..the inner and outer cast / forged iron rings must take huge presses to work the band of rubber between them.
This idea would take a lot of trouble to produce and the results would be questionable.
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Old 04-04-2013, 05:00 PM
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Default RE: Throttle servo

Well, I flew the plane with a new HS-85 and a new West 36...

The servo lasted 2 ground runs and 3 short flights. That servo is going into the wing where the elevon servos are.

And on a side note, the new engine was ridiculously tight...I'll post some video of it, it took me about 45 minutes and a lot of propane torch to get it going! And I don't trust the rod, again, after yanking it out of TDC about 50 times... [:'(]
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