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They All Failed At The Same Time????

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Old 07-26-2004, 08:51 AM
  #1  
hilleyja
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Default They All Failed At The Same Time????

I had something very strange happen to my GP Christen Eagle II on Saturday.

I finished this airplane in early May but have only had it in the air 3 time before Saturday. I've been having tuning problems with my Fuji BT-50SA gas engine. I finally got her tuned Saturday and flew it for the 4th time. She is no longer what I would consider under-powered but she is also certainly not ready to duplicate Chip Hyde's Ultimate 3D antics. The 1st takeoff and landing where great -- it looked like I finally had my bi-wing aerobat. I noticed some sluggishment in turning on the subsequent 2 takeoffs and landings. After the third landing I taxied her back to the pitts. At least 2 of my Hitec HS-5475 digital servos where screaming like banchies and one of my 4 airlons was hanging by a single hinge. Upon examination, I found that 2 of the 3 hinges broke in the middle and the servo was totally stripped -- this stripped servo was understood because of the failed hinges. What I can't understand is the other 3 airlons, each with a separate servo, where what was making the most noise. Every single one of them has at least 1 tooth stripped and the servos where oscillating around neutral.
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Old 07-26-2004, 10:32 AM
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Default RE: They All Failed At The Same Time????

You are very lucky to still have the airplane. You no doubt had a flutter problem with the ailerons. It could have easily destroyed your airplane. Find an expert in your area and get his advice on how to fix it. Otherwise it will just bite you again. Without seeing your airplane, its hard to guess what your problem is, other than that you might just have too much power for the airplane.
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Old 07-26-2004, 01:08 PM
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hilleyja
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Default RE: They All Failed At The Same Time????

Definitely not too much engine for the airplane. Engine is Fuji BT-50SA (Mejzlik 20x10 prop) and the airplane is a Great Plains Christen Eagle II (~18.5 pounds). Based on the 1st takeoff and landing my performance was what one would expect from a 2D acrobatic airplane. I had good takeoff power and climbout was positive. What I didn't have is greater than 1:1 power:wait ratio. You could not hover this airplane with my current configuration. Entering a loop from straight and level requires full throttle.

Flutter -- from what? Certainly not from too much speed or too much control surface. As you can see from the picture below, the airlons make up a very small percentage of the wing area -- each airlon is about 1/3rd the length of the wing half and are less than 1/5th of the wing cord in width. Aside from that, I flew her like she was made of egg shells; my maneuvers were wide and gentle -- I was still evaluating the performance of the engine, which until that flight barely kept her flying.
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Old 07-26-2004, 01:17 PM
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Default RE: They All Failed At The Same Time????

So do you think it fluttered or not? Did you here it flutter in flight or notice anything while it was still airborne?
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Old 07-26-2004, 01:36 PM
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Default RE: They All Failed At The Same Time????

Jim, flutter can be caused by many things. I had a felony flutter problem on a Goldberg Matrix. This airplane certainly isn't overpowered with a 70 Surpass, and in fact isn't that fast.

What caused the flutter was the aileron horns trashing the soft balsa where they were mounted, and allowing the aileron to move by itself. Took about 30 flights for the wood to get chewed up enough, but I too was lucky and didn't lose the model, so there was no crash detritus to obscure the cause.

Sealing the hinge gap will do more to stop potential flutter than just about anything else. But its not bulletproof; I seal all hinge gaps on my models, kit-built or ARF, regardless of how close the fit is. This Matrix was done that way, and is the first model, out of 35 or so I've built/assembled, to experience a flutter problem.
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Old 07-26-2004, 01:59 PM
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hilleyja
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Default RE: They All Failed At The Same Time????

I wish I knew. I certainly had flutter after the 2 hinges failed on one of the airlons. I did notice some abnormal indications from the airlons but attributed them to sun flashes off the white surface. Hind sight tells me I was seeing the flutter from the top left airlon, probably after the hinges failed.

My understanding of what causes surfaces to flutter in the 1st place makes it difficult for me to believe flutter was the cause of the problem, rather than the result of the problem. I now think one of the causes was not enough gap between the wing trailing edge and the airlon leading edge -- this probably caused the hinges to fail. If that is the case the traditional cause of flutter is absent -- there is little to no gap between the surfaces. The other airlons do not have any failed hinges but every single one of the servos are missing one or more teeth around neutral -- I haven't opened them up yet but when I move the surface they click around neutral and when I powered them up they scream like a banchie because the higher-frequency digital updating are moving the servo between a larger than normal gear gap at neutral. If you have ever heard a digital servo whine around neutral, try magnifying that noise a hundred fold and then do it to 3 servos at the same time -- you'll get the mental sound picture I faced when I taxied her back to me after landing.
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Old 07-26-2004, 02:25 PM
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Rodney
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Default RE: They All Failed At The Same Time????

I have to dissagree with the comment above that sealing the hinge gaps eliminate flutter. Actually, sealing the gap has little or nothing to do with eliminating flutter. Unfortunately EVERYTHING WILL FLUTTER given the proper stimulus. The most common causes in models is 1:sloppy or limp (undersized) control push rods 2versized holes in the horns (Zbends commonly have this problem)3:flexible surfaces (keep the surfaces as stiff as possible, i.e. no soft wood or easily flexed control surfaces) or 4; excess play in any part of the system. Even a wing that easily distorts or flexes can cause a stiff aileron to flutter. Often, the place you mount the horn to the moveable surface can minimize the onset of flutter. Never put the horn at 50% of the span on ailerons, always offset it to about 30 or 70% of the span if you can.
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Old 07-26-2004, 03:31 PM
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Default RE: They All Failed At The Same Time????

Let's rule out the flutter problem and the only common denominator is a servo or servos fault.
What is the possibility of three servos failing? Most unlikely but it could happen if one servo failed and put a strain on the other two. Just a thought.
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Old 07-26-2004, 04:46 PM
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Default RE: They All Failed At The Same Time????

Maybe sealing the gap doesn't do anything. Folks with a lot more experience in this hobby than me say it does. I've done it religiously since 1987, and the only time I've experienced flutter was on that Matrix; and I knew better than to screw those aileron horns into soft balsa. I normally fit 1/64 ply "scabs" at horn attachment points on ARFs if there is no hardwood in the proper place. For several reasons, I didn't do it on this one, and it bit me. BTW, the servo hardware was DuBro HD horns and 4-40 pushrods.

What is the reasoning behind not locating a horn midway along the span of an aileron?
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Old 07-26-2004, 05:09 PM
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Default RE: They All Failed At The Same Time????

Rodney's post is right on the money. Sealing gaps helps improve effectiveness, but in all likelihood has little to do with flutter.

As Rodney pointed out, you also have to have flexibility in the main surface itself (this case the wing) in order for the energy coupling inherent in fluttering to occur.

ANYTHING will flutter at some speed.
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Old 07-26-2004, 05:50 PM
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Default RE: They All Failed At The Same Time????

An undersized servo will cause flutter, especially when they snap at the shafts. I found that out on my CAP232 120 recently. I used 635's on the ones that failed. Both ail's failed at the same time. One oif the elevator's failed too. I went to 1:1 mechanical advantage on the control rod and used 5645's. Let's hope that holds them next time.

I was so lucky to get this thing down in one piece. Just broke the right side of the wing. Could have been much worse.
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Old 07-27-2004, 06:24 AM
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Default RE: They All Failed At The Same Time????

How would the failed servo put a strain on the other three? None of them are linked together mechanically. This is a 4-airlon setup with the right side y-connected together and the left side y-connected together.
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Old 07-27-2004, 09:18 AM
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Default RE: They All Failed At The Same Time????

I don't no why all three servos failed that is why I asked this question. The poster claims flutter did not cause this problem. I'm at a loss for an explanation. The only biplane I have flown was a Sig Miniplane and it had airlerons on the bottom wing only.
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Old 07-27-2004, 11:25 AM
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Default RE: They All Failed At The Same Time????

If you can 3D an aircraft you have too much power for the airplane in my opinion and you had better be ready with the throttle to be able to keep the airplane.
My first R/C, built in 1961, had a flutter problem. It was a DeBolt Persuit powered by a K&B 45. The ailerons on that were tiny. I static balanced the ailerons and eliminated the flutter.
Static balancing is the only sure cure for flutter. All full scale aircraft static balance their control surfaces.
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Old 07-27-2004, 11:44 AM
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hilleyja
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Default RE: They All Failed At The Same Time????

Come on guys, we are not talking about a 3D airplane. This is the Great Plains Christen Eagle II. It is similar to a Pitts. It is a short stubby Bi-plane that weighs 18.5 pounds dry. Believe me, there is no 45-50cc engine in the world that would over-power this airplane.

Traditional flutter is the result of loosly coupled control surfaces and/or extra-large control surfaces and highspeed flying. That is also not the case here. Actually one of my possible problems is I did not leave enough gap in the airlons to allow the CA hinges to flex -- probably why 2 of 3 hinges on one surface broke, brittle from the CA and not enough gap for proper flex. Each of the airlons make up less than 1/3rd the wing-half length and less than 1/6th the wing chord -- they are a fraction of the size normally seen on 3D airplanes.

That still doesn't explain why the other 3 airlon survos are now missing one or more teeth in their gears. These Karbonite gears are suppose to be Hitec's answer to both slop from metal gears and wear from nylon gears. All four of these airlons are independent mechanically to each other. The two servos on the left are y-connected and installed in RX channel 6. The two servos on the right are y-connected and installed on RX channel 1. The TX is setup in Flaperon mode.
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Old 07-27-2004, 12:04 PM
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Default RE: They All Failed At The Same Time????

I would personally NOT use anything other than metal gear servos in this size of a plane. I learned that the hard way. Also I'd stay with at least 100 oz-in or torque.
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Old 07-27-2004, 12:33 PM
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Default RE: They All Failed At The Same Time????

Jim I think you are in denial :-) It fluttered, took out the hinges and the servos all in one shot.
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Old 07-27-2004, 01:03 PM
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Default RE: They All Failed At The Same Time????

ORIGINAL: mr_matt

Jim I think you are in denial :-) It fluttered, took out the hinges and the servos all in one shot.
Your 100% right. I am in denial. What other factors can cause the servo damage other than flutter? That is why I'm trying to understand why it fluttered. What could I have done to prevent it aside from the obvious popular 'seal with monocote'. And, lets not forget that I did not fly go-fast with this airplane -- couldn't, even if I wanted to without simply doing a full-throttle nose dive, which most certainly did not happen. Look at the pictures of this airplane and tell me where the sensitivities are for flutter. I have flown this bird a total of four times with 6 takeoffs and 6 landings. Only one of those landings could be characterized as hard -- the plane bounces a couple of feet. The 1st three takeoffs and landings were done when the plane was significantly under-powered (6200rpm from a Zinger 20z10 prop). Flying during those flights were instinctively extremely mild in control movement because she never did build up enough speed for normal maneuvering. One loop attempt during that period resulted in the airplane falling back before it reach peak -- it couldn't go over the top. The last three takeoffs were more realistic (~7200rpm form a Mejzlik 20x10 prop). The first of them was good responsive flying, but again not dynamically challenging. The last two of them were probably post failure of at least one of the airlons because I began to notice attitude response was marginal. I didn't realisze the nature of the problem until I taxied her back from the last landing.

So, again, what caused the flutter that caused the servo damage?
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Old 07-27-2004, 04:15 PM
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Default RE: They All Failed At The Same Time????

Most likely cause of the flutter was surfaces heavy at the trailing edge or slop in the likage. I once cured aileron flutter on a pattern ship by adding a mass balance at the outboard end. Every control system has a critical flutter speed. Either fly below the flutter speed or modify the control surface, servo, and linkage to change the natural vibration frequency. This means balancing the control surfaces the way the helicopter boys balance their rotor blades.
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Old 07-27-2004, 05:16 PM
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Default RE: They All Failed At The Same Time????

Hi Jim,

ORIGINAL: hilleyja
So, again, what caused the flutter that caused the servo damage?
When I worked in aerospace on a flying wing UAV, one of the PhD's was demonstrating a flutter mode with a little balsa glider. He weighted it right and he could toss it and you could watch it flutter right across the room. You do not have to go fast to have a flutter, but most reasonable well built planes need to go fairly fast to exhibit it.

You might have done nothing wrong, there could be a design flaw. Either a very flexible wing, a flexible servo mount, a flexible linkage, anyhing like that could cause the flutter speed to drop very low (remember basically everything has a flutter speed).

The only way to almost completely eliminate the chance of flutter is to balance the surface, and that involves making the CG of the movable surface coincident with the hinge line. I do this on very fast turbines, but in my experience most models do not need it if the wings are still enough.
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Old 07-27-2004, 06:15 PM
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Default RE: They All Failed At The Same Time????

Flutter can be amazingly destructive. A friend of mine named Jerry Ritz was killed when a control linkage problem drove the wing of his untralight into destructive flutter. The wing used geodedic constrution and appeared to be much stronger than other ultralights. I saw the wing before he covered it and I would have never suspected a flutter problem. Unfortunately, the aileron cables were misrigged and allowed some of the cables to go slack at some deflection angles. That was all it took to drive the ailerons into flutter and explode the wing.

Some of you old timers may remember Jerry Ritz as a world champoin free flighter about 50 years ago. He also designed the Ritz airfoils and manufactured Ritz props.
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Old 07-27-2004, 09:35 PM
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Default RE: They All Failed At The Same Time????

A couple of years ago, I saw a Goldberg Ultimate destroyed by flutter. After some discussion, we reached the conclusion that it was due to sloop on the top wing connection.
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Old 07-27-2004, 11:40 PM
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Default RE: They All Failed At The Same Time????

These are the balancers I put on my very fast jets. No flutter.

Before I used these, I had flutter on my fin twice. THese were all composite hi tech structures, top of the line digitals, etc. When the flutter hit before I could slow down all 4 robart pin hinges were gone, servo stripped and the rudder was gone.

No more flutter now, knock on wood.
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Old 07-28-2004, 11:33 AM
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Default RE: They All Failed At The Same Time????

I lost a very nice sport plane 3 days ago to elevator flutter. Mr Matt and ChuckA are right; everything has a flutter state regardless of linkages or servos.

My plane experienced flutter a few times without damaging much. It finally damaged a servo with some down angle applied, but I landed the plane okay. We repaired the nyrod clevis end and put in a Futaba S3151 digital servo. It helped for a few flights, but fluttered again and I had a lot of down elevator and no control. The plane hit in a pond and we rebuilt the wing. At that time, we ran straight 4-40 wire with heavy clevis hardware. It flew 11 flights just fine. On flight #9 I did quite a few loops, humpty-bumps, rolls, etc. I suspected that the plane was out of balance and put it on the CG machine. Sure enough, it needed some tail weights. Did a short test flight and it flew better than it ever had. Fueled up, and took off to have some real fun. A buddy asked to chase me a little and I was flying simple stuff WOT. The .46 doesn't exactly overpower the plane. I didn't hear the flutter, but my buddy did. It went nose down again and was totalled. The digital servo was stripped. That is the end of that plane. The elevator is still installed perfectly with CA hinges. It looks good. No sign of stress, just an innocent looking elevator.

The guy who built it was my instructor when I joined the club. His brother is a flight surfaces stability engineer. His advice is that the elevator somehow happened to resonate into flutter in the ranges that the plane flew. It seems that it was prone to flutter when balanced dead on. Nothing to do except build a new elevator or change the design. We didn't know that before the crash. I don't think there was a hardware sympathetic resonance issue here as it fluttered with 2 different brands of engines. I think we can rule that possibility out...if it could exist.
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