Airframe/wing flutter


22 posts in this topic

Posted

Hey everyone...

 

I am in need of some help/advice with my Cat.....I have been busy getting my Catalina flying and trimmed out properly. On my last flight I experienced what I think is wing flutter (no vibrations in stick) while doing some benign maneuvering in cruise ( calm air, flying level,cruise pwr and banking right/left 20deg keeping my heading and compensating adverse yaw with rudder....then it suddenly airframe shook like crazy). Chopping the power straight away and slowing down got rid of the vibrations.

Engine run smoothly and prop has been checked for play and damage...all good.

I checked everything else I could (flaperon balance weights, play, trim tab, rigging, engine mounts, wing, flight control play ...etc) and found four items...

 

 

1. engine mounts are a bit soft and old

2. flaperon balance weights need 100gr more weight to be slightly nose heavy (I don't get the trailing edge to lift the required 1/4 in)..

3. when I move the flaperon from full down to full up, my rib ends move a bit

4. Found out in my Cat there is no spar stiffener installed inside the spars!!!! I am not sure but I believe the spar stiffeners have to be fitted as per plan?? Do you have spar stiffeners?

 

 

I would really appreciate your precious input, best regards, Laurent

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Posted

Just a thought here, If you were doing a lot of banking back and forth could have the gas in the float bowls sloshed back and forth and flooded the engine for a sec, Was it that kind of vibration?

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Posted

Hey TJAY, thanks for your reply. 

I can confirm the engine was running steady, purring without any drop in rpm or change in noise....Also the bank angles were quite shallow and slow in motion (not doing the rough aerobatic style). It really was a strong airframe shake at a steady RPM. I only suspect wing flutter but i really had no time to actually "look" ....had to act straight away as I thought we were about to shake appart..

Looking forward for some more inputs...I am grounded as it stands now.

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Posted

Grounding for suspected flutter is the right thing to do, it is nothing to play with.  Make sure all the control surfaces are properly weighted and balanced, if so then the spar stiffeners may be needed.

Mark

 

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Posted

It is confirmed, the Catalina has the spar stiffeners, they are needed

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Posted (edited)

I would think that with those longer wings than most Avids, that spar inserts "stiffeners" would definitely be needed, and probably longer ones than were used on the short-winged planes.  The aluminum Just Aircraft inserts that TJay used would be the ideal ones to use.   Plywood and water don't mix well.   If your spars are .065 thick, then they should fit easily, but if they are .083 wall then you have to shave the inserts down a little like TJay did.  EDMO

Edited by EDMO

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Posted

About what frequency was the flutter?  I had a loose elevator trim tab yield about a 5 hz "thumping" that was a little hard to chase down it was so intermittent.   

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Posted

Hey Cowlove,

I would say it was a 5-7 Hz kind vibration. I did check my elevator trim tab and elevator for play....no play there. Also I did not feel the vibration in my stick, instead it felt the hole structure shake. As it stands now I suspect my engine mounts as they were way too soft and also had too much play in the bushings (homemade) where the bolt went through...

 

Thanks for your input.

P.s did you consider balancing your elevator?

 

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Posted

I had the arm come off between the electric trim servo and the trim tab once.  I definatly felt that in the stick.  Forward and back.  Got the heart beating faster for sure.  JImChuk

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Posted

Flywise,

As a former helicopter test pilot, I have had my share of similar experiences, they are the meat and substance of flight test, frankly. Can I suggest a few pointers?

1) See if you can duplicate the vibration on the ground. Have a friend shake the engine firmly in first up-down and then lateral motions at a fast frequency, close to what you felt, while you sit in the pilot seat. Have him stop the shaking and let go of the engine so it "bounces" a few more times on its own. Try to think back as to what it felt like, and concentrate as much on the extra bounces because those are the natural frequency of the fuselage that the vibration was exciting. In other words, a vibration gets the airplane shaking, but the airframe shakes at the frequency it likes, not necessarily the frequency the shake started at. Then have the friend go to the wing tip and do the same thing both vertically and at the training edge in a wing twisting manner (torsional). Be sure he shakes it fast enough to make the structure vibrate. Again look for the feeling in the cockpit that you experienced.

2) If the engine were shaken at 5 hz, I am guessing the carbs (582 with Bings?) would bounce too, and make the engine change its throttle setting a little, so the engine would cue you with an rpm change, most likely.

3) The spar is probably not involved, most bad aircraft vibes are true flutter, where the ailerons wiggle and make the wing twist torsionally, and dump lift as they down twist, then increase lift as they up twist. That makes big G's in the cockpit and also big wing stresses. I believe there is no reason not to overkill the flaperon weights, and make them too heavy as a safety/troubleshooting fix. Heavy weights make the flaperons want to not rotate, so they would make lateral controls a bit heavy. 

4) Because you got there already, it might be a good idea to make the next flight at higher altitude, with a parachute. Gulp. 

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Posted

Hey Nlappos, thanks for your advice. I will try to "shake" the thing....In regards to engine (rotax 912) rpm drop, everything went so fast I did not notice any rpm drop "during" the event...also, first thing I did was to chop power.

In the meantime got to take the old mounts off and am sending the picture of the oversize bushing in the mount.....I think it is very probable the softness and the bushings could have induced an engine oscillation/vibration I felt through the airframe.

New mounts are shown as an example (barry mount series 22002)

Anyone have a shute???

P1100121.JPG

P1100123.JPG

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Posted

That's alotta slop! :o

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Posted

Here to follow up my work on this flutter problem....

1. replaced engine mounts

2. added ~125Gr on my flaperon balance weights (rh +lh) ....suspect they were balanced before paint.

3. found some play (about 1/4in) between both flaperon in the mixer unit.

I can't find anything else and I will go out fly the thing next...will follow up on the result

 

P1100126.JPG

P1100129.JPG

P1100131.JPG

P1100147.jpg

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Posted

One thing you got going for ya, I would feel alot better about flutter over open water than over houses and concrete

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Posted (edited)

Has anyone ever weighed the standard flaperon weights, and how much do they point the leading edge of the flaperon down when correct?   I don't see as many rivets in the photo above as are on Kitfox weights?   EDMO

Edited by EDMO

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Posted

when i built mine think they were around 2 lbs maybe a bit less, and the tail end can be 1/4 high if i remember right

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Posted

Hey guys, I finally went out for a 2 Hr flight........yessssss all is good now, no more flutter and flaperon control feels much tighter/precise.

To sum it up :  got rid of all the play in the flaperon mixer unit, flaperon balance weight where tail heavy (added weight to get them balanced) and my engine mounts were shot (replaced).

 

Feels good to be back in the air :)

 

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Posted (edited)

Hey guys, I finally went out for a 2 Hr flight........yessssss all is good now, no more flutter and flaperon control feels much tighter/precise.

To sum it up :  got rid of all the play in the flaperon mixer unit, flaperon balance weight where tail heavy (added weight to get them balanced) and my engine mounts were shot (replaced).

 

Feels good to be back in the air :)

 

Glad to hear that - ice should be about melted there by now - at least it is melting in Antarctica - 103F here today - Donno what that is in C, but it is HOT!   Hope you have great flying summer.   :BC:  EDMO

Edited by EDMO

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Posted

Nice work, flywise! And an important thread for us to use in the future.

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Posted

May I ask where you got the new motor mounts? I had my cut at a local rubber shop, after a few hours in the engine they got a little slack in them, so I when back and had some thinner rubber to tighten them up. I would like to have some higher quality mounts. Thanks!

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Posted (edited)

May I ask where you got the new motor mounts? I had my cut at a local rubber shop, after a few hours in the engine they got a little slack in them, so I when back and had some thinner rubber to tighten them up. I would like to have some higher quality mounts. Thanks!

Donno how it would apply to your 2-smokes, but the bushings on my Soob are red - guessing Polyurethane, or Polyethaline - like the aftermarket bushings commonly sold at auto parts stores for trucks.   EDMO

Edited by EDMO

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