My wings began flapping - scary - what was it?

[ofergd]

This is not the textbook wing flutter. It is initiated by bumpy conditions, and when the wing flexes upwards the flapperon hinge moves up which causes the the flapperon actuation leaver to move down and the flapperon moves up and reduces the lift on the wing causing the wing to flex down and the same thig happens in the oposite direction and so-on. If you have excesive play in the the control linkage or in the flapperon hinge support bearing (the flat plastic support under the hinge at the wingroot and the top part in the turtledeck).
I had a similar experience, I had eliminated all the freeplay at all other joints and only when I replaced the plastic bearing it was eliminated.

[jdmcbean]

Flutter is a VERY serious issue and should never be discounted.

Do not confuse flutter with the wing flex. The Kitfox wing does flex.. so does a 787 wing.

The model 1 through 3 controls are very different than the Model IV through current.

Prior to your next flight follow the suggestions previously stated and start with all the control connections for correctness and tightness. You should not have any that are loose especially in the Rod-Ends.

Sounds like the wings were flexing with the thermal activity and this was the first experience for you.

[Micro Mong Bldr]

Well, I did find one slightly loose joint in the control system, bearings at the inboard ends of the flaperons look fine. Really don't know what else to try. Thanks for all the help. Maybe I'm just not used to the flexibility of the outer wing panels.
Cheers,
Larry

[Micro Mong Bldr]

Took it up again yesterday - got some flapping on climb out, again on final. Both times it quit with just a bit of aileron input. The outer wing panels on these things seem quite flexy - when you hit a bump do they tend to go through a number of cycles before damping out? Am I just over reacting to something normal? I'd guess frequency about 1/2 second, not a whole lot of movement maybe 4 - 6 inches...

The only thing "different" I've done is lube all the control pivot & bushings points with synthetic oil, which tends to be slicker, so when I got down I put WD-40 on all the aileron bushings, as that dries to something a bit thicker, "greasier". Suggestions?

Cheers,

Larry

[Peteohms]

Maybe some of your wing ribs came loose from the spars.

[Micro Mong Bldr]

Ribs loose - hmmm - it seems to be straight up and down, not a twist, I think.....not 100% sure. If I grab ahold and wriggle the wings U/D & all at the tip during preflight, I don't notice any looseness anywhere, maybe I need to try and wriggle each rib U/D at the trailing edge?

Thanks,

Larry

[Micro Mong Bldr]

Well, I've worked on all the linkages & removed play, new tight fitting turtledeck bushings - I still get it. Not the classic high speed flutter - slow & with lower RPMs - got it on short final only about 10' off the ground, seems to be correlated with low 3000's RPM. It has a HKS, 2 blade IVO, the lower reduction ratio (2.47:1?). Thoughts?

Thanks,

Larry

[Peteohms]

Have you tried flying with about half flap setting?

[jrevens]

I wish you could take a video of it, so that we could see what you're experiencing. The more experienced guys on this forum could probably tell you if it's unusual & a real concern if they could actually see it.

[Peteohms]

Have you tried flying with about half flap setting?

The reason I asked is because with more than 700 hours flying my Kitfox III, on my Oshkosh trip, I had the same thing happen to me. I was approaching Drake Field in Fayettvile, AR doing between 85 and 90 mph, when my stick started shaking. I immediately pulled the throttle and raised the nose and at some point the shaking stopped. After landing I nearly had to clean my shorts.

On the rest of the trip I tried to keep my speed to 85 or less so as to reduce my chance of a repeat. A couple times when I hit turbulence I felt maybe a one cycle shake, or maybe it was just in my head.

On my way home, it occurred to me that if I carried some flap I might change the aerodynamics enough to stop the flutter. So for the last couple hours I flew with about half the flaps.

A couple days ago I had the chance to check the flap angle per the manual and discovered that I had been flying my plane all these years (since 2008) with reflexed flaps. I plan to go through all the linkage and check everything else carefully. Hopefully I have found the problem at least with my Kitfox.