My wings began flapping - scary - what was it?

[wildirishtime]

So here's the scenario, looking for feedback on what
I experienced:

Model 3, 90hours total time, I have 50 of those since I purchased it.
9am in Oregon
winds @ 800ft AGL: 10-15mph from the north.
Direction flying: South
Airspeed Indicated: 75mph
Thermals: Moderate to Heavy, it's a 100degree day now.
Engine: 582
RPM: 5000ish
Load: 950lbs gross

While minding my own business :) the wings suddenly
began oscillating up and down, both sides it appeared. Lasted
3-4 seconds. I was about a mile south of some rolling hills.

I pulled the power to 4000, pulled the nose back a touch to drop
it below 70mph... and waited. It stopped. Full inpsection shows
NOTHING loose etc... remainder of flight was normal (but quite
gusty, bumpy, and not much fun due to weather).

So please avoid wild speculations (I can do that on my own haha!)
if you've had this happen or KNOW what this was from experience
preferably in a model3 please let me know... just wanting to double
check this isn't a known issue of any kind and was truely just the
airflow/thermals that prevailed.

Thanks in advance!
~Wild

[cap01]

i have the long wings on my model IV and they will move up and down at the tips when i get into choppy air . it is a little unnerving at first , but you get used to it . ive been in some pretty rough air and they have really been flapping but nothing has happened .

[catz631]

Chuck,
I venture to say this is normal movement in the wing. My model 4 wings do that also. I was a little supprised in the beginning when I saw that much movement but it dosn't bother me at all now. Your eyes are almost level with the bottom of the wing and any flex in the wing you will see right away. The wings are just a couple of aluminum pipes with non rigid covering (unlike a metal aircraft) so really they should be expected to flex. Bottom line is there have been no inflight structural breakups of the Kitfox (see new Kitplanes mag) so I think you have nothing to worry about.
Flutter,on the other hand,is much more violent and a whole different ball game but that's not what it sounds like from your discription.
Talk about wing flex the B-737 I used to fly would flap its wings all over the place in rough air !
Dick

[Mnflyer]

HI dido on the previous posts, as Dick posted the main spars a alum tubes very strong and flexible, I've seen the flex on my plane also does get your attention the first few times.

[wildirishtime]

Well I'm not sure where "flex" ends and "flutter" begins. I've seen the wings flex a little, and that's ok.... but it was vibrating my control stick in my hand pretty hard - had to hold on pretty tight - It was the wings, not the flaperons.

It was harsh enough I thought I had a prop blade change pitch.... does that still match with 'flex' or would someone call that 'flutter'? and if it's flutter, was that still acceptible behavior for the model 3?

[cap01]

i have never felt anything in the stick that felt like a flutter so im just seeing flexing . they can be a wild ride in rough air .

[Dorsal]

An engineering response (not based on my limited experience as a KF pilot)
IF the motion was up and down as though someone had grabbed the plane by the wingtips and shook it (vs porpoising) and IF it was symmetric (not much roll involved) then I expect the following.
All mechanical systems where flex is involved have a natural response such that if you could grab the plane by the tips there would be some frequency that you could shake it at to excite that response. IF the thermals you where experiencing where the sharp staccato type then it is possible that you hit a set at just the right speed to excite this behavior. If this was the case then slowing down, speeding up or just getting out of that set of bumps would cause it to stop.
There is allot of hand waving and assumptions in this theory and I may be way off the mark so take it for what its worth.
Thoughts from others who may know better?

[catz631]

WildIrish,
You "might" have something else other than "flapping". Is there anyone in your area that has a Kitfox you could check with ? How about giving John McBean a call to see if anyone has had that problem before. He might have some ideas of locations to check on the wing for loosness.
IF it is flutter, I would expect the wing to be moving much like a "venetian blind "in the wind rather then just up and down . In any case the normal flapping is not a violent action and I don't think you would feel it too much in the stick.
Flutter is normally caused by a control surface that is loose (such as a trim tab,aileron,etc) causing the main surface to move.
If it was my plane I would check every nut and bolt at the wing attachment,strut,flaperon,etc.
It is tough to analyze tihs without being in the aircraft. In any case,have it checked out. I still think it is "flapping" but better safe than sorry.
Just my opinion.
Dick

[Andrew G]

As a potential Kitfox owner... I'd like to know how the plane handles in rough air? On windy days, one gets bounced around pretty well in a 152 (my trainer)... but the 152 has 200-300 lbs on the later model Kitfoxes.

I don't mean to hijack the thread, but perhaps it's all related.

Thank you in advance.

Andrew

[catz631]

Andrew,
Does the term "cork" for the Kitfox strike a note?
Dick

[HighWing]

My thought is that if you felt it in the stick and it took a tight grip to control it it is likely flutter. As others have suggested, check all control connections, security of the mass balance weights including the flaperon wing attachment fittings. I have flown lots of hours in some pretty rough mountain conditions, and I don't recall any wing movement that affected the stick.

Regarding the Kitfox and stability in rough air. I think it relates to wing loading. The Kitfox can get pretty bouncy as they are light, especially the earlier models. keep in mind, though, there have been no reports of inflight structural failure. That is what keeps me going.

Lowell

[Andrew G]

haha... yeah re the cork, I hear you...

The wingloading comment makes a ton of sense, bottom line, no burritos on a windy day...

[Newkid]

Our first Kitfox was a Model 1 with the same wing/flapeon setup you should have. The kit provided no mass balances on the flaperons and it flew fine, even at VNE. Later my dad put mass balances on it because they were supposed help prevent flutter. Well toward VNE (upper 90s) the flapperons, with the mass balances, would flutter. They were originally placed towards the outside tips of the flaperons, he moved them more towards the middle and never had flutter again. In the IVs on up the flapperons are symmetrical, and I have never heard of a flutter problem with them.

[bbryan]

Check out this YouTube link. If this looks like yours you may also want to check your pants!
http://www.youtube.com/watch?v=kQI3AWpTWhM

[carlisle]

Chuck...Sounds scary. Never experienced anything like that in my model 2. If you felt it in the stick, I wonder if the flaperons were the culprit and the wing flexing you describe wasn't a result of that rather than the primary problem. Do you have counterweights on the flaperons?

Chris

[dholly]

As a KF-III owner I should know this, but, I believe my spar insert is a 3' or 4' extruded aluminum 'I' Beam. Compared to other twin spar tube wings --and my old Avid Mk-IV comes to mind with flat 18" long plywood inserts-- I would have thought any wing "flapping" would be minimal and visible primarily out at the tips a couple/few feet beyond the lift strut attach. For KF-III owners who have experienced 'flapping' on occasion, for reference just how much flex are we talking about here? The comment about outboard flaperon weight position did catch my attention, interesting to note that the Avid weights were toward the middle and I don't recall too many (any?) flutter posts.

[wildirishtime]

I did find some looseness and something that concerns me in the Aileron swivel area near the stick, I started a new post if anyone can help me by sending photos that would be great - please find the link to that new thread HERE (http://www.teamkitfox.com/Forums/showthread.php?t=1730): http://www.teamkitfox.com/Forums/showthread.php?t=1730

Thanks to anyone who can help answer my questions on that thread!
~Wild

[Micro Mong Bldr]

Hi Folks,
I got into some very strange flapping in my 'Fox Model 2 - but at low speed, about 50 MPH, I changed RPM, put a bit of aileron load on the stick & it quit, not sure the cause. It was only outboard of the outer wing strut attach on both sides. It has an HKS and it was passing through an RPM range where the engine's a bit rough a bit rough, but I wouldn't expect that to be anywhere near low enough frequency to cause what I saw. Thoughts?
Larry

[SkyPirate]

curious...

was it a twisting action?

have you towed your plane without the wing supports in place?

[rogerh12]

I had the Zenith 601HDS, during the time the new 601XL’s were breaking the wings off in flight, so I saw the whole thing develop on the builders group. Talk about your believers and disbelievers going to war, it got real ugly. Eventually, a bunch of builders got together and had a German company analyze the design, out of their own pocket mind you, to try and find a cause.

It took a solid year for Zenith to do anything about it, and only after the NTSB came in urged the FAA to ground all the xl’s. Zenith just stuck their head in the sand and said nothing was wrong, meanwhile wings came off. Well, to the point, for every wing that broke, 10 others fluttered, some violently, some not so much. Often times, this happened in the pattern, so it wasn’t all about speed, like you might think.

So what caused the flutter? Zenith still says they don’t flutter, even though Zenith now sell a wing fix (a fix for a problem that does not exist?), so for the explanation, you have to go to others who were involved in the investigation. The issue is, Zenith changed from a thick bulky wing, used on my HDS to a very thin wing, used on the XL, and also changed the attachment points, which made the wing panels longer, but reduced the amount of material they attached to (so the plane would go faster). Finally, like on my HDS, the ailerons were not mass balanced, but unlike on my HDS, the XL ailerons were connected by cables, cables that could get loose. So, here is what would happen, loose aileron cables in conjunction with unbalanced ailerons would allow the ailerons to start oscillating. The thin wing allowed the oscillations to couple to the wing, and because it was thin it wouldn’t dampen out the oscillations, and under the right conditions, the wings would flutter (conditions which were never specifically determined). Zeniths “fix” add metal to the wing and attachment points, mass balanced the ailerons and added requirements to maintaining aileron cable tension. These fixes seem to have worked, as wings stopped coming off.

So back to the Kitfox issue, I suggest rebalanceing the ailerons and removing any slop from the aileron controls, but that is just my two cents worth. and an aluminum low wing planes problems might have little to do with a tube & fabric high wing (but it couldn’t hurt).
Roger

[SkyPirate]

2 completely different construction formats,..the zenith also used the skin for the aileron hinge,..their fix was to make the top and bottom of the spar stronger,..and beefed up the carry thru's

[SkyPirate]

Im curious if the wing was twisting or just rapidly flexing up and down

[SkyPirate]

an example of how much a wing with round spars can flex,..the zenith could never with stand this

http://www.teamkitfox.com/Forums/picture.php?pictureid=1197&albumid=72&dl=1374100964&thumb=1 (http://www.teamkitfox.com/Forums/album.php?albumid=72&pictureid=1197)

[Av8r3400]

In 30 years of history, with thousands of aircraft flying, there has never been an in flight breakup of an Avid or Kitfox.

There are some pretty shoddy built airplanes out there. (Honest truth) Not one has failed. That says a lot about the design. Even built poorly they are still a safe aircraft.



This is one of the many reasons I chose to join the family.

[SkyPirate]

amen......

[rogerh12]

I too think that if there was a flaw in the kitfox design that allowed a flutter to occur, it would have been found out years ago. So, if it did flutter, something must be "wrong" or loose, or worn out. But one thing I do find interesting, sadly, is that when the first Zenith XL guy reported that he was "scared to death" seeing his wings flutter, the response from most of the others on the forum was: No it didn't, you don't know what flutter is, your a bad pilot, your an idiot,,,, it just got worse from there. Then another guy reported the same problem, and got the same response, and again, and then the wings started breaking off.....

We, as a group, need to take any report of a possible flutter seriously, and not point a finger back at the pilot, or poohoo his reports. I don't want to see this happen again. Not on the Kitfox too.
Roger

[Micro Mong Bldr]

Hi Folks,
I haven't owned the 'Fox very long, previous owner said he'd never seen any flutter, I've only flown it so far on "perfect" days - as flexy as the outer wing panels are, maybe it was my first encounter with its normal response to bumps?
Cheers,
Larry

[Dave F]

So here's the scenario, looking for feedback on what
I experienced:

Model 3, 90hours total time, I have 50 of those since I purchased it.
9am in Oregon
winds @ 800ft AGL: 10-15mph from the north.
Direction flying: South
Airspeed Indicated: 75mph
Thermals: Moderate to Heavy, it's a 100degree day now.
Engine: 582
RPM: 5000ish
Load: 950lbs gross

While minding my own business :) the wings suddenly
began oscillating up and down, both sides it appeared. Lasted
3-4 seconds. I was about a mile south of some rolling hills.

I pulled the power to 4000, pulled the nose back a touch to drop
it below 70mph... and waited. It stopped. Full inpsection shows
NOTHING loose etc... remainder of flight was normal (but quite
gusty, bumpy, and not much fun due to weather).

So please avoid wild speculations (I can do that on my own haha!)
if you've had this happen or KNOW what this was from experience
preferably in a model3 please let me know... just wanting to double
check this isn't a known issue of any kind and was truely just the
airflow/thermals that prevailed.

Thanks in advance!
~Wild

>>... just wanting to double
check this isn't a known issue of any kind and was truely just the
airflow/thermals that prevailed.<<


I am going to agree that is sounds like wing flexing .

Cheers

[ofergd]

Hi
I have a KF-II and I had the same experience. It only happens in rough air. It is caused by free play in the aileron control system. In my case it was caused by exra play in the plasic bearing between the turtledeck and the body at the wing root. Eliminating the free play in the control system stoped the vibrations I experienced.

[Peteohms]

I have a Kitfox 3 and have about 660 hours flying it. I have never experienced any wing flutter. I cruise about 90 mph.

[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.

[Paul Z]

Pete, I am glad you made it back to Leander! I missed you at Oshkosh. I walked and walked and walked, by Thursday this Old Man had had enough!

[Peteohms]

Paul, I stopped by, but you were out. I loved your plane. I want one when I grow up.

[ofergd]

If you think the vibrations come at certain RPM, check you engine mount.
Sometimes it is hard to find the source of vibrations.

[Paul Z]

I walked by your plane. Sorry I missed you. I will probably be flying to San Antonio to see my sister soon, maybe we can get together on my trip down to Dirty SA.
Paul, I stopped by, but you were out. I loved your plane. I want one when I grow up.

[Micro Mong Bldr]

I will try the half flap trick. After some thought, I'm going to also try running the engine at various RPMs on the ground and see if I can find a range that excites some movement. With the damping from sitting on the gear it shouldn't shake as much but I think I'd observe something. If there is a consistent RPM range, I'll put a yellow arc on the tach.

[Micro Mong Bldr]

And I will check the motor mount, fasteners etc again. Maybe the rubber bushings have gone soft. Thanks for all the advice, folks.

[Micro Mong Bldr]

Haven't found any problems w engine mount, etc, fixed all looseness in the control system - but I think I have a harmonic with the engine/prop at about 3600 - 3800 RPM. Ground runs in that range cause some wing tip movement. The wiggle looks a lot less severe at 0 feet than 3000! :) I'm sure that's all perception on my part. My changing RPM and airspeed stopping it makes sense, low airspeed occurrence is when the RPM would be that low, and all I really needed to do was move the throttle. Looks like I need a yellow line on the tach. I'm running an HKS with the lower reduction ratio (2.47:1, I think) and an IVO 2 blade. I don't think that's a common combo, so that's probably why others haven't seen it..

Cheers,

Larry