Uncontrolled Roll on Full Flaps

[Osprey220]

Hi Guys,

I'm a new member - in the Davis/Woodland California area.

I've been helping a friend get his Kitfox Model II (Rotax 582) flying and correctly rigged - and having lots of fun flying it too.

But with the fun has come a bit of undesired sphincter puckering excitement.

A couple of weeks ago, the flapperons went into full flutter at ~75mph! Recovered quickly with pulled power and gentle up roll. Yikes.

I found a lot of slop in the remarkably complex control linkages I could tighten and remove. Additionally, the shoulder bearings on the flappers were not tight and I've fixed that. So far, no more flutter.

BUT -

On full flaps, I get a uncommanded and _almost_ uncontrollable roll to the left. Almost full right aileron controls the roll, but it does not seem like a condition I would want to approach landing in! Half flaps seems fine with no roll tendency.

Any insights or suggestions?

I did a quick search and have seen that others mention limited roll authority on full flaps, but I've seen no mention of uncommanded roll.

I look forward to your input - I really need the help.

Also - if there are folks around wanting to fly out, get in touch.

Cheers,
Owen Hughes

Attached is a picture of the plane:

[jiott]

From your photo it appears there are no mass balance weights on the flaperons; there should be two, one in midspan, the other on outboard end. These weights are extremely important to prevent flutter, and I believe there was a factory bulletin put out to require their installation on the early models that didn't have them.

The uncommanded roll I have no idea about, except it sounds like maybe a very bad rigging problem.

[avidflyer]

Maybe I'm wrong, but I believe the early Kitfox models only used one balance weight per flaperon. They certainly should have them installed to prevent flutter though. JImChuk

[Osprey220]

There is one mass balance per flapperon.

[David47]

Uncontrolled roll to the left when flaps are deployed: I'd be checking for correct part numbers used in the flap/aileron control circuit first of all and if all looks ok, do a ground check to ensure the flaperon angles are the same on both sides for flaps deployed with no aileron. If that's ok, sounds like it could be a rigging issue as Jim suggests. In that case, check the rigging.

[TY2068]

http://kitfoxaircraft.com/images/servicebulletins/SB-3.pdf

[av8rps]

If it doesn' t have them, I wouldn't fly that Kitfox unless it had counterbalance weights put on it. Continuing to do so could cause catastrophic in flight airframe failure. Most of the time the flutter just tears the flaperon off the trailing edge of the wing. But what happens if its bad enough to tear off part of the wing with it :eek: I commend you for tightening up all of the control mixer points, but that will not stop flutter. Balanced flaperons along with a nice tight control system is the best way to avoid flaperon flutter.

As far as rolling one direction when full flaps put in: it isn't at all uncommon to see the ailerons move a little with flap actuation, especially on Model 1 through 3 Kitfoxes. But typically all you have to do keep a reasonable grip on the stick to control that from being an issue. If your Kitfox is worse than that, until you are able to figure out the fix, stop using full flaps. The Kitfox slips so well that most wonder why they ever put flaps on it to start with? The flaps really didn't start working on a Kitfox until the Model 4 was designed. And even then they still aren't great for descent control. But all model Kitfoxes benefit from the flaperons for additional takeoff lift. So if you feel the need to use flaps for takeoff, limit that to no more than 1/2 flap. If you don't find anything wrong with the control system, I would take a hard look at how the flaperons are set in relationship to one another as well as when neutral. You can get flaperon reversal issues if they are set wrong, so take the effort to go through the flaperon setup carefully using the manual used to build the Kitfox.

Aside from the issues mentioned, I'm glad to hear you enjoying that early Kitfox. For pure flying fun the early and super light models were some of the most fun. But they were a bit quirkier than the current models, and manuals and tech info wasn't quite as refined as what we have today, so there can be a little more experimentation required to get them operating properly.

[Osprey220]

Again, the offending Kitfox does indeed have one mass balance per flaperon.

I eliminated/reduced linkage slop and tightened the shoulder bearing and so far no flutter.

I have not found clearly offending causes of the uncommanded left roll. Flaperon attachments and associated ribs are all firm with no unexpected movement when stressed. The flaperons appear to be at same angle (left compared to right) before and upon full flap deployment - and travel appears to be the same.

Owen

[TY2068]

Does the plane have a slight tendency at all to roll left in straight and level flight with no flaperon set ?

[avidflyer]

When setting the flaperons, it is helpful to clamp a stick on each flaperon bottom the same distance out from the fuselage. Have the end of each stick project back the same distance from the flaperons. 18" - 24" is fine. With the aircraft level side to side, and the joy stick centered, ends of both sticks should be at the same height. You should also have the same amount of travel up and down at the ends of each stick when the joy stick is all the way to the right, and all the way to the left. The nice thing about using the sticks is it magnifies the differences between the two sides, and gives you something to measure to. JImChuk

[ppilotmike]

On flutter, have you removed the flaperons and verified that the "mass balance" weights truly balance the flaperons. I would suspect the uncontrolled roll is due to a rigging problem, Jims post is a good way to check that.

[kitfoxjim]

As I recall many years ago, when I did the rigging on my Model 2 (90-91) there was a possibility that one of the aileron push rods could go over centre if the flap handle was not limited in travel. Went back to look at the manual rigging instructions but that section is now missing.
I would suggest looking at the aileron mixer assembly while deploying full flaps as there may be this over centre condition.
The aileron push rod could be too short but the best cure would be to limit flap handle travel.
Jim

[Guy Buchanan]

Two possibilities for uncontrolled role. Miss-rigging, which you seem to have eliminated, and un-symmetric stall. It's possible that one of your flaperons is stalling. It's also slightly possible you're getting a partial stall on one wing, though I'm not really seeing how that would happen. As for the stalled flaperon, the most likely culprit would be a bad airfoil, possibly a kink or other hard edge on the top of the leading edge somewhere. It wouldn't have to be the entire flaperon. I can't remember if the 3 flaperons were symmetric but if they weren't you'd want to check to make sure they weren't installed upside down.

[avidflyer]

The Kitfox 1 to 3 model flaperons all have flat bottoms. Also, it would be pretty near impossible to install them up side down. JImChuk

[HighWing]

I am likely revealing my age, but back in the really olden days when these puppies were being actively built, it was common practice to put an Adel Clamp on the sloping tubing next to the flap handle that limits the flap function of the flaperons to 20 degrees only. I don't remember if the uncontrolled roll was the gremlin, but I did it on my first IV and the one I fly now. Never an issue.

[LSaupe]

My Model III never had an issue (though I had the same Adel clamp at 20 degrees like he describes, also a stop at zero)). Along with High Wing's comments I did ensure max deflection of 20 degrees though (in case you havent yet looked at this). I do remember something about flapperon stall being mentioned for deflections above that value. I set my zero degree using a laser bolted to the bottom of the flapperon to ensure a certain distance below the leading edge (per build manual and subsequent service letter - forget that value now, the value was increased above the initial build manual value).

Fix the control stick to prevent side to side movement, then add in your 20 degrees and ensure both are the same (at 20 degrees).

In reality, the only time I used flaps was when I was two up (used 50% for take off and 100% for final). When flying single never used them except for short fields (would just float otherwise). I did have the enlarged elevator which helped with a 3 point with full flaps.

Please keep us posted with what you find out. Could always put some tell tails on your dropping flap to see (maybe) if your flap is stalling more compared to the other.

[TJay]

If the center mixer that controls the flaps is not level in the plane one side might drop further than the other. I would check that first also if you have one flap bearing on the wing that needs oil it may cause the stick to move when pulling on the flaps. But most of all you keep the plane level with your feet not the stick. If it starts to roll step on the pedal dont be shoving the stick around that will just make it worse ( adverse Yaw):)

[LSaupe]

In addition to my previous, there is also a criteria for max flap extension whereby you set your stick to full deflection to left or right and pull flaps on until the stick starts to move towards center. That becomes your new max flap extension, which might be less than 20 degrees. 20 degrees or stick motion being the limiting factor.

[TY2068]

I read were some guys with early models would tweak the trailing edges of the Flaperons (one up, one down) to help with wing drop. I guess doing that was a last resort when nothing else helped. I'd also guess some of these planes were slapped together and not assembled correctly.