Dihedral Question

[Robin G]

My mod 3 wants to roll to starboard. We have checked and rechecked the rigging and the only thing we can find is a difference in the dihedral of the starboard and port wings. The port wing has an angle of 2.2 degrees and the stbd is 1.5 degrees. In the opinion of anyone more informed than myself, is that sufficient to induce the roll? Also does anybody with a 3 know the recommended dihedral for the wings. By the way, the washout is almost identical on both wings.

Thanks in advance for any input.

robin g

[SkyPirate]

generally if your dehidral is more on one wing the plane will try to center itself raising the lower wing if the wash out is the same unless your holding the stick in the center position then yes it will roll towards starboard,..I'm not sure on the right dehidral angle ,..but they should both be the same regardless ,.. I'd say no more then 2 on each wing which would give you 4 degrees total,..less dehidral ,..less natural stability,.more dehidral ..more natural stability...

I'd level my two front wing mounts left to right on the fuselage by jacking and supporting the airframe with tail level too ,,then set the wing dehidral,..doing it while setting on tires alone can be misleading sometimes,..only 1/2 a lb difference of air in the tires can change the planes stance.

then I'd check the flaperons to see if they are nuetral ,,meaning at the same angle of attack with stick in center position
hope this helps

Chase

[SkyPirate]

if the wash out is correct on both wing's, lengthening the aft strut connect point alone on one side will change the wash out making the less wash out angled wing ( the opposite of the one that was changed) stall first when approaching a stall which could induce a dropped wing at first ( the lesser wash out angled wing) then snapping back the other way into a spin,..the advantage of flaperons will help prevent this to a point if counter inputs are applied prior to fully stalled atitude,.. if the stall isn't recovered before that point ..it will snap roll to the opposite side.

Keep the wash out the same on both wings.

Chase

[SkyPirate]

Robin ..just curious ..but is your wing sewn or just glued,..if it is just glued ..the wing that is raising ..have you checked to see if there is seperation from the fabric to the ribs? ,.in flight it will increase the camber creating more lift in turn raising the wing that has fabric seperation,..
not being able to see the wing ,..I'm just trying to think of all things that might cause your problem.

Chase

[Robin G]

Thanks everybody. The wings are stitched. This weekend I will push up the low wing from 1.5 degrees to 2.0 and pull down the high wing from 2.4 to 2.0. I suppose those long heavy cambered wings could be reacting to the difference.
I have fooled around with the washout and nothing has had any impact. So the dihedral might be the issue.

rg

[t j]

Robin, just a heads up. Be sure to check the witness hole in the strut rod ends when you push up that low wing. I don't have the measurements on a model 3 to calculate how much that 0.5 degree will screw the rod ends out but seems it may be quite a bit and not have much holding after the adjustment.

For what its worth my classic 4 has a slow roll to the right if I fly it hands and feet off. It takes a nat's eyebrow of pressure on the left rudder to fly straight and level hands off. I attribute the right roll to spiraling slip streem from the left hand prop. If you tried adjusting the washout with no success A fixed trim tab on the rudder may be an easy fix if you run out of adjustment on the wing struts.

A small wedge of wood glued to the top of the trailing edge of the flaperon has been used to correct a heavy wing too. Paint it to match and no one will notice. You can duck tape one on to test for size needed.

[JimS]

After trying playing with the wings and even the tail of my KF-4, I tried a trick that I read about in an old RV-ator I had. Seems that by squeezing the trailing edges of the RV series ailerons you can cure a rolling tendency. ( I forget which aileron you squeezed to accomplish what you wanted, the heavy or the light wing) Anyway, I started tweeking the trailing edges of my flapperons (as if the trailing edge was a trim tab) and it doesn't take much to take out a rolling tendency. Just apply pressure to the trailing edge all along the length of the flapperon with your thumbs. Not enough to even see. If doing one side is not enough / Do the other and fly in between. It's great to be able to fly along with just the occasional nudge from your feet to keep you on course.
JimS

[HighWing]

I've been on Kitfox forums for over 15 years and Jim's post proves there is occasionally something new under the sun. Thanks, Jim, I'll remember this tip when I begin test flying my new Mod IV. Regarding dihedral. In the early Kitfoxes, up to the late Model IV it was in the manual, that after temporarily setting the dihedral with the hose clamps at the lift strut to spar attachment brackets, the wings were to be folded to check for the alignment of the wing tips in the folded position and adjust the location of the strut attach brackets to make the wing tips level. The thought here was to make sure the airplane looked pretty with the wings folded as well as ready to fly. Typically there was enough warpage in the jigs while welding that the tips could vary by up to six inches. Further, it was thought that the fuselage would just hang below the wings realigning the dihedral. This might explain why there is a tad of dihedral difference side to side.

Also, a significant reason for the need for a bit of rudder in cruise - right rudder for a 912 and left for a counter rotating prop is because there is no thrust offset -vertical or horizontal in either engine's mount. I found that a simple trim tab on the rudder greatly simplified flying especially in the pattern.

Lowell

[SkyPirate]

C5 ..I was talking aerodynamically,.. the kitfox's wing design,..luckily gives you a truck load of forgiveness,..not so in all wing designs,..
I'd prefer to get the dehidral and washout right first ,..then if tweaking needs to be done,..so be it ,..
don't get complacent with what you "think" will happen ..be ready for the worst.

Chase

[JimS]

Lowell,
Your right about the vertical stab not being offset on the KF 4 on account of the plane being able to be equiped with a 582 or a 912. When I swapped out my 582 to the 912 the trim tab came off one side of the rudder and went on the other side. ( for those that don't know, the engines turn the prop opposite directions)

JimS

[SkyPirate]

I don't believe any of the model's of kitfoxs have an off set on the vertical stab. unless someone modified it after it left the factory.

Chase

[Robin G]

OK, Boys, here's the latest on my heavy wing 3. We brought the high wing down to the same degree as the lower wing. both are 1.8 -1.9 degrees. The washout slightly favors a correction to the problem and the sweep is fine. I flew it this morning and the problem has not been rectified in the least. So I'm thinking that if the wings are good then it must be something influencing the wings, ... the flaperons. Would the recommendations be to adjust the flaperons of use a fixed trim tab? Or any other suggestions.....?

thanks, R

[HighWing]

For what it's worth, this is what I would do first if this was my airplane. I'd adjust the washout to see what effect that has. It just seems to me that if flaperon input is needed to correct a roll tendency, there is some fundamental wing issues creating that tendency. Keep in mind that the jigging for the early Kitfoxes was not the massive box jigs that were made for the V (when certification was being considered) and later and welding warpage is real. I seriously doubt that every fuselage is identical in measurement - the problem might just be the angle of attack at the wing root. To have the dihedral the same and the washout the same and still a roll tendency, there still has to be a wing issue. The wing would be my focus unless, as was mentioned previously, you find yourself at the limits of the rod end fittings, then a trim tab or as was mentioned tweaking the trailing edge.

We put a cockpit adjustable tab on the Lancair ailerons, but this was to compensate for wing rigging - not correctable after assembly - and differential fuel burn as all fuel is in the wings. In other words it was at first a patch, then a convenience.

Just an opinion.

Lowell

[SkyPirate]

good point Lowell ,..if it is the root of the wing ..if the wing isn't often folded ..either front of rear connect point could be modified to correct the AOA,.prefered the rear because it stays bolted regardless if the wing is folded or not ..
you could grind off a little top or bottom of the connect point to correct the AOA then put a shim on the opposite side ..but you might have to make a new inner spar tab due to the bolt hole that goes thru the spar and connect point might end up being elongated

[Robin G]

Here's the latest on my heavy winged 3. I put 2 turns in the washout with no apparent results, still wanted to roll smartly to the right. Then I taped a trim tab on the heavy side flaperon to push it down and the wing up. Again, little to no results. So I moved the trim tab to the underside of the other flaperon and "bingo", there was a notable improvement. It was not a complete fix so I added another couple of inches of tab which added even more correction. I need to adjust the location and size of the tabs as they are adding some drag, but the machine will finally fly straight.

Thanks to you all for your input.

robin g

[JimS]

Robin,
Right wing heavy, tweak the trailing edge of the RIGHT flaperon UP and the trailing edge of the LEFT flaperon DOWN. Like I mentioned earlier, work your way along the trailing edges using just hand (thumb?) pressure and push just hard enough to feel the aluminum give a bit. Probably not even enough to really see. A little bit goes a long way. Try..and...fly......try...and...fly ...you get the picture?

Good luck!
JimS

[jdmcbean]

I have not been watching the threads very closely so this may have already been mentioned.

There are several issues that can cause roll (heavy wing) one being wing rigging and another being rudder. How is the yaw in level cruise flight ? if you were to hold the wings level and take your feet off the pedals then switch hold the yaw straight and release the controls. One will affect the other. The trick can be to determine which one and how much.

[HighWing]

John,

You make an excellent point. There was an article on dihedral it's effects on rudder input and roll in the Wind Tunnel column in Sport Pilot a month or two ago. Definitely worth the read. It cleared up some misconceptions for me.

Lowell