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Performance without flap fairing



I was installing flap fairings today. The ones on the top trailing edge of the wing, above the flap. And I got to wondering if anyone has tested with and without the fairing installed and come up with any difference.

The reason I ask is because some folks believe the flap and aileron need some airflow over the top of the control surface. Removing this fairing would certainly increase flow over the leading edge and top of the flap.
flap gap seals

I have lots of experiance with exactly what you are inquiring about. Rebuilt my Cub a few years ago, new everything; fus, tail, gear, and wings. In haste to get back in the air I didn't take the time to put my new flap gap seals on ( couldn't spare the extra hour). She flew just fine after a few tweaks to the rigging. Never gave them gap seals another thought until I made it to New Holstien this past summer when I noticed my -18 was the only one there without them. After several questions & theories were discussed the only logical thing to do was to try someone elses bird as well as let them try mine. Then compare flight caracteristics. The conclusion I came to is that as long as you don't use more than 20 degrees of flaps not having seals is fine. The air flowing thru the gap over the flap lets it work as its own airfoil, creating lift. However in the hands of some of the more experienced guys that flew it we found that when you pull 30 or 40 degrees of flaps the higher angle of attack of the flap itself would cause it to stall, lose lift. Also this turbulent air flow coming off the flap would raise heck with the elevator authority. Needless to say I was scrounging the hanger when I got home looking for them gap seals. On a very nice day in August I did several T.O.s & landings, then put the gap seals on & did the same. Night & day difference when using 30 degrees and full flaps. I would not think twice about it, put 'em on !!! Maybe a few of the guys that flew mine will chime in (Wilbur, Bill R., Lance, SJ) Does it make any sense or have I lost it again ??? :crazyeyes:
Nope, It makes sense. Your airplane flew great in all aspects as soon as I got in it.......jumped off the ground, good climb out, handled great ...... when I tight turned final and pulled all the flaps....couldn't put a finger on it because I'd never had one act that way, but it didn't feel right, slow, with full flaps. I wouldn't have had any idea what it was, if you hadn't mentioned the gap seals. Wish I had asked to fly it at the Fall Colors shin dig, to see the difference.
I was just talking with someone the other day, as to whether there would be any advantage in trying to seal things up around the other control surfaces.
I know we've seen the different fabrics and tapes people have used to seal up the fast or aerobatic airplanes.
If a fellow felt a night and day difference putting the flap gap seals on, it makes me wonder if there would be an improvement by tightening up the tolerance of all cub flap gap seals.
Jerry Burr, any ideas as to the effect of gap seals on airplane handling, when they are installed on all the surfaces?
Yep, well, there's a lot to be said for getting a good warning........ I'm not one to experiment, when I could be flying :) I just always wondered.
Seals and things.

T.J. is right about the fabric seals not being good for your Cub. Only they don't lower the stalling speed, the raise it 1 to 3 mph depending on flap deployment. Which is probably what he was feeling. On the flaps the stock Piper seal turned out to be the best. But in a different manner than mentioned above. Up to 1/2 flaps the seal works by improving flow over the top of the flap. From 1/2 to full flaps their function changes to air dam to impede airflow through the gap. At full flaps the flap is fully stalled and it's primary function is to provide an air dam to improve circulation around the wing. A piece of flat stock attatched to the top of the false spar will improve the function of the ailerons at low speed. I don't remember for sure, but I think it was 1 1/4" beyond the rear of the top of the false spar. This is a mandatory addition if you have droop ailerons. If the Top Cub kit would of had them I believe it would have had wider acceptance. As it was the ailerons were very weak. :) Jerry.
I had the same experience as you guys without the flap-gap seal/fairing.....flew fine, but something not right at high AOA.

Jerry, I haven't seen the "piece of flat stock" ahead of the ailerons at the false spar. Can you describe it some more.

Also, about the fabric gap seals that TJ brought up.....I had heard they are quite effective for sealing the elevator hinge line for better elev. authority. Jerry, could you speak to that?
.....and for an extra beer Jerry.......your thoughts on aileron gap seals identical to the stock flap gap seals?
Wouldn't aileron gap seals limit the upward travel of the aileron? Or are you thinking in terms of a hinged seal that would deflect upward when the aileron is deflected up?
gap seals

Hi Dave. I probably used the wrong name. Just a 2 1/4" inch piece of .020 4' long screwed to the top of the false spar. I used old leading edge material. It should hang out over the aileron 1 1/4" or so. Mine is not handy to measure so don't know exactly. Should leave a 3/8" slot between it and the aileron in both the up and down position. The elevator seal works fine. Everyone has a different way of doing it. Try to do it so it won't collect dirt. The only drawback that I have noticed is that in rough weather it will feed forces from the elevator back up into the stick. The additional authority works both ways. Stick to elevator and elevator to stick. With the VG's on the bottom of the stab and the gap seals I can usually get a clean stall break where it used to mush. That of course helps the three point. The (fabric) seals that I said raise the stall speed are the flap aileron seals.
Cubdrvr. I haven't tested the stock seals on the ailerons. I didn't have that much material, I didn't want to pull the ailerons to put them on and they are heavier than a flat piece. I did have to cut holes in my seals where there were large screws or stuff on the top of the false spar that wouldn't let them lay smooth. :) Jerry.
Thanks Jerry!

It's always great to hear from your tested and tried knowledge base.

On the elevator seals and keeping dirt out: do you mean to lay the fabric so it goes from the top of the stab down and under to the bottom of the elev. because the other way would catch dirt coming up from underneath the stab.? I'm pretty sure I think I know what I think you mean. :)


Hi Dave. I usually don't care for the down under part as that is usually where the dirt goes. I took two pieces of finishing tape and made two seams down the center of the tape the same distance apart as the slot between the elev and stab. Sort of an H. Then doped the top of the H to the stab and ect. It's more work than sticky tape but has worked great for years. :) Jerry.
Ok, I get the H concept. I use something like this for model airplane control surface hinging.

I've owned my L21B for three years now and found myself slipping instead of using the flaps, I just thought that I liked slipping since I was use to flying champs and a J3 without flaps. Last summer at Sentimental Journey I noticed all the Super Cubs had flap seals which mine does not have. So now I wondering if not having the flap seals just made the slips more comfortable to me. Since I have not flown a supercub with them to compare I just thought it was the way they are. After reading this thread I ordered a set from Univar and am getting ready to put them on. There are no holes in the fabric so I'm assuming they were not put back on after the last covering job in 1990. I don't want to drill any more holes than I need to in the plane. I have the drawing which shows where the holes in fairings should be but I'm not sure exactly where the seals mount and there isn't a local SC around to look at. Can anyone post a few close up pictures of where and how they are attached ? And does anyone know what type and size the screws should be?

Many Thanks,

cubdrvr said:
.....and for an extra beer Jerry.......your thoughts on aileron gap seals identical to the stock flap gap seals?

At rebuild we did exactly that. We also ran a 6" tape over them (flap and aileron seals) all the way out to cover any gaps and screw heads. Looks very clean and my aileron response is very quick. Adjusted the seals as close to the flaps and ailerons as possible without rubbing during movement. A mod no one notices but really works. Crash
#4 stove head PK screws like the leading edge is screwed down with. Just uncovered a SC wing where they installed the gap seal and covered it.
Thanks for the information Crash & Steve. Does that mean they should be covered when the wing is covered or is it OK to attach them now after painting them, then screw them on over top of the fabric ? Sure sounds like a good idea to have them covered and nice clean lines over them, But I'm about 3-5 years away from recovering and would rather not open the wings up right now. How were they from the factory?

Ron, I see no problem with doing it that way. Just check the fit before covering. Does make for a nice clean installation.

Tim, All the ones I have seen came from the factory with the fairings screwed on after the wings were covered.

If I go to the airport tomorrow I will try to get some pics for you