Why is my Flair J3 Cub Nasty?
 

I bought out a guy's hobby and got a Flair Cub out of the deal. I put an ASP 46 on it and the CG is 85 cm back from the leading edge per the plan on the 290 cm chord wing. (29% back) The fuse does have slight twist to it but it doesn't seem that significant. Problem with the plane is that it snap rolls something fierce. If I'm not careful, it is easy to get a secondary snap on the way down when recovering. I've checked the wing and it is quite true. It is actually a fun plane to fly, but a J3 should not have this kind of nasty attitude! Any thing I should check out?

I've two Cubs about that size, the first is s Goldberg Anniversary and the second a World Models, both full wings versions. The Goldberg is rock stable and has no tip stall tendencies and stalls straight forward but the World Models has the nasty tendencies you describe. The Goldberg was built with washout and has strip ailerons and the World Models has no wash out and barn door ailerons..

I was told by a man who had owned a full scale Cub, that it was common for owners who weren't concerned about a slight bit more drag, to trim the ailerons up a bit to reduce the tip stall issue and it indeed works quite well for the models as well but this would depend on whether your version has barn door ailerons, and likely the Flair version does.

Do a throttle to aileron mix where about 1/4" up aileron trim occurs at low throttle. The trim should start when throttle is reduced to 1/2-1/3. The mix simulates having wing washout and is a nice way to do things especially if flown much inverted as washout is then wash in and undesirable.

In conjunction, it is also very important on a Cub to use generous aileron differential with around twice as much up than down. This because the wings being so long, have a long lever arm for the greater drag of a down aileron, producing an undesirable yaw opposing the turn.

Last, Cubs very much enjoy a little aileron to rudder mix. By adding the two mixes and the differential, my World Models Cub got the nasty tamed out of it;... hope it will do the same for yours.

This Cub has strip ailerons, but deflecting them upward a tad did help the nastiness a bit. I compared the CG to my other ships and the Cub is the most forward at 29%, my others being between 35% and 39% back. The Cub has the highest wing loading however, and I am convinced it is the twist built into the fuse. I can slip it fairly slow with the nose to the right, but slip to the left and it doesn't hesitate to roll over on its back. Building straight and building light are indeed important in this hobby!

Tovio isn't it fun having a bug in the plane, it's a good way to learn how to use the radio to tame it down some. However a straight build does eliminate some of these problems from the start. Glad you had enough altitude to save it.

Leroy

Yep, that is true! I've gotten it to the flyable point by reducing the control throws quite a bit, and it is turning out to be one of my most fun planes to fly. Vertical snaps are outrageous as it ultimately ends up trading ends and i have to wait for it to sort itself out and find its way nose down. I put 4.5 inch wheels on it for my rough runway and it does well with those. I have about 220 feet of runway with fences on each end so it has been a handful landing, know it might snap if I get it too slow and knowing it is a tight fit. Still working on landings and take-offs too for that matter.

Have you thought about using your heat gun to add some washout to the wings? You just hold about twice the twist in it that it needs and shrink the wrinkles away. It may take a couple of tries to get them even, but it's not hard.

That is a good technique for built-up wings, but this is a straight foam core plank that is fully sheeted. Not sure if the covering could overcome the stresses in the sheeting but might be worth a try.

It won't. But reflexing the ailerons up should help too. It may even be possible to warp some washout into them.

The plane snaps after the wing stalls.
The wing stalls after the elevator makes it reach a high AOA.
It will help reducing the elevator deflection to the minimum necessary to flare the plane during a dead stick condition with full tank of fuel.

Even with ailerons off-set up some (with which you reduce camber and incidence of the wing), a strong elevator still will take the wing beyond the critical AOA easily.

The deflection of the rudder, however, should be strong, so you have capability to effectively counteract any vices coming from that twisted tail.
As you know, in a snap roll, is the action of the rudder what determines what half-wing flies slower and stalls first.

Finally got the Cub tamed down and it is turning out to be a fun flyer. Reflexed the ailerons up a bit, reduced the elevator and rudder throws, and took my time getting it trimmed out properly so everything works with that twisted fuselage. Put on 4.5 inch wheels and the CG is at 29% of chord back from the leading edge with no fuel. I am still having trouble getting it slowed down for landings based on my experience flying it so far. Nothing gentle about the way this thing stalls, but I am sure with a bit more flying I will get it slowed down enough for decent landings on my 210 foot field. Here's a short video.....

https://www.youtube.com/watch?v=TwmYJpl1CaQ

Hi!
You would get much better performance if you used a two blade 12x4 RAM or Graupner G-Sonic prop or even an APC.

Agreed! It has enough power with the 3 blade and I like the look of it, even it it is rather non-scale. Might give the 12-4 Graupner a try if my shop can get that size.

Ran across this video of a non-nasty Cub! Pretty cool to see scale J3 flight and then 3D.....

http://www.youtube.com/watch?v=I7z4KivDedo

Even better than reflexing the ailerons, shouldn't you adjust the strut lengths to induce the 1 deg or so of beneficial washout? That's what they are there for. Depending on the construction of the struts and how they are connected to the wing, you should be able to lengthen the rear struts slightly without too much difficulty, say with washers between the wing surface and the strut.

Well, those are good ideas, but if you've read the thread you would know this is a plank foam core wing with full sheeting. No struts at all on this ship either.

I'm getting used to flying it and watching it coming in on approach all dog-style. It certainly has activated my yaw awareness and rudder fingers!

Well, there is a new Cub on the flight line. Our old dog Stormy was poisoned by a local farmer last July and we didn't think we were ready for a new pup, but a friend gave Tammy to us last week and she is doing well.

I've taken the 4 ozs of the added weight out of the nose now and the J3 still snaps the same way with the CG a bit further back. It is a fun flyer, but not a bird I will ever trust completely.


. http://www.rcuniverse.com/forum/atta...mentid=1953625

Lazer, Sorry to hear about your dog. The new girl looks good. I had a Sig 1/6 scale Cub. It had some nasty stalling/snapping tendencies and needed a lot of down trim to fly level when I first built it. Turns out I had built in too much positive incidence into the cabin where the wing mounts. I made some 1/8" shim plates and put them under the trailing edge of the wing. It trimmed out normal and tamed it's stall quite a bit. It still would drop a wing if stalled in a turn but that's normal cubiness. Try too shim the back of wing up a bit. It will require a more nose up flare before it stalls. I too would recommend a 12-4 prop even a 11-6 will help it slow down better to land than the 10-7 three blade. Looks like you are having fun with the old Flair Cub.

Thanks Mr. Punk! Yeah, Tammy is turning out to be a great pal.

The Cub is a fun plane to fly, but it seems exactly opposite of my Goldberg Tiger in terms of manners. The Tiger tracks perfectly, is 100% predictable, and does everything well. The Cub needs to be watched all the time like a bad dog! I am finally getting it slowed down for landings and have even done some slips to lose altitude. Since it's a garage sale airplane, I don't have much invested in it so I am finally taking some more risks with it. I think if it had been built straight it would be a fine model.

After putting more time on the Cub, have found that the elevator control is the one to make the bird expose its twisted nasty nature. This Cub can fly knife edge and I can slip it all the way to landing. So it can fly in an uncoordinated fashion no problem as long as the angle-of-attack on the wing is low. But..... Just pull back on elevator stick during a slip at altitude and it is snap roll time. My other models are more forgiving, but this J3 has taught me a new criteria for checking out a model's stability.

Update: Have reduced the elevator throw to half of what it was and the plane is a much more predictable now. Using only 1/4 inch up and 1/2 inch down (for inverted snap rolls), 40% exponential, and it now only snaps when I want it to. I also got rid of the long balsa push rod on the elevator as there was too much play at the control surface and put a servo right at the tail with a 4 inch rod for connection. The improvement in speed control during approaches has been dramatic.

A Cub has large control surfaces at the tail and not much movement is needed for large pitch and yaw changes. I think I had it set up with too much control throw initially and that accentuated problems with the twist in the fuselage.

It sounds like you've done some good experiments and learned a lot from the plane. But I still think all the washout comments you got were right on target. Full scale Cubs were manufactured with washout. Since it's a garage sale plane anyway, why not cut the ailerons about 6-12 inches from the end of the wing and re-glue them up a bit relative to the rest of the ailerons? It would be an interesting experiment.

On the other hand, I guess full scale Cubs didn't do a lot of inverted snap rolls!

Jim

Hi!
High weight and not using wash-out in the wing is the why your plane stalls!

I fixed similar conditions on several planes (planked wing) by taking a sharp knife and making 4-6 cuts (30-50cm long) in the balsa sheeting (on the bottom side) on the wing. Then twisting the wing ( the plastic covering removed of course) to get some wash-out , and then pouring some Ca glue into the crack.

Sheeted foam wing on that old plane. I sold all my glow planes a few weeks back and that Cub has a new owner. It was a fun flyer in the end.

Just to add a little after the fact here. I have been able to remove a twist in a sheeted foam wing by jigging up the amount of twist required and using heat. Most times the adhesive whether it was contact cement or epoxy will take a new set after being heated up.

That is good to know. The wings were true but doing that could have added some washout.