Wing Design and Roll Stablity?

[Ron Kay]

I am working on a small 6 mm blue foam fan fold project that I need some design help with.

I have a 60" span flat wing with no dihedral angle. I don't want to install ailerons as I'd like to keep it as simple and light as possible but I do need roll stability. So the question is, how do I provide roll stability, add winglets?

Does anyone know if there's a rule of thumb for sizing winglets and the optimum angle? Perhaps expressed as some % of wing area? How about the angle? Zero is obviously too little and ninety degrees is too much. Perhaps it's 45 degrees? Don't know for sure?

Picture of the half span is attached...

Any help would be greatly appreciated.

Thanks,
Ron

[JetPlaneFlyer]

Dihedral is really the simplest way to add 'roll stability'. Adding dihedral to the outer portion of the wings (winglets) will also work but this is not as simple as straight dihedral as it involves making dihedral joints on both wings... If using straight dihedral then about 6 - 8 degrees is a reasonable average figure to use. If adding dihedral to outboard panels only then look to put the tips roughly where 6 - 8 degrees of straight dihedral would have placed them..

All rough rules of thumb only ;)

Steve

[Brandano]

hmm, that looks like a pterodactyl wing? If that is so, you might find you don't have enough vertical surface area, and what you are really missing isn't roll stability but yaw stability, that will lead to a side slip and a spin. Some sweep might help by lifting the wing inside the side slip, but it's very difficult to make a tailless wing that is naturally stable in yaw. Not impossible, but hard to do, and even harder if you are constrained by scale issues.

[Ron Kay]

Thanks for the replies!

JPF, that's great. Thanks... I'll give it a try. Still would like to know if there is some mathematical rule that wold describe this? Sounds like a bit of trial and crash... Will work on it..

Brandano - Thanks for the input. Yes it is a Pterodactyl.. Actually I am building it with a "V" tail so it's not exactly tailless. With my design I should have pitch and yaw stability. My only concern is roll and that's why I wanted to build in the tips with some dihedral. I think JPF is on the right track. Only question is how large, area should the winglets be to provide stability. Perhaps trial and error...

Thanks again for the replies.

Ron

[JetPlaneFlyer]

Only question is how large, area should the winglets be to provide stability. Perhaps trial and error...

Thanks again for the replies.

Ron
If the centre panel is flat then the outer panels would need to be about 50% of the semi span or 1/4 total span.. (I'd not refer to these outer panels as winglets because that's not what they are) ... Any smaller and the dihedral angle you had to put in them would be excessive.

Steve

[vintage1]

I only know two ways to get natural roll stability, sweepback or dihedral. Fins are required in both cases as far behind the CG as possible.

[Brandano]

Well, adding winglets to a wing essentially also adds some dihedral as long as the winglet is only projecting upward

[Texas Buzzard]

Vintage says Sweep Back and Dihedral are the accepted way to add roll stability. I agree.

The formula for Free Flight Roll Stability was/is 6 to 8 degrees of dihedral in a V-type dihedral wing and a vertical fin of about 5% of the wing area. Too small a V. fin allows Dutch Roll, and too large prevents the side slip needed for recovery.

Poly-dihedral is the same as V. dihedral but Poly-dihedral wings need more degrees of dihedral.

Why are you wedded to having a Flat (no-dihedral wing) wing? If flying is more important than how it looks then consider when it's in the air the dihedral isn't so noticable.

The standard formula for Sweep to produce stability is this: 3 degrees of Sweep = 1 degree of dihedral. You'll see that on the wingtips of many sailplanes.

[Ron Kay]

Texas Buzzard,

The wedding is off :)

After reading over the replies, and rethinking the project, I'll take a step back and build dihedral in the main center section with additional dihedral in the outer sections.

Sometime we/I get too close to the forrest and ... well you know the rest ...

Thanks everyone for the input.

Ron

[Texas Buzzard]

Ron, it will fly better and your "plane" will live to fly another day with some dihedral.

You could experiment with some balsa or flat foam gliders of about 18" span.
This might help with determining the size of the fin/rudder. In the past I was into hand-launched gliders ( no radio) for a couple of years and it is amazing how small the v. fin has to be to achieve stability. We'd cut the v. fin off until we got some Dutch Roll and then add a bit back on. This allowed the glider to slip and/or roll a bit so the dihedral would take over to prevent a spiral.

Have fun experimenting.