Dihedral

[ivn1188]

@ElvisL1ves

Next, you’re going to ask why high-wing airplanes like the C-5 have zero to negative dihedral, aren’t you? That’s because pendulum stability (from the CG being below the wing) is enough to provide roll stability. Adding dihedral would make the plane *too *hard to roll and would introduce control problems, including Dutch roll. Low-wing planes have negative pendulum stability and dihedral is needed to counteract it.

Sort of. But planes like the Tu-134 and Tu-154 are low wing planes with anhedral. Highly swept wings increase longitudinal axis stability, and anhedral helps correct for it.

[Magiver]

@Johnny_L.A

Yeah, that’s the thing. There’s the Culver, and there’s the T-18, and there’s the… ? It’s like you notice them when you see them, but when you try to think of them they’re gone.

I see that the Culver, like the Thorpe, has a flat center section. On a light aircraft I can see that it would be easier to build it that way than to have dihedral on the outer panels and the center section as well. And I understand that it makes no sense to have the extra bend if you don’t need it. But I’m still not sure what the advantage is of having outboard dihedral in the first place, over the conventional layout.

And I’m still interested in the actual structure.

I talked to a guy who bought one after the war. He said it was easy to fly which I took to mean stable. I fly a short wing Piper and it could use a bunch more dyhedral. Haven’t seen any at Oshkosh for at least 5 years.

[Sam_Stone]

@LSLGuy

I suspect the higher AOA explanation has a dynamic element that isn’t explained.

Imagine an aircraft with 3 degree dihedral. Imaigine it is in wings-level flight. The AOA on left & right wing is the same. Now magine it is in a steady-state bank of 10 degrees right wing low. The AOA on left & right wing is the same.

Now imagine it is rolling to the right at 10 degree/sec and now passing through 10 degrees right wing low. At that instant the AOA is greater on the descending wing. Not becasue of dihedral or bank angle, but because of roll rate.

That’s what I was trying to say.

[Richard_Pearse]

I think the AoA explanation also includes an assumption of slip. The aircraft is flying straight and level, it is then upset by something, turbulence for instance, the right wing drops and the aircraft slips toward the low wing. The low wing has a higher AoA in the slip due to dihedral and this gives it the extra lift to return to wings level.

[ElvisL1ves]

The pix in Johnny’s NASA link suggest that, too - the story only makes sense if you mix directional stability in with longitudinal stability.

[GusNSpot]

Now, lock the ailerons and steer with rudder only… With dihedral and without…

[Xema]

@lazybratsche

Care to elaborate on those performance and handling advantages? You’ve piqued my curiosity…

I’m going to refer this question to Prof. Mark Maughmer of Penn State University - definitely one of the world’s experts on this subject, and a contributor to the design of various sailplane models. I’ll report his response when I get it.