Dihedral

Dihedral
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Dihedral - 1/25/2002 3:47:00 AM

Ed Smith

Posts: 2835
Joined: 12/29/2001
From: Brantford, ON, CANADA
Status: offline

I understand the theory of Dihedral.

Why do some aircraft have anhedral and how does it work?

Ed S

Post #: 1

Anhedral - 1/25/2002 6:46:00 AM

Ed_Moorman

Posts: 3684
Joined: 1/5/2002
From: Shalimar, FL, USA
Status: offline

Modelers are so used to hearing about stability and comparing it to full scale aircraft they forget how we like to fly. Most RC fliers want neutral roll stability. We want the plane to go where we point it. If we put it into a bank we want it to stay there. In a full scale, the roll stability if very positive to keep you right side up and to keep the plane on a steady course. RC trainers have positive stability like most all full scales and you know how bad we all thought a trainer flew after we graduated to a Stick of some other second airplane. Neutral stability.

Basically, dihedral, sweep back and a high wing location cause a roll in the direction of the applied rudder. Think 3-channel and trainers.

Anhedral works the opposite. Anhedral, sweep forward and low wing location cause roll opposite to the rudder.

For these reasons, low wing planes need sweep back or dihedral to be neutral for knife edge, point rolls, etc. High, or shoulder wing, planes, like Sticks, need anhedral to counter the high wing location.

Check the anhedral planes on my web site. One of my present planes, a Joss Stick, was stock to start with. It rolled excessively in knife edge. I finally, sawed the wing in two, added about 3 inches anhedral and glassed it back together. Now it flies like a low wing. Knife edge with no, or very little, correction.

I have flown and modified many high wing planes with anhedral. I have yet to find a down side to the mod. I hear people saying, "My God, it's going to be unstable!" You can't prove it by me. Everyone I have flown, flew better. It is aerodynamically balanced. It point rolls better, it knife edges better, it stall turns better, it does outsides better. My buddies say it lands slower because the air gets packed under the drooping wings, but I can't really attest to this.

Finally, turn a low wing plane upside down and what do you have? A high wing plane with anhedral!!

I'm probably the anhedral proponent of the world, but you did ask. Check my web site.

_____________________________

Ed Moorman, AMA 553, KD4QBM, Revver Bro #156
R/C Report Magazine, Fun Aerobatics Column

(in reply to Ed Smith)

Post #: 2

Dihedral - 2/16/2002 2:05:42 AM

Rotaryphile

Posts: 271
Joined: 2/15/2002
From: Fredericton, NB, CANADA
Status: offline

Low wing models need a little dihedral; otherwise they will have negative yaw:roll coupling (right rudder causes left roll). High or shoulder wing models need a little anhedral to get rid of positive yaw:roll coupling, the amount of anhedral depending on the depth of the fuselage near the wing.

(in reply to Ed Smith)

Post #: 3

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