Dihedral Question

[Don Cooper]

I am starting on the design of a plane with the following major parameters: general config - low wing open cockpit; wing - rectangular, span = 26", chord=8.5" including full span ailerons; airfoil - WASP; target weight - 4 oz including everything but battery; power- IPS, specific version not selected yet. I would like to have 'stay where I put it' stability along the roll axis for moderate bank angles. I have gotten this stability with no dihedral for similar planes with the wing on or slightly above the vertical C.G. Will I need to add dihedral to get that stability with the low wing plane?

[Dave Robelen]

Hello Don,
The short form is yes, you will need dihedral. When the wing is right on the vertical CG, and the side area is pretty well balanced, You will have about zero dihedral effect. Moving the wing below the CG creates a negative dihedral effect with a flat wing. I would add about 3 degrees per panel for a starting point. This is one of those areas where the answer cannot be arbitrary, since so many small things enter in. Back when pattern flyers were flying low wing models, it was not unusual for a hardcore enthusiast to saw the wing in two and change the dihedral until it flew to suit.
Take care, Dave

[CalmAir]

Hi Don,

On the Profile RV-4 I designed, the wing is below CG. I made a flat wing for it for a couple of reasons. I wanted it to go where I pointed it, and I was being lazy and wanted to build the wing up in one piece. It goes where I point it, but with the wing so low it does have some roll instability and is very touchy. It's more stable when it's inverted.

If I had it to do over I would put in 4 degrees or so of dihedral. I do not think it would have hurt "holding the bank", and I believe it would have kept it from being so twitchy.

Thats just a guess, since I have not tried it, but I feel confident that it would have been better than the flat wing.

Dale

[Don Cooper]

Dave and Dale,

Thanks for your responses. Dale, you hit exactly on my reasons for wanting to use a straight wing, but I will take the advice of both of you and add some dihedral.