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Old 08-03-2019, 12:06 PM
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Join Date: Jul 2019
Posts: 3
Default Dihedral effect

I've been playing with some forward swept wing free flight catapult gliders.

Fwd sweep reduces directional (yaw) stability and also has an anhedral (negative dihedral) effect. The fwd swept wing also offers up some structural challenges, may I recommend that the wing structure should concentrate the stiffness,(resistance to bending) int the forward area of the wing structure, especially so in the outboard area of the wing.

When an aft-swept wing bends upward under load they usually twist and add washout. This is good because it "vents" or unloads the tip area.
But the fwd swept wing will twist the other way, adding wash-in to the mix, which can cause tip stalling or structural failure of the wing.

As a general rule aft-swept wings have, in addition to a dihedral effect, a wash-in effect as well, so some wash-out twist is required just to get to neutral. Look at a flex-wing hang glider in flight, it appears to have a lot of wash-out twist in the sail but what your eyes see is not the same as what the airflow "sees". You can get by with little or no washout in a straight or forward-swept wing, but the aft-swept wing demands extra twist in the wash-out direction. If there's not enough the wing will get revenge by tip-stalling right when you're flying low'n'slow on landing approach.
Add excessive taper ratio and it gets worse.

Building and flying fwd swept catapult gliders has been a real eye opener for me. I don't have the smarts for much book learnin' and so it's trial and error for me. but since I don't consider a crash to be a failure, but instead just another lesson, I'm good to go with that.

I've been amazed at how much larger a vertical tail has to be on a fwd swept glider, and also how much more nose weight (or longer nose) is required for the c of g to be right. I have learned that patience pays off though, 'cause once I get the fwd swept glider finally dialed in I'll have a sweet flying glider.

My advice to flyers with problematic planes is to simply treat the symptoms, Tip stalling? Add washout, and/or make sure the leading edges in the outboard wing span are nice and round, if they're too pointy the stall comes sooner.

Plane flies mushy and it's hard to hold the nose up at low speeds? move the c of g aft a little at a time, keeping it in mind that while nose heavy planes fly sluggishly, tail heavy planes fly once.

While researching forward swept wing aircraft I stumbled upon an elegant way to fix the structural issues associated with both aft and fwd swept wings:
"Joined Wing Aircraft". Highly efficient long skinny high aspect ratio wings suddenly lose their torsional rigidity problems when the wing tips of the aft and fwd swept wings are glued together, I'm having ever more fun playing with those things.
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