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Dihedral effect on swept wing

Old 02-15-2019, 02:57 PM
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Default Dihedral effect on swept wing

A buddy built a powered glider type plane with forward swept wings and had some stability and turning issues with it. We got most of these sorted but still have a question we don't know then answer to.

"Miss Tempe" has a 60" span ( two 30" panels), 8" root chord, 5" tip chord, flat bottom conventional airfoil, 4 per panel dihedral, and 5 forward sweep as measured at the leading edge. No tip plates or fences, high wing, rudder, elevator, and throttle only. As built the C/G was about 33% of the MAC but she flew better with the C/G moved forward to around 25%. Weight all up was 19 oz. See the rather poor photo.

I understand that rear sweep adds effective dihedral, found references to anywhere from1 per each 5 to 10 sweep. (References are inconsistent but may represent unknown factors in the overall design.) But what about forward sweep? Does that add effective dihedral or subtract dihedral? I'm thinking it subtracts which would account for the turning difficulty as the effective dihedral was down to 3.5 or 3.0, depending. I also think a fence along the root would have kept any span wise air flow on the wing.

The builder is leaning toward replacing the wing with a 0 dihedral KF2 and ailerons. I'm thinking he could cut the original wing and add a couple more degrees dihedral and would be OK. Another option is a second KF2 with dihedral and no ailerons.

Thoughts and comments? All appreciated.
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Old 02-15-2019, 04:10 PM
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I can confirm from actual flying a model that sweep-back does add 'dihedral' effect.

I had a Pavel Bosak 20 powered Mig17 back in the 80's that had very little dihedral but the sweepback made her effective rudder / elevator machine.

If sweepback aids the turn ... then logic would say sweep forward would detract from the turn ... I know the couple of sweep forward wing models I have flown - I don't like ... which includes the 540 racer which has a forward taper wing ...
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Old 02-15-2019, 04:39 PM
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Thanks, Nigel.

I found there are a couple sailplanes with forward swept wings but they were thus designed to get the main spar out of the cabin area. Forward sweep may work best in high speed designs and just be a novelty with ours.

The buddy also tried a flying wing design with forward sweep. To the best of my knowledge it would snap roll at the slightest hint of a stall and the longest flight I saw must have been under 10 seconds.
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Old 02-15-2019, 09:23 PM
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The Blanik has forward swept wings because it is a 2 seater. This is to move the CG position forward for the extra weight.
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Old 02-20-2019, 08:32 PM
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Old 08-03-2019, 12:06 PM
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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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