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Swept wings

Old 10-02-2009, 05:40 AM
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pattern14
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Default Swept wings

Just wondering..... Saw a swept wing easy star on a U.K site just recently. If a straight wing was cut, angled and rejoined at , say, 35 degrees, as was this plane, would not the airfoil crossection be also at the same angle to the wind? The plane apparently flew well, but I am curious as to how efficient it would be, and would the angled airfoil shape create any problems. I was contemplating doing the same to one of my experimentals;, anyone else tried this?
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Old 10-02-2009, 10:09 AM
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slipstick
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The "angled airfoil" just ends up as a slightly thinner version of the original airfoil because the effective chord is longer. Not likely to cause much of a problem.

The sweepback also gives the effect of some dihedral (and it still works when inverted ).

Provided you get the new CG right there's no reason for it to cause any trouble. Why not give it a try ?

Steve
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Old 10-02-2009, 10:30 PM
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pattern14
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Default New COG

Thanks Steve, will definitely give it a try. The straight wing on my He 162 semi scale edf is already fairly battered, so I'll cut and rejoin them. It could not fly any worse. I intended to make it a "C" model with swept wings and a V-tail anyway, and needed to cut new wings, but ran out of epp. The only other question is the COG. My other deltas and swept wing planes are tailess- Do I simply move it back 20% from the original balance point, as I have read, or is there a more accurate method? cheers.... Included some pics of it in it's current form
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Old 10-03-2009, 04:04 AM
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Sky Sharkster
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Default Swept Wing Center of Gravity?

Hello Pattern,
Here are two links to help with swept-wing C.G.;
http://www.bakehead.com/rc_cg.htm
http://sky.prohosting.com/air2/cg_calc.htm
Once you find the neutral point, the C.G. should be slightly in front of it, this is called the Static Margin.
Hope this helps!
Ron
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Old 10-03-2009, 04:58 AM
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groundrushesup
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Originally Posted by pattern14 View Post
Thanks Steve, will definitely give it a try. The straight wing on my He 162 semi scale edf is already fairly battered, so I'll cut and rejoin them. It could not fly any worse. I intended to make it a "C" model with swept wings and a V-tail anyway, and needed to cut new wings, but ran out of epp. The only other question is the COG. My other deltas and swept wing planes are tailess- Do I simply move it back 20% from the original balance point, as I have read, or is there a more accurate method? cheers.... Included some pics of it in it's current form
I recently sectioned and swept a foam wing for one of my frankenstein projects... The wing was initially straight along the LE and tapered in the back, but I cut the wing at such an angle that this was effectively reversed (TE is now virtually straight along the length).

I did notice that because of the repositioning, the (symmetrical) the inside edge of the ailerons is more forward on the airfoil, and consequently the model has a slightly more pronounced adverse yaw at lower speeds.

Mixed in a bit of rudder and it seems to smooth it out - but unless I am mistaken it is something to think about when you change the shape of the airfoil.

GRU
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Old 10-03-2009, 10:10 PM
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pattern14
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Default ailerons and mixing

Thanks Ron and GRU, I wondered about the aileron set up, as I have changed the tail in the last mod. The twin tail on the He 162 had twin rudders activated by individual servo's. I mixed in the ailerons and rudder(s) and it flew quite O.K. Then I changed the tail for a more scale one, with dihedral, and left the rudders out to save weight, while increasing the size the ailerons. I also gave the wing no dihedral at all, at the same time. It snap rolls and is very responsive, but not all relaxing to fly. More scale like, but gives me a dry mouth everytime I launched it. Having a V-tail means I can mix in some rudder again, I will make the ailerons smaller and the swept wing will give some sought of dihedral effect. Hopefully it won't fly like a piano. The "C" model had a forward swept wing option, so there is allways more room for experimentaion, and the "D" model had a gull wing. Might find it hard to stop experimenting...... cheers
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Old 10-04-2009, 01:27 AM
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JetPlaneFlyer
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Achieving a proper CG is likely to be difficult. As you sweep the wings back then the CG must go back too. For wings with no taper then the CG moves by 50% of the sweep back.. So if the tips were swept back 100mm the CG would move back 50mm. If the wings taper then the CG moves by less.

Also sweep back reduces the tail moment so directional stability will reduce. This may along with the dihedral effect lead to dutch roll (a 'wagging' of the tail coupled with 'rocking' of the wings).

It would be better if you could keep the CG in the same place and move the wing forward on the fuselage but this would be very difficult to do.

The other likely issue if you add significant sweep back (say over 30 degrees), especially if the wings are tapered, is that the model will tent to tip stall, which could make it very unpleasant to fly.

Good luck with your experiments.............

Steve
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Old 10-05-2009, 09:29 AM
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pattern14
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Default worst flying plane anyway

Thanks Steve, but it could not be more unpleasant to fly . It tends to land inverted in trees and dams, through fences and generally is nerve wracking. Will let you know how I get on
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