FSW vs delta for controlled high alpha ? - RC Groups
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Mar 08, 2005, 05:49 PM
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FSW vs delta for controlled high alpha ?

It is generally accepted that a FSW (forward swept wing) has very good high angle of attack handling because tips stall long after the root. As you change the sweep angle towards the conventional backward sweep the tip stall margin decreases.

However I've seen several references on delta wings being very controllable at high alpha which seems to contradict the tip stall theory (since delta is an extreme case of backward sweep). Is it really true ? If so, at which point a backward swept configuration goes through the minima and starts improving stall characteristics and why ?
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Mar 08, 2005, 06:22 PM
Needs brain lubrication
biber's Avatar
On deltas with a TE sweep of more than about 30 degrees or so the swept TE at high AoAs generates big vortices over the wing wich produce lots of lift and lots of drag to. Then the lift depends no more on any airfoil but merely on the TE sweep generating these vortices. Together with the typicaly little aspect ratio of deltas the situation not comparable to less swept higher aspect ratio wings. Btw, strakes like e.g. on the F-18 do exactely the samething, producing these vortices at high AoAs they enable the wing to still provide lift when it would have stall allready without them.
Mar 08, 2005, 06:59 PM
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Does it maintain aileron control in this mode ?
I am looking for an optimal wing planform that would allow precise and very steep descends in adverse wind conditions and moderate to high wing loading for AP. I do not think I can get enough drag from flaps alone so I contemplate high alpha landing mode but the plane must maintain some controlability in this mode.
Mar 09, 2005, 01:36 PM
Needs brain lubrication
biber's Avatar
A delta is a good choice for your task. It will still be fully controlable, even at very high AoAs / Ca. The tremendous amount of drag in that situation makes landing easy. Don't likely need any additional airbrakes for landing, neither do flaps make sense on a delta (would act like down elevator, not good for a smooth landing...). The deltas weak point is that it needs some power since it's quite draggy compared to high aspect ratio wings. The good thing about deltas is that they are very easy to built ( actually any airfoil will work, even a flatplate suits nearly as good as any other airfoil) and they are easy to fly.

Mar 09, 2005, 09:09 PM
Registered User
Thanks for the help. I found more info on delta aerodynamics and vortex lift at high AoA. Delta tendency to dissipate energy in turns is something I can live with (it is not a combat or 3D plane). What concerns me is Cl/Cd in level flight (duration). I have size limitation (portability) and can not make wing loading low enough. This means I have to either fly at very high speed (not convenient for AP) or in draggy vortex lift mode (inefficient).

As an alternative approach, if I manage to build FSW with sufficient torsion rigidity it should be more efficient in normal flight carrying the load yet also have high AoA capabilities. I was thinking about moderate LE forward sweep (something between BOP and Alula) and have some ideas to try for the FSW specific spar structure (foam wing). There is much less info available on FSW and what to expect at high AoA so any input would be appreciated.

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