Why is downwash on a wing not desired

Nick R · November 11, 2013, 03:04

Hi guys,

I simulated a finite wing and sure enough there's downwash behind the wing which lowers the lift coefficient compared to a 2d wing. Something doesn't make sense to me though. If a finite wing creates more downwash, that is more fluid is pushed down behind the wing then it means there's more high momentum fluid going down compared to the 2d case. So if the wing is pushing more fluid down shouldn't the fluid push the wing up more than the 2d case (momentum balance+newton's third law)? What am I missing here.
cheers,
Nick R

truffaldino · November 12, 2013, 17:22

Think in 3d: rougly speaking, due to downwash air goes down along the wingspan, but up if you go away from tips (look e.g. at wingtip vorices).

Nick R · November 16, 2013, 09:52

Yes. That's a good point. Thanks.

November 11, 2013, 03:04	Why is downwash on a wing not desired	#1
Nick R Senior Member Nick Join Date: Nov 2010 Posts: 126 Rep Power: 16	Hi guys, I simulated a finite wing and sure enough there's downwash behind the wing which lowers the lift coefficient compared to a 2d wing. Something doesn't make sense to me though. If a finite wing creates more downwash, that is more fluid is pushed down behind the wing then it means there's more high momentum fluid going down compared to the 2d case. So if the wing is pushing more fluid down shouldn't the fluid push the wing up more than the 2d case (momentum balance+newton's third law)? What am I missing here. cheers, Nick R

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November 12, 2013, 17:22		#2
truffaldino Senior Member Join Date: Jan 2011 Posts: 249 Blog Entries: 5 Rep Power: 17	Think in 3d: rougly speaking, due to downwash air goes down along the wingspan, but up if you go away from tips (look e.g. at wingtip vorices).

November 16, 2013, 09:52		#3
Nick R Senior Member Nick Join Date: Nov 2010 Posts: 126 Rep Power: 16	Yes. That's a good point. Thanks.