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How is downwash affected by deploying flaps?
11-22-2009, 11:52 PM
Post: #1
Quote:When deploying the flaps the effective angle of attack:
increases.

I assume that flaps reduces downwash, thus increasing the effective angle of attack. But how?
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11-23-2009, 12:47 AM
Post: #2
The main wing generates downwash as a by-product of lift generation; the amount of downwash is broadly proportional to the lift created. (Think of the momentum change in the airflow as being the 'equal and opposite reaction' to the lift force; a crude but useful way of thinking of it.

If the tailplane were not behind the wing, it would be in the undisturbed flow field, and the angkle of attack at the tailplane would be the angle of attack of the flow (alpha) plus the angle of the tail relative to the fuselage.

Behind a wing, however, you have to allow for the downwash in determining the flow field. So the tailplane angle of attack is equal to the fuselage angle of attack, plus the tailplane angle, minus the downwash angle.

As the downwash is a function of the fuselage angle of attack, this creates a situation where the effect of a (say) one degree shift in angle of attack for the plane overall may only cause a half degree shift in tail angle of attack, as the downwash cancels some of the change.

Allowing for this effective change in tail angle of attack is important, as it is often a large effect (50%, or so, is not unreasonable for quite normal configurations).



this is what i got from my aircrft studies and google search. . read Smile

*CORRECT ME IF WRONG*
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11-24-2009, 04:25 PM
Post: #3
Forget about it, I think the answer given is wrong, thus the confusion. My instructor too agrees that it should be decrease instead.
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11-24-2009, 06:21 PM
Post: #4
please explain how would it be decereased..ty
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11-29-2009, 11:51 AM
Post: #5
When deploying flaps, the chord length increases as well as the camber of the aerofoil thus increasing the effective Angle Of Attack.

On Take-off : Flaps is deployed to increase the camber of the aerofoil thus getting the aircraft airborne quicker.
In-flight : During Finals, Flaps is deployed to provide a better view for the pilot thus pitching the aircraft the slightly downwards.

Its just a simple explanation.
Cheers!
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11-29-2009, 12:20 PM
Post: #6
So, nothing to do with downwash? Flaps increase camber, and in turn increases the effective angle of attack, that's all? Thanks for explanation Smile
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03-02-2011, 06:34 PM
Post: #7
Flaps up - Assume wing chord line is parallel to aircraft longitudinal axis. Downwash from wing assume to be along the chord line therefore downwash is straight along the fuselage longitudinal axis

Flaps down - Chord line is now going to be at a higher angle compared with flaps up. Downwash will still be along the chord line, but which will now be downwards slightly since chordline has now changed.

Its easier to see if it could be drawn...
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