Re: Jet Engines above the wing: why not used?

From: (Mark Drela)
Organization: Massachusetts Institute of Technology
Date:         02 Jun 96 16:20:13 
References:   1 2 3 4
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In article <airliners.1996.831@ohare.Chicago.COM>, domk1031@cetus.zrz.TU-Berlin.DE (Burkhard Domke) writes:
|> The flow of engines pod-mounted beneath a wing counteract the
|> circulation of the wing, thus reducing lift, whereas engines mounted
|> above the wing would enhance lift.

Careful here.  You cannot relate the lift directly to the circulation if
blowing is used.  The derivation of the Kutta-Joukowsky relation between
lift and circulation...

   Lift/span = rho V Gamma

the circulation "Gamma" describes the flow behavior in the farfield of
the airfoil (i.e. from far away, the airfoil looks like a point vortex).
A jet of air is not a potential flow, so you get different values for
Gamma depending on how you draw the contour.  Also, the derivation of
the above formula assumes that there are no momentum sources in the
flowfield.  A jet engine is a momentum source.

  Mark Drela                          First Law of Aviation:
  MIT Aero & Astro          "Takeoff is optional, landing is compulsory"