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

From:         domk1031@cetus.zrz.TU-Berlin.DE (Burkhard Domke)
Organization: Technische Universitdt Berlin
Date:         27 May 96 13:35:39 
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Followups:    1 2 3
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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.
The german VFW614 used the latter concept to demonstrate lift
coefficients of 4,0 and above. As the VFW614 was designed for service
from unpaved surfaces by third-world operators, FOD-avoidance was
another driving factor for the configuration.
The major shortcomings of the concept can be found in wing-pylon-pod
interference.  Pod and pylon shapes would have to be 3D-optimzed (i.e.
non-symmetrical) with respect to singular operating conditions
whenever sweep is to be applied to the wing in order to minimize
degrading interferences with the latter's pressure distribution. The
build-up of complex shock patterns seems to restrict entry into the
transonic filght regime anyway.
Another factor is cabin noise, as stated before, and certification
criteria regarding turbine disc fragmentation zones. Passengers could
no longer be shielded by the wing structure.
Another factor contributing to the demise of the VFW614 was a
deficiency in the internal aerodynamics of the RR-Turbomeca M45
engine, which led to intermittent bursts fo the jet efflux pounding
the horizontal stabilizer, accompanied by some respective discomfort
felt in the cockpit.
Besides, there was another remarkable (paper-)concept with engines
pod-mounted above the wings. In the late seventies Lockhhed proposed a
transport for hauling caterpillars and similarly oversized equipment
with the crago sitting IN THE OPEN flat-bed style.

Burkhard Domke
TU Berlin, ILR F2
Marchstrasse 14, D-10777 Berlin, Germany
Phone  +49 30 314 24460
FAX    +49 30 314 22955