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

From:         Tom Speer <speer%do.edw@mhs.elan.af.mil>
Organization: 412th Test Wing / TSFF
Date:         09 May 96 12:50:14 
References:   1 2 3
Followups:    1 2
Next article
View raw article
  or MIME structure

Tim Hills wrote:
>
> >I think that the main reason for underwing poddded engines is ease of
> maintainance...
> Finally, it is aerodynamically  advantagous to have the engine thrust
> line below the longditudenal axis, as a power increase will then result
> in a nose up pitch. I notice that the USAF finally went for the
> conventionally engined C17, although I admit that the YC14/15 were good
> looking designs!

Besides the aerodynamic aspects, the use of pod mounted engines ahead and
below the wings (first used on the B-47, I believe) was a major advance
in raising the flutter speeds of jet transports.  The mass balance
provided by the engines allowed the wings to be lighter and longer, while
controlling the flexibility.

I doubt that causing a pitch up with the application of power is a major
design driver in the positioning of engines.  I know of no flying
qualities requirement for this.  Even if were required, the aircraft
speed stability may result in a pitch up even for engines mounted
some distance above the c.g.

The YC-15 had straight turbojets on pylons, which is about as
conventional an engine arrangement as you can get.  What was unique was
that the flaps were not cut out behind the engines so that they were
externally blown.  The C-17's externally blown flaps are a direct
descendent of the YC-15's.

The YC-14, YC-15, C-17, QSRA, and Augmentor Wing (AWJSRA) are all
examples of powered lift aircraft.  Two blow their exhaust above the
wing, two below the wing, and the AWJSRA sent 60% of its thrust through
the wing.  All are viable configurations for STOL performance while still
retaining the capability to cruise efficiently at high subsonic speeds,
and both the upper surface blowing and externally blown flaps have seen
operational service.  Maybe someday, if they can achieve critical mass,
De Havilland can finally put the augmentor wing into service, as well.

TS