Model Forum / Radio Controlled / Air Models / November 2006

Yo are in luck, because its mainly a high frequency hiss with very
little LF roar.

Suggest you blag a spectrum analyzer and toddle down to a club where the
things are actually flying. They are loud close up, but in the sky are
remarkably quite.

I'd say its more 'blue noise' with a spectrum in the KHz only region.

You should be able to compute efflux velocity from the diameter and
thrust. My guess is its well over 100mph./.probably nearer 200mph Dunno
what exhaust temps are, but a bit of bypass may be possible to drop that
and the efflux velocity as well. Wren make a geared turboprop, so they
may be worth contacting about a turbofan setup.

Sorry for the skepticism in my reply, if unwarranted.   Your brief
description of the UA you are designing seemed to indicate an
intention to focus on performance goals that are suggestively military
(or paramilitary) in nature, i.e., speed and stealth.  I hope your
project is legit and you don't wear a towel around your head.

The engine parameters you seek (i.e., exhaust gas temperature and
velocity) should be quite easy for the member of your design team
charged with propulsion to measure or derive.  Re taipipe gas velocity
for example, simple momentum theory will readily yield the answer.
Thrust (a given) is by definition the axial rate of change of momentum
of the ingested air mass, the tailpipe cross-section area is known,
ejecta density can be accurately known after measuring the
temperature, so exhaust velocity can be determined for any flight
velocity where the the thrust is known - including zero flight
velocity if that is all you have available.

Hey, if this is a design team effort, it only makes sense to get the
responsibilites for design data assigned to the proper member of the
team.

Abel