Help needed understanding turbojets and airplanes

On Jun 19, 3:52*pm, es330td wrote:
Nicolas Charmont equipped a Cri-Cri homebuilt with two AMT Olympus
turbojets each producing 51 lbf of thrust that were originally
designed for model airplanes and it flies at 130 mph. I have since
read about another company, FTT (www.fttinc.com) that has an engine
for UAV's producing 37 lbf of thrust, with models producing 100 and
250 lbf under development. *This made me wonder how much plane could
one usefully fly using 1 or 2 of the 250's. *In comparison, I know the
VLJ market uses the Williams FJ22 (or similar engines) producing
550-700 lbf or more of thrust.

Say, for example, one took a 250 lbf engine and mounted it on top a
C172 wing. *Ignoring such changes as weights & balance, etc (in other
words, I am just looking at applying the thrust to the airframe with a
normal fuel and passenger load) how would the performance of this
plane compare to a standard C172 with a 180 hp O-360 piston engine?
Would it get off the ground in a normal distance or do I need a 5000
foot runway? *Would it be underpowered as compared to the O-360 or
would this actually give better performance? *Most importantly, what
could I read that actually discusses the engineering questions
involved in answering this question? (By the way, I was an engineering
major for three years before switching majors so I am not afraid of an
engineering level textbook if I have to go that route.)

Thanks.

I don't think turbojets or turbofans make much sense for typical light
plane operations. The specific fuel consumption is just to high. The
high fuel burn only makes sense at very high cruise speeds.

There's another area where surprisingly enough, they do make sense and
that is sailplanes. Here they are used in two very different ways.

First is the self launcher where a ~200 Lbf thrust jet burns up the
entire fuel supply getting the glider to say, 4000 feet AGL. The
residual weight after fuel depletion is much smaller than a piston/
prop system and retracting the tiny jet engine requires a far smaller
door in the fuselage.

The second way is as a "sustainer" engine. Here a tiny 50 Lbf thrust
jet can push a sailplane along at over 120 knots burning something
like 16 Gal/Hr once the glider is launched by some other means. Most
sailplanes have wing tanks for at least 40 gallons so the range is
240nm +. That's much faster and further than the typical retractable
2cyl 2-stroke turning a 40" prop can go. The strategy is to launch
with a tow plane, soar all day until the thermals quit, then fire up
the little jet and fly home before sunset. That would get you home in
time for the barbecue without having a retrieve crew hit the road with
your trailer.

The jet sailplane idea is under intense development by all the major
glider manufacturers.