Help needed understanding turbojets and airplanes

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.

"es330td" wrote in message
...
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

Here is another GA application of AMT jet engines:
http://www.silentwingsairshows.com/jet.html

Vaughn

These aren't turbojets; they are turbofans. My mistake.

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.

It might fly (with enough runway) but there isn't enough room in the
airplane for enough fuel to keep it in the air long enough to get to the
next airport. :-)

Turbines don't scale down in size very well, and even the bigger ones
aren't very fuel efficient at light plane altitudes.

Charlie

On 20 June, 03:22, Charlie wrote:
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.

It might fly (with enough runway) but there isn't enough room in the
airplane for enough fuel to keep it in the air long enough to get to the
next airport. :-)

Turbines don't scale down in size very well, and even the bigger ones
aren't very fuel efficient at light plane altitudes.

That is my understanding too, although I do not
fully comprehend the issues.

http://www.deltahawkengines.com/

or similar seems to me to be a likely future path
for GA engines.

What's not to like? Many fewer moving parts, more fuel
efficient, uses fuel that will be available for ever ...

On Fri, 19 Jun 2009 14:52:42 -0700 (PDT), 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.


last time I looked a williams engine started at half a million
dollars. disincentive enough for most of us.

the cri cri with two turbojects etc etc sounds interesting. however
ask what it's fuel endurance was. ....any takers on 18 minutes?

there is a model jet engine made local to me which is rebadged and
sold all over the world. suave piece of kit. fuel consumption is
triple that of an O-200 for far less thrust.

you could indeed put a jet on a cessna 172 but where would you go with
so little range?

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.

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. ...
Say, for example, one took a 250 lbf engine and mounted it on top a
C172 wing. ...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 ...
Thanks.


The underlying principle is straight-forward.
Recips are (more or less) constant max HP devices.
Turbojets are (more or less) constant max thrust devices.

Horsepower is a constant times thrust times speed.

Or if you can work in SI units, it's simpler:
using power in watts
using thrust = force in newtons
(Newton is about the weight of an apple)
using speed = meters per second,
THEN power = thrust X speed

On the face of it, that leads to a paradoxical result for recips:

At a slow enough speed, the thrust from a given horsepower is sky-high!
(But props stall out at a fair fraction of stall speed, then thrust
drops off at slower speeds.)
At a HIGH enough speed, the thrust from a given horse power is
teensey-weensey.

Which leads to the first deduction: recips are best at slow
airspeeds and jets are best at high airspeeds.

There's a paradoxical result for turbojets too.....
For a constant thrust, the higher the speed, the higher the power it
represents!

Now an example - the one you offered:
2 X 51 lbs thrust allows a top speed of 130 MPH from a cri-cri

How many horses does this mean at this particular speed?
(I'll convert to SI for convenience)
51 lb = 51/2.2 kg = 51/2.2 X 9.8 newtons = 227 newtons.
130 MPH = 130 / (60X60) miles/sec =
130 X 1760 X 36/39.37 /(60X60) m/s = 58 meters/sec

Remembering power = thrust X speed and we already
know speed and thrust, so we can find power, like this:
power = 2 X 227 X 58 = 26.4 kW = 26.4 / 0.760 HP = 34.6 HP

Is that ALL? It implies the cri-cri is a lowish drag machine!
But there are 1-seaters that make 190 MPH on a VW of similar power.

Now, we ought to work your other example: the C-172 with a 360 engine.
This is (say) 180 HP. Can't remember the top speed of the C-172,
so I'll guess 135MPH

Here we have numbers for power and speed, so we can work out thrust.

I'll do it again in SI for convenience:
135 MPH = 60.2 m/s
180 HP = 137.3 kW
so THRUST = power / speed = 137300 / 60.2 = 2281 newtons
2281 newtons = 2281/9.8 X 2.2 lbs =
512 lbsf. As much as THAT?
This implies that the C-172 is a pretty draggy airframe,
about four times as draggy as a cri-cri anyway.....

Well, by now, we are about ready to guess an answer to your question:
how fast will a C172 go with HALF the thrust it has at top speed with a
180HP engine?
Almost ready.
We also need to remember that air drag varies as the square of speed.
So we can say that the thrust needed at any speed is the drag at that
speed, and for the C-172, drag is 512 lb at 135 MPH
so at some speed V, the drag is 512 times (V/135) squared

For 250 lbf thrust, we get 250 = 512 times (V/135)^2
Rearranging:
(V/135)^2 = 250/512
V/135 = 0.70
so V = 94 MPH top speed.

Not much of a range between stall and top speed in this low power
c-172 huh?
To finish up, we can work out the power delivered to a C-172 by some jet
at 94MPH top speed

power = speed times thrust.
power = 94MPH X 250 LBF X some unit conversion constant
We know 180HP = 135MPH X 512 LBF x the same conversion constant

So the conversion constant is 180 / (135X512)

And FINALLY:
the power needed to drive a C-172 at a top speed of 94 MPH =
speed X thrust X conversion constant =
94 X 250 X 180/(135X512) = 61 HP


This is a third of the horses to get over 2/3 the top speed!

Hope this helps

Brian Whatcott
Altus OK

On Jun 20, 12:18*pm, Brian Whatcott wrote:
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. ...
Say, for example, one took a 250 lbf engine and mounted it on top a
C172 wing. ...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 ...
Thanks.

The underlying principle is straight-forward.
Recips are (more or less) constant max HP devices.
Turbojets are (more or less) constant max thrust devices.

Horsepower is a constant times thrust times speed.

Or if you can work in SI units, it's simpler:
using power in watts
* using thrust *= force in newtons
* * * * * (Newton is about the weight of an apple)
using speed *= meters per second,
THEN power = thrust X speed

On the face of it, that leads to a paradoxical result for recips:

At a slow enough speed, the thrust from a given horsepower is sky-high!
(But props stall out at a fair fraction of stall speed, then thrust
drops off at slower speeds.)
At a HIGH enough speed, the thrust from a given horse power is
teensey-weensey.

Which leads to the first deduction: * * recips are best at slow
airspeeds *and jets are best at high airspeeds.

There's a paradoxical result for turbojets too.....
* * For a constant thrust, the higher the speed, the higher the power it
represents!

Now an example - the one you offered:
2 X 51 lbs thrust allows a top speed of 130 MPH from a cri-cri

How many horses does this mean at this particular speed?
(I'll convert to SI for convenience)
51 lb = 51/2.2 kg = 51/2.2 X 9.8 * newtons = 227 newtons.
130 MPH = 130 / (60X60) * *miles/sec =
130 X 1760 X 36/39.37 /(60X60) m/s = 58 meters/sec

Remembering power = thrust X speed and we already
know speed and thrust, so we can find power, like this:
power = 2 X 227 X 58 = 26.4 kW = 26.4 / 0.760 HP = 34.6 HP

Is that ALL? It implies the cri-cri is a lowish drag machine!
But there are 1-seaters that make 190 MPH on a VW of similar power.

Now, we ought to work your other example: the C-172 with a 360 engine.
This is (say) 180 HP. Can't remember the top speed of the C-172,
so I'll guess 135MPH

Here we have numbers for power and speed, so we can work out thrust.

I'll do it again in SI for convenience:
135 MPH = 60.2 m/s
180 HP = 137.3 kW
so THRUST = power / speed = 137300 / 60.2 = 2281 newtons
2281 newtons = 2281/9.8 * X *2.2 lbs =
512 lbsf. * *As much as THAT?
This implies that the C-172 is a pretty draggy airframe,
about four times as draggy as a cri-cri anyway.....

Well, by now, we are about ready to guess an answer to your question:
how fast will a C172 go with HALF the thrust it has at top speed with a
180HP engine?
Almost ready.
We also need to remember that air drag varies as the square of speed.
So we can say that the thrust needed at any speed is the drag at that
speed, *and for the C-172, drag is 512 lb at 135 MPH
so at some speed V, the drag is 512 times *(V/135) squared

For 250 lbf thrust, we get 250 = 512 times (V/135)^2
Rearranging:
(V/135)^2 = 250/512
V/135 = 0.70
so V = 94 MPH top speed.

Not much of a range between stall and top speed in this low power
c-172 huh?
To finish up, we can work out the power delivered to a C-172 by some jet
at 94MPH top speed

power = speed times thrust.
power = 94MPH X 250 LBF X *some unit conversion constant
We know 180HP *= 135MPH X 512 LBF x the same conversion constant

So the conversion constant is 180 / (135X512)

And FINALLY:
* the power needed to drive a C-172 at a top speed of 94 MPH =
speed X thrust X conversion constant =
94 X 250 X 180/(135X512) = 61 HP

This is a third of the horses to get over 2/3 the top speed!

Hope this helps

Brian Whatcott
Altus OK

What a great post. It took me a minute to get back into physics mode
mentally but this was a great explanation.