Thrust Needed For Self Launching...

Does anyone know (not a guess) how much thrust in pounds from a jet engine
would be needed to self launch a 1000 lb sailplane to 2500 AGL?

Or perhaps know a link whereby I can find the information or formula
needed so I can determine the answer myself.

I've looked ..googled..etc...can't find a thing.

I did read some where online that 3,600,000 joules would be needed.....but
can't find a conversion formula.

Thanks
Tom

1000 lbs at 25/1 Lift to drag needs 40 pounds of thrust to remain in
level flight. Any thrust above that number would result in either
climb or a faster level speed. So in theory, using those numbers,
40.0000001 pounds of thrust would get the job done - eventually. 100
pounds of thrust (2 large model turbines) would probably give a
reasonable rate of climb for a self-launch.

Rather than do all the math just download a demo version of X-plane,
add a jet to the glider and see what happens. www.X-plane.com

Just remember that the simulation is no better than the data fed into
it............................
===============
Leon McAtee

wrote:
1000 lbs at 25/1 Lift to drag needs 40 pounds of thrust to remain in
level flight. Any thrust above that number would result in either
climb or a faster level speed. So in theory, using those numbers,
40.0000001 pounds of thrust would get the job done - eventually.

The above 25/1 occurs at only one airspeed - faster or slower will
require more power. To self launch, you need, minimally, to accelerate
to that speed. Oh yeah, ya wanna launch from grass too? So, how much
power do you need? I dunno, but suspect that it will be a lot more than
40.0000001 pounds of thrust.

Tony V.

I know.

The answer to your question is not as simple as it
seems. If you are serious about a jet sailplane, send
me an e-mail.




At 22:06 12 March 2006, Tinman2006 wrote:
Does anyone know (not a guess) how much thrust in pounds
from a jet engine
would be needed to self launch a 1000 lb sailplane
to 2500 AGL?

Or perhaps know a link whereby I can find the information
or formula
needed so I can determine the answer myself.

I've looked ..googled..etc...can't find a thing.

I did read some where online that 3,600,000 joules
would be needed.....but
can't find a conversion formula.

Thanks
Tom

In article
outaviation.com,
"tinman2006" wrote:

Does anyone know (not a guess) how much thrust in pounds from a jet engine
would be needed to self launch a 1000 lb sailplane to 2500 AGL?

Or perhaps know a link whereby I can find the information or formula
needed so I can determine the answer myself.

I've looked ..googled..etc...can't find a thing.

I did read some where online that 3,600,000 joules would be needed.....but
can't find a conversion formula.

I've done calculations for rocket engines before, but jets are
essentially the same.

http://groups.google.com/groups?selm=an_595515430

--
Bruce | 41.1670S | \ spoken | -+-
Hoult | 174.8263E | /\ here. | ----------O----------

[quote=tinman2006]Does anyone know (not a guess) how much thrust in pounds from a jet engine
would be needed to self launch a 1000 lb sailplane to 2500 AGL?
Hi Tin person, remember-
Close data is always time consuming so, the jet would only be TOO BIG if:
1- it doesnt fit
2- its too heavy
otherwise its TOO SMALL!

like fuel, you have only got too much when your on fire

Bagmaker :-)

The Calif A21SJ with a max t/o mass of roughly 1700 pounds has 200 pounds of
thrust.
A DG800 with approx 1000 pounds also has 200 pounds of thrust.

That gives decent climb rates, but in the first place it allows to
accelerate to max climb speed on grass before you hit the fence on the far
side.

"Tony Verhulst" wrote in message
...
wrote:
1000 lbs at 25/1 Lift to drag needs 40 pounds of thrust to remain in
level flight. Any thrust above that number would result in either
climb or a faster level speed. So in theory, using those numbers,
40.0000001 pounds of thrust would get the job done - eventually.

The above 25/1 occurs at only one airspeed - faster or slower will require
more power. To self launch, you need, minimally, to accelerate to that
speed. Oh yeah, ya wanna launch from grass too? So, how much power do you
need? I dunno, but suspect that it will be a lot more than 40.0000001
pounds of thrust.

Tony V.

Bob Carlton converted a Silent to jet power (and it is anything but
silent!):

http://www.silentwingsairshows.com/jet.html

He uses two 45 lb thrust model a/c jets. The engines are small and
light, but it burns about 18 gallons per hour. For a 1000 lb glider you
will need 4 of these puppies, pushing fuel burn to 36 gal/hr (my DG-400
burns 5 gal/hr). Going with less than 4 means a longer takeoff run (a
LOT longer!). How long? Try F = M*A, or A = F/M, where F is the USEABLE
accelerating thrust, M is the mass of the glider and A is acceleration.
Remember that for useable acceleration you need to subtract off the
rolling resistance from engine thrust (pull your glider with a rope and
a BIG fish scale to measure this). Four engines also increases the risk
of engine failure by a factor of 4. Of course, you could still climb
with only 3, just slower.
Also, the 25:1 glider ratio doesn't apply because you have the
additional drag of the engines and pylon. Jets help here because of
their small size, but 4 of them will probably knock your glide angle to
under 20:1.

Tom Seim
Richland, WA

Also keep in mind that these model engines have a TBO of 25 hours.

Mike Schumann

wrote in message
oups.com...
Bob Carlton converted a Silent to jet power (and it is anything but
silent!):

http://www.silentwingsairshows.com/jet.html

He uses two 45 lb thrust model a/c jets. The engines are small and
light, but it burns about 18 gallons per hour. For a 1000 lb glider you
will need 4 of these puppies, pushing fuel burn to 36 gal/hr (my DG-400
burns 5 gal/hr). Going with less than 4 means a longer takeoff run (a
LOT longer!). How long? Try F = M*A, or A = F/M, where F is the USEABLE
accelerating thrust, M is the mass of the glider and A is acceleration.
Remember that for useable acceleration you need to subtract off the
rolling resistance from engine thrust (pull your glider with a rope and
a BIG fish scale to measure this). Four engines also increases the risk
of engine failure by a factor of 4. Of course, you could still climb
with only 3, just slower.
Also, the 25:1 glider ratio doesn't apply because you have the
additional drag of the engines and pylon. Jets help here because of
their small size, but 4 of them will probably knock your glide angle to
under 20:1.

Tom Seim
Richland, WA

On Sun, 12 Mar 2006 17:02:46 -0500, "tinman2006"
wrote:

Does anyone know (not a guess) how much thrust in pounds from a jet engine
would be needed to self launch a 1000 lb sailplane to 2500 AGL?

Or perhaps know a link whereby I can find the information or formula
needed so I can determine the answer myself.

I've looked ..googled..etc...can't find a thing.

I did read some where online that 3,600,000 joules would be needed.....but
can't find a conversion formula.

Thanks
Tom


I have a nice spreadsheet to do these calculations but I'm not yet
prepared to release it to the world.

Try Googling "aircraft performance" and you will get some references
which let you do the calculations. Prof Ilan Kroo at Stanford's aero
engineering course outline has some useful stuff. It really is just
high school physics.

My conclusions are that a thrust/weight ratio of 0.1 is the minimum
you want for self launch off smooth hard surfaces. This means for a
500Kg(1100lb) glider you need a total of 50Kg or110lbs of thrust.

If you go to about 0.14 you will be able to handle short dry
grass.(two 35Kg - 77lb thrust engines)

Even the 50Kg thrust on two engines will give a positive rate of climb
on one engineat 500Kg. This is for a Ventus C but the results are not
that sensitive to glider type as long as we are talking about modern
composite gliders at the same weight
..
The surprising thing until you understand what is going on is that the
best rate of climb speed is around 100KIAS. Faster for the larger
engines and the curve is quite flat. It doesn't matter much whether
you climb at 85 or 130 knots.
SE best rate is at around 75 to 90 knots depending on engine thrust.

Fuel for a 2000 foot launch is estimated at less than 4Kg(5 liters) of
Jet A1/jet oil mix (at SFC of 1.4)

Retrieve is on one engine at about 50% thrust at 90 to 100KIAS and we
should get at least 150 nm range in this mode if we start with 50
liters(40Kg) of fuel.

These numbers are preliminary and will depend on engine test results.
Hopefully coming soon.

The Ventus currently has a TOP module. I have the thrust/airspeed
curve for that.. Static thust is 750 Newtons(about 73.5Kg) rapidly
reducing to about 56KG at just over 20 knots so with two 35Kg engines
we will have more thrust during almost the entire takeoff run. With
the TOP the aircraft flys at 400Kg (no water)and at sea level is
reasonably lively and takes off easily on grass.

I also have about 20 hours in the Jet Caproni for comparison and
having seen Bob Carleton's films and a log of takeoff (thanks Bob) I'm
convinced this concept is a winner.

I expect the takeoff to be a little slower at first but the jet thrust
drops off much slower with airspeed (only about 10% at 100Knots) so
the latter stages of takeoff (where the distance is being used up)will
be much better. I project being able to make JAR 22 takeoff
performance at 500Kg at sea level on hard surface(15m at 1.3Vs 500m
from start of roll).

Mike Borgelt