At 01:31 AM 30/05/2010, you wrote:
STATIC thrust is thrust at zero airspeed! Jet engines produce a low
thrust at zero airspeed that increases with increasing airspeed up to
a fairly high speed - so the acceleration of the jet ASW20 increases
significantly as airspeed increases up to about 80 kt from my memory
and observation at Waikerie - I saw the same flights as Dion did.
Best rate of climb came at 80 - 90 kt as I recall Allan telling us.
No, No, no.
The jet thrust decreases with increasing airspeed just like a
propeller only more slowly because the exhaust velocity of a jet is
much higher than for a propeller.
In both cases this can be modified somewhat by problems. In the jet
case by poor inlet design causing inlet losses at low speed or in
the prop case by part of the blade stalling at low forward speed due
to the twist in the blade. A prop designed for static thrust would
have little twist in the blade. (I still get annoyed when I see
ceiling fan blades though).
Once you get to high subsonic speeds you may benefit from compression
effect at the inlet with a jet. Mach 0.8 or so.
To a quite good first approximation this is a simple physics problem.
Thrust is change of momentum through the prop disc or through the jet
engine (mass of air x final velocity - initial velocity).
Power(energy per unit time) to do the acceleration of the air in both
cases is mass of air x (final velocity squared - initial velocity
squared). It is easy to see that you can get the same thrust by
moving half as much air through twice the velocity change but you
need twice the power to do so.
For efficient low speed propulsion you want large mass of air changed
in velocity as little as possible. A glider wing is the extreme
example of this. The greater the span the more mass of air is
influenced so it needs to change velocity less to generate the
thrust(lift) to oppose gravity. Changing velocity takes power which
in the case of a glider comes from its mass moving downward through a
gravity field (loss of potential energy). Minimise the power and you
minimise the sink rate. As the glider flys faster it influences more
air per second so needs to change the air's velocity less so induced
drag decreases with increasing airspeed. Which we always knew but now
you know why.
Now if you assume that the power inside the engine(jet or piston) is
constant (not quite true but close enough) it becomes easy to
calculate the drop off in thrust with airspeed.
For a jet exhaust velocity of 600m/s I get 8% thrust decrease at
about 100knots, 4% at 50 knots.
For a typical piston/prop self launcher the numbers might be a
halving of thrust at 50knots which explains why initial acceleration
is better for the prop and the jet is better at high speed.
When you plot from the glider polar the power required to remain in
level flight vs airspeed and the power available ffrom the jet
(thrust x airspeed) you find the maximum excess specific power
occurs at typically 80 to 120 knots depending on the glider and the
thrust of the jet. The lesson here is that profile drag of the jet
installation will have an effect on rate of climb so closing the
doors after the engine/s are extended is a good idea.
I'll find out a price, battery and maintenance issues on the PBS 100
engine. Bob Carlton was very positive about that engine. It does have
a built in generator and a real oil system.
Splitting the exhaust system will have little effect on thrust(or
with a twin pointing the engines slightly outboard). It is a straight
cosine law. Less than 0.5% for 5 degrees and about 1% for 7.5 deg.
Bob is using about a 15 degree split so he's got 7.5 deg per side.
Mike
Mike
Borgelt Instruments - manufacturers of quality soaring instruments since 1978
phone Int'l + 61 746 355784
fax Int'l + 61 746 358796
cellphone Int'l + 61 428 355784
email: [email protected]
website: www.borgeltinstruments.com
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