"John Ings" wrote in message
...
All the answers I've seen so far seem to be forgetting a few things.
1. If it's thrust against the front compressor blades, how did engines
with centrifugal compressors ever get an aeroplane off the ground?
When the air makes a right angle turn towards the combustor
2. What about afterburners? All they do is dump lots of fuel into the
hot gasses just behind the turbine. How does this increase thrust?
There's still pressure after the turbine so might as well add some air (um,
I've never heard of an engine being ran extra lean when afterburner is
added, but it would have to be, no?) and fuel, plus some extra exhaust
nozzle to make use of the burning, expanding gas and, um there you have it.
(I'm too tired to correct that paragraph gramatically.)
3. Many military engines have variable orifices at the very end of the
tailpipe to adjust thrust.
Probably something like putting your thumb over the end of the garden hose.
4. In plain rocket engines, like those on the shuttle, there are no
fan blades at all, but lotsa thrust.
But you *are* moving thousands of pounds of fuel from zero (relative the
engine) to several mach, aft-ward. That makes for a nice reaction force.
Same goes for *any* other jet engine, or fluid mover (propeller in air or
water) for that matter: the net effect is the fluid medium being thrown
backwards with respect to what's throwing it. Note that drag (parasite
drag, induced drag, drag of a turbine to spin the compressor, etc.)
displaces this air foreward, or at least less aftward than the thrust. So
thrust has to be that much more to counter it.
So for engines that don't use bypass fans:
"Gas turbine engines for aircraft have an exhaust system which passes
the turbine discharge gases to atmosphere at a velocity in the
required direction, to provide the necessary thrust. The design of the
exhaust system, therefore, exerts a considerable influence on the
performance of the engine. The cross sectional areas of the jet pipe
and propelling or outlet nozzle affect turbine entry temperature, the
mass flow rate, and the velocity and pressure of the exhaust jet."
Makes sense because for a given source of limited pressure and flow rate,
there is an ideal nozzle dimension which produces a maximum velocity output
(without compromising flow rate by restricting, nor pressure by being too
open). If you had some detailed spec's on the engine's output behavior, I
bet it'd be pretty easy to find with some calculus. (What can I say, I'm in
a calc. class, everything's starting to look like a derivative, erm, slope
now...)
So I say the thrust is against that whole tailpipe assembly, including
the cone just behind the turbine. Probably the combustion chamber
takes some too.
I would bet that the places of main thrust production are those most highly
pressurized: the compressor, because it's producing the pressure in the
first place; the combustor, because pressure again increases here; the
exhaust nozzle because the air is able to do more physical work before
exiting the engine.
In each place there are surfaces whose normals are pointed in the general
direction of thrust, although some mildly. Obviously these won't contribute
much thrust, instead having to simply retain their internal pressure. (Take
a piece of pipe for instance: blow air through it -- its walls are parallel
to the flow direction so despite the pressure inside it, what net force, if
any, is acting on the pipe? However, if you curved the pipe around 180° so
it points back at the source, the back side will be contributing a net
outward force. You might also be able to argue that the inside of the bend
is contributing "negative" force (um... double negative kind of "negative")
if the conditions are able to reduce its pressure below outside pressure.)
That was too long. I'm going to sleep. lol
Tim
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