Matt Prather wrote:

Let me ask this differently...  At low IAS, is more thrust going to be
available from accelerating a large volume of air a little bit (from
converting exhaust gas energy into propellor thrust), or by
accelerating a small volume of air a lot (directly, but efficiently
ejecting the exhaust gas stream)?  Somewhere I think I have read that
this effect is why turbojets don't work very well at low speeds.

Being an EE, I see this as analgous to impedance matching..

Regards,

Matt-

Matt Prather wrote:

Would it be safe to say that if you are building an airplane that

will spend most

of its life at low altitude, with high indicated airspeeds, don't

turbo compound,

but do try to take advantage of exhaust thrust power?  If however,

you are building a long distance, high altitude, low IAS airplane,
tc is the way to go?

Matt-

Turbo compound all depends on how much fuel money and extra range is

worth to you.

Exhaust gas thrust on the other hand is free.

Paul Lamar

Matt Prather wrote:

Let me ask this differently...  At low IAS, is more thrust going to be
available from accelerating a large volume of air a little bit (from
converting exhaust gas energy into propellor thrust), or by
accelerating a small volume of air a lot (directly, but efficiently
ejecting the exhaust gas stream)?  Somewhere I think I have read that
this effect is why turbojets don't work very well at low speeds.

Being an EE, I see this as analgous to impedance matching..

Regards,

Matt-

That is true Matt. The faster you go the more effective direct exhaust
thrust becomes because thrust is more or less constant. HP is thrust
(pounds) times velocity (FPS) divided by 550. If you take the same
thrust and feed it into a high speed turbine it becomes a lot of HP
regardless of how fast the airplane is going. You then feed it into a
large diameter efficient propellor and get a lot more thrust. Large
volume of air accelerated a little bit. A helicopter is a prime example.

Looking back at these graphs the R3350 TC in the Connie at about 250 MPH
 got 20% more HP or BSFC  at altitude while the P51 got about the same
20%   HP increase at about 375 MPH just using  the thrust directly. What
the  Connie pilot chooses to do is back off on the HP and derive the
benefit in  lower fuel consumption. You don't have that option with
direct exhaust gas  thrust. If you back off on the HP the speed drops
and so too the thrust greatly.

You need to run the numbers in both cases to see the cross over. This
situation is also intrinsic on why a pure jet becomes better HP wise  at
high speed than a propellor. TSFC Thrust Specific Fuel Consumption is a
better way of comparing jets to props.

Does this answer the question or am I still not getting it?

Paul Lamar

Matt Prather wrote:

Precisely..

Correction that should be:

"If you back off on the engine HP the speed drops and so too the additional HP greatly."

Paul Lamar

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