Comparison prop to jet propulsion


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11 March 2006
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An afore-mentioned comparison is a little bit tricky, partly because performance
of a propulsion system using a propeller is usually given by the engine power, measured
in W (a physical power), whereas for a jet propulsion the thrust ist given, measured
in N (a physical force). If the maximum speed and engine power for a prop a/c are
known, is it correct to state, that this power is translated into a driving force, which is
determined by the equation F=P / v ( with P: power,F: force, v: speed) ?

As an explanation for my question, I'm trying to compare aircraft, which were modified
from prop to jet propulsion, take the Hispano HA 100 and 200 as an example.
You need to include prop efficiency in the equation, which is itself a function of speed.
If I have the maximum speed of a prop driven aircraft, which quite probably
is achieved with maximum available power, do I really need the prop efficency ?
If it would be lower, thrust generated by the prop and so max speed would
be lower, too, I think.
As an afterthought it came to my mind, that on the other hand the thrust data
of a jet a/c may be misleading, because generally given is just the nominal thrust
of the engine, without regards of losses due to, say, a long jet pipe.
Is there something in particular that you are trying to compare? Like airframe drag? Otherwise it's difficult to make an apple-to-apple comparison.

Anyway, prop efficiency becomes important near maximum speed. Take the example of P-51s in the Reno Races. They might have 50% more power than a stock Mustang, but the prop efficiency is so poor at high speed that they might only be 10% faster. Unfortunately, only prop manufacturers have precise numbers on efficiency. You would have to look at the propeller map to figure out efficiency. The formula for max speed would be:

Vmax = Max_Power * Prop_eff / Drag (requires conversion factors depending on what units you are using)
"Is there something in particular that you are trying to compare?"

I'm just looking for a way to make a plausible guess about the performance
of an aircraft, when the piston engine/prop system is substituted by a jet
engine, with minimal changes to the basic layout. Examples may be the already
mentioned HA 100/HA 200, or to name others, the Saab 21/ 21R, Bede BD 5/
BD 5 Jet, or the one with the least changes at all, the Cri Cri/Cri Cri Jet.

You're right about Vmax, I think, but to my opinion (although I'm still not absolutely
sure about it !) all I need is Vmax of the prop version and that's given, as the prop
version was actually built, but the jet version not.

I'm aware, that it will be a very rough estimation, but to know at least the magnitude
of the increase of Vmax would be quite sufficient.
Ok that explains it. Interesting problem. have to somehow extrapolate what the drag of the airframe is and then guess at what the drag for the jet version would be.
The maximum speed of the jet is the speed at which drag equals max engine thrust.

There is also the problem that the max speed of a prop and jet aircraft might occur at different altitudes (thus different air density). Do you know at what altitude the Ha-100 reached its max speed?
Actually, there is a larger example of prop to jet for comaparison: The Dornier 328/328 Jet. There should be plenty of comparison data since both aircraft saw plenty of service.
Yes, you're right, I should have been more precise and said earlier, that I
meant single engined aircraft (although the CriCri still is an exception then).
Nevertheless, you're clue could be really worthwhile, as generally there should
be no differences in comparing prop to jet propulsion.
So, thanks for this suggestion ! ;)
Did a quick look up on performance for the Dornier 328 and 328 Jet; max cruising speed for the 328 was about 350 mph and about 440 for the 328 Jet (There were variations at the given cruising altitude, the 328 was lower). Other than maybe slight changes to the wing, the airframes for both are virtually identical.
I would like to suggest the book "Aerodynamics for Aviators" by Jeppesen Inc. (as the name suggests it's an aerodynamics book for pilots), it includes a section that's a nice introduction to this topic.

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