Quote:
Originally Posted by Artist
Viper2000,
thank you for your knowledgeable input!
May I press you for an elaboration of these "various reasons"?
Artist
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If you're going faster then you'd need to turn the prop faster to keep the velocity triangles similar.
The amount of engine power required to do this varies as
(C*roh*(v^3)/2)*S
This means that you need more and more power per unit prop area in order to keep the velocity triangles the same as you go faster. So you either have too much prop to maintain rpm and efficiency at high speed, or too little prop to absorb all your engine power at low speed.
If you climb then you can reduce the air density to compensate for this (though this then causes further matching problems because lift & drag vary as roh*v^2).
However, your ability to speed up the prop in order to keep the same velocity triangles is limited by the fact that this will increase your tip Mach number, which will eventually lead to shock losses.
If you started out turning the prop at a low tip speed for the low speed case then it will be pretty inefficient because of its low dynamic pressure; if you started out with a high tip speed then the prop is smaller and lighter but you can't get much faster without shock losses.
Since infinitely variable transmissions are a real pain (and tend to be quite inefficient, which increases your cooling requirements as well as reducing useful power output), you would probably end up with perhaps two or three fixed gear ratios, and you would then have a constrained set of running lines because if you were in the wrong gear you'd either overspeed the engine or suffer detonation due to excessive boost at low rpm.
Changing gear requires a clutch (heavy) and it also kills power to the prop during the gear change.
Gear boxes capable of handling high powers are not easy to design, even with modern technology; just look at the problems with certain helicopter gearboxes for example - and they're only single speed (albeit with much larger reduction ratios). Making a multi-speed gearbox capable of handling 1000 bhp+ for 300 hours safely at flight weight just wasn't within the reach of 1940s technology; supercharger gearboxes were hard work, and they handled about an order of magnitude less power.
Changing the blade angle of the prop is so much easier, and gives the opportunity to continuously vary rpm and boost independently of one another. It also offers the opportunity to feather the prop for reduced drag in case of engine failure with relatively little additional effort. Finally, it allows you to actually unstall most of the blade at low speed & high power, which gives much better takeoff performance.