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The future is direct drive

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Starman

Well-Known Member
Joined
May 16, 2009
Messages
2,011
Location
High in the Andes Mountains
The new auto and truck engines that are coming out are light enough so that they can give good performance without a reduction gear. I think many people assume they need a PSRU because they are good followers, the older car engines were heavy, and it comes from looking at little charts with blinders on :) Instead let's be leaders and look at the bigger picture.

Of course little two strokes that need to run at 7000rpm need a PSRU, but I'm referring to the new aluminum long stroke car engines.

Looking at the issue from a standpoint of the PSRU itself I'll just pic some numbers out of the air to make some points.

Let's say a typical good PSRU will weigh about 1/3 of the weight of one of the light aluminum engines. One of the rarely seen good PSRUs will be heavier than a normal one because it will be designed to handle gyroscopic loads of the prop.

First off a smaller prop will be a little less efficient, but it will allow the engine to put out more power for an overall increase in thrust, so we will be using smaller diameter props unless we are building one of the popular Cublike planes.

If we assume that the PSRU will give you 50% more horsepower than if we are running direct drive then we get only 67% of the potential power of the engine, but we also have only 75% of the weight, so it's pretty close. Close enough to say the hell with reduction gears whenyou take the other benefits into consideration.

I know you can get some more thrust if you use a PSRU, I'm just sayin', look at the bigger picture, a proper direct drive setup will work really well with the right auto engine just like it did 40 or more years ago when Steve Wittman put one in his tailwind, and that's the kind of simplicity and reliability that will make auto engines take over from airplane engines.
 
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