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Cactus521

OK, question for the engine experts...

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I've flown in a number of aircraft that have ground adjustable props. Often the owner will adjust the prop for cruise vs. climb and this has two effects--faster cruise but also notably lower top end rpm during flight.My question--most engines have a redline rpm (let's say 5800 for the Rotax). But if the rpm isn't exceeding the rpm for max continuous power due to the prop adjustment, can the engine be run full throttle for an extended period? Seems to me the engine is working just as hard, simply at a lower rpm.Anyone who can help (or understand what I am asking), I would really appreciate it.Regards,John

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Well, as for any engine- It would really depend on what RPM makes peak horsepower. Most engines make peak HP below the redline, so you would probably not want to be at redline. Also, during cruise flight you don't use max throttle, so it is more efficient for the engine to turn lower RPMs. High RPMs are not necessarily good for an engine- that would be like you driving your BMW on the highway in 3rd gear. Every Rotation of the crankshaft sucks in X amount of air, so you need X amount of fuel to overcome the mechanical friction AND produce any work. The lower your RPMs, the less mechanical friction and the higher the efficiency of the engine. The cylinder heads are meant to handle a certain amount of pressure, so you can safely run at a lower RPM. If you have too much power, and too little RPM you can destroy the cylinders however- that's like "bogging down" your car by driving in 5th gear at 10 MPH and giving it full throttle- it's not good for the engine.Hope this helps a little?

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Ground adjustable props? Can you give a few examples?Thanks,Jeff

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>>Twisting of the prop before you start it up.<

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"but are we talking about pitch adjustments as in a constant speed prop, only it can only be adjusted manually, while on the ground of course?"Yep, ground adjustable props are common on light sport aircraft, and usually adjusted for cruise or climb performance. Reason why they are ground adjustable and not in flight adjustable is Sport Pilot doesn't allow in fligth adjustment of a prop. -John

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