Engine power

·         bob

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My 3300A engine is rated for 3300 RPMs continuous is there any reason not to run it that high?

Also it has to turn that high to get full HP ,why does everyone set prop to get 2850 rpm?

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·         Doug Smith

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Hi Bob,
Structurally there's no issue with the engine running that sort of RPM and you can do it if you like, though most folks object to the noise and vibrations.

Manifold pressure (or power setting in other words) is a slightly different story - if you run at full power and full RPM then your fuel use will be huge. I'm guessing that what you're thinking of would more along the lines of setting the aircraft up to be able to cruise at a sensible power setting but at 3300 RPM - meaning that full power would take you somewhat above that. It would be a bit of a unique setup and you'd have to adjust your carburettor tune, plus you'd have to be on the ball when flying it to avoid exceeding redline but it could be done.

There are a few other things you'd have to be aware of.. for example the oil pump can suffer cavitation if you run high RPM while the oil is still cold and thick which can cause issues with the oil cooler (splitting) and lifters (oil foaming).

Regards,
Doug.

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·         bob

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Hello Doug, I have engine #33A1618 in a Jabiru sp250. I am flying off grass with trees on both ends therefore need all the power I can get. I have installed a new jabiru adjustable prop @ 20 degree pitch
with that I am turning 2980 to 3010 rpm on climb out
plane preforms much better with this prop and pitch and I true @115 knots around 3100
Others seem to get better performance @ lower rpm and higher pitch

I would like to understand how

Thank you for your response and help Bob Gilliam

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·         Doug Smith

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Morning Bob,

I did forget to mention a big factor with running that sort of rpm: you will lose some efficiency unless your propeller is designed for that rpm. While you can adjust the blade angle of a ground adjustable prop you can't adjust the blade twist - which is optimised for a particular combination of rpm and airspeed. Blade area is another factor which is optimised during prop development: high rpm props for jabirus generally do better with less blade area than a you would use on a STOL prop for example.

Your settings sound about right... Be a bit careful with comparisons unless you're flying side by side with the other aircraft. The number of things that affect performance is huge so it's very hard to be sure that you're comparing apples with apples. And ultimately, if you're setting your aircraft up to get off a grass strip with trees at either end you're going to have to compromise a bit on cruise.
Regards,
Doug.