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R22 engine

Old 25th Oct 2014, 05:34
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R22 engine

I never thought and I was never told about this so sorry if my question is somehow dumb.

Why piston engines rev at such low rpms?
Robinson always struggled to make the R22 a light aircraft so wouldn't have been better to put a smaller, lighter block instead of a 5.9 litre and make it rev at higher rpms to produce the HP required?

The answer I gave myself is because he (Robinson) used a fixed-wing engine and those kind of engines rev at low rpms otherwise the tip of the blades would go supersonic or something like that..
But I don't have anyone telling me if I'm right or wrong hence this post.

If this is the case, do other piston helicopters out there use engines made for helicopters?
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Old 25th Oct 2014, 08:50
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The engine used isn't actually that heavy and is reduced in power by limiting the MAP allowed.
A large, slow revving engine is going to be more reliable than a smaller harder working one.
I've just measured an R22 piston for you, it has a diameter of just under 130mm and weighs about 1.3kg.
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Old 25th Oct 2014, 09:00
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I feel the answer may be , its a pretty Bullet proof engine, been around for many years, so has much history of service/ failures history of such things all help to make it the engine of choice for the R22. like its big (6Cyl) brother engine from the same maker and design is used for the R44.

Also both engines are already cleared to fly by the relevant World authorities , plus spares are available all over the "Civilised" World.

To develop a new and untested engine in today's market would cost an awful lot of Shekels..!
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Old 25th Oct 2014, 15:19
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MitchStick, helicopters have transmissions, so the problem of the blades going supersonic is not valid. You would just need to design the helicopter for a high rev engine.
The problem is reliability, and certification, as stated above.

As for the weight, the Lycoming 360 used in the beta II weighs approx 130kg. A motorcycle engine, for example the Yamaha R1 engine, producing about the same max power, but less torque, weighs about 80kg.
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Old 25th Oct 2014, 22:52
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Great answers, guys. Also, higher rpm will require another stage of gears in the transmission, adding weight, cost and loss of power. A turbine engine requires that extra transmission stage, but its power to weight is much higher than a piston engine.

R88 is right on, let me add that the CTS800 turbine weighs 140 kg and puts out 1400 horses
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Old 25th Oct 2014, 23:29
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Thanks all.

I didn't think about the extra weight added with a bigger gearbox so probably with a lighter engine but heavier gearbox the weight difference would be little.

It make sense what you're saying about the spare parts and reliability.

the beater
When I went to the Robinson factory I actually remember them saying that Robinson chose that engine cos it had longer life-limited parts than others, so you're right.

In regard to the tip speed I was actually referring to fixed-wing aircrafts, cos I know the propeller is mounted directly to the engine with no gearbox so the rpms are limited by the speed of the blades..
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