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Do RPM change if no prop is attached?


duncan_burdorf
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Today I ran the motor without a prop to see what the RPM are but am not sure if the RPM changes due to no prop attached. I cant attach my prop as I have a faulty pulley and waiting for a new one.

My F200 without the prop runs idle between 3800 and 4050. Its this normal or should this be only measured with a prop?

If so, I am also awaiting a new wooden (two blade) prop will my RPM change between the 3 blade carbon fibre and the 2 blade wooden prop?

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I'm no expert but yes, there will be a dramatic difference between the rpm of the motor under load between having a prop on and not having a prop. This applies both at tickover and higher revs.

Be very careful when applying higher revs as there is a real danger that you could over rev and damage the engine.

Do you know what rpm the motor ran at full throttle before removing the prop? If you have a tacho you can then at least ensure that you don't exceed that, but the revs will rise very quickly without a prop!

I'm sure someone more knowledgeable will be along in a bit.

Ian.

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Hi Ian,

I do not know what the RPM with the prop was as I just bought this motor 2nd hand.

I did use a bit of throttle but did not exceed 6000 RPM as I don’t want the faulty pulley to break and cause more damage.

I did suspect that the RPM will be influence as the prop adds weight and resistance but wanted to be so sure.

Do you know if the new wooden prop which has only got 2 blades will make a difference?

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Having had a quick google search (so far from gospel!), it seems the max rpm for that engine is 8,000 rpm. It would definately be worth trying to get a copy of the owners manual for your particular paramotor to make sure though.

Measuring the idle rpm without a prop is pretty pointless as far as setting the motor up. As soon as the prop is fitted the load will slow the motor down and any settings will have to be re-made.

I worked on 7,600 rpm max for my parajet when I was setting up the pitch on my 3 blade ground adjustable prop. I would think that there will almost certainly be a difference between the performance of your original 3 blade carbon and new 2 blade wooden prop, but if it is designed for your machine then the best way to find out is to fit it and give it a run up/fly with a tacho to ensure that it doesn't over or under rev.

Over revving will possibly cause early engine wear, under revving will possibly not allow you to achieve max engine power and so max thrust.

Ian.

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Just remember that your machine has no forced air cooling (like a Ros or Top80) so you are dependent on the prop to draw cooling air over the fins. This is why the redrive takes the hub to the other side of the crank from the cylinder whereas forced air cooling machines can be more compact by taking the prop shaft straight behind the cylinder.

What are you really achieving by running it now rather than waiting for the new prop? I can only think of problems.

Another F200 with pulley problems :roll:

We did try to warn you! The previous owner list doesn't automatically mean it is a good machine!

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I highly advise that you wait for a prop before running.

Same advice for any paramotor.

Enjoy your new motor! May you have many hours of epic flying with it!

SW :D

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