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Prop reving high then low


ralphwishart
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Hi there,

I had an incident involving lines in the prop damaging it. I ordered new carbon props to replace my damaged one then meanwhile I installed my reserve wood prop which I had never used before. Running the engine with thís prop, it would go up to 9100 RPM but after a few seconds it would settle at 7000 RPM. Never suspecting it was a prop issue, I called my dealer and he told me he suspected it had to do with retorquing the cylinder head nuts. I tried that but the problem persisted. Upon arrival of my carbon prop, I installed it an voilá, all is back to normal. Can someone explain how come the prop could reach max RPM and only after a while under rev? I am not going to use that wood prop again but I´m intreaged by this issue.

Ralph

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If I was trying to match a fault to those symptoms, I would say- a slipping belt driving a wooden prop with more pitch than the carbon one it was replacing. As you power up the belt can't initially cope with the torque, the belt slips raising the revs, but then grips, so the revs die back.

If you're gear driven..........then I haven't a clue!

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It could actually be the blades stalling from the center if they are over pitched. That stalled area will then extend as the prop slows down more. All of that happening = more drag = slower prop and more stalled area and repet

Maybe... its a talking point for sure!

SW :D

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Had a similar thing with a prop that was slightly too high pitched, it wouldn't quite reach normal full rpm and then would slowly slow down. Never did figure why it slowed down, I could understand it being too draggy to reach full rpm but not the slowing..

Nige

There's got to be some black magic involved...

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It could actually be the blades stalling from the center if they are over pitched. That stalled area will then extend as the prop slows down more. All of that happening = more drag = slower prop and more stalled area and repet

Maybe... its a talking point for sure!

SW :D

That's the line of thinking I was on, only the air flow should be more laminated at slower speeds and separate at higher revs.

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OK to mash it up even more!!

Now 'add the fact' to the above speculation that on the ground your prop is static while when flying your prop has air mass flowing through it from your airspeed.

The main point being, while on the ground the second blade of the prop can catch up with the shitty air left behind from the prop before it, more easily than when flying or moving through the air.

Again, just adding to the theory pot. :-)

SW :D

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How's about another angle.

A cold engine is more powerful than a hot one.

A correctly pitched prop will reach operating temp at max revs and stay pretty much there.

An over pitched prop will spin up to max revs when cold. but will then quickly loose power and slow down as the motor is forced to work harder and hotter.

Probably :-)

Sent from my iPhone using PMC Forum mobile app

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How's about another angle.

A cold engine is more powerful than a hot one.

A correctly pitched prop will reach operating temp at max revs and stay pretty much there.

An over pitched prop will spin up to max revs when cold. but will then quickly loose power and slow down as the motor is forced to work harder and hotter.

Probably :-)

Sent from my iPhone using PMC Forum mobile app

I thought it was an Engine issue, and that prompted me to remove the cylinder head to look for a faulty gasket and found nothing. I chased ghosts until my new prop arrived. My next theory was stichy rings, luckly it didn´t come to removing the piston.

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