Propeller cavitation?

If the overspeed is 1 second or less I would question first if the engine is actually doing this or if the tachometer is faulty (more likely). It takes a little time for the engine to speed up and slow down. Can you confirm the overspeed acoustically?

I think cavitation is a marine propeller phenomenon in which the water vaporizes. This cannot happen in air, as far as I know. Please advise if I am wrong.

Bill is correct, cavitation is a mixed phase flow of liquid and vapor within a rapidly changing pressure zone of liquid.  It can be very destructive of water pumps and marine propellers.

It does sound like fan surge, where the flow rate through the fan becomes unstable.  How many times has this occured, only once?  What is the pressure altitude you are flying at when this occurs?

What diameter is the 4 blade propeller?  What rpm is the propeller turning at?  Could you post a couple of pictures of the installed propeller?

My first guess is you might have to get a different propeller.  Have you asked the people at Warp Drive for their opinion on this phenomena? 

These opinions are based on my experience in ventilation design, not aircraft design, by the way.  I would also be interested in what others have to say on the issue.

Dave,

Here's how to determine if your prop is cavitating.  Prop tip speed MPH = (RPM x Prop Dia") / 336.15.

The transonic range begins at about 625 MPH (Mach .85) and continues to Mach 1 (736mph).  A tip speed of 625mph is fast enough to cause compression of the air and at least two mini sonic booms per revolution.

This info is from an article by Syl Heumann in the Sept/Oct 2004 issue of Cal Pilots Assoc publication.

 "Can you confirm the overspeed acoustically?"

Yes. It is distinctively audible and correlates with the tach reading.

"I think cavitation is a marine propeller phenomenon in which the water vaporizes. This cannot happen in air, as far as I know."

Wrong term, I guess. My thoughts are that  the propeller air foil is stalling, briefly, induced by turbulent air flow around the engine and muffler.

"It does sound like fan surge, where the flow rate through the fan becomes unstable.  How many times has this occurred, only once?  What is the pressure altitude you are flying at when this occurs?"

 My thoughts, exactly. This phenomena occurs every time I fly, therefore varying conditions. It is always during acceleration during take off (~25-35 mph) and occasionally early in climb out. I suspect the problem may be worse with heavy loads and warmer air temps although it occurs with lighter loads and cold air.

 I have calculated the propeller speed, although I do not have the info handy, and it is well below Mach 1.

 I have a 3 blade Warp Drive that I will install soon and will give an update on performance.

 Thank you all for your input . Any further input is appreciated.

It could be stalled at low speed, but a stalled airfoil has more drag, not less. Same for transonic.

This a mystery, I have not heard of this.

David, RPM surging with a 582 can be caused by the big gear slipping on the prop shaft.  I would check to see that your gear box has had the updates done to prevent this.  I can't figure out how to post links here but it is Rotax Service Instruction Bulletin #SL-06-98.  The procedure is also referenced in California Power Sports tech articles "Part 55 and 56" near the bottom of part 56.  www.800-airwolf.com

Bill and David

Ried

Tom,

Good post, I did not think of a mechanical slipage between the engine and the prop, and this would be a better and simpler explanation for the problem.

Ried

Thanks to all for your input. Tom, very helpful info. I will be checking the gear box ASAP and report back.

Dont exactly know if my info will help, but here is what happened to us.

We were running a turbo charged honda motor, and if I remember correctly, it was a Warp Drive prop. We had set the pitch high so that at cruise we had a decent RPM. On takeoff, you could hear the prop stall due to the high pitch. We just learned to go about 3/4 throttle until about 40MPH or so then opened her up. But I dont ever recall the prop stalling on climb. Of course, the turbo'd Honda had a LOT of power. I am leaning towards the gear box theory.

If you can get your hands on an optical tachometer and verify the rpm change in the prop you may be able to isolate the problem to the engine or propeller. I assume the same problem is encountered on a full power static run up. You may be able to detect a drop or increase in thrust associated with the rpm change when the aircraft is stationary.  Blade stalling should be consistent and repeatable in calm winds and be affected by a head or tail wind gusty conditions.  

Problem solved! A special thanks to Tom Jones for your research finding the service bulletin. That was exactly what the problem was.

We dismissed the theory of mechanical slippage because the prop is gear driven and we assumed that the prop shaft was coupled to the drive gear with a moon key as most other shaft/gear assemblies are. That is what you get for assuming.

"I assume the same problem is encountered on a full power static run up."

 Grant, that was one of the confusing points in this ordeal. It never did this in a static run up and I do this every time before takeoff.

 You guys were a great help. Thanks!