Hi Ed;

    This talk of stalled props brings up another question. Last week when I was doing circuits, on one of the touch & goes, moments after I'd applied power, the engine suddenly revved up momentarily much the same way as when you hit a patch of ice while driving a vehicle with a heavy foot. This happened very fast so I wasn't able to check the RPM (sure wish I had a datalogger), but both my buddy & I heard/felt it. My first thought was the PSRU had slipped, but it had no accompanying mechanical noise (like broken gears) then second thought was prop stall. I held it on the ground a little longer without reducing power, but as we pulled through 90mph with no further indication of a problem ( I still had several thousand feet of runway ahead of me) I let her lift off and then went on to complete another dozen circuits with no further incidents. But afterwards we discussed it further and I recalled Dave's broken PSRU shaft, but if I recall his was a clean break without any sort of preceding slip. This just leaves a prop stall as the likely culprit, but I wouldn't expect that a prop would stall when at approx. 50mph. At the time my electronic prop governor was on auto and had been performing well and in any case the electric IVO prop is too slow to have gone full fine momentarily so I can almost discount this as being related.

    Any thoughts on whether this could have simply been a momentarily stalled prop?

Todd   

 

While stationed at Mustin field, in the Philadelphia Navy Yard, I became a professional prop staller. I was a plane captain on a Beech D-18. So I would fire up the engines, Listen to the LF radio (rock and roll) run the oil up to temp, check the mags and feather the props twice per engine. You just put your thumb over the breaker and hit the red button. When the revs begin to drop enough to make that awful sound you pop the breaker, and let the revs come back up. 

 

Airfoil stall is a function of angle of attack. That can be produced in a prop that is at the limit already, by a slight gust that is slightly off the centerline. If nothing changes, the stall propagates along the blade as the lowered thrust reduces airflow into the prop arc. If enough of the blade remains unstalled to keep the plane accelerating, the prop unstalls and all is well. Otherwise, the plane stays at the same speed, or slows.

 

Reducing power, reduces prop RPM, and that lowers effective angle of attack (like putting the nose down) and the prop unstalls, and all is well. Another good reason for the longest prop available, in that it limits how many revs you can turn the blade (and reduces effective angle of attack).

 

Direct drive VW engines do this very well. Also another good reason to avoid wake turbulence

on take off, where there will be gusting from many directions for as long as you are in it.

 

It is also counter intuitive to power back while to doggy do is hitting the fan (so to speak).

The engine is revving up, and the plane is slowing.

 

Extend that into situations like a low wing and crossed rudder to hold a takeoff heading in a cross wind, Vice letting it weathervane with wings level. In one case the high pitched prop will be exposed to stalling, in part of the arc, as well as being in the turbulent air of part of the cowl. Plus the asymmetrical disc loading, that adds a bit of nose torque up when from one side and nose torque down when from the other, through precession.  

 

Lynn E. Hanover