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Old 19-04-2015, 16:41   #16
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Re: Prop walk depending on prop size?

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Originally Posted by CAELESTIS View Post
What causes the prop walk in the first place? P-factor. The steeper the angle of the prop shaft the more prop walk you will have. This is due to the difference of the ascending and descending blades angle of attack to the water.

P-factor is the term for asymmetric propeller loading, that causes the airplane to yaw to the left when at high angles of attack.

Assuming a clockwise rotating propeller it is caused by the descending right side of the propeller (as seen from the rear) having a higher angle of attack relative to the oncoming air, and thus generating a higher air flow and thrust than the ascending blade on the left side, which at the other hand will generate less airflow and thrust. This will move the propellers aerodynamic centre to the right of the planes centreline, thus inducing an increasing yaw moment to the left with increasing angle of attack or increasing power. With increasing airspeed and decreasing angle of attack less right rudder will be required to maintain coordinated flight.

This occurs only when the propeller is not meeting the oncoming airflow head-on, for example when an aircraft is moving down the runway at a nose-high attitude (in essence at high angle of attack), as is the case with tail-draggers. Aircraft with tricycle landing gear maintain a level attitude on the takeoff roll run, so there is little P-factor during takeoff roll until lift off.

When having a negative angle of attack the yaw moment will instead be to the right and and left rudder will be required to maintain coordinated flight. However negative angles of attack is rarely encountered in normal flight. In all cases, though, the effect is weaker than prop wash.
This is a really good explanation. Please be carefully not to over prop, I had a friend who did it on a new engine that failed after 500 hours. You need to get to hull speed at 90% max rpm without overheating if motor is close to required size.


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Old 20-04-2015, 07:04   #17
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Re: Prop walk depending on prop size?

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Originally Posted by Sailor Doug View Post
This is a really good explanation. Please be carefully not to over prop, I had a friend who did it on a new engine that failed after 500 hours. You need to get to hull speed at 90% max rpm without overheating if motor is close to required size.


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Thanks for the advice. I'm aware that over-propping is not good, but somehow the engine survived for 35 years and is still in good shape. I suppose everybody was careful with not running it too hard by managing to fit the fuel injection with power output and cooling water flow.

And yes, the P-factor explanation puts a new angle on the story. However, as I understand it, it becomes more relevant when acting in flowing medium (e.g. while moving the propeller through water or air), while what I was thinking of was sideways movement of the propeller while the boat is standing still, or almost still. Could it be that in these cases the angle of the prop shaft might not have a too big effect?
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Old 20-04-2015, 08:06   #18
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Re: Prop walk depending on prop size?

P factor comes into play when the prop is not 90 degrees to relative flow.
Some boats like my IP, have very little angle difference, and therefore would have very little if any P factor. And yet, they can have significant prop walk.
I believe the high pitched props exhibit prop walk more than flatter pitched props do, I think the high pitch acts sort of like a paddle wheel and "walks" a prop when there is little induced flow, IE almost no boat speed.
I know when I went from a high pitched prop that had a lot of prop walk, to an Autoprop, my prop walk essentially went away. An Autoprop when there is no induced water flow, is an almost flat prop with very little pitch, it adds pitch as boat speed (induced flow) increases.

I think over propping is very common, My boat with the 17X16 that was on it would only turn 2600 RPM at full throttle (3600 is spec) it was so overpropped. If you don't overload an engine that is overpropped it won't damage it, just you have about 1/3 less usable HP than an engine that is correctly propped.
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