Props...

Guy's I have read that it is correct to change the pitch from BOTH the front and from the back of a prop.
What do the expert's say, and why do it the way you suggest ?
T I A..

Gil,

This is a fairly recent thing to do with the composite props we have now. When heated enough, you can twist in the desired pitch and it stays. This is an excellent 'tuning' item. I expect several who do this regularly will offer you more about the technique, and the reasons they use it.

I'm lookng forward to reading through that, too.


The old way, with wooden props, could get tedious. I'd always heard that you don't work on the back (engine) side. However, I have repitched a few props. It usually required a change in the 'planform' to find enough depth of wood to raise or lower the leading or trailing edge enough to get the desired pitch change.

Of course, all this - heating and carving to change pitch - requires you have a good pitch guage at hand.

Many earlier Zinger props had a noticeably square LE and TE, often enough to repitch a half-inch or so at the TE. I marked the prop at the stations available with the pitch guage. A round file - like one used to sharpen chain-saw teeth - can cut a 'template' depth at the new pitch. Then smoothing the back face to these grooves sets the new pitch.

As I said, can be tedious...

I also usually cleaned up the LE and refined the airfoil a bit. That means assuring the high point is about 30% and the LE entry is a bit sharper.

In fact, I did more cleanup on the front face on props with good rear face pitch, than I did repitching.

Next!

> Guy's I have read that it is correct to change the pitch
>from BOTH the front and from the back of a prop.
> What do the expert's say, and why do it the way you suggest
>?
> T I A..

We usually measure pitch from the flat side of the prop, although airfoil will also affect it. If you round the leading edge up from the flat side, then that effectively raises the pitch. If you have a Clark Y or similar airfoil, then the effective pitch will be closer to what you measure on the flat side. Just remember, that the prop blade is, in effect, somewhat of a wing. Even if you measured zero pitch on the flat side, there would still be some "lift" as a result of the wing "flying" through the air that would result in a certain degree of thrust.

That said, there is always slippage as well. We never get 100% efficiency out of our props due to the drag of the airplane behind. Again, it will depend on a lot of things, but I usually just assume that drag and extra "lift" (or thrust) gained by the airfoil pretty much cancel each other out. (Of course, drag could be greater, again, depending on the size of prop, etc.)

That said, I would set the pitch on the flat side of the prop, and then dress up the front, or airfoil side to where it seems right, including a little bit of rounding (call it a Phillips entry) on the leading edge beginning at the flat side. Thinning the blade will decrease the "extra" pitch, but also decrease the load on the engine. Power Point props have a narrow blade that is quite thick. We have modified these by cutting down a 13 x 6 to 12.5 inch diameter and then thinning the airfoil resulting in a good drop in replacement for a 12.5 x 5.5 Bolly. I call it a poor man's Bolly. A lot of work, though, especially trying to make these repeatable so that you have an immediate replacement should you break one.

Do you want to increase or decrease the pitch? This effects which edge you work on.

To increase pitch you have to raise the flat side of the leading edge. Working on the airfoiled side on the trailing edge has the effect of increasing the angle of attack of the wing, but not the measured pitch.

to decrease the pitch you raise the trailing edge. A lot of props don't offer the opportunity of lowering the leading edge.

The work on the flat side is easy to measure with a pitch gauge, and can be duplicated on a second prop. Work on the airfoiled side is harder to measure and duplicate.

Carl

Just my 0.02 cents worth

I have enough of a challenge just trying to balance a prop without inadvertantly changing the pitch, symmetry or appearance. I usually try to thin the heavy blade on both front and backside without changing the airfoil.