Propeller Efficiency

[Chuck Irby]

Question:

Since we pitch our air props in degrees, can someone tell me how to convert degrees to inches so I can determine slippage?

This probably makes no difference as far as the question and it's answer, but I am turning a three blade, 60" Warp Drive on my Dominator at just over 16 degrees of pitch. To me, this seems like a lot of pitch. However, my climbout rpm is 6640, which is where I want it. (Rotax 618 with 3:1 redrive)

When I was racing outboards, it was easy to determine the efficiency of the prop, because they were pitched in inches. Some of my better props would perform at about 92% efficiency. This was on a light weight 20' race boat at 110 MPH.

[C. Beaty]

The advance ratio of a prop is simply the tangent of the incidence angle times its circumference at whatever station measured.

The efficiency of a prop is the ratio of power applied to the prop Vs. thrust power applied to the airframe. Has more to do with propeller disc area than pitch setting.

The picture below shows how to obtain the tangent of 10º from the calculator included with Windows. Enter 10 and mash the tangent button.

[Chuck Irby]

Wow, thank you Mr. Beaty. When I find time to do the math, if you don't mind, I'll report back to you with my findings so you can check it for me.

[Russell]

Chuck I think I had 15 deg.on the prop and it was turning appox 6800. And did it ever climb at that setting!
Russ

[Chuck Irby]

Russ, thanks for that, somewhat of a conformation. I pitched the prop to 6640 on climb out, and then checked it and determined that it was slightly more than 16 degrees.

What caused me to start wondering about propeller efficiency was when I discovered that I can maintain altitude 100 feet over the runway at 60 MPH at just 5120 RPM. At that RPM my fuel burn is only 3.5 GPH (I have a fuel flow meter on it). To me, this is amazing! Were your results similar, even though you were running a little less pitch?