OK, just to try to stir something up, and maybe for a little personal edification:

We've all seen various estimates of the actual advance a prop makes relative to its geometric pitch. Something like 85% is the most common estimate I've seen. We have also read that a flat bottom airfoil delivers lift even at a 0 degree angle of attack (actually down to some small negative angle, if I'm reading the charts right).

SO, if we bolted an 8x0 Master Airscrew on our OS .15 FP, what would the advance ratio be? If it's making lift it will move the thing forward, won't it? What's 85% of 0? Does that nominal 85% figure change with different pitches?

Just wonedrin'

Prop efficiency?
tricky one, that.
Why?
In actual practice, it is firmly related to the airframe being dragged along.
or worse - pushed along (ever tried to push a piece of chain?)

Example:
a prop which reaches a theoritical 85% of it's possible screwing speed- may do so in a given super slippery airframe.
Now put same engine and prop in a big draggy airframe -
It will never reach that 85%.
So - prop efficiency should always be a statement of prop to airframe relationship . (efficiency)
No/Yes
In my opinion most prop charts are best used -if printed on soft , double ply tissue.

The effieciency would be infinate because the airfoil shape would make "some" foreward lift, and the pitch is zero... any number divided by zero is infinity. Well that's one way of doing it anyways.

The idea of pitching a prop at zero is silly. There are basically two ways to look at prop efficiency. One is to use the slip to calculate how far the prop moves foreward per rotation compared to the pitch of the blade... example an 18x10 prop that is only moving foreward 8 inches per rotation has an efficiency of 80%(8/10). But, this method is flawed in some ways. An 18x10 prop that is mounted to a plane in a hover for example is not moving foreward at all so it's efficiency would therefore be 0/10 which is zero... yet the model stays in the air... hmm that doesn't make sense. A better way to measure prop efficiency would be to compare brake horsepower to thrust horsepower. THP=BHPxprop efficiency. This can be done static by using a fish scale to meauser the thrust, and compare that to the brake horsepower if that data is available... I don't know if anyone has ever put an OS on a Dyno to create the tables needed to do these calculations.

Ty

The term for the difference between how far forward the prop WANTS to pull the airplane in one revolution and how far forward the aircraft actually gets is the "Slippage factor."

This is not a constant for any prop, any airplane or any engine... because it varys with too many things. If you simply increase throttle from 60% to 65%, the slippage factor will increase. If you go from level flight to a climb, slippage increases. If you dive slippage can decrease and may become negative. (the airplane can be pushing the propeller.)

Slow flying FF models of the 1930's to 1940's expected a 50% slippage factor durring thier climb. (and it was far from a vertical climb as seen with modern FF competition models)

The modern FAI Pylon racers would be severely dissapointed to have a 15% slippage factor in level flight... That would indicate they had not properly streamlined the model somewhere. (or there was some other setup error...)

As to the airfoil effect on a prop's thrust... you can easilly demonstrate that it exists. Put a prop in a phillips screwdriver as if that were the engine crankshaft. Point the front of the prop into the wind and let the prop windmill. If the wind speed is above 10 mph, a 10X6 prop will pull itself into the wind, and off the screwdriver. The problem is... its a very difficult factor to quantify. Different propeller designs will be more effective at this than others of the same diameter and pitch. ( A 10X6 APC will "climb" off the screewdriver more rapidly than a Zinger 10X6 wood prop even though the APC takes longer to get to the same rpm...)

Strat,

The pitch is normal associated with the 75% blade section...and a zero pitch prop will be very close to reaching the zero thrust speed. Further, after your prop reachs an AOA of about 5 or 6 degrees the thrust starts to fall off pretty quickly. Now if you take the Thrust generated at a given speed this will give you the power that is being used to drive the model. (Thrust*Speed=Power).....Lets say that our plane reach zero thrust speed (20fps) and let say that at 10 fps and is generating .5 pound of thrust then the power would be 5 p/fps...lets say it is a .40 the manufater rates the engine at .9 HP which is about 500 pfps. 5/500=eff..... 1% eff. Actually for full size airplane 85% effeciency is the normal number used. If you are using the Manufatures HP rating for most RC engines, I would be suprised if you will find a prop that will even come to 30 percent effeceincy. With the correct dyno HP you might be able to get to 40%. So in general to have a comperable plane compare to there big brothers you will need about 2.5 time the HP/wieght ratio to make up for the difference in prop effecency.

Steve

Is propeller pitch relative to the chord line or to the angle of zero lift?

The Cox Lazy Bee ARF came with what was close to a zero pitch prop... Lots of noise, no thrust.
We found putting on a real 6x3 and the plane flew just fine.
Your OS 15 would melt with an 8x0 prop.

Pitch is measured relative to the chord line.

For simplicity... some people that hand carve thier props measure it by the "bottom" of the airfoil. Since the typical prop has a flat bottom airfoil this is reasonably accurate. But the rounded LE of the blade does make it so its off by a little.