>
"...However, this one is particular applicable:
>
>http://www.supercoolprops.eftel.com/ARTICLES/article_4.htm
>
>That's why you have to spin a 4" pitch Bolly 1500 rpm faster
>than a 4" pitch Eather UCT to get the same speed. Just an
>example.
>
>Brett" >
>Whoa... I was "sort of" following it up until he started on
>"Experimental Pitch", and he completely lost me at the end.
>Which type of pitch are you measuring if you use the
>laser-beam method in the recent Stunt News article?
A sort of geometric pitch, or "back-of-the-blade" angle. For flat-back props, it's just exactly the same as what you get using a pitch gauge.
The term "experimental pitch" is maybe not obvious unless you read the rest of the articles. But think of it like this - you get a propellor spinning at some fixed RPM, then blow air at it in a wind tunnel until it's putting out exactly 0 thrust. Then divide the air velocity by the RPM, and you get the "experimental" or "true" or whatever you want to call it, pitch - the advance rate at which it travels at 0 slip.
With cambered airfoils, I think the "experimental pitch" (I had called it "effective pitch" in the past), this is always *higher* than the measured or geometric pitch. Right off the bat, you can explain one rather puzzling phenomena - airplanes going faster than they are "supposed to be". Take a situation such as this: I have a 11.5-3.7" *measured* pitch prop with a lot of undercamber (Eather UCT "red"). The airplane is timed in flight going 78 fps, and through the "sync method" I know that the engine is turning 10800 rpm in flight. 3.7" = .3083 feet, and 10800 rpm = 180 rev/second. .3083*180 = 55.5 fps - or WAY slower than the observed speed. It's like the prop is travelling further forward per rev (higher pitch) than it's supposed to be.
In reality, you know that it's probably only travelling forward about 75% of the *real *pitch because you know it slips some (in fact, if it doesn't there's no thrust). So, in fact, the "experiemental" or "effective" pitch is in the range of maybe 7". But how can that be? Well, that little equation about halfway down the page tells how much angle you have between the chord line (which is what you measure) and the "zero-lift-line" where the airfoil actually does produce no lift (and is exactly equivalent to the "zero-thrust" condition in the second paragraph above). It's a function (to a first approximation) of camber and high point of the airfoil. Well, I sacrificed a 12-3.75 UCT prop and cut it into slices along the length so that I could scan it in and figure out the camber and high point from an actual prop. I then figured out the relation of the "zero-lift-line" to the geometric pitch angle. It amounts to a "correction angle" at each station. Then I put it into an spreadsheet that did all the calculations for me, and this is what I got:
Brian Eather 12-3.75 UCT "red" , 3.700 pitch measured using calibrated Prather gauge
r....true pitch....zero-lift angle
1.0...5.6920........11.68079884
1.5...5.2297.........7.59146369
2.0...5.3229.........6.55022171
2.5...5.6812.........6.629379469
3.0...6.3689.........7.563645591
3.5...6.9391.........7.961950581
4.0...7.289..........7.71402829
4.5...7.7332.........7.841299863
5.0...8.1085.........7.755171075
5.5...8.8681.........8.28137634
6.0...8.4587.........7.040886411
where r = radius in inches, true pitch = the pitch as determined by the "correction angle" method described above and on the Joe Sueprcool site, and zero-lift angle = correction angle as computed using the high point and camber. Lo and behold, the "effective pitch" is around 7" - just like you might guess from the observed results!
When I did this, it was sort of a Eureka! moment, as it explains a lot of what we have been observing over the years. A Bolly, for instance, has a different airfoil with a lot less camber, and thus it's somewhat closer to what you measure - or alternately, if you want your Bolly to perform more like an Eather, or your Eather to perfrom more like your Bolly, you have to set the pitch to something quite different. Then, and only then, can you begin to understand ANYTHING about the effects of the airfoil shape, L/D, etc. If you don't correct it, then the difference in effective pitch washes out the much smaller effects.
Note that this also explains a lot about why you just can't take a PA61 set up for a Bolly, and just slap on an equivalent Eather, and expect it to work very well, or vice versa. Most of the talk about "efficiency" or "load" of one prop vs. another is a function of the "effective pitch" instead of any intrinsic characteristic of the prop. It shouldn't surprise anyone that a prop with 7" of effective pitch might produce more "load" than one with 5.5" of effective pitch.
I would also add that this just begins to open the floodgates of a whole lot of other questions.
