ORIGINAL: grubs4flying
I always thought inches of prop pitch X RPM equal the maximum speed of the aircraft in a perfect world where there is no drag. Since there IS always lots of drag (more the faster you go), you multiply your maximum speed by a percentage of efficiency of the prop, say 80%. This depends on lots of variables and keep in mind drag comes from lots and lots of places. Props, unclean surfaces, airfoil design, control surfaces out of trim or fighting each other, the angle of attack the wing has to maintain if the loading is high, gear, wheels, etc etc etc.........
..........sorry, wasn't trying to school anybody, just want to make sure I hadn't been steird wrong.....
A lot of people use the pitch and rpm to get a ballpark speed. But as you say, "lots of variables".
This was posted awhile back by Jeremy F. and I thought it was interesting.
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published in MAAC magazine a few months back.
Aircraft.....................Engine............RPM ........Pitch.................Theoretical mph............Actual mph
6ft Chipmunk............90 4 Stroke.....9540.......14x6..................54.20. .........................84.7
Hanger 9 mustang.....100 4 stroke...9240........14x7..................61.25.. ........................72.43
1/A wing...................061 Norvel.....21,990.....5x3....................62.5. ...........................96.4
Quicky.....................40 2 stroke.....18,500.....9x8....................141.. ...........................139
Diamond Dust..........40 2 stroke.....24,400.....7x8....................184.8 ...........................165
So go figure........
For interest, the article says:
Each competitor must fly straight and level over a 528 foot course, both ways and is timed from two pylons at each end. The timing system used is manual timing to a single stop watch, hard wired from each end. This method has proven more accurate than a radar gun that was used in the past. The flying mile is comprised of five timed laps around the pylons.
So yes, there is room for error etc, but I just found it quite interesting when I read it
Another short excerpt from the article that actually made great sense to me when explaining why an aircraft would move faster than the theoretical pitch would allow was
" 'it's perfectly understandable, it's a rotating wing with an airfoil .' As I had a very good understanding of boat propellers of which always had slip factors of from 12% to 30% it became very apparent that air and airfoil s were very different"
I guess when you consider that a wing in forward motion creates lift, it stands to reason that a forward facing rotating wing will create more pull than just what the pitch of that prop is giving as it's creating lift as well.
I don't understand the physics behind it, nor do I want to, all I know is, I've had airplanes that were radar clocked faster than what they "should" have been going and never understood why, this sort of tells me why it COULD be anyway