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A prop is a wing that flies in a circle. Right? The most efficient wings (the ones that put on gliders), are very long and thin to get a high aspect ratio. Right? Given this premise, it only stands to reason that the best prop would be the longest one that would leave some ground clearance, and then absorbs any remaining Hp by increasing the chord.
That's my story, and I'm sticking to it (unless I'm wrong).
Ed Anderson wrote:
Lynn, I've looked into trying to calculate such several times and after getting a headache each time I gave up. Prop calculations makes duct calculations appear simple {:>). Having said that here is one equation (below) that relates Thrust "T" and propeller diameter "D". There are only three other significant factor in the equation. There is the speed of the air stream into the prop disc "v" and the acceleration imparted to the air stream by the prop "Dv" - air density is what it will be and you can't affect that (except by where you fly).
so if you look at the factors one could conclude that Prop diameter appears to be the most signficant factor (at least in this momentum equation). The Thrust is directly proportional to the square of the diameter. So if all else is equal then a 74" dia prop would produce 74^2 = 5476, whereas 76 " dia prop would produce 76^2 = 5776
or approx 5467/5776 = 0.9467 giving approx 5% more thrust for the slightly longer prop.
I went from a 68" prop to a 76" prop when I switched from my 2.17 to the 2.85 gear box. I was very impressed with the change in take off performance. So comparing the two (Yes, RPM went down a bit on the prop with the 2.85 so in real life the comparison is skewed), but in any case at the same PROP rpm. The 66^2 = 4356
going to the 76 gave me 5776, comparing thrust for the two 4356/5776 = 0.75 or a gain of 25% in thrust for the same prop rpm - now that will get your attention. However, this equation clearly does not take prop chord into effect and based on the simple chart of three data points - it would appear to play a major role in static rpm and thrust.
*From:* Lehanover@aol.com <mailto:Lehanover@aol.com>
*Sent:* Friday, October 08, 2010 1:54 PM
*To:* Rotary motors in aircraft <mailto:flyrotary@lancaironline.net>
*Subject:* [FlyRotary] Re: Prop Chord vs RPM
In a message dated 10/8/2010 11:48:42 A.M. Eastern Daylight Time, eanderson@carolina.rr.com <mailto:eanderson@carolina.rr.com> writes:
I did a plot of chord of the three props mentioned recently. The
props were of 74-76 inches in dia and made by Catto (2) and
Performance Prop (1). Note my Performance Prop was shorter only
74 compared to Sams and Dennis Catto Props. When mine was 76" in
dia my static was lower than 6000 which would have all three props
pretty much on a straight line. I don't know if one could make
much of a linear relationship between prop chord and rpm based on
just three data points, but thought some might find it interesting.
And now how about the thrust for each diameter and blade chord and pitch. What is the most effective prop for the most common diameters?
Lynn E. Hanover
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