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tl;dr: trying to find propeller efficiency with thrust, velocity and power and see which propeller length to pitch ratio is efficient.

Hello, I'm a high school student working on an extended essay for my physics class. What I've basically done is took a bunch of propellers (7x4E, 7x5E, 7X6E, so on and so forth). Propellers of lengths: 7, 8, 9, 10 inches and pitches: 4, 5, 6, 7, 8. (Fixed pitch propellers).

I've taken these propellers and hooked them up to a motor and a weighing scale to try and find how much thrust they produce. Then, I put them on to a control line aircraft to see how fast the plane goes.

Basically, Im trying to find the efficiency of these propellers and the data I have is the thrust produced by the propeller at different power levels and the velocity at different power levels. I'd really like to know if there's a relationship that I could employ to find the efficiency!

It doesn't have to be complex! Just needs to work from a physics standpoint. My deadline's coming up soon and any help would be greatly appreciated.

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    $\begingroup$ This answer should give you the needed theoretical background. $\endgroup$
    – sophit
    Feb 23 at 11:44
  • $\begingroup$ I'm voting to close because of the general lack of quality of this question. I think it's great that HS students come here for information, but it's also incredibly important to meet side guidelines. This question will far outlive the student's HS career, and will linger around the internet for possibly decades. The question decreases the SNR at SE.Aviation, as the poor title and body clarity makes it difficult to understand without taking a lot of time to read it. This nukes search engine results. I would retract this vote if the OP could go through and clean up the text and title. $\endgroup$
    – Kenn Sebesta
    Feb 23 at 11:57
  • $\begingroup$ Paragraph 4 qualifies this question. Let's give it some time to improve. $\endgroup$
    – Robert DiGiovanni
    Feb 23 at 12:00
  • $\begingroup$ Suggestion: can you change the slant of your project, to trying to prove or falsify the idea that the propeller that is most efficient in the static situation, is also most efficient in flight? Or maybe show that starting from knowing which one is efficient on the ground, you can predict that one with a higher pitch will be most efficient in flight? Just brainstorming here, hope it helps. $\endgroup$
    – quiet flyer
    Feb 23 at 13:07
  • $\begingroup$ Your best bet for demonstration purposes should be to take it to the extreme. Compare 6 foot propeller with a 30 foot helicopter rotor blade. They are both the same thing in the end. The 6 foot prop will make maybe 600 lbs of thrust with 150 hp, whereas a 30 ft "prop" turning way slower, will make 4x that. $\endgroup$
    – John K
    Feb 23 at 17:35

2 Answers 2

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Pitch, diameter, rake, cup, material and # of blades will need to be factored to match the manufacturer's optimal wide open rpm of the engine. Lugging or overrevving the engine is not anything you want. A test stand will not do this. Too much pitch, cup, rake and diameter will lug the engine and too little of all or some, will overrev. Also a propeller thats gives good acceleration may over rev once it gets going... and on the other side, one that gives greater top speeds over a longer flight may never reach its optimal recommended rpm. It must be done on an aircraft in flight. Also prop slip, blade cavitation, aerodynamic drag, temperature and altitude will affect results.

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You may already have enough data for a very good report.

But know that propeller efficiency changes with airspeed!

Expect the finer pitch (4,5) to produce more thrust on the test stand, but the coarser 6 or 7 will produce more thrust in the air because forward motion changes the angle of attack of the rotating propeller.

Propeller length depends on engine power. A really small engine can not turn a huge propeller fast enough$^1$. Reynolds number also plays a role. Efficiency decreases if the prop rotation speed is too slow. Though it is better to "move a lot of air a little", there is a lower limit to efficient prop speed, with the upper limit being its critical Mach number.

So, just plot your results, include the horsepower (or kilowatt) of your engine, and get your good grade.

Top speed will be your best prop for cruising, but a finer pitch may be better for climbing (at a slower speed).

For a A grade, measure RPM of your prop on the test stand. Tachometers are readily available for this purpose. Feel free to share your raw data with us, so we may help interpret and draw conclusions.

$^1$ though the Wright Brothers may have disagreed. They turned 2 huge propellers with a really small engine. But, with fixed pitch this will produce maximum efficiency only within a very small speed range. Variable pitch props help solve this, and extend the range of airspeeds at maximum efficiency$^2$.

$^2$ see V-22 Osprey.

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