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Propeller efficiency


David Hall 9
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I recall reading (perhaps on this forum) that at the prop sizes we might use, the efficiency (I guess the conversion of input energy to thrust) increases by a few % for each increment in prop diameter....

I can't find the thread now, If i recall correctly, it claimed that efficiency increase by around 5% for every inch increment in diameter when around 10 inches.

Anyone know if there is a rule of thumb for this increase?

 

 

Edited By David Hall 9 on 18/12/2019 11:01:09

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Posted by David Hall 9 on 18/12/2019 10:53:13:

I recall reading (perhaps on this forum) that at the prop sizes we might use, that the efficiency (I guess the conversion of input energy to thrust) increases by a few % for each increment in prop diameter....

I can't find the thread now, If i recall correctly, it claimed that efficiency increase by around 5% for every inch increment in diameter when around 10 inches.

Anyone know if there is a rule of thumb?

 

 

im not sure there is one that can be directly applied as different prop brands use different sections so you might find brand A is more efficient than brand B no matter what size you use. That said, brand B might be as efficient as brand A but only up to X rpm at which point it all falls apart and just starts turning rpm into noise.

 

Edited By Jon - Laser Engines on 18/12/2019 11:02:46

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I use the rule of thumb that 1 on diameter = 2 on pitch when it comes to prop load at normal RPM.

There are significant differences between makes, for instance I find APC props to be more efficient than Master Airscrew, the black ones with the white tips. Graupner props which are a similar to MA props are nearly as good as APC so you can't tell just bu looking at the shape.

A.

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Ignoring differences between makes, a larger diameter will move the same mass of air but at lower velocity. If MV2 is to be believed then you can shove a LOT more air at lower velocity for the same power level, or as we all know a big fine pitch prop works better for slower models. Once your model approaches the speed that big prop is moving air the only way to go faster is upping pitch which means going back to the smaller, less efficient prop.

If you want to go fast then Jeremy Clarkson has it right - "MORE POWER!" devil

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"Anyone know if there is a rule of thumb for this increase?"

Personally, I'm not sure efficiency changes very much at all by prop size (yes, a tiny bit, but nothing like 5%). If you mean static thrust then I think that 5% rule of thumb may be about right.

However the converse is true when travelling fast. That bigger prop - turning slower - will hurt your top speed.

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Thanks guys.... my purpose here is considering a slow climbout of a powered glider, so it may be closer to the static thrust condition than a high speed model.

It's been my observation that there is a marked increase in efficiency as the prop size is increased, but it's not an observation based on measurements. I suspect that the Open class F5J flyers tend to run higher speed motors through a reduction gearbox for the same reason (although the airframes have now evolved so that even a 28mm outrunner wouldn't fit anyway).

 

Edited By David Hall 9 on 18/12/2019 21:24:49

Edited By David Hall 9 on 18/12/2019 21:25:06

Edited By David Hall 9 on 18/12/2019 21:25:29

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One shouldn't confuse the power density (Watts absorbed per unit area) of a prop and its efficiency (the thrust delivered per Watt).

Yes the tip losses are proportionally less on a bigger diameter but for a given RPM so is the tip speed and the aerodynamic friction losses.

For a given diameter and pitch a prop will be most efficient at a specific air speed and RPM however this point will not necessarily be the point of maximum thrust or give the maximum plane speed.

To make the best use of every Watt you would need at the very least a constantly variable pitch.

Without such it comes down to selecting a diameter and pitch that gives the best overall performance 'preference'.

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