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Prop size and efficiency

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eflightray19/08/2019 20:07:56
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574 forum posts
128 photos

Many many years ago, (nicads and brushed motor), I did some thrust testing for I think it was my first electric model, found and used a prop that gave the best thrust.

The plane wouldn't fly much beyond a sinking powered glide.

Eventually it dawned on me, (Please excuse the the following, I'm not singing it).

'Thrust - It don't mean a thing if it ain't got that swing ....'. for swing read rpm, for rpm read pitch speed.

I'ts the right prop turning the right speed (pitch speed), that flies the model how you want, not just how much static thrust it measures.

Ray.

Andy4819/08/2019 21:33:46
1358 forum posts
1 photos
Posted by eflightray on 19/08/2019 20:07:56:

Many many years ago, (nicads and brushed motor), I did some thrust testing for I think it was my first electric model, found and used a prop that gave the best thrust.

The plane wouldn't fly much beyond a sinking powered glide.

Eventually it dawned on me, (Please excuse the the following, I'm not singing it).

'Thrust - It don't mean a thing if it ain't got that swing ....'. for swing read rpm, for rpm read pitch speed.

I'ts the right prop turning the right speed (pitch speed), that flies the model how you want, not just how much static thrust it measures.

Ray.

Its not just about the "best" thrust. I work on the basis that if the static thrust is the same or higher than the all-up weight of the plane it will fly well. So far I have not been proved wrong with a whole variety of planes.

Martin Harris19/08/2019 23:02:00
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8741 forum posts
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I suspect that things are very different with greater than 1:1 thrust to weight ratios than when you're relying on aerodynamics for flight.

PatMc19/08/2019 23:10:58
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Posted by Andy48 on 19/08/2019 21:33:46:
Posted by eflightray on 19/08/2019 20:07:56:

Many many years ago, (nicads and brushed motor), I did some thrust testing for I think it was my first electric model, found and used a prop that gave the best thrust.

The plane wouldn't fly much beyond a sinking powered glide.

Eventually it dawned on me, (Please excuse the the following, I'm not singing it).

'Thrust - It don't mean a thing if it ain't got that swing ....'. for swing read rpm, for rpm read pitch speed.

I'ts the right prop turning the right speed (pitch speed), that flies the model how you want, not just how much static thrust it measures.

Ray.

Its not just about the "best" thrust. I work on the basis that if the static thrust is the same or higher than the all-up weight of the plane it will fly well. So far I have not been proved wrong with a whole variety of planes.

Yes "if the static thrust is the same or higher than the all-up weight of the plane it will fly well" but only sometimes, it has to be always in order to say that it is a rule.
Ray and myself have previously recounted examples when static thrust that was the same or higher than the all-up weight of the plane was not sufficient to fly the plane well or even at all in level flight.

Ref **LINK**

Bob Cotsford20/08/2019 10:32:42
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7931 forum posts
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To take an extreme example, using a big enough prop you could achieve the model's mass in thrust by moving a lot of air at say 10mph. Great acceleration from a standstill up to 10mph (ish) but unfortunately that also limts the model's flying speed to a similar figure. Moving a smaller mass of air at 60mph for the same static thrust (M*V*V and all that lark) may give lower acceleration but a practical achievable airspeed. Stalled props also have a big effect on the static figure as Martin pointed out.

Static thrust is a guide, but as has been said, pitch speed counts for a lot. It's all a compromise to suit the airframe and how you want to fly it.

Nigel R20/08/2019 11:53:17
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2986 forum posts
472 photos

"This is why I posted - it made me go back and re -examine the formulae I used - I had the square in the wrong place!! Amended figures below. However, the general principle and the question is still valid."

Off the top of my head, the simple formula used takes no account of inefficiency around centre of prop, inefficiency around tip of prop, speed differential between static body of air and the air moved by the prop, drag of power train components in the airflow... among what are probably many other real world effects

A question - do you really believe a 9" prop moves over twice as much volume of air as a 7" prop, give a very similar power consumption?

As a more "real world" test, what about measuring energy consumed over an entire flight on a smaller prop, then repeating with a larger prop, I would hazard the suggestion there will be only a little difference over the sort of prop size variations you are looking at here. I think you would need to fly an identical pattern from flight to flight, i.e. a "minimum power to stay aloft" type pattern of easy circuits. It takes a certain amount of power (thrust?) to pull any given airframe through the air at a given speed, so this should in some way serve to assess the two props at the same output.

Toni Reynaud20/08/2019 13:29:47
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387 forum posts
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" Off the top of my head, the simple formula used takes no account of inefficiency around centre of prop, inefficiency around tip of prop, speed differential between static body of air and the air moved by the prop, drag of power train components in the airflow... among what are probably many other real world effects "

Absolutely right, BUT if all calculations are made on the same basis and contain the same errors, then the comparitive results are acceptable. It all started ages ago because I wanted some idea of the different size of prop I would need when changing from a direct drive Speed 400 to a 2.5:1 geared Speed 400 - about 40% less rpm, approximately the same power, so what size prop would work?

 

Edited By Toni Reynaud on 20/08/2019 13:42:10

Nigel R20/08/2019 15:08:49
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2986 forum posts
472 photos

Well that's just it, I don't think you can easily determine that they contain the same magnitude of error...

I guess the answer to the second question could be determined empirically - "whatever size results in the same power draw". But that is drawing on the "W/lb" rule of thumb, rather than trying to use a 'volume of air moved' type method.

Interesting topic!

Nigel R20/08/2019 15:52:54
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2986 forum posts
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Propeller Efficiency Rule of Thumb

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