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Prop size and efficiency

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Toni Reynaud19/08/2019 09:04:38
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Disconnected jottings - I have seen one or two comments on this forum that a bigger prop spinning more slowly is more efficient. A good few years ago I was involved in testing some brushless motors and producing performance graphs for them. At one time or another I have wondered about just how much air a prop actually moves and made up an Excel spreadsheet to help the thought process.

Add that lot together, and in a moment of boredom (or maybe curiosity) I took some information from three of the said performance graphs, put it into the air moved spreadsheet, and came up with some interesting figures. See the next post.

 

 

Edited By Toni Reynaud on 19/08/2019 09:26:42

Toni Reynaud19/08/2019 09:19:03
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prop size vs eff 01.jpg

Toni Reynaud19/08/2019 09:21:29
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It is evident from the figures that a big prop turning more slowly moves more air for less power. Should I therefore go to 2s LiPos or lower Kv motors with bigger props, or am I missing something?

Over to those who know about these things for discussion.

Edited By Toni Reynaud on 19/08/2019 09:26:05

Cuban819/08/2019 10:12:11
2641 forum posts
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Interesting data - I can only comment that when designing an electric installation and taking into account the model and its expected performance envelope, I always start with the largest diameter propeller that will give adequate ground clearance for a conventional tractor arrangement. Compromises may need to made for pushers. Another benefit is that the model will be much quieter with a larger and slower prop.

Andy4819/08/2019 10:38:49
1358 forum posts
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I think those charts are over simplistic. I tried something similar but using a simple practical test rig that measured the actual static thrust and current for any throttle position. I came to the conclusion that there was no simple answer, and now I test every new motor type with a variety of props at a range of different currents. At full power, results are very misleading as efficiency drops off considerably. I do know, contrary to Cuban8's post that the largest prop is not always the most efficient, and there is some difference between different makes of propellor. Also one must consider the maximum current the system can cope with. Too large a propellor can take the current consumption way above what is prudent.

Also in every case, a higher voltage battery (within the propulsion system limits) produced a far more efficient system at half power.

Incidentally, before someone comes in and says static thrust does not relate to what happens in the air, I tried a few tests on a very windy day >25mph, and found the static thrust to be within a few % when pointing into the wind and when pointing with the wind.

Bob Cotsford19/08/2019 10:50:50
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One feature missing is the air velocity as this would reflect on the sort of speed your airframe can achieve. I'd agree that this table gives a fair representation for lightly loaded airframes where speed (hence drag) isn't critical but when you want to go fast you need air to be displaced even faster (to account for that airframe drag). That takes low pitch and very high revs or higher pitch at lower revs. Which means a smaller prop/fan diameter to keep power within acheivable limits.

Chris Freeman 319/08/2019 11:52:31
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305 photos

This is the topic that resulted in me building another Spitfire early this year and the conclussion that I reached was that a larger prop is more efficient. You also need to consider the airframe to be used as some airframes will require a smaller, higher pitch prop.

Mike Blandford19/08/2019 12:23:24
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I'm curious as to what formula you used for the "Volume of air per rev (cubic inches)" value.

For a 7x5 prop you have 3023.35. I would expect the calculation to be 3.5 * 3.5 * pi * 5 (as in pi * radius squared * pitch) for which I get 192.42.

You appear to have used (3.5 * pi * 5) squared!

My understanding is you need to consider the velocity of the air moved as well as the volume (mass) moved. The energy imparted to the moved air is kinetic energy, so is 0.5 * mass * ( velocity squared). This means a larger prop of the same pitch as a smaller prop, but rotating more slowly, will move a greater volume (mass) of air, but with less velocity. The energy imparted to the air may then be less.

Calculating this energy for the 7x5 and 10x5 props for the first motor in your list, I find the 10x5 is only about 16% "better" than the 7x5, as opposed to about 53% from your spreadsheet.

For electric power, there are electrical losses, and these are mainly due to the current flowing, and proportional to current squared. Moving to a lower voltage, and hence a higher current is likely to increase these losses, and the extra loss may be more than any gain from a larger prop. Personally, I try to use a higher voltage with a lower Kv motor to reduce the current while driving a larger prop.

Mike

Frank Skilbeck19/08/2019 12:30:22
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Ecalc is a fairly good comparison tool, here's some selections I ran for looking at prop motor combinations for a scale model

5065-450 Kv

Prop 16x8, battery 5000 mah 6s

Thrust – 5.7 kg

Power – 1300 w

Amps – 68 A

Rpm – 8,100

5060-380 Kv motor

Prop 15 x 10, battery 5000 mah 6s

Thrust – 4.8kg

Power – 950w

Amps – 45 A

RPM – 7,280

Put a 16 x 10 prop and

Thrust – 5.5kg

Power – 1130w

Amps – 55A

Rpm – 7100

5065-420kv

Prop 16 x 10, battery 5000 mah 6s

Thrust – 6.2 kg

Power – 1300 w

Amps – 68 A

Rpm – 7,500

With a 16 x 8 prop

Thrust – 5.3 kg

Power – 1200 w

Amps – 58 A

Rpm – 7,750

note Ecalc also gives you a theoretical speed but as all motor/prop combinations gave adequate speed I didn't note that down.

Geoff Sleath19/08/2019 12:49:10
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Posted by Mike Blandford on 19/08/2019 12:23:24:

I'm curious as to what formula you used for the "Volume of air per rev (cubic inches)" value.

For a 7x5 prop you have 3023.35. I would expect the calculation to be 3.5 * 3.5 * pi * 5 (as in pi * radius squared * pitch) for which I get 192.42.

Mike

Yes, that's what I get too (to umpteen decimal places on my Poundland scientific calculator )

When I first got interested in aeromodelling (not as long ago as my age might imply) I did a similar exercise because I was trying electric flight. I'd just retired so, as an exercise to prove to myself I hadn't totally lost it I designed a tacho with a Microchip embedded processor. I wrote some of the software to calculate the thrust for a measured rpm. It was an interesting project at the time but not really useful as it would only estimate static thrust which isn't necessarily of value except for initial take-off or (say) prop hanging.

I still have my list of air volume/rev for different props and use it occasionally as a first step to see how different props will load a motor. It's mainly to give me an idea of what props to buy and save money.

Geoff

Martin Harris19/08/2019 13:29:10
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Posted by Andy48 on 19/08/2019 10:38:49:

Incidentally, before someone comes in and says static thrust does not relate to what happens in the air, I tried a few tests on a very windy day >25mph, and found the static thrust to be within a few % when pointing into the wind and when pointing with the wind.

This is an extremely interesting observation - it certainly flies in the face of accepted wisdom and I'm surprised it hasn't raised any comment. Could you post a picture of your test rig?

Toni Reynaud19/08/2019 14:26:31
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387 forum posts
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Thanks for all the replies, gents

I'm curious as to what formula you used for the "Volume of air per rev (cubic inches)" value.

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.prop size vs eff 04.jpg

Toni Reynaud19/08/2019 14:49:14
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387 forum posts
46 photos

The test system was fairly basic - a 90 degree rig with one arm vertical with the motor mounted on it, and the other horizontal reaching out to an electroni kitchen scale for thrust measurments. We were using a Medusa Research Power Analyser Pro connected to a PC, and a selection of battery packs and props. Sanyo SCR 2400 NiCd cells and a Castle Creations 25A ESC were used for all tests.

fig 1 test stand overview.jpg

This graph is representative of the results we obtained.

graph.jpg

Andy4819/08/2019 14:50:46
1358 forum posts
1 photos
Posted by Martin Harris on 19/08/2019 13:29:10:
Posted by Andy48 on 19/08/2019 10:38:49:

Incidentally, before someone comes in and says static thrust does not relate to what happens in the air, I tried a few tests on a very windy day >25mph, and found the static thrust to be within a few % when pointing into the wind and when pointing with the wind.

This is an extremely interesting observation - it certainly flies in the face of accepted wisdom and I'm surprised it hasn't raised any comment. Could you post a picture of your test rig?

Check out an issue of RCM&E some years ago it was in there. No idea which one now, I've thrown it out. Basically all you need is a simple but reasonably accurate luggage scale.

Has anyone ever done any tests on models as opposed to real aircraft, because most data seems to apply to the latter. Take this graph for instance:

thrust.jpg

Notice the efficiency at 15mph is roughly that at about 75mph. Its only above that that airspeed has a significant effect. Most of my planes fly at up to 60-75mph as measured with a pitot tube air speed indicator. Thus this graph would seem to back up exactly what I found.

Toni Reynaud19/08/2019 15:05:47
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387 forum posts
46 photos

I suppose the bottom line is that it's always worth trying different props, sometimes at odds with what we might expact to work. As long as the plane gets off the ground and can fly safely enough to check the characteristics with the new prop, all should be well. LiPo duration also comes into play. I will go away and try a 3S and bigger prop instead of a 4S in my Travelair at some stage.

Chris Freeman 319/08/2019 15:06:10
235 forum posts
305 photos

Static thrust is not always a good indication of what to expect in the air. A few years ago I test flew a 1/5 scale Spitfire for a mate and it was powered by a ST 3250 and the static thrust was good on the ground and the motor sounded good so I elected to test fly. The Spitfire accelerated well and I lifted off but the aircraft did not continue accelerating but stayed at the same spped which was just on the stall. I managed to do a circuit and land and when questioning the owner I found that a 20x6 prop was used which was why the plane did not accelerate as it needed more pitch.

A watt meter give a good indication of what to expect but the truth will only be revealed in flight. I have a Hanger 9 ME 109 with a AXI motor that needs the throttle to be opened slowly otherwise it will cut out and the amp draw is high showing that only short flights will be possible yet in the air I can fly for 7 minutes and still have 35% left in the 6 cell lipo indicating that the amp draw is a lot lower once the aiframe is at speed.

I have also found that many of the chinese motors specifications are not accurate and the prop size they recommend appears to be wishfull thinking. The other surprise is that the thinner air in Johannesburg South Africa normally means that we can run a slightly larger prop or one with a little more pitch.

Cuban819/08/2019 15:09:57
2641 forum posts
13 photos

You can really tie yourself up in knots with this, so I like to keep things fairly simple but with an eye to the fact that the subject is far from simple!

Through trial and error (but with test equipment -wattmeter and electronic fishing scales) I have found that for sport type or club aerobatic 'planes, initially designing for a prop towards the larger size range of what's practical is a good place to start. I then aim for suitable static thrust/rpm/current/voltage depending on what I'm after and select components to suit. The APC website gives useful prop info and there's some excellent performance preditor websites where prop size and RPM can be plugged in to give expected static thrust and power figures. Given all the possible variables, they're usually good to better than 15% of real world performance - which isn't bad and gives a bit of 'tweaking room'.

Cell count/motor Kv/ power can then be juggled around to match what your after or what's parts are available from suppliers.

 

 

 

Edited By Cuban8 on 19/08/2019 15:19:06

Martin Harris19/08/2019 16:12:51
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8741 forum posts
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Posted by Andy48 on 19/08/2019 14:50:46:
Posted by Martin Harris on 19/08/2019 13:29:10:
Posted by Andy48 on 19/08/2019 10:38:49:

Incidentally, before someone comes in and says static thrust does not relate to what happens in the air, I tried a few tests on a very windy day >25mph, and found the static thrust to be within a few % when pointing into the wind and when pointing with the wind.

This is an extremely interesting observation - it certainly flies in the face of accepted wisdom and I'm surprised it hasn't raised any comment. Could you post a picture of your test rig?

Check out an issue of RCM&E some years ago it was in there. No idea which one now, I've thrown it out. Basically all you need is a simple but reasonably accurate luggage scale.

Has anyone ever done any tests on models as opposed to real aircraft, because most data seems to apply to the latter. Take this graph for instance:

thrust.jpg

Notice the efficiency at 15mph is roughly that at about 75mph. Its only above that that airspeed has a significant effect. Most of my planes fly at up to 60-75mph as measured with a pitot tube air speed indicator. Thus this graph would seem to back up exactly what I found.

I'm wondering if your 25mph winds have hit "the sweet spot" and given unrepresentative results in compensating for the results into and down wind? I'm not questioning your observation - just the conclusion that airspeed has no bearing on results versus static testing. I do know that high pitch props are stalled at low airspeeds - my clubmate's F5b 7 or more kW powered 18 x 20ish prop (or thereabouts) accelerating from low airspeed demonstrates this perfectly...

Denis Watkins19/08/2019 18:42:48
3814 forum posts
54 photos
Posted by Toni Reynaud on 19/08/2019 09:21:29:

It is evident from the figures that a big prop turning more slowly moves more air for less power. Should I therefore go to 2s LiPos or lower Kv motors with bigger props, or am I missing something?

Over to those who know about these things for discussion.

Edited By Toni Reynaud on 19/08/2019 09:26:05

Please continue this discussion as it is very interesting

I don't know the numbers, but do know that a previously marginal model has been improved by a change in pitch or diameter by as little as 1"

3 times I have transformed marginal models, to a pleasure to fly, and going up or down a size, against the numbers

Even against the Calcs, and the model has clearly improved.

It would be such a bonus to have a spreadsheet guidence, as well as wattmeter and a handful of props

Edited By Denis Watkins on 19/08/2019 18:44:36

Denis Watkins19/08/2019 18:42:50
3814 forum posts
54 photos
Duplicate post

Edited By Denis Watkins on 19/08/2019 18:43:37

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