Prop Size vs Power Reqd

[Steve Hargreaves - Moderator]

Does anyone know of a wesite or an spreadsheet that shows the sort of power (in watts) required to turn a specific prop size at a certain speed (rpm)?

As we all know one of the joys of electric flight is the ability to alter the performance of a model completely just by changing the prop. Manufacturers/suppliers often quote the best prop sizes for MAXIMUM power & current draw but what if you don't need that? What if you take that motor out of your sports aerobatic model & stick it in something else that only requires (say) 100watts to fly it rather than the 170 you had previously? How do you find a good starting point for your prop size given the power you want to consume & knowing the kv of your motor?

I suspect the answer will be experience & guesswork but a graph of the power consumed against the revs turned would be invaluable to choosing a starting point for yer prop....yes it would be a huge task to collate all this info which is why I'm hoping someone has already done it for me!!!!!!

[Tim Mackey]

Not quite the answer you are looking for I know, but remember that efficiency will fall off quite a bit if you prop a particualr wind motor to rev at a reasonable different range than that which it was designed for.

AXI for instance give quite informative charts about their motors efficiency levels when using different props.

[Steve Hargreaves - Moderator]

I'm sure what you say is true Timbo but my understanding of electric motors is that they are essentially a fixed speed device & will turn at what their kv figure dictates. If we take a 1000kv motor & put 10 volts across it it wants to turn at 10,000 rpm whatever prop you stick on it. Granted a load (ie a prop) will stop it reaching this rpm & the greater the load the further away it will be. This "slip" is all very necessary to produce the torque (simply put, more slip equals more torque...up to a point) but more slip equals a higher current & hence more power to drive the load.

Lets say our motor is rated at 300watts & it will turn a 10 x 6 at about 9500 rpm & consume 30A whilst doing it. Put a 12x6 on it & the revs go down to say 8500 but the current goes up to say 40A & we risk melting the motor because it is too far away from its rated kv. The opposite is also true but in a different way......lets put an 8x6 on it. It spins at 9800 & consumes 20A....lets be daft & put a 6 x 4...it spins at 9,999rpm & consumes 5A.......no prop at all & it hits 10,000rpm & consumes 1.5A.

My point here is that if you over prop & get too far away from the rpm dictated by the kv the current will shoot up & you will melt the motor....under prop & you simply consume less power, the revs will never go over the kv value...

Does this make sense or am I talking complete rubbish as usual......?? (am now going to look at the AXI charts in an attempt to shoot myself down or rather increase my understanding!!!)_

[Tim Mackey]

No...you are completely correct and have to a large extent answered your own question.

My point was merely about "efficiency" and as we know ( dont we ? ) brushless motors are far more efficient than brushed ones, but all of them have a certain performance target ( in rpm and current flow ) to achieve the maximum efficiency in reference to power in ( watts ) versus actual power out ( also watts...but sometimes converted into good old fashioned horsepower )

[Steve Hargreaves - Moderator]

H'mm...just looked at the info suggested Timbo & it makes interesting reading & (I think!!) supports my theory.....

 I looked at the AXI 28/20/10 as a good middle of the road motor. They show values for props from 9x6 up to 12x8 & power from  116watts to 420 watts (on varying cell numbers/voltages) & interestingly the efficiency only varies by 8% or so with the least efficiency (69%) coming with the biggest prop at the furthest point away from the motors rated kv & the highest efficieny (76.2%) coming from props that pull the "rated" current as we might expect. "Under propping" to achieve a mere 116watts still shows 72% efficency.......

[Tim Mackey]

Yes.... but some of the bigger motors will show greater variations in efficiency levels....anyway, yes, thats the point I was making.

Overall, still feel its better to try and get a motor which is best suited to the likely prop and model for its home.

There is also of course the eventual limit which could be exceeded in RPM. A 4900 KV motor for instance which is recommended for say.... 3s voltage on a small fan, may rev itself to death if some boy racer sticks a 4 or even 5s pack on it, or perhaps decides to convert the 5 blade fan to a 3 blader and so on...

Obviously the manufacturer must have a likely or recommended maximum RPM in mind, and KV figures should be used in conjunction with those parameters.

This is probably where the difference between the budget motors and something of hgher engineering quality would show up, with the cheapies turning into BB guns as the rotor spins at 3 zillion rpm !

[Steve Hargreaves - Moderator]

a 4900kv motor on a 5s......Mmmmmmmmmm!!! That would be quite a few rpm then!!!!

I would assume most manufacturers would  quote a max voltage figure & this would limit the max rpm. It is physically impossible to exceed the rpm dictated by the kv figure for a motor at a given voltage. Provided the bearings are rated to the rpm dictated by the max voltage permitted times the kv value then there shouldn't be a problem.

Don't get me wrong...I'm not wanting to put an AXI 5330 into a shockie but I had a particular instance where I had a 200watt motor floating around that had previously powered a fast sporty aerobatic job (8 x 4 pulling 20A on a 3S) & wanted to power a similar sized high wing cabin job which I reckoned would only need 80-100 watts to fly nicely.....what prop to use?? I had no idea hence my question......(ended up with an APC 6 x 5 which pulls a bit more than I want but being a good boy I used the throttle stick thingy & set it in the middle after take off!!!!)

[Tim Mackey]

Absolutely.... but the key in that statement is.. "Provided the bearings are rated to the rpm dictated by the max voltage permitted X the kv value then there shouldn't be a problem".

Wouldnt the 6 X 5 pull slightly less than the 8 X 4....and it also seems a little small for a high wing cabin style model ?

[Steve Hargreaves - Moderator]

Yeah, the 6x5 pulls about 12A (hence 120watts) rather than the 20A the 8x4 pulls. Agreed you would expect a bigger prop on a high wing jobbie but its quite a high kv motor (1500)  hence the small prop. Its only a 32" span so the smaller prop is in keeping.

As I say this prompted the question in the first place......I tried all sorts of sizes from my drawer to get the 6x5...7x4ic props.........7x6 slow flys(which thrum & wobble in a very exciting way at max rpm by the way!!!!!). an ancient 6x4 Cox prop...even a 5x3 3 blader I found wedged behind a 1978 copy of Aeromodeller!!!!!!

My best option would probably have been a 2S battery on the same prop but I don't have any 2S Lipos.....

[Tim Mackey]

Yeh, the 2s would be the way to go.....

You could just get the bandsaw out on a couple of old 3s jobbies, and bingo - 3 X new 2s batteries

PS Ask Myron if you could use his campervan to work in whilst doing it...bet its nice and warm in there, and if it isnt, it would be after

[Steve Hargreaves - Moderator]

Its a thought Timbo...its a thought!!!

How about it Myron....?? Fancy seeing the sparks fly.....I'll just stand behind this sandbag if you don't mind

[John Ogier]

I would suggest gentlemen that with the advent of lipos that the need for efficiency is not as great as it was with nicads & nimhs. Efficiency will give duration at the expence of performance. When I buy a motor I look at the instructions to see the max. amps., and prop. it to that figure using an ampmeter. This gives the best performance, if you want more duration use a bigger lipo

[Steve Hargreaves - Moderator]

Indeed John but I am in the unusual position of having too much power & am using the inherent flexibility of an electric motor to effectively de-rate the power available by fitting a smaller prop.....

This is my problem.....all suppliers who offer recommended prop sizes assume you want to squeeze every last watt out of the motor & to be fair most people probably do.......I however want to use about half of the motors available power but have no idea of the prop needed to achieve this other than by trial & error.....

[Tim Mackey]

I reckon a reduction of about 30% in pitch and diameter of the suggested "max prop" would be a good starting place...after that, its down to using the ammeter.

[John Ogier]

Yes Steve. you can put a smaller prop. or you could put a limiter on your tranny so that the sc. will not open fully, using a P-mix. This way you can use whatever power you fancy.

[wess]

hi  all.... i found this little program some time ago and i think it is very useful

It gives thrust against  rpm...speed of aircraft ....thrust....against diameter and pitch..you can enter any sizes u wish and compare 2 different  selections over any rpm  range u desire. ......Remember u won't get the answers unless u print them out which is a bit weird.

as i see it by choosing   a voltage input to the motor times the Kv u will get the shaft speed that you can then take off the appropiate thrust and aircraft speed printout .....i think the aircraft speed is the theoretical calcs of advance of the prop against any particular pitch/rpm......... I an not quite sure this is what u want but have a go it is free and there is another useful conversion program available

http://freespace.virgin.net/barry.hobson/

[Tim Mackey]

dead link wess.

[wess]

hi Timbo and guys.......sorry about that....seems his earlier website is defunct.   but WEB search google on hobson thrusthp and u get a page like this with free download .......well worthwhile

[b]ThrustHP[/b] V 2.0d

- [ Translate this page ] ThrustHP V 2.0d (Barry Hobson) Download: thrusthpv20d.zip (841 kB - FREEWARE) ... Met dit aardige programmaatje van de Engelsman Barry Hobson kun je de ...
www.hoppenbrouwer-home.nl/ikarus/software/thrusthpv20d.htm - 5k - Cached - Similar pages

let me know if u can download it ok and if it is what u want.

wess

[Steve Hargreaves - Moderator]

Thanks Wess...an interersting little program......I was looking for something that would give me watts consumed against speed & prop size.....fer instance if you have (say) a 200 watt 1534kv motor on a 3S LiPo but only want to consume 120watts what prop would be appropriate.... From the above you could deduce that the motor was fairly lightly loaded so would probably be turning around 13500 rpm.....what prop would take 120watts to turn it at that speed???

[wess]

Hi Steve...huummmm...... take some thinking about!

[Steve Hargreaves - Moderator]

Yep....thats why I'm hoping someone might have done it for me!!!!!!!!!!

I knew it was a long shot but Hey! if you don't ask.........

[wess]

hi steve...........what diameter and pitch are u using when u register 200 watts........i think we can crack it from the data available on the program........also  what is the flying speed /  ie.airspeed likely on the low powere 120 watt bird?.......still thinking.....

reg.wess

[Steve Hargreaves - Moderator]

Hi Wess.....an 8x4 APC on my motor clocks just over 200 watts...about 210 actually. a 6x5.5 APC registers about 120watts......flying speed??? I have no idea!!!!!!

[wess]

Hi Steve..... using the ThrustHP calculator  you need to enter Case 1 details and u will then get the HP and static thrust for the 8x4 namely 2.76 lbs thrust that u measured at 210 watts ie 20a at about 10 v the chart shows 0.395 HP

  As u want to hit  12 amps at 10 volts  ie 120watts u will need to find a prop that absorbs about 43% less  ie  0.225HP at the same rpm.

  If u enter Case2 looking for a prop that  absorbs 0.225HP at 15000rpm u will find a 7x4 meets that  hopefully  would absorb 120 watts in so doing.

 I expect u have got there before me and i can't explain why the HP figs are greater than the calculated equivalent in watts.

 Your 6x5.5 probably would do the same thing but at a greater airspeed and possibly not as efficient as the 7x4....

 Over to you

wess

       

[wess]

hi all.......continuing on with Steve's question about prop sizes at various input Watts the free software at the following address just about answers a maiden's prayer for just about everything u need to design model aircraft....... it is better if u can speak Dutch but download the propeller sizing freeware and all will be revealed......

http://www.hoppenbrouwer-home.nl/ikarus/software/software.htm