any calculator tools to optimize propeller size for "found" electric motors?

[wfairley]

any calculator tools to optimize propeller size for "found" electric motors?

I was thinking about scratch building an aircraft, and using a pair of Dewalt 60v FlexVolt 7 1/4 cordless saws for propulsion as they report the motor produces 1600 watts at 5800 RPM, but I am not sure of the ideal propeller diameter to avoid burning up the motor, nor am I sure about the pitch that will provide me with a cruising speed between 50-80 knots.

I am posting here because I think a spreadsheet to calculate these parameters might be useful to others, and if anyone does not have one to share, then perhaps I can develop one (please be patient if this is the case).

I wish to optimize thrust without the risk of burning up the motor, and I would like to be able to estimate the achievable speed given the motor's parameters:

The "found" motor must have the specifications label still attached for this to work, and the label must contain the following minimum specifications to avoid mounting the wrong propeller and burning up the motor:

wattage
amperage
rpm
max voltage

I think that might be all I need to calculate the optimum propeller diameter for maximum sustainable thrust. With the motor's rated rpm, I should be able to calculate the forward speed of the craft at various propeller pitch settings.

I've read several highly valuable articles on the topic of propeller calculations, but so far I have not found a spreadsheet that calculates for a "found" motor.

For reference, the Colomban CriCri is a tiny aircraft that, with twin 15hp engines spinning 27-inch propellers of unknown pitch at 6000 rpm achieves a cruising speed of 115 mph, pilot on board. Just search for "Colomban CriCri" for more information.

I will be very grateful if anyone can direct me to such a tool, as I find lots of motors that seem quite promising. Thank you in advance.

PS - the Dewalt FlexVolt saw incorporates everything necessary to produce power: control switch, battery, and motor. I think it would be easy to run a cable to the power switch, and construct the propeller interface. There are a number of cordless power tools available on eBay and Craigslist for very low prices, but the motor specs vary, and there are some quite useful calculators for thrust but I have found none for the reverse calculations thus far... any help?

 

[ProfessorFate]

Hi
I enjoy these problems about motors n thrust your Q w/ variables 5800 rpm 1600 watts sounds like it's very close to the apc 15x10 I was planning 4 to 6# thrust 24 volts and a 325 to 350kv motor to top out around 6400 rpm about

This info came from apc mfr page static tests at specified rpms, I only listed the ones in range that concerned me but you may find this very helpful

And you can find at apc other prop sizes and data for in-flight not static measurements, looking at the prop and what it draws at the rpm you mentioned, got to be the apc 15x10 or something smaller like the the 14x8.5

I'm locking in on the power and rpm, not having other things to go by like how much thrust you want



I know it's not exactly what you asked for like a spreadsheet or prgm but may be the ans
Down around the lower rpm range before the power skyrockets, 5800 may be predictable and realistic power limit

Well I looked again at the 15x10 chart and so you may want to look at the next size up if you want more thrust just thinking of the power allowances and they may be counted as estimations

I've enjoyed working on most thrust for least amt of power but in the 2-3# range and find that around the 6000 rpms with a 12x8, 2# 107w and 3# 200w also true with apc as well as my own wound motors





Hey the funny thing makes me feel good here is that they get 3# for 200w agrees with my experience but some of my motors approach 60oz for 424w, I got 54oz for 242 watts, much better than their test motor, that's bottom right chart d4023 729kv. So looks like my motors tend to give more thrust for less power, a kind of departure from agreement w/ apc around 6500 rpm... their test motor starts to go thru the roof for pwr

Maybe this is very close to the ans you would want your pgm to give you, working it backwards from prop tests

See also the very informative and honest Cobra and Scorpion motor test charts, no joke, they are good.


OK by looking here it seems you could really go with much larger props even than 15x10
So this is how much we can tell by good motor test charts
you might want to look at charts for larger motors

The Cobra 4130 540kv comes closer to defining your power limits see the 18v list 16x8


So I'm answering your Q with a prop and motor test chart your prop by the power it demands at specified rpm must be the 15x10 or very nearly you can refine with further study
Really hope this approach helps you. I find those charts very helpful.

Well then again see Scorpion bottom swings an 18x8 comes closer to your power limit 219oz at 7001rpm 1355watts, you get an 18x10 that would speed things up and be closer to your motor's limit

We need to get a chart for a bigger motor n see about 5800rpm and 1600watts limit, that it might allow say a 19 or 20 inch prop, no programs, just reliable test charts. If I go look and find bigger Scorpion or Cobra motor charts I'll post them, we're lacking just that last little extreme, how far can we go?

Leonard




I'm returning with a quick ans from innov8tive.designs.com this motor see the 20 and 21 inch props for the Cobra C- 5340 160kv


There's your rpm lim 5800 and power lim 1600 watts, so this would tell you what your motors can do


The APC 20x13e looks like your prop see on the 1st chart 5511rpm 1402w 233oz or 14.56# thrust



You also mentioned 27 inch props, you would need lots more pwr to swing a 27" prop 5800rpm

Have you seen Skylars brushless motor build for a paraglider on rcgroups? Stator is 6" dia x 1" thick pwr takeoff est 3000 watts

A few yrs ago I made up a chart for gws 9x5" props several motors, maybe you could make a similar chart with these tool motors if you get any real good info or specs on.
This is just an idea, it's not on the size or scale as yours


Concluding, you really have a lot if you specify how much max pwr, rpm max, and speed
If your motor can sustain 1600w constant then your max prop size would be 20x13, remember you did supply the rpm 5800.

I have often found good efficiency with large props at low rpms around 6000

Leonard

 

[wfairley]

Thanks Leonard, your post is very informative. It is helpful to know the parameters for the two propellers.

My intent is not to achieve a target thrust but instead to calculate the desired propeller dimensions for optimum thrust for any motor. The motor is the known device, and the propeller is the unknown, if that makes sense. It sort of flips the calculations upside down.

Imagine finding an electric motor in a yard sale, and using a spreadsheet to calculate which propeller would deliver the optimum sustainable thrust (and how much thrust and speed can be achieved by varying the propeller dimensions in the calculator).

Keep up the good work.

 

[Hai-Lee]

You might also need to find out the Max Duty Cycle at Max Power. A lot of electric tools can develop amazing amounts of power but cannot be loaded continuously with maximum power because the cooling is less than ideal. Often the result is a burn out or bearing failure.

You might note some tools have disclaimers about its lack of suitability for industrial usage because of this very issue.

Just beware!

Have fun!

 

[wfairley]

You might also need to find out the Max Duty Cycle at Max Power. A lot of electric tools can develop amazing amounts of power but cannot be loaded continuously with maximum power because the cooling is less than ideal. Often the result is a burn out or bearing failure.

You might note some tools have disclaimers about its lack of suitability for industrial usage because of this very issue.

Just beware!

Have fun!

Outstanding advice, thank you! I agree that the published rating on a found motor may in some cases be assumed to be the max duty cycle at max power. I am interested in calculating the propeller parameters for the Dewalt DCS575 60V saw because it is brushless, but I agree it is not designed for the continuous duty of powering aircraft, only for cutting wood.

Thank you for contributing to this post, and to Flite Test!