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Propeller Matching A Brushless Motor

Introduction

This is going to be technical, nerdy and not for the faint hearted. If your favourite relaxation revolves around translating the Iliad into Kanji then close this file down and find consolation elsewhere.
There are a few motor matching software methods available. The Micron site probably has the market leader but requires the user to input motor, prop and model data and leans towards RC models.
The method described below is first principles. Not only will you be able to match propeller and motor, you will understand why,

Buying a motor

If your intended model was IC driven and you want to go electric FF, then assume that 75Watts/cc of electrical input power will be needed. Find a supplier who offers energy input data and recommends a suitable propeller. If he does not supply this data , find a supplier who does. If you still wish to buy a 'pig in a poke' motor then at least make sure that you have three pieces of information.

Motor speed constant (kv)
Resistance of the coils (Rm)
Zero load current draw (i0)

If you are in luck the motor you have selected may be listed on the web site www.flybrushless.com. If you have bought your motor from Hong Kong, it won't be listed and you might as well throw it away.

With these 3 variables you can predict or characterise any motor using a method proposed by Mark Drela 'First Order Electric Motor Model'. Click on the link to view this article. You do not have to bother to code this as I have done this for you in Excel. Contact me if you want a copy via the contact address at the end.

However, here are the results of the analysis using an S2503 Scorpion F3P motor.

The graph shows calculated current draw versus motor speed and is absolutely linear. This performance is nothing like an IC engine where the operator will try progressively smaller propellers in an endeavour to produce maximum horsepower. If you do this with an electric motor you will end up at very high speed drawing no current at all. Electric motors want to run slowly with very high current. A screaming electric motor is impressing your friends but not doing anything useful.

It also tells you that if an electric motor does not start and fidgets about at very low speed, it is drawing very high currents. In fact, 5 secs of dithering and most motors will fail. So flick the propeller or disconnect the battery if this happens. Run an electric motor on a power supply with some caution, the PSU WILL supply massive 'dither' current, a battery will not as the ESC will shut down on voltage low.
Note that to check the analysis, a series of tests were made with various batteries and propellers. Good agreement can be observed between measured and predicted results.

The method predicts the motor efficiency and shows that for peak efficiency a propeller giving just less than 12000 rev/min would be preferred. Here is where you have to rush off and buy a tachometer from Ebay at £12-£15 and start testing propellers to get to 12000rev/min. If you do this then you will find that a GWS 6 x 3 would be a starting point. This might be a specification for an RC modeller anxious to maximise flight times.

Flushed with success you also buy a wattmeter to find out what the motor is doing as being a responsible fellow you would not want to exceed the suppliers recommendations. You find that the motor runs at 5.2amps and 40Watts input way below the suppliers recommendations. You have actually chosen the wrong motor for sport RC as this is an f3P motor capable of twice the power input.
Armed with the 12000rev/min and 6 x 3 data go to the www.flybrushless.com web site and get a thrust prediction.

An alternative matching method might be to match at peak shaft (output) power of 7000 rev/min. This is also the speed at which peak thrust occurs.
A propeller for this would be an APC 8 x 3.8 drawing 13amps and 85Watts of input energy and offering 347gms of thrust. In practice a static speed of 6000 rev/min might be a better match point as the prop will unload in the air. This would be a specification for a pylon duration FF model. The power input exceeds the supplier's recommendations but on a 15 second power run, the motor is only just warm to the touch.

This raises another point in that suppliers recommended power input figures are invariably steady state, RC and conservative.

Please note also the way in which power claims are quoted:-

IC engines are quoted at output power
Electric motors are quoted at input power

You can now compare the electric motor predicted shaft power with the 75Watt/cc rule of thumb offered at the beginning of the article. The electric motor may be capable of sinking 85 Watts of input power but the shaft power of 52 Watts is exactly the shaft power of the IC engine. The flight performance should be the same.

If you have understood this you are now able, with a copy of my Excel spreadsheet, to get a first estimate for an electric motor/propeller match. You will also know the load you are putting into the motor and you can estimate the thrust from the www.flybrushless.com website.

Conversations that I will no longer have to endure.

Modeller: Tell me all about electric then?
This is akin to asking me to precis the Theory of Relativity in 60 s. It usually happens on the flight line of an otherwise perfect day when the windspeed is 2 mph and the thermals are popping off with regularity just when I am rigging something and have not flown. One nevertheless spends 15mins explaining such. The usual response is….

Modeller: I do not understand Electronics.
Said with a smirk as gross ignorance appears with some to be a badge of office.

Me: Go away , fill your Ajax with rubber and stop wasting my time.

Modeller: I got this motor for £2 off Ebay. What prop do I need?
Me: What kv is it?
Modeller : Dunno
Me: What is the serial number?
Modeller: Dunno
Me : How much does it weigh?
Modeller: Dunno
Me: What ESC is recommended?
Modeller: Do I need one?
Me: Go away and stop wasting my time

Modeller: What KPxx motor do I need to drive this model?
Me: ???!!!!**!?^^!

Modeller: I cannot solder
Me: That's like me saying I cannot glue two pieces of wood together.
Modeller: What can I do?
Me ; Go away and practice until you can.

Never again…………'its on our website' I will remark.

Ian Middlemiss Dec 2012