Understanding electric principles

[Chumbawumba]

Hi there, having cut my teeth on electric ARTF's, I am now wanting to build my own electric aircraft from plans.

I was looking for a good source of information about the principles of electric flight, particularly as to the relationship between propellor dimensions, thrust, motor specs and battery spec. Any advice?...

[Alistair Taylor]

Guy

If you're using NiCads/NiMH and brushed motors, check out the Technofiles on the Carlisle and District MAC website;

http://www.cdmac.org/

Written by Nigel Hawes quite some time ago..

If you want to know more about brushless motors, using NiMH and NiCads, you could do worse than check out the motor stats available on various suppliers websites, e.g.

http://www.conceptmodels.co.uk/TORNADO%203615,3618,3630.htm

If you want to use brushless motors with LiPos, check out similar stats available from the motor manufacturers, e.g. Axi, Hyperion, and Balsa products for the cheapy bell brushless motors.

Once you've got a motor/lipo setup in mind, check out the forums (including this one of course, although this not specifically for electric flight). E.g. the Ezone, or RC Groups, for advice on how the motor/ESC/Lipo actually performs.

You'll probably find a lot of guff, but there's often some useful stuff in there somewhere.

AlistairT

[Doug Ireland]

I'm a devout IC flyer, a 27% CAP 232G being my latest love affair. I have a question about electric motors for model aircraft. I'm confused as to the use of the term Kv with regard to performance of these motors. As a professional electrician I would be a little concerned about getting any where near a 5400Kv motor (5,400,000volts)!What am I missing here?

[Alistair Taylor]

Kv isn't Kilovolts (KV), it's a motor constant, expressed as RPM per Volt;

http://www.slkelectronics.com/help/Eca00004.htm

It indicates how fast your motor is going to want to try to spin at a given voltage. i.e. 5400Kv motor will want to spin at 27,000 rpm on 5 volts, or 54,000 rpm no 10 volts (assuming no load).

This can help you decide whether the motor is the right one for the application envisaged - e.g. spinning a ducted fan at lots of rpms, spinning a large prop via a gearbox, or spinning a tiny prop very quickly (5400Kv motor would be ideal for these applications in theory).

In IC terms think of a low Kv electric motor (around the 800 - 1500 mark) as a four stroke, a medium Kv motor (1500 - 2000) as a sport two-stroke, and the higher Kv motors (2500 and upwards) as highly tuned two-strokes running on high nitro and tuned pipes. The sort of motor guaranteed to get your collar felt by the club noise pollution officer....

Of course - even high Kv motors can be made to spin slowly if you only use low voltages....but then you need lots of amps to get adequate watts...which can be problematic.

AlistairT