Still confused

[fly boy3]

Hi all, the spec.on my HK motor for prop sizes state, lipox2, and lipox4,I assume these are cell counts so if I use 3 cell, would my prop size fall between the 2 and the 4cell. Part 2 of my question is why when the lower voltages are stated the prop sizes are bigger ? Thanks

[Frank Skilbeck]

FB3, yes but there are programs you can check it with, e.g. drivecalc (it's free)

As regards prop size the motor speed is proportional to the voltage, so a 4 cell lipo will turn the motor twice as fast as a 2 cell lipo, power with the same size prop is roughly equal to the speed squared, so a 4 cell set up will pull 4 times the power as a 2 cell set up (same prop), as power = volts x amps, this means that the amps on a 4 cell set up will be double the amps on a 2 cell set up, so you have to go to a smaller prop size to prevent overloading the motor. Similarly this means you can use a bigger prop on a 2 cell set up without exceeding the motor amp rating.

If you post the motor and prop you intend to use on here I'll run it through Ecalc and let you know what it estimates.

[fly boy3]

Very well explained Frank, I m feeling a lot better now after that. LoL Motor spec,Turnigy, D2830/11. 1000kv, 210watt. I was thinking (with very little elect, knowledge). 8x5 ? Thanks again.

[Steve Hargreaves - Moderator]

Sounds a reasonable starting point FB.....the thing is to make an educated guess at the right prop & then try it out on the motor with the wattmeter in place.....you'll soon see if you're in the ball park. Then swap props to fine tune...

A good slection of prop sizes is definitely a must in electric flight.....

What is the model & what power are you aiming for?

[PatMc]

Posted by fly boy3 on 21/10/2017 12:57:49:

Very well explained Frank, I m feeling a lot better now after that. LoL Motor spec,Turnigy, D2830/11. 1000kv, 210watt. I was thinking (with very little elect, knowledge). 8x5 ? Thanks again.

The spec quoted for Watts is misleading for all motors, real limiting parameter is the max current.

The Turnigy motor is rated at 210W but that's must be on 3s lipo [HK have allowed for a volt drop to 10v]. This means that on 2s lipo the max power for this motor would be 2/3 x 210W = 140W - but that would be on a bigger prop.

Edit - just noticed the weight of this motor. I think that 21A is unrealistic without the motor overheating, except for a short burst say 20 - 30secs. The 210W figure quoted is probably on 4s giving a realistic continuous max current of around 14A. So the continuous max power on 3s would be 3/4 x 210W = 158W

 

Edited By PatMc on 21/10/2017 14:23:31

[Frank Skilbeck]

FB3, here you go from Ecalc, on a 3s the 8x5 might be a bit lacking

3s 2200 mah lipo

8x5 :- Amps 8.15A, Volts10.8V, Power 88W, rpm 9505, Static thrust 622g

10x5 :- 15.23A , 10.54V, 160W, 8409 rpm, 951g

10x6 :- 17.03A, 10.48V, 178W, 8130 rpm, 1067g

4s 2200 mah lipo

8x5 :- 13.23A, 14.19V, 188W, 12186 rpm, 1023g

On a 3s setup I'd start with a 10 x 5. But what plane are you looking at.

[fly boy3]

.Thanks both for your input, I need it lol. Frank the motor is for a low aspect ratio glider designed and flown successfully by Mr Peter Miller. Called Easter Eagle. I think his power train was around the 200w figure. Picture of model in the "ask Peter Miller" thread. As its a glider (and a good one ) I really just need a tidy set up to get the glider aloft. Cheers ps exellent Ecalc figures Frank.

[Frank Skilbeck]

I'd go with the 10 x 5 you can always throttle back, a folding prop and setting the ESC brake to on will reduce the drag on the glide.

[fly boy3]

Thanks Frank will order one today. Cheers