Power assist glider motor selection

[fly boy3]

Hi all, does the watts/llbs equation still stand for motor selection for power assist only gliders. Cheers

[Pete B]

Yes, fb3 but for a sedate climb 70W/lb should be fine. If you want more vertical performance then 100W/lb or more is needed. It's quite important to get the motor Kv right, too. If you are going for a larger folding prop, fitted in the nose, you'lll be looking for a lower Kv than if you were mounting a smaller prop on a powerpod.

Pete

[PatMc]

There's a number of E-gliders listed in the Electric Flight Database that should give you some idea of what others have used.

[ross blackwell]

Please excuse:watt/lb is that say for my 60 size cub I might need a 2000watt low KV motor? So would you then not use a 11.1 2200 3C battery but a 3000 3C or step up in volts?This is where I'm struggling with "electric" flying.

[Steve W-O]

Posted by ross blackwell on 31/03/2012 12:54:05:

Please excuse:watt/lb is that say for my 60 size cub I might need a 2000watt low KV motor? So would you then not use a 11.1 2200 3C battery but a 3000 3C or step up in volts?This is where I'm struggling with "electric" flying.

The difference between the two batteries you mention is minimal in the setup you suggest.

Can't be more exact, I don't know how big (wing span and area) and how heavy a 60 size plane is at a guess you will need a 4 cell battery at least 4000mAh capacity

[fly boy3]

Thanks Pete, and PatMc for the info . Sorry for delay in thanking you both. Cheers

[Devon Flyer]

You certainly don't need 2000W to fly a 60 sized Cub - not unless you intend entereing the World Rocket Championships!

2000W equates to 2.68 (real) horsepower ie . about the same power that a 120 two stroke actually puts out on a useable sized prop, flat out.

A Kilowatt (1000W) of real electric power would be enough to fly a 60 sized Cub very nicely.This would be more real power than a decent 70 four stroke.The way to do it is to go to a 4S or 5S sized battery pack of about 4000mAh, or more, capacity. This would reduce current drain and give longer duration. It gives the motors an easier life too. Current tends to kill motors, not voltage.

[ross blackwell]

That sounds like good advice Devon:does it follow that if I actually used a 6 or 7 cell battery with a "suitably able" esc to control amps/current,I could increase my flight time even further. Haven't thought about motor have I ?

[Devon Flyer]

You could go down the high cell count route, but you would need to reduce the Kv of the motor as the voltage input got higher. (Kv = rpm per volt input ie. a 1000kv motor does 1000rpm, unloaded, for every 1 volt of input).

As power = volts x amps, you can see that you can get the same power in two different ways.

70A x 11v is the same as 70v x 11A. ie.770 watts. ie over 1HP (1HP = 746 watts)

However, as high currents tend to kill motors more than high voltage, the best option from the example given above would be the second one.

It all becomes a bit of a balancing act when deciding on motor/battery combinations for any given application. ie. how big a motor will fit, how big a battery will fit, what size prop you can fit etc. Costs come in to the equation as well.

For something like a 60 size Cub, I would go for a lowish Kv motor ( around 700 Kv - 800Kv or so), probably a 5s/4000+mAh battery, a 60A or higher ESC and as large a prop as I could get away with whilst keeping current drain to an acceptable level. At 50A this set up will give near as dammit 1.25 HP. Bearing in mind that you will be throttled back for most of the fligh and only pulling 20A or so, duration will not be a problem

Don't confuse the electric 1.25HP with the figures quoted by i/c engine manufacturers. eg. OS claim 1.75PS @ 16000rpm for their Max 65AX . Bearing in mind that a PS is a slightly smaller unit of power than a HP.

However, they also say that the practical RPM is between 2000rpm and 12000rpm. They don't quote the power output at 12000rpm because it would not be a headline figure. They know that most modellers are dazzled by the top power figure tested, albeit on a prop which would be useless for the application that the motor was intended for. It's probably producing around half the quoted horsepower at 12000rpm.

 

 

 

 

Edited By Devon Flyer on 01/04/2012 08:24:47

[ross blackwell]

Thanks for that info Devon: will print out for future refernce. Will start by checking total weight of model & work from there.One of the other considerations is our flying site as we have regular strong winds so would like a little something in reserve for landing on windy days.