motor for 72" trainer ...

DennisZ

Hey All,

I am in a build of 'classic' high-wing trainer plane...
it is a big one - 72" wingspan (to accommodate my future projects)...
so, what electric motor will you recommend ?
big one like http://brchobbies.com/?page=shop&action=additem&item=869
or smaller http://brchobbies.com/?page=shop&action=additem&item=804

any ideas ?

Thanks

coolwings

Neither. It's a typical high amps setup. I use higher voltage low amp setups.

I wouldn't want 25% of my power going in wasted heat. (budget outrunners are 75% efficient at best). I'd get an 80% - 88% efficient motor myself and waste 1/3 less power, and then run on lower throttle settings for the same power (longer duration flights), but with 1/3 rd more power available when required.

Generally speaking a 40 size engine needs 600-700 watts to replace it. A 600W low efficiency brushless on a similar speed controller would be going flat out, and therefore probably have a short life, due the the glue holding the motor windings softening, and/or heat demagging.
So of those two budget motors you would need to choose the 1.2 kilowatts setup. Overkill and heavy because 800W is plenty.

Isn't a 72" plane usually a "60" engine? Maybe it's a Cub style floater in which case the 40 was always underrated and a 45 (800W) is a better solution.

A mismatch: those motors have a low kv (revs per volt). So they spin a big prop slowly. That is a glider setup. So that 1.2 KW motor with 400 rpm/volt would be (on 5S) turning the 13" prop at 7000 rpm wide open throttle. Is that enough? (Might need deep pitch props to get airspeed up)

You probably want a higher KV motor, spinning your prop faster.
If you got a 6S lipo and rotated the motor faster, then it would use a "normal size" prop, that motor would work ok that way.
But if you select a motor (for a 40 - 50 size plane) with a kv of about 800, the 5S batteries required will be more reasonable cost, and more "standard" in dimensions for the model size.

EDIT: I just thought .... a slow rpm can be made to give more power if a 3-blade prop is chosen instead of a 2-blade prop. This would be similar in effect to increased rpm. There is a smaller selection of prop sizes available in 3-blade though. C~

DennisZ

here she is


fullsize image

coolwings

Hi Dennis

With the built up fuselage you have made that plane will come in lovely and light. A good bit lighter than the normal ARF trainers.
So you won't have to put in so much power. Unless you want to

The usual ARF type 40 trainers with plywood sides weigh 2 1/4 kgs all in.
You will finish up with lower weight, I'm betting.

We allow 200 watts per Kg plus a bit to spare, for trainer glowfuel performance, and 250 - 350 watts per Kg for more lively glowfuel performance.

So my suggestion is finish building up the airframe, then pop it and the RC gear on the scales, and add on the extra for battery/motor/prop.

Then give yourself about 250 W/kg and it will all fly perfectly when the stuff goes in and you takeoff.
Probably with 4S but 5S is more handy for later when you have a heavier built model waiting for the power set.

DennisZ

finished product

http://www.leinster-model-flyer.org/wordpress/?attachment_id=111

coolwings

That turned out fine.

Did you give it a maiden flight?

DennisZ

coolwings wrote: »

Did you give it a maiden flight?

not yet - it was a bit too windy ... above my "comfort zone".
have a look at these landings on that day

http://vimeo.com/3667589
http://vimeo.com/3669031
http://vimeo.com/3669698

DennisZ

she made it
http://www.vimeo.com/3804148

coolwings

Well done!
The first flights of a new model always have that extra bit of excitement, don't they?