Choosing the right motor

[RandomChaos]

Being new to the RC Plane world, with a little experience in flying a custom built quadcopter, I've gotten a little confused on how to choose the right motor for a setup. I have a spare 980kv motor that I would like to try using on a plane, specifically my Blunt Nose Versa Wing, but am not sure if it would give me the results I am looking for.

Currently, I am running the Hextronik 1300kv on it, but it seems to be slightly under powered for my setup, especially once I start to try adding a camera and such. I'm not really looking for all out top speed on it, more of looking to have a fairly responsive motor at lower voltage levels, not an all out screamer. I was thinking the 980kv motor with its 10x4.7 prop would be nice, but i'm not sure. There is plenty of space for the 10 inch prop, so that is a non issue. In the world of quadcopters, with my limited understanding of it, usually as the weight of the quad goes up, you need to look at fitting larger props to get more "torque", thus needing a lower KV motor to handle the larger prop. Is it the same concept with planes? Do you go for a smaller prop / higher KV motor more top speed thrust, and larger prop / lower kv motor for more thrust at a lower voltage?

Sorry if this has been answered before.

 

[Craftydan]

RC,

Been asked many times before, but it's a complicated topic -- reading one person's Q&A thread doesn't mean it'll make sense to soemone else.

A lot of these concepts are similar, but they differ a bit due to the goals.

In Multirotor it's all about managing sheer power -- your trust:weight ratio is always much greater than 1, and the subtlty of control is in how well you can change your thrust.

In fixed wing, we're going to use the airframe to generate our lift, so thrust:weight doesn't need to be that favorable, and running for extended periods at 1/2, 3/4, WOT or even dead sitck, can be normal . . . the subtlty of control is in how the airframe cuts through the oncomming air. KV, as such, will affect your prop size -- potentially your efficient speed range -- but not as significantly your flight performance.

What you'll be more interested in is the physical power output the motor is capable of efficiently dumping into the air, either in thrust or in Watts. A good starting place is the 100W/LBS Rule Of Thumb. With this, you should be able to perform light aerobatics and sport flying. I typically look at the motor's max efficient current and the voltage I expect to drive it at (cell count * 3.7v -- while a fresh unloaded battery can give you 4.2v, under a strong load it's seldom that high). So for the right prop and WOT, I can expect:

Pmax = Imax * Cell Count * 3.7v

From this and the airframe's expected AUW, I can gage how the motor will perform.

Two other prop-based points to consider:

-- KV isn't as critical, but you shouldn't ignore it, becasue it directly impacts speed through the prop's pitch. A 10x6 prop on a 900kv motor might pull just as much power as an 8x6 prop on an 1500kv motor, but the higher KV on the same pitch means both the top speed and the efficent speed range of the higher KV motor/prop combo is *much* higher.

-- Prop length might be fixed by the physical limitations of the airframe. putting a prop on a nacell on a wing? putting it between two booms? putting it on the nose of a FAT fueselage? Too big of a prop might cut into the airframe, or too small might waste too much of it's thrust blowing onto the fuselage and not out into open air.