Motor Suggestions for Endurance Solar Flight
 

I am looking for an efficient motor for an aircraft around 15lbs that is capable of low wattage cruise speeds. My background is mostly in airframes and design so any advice for a motor, ESC, battery system would be huge! My aim is to design the airplane around the best motor I can find (focusing on the elements I cannot easily adjust, ie. motor, battery, esc) so as to get the best efficiency out of the electronics and use solar panels on the wings to charge the batteries and sustain flight. This is a huge project and there are a team of us working on it, unfortunately we are all mechanical engineers and do not have an electrical engineer involved at the moment.

I have researched the AtlantikSolar plane and some others like it that have taken on this same feat but would like to know if there are newer motors or tests people have done with newer systems that are worth looking into. Thanks!
-Michael M.

Not sure if your asking on the right Forum for you answer? This is sailplanes and powered gliders. Is your aircraft going to be multi rotor or fixed wing? There are lots of motors on the market that will fly a 15 to 20lb aircraft like the Eflite power 180 and Rimfire has big motors, but as far as solar charging LiPo batteries, that’s out of my league.

Even if probably too late:

Seems you forgot the most important part of the drive: the propeller. And you have to decide for the best overall design, airframe and drive combined. I think you won't find an ideal motor anyway, and normal motors will be suitable.

Solar panels for sustained flight means you need a big wing for low wing loading so little power is needed to keep the airplane afloat. That in turn requires a big slow-spinning prop, which makes for quite some torque but still little power. It is very likely that a small fast-spinning motor with a reduction gear is the best solution in this case. You need a low-Kv motor but these are all quite heavy and too powerful for the task. At least I don't know of low-power low-Kv motors and that's why I recommend you look for small powerful inrunners and high-reduction-ratio gears. The drive's Kv is simply motor Kv divided by gear ratio.

Indeed there are not many options (not easily adjustable, as you say), just as for the propeller. There are not that many big propellers for low-power applications, and you'll need a big one (think of 20" or more) for slow flight. It should have big pitch, about 1/3 the diameter or more, for best efficiency. The prop is what determines overall drive effiency in the first place, so you might choose a prop (or even two, counter-rotating but not in line) first and then search for a suitable motor-gear combination. Battery might be the last thing to choose.

Anyway, expect not much more than 50% overall drive efficiency, meaning propulsion power (thrust times airspeed) divided by electric power (internal battery voltage times amperage). That is achieved only in a small range of airspeed, so the airframe should be designed to have best efficiency (lift/drag ratio) at that speed as well - or the other way around. For economic cruise, my rule of thumb is that prop rpm is half full-power rpm, amperage is a quarter of full-power amperage, and power is 15% of full power.

So you can lay out the drive for a decent climb and still have an economic cruise. You just shouldn't overdo by choosing a too strong drive because its weight would mess up the whole design. A good starting point might be 50 Watt in (electric) per pound weight, meaning a 750W drive. The better (lighter) the airframe the lower the power needed and the smaller and lighter the drive. The top design goal should be low weight (airframe and drive), IMO it's more important than airframe sleekness or drive efficiency alone.

We had a thread here about 6 years ago about a similar problem: here. My contributions start with post #70 at page 3.
You may look at Martin Hepperle's website for information about propellers.
My drive calculations are offered at my download page, together with an article about the equations used and examples.
A prototypical explanation of these calculations is in my Sr. Telemaster Plus review page.
Very good drives are made by Leomotion; you might even ask Urs (the owner) for advice.
Drive Calculator is great to search for suitable drives and has good information as well.
You might look at Ken Myers' Ampeer website for a lot of electric drive information.

By the way, I'm a mechanical engineer as well (even industrial) but all this is no rocket science so I learned it and I can tell you that it's even fun. :)