Single vs Twin motor for an electric RC airplane

Question

I'm designing a fixed wing UAV for a project. I need an airplane that can fly for at least 200 minutes. Obviously, it means a big battery and thus a lot of added weight. I've been debating whether or not I need a twin motor configuration. The idea came after I saw a video of a modified "My twin dream" airplane that has a range of 269 km and can fly for more than 5 hours. The extra thrust might be the reason it can fly for that long but I can not find any research for electric twin motors whatsoever.

Let's say the cruise speed would be 15 m/s. If it only has one motor, maybe it would need to run at 90% of its power vs if it has two motors they would need to be at 45% of their power to match the required thrust for that cruise speed. This could mean, two motors are more efficient than one, even though the extra weight of the added motor.

I'm not sure how to solve this problem.

Answer 1

You should keep in mind that adding an engine adds wiring, extra propeller (a larger single prop might to be lighter in RC than an equal setup of two), overall complexity etc.

As I understand it, electric motors have a "sweet spot", or rather an area of (iirc) certain load and rpm. You should desing your configuration so, that the motor(s) would operate in this rpm/load range in cruise. This sweet spot is (again, iirc) much higher than half load. So it is likely you are better of with a single engine than two that are only utilizing about half their rated pwr.

This sweet spot is very motor specific, so you need to dig deep into spec sheets.

Answer 2

Your intuition is right, that aircraft design, large or small, is a mess of tradeoffs. One way to experiment with such designs is software such as the venerable Motocalc, which lets you choose the airplane's attributes (minimum flight duration), some constraints on its design (up to two motors, weight range for batteries, etc.), and then gives you many designs that optimize your attributes.

Answer 3

The single prop should be more efficient if both systems are designed properly. My understanding is that the benefit of a twin on UAVs is ease of mounting the payload (counter-rotating props is nice too).

Note that one larger propeller is more aerodynamically efficient than two smaller ones totaling the same area. That is a big drawback in efficiency for using two electric motors instead of one.

From the RPM-efficiency curves I have seen for DC motors, and from what I have read, the efficiency tends to peak at right around 1/7th the no-load RPM while the power peaks at 50% no-load RPM. Better to load it a bit too much and have it run a bit slower than load it a bit too little and have it run faster since the slope on the side that is underloaded drops off much faster than the slope on the side that is overloaded.

If you want to get peak efficiency when it occurs at such a high RPM combined with the efficiency of large propeller, you will probably need a gearbox. Gearboxes are expensive so you probably don't want two of those.

If the mission profile allows, you might benefit from designing the motor-prop for maximum efficiency during a climb and then operate it to repeatedly climb and descend in an unpowered glide rather than design it for maximum efficiency in cruise and then cruise under power the entire time.

A motor-prop combo designed to be most efficient during cruise will produce just enough thrust to overcome the drag so that it can be running the entire time with no efficiency penalty, but such a combination will probably have a lot of trouble getting off the ground.

Designing things for optimal efficiency during climb and then operating like a sailplane lets you use a motor-prop combo that is better suited for takeoff while also enabling you to only ever run it at the point of optimal efficiency during travel.