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Nov 09, 2017, 02:34 AM
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An unconventional power system

Hey all.

I'm building a Solar Plane for a college engineering project, and while I'm not new to flying or building RC aircraft- selecting a power system has always been confusing.

My objective is to power a ~3 meter sailplane type plane that weighs less than 1.5 kg with the most efficient set up possible, and because larger props = better efficiency, I decided to go for a 320kv multirotor motor turning an 18x8 prop.

As far as I can tell, this should function just fine- yet I have seen no one run such configuration when looking to maximize efficiency, or at least not one with an 18 inch prop. This should create over a kg of thrust, which is plenty for my purposes.

So my question is will this function? And if it does- will it even be more efficient. Hopefully able to cruise at ~3-4 amps?

Here's the setup:

Motor- Turnigy Multistar 4112-320kv 22 pole w/ a max draw of 20A
Prop- TCF 18x8
Battery- 4S 3200mAH 30C
and a 30amp rcmc ESC
Last edited by KiwiDelta; Nov 09, 2017 at 05:37 AM.
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Nov 09, 2017, 05:12 PM
Registered User
Some more information about the plane would help.

Is this a solar powered plane? If so, some information about that would help.
Wing Area?
Mode - for time or distance or ?

Efficiency is a ratio and is meaning less without context. So under what conditions will the plane be flying when the stated current (Amps) conditions are required (air speed,etc.)

Is the 1 Kg of thrust at maximum power or for some other condition?

Here is some preliminary full power estimates of thrust as a function of air speed for that motor and the 18x8 prop.

V, km/hr------T, Kg
Nov 13, 2017, 12:50 AM
Registered User
Thank you for responding. Because of short deadlines, the plane wont be finished before the power system is ordered. Regardless, we already are settled on the wing config and I can estimate final weight.

We are looking to fly a plane for maximum duration using 30 Mono crystalline 125mm (5x5) flexible solar cells (Maxeon). Altogether, these produce 100W under ideal conditions, but during a north Californian winter, I cant expect more than 35W at mid day. So indefinite flight is probably impossible at this time of year on any aircraft without adding more solar cells.

The plane is most similar to a large wingspan sailplane or slope soarer. It has a minimal fuelselage, carbon tail spar and very long wings with 3 degrees of dihedral.

The plane will have a 3.125m (10 ft) wingspan and an average chord of 200mm (~9inch). The wing area should be about 62.5dm.

The weight will be as low as possible, but the AD Inventor model gives it an estimate of 2 kg with EPP #1.9 foam wings. I expect this to be higher, and this is a more accurate estimate than the above 1.5kg.

For calculations I was using the old WeboCalc (I really dont know much about rc power systems)

I had the max speed set to 50kph and max current at 18A (motor maxes out at 20A)

I don't see why I wouldn't go for the lowest kv motor reasonable turning a prop big enough to provide enough thrust to fly the plane well above stall speed at max. Is my logic flawed in this aspect?

In revision, I've reduced prop size to 17x8, just to keep the motors amperage in the safe zone.
Nov 14, 2017, 03:06 PM
Registered User
You are correct, large props with low Kv motors tend to be more efficient than small props with high Kv motors.

Here is some data from a personal program designed to estimate full power current, power, climb rate and climb angle a a function of airspeed. The program also estimates current, power and flight time assuming level, constant airspeed, flight.

Turnigy Multistar 4112-320Kv motor, 4S 3.2 AH 30C battery ( 16 Volts assumed), 17 x 8 generic prop. 16 Volts assumed.

Wing span 10 ft, area 1080 sq inches, weight 2.2 Kg, sailplane airfoil, clean fuselage and tail ( no landing gear, etc.)

Full power Results
Vmph------rpm-------thrust(oz)-------total drag(oz)------Pbattery-----------I amps----------Climb rate (ft/min)-----------climb angle(deg)

Level flight, constant airspeed results
Vmph---------rpm----------drag(oz)----------Pbattery(W)----------------I amps----------------Time (minutes)

Note that 16 volts was assumed for the analysis. If the battery is not kept at full charge, the voltage will decrease and the corresponding full power operation will be less. The level flight, constant airspeed results are not affected other than to decrease the flight time.

An alternative would be to go to a 17 x 12 prop which would provide better climb performance (about 1,000 ft/min) at the expense of
greater current (approximately 20 amps at 16 Volts). The level flight current would also be lower at 20 mph ( ~ 1.5 Amps), increasing flight duration to about 2 ideal hours. Top speed would increase to about 45 mph.

There is no provision for the effect of having solar power available in this analysis. It assumes that the model is flying on the battery alone.

I hope this helps.
Nov 15, 2017, 04:40 AM
Registered User
Wow, seriously great stuff! Exactly what I needed, thank you. I'll look into increasing the pitch for sure, if I can get the improved efficiency suggested then that's a game changer for me.
Nov 15, 2017, 04:52 AM
An itch?. Scratch build.
eflightray's Avatar
A quick search across the 'Airplanes' and 'Sailplanes' RCGroups forum channels found over 120 threads with 'solar power' in there titles.


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