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Aug 15, 2011, 12:18 PM
Frozen Flyer
moes's Avatar

STOL power set-up


I'm looking for opinions on how to power my STOL (short takeoff and landing) project. It will have roughly a 78" wing span, twin motors (one on each wing) in a normal configuration similar to a .40-60. size trainer/sport plane (except a twin), estimated weight will be 3-4 lbs roughly ready to fly. Iím mostly concerned with generating maximum thrust during take-off and significant power to lift payloads (2-3 lbs+).

I would like to get your opinions on power systems for this project. Iíve settled on brushless outrunners and would like to use 8-9Ē three bladed props.

- What I donít know is if I should look at 3S, 4S, 6S voltage. (I couldnít find much for proís vs. conís for the battery voltage wile searching)
- Also pitch for propellers, Iíve read that lower pitch would generate more thrust with a larger prop, but not sure if I should look at 9x2x3, 9x3x3, 9x4x3 (if they even make these sizes) your thoughts?

Also for this type of set-up where your goal is to generate maximum thrust for extremely short take-offs what other factors (power-wise) I should be considering? (fyi I have the aerodynamic part extensively researched, but short on the electrical side.)

Thanks again for your ideas/ and or suggestions.
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Aug 15, 2011, 03:57 PM
An itch?. Scratch build.
eflightray's Avatar
You may need to define what you actually require in the way of the model flying 'STOL'.

Many lightweight electric models can just about take off in less than 4x their own length.To the point where some just fly without any noticeable roll out.

As for landing, given a very light breeze, I have a ducted prop model that can just about fly backwards on very low throttle. Many others have models that can fly very slow.

You really do need to define exactly what you hope to achieve.

Oh, and there is also plenty of VTOL models in the VTOLs forum , you can't get much more STOL that that.
Aug 15, 2011, 04:05 PM
Registered User
Doug Bartley's Avatar
Hi moes: Why such a small prop diameter? Is it a typical high wing plane? A 78" wingspan should be capable of spinning a 12" without issue. Larger props are far more efficient. the Apc e 12/6 is one of the best on 3/4S, and a big surprise was a 12/8/3 blade EMP.
I put two of these EMp's on my 84" Twin Otter, running 4240-900 Turnigy's on 3S, plane climbs out at 60+ degrees for 250 ft. AUW is 8lbs,6ozs running 4 2200/3S in parallel for 15 minute flights. Take off is 50 ft, landing in about the same with flaps. These motor turn 485 watts each. that about 116w/lb
For unlimited climb you would need 125-150 watt/lb, so at 7 lbs, about 1050watts max.
A turnigy 3548-900 on 4S with a12/6 would deliver this or very close. The other consideration with a twin is battery capacity. A single 3600/3S/25C would run each motor well without undue stress on the battery, but two motors would quickly tax this system, so you have to double the capacity (mah) to keep amps down on each battery.
Hope what I've said makes some sense. FWIW Doug B
Aug 15, 2011, 04:25 PM
Frozen Flyer
moes's Avatar

I might have said too much or not enough.. always hard to tell. But to give you a better idea, Iím going to be building something very similar to a twin otter, for my next project Iím looking to see how far I can push a twin otter style airplane in respect to payload (lead shot / weights for tests) so Iíll be building as light/strong as possible but will be flight at significantly higher weights, Iím looking for what characteristics I would want in a power system to achieve maximum thrust wile minimizing weight. I was leaning towards 3542-400 motor (low KV) 10x3 3 bladed prop, and maybe a 3S 4500-5000 or (2) 3s 3000 packs, but I would like to know if there are any proís in going to a higher voltage set-up..I know three bladed props are less efficient but I figure with a proper low KV motor this might be practical to move more air with less diameter ect..

Aug 15, 2011, 11:12 PM
Registered User
Doug Bartley's Avatar
The smaller the prop, the higher the rpms has to be to create the thrust! You also have to deal with pitch speed, you have to to have enough forward speed to maintain flight. the normal consensus is 2 1/2 times (minumum) stall speed. A plane as light as your at 78" would need to fly about 30mph at full throttle and have a landing speed of 10-15mph. that stall speed goes way up as you load the plane with your weight lifting attempts.
The right 12" prop can make twice the thrust of a 10" at the same rpms. A low Kv motor as you suggest is really designed for higher cell counts than 3S. The advantage to higher cell counts is lower amps on the same prop. Higher cell counts usually require much smaller props to stay within the amp ratings. Only if the chosen prop gives the thrust you require at the higher cell counts is there any real advantage.
I've never heard of a 3542 400, I have 3542/1000 that works well with a 11 x55.5 or 12/6 (2 blade)(max) 400kv x 11 volts = 4400rpm, a 10 x ? x 3 blade wouldn't make enough power to fly your plane at its lightest Imho.
Aug 17, 2011, 10:55 PM
Frozen Flyer
moes's Avatar
Thank you Doug for your reply,

On my question about how pitch will effect thrust, on full size aircraft you would adjust the prop for min. pitch for max RPM in a landing or take-off config. So thats why I was wondering if going with a 2,3,4 blade pitch would be optimal compared to a 5,6,7 that would be more for cruise to get the most thrust from a said prop. I might look at a 11" prop for my plane but 12" might be a bit big for the scale i'm looking for. Sticking with a three blade configuration I can expect an 10-11" to act nearly equal to a 12" and also save on ground clearance issues with a weighted plane.

On the 4-6 cell count your saying a higher cell count = lower amps as everything equal resistance would be the same, higher voltage would give lower amps with the same prop. But you also say with higher cell count you need to run a smaller prop or less pitch? otherwise amps raise? you mean I would need to run a smaller prop because the RPMs would increase with a higher voltage battery assuming the same KV motor. So with a 42-50A 700KV on 4's or 5's should be good then?

Aug 18, 2011, 12:35 AM
Registered User
Doug Bartley's Avatar
Hi moes: Because there are no variable pitch props available that adjust in the air like real planes, you have to select a pitch that is a compromise. It must give adequate takeoff and climbing thrust while giving the plane ehough forward speed to fly.
Unlike nitro, electric motors produce much more torque at lower rpms, so can spin both a larger and higher pitch. Use the largest prop you can spin for efficiency at the pitch that attains the air speed the particular plane requires.
Higher S rating require you run a smaller prop at higher rpms to achieve the same thrust levels, and speed goes up with it.
I look for a motor that spins the prop size I need at the watts desired first. A higher voltage will be heavier and use less kv to achieve the same watts on the same prop at a lower amp draw. The trick is in finding the perfect match of higher volts, not too heavy, at the right kv to get only the watts you need with the desired prop. I wish you luck. DOug B

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