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Old Dec 27, 2012, 12:24 PM
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Crazy motor sync question

This wild idea occurred to me for use on my multicopter.

On multis coaxial props are often used as a means of redundancy, The problem is having two motors/props stacked and spinning opposite directions results in a loss of efficiency from 15-20 percent.

So here is my idea; Same type of physical coaxial setup but have the props spinning in the same direction. So far no big deal but here is the trick. I want one ESC to control both motors and I want them perfectly synchronized.

My thought is this will aerodynamically just function as a 4 blade prop but two props are driven by one motor and the other 2 props are driven by a second motor. This gives redundancy if you lose a prop or motor but should not suffer the coaxial loss from running a counter-rotating prop.

I have attached a picture of a normal coaxial quad so you can see what the setup looks like.

Is this possible with one ESC? What is needed to synchronize the motors?
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Old Dec 27, 2012, 12:43 PM
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Some searching led me to this thread. http://www.rcgroups.com/forums/showt...ght=sync+motor

It looks like it will work just fine. But the problem is the failure of one motor will kill the second motor.

So I'm changing my question.

Now I want to use 2 esc's that are driven by the same radio signal (I'll just put a Y on the signal from the receiver to the ESC's). How do I keep them synchronized?
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Old Dec 27, 2012, 03:38 PM
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Quote:
Originally Posted by milo12
I want to use 2 esc's that are driven by the same radio signal (I'll just put a Y on the signal from the receiver to the ESC's). How do I keep them synchronized?
Couple the shafts of both motors together, then they will always be 'in sync' and you can set the rotational position of each prop relative to the other (for best results the blades probably should be directly in line vertically).

Quote:
The problem is having two motors/props stacked and spinning opposite directions results in a loss of efficiency from 15-20 percent.
I think you are trying to solve a problem which doesn't exist.

Contra-rotating propellers
"Contra-rotating propellers have been found to be between 6% and 16% more efficient than normal propellers"
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Old Dec 27, 2012, 04:14 PM
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Coupling the shafts seems like it would cause an issue that if a motor fails it takes the other with it.

I agree about the efficiency issue. In fact I have posted studies in the forum that support the contrarotating are more efficient case.

Nonetheless the proven fact is on a multirotor using the normal airplane props that everyone uses there is a large loss in efficiency. My guess for the cause is all the real studies were done on full size controllable pitch aircraft propellers and the little plastic fixed pitch model props do not act the same.
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Old Dec 27, 2012, 07:43 PM
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It looks to me that the copter wouldn't fly at all with the props as they are!

How about a single motor on each arm with along shaft and a prop on each end, you still cut your esc count in half.
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Old Dec 29, 2012, 03:55 AM
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The props need to be designed from the get go to perform efficiently as counter rotators. The rear prop in the drive photo has four blades. Although even in the three blade configuration the blade profiles of the front and rear prop are considerably different. The coaxial heli's have upper and lower blade sets and the differences are not as pronounced but the necessary differences are very visible in props run in water. Maybe experimenting with two different props could lead to improvements.
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Old Dec 29, 2012, 05:07 AM
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Originally Posted by milo12 View Post
Nonetheless the proven fact is on a multirotor using the normal airplane props that everyone uses there is a large loss in efficiency.
I searched several forums, but was unable to find any definitive proof of that claim. So I ran a little test:-

Motors: Turnigy 2217-16
Props: APC 10x3.8SF & 10x3.8SFP
ESC: Hobbywing Flyfun 30A x 2
Batt: Bench Power Supply

I mounted the motors on the front and rear of my thrust test rig (pusher at rear) simulating one arm of a multi-rotor platform. The distance between the front and rear props was ~140mm.

First I ran just the front motor (with no prop on the rear motor), then both motors together, and finally just the front motor but with voltage raised to get the same thrust.

Results:
Front only: 6.9V, 11.3A, 78W, 570g, 73% motor efficiency (calculated)
Front&Rear: 6.9V, 21.8A, 151W, 965g, 73% motor efficiency
Front only: 9.4V, 18.4A, 173W, 962g, 70% motor efficiency

That's 15% less power required to get the same thrust with coaxial motors vs a single motor. Even after taking into account my motor's 3% lower efficiency at higher power, the coaxial contra-rotating setup still has a 12% advantage.
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Old Dec 29, 2012, 06:08 PM
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Contra-rotating propellers is a good way to improve efficiency because the rear propeller can recover some energy out of the turbolence made from the first propeller , for BEST efficiency and LOWER noise the two propeller should be different in pitch and blade number but a plain setup with two identical (CW and CCW ) propeller is a huge step forward in efficiency because power have a greater disc area so can use more air
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Old Dec 29, 2012, 10:33 PM
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a plain setup with two identical (CW and CCW ) propeller is a huge step forward in efficiency because power have a greater disc area so can use more air
My tests show that a pair of independent motors and props would produce 15% more thrust for the same power, but that requires going to a radial octocopter configuration which is bigger and heavier. A coaxial configuration is used to provide greater lift and/or increase redundancy without increasing size. It has been suggested that running both props synchronized in the same direction should improve efficiency, but I suspect that this would actually be less efficient than a contra-rotating setup.

Quote:
Originally Posted by milo12
My guess for the cause is all the real studies were done on full size controllable pitch aircraft propellers and the little plastic fixed pitch model props do not act the same.
More likely a misunderstanding of the constraints involved.

When you want more power without increasing size, coaxial contra-rotating props have an advantage. If you have more room available then separate props are more efficient. Both are better than just pumping more power into a single prop of the same size.

Full-size helis use contra-rotating coaxial blades to get better efficiency at higher power without increasing size. It eliminates tail rotor losses and allows higher disc loading for heavy lifting. A larger rotor diameter is generally even more efficient, but size brings its own problems, and having more than two rotors does not significantly improve reliability or lower operating costs.

The aerodynamic principles are the same for models as for full-size aircraft, but we don't have to do things the same way. Multi-rotor platforms with lots of motors, props and ESCs are easier for us to implement than complicated mechanisms with swashplates, servos, flybars and gearboxes. At this smaller scale materials are stronger relative to their size, so we can use structures that wouldn't be appropriate for full size helis, and electric motors throttle rapidly so we don't need variable pitch mechanisms.

Quote:
My thought is this will aerodynamically just function as a 4 blade prop but two props are driven by one motor and the other 2 props are driven by a second motor. This gives redundancy if you lose a prop or motor but should not suffer the coaxial loss from running a counter-rotating prop.
4 blades are known to be less efficient than two, so I doubt that it would be better than running contra-rotating props. However you do it, there will be some loss of efficiency due to the limited disc area.
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Old Jan 15, 2013, 05:48 PM
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Thanks for that excellent explanation. I have always liked coaxials and I know all the research shows they do have some advantages.

The problem is the mulirotor crowd looks at it very simply.

If I build a flat hex multirotor I can fly longer than if I use the exact same motors, props esc, and batteries and make a Y6 multirotor.

That has been done and proven many times. So the multirotor crowd accepts the conclusion that coaxial is less efficient compared to separate props. And in this situation I can see why there is an efficiency drop. Separate props will have twice the disk area, half the disc loading, than the coaxial arrangement.

Regardless, thanks for your input as I see how my initial concept was no good and a waste of effort. I will stick to a coaxial arrangement.
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