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Old Apr 24, 2012, 05:44 PM
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Twin or single motor cargo plane

Just one question, I have learned growing up that twin motor planes are more "inefficient" and it never quite made sense to me. If you have two propellers, shouldn't you be outputting twice the amount of thrust that one propeller planes can produce? And if you're outputting twice the amount of thrust, you should be also putting out twice the amount of force. By that principle, you should be able to accelerate two times faster. How does this fact make twin props more inefficient? Also, does two props give you more maximum speed over single props? Or does it just help you accelerate up to maximum speed faster?

If I want to build a heavy lifting cargo plane, should I go with a 4 motor, 2 motor, or single motor design? Also, what Kv would you recommend if I use a 3s Lipo.

Thanks for all the help and support.
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Old Apr 24, 2012, 05:57 PM
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I would also like to know the answer to this question, I have posted a similar thread in power systems a couple of hours ago.
My hypothesis is that with a twin, you have twice the power output, but the pitch speed remains similar. It is also known that a larger prop spinning slower is more efficient than a small prop spinning quickly. I am not sure how that translates to acceleration though, using f=ma, we know that the force (in this case the tractive force from the props) is proportional to the acceleration, so in theory it should increase with the increase of force, that is of course assuming that mass would be constant, and it wouldn't because two motors and esc obviously weigh more. How drag from the extra nacelles and propellers affect the aircraft also need to be taken into account :/
James
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Old Apr 24, 2012, 06:04 PM
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Originally Posted by -james- View Post
I would also like to know the answer to this question, I have posted a similar thread in power systems a couple of hours ago.
My hypothesis is that with a twin, you have twice the power output, but the pitch speed remains similar. It is also known that a larger prop spinning slower is more efficient than a small prop spinning quickly. I am not sure how that translates to acceleration though, using f=ma, we know that the force (in this case the tractive force from the props) is proportional to the acceleration, so in theory it should increase with the increase of force, that is of course assuming that mass would be constant, and it wouldn't because two motors and esc obviously weigh more. How drag from the extra nacelles and propellers affect the aircraft also need to be taken into account :/
James
Well, with a larger prop, I guess you have to take into account of torque, as T=Ia, the I increases as mass increases, so to get the same amount of torque, the smaller prop would need a lot more angular acceleration to compensate for its smaller rotational inertia.
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Old Apr 24, 2012, 09:24 PM
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extra bearing friction, extra drag from another prop.
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Old Apr 24, 2012, 11:23 PM
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Okay, but for the extra torque you're getting, it would probably not be too inefficient a choice right?
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Old Apr 25, 2012, 02:58 AM
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It's generally accepted that larger diameter props are more efficient at creating thrust than small props. For the same total thrust you're looking at the difference between one large prop or two or four much smaller props.

But acceleration is governed by how much total thrust you have. If the total thrust is the same the acceleration will be too .

Speed is basically related to the pitch speed (prop pitch x rpm). It doesn't matter what diameter the props are from that point of view though if the thrust is low the plane may have trouble ever accelerating up to the theoretical maximum.

And of course efficiency really means how much power do you need to put in to get out the thrust/speed that you're looking for. Multiple motors with small props will almost always need more input power than a well sized single motor with a larger prop so less efficient overall. In our terms that basically just means that with the same size battery you'll get a shorter run time with the less efficient multi-motor system. But the difference won't be huge.

Steve
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Old Apr 25, 2012, 05:35 AM
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The main reason to go multi-engine is to get more power through the props. With a single prop, the amount of power you can put through it might be limited by the maximum diameter you can fit on the aircraft without the prop hitting the ground.
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Old Apr 25, 2012, 05:47 AM
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I was considering a 4 prop setup with the two inner props being low pitch for good low-speed response, while the two outer props being high pitch for good speed.

Can anyone think of any major problems that such a setup might cause? Is it worth bothering with?
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Old Apr 25, 2012, 05:52 AM
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Quote:
It's generally accepted that larger diameter props are more efficient at creating thrust than small props.
That's not true in this case. One large prop is more efficient than one small prop, but only because it's got to move more air through a smaller disk

One large prop is the same as two smaller props of the same area.

The reason big planes have multiple engines is that they don't have enough ground clearance for one huge prop. It's safer, as they can continue to fly if one engine fails, and there are structural benefits to moving heavy weights out onto the wings. (the wing doesn't have to be as strong if the weight is in the same place as the lift). Also many multi engined planes have been designed around existing engines, many WWII bombers used 4 engines of similar types to the fighters.

Very little of this is relevant to models, except possibly ground clearance. If you have a single motor, the long landing gear is going to make it look like a small plane.
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Old Apr 25, 2012, 07:34 AM
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That's not true in this case. One large prop is more efficient than one small prop, but only because it's got to move more air through a smaller disk
Except it is very often true in precisely this case. E.g. when you look at the likely disk area of a single prop vs two or more props using the same total power the single prop will almost always have more area. E.g. replace one 10" prop by 2 x 7" props - slightly less area. If, as is more likely, you use 2 x 6" props that's over 30% total less disk area.

Obviously it would theoretically be possible to replace one 10" prop by two larger props turning very much slower (and either use very high pitch props, or lose a lot of pitch speed)...but very few people do that .

Steve
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Old Apr 25, 2012, 07:35 AM
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Originally Posted by ulogix View Post
I was considering a 4 prop setup with the two inner props being low pitch for good low-speed response, while the two outer props being high pitch for good speed.

Can anyone think of any major problems that such a setup might cause? Is it worth bothering with?
I can't give you any rules, or formulae, or anything, but that doesn't sound like a good idea.

First of all, the props probably won't be at their most efficient if they're trying to do different things at the same time. Then, when you want a quick response, it still won't be very quick because only half of your power is coupled to low-pitch props. I'm guessing it's probably best to put "compromise" props on all the motors.
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