Twin Engine Efficiency

[Flying Cheetah]

I'm not sure if I'm posting this question in the right forum, but I have a question about the efficiency of twin engines depending upon their placement on an airplane. The DoubleEnder uses a push-pull method (one engine behind the other) which is contrary to most other twin engine setups. I plan on building a plane in the fairly near future that will be able to carry a sizable load, but I don't know if I would get comparable lift by using either placement of engines. I prefer the concept of the DoubleEnder versus side by side so that an engine failure wouldn't be a big issue. Does anyone have an answer to which placement of engines gives more lift?

 

[Foam Addict]

More lift? None really, in an electric aircraft, you will almost never have an motor out situation unless both are out.
Efficiency? I would think that the wing mounted motors would be more efficient since the rear one isn't in turbulent air.

 

[510thousandths]

On a push/pull setu up, you would run a higher pitch on the rear in the prop wash?

 

[xuzme720]

That might be a good experiment. I know the full size run the same props but at our reduced fluid scaling, there might be some benefit...

 

[Flying Cheetah]

More lift? None really, in an electric aircraft, you will almost never have an motor out situation unless both are out.

I was planning on using a couple of gas engines (I know, they might not be very commonly used in rc planes). Are those reliable or would I have to worry about engine failure?

Another question I have is "do props create a cork-screw of air behind them? If so, if one engine would be behind another, would there be less efficiency when the props are spinning the same way and more when they're not?"

 

[Flying Cheetah]

Would that help?

 

[xuzme720]

There are only a few aircraft that run twin engines in a coaxial configuration as you are considering. There are however many, many aircraft with twin engines in a side by side configuration, even though it creates a possible yaw problem in a single engine out scenario. I can think of 2 significant reasons for this. One is the loss of fuselage area for passengers/baggage because of the engines taking up real estate in the cabin. Two is probably efficiency since we know the coaxial props on y-hexes or octo quads are less efficient. Now granted, those are turning in opposite directions, and NOT having the props counter-rotating on the centerline might just induce torque roll of a significant strength. There are many other engineering aspects that must have been considered as well as noise in the cabin.

 

[Billchuck]

There's also balance issues; if you put the motors on the wings you can easily adjust them forward/backward (in the design) to get your cog where you want it. In a coaxial configuration that's much harder to do, you're heavily constrained by everything else that needs to be in the fuselage space.

 

[andybenton]

coax seems to cause a weird thought in my head... spinning props act like a disk right? so wouldnt the coax have less drag than twins on the wings?... the front profile would be the same as a single engine, but i feel like some thrust would be wasted pushing against the rear motor

although i dont know anything about this sort of stuff

 

[xuzme720]

coax seems to cause a weird thought in my head... spinning props act like a disk right? so wouldnt the coax have less drag than twins on the wings?... the front profile would be the same as a single engine, but i feel like some thrust would be wasted pushing against the rear motor

although i dont know anything about this sort of stuff

There would be less wetted frontal area by having them inline which would certainly create less drag. The question then is, will it be enough to offset the efficiency loss due to the rear prop being in the wash from the front prop? I have been basing all this on full scale craft though. I'm not sure how this would scale down or what kind of difference that would make.