Counter Rotating Prop Direction

[rcspaceflight]

I'm building my first twin engine plane this week and I have a question. Does it matter which engine spins counter clockwise and which spins clockwise?

I know for a fact that the flying pancake, pictured below, has the propellers spin so the air is being pushed down and under the wing. This is a key feature of the flying pancake because it allows for minimal air spillage over the wing tips which increases lift and efficiency which is why it can fly so great with such stubby wings.

But I'm not mounting the motors close enough to the wing tips to gain that advantage. So I'm guessing it doesn't matter at all which motor spins in which direction. Or does it?

Along the same question, I also want to ask about a plane with 4 engines. Would the two engines on each wing spin in the same direction? Or would the two engines near the fuselage spin in one direction and the two outside engines spin in the other? Or does it not matter, at all?

I'd probably be best off just copying what real aircraft do and this video shows that it's the same rotation set up as the flying pancake.

But I'd still like to know if it even matters. If anyone knows that is.

 

[vk2dxn]

Any aeronautical engineers on board?.

My guess is to copy the prop direction of the vid posted as it makes sense to me however
this is only my opinion.

 

[c172ae]

I think I remember reading a long time ago about the twin mustang not taking off at first, but when they switched the rotation direction on both engines, it took off just fine.

I don't know how big of an impact this has with rc planes, but i would set them up so that the props "meet" when they are going downwards. This is the setup most light twins use .

 

[rcspaceflight]

I remember hearing that when the P-38 first came out they made a trainer version with the props rotating in the same direction. I think it was because they didn't yet have an engine for the plane that could rotate in the needed reverse direction. Anyway, I guess the plane was horrible and the people that ended up with it HATED it. lol.

Interesting about the Twin Mustang though.

 

[pgerts]

The "propwash" is trying to turn the plane in one direction. That is the reason for "normal 4°" right angle on a motor is to counter react to the left turn the "normal turning" prop is generating.

The right motor (alone) is trying to turn the plane to the left?
I would have choosen a (counter) prop that was helping the plane turning right - and also turning the right motor outward a little.

The opposite for the left motor.

Take a look at this thread with video and discussion http://www.rcgroups.com/forums/showthread.php?t=656493

 

[xuzme720]

I say go for it the way the full scale is done. Then try it swapped and see if it makes a difference to our sizes. It's only a matter of swapping 2 leads per motor and swapping the props...

 

[rcspaceflight]

I say go for it the way the full scale is done. Then try it swapped and see if it makes a difference to our sizes. It's only a matter of swapping 2 leads per motor and swapping the props...

...but that's work.

 

[DDSFlyer]

...but that's work.

Lol!

 

[SanderHG]

Looking from behind the plane, make the left engine turn clockwise and the right engine counter clockwise (downward going propeller blade next to the fuselage). In real twins this is done to reduce the yaw moment you get should one engine fail, and thus the size of the rudder you would need on the airplane to counteract it. The downward going propeller blade produces more thrust than the upward going one due to the airplane having a positive angle of attack during most flight phases (down going blade has a larger angle of attack than the upward going one). Becaue of this, the total resultant thrust force a propeller produces is not in line with the center of the prop, but on a clockwise turning prop it is right of center and on a counter clockwise one it's to the left. With the configuration I mentioned above, you end up with the smallest yaw moment possible, should either engine fail.

Another effect having 2 props that turn in the same direction (and you don't have when one is turning clockwise and the other counter clockwise) is a yaw moment when you add power, even with both engines running. This is again due to the fact that the thrust a prop produces does not act out of the center of the prop but a bit off center. If for example a plane has 2 clockwise turning props (and thus the thrust force acts right off center), this would mean that the thrust of the left engine would be closer to the center of gravity than the right one, resulting in a yawing moment to the left. I don't know how noticable this effect in a model airplane would be though. I fly a Bombardier Dash8 Q400 for a living and there it is really noticable. You need the rudder trim with every power change to counter it.

The P38 you see in your video is an exception to the rule. There they made the left engine counter clockwise and the right one clockwise to reduce the amount of propwash on the horizontal stabilizer to make it more stable and easier to shoot things.

Just google "critical engine" for more info. Here is the wiki article about it: http://en.wikipedia.org/wiki/Critical_engine

So tl;dr, looking from behind of the model, left engine should turn clockwise, right engine counter clockwise (down going propeller blade closest to the fuselage).

 

[rcspaceflight]

I guess I better listen to pgerts and SanderHG. They said the same thing with the same, sound logic.

However I have one question about that. I get that that set up would mean it's the most stable, especially for an emergency like when one engine fails. But we're talking about RC planes. I shouldn't have ONE engine fail. But ignoring that, let's say that I want to use differential thrust to get super crazy flat turns. Wouldn't I want the motors the opposite then? Wouldn't I want extra pull in one direction when only using a single engine for a sharper turn? If I went with the stable set up, then differential thrust wouldn't work as well, right?

At least my take away is that I want the Flying Pancake / P-38 set up for an RC plane if I'm going for super tight differential thrust turns. But if I want it as stable as possible, then the opposite. But I'm also, again, wondering if it even matters. Either way the twin engines still counter act each other. Maybe it simply boils down to how the prop wash is hitting the control surfaces. So then I'd want the P-38 set up for the reasons SanderHG mentioned about the real P-38.

.... I think I'm going to go with the P-38 set up. Even though I'm not going to set up differential thrust, at least not at first. I also think I'm going to solder the motors directly to the ESCs so I can't simply try the opposite to see if it flies differently. I figure I can just solder one motor to an ESC and the motor direction doesn't matter. Then flip around two wires on the other motor/ESC when soldering. That should guarantee me counter rotating motors without any testing.

 

[quorneng]

rcspaceflight
Just a historical note.
The P38 prototype had its propellers counter rotating in the 'conventional' way (over the top towards the fuselage) but it caused a buffet problem so was reversed to over the top away from the fuselage on all production planes.
http://p38assn.org/images/p38s/XP-38-orig.jpg
Although this solved the problem it left the P38 with less than ideal single engine characteristics at slow speed as the asymmetric thrust combines with the engine torque resulting in a strong rolling component. Indeed an engine failure just after take off in a P38 required the good engine to be throttled back a bit to prevent it rolling over until it had gained sufficient speed. Several pilots were killed before this problem was identified.
So unless you are building a P38 and want absolute scale set the props 'over the top towards the fuselage'.

 

[rcspaceflight]

set the props 'over the top towards the fuselage'.

That's actually the way I'm going. I changed my mind last minute. Having the tops of the props going towards the fuselage seems more naturally to me for some reason. It just seems to look better. If I ever make a Flying Pancake I'll go the other way in order to keep the air under the wings. But I agree with you guys that that would probably be the only exception.

 

[wb4qiz]

Josh, what do you say about the prop rotation for the FT Cruiser?

 

[Maingear]

Hope this video helps explain why the top tip should rotate inward - Slipstream/
http://youtu.be/yxKY6UOGgWs

http://youtu.be/QGWbjGqmoWo

Required knowledge for pilot or technition rating granted by the FAA.

Justin

 

[joshuabardwell]

In real twins this is done to reduce the yaw moment you get should one engine fail, and thus the size of the rudder you would need on the airplane to counteract it.

Are RC twins typically flyable on one engine?

 

[Maingear]

I was asked to perform an engine out via diff thrust on my Kraken at the club back in December. First engine, no big deal aileron added and limped along. Cary said "Ok, try the other." I threw the rudder to other side and it looked like a wounded bird going down! I was shocked! The props were the same standard APC 9x4.7 and not counter rotating. I don't know if flying wings could be worse because they don't have an empennage?

As soon as I got home I ordered several spare pusher props and added to the minimum equipment list!

Justin