Why Use Contra-Rotating Propellers in Aircraft Design?

[DaveC426913]

Upon exiting Avatar I was discussing with my biologist sister some of the flubbs in the film - she biology, me technology. I made a joke about the helicopters but now it's become a topic of discussion and I thought I'd check my facts.

I noticed while watching Avatar that the http://james-camerons-avatar.wikia.com/wiki/Scorpion_Gunship" . So, the copters have one prop pod per side AND each prop pod has two rotors (for a total of 4 rotors). The rotors in a pod rotate on the same axis but in opposite directions.

IIRC, some carrier-based craft used this prop design in WWII. Funny, Wiki doesn't pull up ones that have familair names, but here's the http://en.wikipedia.org/wiki/Fairey_Gannet" .

Anyway, it is my understanding that the primary purpose for using contra-rotating propellors is that the engine torque is cancelled, i.e. it does not pull to one side.

So, it seems to me, on a craft that is symmetrical and therefore can use two http://en.wikipedia.org/wiki/Counter-rotating_propellers" , the torque is canceled anyway, meannig there's no advantage to using contra-rotating props.

 

[DaveC426913]

:sigh: Further research (read: Wiki surfing) turns up an airplane with exactly the configuration as the Scorpions - the http://en.wikipedia.org/wiki/A2J_Super_Savage" .

So, yes, I guess there is an advantage, else they would not have built this craft at all...

 

[Cyrus]

Upon exiting Avatar I was discussing with my biologist sister some of the flubbs in the film - she biology, me technology. I made a joke about the helicopters but now it's become a topic of discussion and I thought I'd check my facts.

I noticed while watching Avatar that the http://james-camerons-avatar.wikia.com/wiki/Scorpion_Gunship" . So, the copters have one prop pod per side AND each prop pod has two rotors (for a total of 4 rotors). The rotors in a pod rotate on the same axis but in opposite directions.

IIRC, some carrier-based craft used this prop design in WWII. Funny, Wiki doesn't pull up ones that have familair names, but here's the http://en.wikipedia.org/wiki/Fairey_Gannet" .

Anyway, it is my understanding that the primary purpose for using contra-rotating propellors is that the engine torque is cancelled, i.e. it does not pull to one side.

So, it seems to me, on a craft that is symmetrical and therefore can use two http://en.wikipedia.org/wiki/Counter-rotating_propellers" , the torque is canceled anyway, meannig there's no advantage to using contra-rotating props.

Yep, that's right. The real difference between the two is that a contra-rotating propeller has a larger effective disk area in a smaller package.

 

[xxChrisxx]

It amuses me that this was the first thing you thought about upon leaving the film Dave.

Not to bump it off topic buuuuuut, but did you feel slightly motion sick until you get used to the 3D? Saw Avatar about a week ago and, I don't know if it's becuse it was my first time watching 3D or because I wear glasses (I do need my prescription updating) but about 15 mins I felt a bit groggy.

 

[tomkeus]

but about 15 mins I felt a bit groggy.

Me too. I don't wear glasses and I never suffer from motion sickness, but during the first 10-15 minutes I did feel sick. I watched it in Xpand3D theater. Some of my friends who saw it later in Dolby3D told me it was a lot easier, both on the eyes and the brain.

As for the science of Avatar, the first thing I noted immediately is that the Na'vi feel somewhat out of place on Pandora when wildlife is accounted. They are anatomically too different from the rest of the animals on the planet yet they still share that LAN-kinda connector.

 

[FredGarvin]

Anyway, it is my understanding that the primary purpose for using contra-rotating propellors is that the engine torque is cancelled, i.e. it does not pull to one side.

So, it seems to me, on a craft that is symmetrical and therefore can use two http://en.wikipedia.org/wiki/Counter-rotating_propellers" , the torque is canceled anyway, meannig there's no advantage to using contra-rotating props.

The counter rotation cancels torque which prevents the SPINNING of the aircraft, i.e. about the yaw axis. It does not prevent slip or side-to-side. Looking at the picture, if you had a single rotor on each wing and they were counter rotating, they would cancel out, but there would be some serious bending (about the vertical axis) stresses on the wings themselves. With two rotors on each pod, they cancel each other out and eliminate the bending moment on the wings.

I can't tell but it looks like they are lacking some form of yaw control. If they have just the two fans, then there is no way to yaw the aircraft without rolling as well.

 

[DaveC426913]

It amuses me that this was the first thing you thought about upon leaving the film Dave.

Well, not the first thing. I was half kidding around. My sister was being a biology nerd, so I played the tech nerd.

Not to bump it off topic buuuuuut, but did you feel slightly motion sick until you get used to the 3D? Saw Avatar about a week ago and, I don't know if it's becuse it was my first time watching 3D or because I wear glasses (I do need my prescription updating) but about 15 mins I felt a bit groggy.

I did not experience any difficulties at all. I was sitting in the third row and I wear glasses too.

As for the science of Avatar, the first thing I noted immediately is that the Na'vi feel somewhat out of place on Pandora when wildlife is accounted. They are anatomically too different from the rest of the animals on the planet yet they still share that LAN-kinda connector.

Yes, this was my sister's complaint.

Elsewhere on the web, there is a hypothesis that the Na'vi are not native to Pandora. The pony-LAN is a carry-over from an ancient bio-engineering mod.

The counter rotation cancels torque which prevents the SPINNING of the aircraft, i.e. about the yaw axis. It does not prevent slip or side-to-side.

Yes, I was being sloppy with my words.

Looking at the picture, if you had a single rotor on each wing and they were counter rotating, they would cancel out, but there would be some serious bending (about the vertical axis) stresses on the wings themselves. With two rotors on each pod, they cancel each other out and eliminate the bending moment on the wings.

I find it implausible that they would add a whole second rotor to the design, vastly complicating its workings, for the sake of an under-engineered wing.

I can't tell but it looks like they are lacking some form of yaw control. If they have just the two fans, then there is no way to yaw the aircraft without rolling as well.

The rotor pods pivot along a common transverse axis. Easy yaw control.

 

[tomkeus]

I can't tell but it looks like they are lacking some form of yaw control. If they have just the two fans, then there is no way to yaw the aircraft without rolling as well.

Perhaps, asymmetrically slowing down rotors on one or both sides of the aircraft to produce net torque, while adjusting the pitch of the blades at the same time to compensate for the lost lift.

 

[DaveC426913]

Perhaps, asymmetrically slowing down rotors on one or both sides of the aircraft to produce net torque, while adjusting the pitch of the blades at the same time to compensate for the lost lift.

Or just tilting the rotor pods.

 

[FredGarvin]

The only thing with the tilting pads is that there would have to be an associated change in pitch to accommodate the lower lift due to the tilting of the lift vector. Still...nice and easy.

 

[Cyrus]

The counter rotation cancels torque which prevents the SPINNING of the aircraft, i.e. about the yaw axis.

Errr...that's not true. It cancels out a lot more than just spinning, it cancels out all gyroscopic couplings due to angular momentum of the rotors:

$$M_T = [0, I_p \Omega_p r, -I_p \Omega_p q]$$

It does not prevent slip or side-to-side.

I don't know what this means.

With two rotors on each pod, they cancel each other out and eliminate the bending moment on the wings.

Good point!

I can't tell but it looks like they are lacking some form of yaw control. If they have just the two fans, then there is no way to yaw the aircraft without rolling as well.

Why? Adverse/proverse yaw behavior will happen contra-rotating propellers or not. Its an effect of variational freestream velocity along the span wise direction due to yaw rate.

 

[DaveC426913]

I don't know what this means.

He was interpreting my original use of the phrase "pulling to one side"; he was thinking I meant side-slip.

So: torque doesn't prevent side-slip; it prevents yaw.

 

[Cyrus]

So: torque doesn't prevent side-slip; it prevents yaw.

If you have asymmetric thrust, yes. Typically, the tail fin prevents side slip by weathercocking the airplane into the wind.

 

[DaveC426913]

If you have asymmetric thrust, yes. Typically, the tail fin prevents side slip by weathercocking the airplane into the wind.

Tell that to the poor slobs trying to outmaneuver a Sopwith Camel in a hard right turn...

 

[Cyrus]

Tell that to the poor slobs trying to outmaneuver a Sopwith Camel in a hard right turn...

That's because of gyroscopic coupling in the equation I gave earlier.

 

[DaveC426913]

That's because of gyroscopic coupling in the equation I gave earlier.

Well ... yes. But weren't you just suggesting that the tail fin will prevent this side-slip?

 

[Cyrus]

Well ... yes. But weren't you just suggesting that the tail fin will prevent this side-slip?

Well, I said a tail provides weathercock stability. But I did not say that it necessarily has enough counter torque if there is a high inertia engine spinning at the nose. I made no claims about the overall aircraft performance, only the contributions of each.

Edit: Going back to your original question and tails for a moment (no pun), the size of the tail is usually design to specifically provide stability in an engine-out scenario for a twin engine aircraft.