In most helicopters there is the anti-torque tail rotor to prevent the body from spinning in the opposite direction to the main rotor.

What's the equivalent mechanism in horizontal takeoff single engine propeller, and jet aircrafts, where the air or the jet coming out back from the turbine or propeller is spinning and will cause such aircraft to roll?

  • $\begingroup$ For twin engined prop craft, there is a critical engine which if fails will tend to force the craft to yaw more. Look at en.wikipedia.org/wiki/Critical_engine $\endgroup$ – ja72 Apr 9 '13 at 20:52
  • $\begingroup$ I'm more concerned about single engine planes, because in this case the torque will be asymetric. In twin engined planes, this effect is obviously not in effect, because the torque caused by the engines will affect one side of the plane, which is far from the center of gravity, and the other engine's direction of rotation could be adjusted to cancel the effect. I will edit my question to clarify this point. $\endgroup$ – Force Apr 9 '13 at 21:18

The motor does exert torque on the fuselage.

The pilot, without having to think about it, compensates by applying right aileron, which has plenty of roll authority.

There's more to it. When a propeller-driven plane is taking off, it has a tendency to yaw to the left, and the pilot automatically applies right rudder to compensate. That left-turning tendency is due to the propeller descending on the right hand side, at a higher angle of attack (thus more thrust) due to the pitch of the airframe, and also because the wind coming off the propeller is a corkscrew flow, and it strikes the left side of the vertical stabilizer.

If you watch this video, you will see how pilots are instructed to operate the F4U Corsair in WW2. It is recommended to tune in some right-rudder and right-aileron trim when taking off. It is also recommended not to take off at too slow a speed because it will seem "left-wing-heavy" due to that torque, and more speed means more control authority.

  • $\begingroup$ So there's no design feature that counter the effects of this torque, so far as aerospace technology has reached? $\endgroup$ – Force Apr 9 '13 at 21:25
  • $\begingroup$ It's just one of those things that pilots learn. Planes are usually designed with some built-in trim, to act as a compromise between different flight regimes, like takeoff (high power), cruise (medium power), and landing (low power). Like if you look at the rudder of a C172, you will see a little plastic tab near the bottom. That's a built-in trim tab so you need less right rudder on takeoff. As far as roll torque is concerned, the ailerons are plenty strong enough to manage it. It's not an issue. $\endgroup$ – Mike Dunlavey Apr 9 '13 at 21:29
  • $\begingroup$ @Jim: Lots of questions here about airplanes seem to arise because of not comprehending how heavy air is. In everyday experience, air seems ephemeral, but that's an illusion. It weighs about a $kg/m^3$ and even at low aircraft speeds it takes a lot of force to muscle it around. That force increases as speed squared, so it's practically as dense as water to a speedboat. $\endgroup$ – Mike Dunlavey Apr 9 '13 at 21:38

If you place two engines spinning in opposite directions equidistantly from the center of gravity, the torque is cancelled. Hence the old Air Force song:

"Don't give me a P-38, with props that counter-rotate They'll loop, roll and spin but they'll soon auger in Don't give me a P-38!"

Single engine planes can compensate with a bit of rudder . Check Martin Beckett's answer below.

  • $\begingroup$ Do you mean the rudder in center position? Isn't the rudder responsible for yawing not rolling? $\endgroup$ – Force Apr 9 '13 at 3:17
  • $\begingroup$ I am not sure actually, I do not know the name, I was just meaning that the torque needs to be compensated for and the only option in single engine planes is to do it mechanically through the use of control surfaces. It would be interesting to investigate how much that control needs to be and how does it affect flight aerodynamically (ie drag). $\endgroup$ – Isopycnal Oscillation Apr 9 '13 at 3:30
  • 2
    $\begingroup$ The tail is slightly angled in some high performance single prop planes to counter torque. In WWII fighters it was a desired feature - allowing the plane to roll more quickly (although in one predictable direction). In very light WWI aircraft with rotary engines (where the engine rotates with the prop) it could be very dangerous $\endgroup$ – Martin Beckett Apr 9 '13 at 3:46
  • $\begingroup$ By tail slightly angled you mean, elevator slightly rolled to be non perfectly horizontal? $\endgroup$ – Force Apr 9 '13 at 3:58
  • $\begingroup$ @Martin: Rotary engines on WW1 fighters were dangerous because of gyroscopic precession. i.e. hitting the rudder for left or right yaw could knock your nose up into a stall or down into the ground. $\endgroup$ – Mike Dunlavey Apr 9 '13 at 21:19

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