Uniform turbine rotation direction?

cfx
  • Uniform turbine rotation direction? cfx

    I'm not an aviator, I guess I'd call myself an enthusiast. I'm also a physics nut. I've always wondered about this but I equally worried my question posed to a commercial airline crew would land me in the Homeland Security Suites.

    On helicopters, torque along the vertical axis is countered by a smaller tail rotor (or a twin main rotor rotating in the opposite direction).

    My question is whether the turbines/fans of multi-engine jet aircraft rotate in the same direction. I.e., do both the port engine and the starboard engine rotate uniformly clockwise or counterclockwise?

    It seems like if that were the case then maintenance might be easier but it would create some instability along the longitudinal axis of the airframe due to torque and moment arms.

    Follow up: I guess, then, that single engine propeller aircraft utilize trim presets to compensate for the longitudinal torque generated by their propeller–correct?

  • Engines on airliners turn the same direction. The torque isn't as much an issue on jets as it is on props.

    A lot of multiengine prop planes have propellers that turn in opposite directions. In turbine engines this can be done in a gearbox to allow the same engine to be used on both sides.

    Some turbine engines, such as the PT6, have two turbines turning in opposite directions, but the propeller friction produces additional torque in one direction.

    Single engine planes sometimes have the engine mounted at a slight angle to reduce the effect of the propwash (and maybe torque?). The propwash causes imbalance at low speed and high power when the descending blade tends to push air down over the wing on that side, and and vice versa for the ascending blade.

    Immediately after takeoff, the rudder of a single engine plane is normally depressed, to some extent, to make up for the uneven torque and propwash. This may be done manually, with trim settings, or both.

    At higher speeds, the effect of the propeller torque is not as noticeable because the wings are held "more tightly" by the airflow.

  • Let's give a short and snappy answer regarding jets: it's all about efficiency.

    If a part of an engine is damaged then the engine will most likely be replaced and repaired while the aircraft is flying with another engine. An engine turning clockwise and an engine turning counterclockwise are two different engines. Let's do some simple economics now: is it more efficient to have only one extra engine you can use regardless of which engine failed or if you need an extra engine for each side? Clearly the most economical maintenance solution is to ensure both jets, regardless of the side of the airframe they attach to, are identical and thus interchangeable.

    Speaking as someone who's not a scientist, the torque effect at jet engines indeed is negligible.

    How about props? The biggest factor affecting props is that an ascending blade delivers less lift (in forward direction) than the descending blade. If you now chose two engines both turning in the same direction and attach these to the wings of an aircraft, one side has the descending blade outboard and on the other side it will be inboard - now you have a critical engine. If the engine where the descending blade is inboard (critical engine) fails, it creates a greater yaw than if the other engine fails. Now you need to evaluate if safety and structural design allow you to have two engines turning in the same direction.

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