Translating tendency

September 23, 2011 by Tim McAdams

If you have ever closely watched a hovering helicopter, you might have noticed that most times the skids are not level with the ground. In other words, one of the helicopter’s skids is lower than the other. Although wind and loading can cause this, the tail rotor thrust determines the base line for either right skid low or left skid low. Tail rotor thrust tends to make the helicopter drift in the same direction and is called translating tendency.

The tail rotor is designed to produce thrust to oppose the torque that tries to spin the helicopter in the opposite direction of the main rotor. Some of this thrust applies a force to the fuselage that causes the helicopter to drift laterally in the same direction. A tilt in the main rotor causes a small sideward thrust opposite the tail rotor to counteract the drift. The tilt can be accomplished by mounting the transmission at a slight angle or designing the flight control system to tilt the rotor disc when the cyclic control is centered.

The direction the rotor spins makes a difference. In a system the turns clockwise when viewed from above, the tail rotor thrust causes the helicopter to drift to the left. Tilting the main rotor disc to the right to counter this causes the right skid to hang low. A counterclockwise turning system will cause a right drift and a left tilt making the helicopter hover left skid low.

23 Responses to “Translating tendency”

  1. Restor8 Says:

    Good information to know as a pilot. Keep up the good work.

  2. Alan D. Resnicke Says:

    Teaching this to students, and why it mattered, was a challenge at times – especially the why. Then we took them into tight, confined areas and suddenly it became much clearer to them!

  3. David Shirley Says:

    I don’t know how many other helicopters this is true for, but in the Robinson R22, translating tendency is manually counteracted by pilot input, in order for the helicopter to hover over a spot, the cyclic control must actually be slightly to the left of center.

  4. Cory Says:

    David, translating tendency is always counteracted by the pilot.Any time you add any pedal input, there is a reaction causing the helicopter to drift, just like you said, when you are in a stable hover the reason you have to add a bit of left cyclic is because if you didn’t, the helicopter would slide to the right due to the tailrotor thrust pushing you to the right, it is the same when you are slowing down..When you decrease collective, you have to add right pedal and a bit of right cyclic to counteract the tailrotor from sliding the fuselage to the left.

  5. Cfiiiclint Says:

    We teach our students in the R22 that translating tendency is compensated in the r22 that the main rotor is offset 2 degrees to the left, but as I begin to get more and more experience as an instructor and building a relationship with the mechanic at our school I’m beginning to doubt if that is entirely accurate.. And there seems to be quite the debate about this subject among all the CFII’s on this subject. So hopefully to put this to rest does anyone know for 100% sure how translating tendency is compensated for in the R22?

  6. Dylan Says:

    Translating Tendency is caused from the thrust generated from the tail rotor. As collective is increased, tail rotor thrust increases resulting in right drift. In most military helicopters, we counteract this with automated flight control systems. It will automatically displace the tip path plane (rotors) to the left to counteract the right drift. Additionally, one final note – GYROSCOPIC PROCESSION IS NOT PHASE LAG!

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