Question about LTE in helicopters?

2 Comments

1. admin says:

   September 14, 2009 at 5:27 pm

   The more resistance you put on the blades, the more torque is produced. The tail rotor needs to counter this torque by increasing the angle of attack. This puts more counter-torque on the helicopter, keeping it going straight.
2. admin says:

   September 14, 2009 at 5:27 pm

   I wish I could give you an answer, but I really can't. I had never looked into it before until I saw your question, and sure enough the textbooks do not offer an explanation. I checked Coyle's Cyclic and Collective and Whyte's Fatal Traps, as well as the sources you mentioned. None of them explained exactly why an increase in angle of attack occurs as the tail rotor moves into the vortices, but then a decrease occurs as you move out of them. They just state that it does happen and the increase then reduction in thrust could cause the problem.

   If we imagine that there is a constant pitch angle of the tail rotor blades, either in a stable hover or a constant rate pedal turn, then the only way angle of attack could change would be a change in the induced flow. I couldn't guess why it is that the main rotor vortices cause a reduction in the induced flow, especially when the opposite problem occurs when the tail rotor's own vortices are blown back into it in tail rotor vortex ring state.

   It's a good question. Let me know if you find an answer.

   (Added)
   Having given it some more thought, the only thing I can come up with is that the vortices coincidentally hit the tail rotor just the right way to change the relative wind enough so that angle of attack is increased. I am really just guessing though because I can't find an answer.

   There are some books and academic journals out there that go much, much deeper into the physics of helicopter aerodynamics. I think you may need to look there to get a truly complete explanation.