Tuesday, November 29, 2011
Height Variations in Right/Left Spot Turns with Rotors Turning Clockwise
Question:Why does a helicopter with rotors turning clockwise (e.g., Lama or Mil Mi-8) gain height in a steady spot turn to right and lose height in a left spot turn?
Answer:In helicopters with a conventional main/tail rotor, any increase in pedal application that opposes the torque effect of the main rotor will increase the total amount of power demanded from the drivetrain. With main rotors that turn clockwise when viewed from above, this is a right pedal input (left pedal in those turning counter-clockwise). Now consider a turbine-powered helicopter such as the SA315 Lama with governed engine/rotor RPM (N2/NR). Upon applying right pedal during a “spot turn” in the clockwise-rotating Lama, the increase in power required is sensed by the governor as a slight droop in the N2/NR. The governor then tries to maintain the N2/NR by adding fuel to raise the RPM back to normal. Keep in mind that the governor is sensing rotor RPM through a system that is fixed to the airframe, so as the aircraft yaws right, it follows the direction of the spinning rotor shaft, and the governor senses a relative rotor RPM that still appears lower than desired. The response is a continued increase in fuel flow by the fuel control, and a resulting higher-than-necessary RPM relative to the air mass you’re flying in. It does not take much yaw rate to cause this. Yawing at 30 degrees/second (that’s 12 seconds for a full 360-degree pedal turn) will change the relative rotor speed by 5 RPM. Since lift is proportional to the square of NR, even this small change in RPM will bring an increase in main rotor thrust enough cause the helicopter to climb. Of course if you add left pedal the opposite is true, and the helicopter will descend.
|Frank Lombardi is a Police Helicopter Pilot,Testing & Evaluation
An ATP with both fixed-wing and rotary-wing ratings, Frank Lombardi began his flying career in 1991 when he graduated from Polytechnic University in New York with a bachelor’s of science in Aerospace Engineering.
Upon graduation, he worked on various airplane and helicopter programs as a flight test engineer for Grumman Aerospace Corp. Frank became a police officer for a major East Coast police department in 1995, and has been flying helicopters in the department’s aviation section since 2000. Frank remains active in test and evaluation, and holds a Master of Science degree in aviation systems-flight testing, from the University of Tennessee Space Institute. He is a regular Rotor & Wing contributor.