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Tuesday, November 29, 2011

Loss of Tail-Rotor Effectiveness - Bell 206

Frank Lombardi

Question:

Could you please address the issue of loss of tail-rotor effectiveness, specifically in a Bell 206. I would like to know why (on some helicopters) when the engine does not have sufficient power in high-power flight conditions the main rotor speed will slow and the aircraft will then start a slow, but controlled descent due to the loss of the engine’s ability to provide that requested power by the pilot. With the 206, it will respond with LTE in the same situation.

Answer:

What is described in this question might be more appropriately called NETR, or “not enough tail rotor,” rather than the over-used term of LTE, or “loss of tail rotor effectiveness.” Let me explain why. In a condition where the main rotor has been “over-pitched” and the power demanded has exceeded what the engine can provide, the helicopter’s response to pulling too much pitch is a slowing of the rotor system. As RPM drops, main rotor torque continues to increase (power = torque x RPM, and the engine is trying to keep power constant). As torque goes up, more tail rotor thrust is needed to maintain heading. But the tail rotor RPM has also decreased, and therefore, so has the amount of thrust it can provide for anti-torque (thrust changes with the square of rotor speed). If the tail rotor is marginally designed, this can eat up all the anti-torque pedal available before main rotor torque can be completely cancelled. At the max power available, adding more pedal droops the RPM more, the aircraft begins to settle, torque continues rising, the pilot hits the pedal stop, and an uncorrectable yaw develops. Keep in mind, the whole time the tail rotor has never stopped producing thrust. Any (single-rotor) helicopter can run out of pedal if torque is driven high enough and the RPM is dropped low enough. Ideally, a helicopter should always have a surplus of tail rotor thrust available to cope with all situations, but if the tail rotor is too small then it is much easier to encounter uncorrectable yaw as in the above situation—this is what critics claim is wrong with the Bell 206 as well as certain other models; it just doesn't have sufficient surplus tail rotor thrust to keep the pilot out of trouble in certain scenarios. Now it may be splitting hairs, but the term LTE deals with aerodynamic circumstances which can adversely affect or hinder the job of the tail rotor: 1) unfavorable wind azimuth, 2) main rotor vortex ingestion, and 3) tail rotor vortex ring state. Some may object that the above over-pitching situation can be a precursor to developing an aerodynamic LTE situation. The counter argument is that a helicopter with an adequately designed tail rotor will have enough available thrust to overcome these aerodynamic effects; whereas helicopters with lesser tail rotors can be subjected to them suddenly, sometimes catastrophically, even within the normal flight envelope, and therefore, LTE schooling continues to be an important part of helicopter flight training.

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.


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