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Tuesday, November 29, 2011

Velocity to Never Exceed (VNE) - Eurocopter AS350

Frank Lombardi

Question:

Why does the Eurocopter AS350 AStar flight manual list a different Vne (never exceed speed) for “power-on” vs. “power-off” flight? It lists 155 kts at SL, Std. Day power-on, and 125 knots at SL, Std. Day power-off. Shouldn’t the maximum speed of the aircraft be the same, regardless? 

Answer:

The Vne, or velocity to never exceed, can be based on many things. Most often, it is based on an aerodynamic limit, such as the onset of retreating blade stall, or a structural limit, such as excessively high loads imposed on the rotor mast/hub. In the power-off case of the AS350, i.e., when in autorotation, this Vne limit is imposed by the ability to keep the aircraft in trim with the pedals. If you look at the aircraft’s upper vertical tail, you will notice the trailing edge is angled, 6 degrees to be exact, to the right. In powered flight above 40 knots, the vertical tail begins to produce an increasingly nose-right side force which aids in counteracting the nose-left torque effect, reducing the need for so much right pedal (and tail rotor power) to remain in trim. However, when in autorotation, the nose-left torque effects are gone, and friction in the transmission drags the nose right, in the direction of the turning rotor. Pedal requirements go from being predominantly right, to predominantly left. As airspeed increases, the angled vertical tail causes an increasingly nose-right yaw, and the available control that the pilot has to keep the aircraft in trim diminishes as the left pedal travel limit is approached. Designers therefore imposed the lower airspeed limit in autorotation so the pilot would always maintain a margin of control. In short, at speeds beyond power-off Vne, you will hit the left pedal stop, and the nose will no longer be yours to command. 

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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