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An aerodynamics principle that affects the way helicopters fly and which helo pilots should be aware of.

A quick recap of how helicopters fly: helicopters have a system of one or more rotor discs, each composed of two or more rotor blades with an aerofoil cross section. When an aerofoil moves forwards through air (blunt end first) it affects the flow of air it passes through. Due to the Coanda Effect (which states that fluid substances tend to follow curved surfaces) the air follows the curves of the aerofoil which results in some of it being forced downwards. This action of forcing air down exerts an equal upward force on the aerofoil (see Newton's Third Law of Motion): lift. The more air that is forced downwards and the higher its vertical velocity, the more lift is generated (see Newton's Second Law of Motion).

Increasing the angle at which the aerofoil meets the air effectively tightens the curvature of the wing (relative to the airflow). The air will follow the curve of the wing and be pushed downwards more steeply and at a higher velocity. Increasing the angle of attack increases the lift the wing generates, up to a point.

As a helicopter's rotor blades (in effect, its wings) rotate, they move through the air at sufficient speed to lift themselves and the helicopter fuselage off the ground. Simple in principle, but in practice there's a great many factors that affect rotor system efficiency and/or the way the pilot flies a helicopter (a major part of helicopter design seems to be the compensating for unwanted forces). One of them is the Transverse Flow Effect.

The transverse flow effect is a reduced efficiency of the rear half of the rotor disc when moving laterally (i.e. forwards, backwards, left or right, but mostly forwards) at low speed, due to the way the air flows into it. When a helicopter is flying, air is sucked downwards towards the rotor disc as it spins, due to the low pressure area this generates above it. In hover this is fine because the air meets the rotor disc at more or less the same speed all around (ignoring wind for simplification). However when the helicopter is moving laterally the rotor disc is tilted, so air flowing into it meets it at an angle and enters different parts of it at varying speeds.

The front of the rotor disc meets the air first so the air is almost horizontal (as in hover) as it enters the rotor disc, although the air enters the disc at a slightly higher angle than it would do in hover. However air that enters the rear of the rotor disc has been accelerated considerably more before it enters, and enters at a steeper angle. This has the effect of reducing the angle of attack of the rotor blades more at the rear of the rotor disc than at the front.

The following exaggerated diagram shows a projection of a rotor at the front of the rotor disc, while the helicopter is flying forwards.

                                                                 @@@@@
                                                            @@@@@@  A
                                                       @@@@@        |
                                                 @@@@@@             |
    Plane of aerofoil (rotor) ------------> @@@@@                   |
                                      @@@@@@                        | <-- Angle of Attack
                                 @@@@@                              |
                           @@@@@@                                   |
                      @@@@@                                         V
                @@@@@@                                ############### <-- Wind direction after downwash
           @@@@@                 #####################              A  
     @@@@@@ #####################                                   | <-- Downwash
@@@@@#######                                                        V 
-----------------------------<--------------------------------------- <-- Wind relative to blade rotation

Because of the forward movement and tilt of the rotor disc, air will enter it at an angle. This angle is above the plane of rotation, resulting in a slightly reduced angle of attack (it is as if the pitch of the rotor blade has been reduced). As the rotor disc moves forward it sucks in more air, however because it is moving forward, much of the fastest-moving air flows through the rear of the rotor disc. This air has been accelerated more than the air entering the front of the rotor disc, which increases further the angle at which it enters the rear of the rotor disc:

                                                                 @@@@@
                                                            @@@@@@  A
                                                       @@@@@        |
                                                 @@@@@@             |
    Plane of aerofoil (rotor) ------------> @@@@@                   | <-- Angle of Attack
                                      @@@@@@                        | 
                                 @@@@@                              V
                           @@@@@@                         ########### <-- Wind direction after downwash
                      @@@@@                   ############          A
                @@@@@@             ###########                      |
           @@@@@       ############                                 | <-- Downwash
     @@@@@@ ###########                                             |
@@@@@#######                                                        V 
-----------------------------<--------------------------------------- <-- Wind relative to blade rotation

This increased downwash reduces the angle of attack of the rotor blades more and increases the amount of air entering the rotor disc from above instead of in front (making it "harder" for the air to follow the curve of the rotor blade - the Coanda Effect states that fluid substances tend to follow gently curved surfaces they contact). These two effects reduce the lift generated and cause an uneven distribution of lift between the front and rear halves of the rotor disc: the rear half of the rotor disc will generate less lift than the front.

Because of an effect called gyroscopic precession (whereby any force applied to a rotating object parallel to its plane of rotation will not affect the object until 90° of rotation later) this reduced lift will not manifest itself on the rotor blade until it has turned a further 90°. The rotor blades in most modern helicopters rotate counter-clockwise, so the area of reduced lift will be on the pilot's right side. The area of greatest lift will be on the opposite side.

Although this effect is constant when the helicopter is moving, it is most noticeable at between 10 and 20kts (knots - nautical miles per hour), when the air is travelling past slowly enough to be significantly affected by the rotor blades. The signs of it occurring are a vibration of the helicopter airframe and a tendency for it to roll slightly in the direction of the advancing rotor blades: if the rotor disc rotates counter-clockwise this will be the pilot's right, if clockwise it is the pilot's left.

This is all dependant upon relative wind speed, so the effect would be the same if the helicopter were hovering in a 20kt headwind as it would be if it were flying forwards at 20kts in calm conditions. This effect is not something that is compensated for automatically so some opposing cyclic is needed at low speed to correct it. Once above a certain speed (this varies from one helicopter to another) the TFE makes a negligible difference to the helicopter's flight characteristics.


See also:
Sources:
  • Dynamic Flight, Inc; "Transverse Flow Effect"; <http://www.dynamicflight.com/aerodynamics/transverse_flow_eff/>
  • usmcug.usm.maine.edu; "HELICOPTER AERODYNAMICS"; <http://usmcug.usm.maine.edu/~miranda/inst/aerodynamics.html>
  • (Author unknown); "Principles of Flight Lesson 1; Transverse Flow Effect"; <http://www.helicfi.com/princ1.htm>
  • AvStop Magazine Online; "Basic Helicopter Handbook; AERODYNAMICS OF FLIGHT"; <http://avstop.com/AC/BasicHelicopterHandbook/ch2.html>

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