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Helicopter aerodynamics involves a complex interaction between gravity, thrust, and directional forces that make them highly maneuverable aircraft, but also much more inefficient than traditional planes as well as having a lower maximum speed and shorter range. The three-directional forces of yaw, pitch, and roll must be considered at all times while a helicopter is in flight. It also operates on unique aerodynamic principles controlled by the main rotor disc, the tail rotor, and translational or ground effects due to its forward movement and changes in thrust when approaching land or buildings.
While the flight principles of most helicopters are well-known to the public of vertical take-offs, hovering, and sideways movement during flight, this is not the limit of a helicopter's performance characteristics. The main rotor disc on a helicopter can be tilted in any direction. Tilting it forward will reduce downward thrust and provide forward momentum. The rotor can also be tilted to the side or rear of the main body of the helicopter, however, making it possible for the vehicle to increase speed at an angle or move in reverse.
This feature of the main thrust mechanism in a helicopter makes an understanding of yaw, pitch, and roll characteristics more important in helicopter aerodynamics than may first be realized. Yaw is movement to the left or right that is often accompanied by pitch, which is upward and downward movement. Roll is a combination of yaw and pitch, where a helicopter angles off of its main flight direction by rolling up or down to the left or right, all of which are directly affected by the tilt of the rotor blade itself as well as the amount of power being applied to the blade.
None of these maneuvers are possible, however, without the tandem effects of the tail rotor. The control of the angle and thrust of the main rotor disc is done through a handheld cyclic, or stick, while the tail rotor's level of spin or torque is controlled by foot pedals. The tail rotor directly counteracts the rotation of the helicopter body, which would otherwise spin out of control to match the rotation of the main rotor. Increasing or decreasing tail rotor speed using the foot pedals will allow the helicopter to change the direction that it is facing while in flight. This is most often done on take-offs and landings, since, once the vehicle has significant forward motion, changes in direction are done using the helicopter aerodynamics principles of roll and pitch. For this reason, most helicopters are not equipped with tail flaps on the end of the tail to control direction, as they are unnecessary.
The other major aerodynamic forces affecting helicopters in flight are that of translational lift and ground effects. A helicopter rotor blade is similar to a propeller on a fixed wing aircraft, but flatter and flexible, where it is designed to push air out of the way as it rotates instead of corkscrewing through it. As the vehicle moves forward and gains speed, the air becomes less turbulent around the body and rotor, allowing for the production of better lift through translational aerodynamics that creates a sort of forward inertia for the vehicle.
The ground effect is the opposite of this, and is a repellant effect experienced as the vehicle approaches land. As the downward thrust hits a solid surface, it creates increased upward thrust for which must be compensated. This can also occur in flight if the helicopter passes close to a building or other solid obstruction.
The main rotor used for helicopter aerodynamics must undergo a variety of competing forces while in flight. Modern helicopter aerodynamics must account for dissymmetry of lift through the use of blade flapping. As the vehicle moves forward, the rotor blade twists while in motion to accommodate greater lift effects generated at the front of the blade than at the rear, which can cause the helicopter to roll. Blade flapping is used to offset this by making a flexible rotor blade that bends upward at the leading edge, and down at the trailing edge. This equalizes lift forces, and such flexibility is visible in parked helicopters where the rotor sags downward at the edge.
The complexity of helicopter aerodynamics also allows them to land safely if full power is lost to the rotor. Unlike the popular assumption that a helicopter would drop like a rock with a loss of power, the shape of the vehicle and still spinning rotor blade allows it to perform an autorotation maneuver in emergencies, otherwise known as gliding. The descent of the vehicle actually powers the rotor at a maintained or increased speed when the clutch system is disengaged, allowing the rotor to spin freely and land the vehicle at a faster-than-normal, but safe speed.