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6.2 Helicopter Aerodynamics

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Aerodynamic Properties

An aerofoil-shaped body moved through a fluid produces an aerodynamic force. The component of this force perpendicular to the direction of motion is called lift. The component parallel to the direction of motion is called drag. Subsonic flight aerofoils have a characteristic shape with a rounded leading edge, followed by a sharp trailing edge, often with asymmetric camber. Foils of similar function designed with water as the working fluid are called hydrofoils.

An aerofoil as seen in the cross-section below is used to shape wings or blades (of a propeller, rotor or turbine) or sails.

The lift on an aerofoil is primarily the result of its angle of attack and shape. When oriented at a suitable angle, the aerofoil deflects the oncoming air, resulting in a force on the aerofoil in the direction opposite to the deflection. This force is known as aerodynamic force and can be resolved into two components: Lift and drag. Most foil shapes require a positive angle of attack to generate lift, but cambered aerofoils can generate lift at zero angle of attack. This "turning" of the air in the vicinity of the aerofoil creates curved streamlines which results in lower pressure on one side and higher pressure on the other. This pressure difference is accompanied by a velocity difference, via Bernoulli's principle, so the resulting flow-field about the aerofoil has a higher average velocity on the upper surface than on the lower surface.

Blade design

The blades of a helicopter are long, narrow aerofoils with a high aspect ratio, a shape which minimises drag from tip vortices (wings of a glider have a high aspect ratio). They generally contain a degree of washout to reduce the lift generated at the tips, where the airflow is fastest and vortex generation would be a significant problem.

Rotor blades are made out of various materials, including aluminium, composite structure and steel or titanium with abrasion shields along the leading edge. Rotorcraft blades are traditionally passive, but research into active blade control trailing edge flaps is performed.

Twist (Washout) and Tapper

Twist that decreases the local chord's incidence from root to tip is sometimes referred to as washout. Twist that increases the local incidence from root to tip is less common and is called wash-in. Washout is commonly achieved by designing the blade with a slight twist, reducing the angle of incidence from root to tip, and therefore causing a lower angle of attack at the tips than at the roots. Washout may also be used to modify the span-wise lift distribution to reduce lift-induced drag.

When a blade rotates, each point on it travels at a different speed. The further away from the root, the higher the velocity. This means that the contribution to lift and drag of every point on the blade differs, with each aspect getting larger when moving closer to the rotor tip. Clearly, the lift distribution over the blade is not constant. This is not a desirable situation, because the contribution diminishes when getting closer to the root. To change this distribution, blades are twisted and, sometimes, also tapered. The twist is such that the angle of attack increases when travelling towards the root, producing more lift. Tapering the blade also contributes to achieving a more evenly spaced lift distribution. With blade tapering, the blade's surface gets larger when travelling towards its root.

Both tapering and twisting can be observed when looking carefully at rotor blades at rest. Note that blade tapering is not always used (especially on metal blades because of a more complicated fabrication process). Rotor blades are constantly strained by moments that try to twist them. This twisting has its origins in the moments which exist between the centre of pressure (due to the aerodynamic forces) and the mass centroid over the chord line.

The blade designer must take these twisting moments into account by designing a blade with high torsional stiffness. They must also ensure that the mass centroid is located ahead of the centre of pressure for all blade angles (in its operational range). In this way, lift tends to lower the angle of attack: a stable condition.