Figure: Total Aerodynamic Force @~copters

A total aerodynamic force
~ is generated when a stream of air flows over and under a blade that is moving through the air
~ the point at which the air separates to flow about blade is called the point of impact
~ a high pressure area or stagnation point is formed at the point of impact
~ normally the high pressure area is located at the lower portion of the leading edge, depending on >angles of attack
~ this high pressure area contributes to the overall force produced by the >blade
~ air is deflected downward as it passes under blade and leaves trailing edge
~ Newton's third law states "every action has an equal and opposite reaction"
~ since air is being deflected downward, an equal and opposite force must be acting upward on blade
~ this force adds to the total aerodynamic force developed by blade
~ at very low or zero >angles of attack,
~ the deflection force or impact pressure may exert a zero positive force, or even a downward or negative force
~ air passing over the top of the blade produces aerodynamic force in another way
~ the shape of the blade causes a low pressure area above the blade according to Bernoulli's Principle,
~ and the decrease in pressure on top of the blade exerts an upward aerodynamic force
~ pressure differential between the upper and lower surface of the blade is quite small - in the vicinity of 1 percent
~ even a small pressure differential produces substantial force when applied to the large area of a rotor blade


Figure: Forces acting on >blade @~copters

Increased speed
~ causes increased lift
~ a larger pressure differential is produced between the upper and lower surfaces
~ lift does not increase in direct proportion to speed, but varies as the square of the speed
~ a blade traveling at 500 knots has four times the lift of the same blade traveling at only 250 knots

Lift increases when the area of the blade increases
~ a blade area of 100 square feet will produce twice as much lift as a blade area of only 50 square feet

>Angle of attack has an effect on the lift produced
~ lift increases as the angle of attack increases up to the stalling angle of attack
~ stall angle varies with different blades and is the point at which airflow no longer follows the camber of the blade smoothly

Air density directly influences lift.

Two design factors, blade shape and blade area are primary elements that determine how much lift and drag a blade will produce.
Any change in these design factors will affect the forces produced.

Normally an increase in lift will also produce an increase in drag.
Therefore, the >blade is designed to produce the most lift and the least drag within normal speed ranges.

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