Blade
A blade of propeller (plane) or rotor (helicopter) is an airfoil in rotation around an axis. It is a device aéro or hydrodynamics intended to create a displacement of the molecules of the fluid in which it moves to exploit the differences of Pression which result from it.
Several blades (generally 2,3 or 4) are fixed at a central hub to form a Hélice or a Rotor.
Almost all the wind mills have three blades, however L exists a system of wind comprising only one blade, balanced by a counterweight. This decreases the collision risk (which is very weak) for the birds.
Form
The form in plan, the Chock (in foot of blade) and the Vrillage (along the blade) of a blade are given according to the force of traction required and number of revolutions of the Hélice or the rotor.
Its aerodynamic effect is due to its profile. Just like a aerofoil of aircraft, it has a Extrados and a Intrados (see Bernoulli) but the speed and the direction of the air being unequal on its Envergure, its profile must be evolutionary.
To the point of rotation lower speed (near to the Tree of drive), the foot of the blade presents a thicker section to hold the efforts (inflection and centrifugal force). In blade tip, the profile is refined, suction face and under-surface can meet to form two parallel surfaces (thus for purpose of neutral Portance). The blades of the planes of new generation as the Airbus A400M can have the shape of a curved saber.
Speed in blade tip
The number of revolutions is limited by speed in blade tip, this one being able to approach the speed of the its, thus causing the noise characteristic of a helicopter ( tchop-tchop which gave him its name familiar of chopper in English). One can partly mitigate this problem by giving at the end of the blade the shape in plan of a wing of supersonic aircraft (see the Gothic profile of aerofoil of the Concorde, example: the helicopter Tiger: strong Arrow and curve downwards to cancel the bearing pressure.
Charge with the disc
By increasing the number of the blades of a system, one can is to increase the airfoil and the force of traction (with wing load equalizes) of a rotor without increasing the length of the blades, also limited by their mechanical resistance (see flying Grue). That makes it possible, for the same bearing pressure, to reduce the number of revolutions of the rotor and to increase consequently the rate of advance of the helicopter (see also the photograph of the hub rotor of the Eurocopter EC155 with 5 blades in the article relating to the cyclic Plateau) of the family Dauphin.
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