AERODYNAMIC FORCES ACTING ON THE BLADE
Aerodynamic forces acting on a blade element tending to make it rotate, these are important parameters for a system engineer. These are several basic types of blades on aeriturbine may have, eg.,sails, planes and aerodynamic surfaces based on the air craft wing cross-section for which there are many kinds. The early history of wind mills is based on the first two; modern higher efficiency wind-electric generators are based on use of blades with aerodynamic surfaces.
Consider the aerodynamic blade shown in fig.,5.12. The blade can be through of as a typical cross-sectional element of a two-bladed aeroturbine . The element shown is at some radius ‘r’ from the axis of rotation. It is moving to the left. Because the blade is moving in the plane of rotation it sees a tangential wind velocity, VT, in the plane of rotation.
This component added vectorially to the impinging wind velocity gives the resulting wind velocity, VR, seen by the rotating balde element. At right angles to VR, is the lift force FL, caused by the aerodynamic shape of the blade. The drag force, FD is parallel to VR. The vector sum of FL and FD is FR which has a torque producing component, FT and a thrust producing component. The former is what drives the aero-turbine rotationally and the later tends to flex and also overturn the aerogenerator.
The vector diagram is centred on the centre of lift of the aerodynamic blade. As is well known from aircraft wind theory, one of the critical parameters is ’a’ the angle of attack of the aerodynamic element, It determines lift and drag forces and hence speed and torque output of the aeroturbine. This quantities can be varied by changing the blade pitch angle ‘β’, and this is the basis torque control method used on large variable pitch wind-electric generators. The torque whould determine the AC output power if a synchronous generator was used.
Since VT, increases linearly as we go out radically, ‘r’, on an inclined aeroturbine blade, it is necessary to adjust ‘β’ with ‘r’ so as to always have a positive angle of attack and to maintain reasonable stress levels within the blade. This mean that at large ‘r’, ‘β’ is made small while at small ‘r’, ’β’ is large. Thus the blade ‘bites’ the air more in close than near the tips. These the considerations result in an aeroturbine blade with an apparent twist in it. The need for twisting wind mills sails was recognised hundreds of years ago and widely used on Dutch wind mill.
Having now achieved an elementary understanding of the basics of what turns the aeroturbine, we see that the airfoil orientation for an aeroturbine drives by the wind is exactly opposite to the orientation of a classical airplane propeller which is mechanically driven and whose function is to go give a lift force in the axial direction. Thus, through aircraft wing theory is applicable to the operation of an aeroturbine, direct of a classical aircraft propeller on an aeroturbine, would not produce the most efficient aeroturbine because the aerodynamic surface is oriented backwards to what it should be.