Page 385 - Wind Energy Handbook

BRAKING SYSTEMS 359

Rotary bearing
Tip shaft
Screw
Blade pitch angle

Compressed spring
Flow

Figure 6.13 Passive Control of Tip Blade, Using Screw on Tip Shaft and Spring

Spoilers

Spoilers are hinged flaps, which conform to the aerofoil profile when retracted, and
stick out at right angles to it when deployed. However, although such devices have
been used in the past, they have to be of considerable length in order to decelerate
the rotor adequately (Jamieson and Agius, 1990). Moreover, unless the design
allows for their operation to be regularly tested, there is a risk that they will fail to
deploy when actually needed.

Other devices

Various other devices have been suggested, such as

• ailerons,

• the sliding leading edge device, or SLEDGE, in which a length of leading edge at
the tip slides radially outwards,

• the flying leading edge device, or FLEDGE, in which the whole leading edge
together with an adjacent section of the camber face is pitched towards feather.

Jamieson and Agius (1990) and Armstrong and Hancock (1991) give useful surveys
of these and other aerodynamic braking devices, and note that the SLEDGE device,
which utilizes only 2 percent or 3 percent of the blade area, is highly effective
aerodynamically. Derrick (1992) examines the capabilities of the SLEDGE and
FLEDGE devices for both braking and power control in more detail. Despite their
promise, these devices have not yet found commercial application.