History of Danish Wind Power - Wind Power for the World: The Rise of Modern Wind Energy - page 56

when the wind speed was high enough, it operated at over-
synchronous speed and acted as a generator, delivering electricity
into the grid. By a careful combination of generator size and blade
profile, the turbine could be “self-regulating”. When the wind speed
got too high, the force on the blade was reduced because of the
“stalling phenomena”, known from aeroplane wings. As en extra
safety feature, the blades were equipped with revolving tip brakes—
a feature that Juul got patented in 1952—and many years later
became “standard equipment” on many modern wind turbines.

Figure 3.20 Johannes Juul's first blade design for the 2-bladed turbine.

Here, a downwind concept was chosen. The design includes
Juul's patented revolving tip brakes (Picture: The Danish
Energy Museum).

The testing of the first experimental turbine gave good
results, but in fact, the choice of two blades was a low cost solution.
A rotor with three blades could have been more stable. When in
1952 SEAS took over the FLS Aeromotor from a small utility