Appendix D: Forces on blades and testing
This information may be useful to those testing the blades presented here.
Forces on the wind turbine blades
There are three main forces acting on the wind turbine blade. These are:
Wind Direction
Hub
Centrifugal
Blade
Gyroscopic Thrust
Viewed looking down from top of wind turbine
Centrifugal
This is due to the rotation of the blades. It is an inertial force which depends
upon the rotational speed. According to the ITDG Fibre Glass blades manual
this force is approximately equal to 100 times the weight of the blade at 500
rpm. This is for the 500W machine with smaller blades.
Calculations for 1kW blade:
Fcentrifugal = m ω2 r
Where m is the mass of the blade, ω is the angular velocity and r is the
distance at which the mass acts (assumed a point source mass).
The angular velocity is a function of the rpm, which is 500rpm at rated
output.
ω = 2π (rpm) / 60
ω = 2π (500) / 60 = 52.4 rads-1
Assume r = 0.9m (approx point of action of weight)
Fcentrifugal = m (52.4)2 0.9 = 2467 * m
This is approximately equivalent to 250 times the weight of the blade (weight
= mass x gravity (9.81)).
Thrust
This is due to the wind pushing the blade backwards as it spins. This is in the
region of 5-10 times the weight of the blade.
Gyroscopic
This is due to the rotation of the wind turbine around the tower axis. This acts
to both pull the blade forward and push the blade back by as much as 5-10
times the weight of the blade.
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