Wind powered turbine

A wind powered turbine of the vertical axis type having rotor blades (3) carried on a rotor arm (2), in which the blades can be reefed to reduce torque on the turbine. The blades have two portions (6, 7) pivotable with respect to one another by the reefing means so as to move from a position parallel to the rotation axis to a position at which they form an angle about a plane perpendicular to the rotation axis.

The present invention relates to a wind powered turbine and in particular 
to an H-type vertical axis turbine having variable geometry as a means of 
power control. 
H-type vertical axis wind turbines such as the Musgrove turbine described 
in U.K. Patent Specification 1,549,767 have straight blades of symmetrical 
aero-foil cross-section uniformly arranged around the rotation axis of the 
turbine and each pivotally connected to a rigid rotor arm. The rotor arm 
is rotatable at its center on a supporting structure. In normal operation 
the blades are held parallel to the vertical rotation axis of the turbine 
and as the rotor arm turns the blades intersect the air currents passing 
across the turbine. The velocity of each blade combined with the air 
velocity produces a relative velocity of the blade through the air which 
due to the aerofoil shape of the blade produces both lift and drag forces 
on the blade. When the blade velocity in relation to the wind velocity is 
sufficiently great components of the lift and draft when resolved in the 
direction of rotation of the turbine show a net positive torque on the 
turbine. The turbine operates whatever the direction of the wind and the 
torque generated increases with increasing wind speed. 
It has been found that high wind speeds can damage the turbine due to the 
high torque generated and to avoid this the turbine is reefed by allowing 
each blade to pivot outwards at one end if the wind speed becomes 
excessive. This outward pivoting causes the blades to describe a cone 
shape which reduces the torque since the effective cross-sectional area of 
the turbine and the blade aerodynamic efficiency is reduced. 
However, when the blades cone outwards it is found that a considerable 
bending moment is applied to the rotor arm and in order to resist this 
bending moment the supporting structure must be made more rigid than would 
normally be required to simply support the turbine, with a consequent 
increase in the cost and complexity of the equipment. 
The bending moment is due to a component of lift and drag forces on each 
blade which is resolved normal to the chordal plane of the blade. This 
component is inward towards the rotation axis of the turbine on the 
up-wind side and outwards away from the rotation axis of the turbine on 
the downwind side. When the blades cone outwards this component is angled 
to produce a vertical load on the rotor arm which reverses direction when 
passing from up-wind to down-wind, resulting in the bending moment. 
According to the present invention there is provided a wind powered turbine 
comprising; 
a support member, 
a rotor member rotatable on the supporting member about a rotation axis, 
a plurality of rotor blades carried by the rotor member at positions spaced 
around and equidistant from the rotation axis, each blade having two 
portions capable of pivotal movement with respect to one another, 
and reefing means capable of pivoting the blade portions in opposite 
directions so as to move them from a position parallel to the rotation 
axis to a position at which they form an angle about a plane perpendicular 
to the rotation axis. 
Preferably, the two blade portions are substantially identical and the 
reefing means pivots the blade portions by substantially equal and 
opposite amounts. 
Various wind powered turbines constructed in accordance with the present 
invention will now be described by way of example and with reference to 
the accompanying partly schematic drawings wherein like parts have 
identical reference numerals and in which:

Referring to FIGS. 1 and 2 there is shown a rotor assembly for a wind 
powered turbine, the assembly being supported for rotation about a 
vertical axis on a supporting column 1. This supporting column is of 
sufficient height to ensure that the wind can blow freely across the rotor 
assembly and the assembly is clear of obstructions at ground level. 
The assembly comprises a rotor arm 2 which is supported at its centre for 
rotation on the supporting column 1, and two aerofoil blades 3. Each blade 
is connected to one end of the rotor arm by struts 4, and further 
supported at a position intermediate the end of the rotor arm and its 
centre by struts 5. The struts 4 and 5 are each pivoted at their 
respective ends to the rotor arm 2 and to the blade 3 at positions equally 
spaced on either side of the middle of the blade 3. 
Each blade 3 is of aerofoil cross-section with the leading edge of the 
blade facing in the direction of rotation of the rotor assembly and the 
chord line tangential to the circle in which it rotates. The blade 
comprises two straight rigid portions 6 and 7 joined by a hinge parallel 
to the chord line at the middle of the blade, which enables the two 
portions 6 and 7 to fold back into a V-shaped configuration as seen in 
FIG. 2. 
The pivot point 8 for the struts 4 on the rotor arm 2 is moveable relative 
to the axis of rotation of the rotor arm in a substantially radial 
direction. Outward movement of this pivot point 8 causes the two portions 
6 and 7 to rotate by substantially equal amounts and fold back into the 
reefing position of FIG. 2 where the cross-sectional area of the rotor 
assembly and the blade aerodynamic efficiency are reduced, which then 
reduces the torque on the rotor arm from the wind. 
In use, the turbine normally operates with the blades parallel to the axis 
of rotation as in FIG. 1. Wind velocity across the turbine combined with 
the rotational velocity of the blades produces an effective relative 
velocity between the wind and each blade which is at an angle to the chord 
line of the blade. This creates a lift force and a drag force on the blade 
the lift force having a component in the direction of rotation of the 
rotor assembly which overcomes the drag and so drives the turbine, and a 
component normal to the chordal plane of the blade which changes direction 
on going from the up-wind to the down-wind side of the rotor assembly so 
as to produce a force along the rotor arm 2. The supporting column is 
sufficiently rigid to resist this force along the rotor arm in the 
direction of the wind. 
If the wind velocity becomes too high the torque generated by the rotor 
assembly as in FIG. 1 will be such as possibly to cause damage to the 
structure of the turbine and therefore the pivot points 8 are moved 
equally outwards from the axis of rotation until the torque has fallen to 
a suitable value. This reefing of the blades causes the component of lift 
which acts perpendicular to the chordal plane to rotate and produce a 
vertical force on each portion of the blades as well as a force along the 
rotor arm. 
However, since the portions of each blade are equal in length and rotate by 
substantially equal amounts in opposite directions the vertical force on 
one portion will be approximately equal and opposite to the vertical force 
on the other portion, producing substantially no overall vertical forces 
on the rotor arm 2. Thus, the bending moment acting on the rotor arm 
during reefing is considerably reduced in relation to that of the original 
Musgrove turbine construction. This reduces the engineering constraints on 
the turbine supports and in particular allows the rotor arm length to be 
increased without danger of producing a large bending moment during 
reefing. 
A slightly modified construction to that shown in FIGS. 1 and 2 is shown 
partly reefed in FIG. 3. In this construction, each end of the rotor arm 
has a raised "beak" 10 at the free end of which is pivoted a link 11. The 
free end portion of the link 11 is pivotally connected to the end of each 
of the two struts 4 and to a link 19 at the end of a push rod 9 mounted in 
the rotor arm 2. Outward longitudinal movement of the push rod pivots the 
link 11 and so moves the pivot point 8 outwards. This pivotal movement 
means that the pivot point does not move exactly radially outwards but 
along a slight curve which tilts the blade slightly during reefing. The 
tilting of the blade will produce a small net vertical force during 
reefing, but considerably less than in the conventional construction. The 
intermediate link 11 reduces the shear forces acting on the push rod 
during operation of the turbine. The push rods 9 are operated by hydraulic 
cylinders 18 at the centre of the rotor arm as seen in FIG. 4. FIG. 4a 
shows in plan view the position of the push rods and cylinders when the 
blades are vertical and FIG. 4b shows them when fully reefed. The link 11 
may alternatively be operated directly by hydraulic pistons positioned at 
the ends of the arm 2. 
The struts 5, struts 4 and beak 10 of the rotor arm 2 are of symmetrical 
aerofoil cross-section similar to that of the blades 3, having their 
leading edges facing in the direction of rotation of the rotor assembly 
and producing a component of lift in that direction which tends to 
overcome the drag force, so as to contribute some positive driving torque 
to the turbine. Each strut 5 carries direct loads and the shear from the 
forces tangential to the rotation. The struts 4 apart from controlling the 
reefing of the blades, assist in maintaining the required shape of the 
blades by providing further support for them and share the inertial and 
aerodynamic loads with the struts 5. This enables light components to be 
used in the blade construction and struts, which reduces centrifugal 
forces and so also enables lighter and cheaper components to be used in 
other parts of the turbine as well. 
In a further modification of the assembly of FIG. 1 the pivot point 8 may 
be fixed at the ends of the rotor arm 2 and the pivot point 12 of the 
struts 5 be made movable inwards along the rotor arm so as to reef the 
blades. However, in this construction the blades would be positioned 
further in towards the axis of rotation when reefed than as shown in FIG. 
2. which would reduce the blade velocity for the same rotational speed and 
at a certain minimum blade velocity the relative velocity between the 
blade and the wind reaches an angle to the blade chord line at which 
stalling of the whole blade can occur. 
Instead of moving the pivot point 8 or 12, in order to reef the blades, it 
is possible to construct the struts 4 or 5 so that they may be extended or 
retracted, the change in length of each strut 4 or each strut 5 being the 
same so as to prevent any tilting of the blade during reefing. 
Various other arrangements of struts and pistons or other actuating means 
are also capable of pivoting two halves of a vertical blade in opposite 
directions so that they make substantially the same acute angle with the 
horizontal and some of these are shown in FIGS. 5 to 8. In each figure 
only one blade and its struts are shown, the other blade and struts being 
the same. The position of the blade 3 and its struts during normal 
operation when the blades are parallel to the axis of rotation is shown in 
solid lines and the position when the blades are reefed is shown in dotted 
lines in each figure. 
Referring to FIG. 5, the blade 3 is supported by two rigid struts 13 
pivoted to the rotor arm 2 and to the respective portions of the blade 3 
at positions equally spaced on either side of the middle of the blade 3, 
and by the end of the rotor arm 2 pivoted to the two portions of the blade 
3 at its middle. Reefing is produced by moving the end of the rotor arm 2 
radially outwards. 
This construction is simpler than that in FIG. 1 but requires a substantial 
outward movement of the end of the rotor arm 2 in order to produce the 
same degree of reefing. 
Referring to FIG. 6, this is similar to the construction of FIG. 5, but in 
this construction reefing is produced by moving the pivot point 14 of the 
two rigid struts 13 radially inwards rather than the end of the rotor arm 
radially outwards. There is a disadvantage over the construction of FIG. 5 
in that the reefed blades are further in towards the axis of rotation 
which, as explained earlier, raises the stalling speed of the turbine. 
Instead of moving the pivot point 14 inwards it is possible to reduce the 
length of the struts 13. 
Referring to FIG. 7, the blade 3 is supported by a rigid extension 15 of 
the rotor arm and a strut 16 which is pivoted to the rotor arm, the 
extension 15 and strut 16 being pivoted to the respective portions of the 
blade 3 at positions equally spaced on either side of the middle of the 
blade. Reefing is produced by rotation the upper portion of the blade 3 
about the pivot on the extension 15. The rotation is preferably carried 
out by means of a piston and cylinder connected between the extension 15 
and the upper portion of the blade 3. 
In FIG. 7, the rotation of the two portions of the blade 3 is in a sense 
such that the V-shape formed thereby points inwards towards the axis of 
rotation of the rotor assembly. Reversing the sense of the rotation of the 
upper portion of the blade 3 would however give a V-shape pointing 
outwards as in FIGS. 2, 5 and 6 and similarly in the construction shown in 
those figures reversing the movement required to reef the blades would 
give a V-shape pointing inwards as in FIG. 7. 
Referring to FIG. 8, the two portions of the blade 3 are not connected at 
the middle of the blade as in the previously described constructions, but 
are free to move independently. The sections are pivotally connected to 
rigid extensions 17 of the rotor arm 2 at positions equally on either side 
of the middle of the blade 3. Reefing of the blade is produced by rotating 
the portions of the blade through equal and opposite angles about their 
pivotal connections to the extensions 17. The rotation is preferably 
carried out by pistons acting between the extensions and the portions of 
the blade 3 to be rotated. As in the construction of FIG. 7, the blades 
may be reefed so as to form a V-shape pointing outwards (as shown in FIG. 
8) or a V-shape pointing inwards. 
In all the above described constructions of the rotor assembly for a wind 
powered turbine there are two blades 3 arranged on opposite ends of a 
straight rotor arm 2. The invention is however, equally applicable to such 
a wind turbine which has three or more blades uniformly arranged around 
the rotation axis of the turbine.