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An improved variable pitch fan comprising a fan hub, with fan blades extending radially outward from the fan hub and mounted for rotation about respective radially extending axes corresponding to each fan blade. Each fan blade has a blade surface extending perpendicularly to the radially extending a

An improved variable pitch fan comprising a fan hub, with fan blades extending radially outward from the fan hub and mounted for rotation about respective radially extending axes corresponding to each fan blade. Each fan blade has a blade surface extending perpendicularly to the radially extending axis of the fan blade, each blade surface lying between respective outer edges of the corresponding fan blade and facing rearward. A pitch shifting mechanism is mounted in the hub and interconnects with the fan blades to control the rotational position of each fan blade about the corresponding radially extending axis of the fan blade. The respective outer edges of each fan blade diverge as the fan blade extends further radially outward; and the blade surface of each fan blade has an angle of attack that decreases as the fan blade extends radially outward. Each blade surface has a constant or increasing radius of curvature as the respective fan blade extends further radially outward. The respective outer edges of each fan blade are straight. Each fan blade has integral moulded counterweight supports and counterweights mounted on the counterweight supports.

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1. A variable pitch fan, comprising:a fan hub; fan blades extending radially outward from the fan hub and mounted for rotation about a radially extending axis corresponding to each fan blade, each fan blade having a blade surface extending perpendicularly to the radially extending axis of the fan bl

1. A variable pitch fan, comprising:a fan hub; fan blades extending radially outward from the fan hub and mounted for rotation about a radially extending axis corresponding to each fan blade, each fan blade having a blade surface extending perpendicularly to the radially extending axis of the fan blade, the blade surface lying between respective outer edges of the corresponding fan blade; a pitch shifting mechanism mounted in the hub and interconnecting with the fan blades to control the rotational position of each fan blade about the corresponding radially extending axis of the fan blade; each fan blade having an integral moulded counterweight support and a counterweight mounted on the counterweight support; and each fan blade is mounted on a shaft and secured on the shaft by clamping two sections of the fan blade onto the shaft, with one of the two sections of the fan blade incorporating the counterweight support. 2. The variable pitch fan of claim 1 in which one at the two sections of the fan blade is smaller than the other of the two sections.3. A variable pitch fan, comprising:a fan hub; fan blades extending radially outward from the fan hub and mounted for rotation about a radially extending axis corresponding to each fan blade, each fan blade having a blade surface extending perpendicularly to the radially extending axis of the fan blade, the blade surface lying between respective outer edges of the corresponding fan blade; a pitch shifting mechanism mounted in the hub and interconnecting with the fan blades to control the rotational position of each fan blade about the corresponding radially extending axis of the fan blade; and each fan blade being mounted on a shaft and secured on the shaft by clamping two sections of the fan blade onto the shaft. 4. The variable pitch fan of claim 3 in which one of the two sections of the fan blade is smaller than the other and further comprising counterweight supports moulded onto at least one of the two sections, counterweights being mounted on the counterweight supports.5. A variable pitch fan, comprising:a fan hub having interior walls defining a cylinder having an axis, the interior walls including an encircling wall and first and second end walls; fan blades extending radially outward from the fan hub and mounted for rotation about a radially extending axis corresponding to each fan blade; a pitch shifting mechanism mounted in the hub and interconnecting with the fan blades to control the rotational position of each fan blade about the corresponding radially extending axis of the fan blade; and the pitch shifting mechanism including a piston that is stabilized within the fan hub by contact of an outer peripheral sealed surface of the piston with the encircling wall and by a guide interconnecting the piston and one of the first and second end walls. 6. The variable pitch fan of claim 5 in which the guide extends from the piston through one of the first and second end walls.7. The variable pitch fan of claim 6 in which the guide lies along the axis of the cylinder.8. The variable pitch fan of claim 7 in which the piston is actuated by fluid injected through a port lying on the axis of the cylinder.9. The variable pitch fan of claim 5 in which contact between the piston and the encircling wall occurs at the outer peripheral sealed surface and at an inner peripheral surface of the piston.10. The variable pitch fan of claim 9 in which the inner peripheral surface of the piston is coincident with inward edges of blade mounts for the fan blades.11. A variable pitch fan, comprising:a fan hub having interior walls defining a cylinder having an axis, the interior walls including an encircling wall and first and second end walls; fan blades extending radially outward from the fan hub and mounted for rotation about a radially extending axis corresponding to each fan blade; a pitch shifting mechanism mounted in the hub and interconnecting with the fan blades to control the rotational position of each fan blade about the corresponding radially extending axis of the fan blade; and the pitch shifting mechanism including a piston that is stabilized within the fan hub by contact of an outer peripheral sealed surface of the piston with the encircling wall and by contact between the piston and the encircling wall at an inner peripheral surface of the piston. 12. The variable pitch fan of claim 11 in which the inner peripheral surface of the piston is coincident with inward edges of blade mounts for the fan blades.