GB2356616A - Helicopter with tiltable rotor assembly - Google Patents

Description

2356616 PATENT APPLICATION 20 October 1999 John Ronald Watkinson and

Mikael Reichel

2, Hillside, Burghfield Common, Reading RG7 3BQ U.K.

ROTORHEAD This invention relates to the rotorhead of rotary-winged aircraft.

In rotary winged aircraft it is well known that the individual blades are attached to a rotorhead which in turn is attached to the main shaft or mast. It is conventional practice that this shaft is constrained by bearings so that its axis cannot change witfi respect to the hull of the craft.

Control of the attitude of the machine is performed by cyclic variation of the blade pitch. This initially results in a tilting of the rotor plane, and the setting up of a bending stress in the mast which seeks to change the attitude of the hull.

This stress is handled in two ways in the conventional art. In the first, the rotorhead, blades and mast are made sufficiently strong (and therefore heavy) to withstand the stress and the resultant control power will be very high.

In the second, where low weight is a requirement, the blades may be mounted on flapping hinges in the rotorhead as moments cannot be transferred across a hinge. The control power is reduced.

However, as the blades move on the surface of a cone and not in a plane, the use of flapping hinges requires that orthogonal hinges known as dragging hinges be fitted to allow the blades to conserve momentum. The hinges are all subject to rotational forces and must be substantially constructed. The blades can resonate about these hinges and damping will be required in the rotorhead and in the undercarriage of the craft. This damping apparatus adds weight and drag.

An exception is the teetering head and the related tri-hinge head in which two blades are allowed to flap so that no moments are transferred to the mast, but no dragging movement is allowed.

A great drawback of teetering and tri-hinge heads is that it is only gravity which keeps the hull below the rotor. In negative-g manoeuvres the hull can fall into the rotor with fatal results. The teetering mechanism is restricted to rotors having two blades.

A further drawback of heads containing hinges is that perfect conservation of blade momentum is not achieved and the result is vibration.

According to the present invention, the hingeing apparatus of the conventional rotorhead is moved to a location where it does not experience rotational loading. The hinges can then be much smaller and lighter. As these hinges do not rotate, their characteristics can be made non-linear or dynamically adjustable. For example in cruise the stiffness of the hinges could advantageously be made low to reduce vibration, whereas if a gust resulted in negative-g, the hinges might automatically stiffen to prevent the hull attitude diverging.

The attachment of the blades to the head in the present invention is hingeless, except for the possible use of feathering hinges. Residual flapping and dragging may be accomodated by the use of blades which are designed to flex in a known manner. The resonant frequency of dragging is designed to be sufficiently high that no dragging dampers or undercarriage dampers are required. This reduces weight and aerodynamic drag.

According to the present invention, the rotor shaft is not rigidly mounted in the hull, but instead its axis may move in a controlled fashion to allow the plane of the rotor disc to be different to that of the hull, although it is advantageous if the centre of mass of the rotor does not move with respect to the hull. This arrangement is geometrically perfect and allows accurate conservation of momentum in the blades. In the hover, rotor vibration will be minimal.

Allowing the rotor head alone to tilt in this way requires a constant velocity joint which must be capable of carrying full rotor torque and the full weight of the machine in order to prevent vibration. According to the present invention this is not necessary because the rotor reduction gearbox and the rotorhead are combined in a single assembly. The rotorhead and gearbox will tilt as one unit with respect to the hull on hinges which are not subject to rotational loads. The hinge mechanism may incorporate means to resist the torque reaction of the gearbox or separate means may be provided.

As the rotor loads are carried to the hull by a stationary hinge mechanism, the drive shaft to the gearbox carries no axial load at all. As this shaft turns much faster than the rotor shaft, the torque is reduced and a small, light constant velocity joint will be adequate. This joint may also require some plunging capability.

Making the gearbox and rotorhead as one assembly has a number of advantages:

More space is available inside the hull. The cooling of the gearbox and the ease of inspection of the oil level are improved compared to a gearbox buried inside the hull. The overall weight is reduced as the heavily stressed mainshaft and rotating hinges are eliminated. Replacement for maintenance is easier.

The use of dampers is not required, allowing further weight saving. In the teetering head, this advantage is offset by the loss of control in negative-g conditions. The present invention does not lose 1 control in negative-g.

The close coupling of the gearbox and the rotorhead means that the gearbox acts as a mass damper to vibrations originating in the rotor. Even with perfect geometry, translational flight causes periodic blade forces and the present invention reduces the vibrations due to them.

One embodiment of the present invention is shown in the accompanying figures in which:

Fig.1 shows the general arrangement.

Fig.2 shows one possible stiffness characteristic of a tilt linkage bearing.

A suitable number of rotor blades 1 are mounted on the rotorhead 2 which is integral with the gearbox 3. The gearbox is supported by a linkage such as that of links 4, 5 which allows the rotor assembly to tilt about its centre of mass in any combination of pitch and roll. A constant velocity joint 6 transmits shaft power from the engine.

The control of blade pitch may be performed by known means such as a spider which is raised and tilted with respect to the hull by the collective and cyclic controls. As the spider is hull-referenced, any disturbance of disc attitude will be self correcting because hub tilt will cause cyclic inputs tending to cancel the tilt.

The rate of response to the cyclic controls will now be controlled by the stiffness of the bearings 7 in the tilting links. The bearings may advantageously be elastomeric therby simultaneously absorbing vibration.

Fig.2 shows that the characteristics of the elastomeric bearing may usefully depart from the normal Hooke's Law characteristic. Instead the stiffness may be low for small displacements so that the response to small corrections in the hover is slow. However as the displacement increases, the stiffness may also increase so that larger control inputs result in a faster response. This characteristic will also provide stability in the event of negative-g.

Alternatively the contol of blade pitch may be by servomotors mounted on the rotorhead or blades, with control information carried by non-mechanical means across the flexing supports. Such means could include but are not limited to electrical, optical or hydraulic signalling or the transmission of numerical parameters.

Claims (5)

Claims:

1. A rotorhead having an integral gearbox and supported by non-rotating means allowing the rotor plane to tilt with respect to the hull substantially about the centre of gravity of the rotor.

2. A rotorhead as in Claim 1 in which the response rate of the rotor is a function of the resistance to deflection of said supporting means.

3. A rotorhead as in Claim 1 in which the resistance to deflection of said supporting means is a non-linear function of the displacement from the neutral position.

4. A rotorhead as in Claim 1 in which the transfer function of the resistance to deflection of said supporting means may be varied by control means.

5. A rotorhead as in Claim 1 in which blade pitch control information is carried across the tilting mechanism by non-mechanical means.