A slew of patent awards for Bell over recent months – including a biplane tiltrotor, a pusher propeller drivetrain and an anti-torque tail rotor that can be turned through 90° – highlight the range of innovations potentially being considered by the helicopter manufacturer.
So far this quarter, Bell has had 45 patents granted by the US Patent and Trademark Office. While many of these are for relatively mundane items such as landing skids or bearings, a breadth of new architectures is also revealed.
One of the newest approvals – granted on 8 December – covers the addition of a tail-mounted pusher propeller to an otherwise conventional helicopter.
Under the potential layout shown, the pusher propeller, with variable pitch blades, would be linked to the helicopter’s engine via a driveshaft and tail system gearbox, which would be shared with the anti-torque mechanism.
In addition, mid-fuselage-mounted stub wings would provide additional lift for the rotorcraft when the pusher is engaged and the main rotors are in an autorotative state.
“Cases in which it may be useful to operate the main rotor in an autorotation state may include high forward speed flight for a pusher rotorcraft in which the engine(s) may provide increased power to the pusher propeller system while the main rotor is unpowered or absorbs less power from the engine,” the patent reads.
One key aspect of the design, it says, is that torque from the propulsion system could be routed directly to the main gearbox or tail system gearbox using a “drop-down gearbox” positioned between the two. That would “reduce power losses” to the tail system when the helicopter is in an autorotative state, “making the pusher rotorcraft more efficient”.
But the pusher design is not the only change to a helicopter’s tail proposed by Bell. In a separate filing, approved on 29 September, the manufacturer outlines an anti-torque system that could pivot to provide directional thrust depending on the flight mode.
Bell has already revealed its Electrically Driven Anti-Torque (EDAT) system – four electric fans embedded in the helicopter’s vertical fin – and this is referenced in the patent, but the variable configuration takes that design one stage further.
Drawings submitted with the application show an array of six “fixed blade pitch motors” arranged in a circle, with a seventh motor in the centre; these are depicted as both shrouded and unshrouded configurations.
However, the patent’s text makes clear that Bell sees the potential for an anti-torque assembly with “15 or more fixed blade pitch motors” depending on the application.
Individual electric-powered motors, or the entire assembly, may be surrounded by a cowl, and all or some could be pivoted to vary the direction of thrust. Bell says the assembly would be in line with the tail boom during hover and “substantially perpendicular” during forward flight.
Thrust directed towards the rear of the helicopter would add “to the forward thrust, which is generally used in high airspeed cruise”, it says, and would “increase cruise efficiency”.
The system’s deployment, and the direction and speed of the fans would be controlled by a flight-control computer “to position the anti-torque assembly system for optimum thrust angle… and magnitude”.
Bell highlights the simplicity of an anti-torque system comprising multiple, fixed-pitch fans, noting numerous safety and operational benefits.
In addition, Bell believes the variable direction assembly could enable the elimination of vertical and horizontal stabilisers from helicopter designs, saving weight and reducing complexity – such as the “down-loading” of the horizontal structure by the downwash from the main rotor.
While many of Bell’s patents are for individual components or structures, perhaps the most striking of the recent designs is for a biplane tiltrotor.
Images submitted as part of the application show a number of embodiments of the biplane design – with the wings variously parallel, or in combinations of anhedral or dihedral orientations. Also shown is a four-rotor design which features two pairs of wings.
Bell argues in the patent – which was granted on 17 Novemeber – that the biplane configuration eliminates some of the “drawbacks” of a conventional, monoplane tiltrotor.
Because both the upper and lower wings provide lift, they can be smaller than that of a monoplane design – for example, the chord of the upper wing can be reduced by “roughly 50%”. This in turn cuts the magnitude of the downward force caused by the rotor downwash on the upper wing in helicopter mode – essentially boosting the lift from those rotors.
This increased efficiency “can be leveraged to support a larger payload, longer missions [or] reduced fuel consumption”, it says.
Wing “down-loading” could be cut further with the use of a forward lift-generating canard, allowing the chord of the upper wing to be made even smaller.
The biplane design would also provide “additional structural support” for tiltrotor components mounted at the end of the wings, such as nacelles and pylons. Because the overall design is stiffer, overall weight could be reduced, says the patent filing.
And, in a sign that Bell has military applications for the technology firmly in mind, one illustration shows weapons mounted on the lower wing. “In this manner, weapon ordnance is mounted in a location that is unobstructed by the blades of the tiltrotors”, it says.
While the patent applications are a sign of interest in those areas, Bell is yet to signal that it is actively developing any of the above technologies.