Future Bell helicopter designs could come with split-tip main rotor blades and anti-torque control provided by a cross-flow fan embedded in the tail boom, rather than a traditional tail rotor, if two recent patents are used.

While the manufacturer has previously revealed the cross-flow fan on its FCX-001 concept aircraft in 2017, the detailed patent application – granted on 28 April – provides more detail of the design.

img12 crop

Source: United States Patent & Trademark Office

Bell split-tip rotor design

Described in the application as a “centrifugal blower system”, the fan sits within the tail boom generating thrust by expelling air through “variable aperture” ducts, whose size would be controlled by the pilot’s foot pedals.

These would be combined with “adjustable Coanda slot(s)” – which would push airflow down the side of the tail boom, producing the lift-inducing Coanda effect – to “direct the appropriate amount of thrust through the appropriate side(s) of the tail boom in order to provide anti-torque and directional control.”

Drawings accompanying the application show the air intake for the system variously located further forward on the tail boom, or at the very end of the structure. Bell says the fan could be mechanically, hydraulically or electrically driven.

As part of its justification for the concept, Bell notes the safety disadvantages of traditional tail rotors and the added weight and complexity of ducted fan designs such as the Airbus Helicopters’ Fenestron. Both also require gearboxes and drive shafts to deliver power from the engines.

Bell also notes the “performance drawbacks” of MD Helicopters’ NOTAR – or No Tail Rotor – system, which “increases complexity” and also requires a less aerodynamically optimised cylindrical tail boom.

“Accordingly, a centrifugal blower system provides numerous benefits over existing approaches, including improved safety, acoustics, and performance, and reduced complexity, cost, and weight, among other examples.”

However, Bell says the centrifugal blower – which it also refers to as a “squirrel cage blower” – can also be used in conjunction with a traditional tail rotor or rudder to “augment or improve the anti-torque and directional control”.

Meanwhile, Bell also appears to be seeking to bring to rotorcraft the performance gains fixed-wing aircraft have obtained from the use of split-tip winglets, such as those produced by Aviation Partners for the Boeing 737NG.

The patent application for the split-tip rotor blade notes that in wind-tunnel testing of the design, lift rose by about 13.5% and drag reduced by about 17%. “Accordingly the lift/drag ratio was increased by about 36.91%”. This, Bell says, “can result in fuel savings of about 15-17%”.

Bell was granted a US patent for the design on 21 April. Although the airframer gives no detail of specific applications for the split-tip rotors, a drawing within the patent documents shows an aircraft bearing a strong similarlity to its 505 light-single.