The US Air Force Research Laboratory (AFRL) has contracted Bell to research a high-speed vertical take-off and landing (VTOL) aircraft.
The laboratory granted Bell a $950,000 contract to conduct applied research into the concept, it disclosed in an online notice posted on 28 April.
Bell acknowledged the contract, but declined to comment further. The AFRL did not respond to multiple requests for comment.
Little else was disclosed in the contract notice titled “Bell’s High Speed VTOL (HSVTOL)”.
The research contract comes several months after the US Air Force Special Operations Command (AFSOC) said it was looking ahead for a VTOL aircraft with “jet speeds” to replace its Bell Boeing CV-22 Osprey tiltrotor.
“At least our initial thinking in AFSOC is that we’re looking at a generation beyond current tiltrotor technology,” said Lieutenant General James Slife, commander of AFSOC, last September. “We’re not just looking at marginal improvements, in terms of speed, range and reliability, but we’re looking at a generational movement for a vertical take-off and landing capability going into the future. I think it’ll be probably something quite different than the V-22.”
In addition to being the lead manufacturer of the V-22, Bell is developing a next-generation tiltrotor, the V-280 Valor. That aircraft is a candidate the US Army’s Future Long Range Assault programme, an ongoing competition to replace the service’s Sikorsky UH-60 Black Hawk utility helicopter fleet starting in 2030. However, the V-280 has a cruise speed of 280kt (519km/h), only 39kt faster than that of the CV-22. AFSOC appears interested in going much faster.
“There are a number of technology and drive system proposals out there that look like they may be within the realm of possibility; that they could provide like a generational step ahead in technology, get us up into jet speed kind of capabilities,” Slife said. “When you look at the future operating environment, where range and access are going to be challenging across the board… I think whatever comes next is going to have to be a generation [ahead] yet again.”
In recent years, the US Department of Defense has stressed the challenge of moving forces across the long distances of the Pacific Ocean for potential combat operations against adversary China. In light of that problem, it has pushed US aerospace manufacturers for new ways to extend the range and speed of aircraft and weapons.
However, conventional tiltrotors, such as the V-22 or V-280, are limited by drag on their large rotors to speeds and ranges far less than turbofan-powered aircraft.
SWEPT ROTORS
Bell has been exploring aircraft that can take off vertically using tiltrotors, but then fly forward in cruise mode using wing-borne lift and thrust from jet engines, according to patent applications published between 2017 and 2020 by the US and European patent offices. Rotor blades would fold back to reduce drag during forward jet-powered flight.
One way such an aircraft might switch between high-speed cruise and VTOL mode is by relying on a “convertible engine”, a novel jet engine that switches between turboshaft and turbofan modes, according to several of Bell’s patent filings published in 2020.
“During operation as a turbofan engine, the bypass fan produces a bypass airflow to provide thrust to the aircraft,” explains a Bell patent application published by the European Patent Office in April 2020. “During operation as a turboshaft engine, the bypass airflow produced by the bypass fan is blocked, allowing other aircraft systems to utilize the power produced by the convertible engine via the power output shaft.”
In turboshaft mode, one or multiple engines would mechanically power rotor blades via a central gearbox. Alternatively, in turboshaft mode the convertible engine could turn an electrical generator that would send power to electric motors which would then move the rotor blades, say a Bell patent application published by the European Patent Office last October.
Because a convertible engine would need to be sized to power the more energy-intensive VTOL flight phase it would have extra power available during cruise, which could be used to power accessories, such as directed-energy weapons or batteries, says the patent application.
“Electrical [battery] storage provides additional capability for increased power during VTOL or other rotary operations by supplying additional electric power for short durations,” the filing adds.
Foldable rotor blades have other advantages too, says a Bell patent application published by the US Patent & Trademark Office last December. Actuators, which would be used to fold and open blades in flight, could independently extend rotors on the end of each wing to create an “active sail blade” that could provide another control surface and would increase the aircraft’s total lifting surface, possibly enhancing stability and increasing range.
Ultimately, Bell may choose to optimise rotor blades – technically known as proprotor blades when used on a tiltrotor – for active sail applications instead of the VTOL phase of flight.
“The airfoils on rotor blades may be optimised to perform better as static wings than as proprotor blades in order to support the sail blade functionality,” says the patent application. “This optimization may degrade proprotor efficiency for hover and helicopter mode flight but would improve efficiency for airplane flight mode.”
