Yesterday, Bell Helicopters revealed a futuristic stop-fold aircraft concept combining the advantages of a tilt-rotor with potentially higher top speeds. We consider the proposal with the help of former Head of Future Projects for Westland Helicopters, Dr Ron Smith.

New ideas in rotorcraft technology never die, but sometimes they remain in a state of metaphorical autorotation until the technology reaches appropriate maturity. The tilt-rotor is a very old idea, dating back to the 1909 Dufaux triplane from Switzerland, but it wasn’t until 98 years later that the concept became an operational reality with the V-22 Osprey. Clearly some ideas take a very long time to leave the experimental phase and enter service; Could the same be true of the stowed stop-fold prop rotor? The advantages of tucking away the huge prop-rotors of a tilt-rotor and switching to jet propulsion are obvious, you frontal cross-section is massively reduced and you can potentially break the 540mph barrier that has long limited propeller-powered aeroplanes.

According to Ron Smith, “This concept dates back to 1967 with the Bell Model 627. In 1967 the US Air Force solicited proposals for `low-disc-loading [Vertical Takeoff and Landing] configurations suitable for high-speed flight.’ Bell responded with a proposal for a folding proprotor design. Development and analysis included design studies, leading up to a 1972 test with a full scale 25 foot diameter pylon and rotor assembly wind tunnel in the NASA-Ames Large Scale Wind Tunnel. The original Bell Helicopter proposed stop-fold tiltrotor design allowed for vertical take-off and landing. Take off was followed by a tilt rotor transition sequence rotating the pylon rotor assembly from helicopter to airplane mode, with wing lift supporting the aircraft.

A final conversion sequence involved slowing and then stopping the rotors and folding the blades rearward along the pylon, with propulsion being taken over by direct jet engine thrust.
The Bell Helicopter report of the full-scale blade fold tests (Report D272-099-002, May 1972) is available here.
The images to the left have been extracted from that report and make an interesting comparison with the imagery recently released by Bell.

The current Bell HSVTOL imagery from their recent press release is shown above.

Bell’s press release summarises the capability objectives as follows:
“HSVTOL technology blends the hover capability of a helicopter with the speed, range and survivability features of a fighter aircraft. Bell’s HSVTOL design concepts include the following features:
Low downwash hover capability Jet-like cruise speeds over 400 kts
True runway independence and hover endurance
Scalability to the range of missions from unmanned personnel recovery to tactical mobility
Aircraft gross weights range from 4,000 lbs. to over 100,000 lbs.

Bell’s HSVTOL capability is critical to future mission needs offering a range of aircraft systems with enhanced runway independence, aircraft survivability, mission flexibility and enhanced performance over legacy platforms.

With the convergence of tiltrotor aircraft capabilities, digital flight control advancements and emerging propulsion technologies, Bell is primed to evolve HSVTOL technology for modern military missions to serve the next generation of warfighters.”

The downwash velocity comment depends on the comparison being made. Tilt rotor designs will have higher downwash velocity than a single rotor helicopter at comparable all up weight. They will, however, have greatly reduced downwash compared with jet or fan-lift concepts. There will be drag penalties associated with the rotor and rotor nacelle assemblies, even when folded, but a speed range up to 400 kt seems quite credible.
There is also no fundamental reason why the concept should not be scalable over a wide range of roles and all up weights.

The artists impressions show designs that incorporate some fuselage faceting to reduce radar signature. Despite this, one suspects that real world excrescences associated with blade fold mechanisms will impact both airframe drag and radar signature. The image also shows three different intake arrangements and optimisation of the engine installation will necessarily depend on both radar and IR signature requirements. It is not clear whether an entirely separate propulsion engine is used in addition to a powerplant to provide shaft power to the rotors, or whether a variable cycle engine is proposed that can deliver a controllable mix of jet thrust and shaft power.

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The starting point for a demonstrator would probably avoid the risk and complexity of developing a variable cycle solution simultaneously with a new airframe configuration.
The Bell report from 1972 lists key risks. Not surprisingly, these focus on static and dynamic loads during the blade fold sequence. Having said that, this test programme extended to 40 start / stop transition sequences conducted at full scale in the Ames Wind tunnel. Even though this data was collected some 50 years ago, it does represent (along with more recent experience) a significant contribution to risk reduction.
The risks identified in the 1972 Bell report were (my interpretation of the findings):
• Proprotor instability (alleviated by folding the blades before reaching the high speed flight regime).
• Potentially large amount of blade flapping during the blade fold sequence, requiring flapping restraint and associated dynamic loads. A short duration blade fold sequence would limit the number of cycles to which the system is exposed.
• Dynamic excitation of the wing and pylon structure during the blade fold sequence. During testing, this was found to be associated with blade / wing aerodynamic interference.
• The report also comments that it was found that the mechanisms associated with the folding-proprotor concept are a significant design challenge from the standpoint of achieving a reliable mechanism at a reasonable weight.
All-in-all, Bell Helicopters appear to be well placed to pursue this development. The challenge will be to secure a stable funding stream for the lengthy design and development program that will be involved. The Army is showing sustained interest in high-speed rotorcraft and one imagines that there would be significant interest across the US armed forces in understanding the options to replace the V-22 Osprey.

This could be a starting point and technology demonstration at lower weights could spin off additional applications.

What is the real-world need do you think?

“Bell would say – Civil: executive helicopter replacement (- like the Bell 609 tilt rotor now moved to Leonardo). Air taxi operations?

Military applications – combat search & rescue, special forces, Osprey replacement, armed UAS?”

What are the advantages?

“Primarily speed, possibly manoeuvrability, external noise. (Could be like Osprey – makes the naval fleet operation less vulnerable).”

What are the disadvantages or potential problems?

“Complex transmission and propulsion arrangements, load prediction, certification, failure modes? (but not much worse than tilt-rotor).”

Is it worth the effort? 

“Only if there is a genuine requirement backed up by funding / political support and timescales that are achievable.”

What will be the hardest aspect to master if it goes ahead? 

“Certification, complexity. Accommodating associated military requirements (crashworthiness? radar & IR signatures, weapon system integration, defensive aids, sensors, operation in adverse environments, survivability, etc.). Weight (issue for anything VTOL), achieving predicted payload and range. Development cost & timescales. (Just like any other military procurement).”

Do you think it will happen? 

“Very hard to answer. If it starts to look like a threat to other programmes, their respective protagonists will lobby against it. I can see smaller variants being pursued because something civil might spin off into an armed UAS. Work on smaller platforms will ultimately de-risk more ambitious developments. I’d say less than 50% chance but higher than that for something at the technology demonstrator level with an eye on actual applications.”

Ron Smith
August 2021