In the early 1980s the US Army had a good think about their helicopters, and how vulnerable they were to modern air defence systems. A vast and ambitious programme was started to address this concern, dubbed the Light Helicopter eXperimental (LHX).
The LHX was required to replace the UH-1 ‘Huey’ in the utility role as the LHX-U, and the AH-1 Cobra and UH-1M in the gunship role as the LHX-SCAT. The SCAT would also supersede the OH-6A and OH-58C for the ultra-dangerous scout/reconnaissance mission sets. In 1982, the US Army had a force of around 2,000 utility aircraft, 1,100 gunships and 1,400 scout helicopters — any replacement could expect enormous orders. Such large numbers meant a big budget for researching new technologies, big profits for the winning contractor and global dominance in the field of military helicopters. The study that led to the LHX noted that there was a lack of original thinking in US Army aircraft procurement and that bizarre, exotic and unconventional approaches to the problem should been encouraged.The use of advanced materials, avionics and new concepts – like stealth and a single-pilot crew – were also to be encouraged.
One way to reduce vulnerability was to make the LHX faster than existing helicopters, and a top speed of 345 mph was suggested. This is extremely fast for a conventional helicopter, even today the fastest helicopters rarely go beyond 200mph (for reasons explained here). All major US helicopter manufacturers leapt into the fray, fiercely fighting to win the golden ticket of LHX. The entrants were quite unlike anything else built before or since.
Bell & Sikorsky’s convertoplanes
Convertoplanes are a category of aircraft which uses rotor power for vertical take-off and landing (VTOL) and convert to fixed-wing lift in normal. Their inflight fixed-wing configuration means they can fly faster than helicopters, but the technology took a tortuous path to the mainstream. Bell had been researching and building experimental convertoplanes since the 1950s and this technology had reached a new level of maturity with the Bell XV-15 of 1977. Bell could make an LHX convertoplane that would be far faster than a helicopter, an idea that Sikorsky would also explore.
Bell’s initial idea was revolutionary — a small fighter-like machine with a butterfly v-tail named the BAT (Bell Advanced Tilt Rotor). It was to weigh slightly more than 3.5 tons, with a maximum speed of 350 mph and armament options included four Stingers, four Hellfire anti-tank missiles or two 70-mm Hydra rocket launchers.
Sikorsky early efforts were, like Bell’s, tilt-rotors. Their convertoplanes concepts were rather larger and heavier than Bell’s. Sikorsky soon become daunted by the technological risks involved in tilt-rotors and moved towards a more conventional solution.
Sikorsky’s next proposal featured a coaxial scheme with an additional pushing screw in an annular fairing. Co-axials were popular with the Soviet company Kamov, who today employ the technology on the Ka-50/52 gunship. Though slower than a convertoplane it was expected to offer greater stability and manoeuvrability. Despite did not winning, the concept is still alive today and can be seen on the S-97 Raider.
One Sikorsky LHX SCAT concept with co-axial rotors and a pusher propeller again engaged in the anti-helicopter mission. New rotorcraft concepts take a very long to perfect, and thirty five years later Sikorsky is developing the similar S-97 Raider.
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Hughes, then producing the world’s most advanced helicopter gunship for the US Army, the AH-64 Apache, felt they were in a strong position to win LHX. Their offer was extremely bold and quite unlike any flying machine before or since. The Hughes LHX SCAT had no tail rotor, instead using the NOTAR system allowing a shape that would have had far less drag than any other helicopter. The fuselage was an aerodynamically wasp-like pod with two sharply swept wings and a nose section closer in appearance to a supersonic fighter than an attack helicopter. Smaller than the other proposals, yet equally well armed and fast at an estimated 342mph. It is unclear what Hughes were offering the utility category for LHX.
Boeing rejected the notion of very high speed, deciding that stealth and advanced sensors were the solution to the requirement for enhanced survivability. Their proposal was shaped for low radar observability — with weapons mounted internally. According to the writer Bill Gunston, the proposal rejected cockpit transparency (windows) in favour of sensors creating an artificial view of the world for the pilot; the reason for this is two-fold, transparencies create problems for stealthy designs and at the time there was a fear of laser dazzling weapons (also seen on the stealthy BAe P.125 concept).
Boeing’s embrace of stealth over speed won out, and a 1984 review of the proposals agreed. An updated requirement was issued – LHX / LOA – which insisted that the new aircraft must be low-observable (to radar and infra-red sensors) . Such a brief immediately wiped out the chances of any tilt-rotor designs with their massive frontal cross-sections.
Boeing LHX SCAT.
Though knocked out of the LHX contest, American interest in high-speed battlefield tilt-rotors would soon return. Replacing the A-10 battlefield support aircraft with a vertical take-off aircraft could prove a boon for forward deployment and potentially offer far greater flexibility. In 1986, Bell and Boeing created a proposal for such a machine, dubbing it the Tactical Tiltrotor. This extremely ambitious machine promised supersonic performance, thanks to an ingenious propulsion system. On take-off, landing and speeds up to 186 mph the aircraft acted as a turboprop tilt-rotor with the engines fed from a central turbojet, above these speeds the rotors folded into the engine nacelle and the turbojet provided direct thrust. In this mode, a top speed of Mach 2 was anticipated. This already radical idea was to be combined with forward swept wings, canards and an internal weapons bay housing eight Hellfire or Stinger missiles. Work continued until 1990, when it was cancelled as the Soviet threat disappeared.
An artist’s impression of an early Bell / Boeing Tactical Tiltrotor concept.
Various Bell / Boeing Tactical Tiltrotor layouts were studied, including versions with two turbojet engines.
The novel internal arrangement of the Bell / Boeing Tactical Tiltrotor.
This artist’s impression shows a glass two person cockpit and as a two-ship attack Soviet tanks on a bridge.
In addition to the battlefield attack variant, a transonic combat utility convertoplane was considered. It appears that this design may have some external features designed to reduce radar conspicuity.
Would the LHX Stingbat have been any good? Find out here.
Back to the LHX
The four big names in the US helicopter industry paired up into two teams: Bell and McDonnell Douglas formed the ‘Super Team’ and Boeing and Sikorsky formed the ‘First Team’.
LHX requirement dropped the utility requirement in 1988, and by 1991 the Sikorsky-Boeing collaboration had been selected as the winner. This aircraft, the Boeing–Sikorsky RAH-66 Comanche, first flew in 1996. The 200 mph 11,000Ib Comanche was a very sleek machine with weapons and undercarriage stowed internally to minimise drag and, more importantly, radar cross section. Three Hellfire (or six Stingers) missiles could be held in each of two weapons bay doors complimented by a trainable 20-mm GE/GIAT cannon. It was intended that the two-person helicopter could sometimes be crewed by one, but this proved dangerous in practice (the single person attack helicopter has proved unpopular- the sole operational example being the Russian Ka-50).
It was the first known helicopter designed with a high degree of low observability and was extremely sophisticated, but despite the $7 billion USD spent, it was not to be. It required substantial modifications to be survivable against modern air defences, dwindling orders were pushing the unit price up and the US Army thought it wiser to invest funds into upgrading existing platforms and into developing unmanned scouts that could do the job without risking a pilot’s life. Some also wondered how useful radar stealth was for an aircraft that would often be slow and low enough to be targeted optically. After a 22 year effort, the Comanche was axed in 2004.
Life after Comanche
Not all was lost however, the LHTEC T800 turboshaft developed by Rolls-Royce and Honeywell for the Comanche has seen considerable use. It powers the Super Lynx 300, AW159 Wildcat, Sikorsky X2 (an experimental co-axial pusher), T129 ATAK gunship and even serves (as a boundary layer control compressor) on a vast flying boat – the ShinMaya US-2.
The tilt rotor technology pursued for LHX (and other US programmes) eventually led to the V-22 Osprey, smaller AW609 and V280 Valor. The co-axial pusher, an idea that dates back at least as far as the Lockheed AH-56 Cheyenne, can be seen today expressed in the S-97 Raider now under development. It is likely that stealth technology developed for the Comanche found a home on the US Army’s secret fleet of stealth helicopters, famously (and accidentally) coming into the public eye during the assassination of Osama Bin Laden.
Attempts to replace Army scout helicopters with the new, far less ambitious, Bell ARH-70 Arapaho also floundered when the project was found to be 40% over budget.
Interview with USAF spy pilot here
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I remember these artistic adverts from Vertiflite, Aviation Week & ST, and (maybe) Rotor & Wing at the time. Before saying anything about these schemes, I’d like to comment on the procurement debacle that resulted in the collapse of the Comanche program.
The ambition of the program was enormous with an initial procurement target of 4,000 airframes. US procurement politics is (from my perspective) a very strange beast. The Army knows that it needs to replace the OH-58 as a Scout helicopter (described by one serving officer thus: “as a Scout, it makes a great General’s air taxi”); they also need to replace the hugely successful (but old) Huey; finally, they want an LHX with lighter weight, good firepower, high performance and stealth – adjectives that (other than firepower) cannot be applied to the AH-64.
As soon as the program gets underway, the Army Program Managers of the Apache and Kiowa (and anyone else with a rotary program that is in service and has development ambitions) immediately see Comanche as a threat that will suck dry the funding for their own programs.
Those industrial concerns that haven’t won the development contract also start to lobby their Congressmen & women to show how damaging Comanche is going to be to their local communities. In the US ‘pork-barrel’ environment the program attracts huge scrutiny.
Before Boeing Sikorsky has had time to freeze their design, they are saddled with Congressional Committee requirements in the form of Contractual maximum weight, minimum performance and maximum cost targets. As weight, cost and performance are pretty much the only design freedoms available to a designer, I felt at the time that the only rational response was to say to Congress – well you go and design it then! (If they already knew what those answers are … they must be able to produce the design)!
Over time the size of the program shrank and (predictably), the specified-by-committee weight, performance and cost constraints proved unachievable. To my mind, this classic Army in-fighting and Congressional pork-barrel politics made it easier to cancel the program than to stick with it.
(It’s all a bit reminiscent of the unintended consequences of applying import tariffs to your home Industry’s raw materials – namely that they are then incentivised to move production off-shore in the interests of maintaining competitiveness).
These adverts go back to happier times, when none of these unfortunate realities applied.
The conventional rotor designs reflect a desire to eliminate the tail rotor (with benefit to acoustic and radar signatures), Drag reduction is also essential (as power increases with speed cubed), so the performance challenges are then going to favour compact designs with faired rotor hubs and shafts and internal control systems (like the Hafner ‘spider’ arrangement used on the Lynx), rather than exposed pitchlinks sat on top of a conventional swashplate.
I should declare a small interest, as developments of the theoretical model of Circulation Control that I had developed during my PhD, were used by McDonnell Douglas for first-cut optimisation of NOTAR blowing geometries (see “Circulation Control Tail Boom Aerodynamic Prediction and Validation” by Hormoz Tadghighi and Thomas L Thompson of Mc Donnell Douglas Helicopters, presented at the 1989 American Helicopter Society Forum in Boston, USA).
The sharply swept wings are completely unecessary aerodynamically, but the F-117 and the RAM wedges lining the SR-71 leading edges should give the hint that these may help in terms of radar signature. Another common feature (certainly there in Comanche) was the use of entrained external air mixed with the exhaust to minimise thermal signature.
When I was looking at low signature helicopters (1982) we made a patent application for internal weapon carriage (pretty much identical to that used on the Comanche design – which we knew nothing about). We were surprised to then have a Secrecy Order placed on the Patent Application by the US Patent Office. (My other vaguely-related patent application has not yet surfaced, but that’s another story).
The Sikorsky co-ax design (presumably Advancing Blade Concept) is very reminiscent of the current S-97 Raider, other than the use of a ducted, rather than open rear propulsor (and tandem, rather than side-by-side seating),.
It is probable that all the conventional rotor desighs use a stiff (or semi-rigid) rotor to aid manoeuvrability and avoid a tall rotor mast. There does not seem to be much sign of metalised cockpit transparencies. Also. none of the designs show any electro-optic sensors (although these could be hiding behind fine mesh screens, al la F-117.
The Boeing LHX SCAT drawing suggests the use of a Longbow style radar sensor and terminally guided missiles (Imaging IR, laser designated, or radar homing – as Brimstone).
None of the designs look credible in tems of weapons packaging and the speed requested would surely not have been achieved with external ATGW carriage (although small anti-air missiles carried externally might be possible.
The tilt rotor designs with their unswept wings do not seem to be addressing radar cross section to any great degree. the Bell Boeing tactical tilt rotor designs look splendidly futuristic and have clearly (from the side view internal packaging diagram) taken IR signature seriously with a large propulsive fan mixing external air with the exhaust flows (as was done on Comanche). Her we have two turboshaft engines with a central combining gearbox driving a buried propulsive fan (for use when the tip rotor blades are folded back). Power is fed forward to a gearbox to split drive to the wingtips when using the rotors. At the tips, there are further boxes to drive the rotor and control their blade fold for high speed flight. To me this looks over-complex.
The futuristic Tilt Wing Air Assault Vehicle does has a shaped fuselage and canted fins, but still looks challenging from the RCS viewpoint.
The small Bell tilt rotors do not appear to offer sufficient weapons bay internal volume and do not portray any target sensors. Similarly, the avionics volumes hinted at on the Armed Recon / Anti Armour scheme look unrealistically small.
Not one of the designs appears to address the Huey replacement requirement – presumably not sexy enough for the Marketing Departments concerned.
If any of these had gone ahead, they would have been great projects to work on – but would they have survived the US procurement cat-fight?
A further comment on the relevance of low radar cross-section. A primary threat to Scout and Attack helicopters is radar directed anti-aircraft artillery, of which the Russian ZSU-23-4 would be a prime example. The ZSU-23-4 had potent and reliably fused ammunition and is effective at rages close to those at which the helicopters can obtain positive target ID. Improved electro-optic performance can allow the helicopter to stand-off, and the use of mast-mounted sights allows the helicopter to hide behind buildings, ridges and vegetation. Nevertheless, when operating at low level, there is always the risk of accidental exposure to this potent threat, Unlike fixed-wing strike aircraft, the helicopter cannot do its job while flying high to stay outside the anti-aircraft gun engagement envelope. Therefore, it remains important to reduce radar signature when operating against this type of threat. Modern electro-optic sensors may use target recognition software to analyse the results of a target area scan and prioritise high threat targets (such as air defence assets).,
When noting that several of the abovementioned projects were supposed to be flown by a single pilot even under combat conditions, a mention must be made of the technology which was envisaged to enable this. At this time, when immense intellectual and economic resources were dedicated to overcoming the still existing Soviet menace, there was also the belief that one outcome of the software part of defense research would yield viable AI solutions within a few years. Under the title ‘Rotorcraft Pilot’s Associate’ went the advanced, pseudo intelligent computer system which would have supported the pilot by filtering information, selecting targets and classifying threats and even acting as a backup to the pilot. Among the intended features was co-training of pilot and AI in order to achieve closer integration and support to a pilot just right for his specific way of performing flight and tactical tasks.