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Lockheed AH-56 Cheyenne: The Incredible Attack Helicopter Decades Ahead of Its Time

Written by George Colclough

https://commons.wikimedia.org/wiki/File:YAH-56A_%22Cheyenne%22_4015.jpg

Introduction:

It would be nice to imagine technological progress as a smooth straight line onwards and upwards, one in which groundbreaking prototypes both absurd and viable are pitched, and allowed to battle it out in the marketplace of ideas, leaving only the best and most advanced ideas standing as champions to carry the heavy mantle of responsibility in mankind’s nonstop onwards march towards progress.

Alas, were it so easy – this channel is littered with examples that thoroughly demonstrate that often, the sophistication and viability of a project has little bearing on its adoption and production. This is because lingering in the background of any project great or small is a foul spectre, a machiavellian demon of the foulest proportions, that cares not for the security of the nation, the fighting ability of the enlisted soldier, nor the noble pursuit of technological sophistication. I speak, of course, of the politician…

The politician, and those who politick, are concerned with their own short sighted survival; securing victory in their next election, securing their next climb up the career ladder, the promotion of their own internalised biases, and the utter destruction of ideas that happen to fall foul of the aforementioned.

We are no strangers to this most wicked force on this channel: the SA80, a dangerously terrible rifle, whose flaws were willingly covered up by promotion hungry bureaucrats, the Miles M.52 which was MYSTERIOUSLY cancelled just as it sat poised to take the supersonic trophy for the United Kingdom, the TSR.2, which was MYSTERIOUSLY cancelled just as the American military industrial complex began to get scared of having a viable overseas competitor… we have covered many examples of the corrosive meddling of the politician on this channel, and today we are leafing through the annals of aviation history to examine another type of toxic politics: interservice rivalry.

Today’s subject was an advanced American attack helicopter that was poised to be the most advanced thing in the sky, to revolutionise helicopter design, and set the benchmark for rotorcraft design for decades to come – only to have the rug pulled from under it, and be unceremoniously cancelled and abandoned thanks to politicking and tribalistic interservice chest thumping most malevolent and capricious.

It was an attack helicopter so advanced that still to this day it stands as one of the fastest ever rotorcraft, had an advanced weapons suite years ahead of its time, and to many is still considered to be leagues ahead of the helicopter that eventually filled its role over a decade following its cancellation: the Boeing AH-64 Apache.

This is the story of the Lockheed AH-56 Cheyenne, the revolutionary attack helicopter killed by politics.

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Background

The AH-56 Cheyenne was the end result of a long line of development, with numerous precursor technologies and prototypes all playing their contributing part to this monumental aircraft.

The main such precursor technology for the AH-56 was the rigid rotor, which is to say a rotor design in which the helicopter blades are fixed to the rotor hub without hinges and the hub is fixed to the rotor shaft, rather than the traditional hinged rotor design found in most helicopters, in which hinges let the blades move independently of one another.

This method of rotor assembly can trace its lineage back as far as 1920, when Juan de la Cierva, a Spanish aeronautical engineer tried to rigidly attach the rotor blades of an autogyro to its hub. Despite his best efforts, he failed to produce a viable design on account of the difficulty in controlling the rotor, on account of the excessive gyroscopic moments of the rigid rotor design.

The idea was again picked up in 1958 by Irv Culver, an American aeronautical engineer who presented his ideas for rigidly attaching the rotor blades of a helicopter to its hub to Lockheed management in 1958. Culver quickly ran into the same problems as Juan de la Cierva, but believed a solution to the excessive gyroscopic moments could be found in the use of a feedback system into the rotor, which would allow excessive pitch and roll movements to be controlled.

Further development by Culver led to the creation of a device known as the “moment feedback system”, which created a “compliance factor” in the blades which gave them forward pitch movement of less than a degree, subsequently allowing the application of a corrective feathering input to the opposite blade. Culver then went on to demonstrate this innovation by incorporating it into a radio controlled model, which thoroughly demonstrated the feasibility of the concept, and he was subsequently given a test flight hanger, a flight test engineer, and two mechanics by Lockheed so he could further test and refine the concept.

Culver’s work progressed sufficiently that Lockheed soon commissioned a full scale flying prototype of his new technologies, the Lockheed CL-475, which first flew in November of 1959.

The CL-475 was very much a technological testbed, in that all emphasis in its design, and the majority of the resources allocated to its development went into perfecting its rigid rotor configuration, with the rest of the aircraft being comparatively spartan and utilitarian, it was a fabric covered design, built around a steel and aluminium frame. The landing gear was a crude none retractable tricycle layout, and it was powered by a modest four cylinder, air cooled Lycoming O-360-A1A piston engine developing 140hp, which ran to a two bladed wooden rotor.

The design proved a hit however, as despite its largely crude nature, the CL-475 proved the real world viability of a rigid rotor design, as Culver’s innovations, in conjunction with an upgraded metal three blade rotor made for an exceptionally responsive and agile, yet stable aircraft.

Lockheed was naturally impressed with these results, and he was formally assigned to Lockheed’s “Skunk Works” Advanced Development Programs division, where he received the luxuries of both the funding and the facilities that came with such a prestigious assignment.

Lockheed’s faith in Culver soon yielded dividends. With the extra resources and personnel of the Skunk Works, he quickly developed a more advanced rigid rotor prototype, the Lockheed XH-51, which first flew in November of 1961, three years and a day after the maiden flight of the CL-475.

The XH-51 was a significantly more advanced design than the CL-475, and rather than being a simple proof of concept, was a full and true prototype, designed to show potential customers the true capabilities of a fully fleshed out and realised rigid rotor helicopter.

Powered by a Pratt & Whitney Canada PT6B-9 turboshaft engine producing a healthy 550hp, the XH-51 was significantly faster than the CL-475, being capable of reaching 257mph in level flight, and 302mph in a shallow dive. For perspective, the XH-51’s contemporary, the venerable Bell UH-1 Iroquois was capable of 127mph in level flight, and 180mph in a shallow dive.

Design and development of cutting edge aircraft is rarely a clear cut process however, and invariably issues were encountered during the testing of the XH-51. One such issue encountered was that despite Culver’s great innovations, the aircraft still proved unstable at high speeds, which fortunately, proved easily remedied by the implementation of a four blade rotor system.

High speed performance was also further improved with the addition of 4.9m long wings on the second XH-51 prototype, which supplemented the rotors by producing lift whilst in forward flight.

With all of its kinks ironed out, the XH-51 proved to be an agile, stable, and above all else blisteringly fast aircraft, and Lockheed felt nothing but confidence when from 1962, they began marketing the XH-51 to both military and commercial potential customers.

Unfortunately for Lockheed, this went badly… very badly. The problem, there is a very big difference between a fantastic and technically ground breaking aircraft, and a commercially successful one. The market didn’t want an expensive 300mph utility helicopter, America’s airlines, executives, and private pilots were all perfectly content with their slower, more orthodox, and above all else, cheaper helicopters, and Lockheed failed to secure a single order.

This commercial failure appeared to be the end of Lockheed’s dabbling into rigid rotor helicopters. The market had spoken, the clerks in Lockheed’s sales department sat twiddling their thumbs, and Lockheed’s profit conscious executives were beginning to turn on the project in the face of its dubious profitability.

But then at the 11th hour, when all seemed lost for Lockheed’s rigid rotor craft, a customer emerged with pockets so stuffed and heavy with plunder that it cared not for such assinites as “cost”, and “affordability”. A customer who wanted naught but the most advanced, fastest, and most lethal helicopter possible. I speak of course, of the US Army.

All the resources Lockhood poured into its rigid rotor prototypes, the CL-475 and the XH-51, finally began to return dividends in March 1963, when the US Army announced a program to acquire a new class of helicopter: a fast, agile, and above all else dangerous helicopter that could carry weapons in a dedicated platform, rather than the jury rigged contraptions that had previously carried armaments – this program was dubbed the Advanced Aerial Fire Support System.

The timing couldn’t have been better for Lockheed. Thanks to Irv Culver, and his team’s dedicated and extensive work on the XH-51, Lockheed had a big bucket full of patents and potential that could (reasonably) easily become the perfect helicopter for this program, and thus, work on what would become the AH-56 Cheyenne finally commenced. 

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Development

The specifications of the Advanced Aerial Fire Support System program were lofty to say the least. It called for a helicopter with a dash speed, (an emergency high power speed which can only be sustained for a few minutes before one damages mechanical systems) of over 220 kilometres per hour, a cruising (normal) speed of 196 kilometres per hour, a ferry range (the maximum range of the aircraft when it has all non essential components stripped, and fuel tanks brimmed so it can be “ferried” from place to place) of 700 miles, and be able to hover at altitudes of 5,000 feet, or 1,800 metres.

12 companies submitted designs to the US Army, with all of the big players of America’s aviation scene making a bid for the contract. But only Sikorsky and Lockheed’s proposals were shortlisted. The former submitted the Sikorsky S-66, and the latter submitted the AH-56 Cheyenne.

The S-66 looked very similar to the Lockheed AH-56 Cheynne, and indeed both were advanced compound helicopter designs leagues ahead of anything then in the US Army’s inventory, But for want of a better description, the S-66 was more of a bargain basement design compared to the AH-56, with the program coming in at a MERE66 million dollars compared to the AH-56 at 106 million dollars. Not to decry the Sikorsky proposal as crude however, rather it is indicative of the absurd levels of sophistication and innovation Lockheed proposed for the AH-56. 

The S-66 had a Rotorprop tail rotor which could rotate its axis through a 90° range to act both as a conventional anti-torque rotor in horizontal flight and as a pusher propeller, thereby transforming the S-66 into a compound aircraft in cruising flight. This was a great innovation for sure, but Lockheed’s innovations in stability and control blew it out of the water.

The proposed Sikorsky S-66 would have produced 3439 horsepower, being powered by a Lycoming T55 turboshaft engine, and able to achieve a dash speed of 390 kilometres per hour, a cruising speed of 340 kilometres per hour, a ferry range of 3,500 kilometres, and was able to hover at altitudes of 5,500 feet. This was a fantastic effort by Sikorsky for sure, but was nothing compared to the Lockheed AH-56, which was leagues above the S-66, both literally and figuratively, and was selected for further development by the US Army over the S-66.

The AH-56 decimated the Advanced Aerial Fire Support System specifications, with a dash speed of over 400 kilometres per hour, a ferry range of 3,900 kilometres, and was able to hover at altitudes of 6,000 feet, or 1,800 metres.

Furthermore it featured a retractable landing gear, wings that spanned 27 feet, and was powered by a general electrics T64-GE-16 turbine engine mated to a four bladed, gyro stabilised rigid motor that produced 3,922 horsepower. It had a tail mounted anti-torque rotor, in line with basically every helicopter ever made, but complemented this with an additional pusher propeller also mounted on the tail boom. This pusher propeller was also reversible, allowing the AH-56 to decelerate and brake during vertical dives, allowing the pilot to enter a dive, then reverse thrust on the pusher to slow the aircraft down considerably, allowing him plenty of time to fixate on the target, fire and then start his recovery.

During vertical and hovering flight all power was sent to the main rotor and the torque rotor on the tail boom, giving flight control relative to a normal helicopter, but upon transitioning to level flight all but 700 horsepower from its mighty power plenty was sent to the aforementioned pusher propeller. This made the main rotor all but redundant for generating any lift beyond residual wind milling, but this was in turn off set by lift generated by the craft’s short wings.

What’s more, while the AH-56 may have had familiar flight control during vertical and hovering flight, its performance was anything but familiar, with Lockheed’s advanced fixed gyro stabilised rotor design giving it performance that is arguably unmatched to this day. The lack of hinges in its main rotor design meant that that airframe would follow the rotors directly, with no delay or inaccuracy, making it incredibly intuitive to control. The gyroscopic control also meant that the AH-56 was immune to all but the most extreme changes in its centre of gravity – being able to handle offset loads of up to 5 metres without any difficulty in control or input responsiveness.

The AH-64’s revolutionary performance was complemented by similarly exceptional weapons systems. It’s primary armament was a fearsome 30mm XM140 autocannon, mounted in a belly gun pod which gave a full 360 degree traverse, with advanced armour piercing rounds, this gun alone more than had the capabilities to ruin the day of anyone unfortunate enough to come into the gunners crosshairs, and in what was probably a big brain move – mechanical stoppers were added to prevent the cannon from coming in line with the rest of the aircraft.

Complementing the XM130 cannon was a front nose turret carrying an additional secondary weapon: either a 40mm XM129 grenade launcher, or 7.62mm M134 Minigun with 100 degrees of traverse either side of the centre line.

The aforementioned lineup is all well and dandy of course, and no doubt would have had any infantry or lightly armoured targets reaching for the toilet paper upon the AH-64’s arrival into a hot zone, but what about tanks? How would the AH-64 fend off the vast armoured columns of the Red Army, that every defence analyst of the time predicted was imminently about to pour through Checkpoint Charlie. Lockheed answered this question with six hardpoints mounted on the AH-56’s short wings, to which could be fitted either six TOW missiles or 2.75 inch rocket pods. 

These hardpoints were also compatible with a range of gun pods, all further increasing the AH56’s lethality towards soft and lightly armoured targets. These gun pods seem to be pre-existing designs bought of the shelf and not designed specifically for the AH56: such as the SUU-11a 7.62mm minigun pod, the SUU-12a .50 calibre gun pod, or the XM-13 40mm grenade launcher pod.

To further complement this terrifying complement of weapons, the AH-56 was fitted with an incredibly advanced weapon sighting system consisting of night vision equipment and aiming cameras that all linked back to a helmet mounted sight for the gunner. This early helmet mounted sight was further complimented by the gunner’s seat being fitted with a full 360’ rotation – wherever he looked, the guns looked.

Furthermore, a fire control computer took the information from the gunners helmet mounted sight – and through the use of two gyroscopes connected to a doppler radar kept all weapons on target. For missiles the computer compensated for wind, and the movement of the target on the ground, meaning that the gunner could ‘fire and forget’ and not have to adjust the missile’s trajectory mid-flight. For the guns the fire control computer automatically compensated for bullet drop and wind.

Thanks to the stability of its rigid-rotor design, and the computer controlled weapons the AH-56 was also exceptionally accurate – with the 30mm cannon consistently being able to hit a 25cm target at 3 kilometres.

To say the top brass of the US Army was enthusiastic about the AH-56 Cheyenne would be a gross understatement, and following our review of its features its not exactly hard to see why – remember that attack helicopter technology of the 1950’s and early 1960’s otherwise essentially amounted to inaccurate rocket pods jury rigged to the side of slow and crude transport helicopters. 

The US Army took to the AH-56 Cheyenne like a pig to muck, and following an initial order of 10 prototype units in 1965, placed an initial production order for 375 aircraft in January 1968.

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Fall from Grace

All seemed rosy for the AH-56, Lockheed had produced a simply phenomenal aircraft years ahead of its time, and the military money men who paid Lockheed’s bills seemed to agree that the AH-56 was indeed the best thing since sliced bread, but alas, a happy ending for the AH-56 was not meant to be. Not even 3% of that initial order of 375 would be produced, with a mere 10 airframes leaving the assembly line before the project was ultimately canned, and this phenomenal aircraft was reduced to nothing but a curioso relic, gathering dust in the corners of aviation museum, as a monument to something great that could have been.

As we briefly discussed at the start of today’s episode, the United States Airforce HATED the AH-56 Cheyenne. A cabale of senior officers perceived the development of the AH-56 as a most grave and personal insult – they hid their emotional knee jerks behind various legitimate and professional facades, but when you go back and read primary documentation from the time, it becomes quite apparent that the Airforce Officers involved were little more than petulant children throwing their dummies from their prams because they were envious of another child’s toy.

This at face value may sound ridiculous, Airforce officers of the time were intelligent and well paid thinking men surely? They cared about the defence of their nation and people above all else right? There’s no way they would sabotage game changing hardware just so their branch of service could beat its chest and have the nicest toys? Of course they would! They were human, and as tribalistic, and inclined towards prejudice as you or me, regardless of how many medals uncle sam slapped across their chest.

One of the key arguments used by the anti AH-56 lobby was the 1948 Key West Agreement. This agreement, colloquial shorthand for the Functions of the Armed Forces and the Joint Chiefs of Staff affirmed the role of aviation within the American Navy, Army, and the then newly created air force, giving clear roles and responsibilities to each of the aforementioned services air arms.

To simplify somewhat, this agreement meant that the army could use rotorcraft for troop deployment and logistics, but that close air support would be provided by fixed wing aircraft from the air force.

The air force argued that the advanced nature and capabilities of the AH-56 made it overlap into their close air support role, and as per the agreement, such overlaps were meant to be avoided in order to prevent the wasting of resources. They pointed to the Douglas A-1 Skyraider they currently had in service, and the Fairchild Republic A-10 Thunderbolt they had in development as proof that close air support needs were already being adequately met by their aircraft.

The Department of Defence launched a study to investigate the Air Forces claims, the study compared the AH-56 Cheyenne from the army, the prototype A-10 from the airforce, and finally the McDonnell Douglas AV-8B Harrier from the Marines, as the airforce was also moaning about that one. This study, published in 1973 concluded that all three aircraft were so radically different, that there was no overlap in their roles; citing the Harrier’s VTOL ability as an essential component of carrier operations, the A-10’s armoured frame as an essential component of close air support operations, and the AH-56’s ability to hover and linger, and precisely hit multiple targets with its advanced targeting computer as something unable to be offered by the air force’s aircraft inventory.

But of course, this study did little to quell the anti AH-56 lobby in the air force.

One Air Force general who was particularly zealous in his crusade against the AH-56 was one General William Moyer. Moyer stands out in terms of his egregious conduct as more so than any other air force general who opposed the AH-56, he was willing to lie and slander in order to see his prejudices satisfied. This is typified by his testimony before the senate on the twenty second of October 1971, in which he used exaggerated, and out of context claims about the role of the helicopter in the failed Operation Lam Son 719 to undermine not just the AH-56, but the very concept of the attack helicopter.

As if the political attacks from the airforce weren’t bad enough for the AH-56, it also had to contend with one of the most frustrating components of American military procurement, corruption! Sorry, I mean, LOBBYING.

This LOBBYING came from Bell, manufacturers of the AH-1 Cobra attack helicopter, which was first procured by the united states army in 1967, as a cheap and cheerful interim measure to tide the army over until more advanced and sophisticated designs could be put into service. MYSTERIOUSLY many senators and congressmen, who were previously very in favour of an advanced, highly sophisticated attack helicopter, moved towards favouring the cheap and cheerful AH-1 Cobra after dinners and outings with Bell executives.

But while we know there was political meddling involved in the cancellation of the AH56, as ever there is two sides to every story – and as we have seen before in similar posts discussing political meddling in aerospace and defence programs, often there are very genuine flaws and downsides to a seemingly ground breaking design, that complicate the story beyond simple political meddling, and we as historians are left trying to decide which factor was more important in killing the design; the flaws simply being used as a weapon by partisan and prejudiced actors determined to kill a project, or these flaws being serious enough to genuinely undermine the viability of a seemingly otherwise world breaking project.

The AH-56 Cheyenne was no exception to this, and the project did have some very real flaws, regardless of ground breaking the overall design may have been.

One of the more pressing concerns was the occasional tendency of the rigid rotor motor to unintentionally vibrate once per two main rotor revolutions in certain conditions – dubbed the “half p hop”. This vibration, and the jolt it would send throughout the entire airframe could cause the pilot to put too much input onto the control stick and unintentionally manoeuvre the aircraft.

Normally this was an inconvenience: throwing the gun off target, or simply requiring the crew to change their underpants after the unexpected jolt, but tragically, this flaw proved fatal on the 12th of March 1969, when the rotor on prototype number 3 struck the fuselage during a “half p hop”, this sheared the rotor from its rotor mast and sent the craft into an unrecoverably dive, killing the test pilot David Beil.

This tragic accident was an absolute godsend for the US Airforce, who fully exploited the incident to help build their case about the AH-56’s unviability, but there is more nuance to the accident that shows the story really is not that simple.

Firstly, the test flight was specifically being conducted to assess the full extent of danger presented by the “half p hop”, and accordingly safety measures normally in place were removed for the test flight, and David Beil was on orders to go out of his way to repeatedly induce the problem to its fullest extent – so the accident, although certainly tragic, was hardly representative of the real world danger posed by the issue.

What’s more, data gathered from the subsequent crash investigation led to the problem being fully eliminated. So in light of the fact this problem was solved before the project’s cancellation, it’s hardly damning evidence of a fatal flaw in the AH-56.

But oh so often in military procurement, when ego and tribalistic chest beating gets involved the technical merits of an aircraft, and the hard work of its designers counts for very little, compared to the spin that can be sold to congressmen, whose pampered and callous free hands know little of the true realities of the military.

The Army tried to fight back, and to display the merits of the AH-56 Cheyenne over its competitors. In January 1972, a special army task force under General Marks reevaluated the requirements for an attack helicopter. Marks’ task force conducted flight evaluations of the AH-56, along with two industry alternatives for comparison: the Bell 309 King Cobra and Sikorsky S-67 Blackhawk. Analysis of the three helicopters determined that the Bell and Sikorsky helicopters could not fulfil the Army’s requirements.

So now we arrive at the pivotal moment. We know all about the advanced specifications of the AH-56, as well as its advocates and detractors. So now we have to ask the question, was the spin of the anti AH-56 detractors good enough to kill the project?

Yes.

Yes it was.

The lobbying eventually proved too much, and the Cheyenne program was cancelled by the Secretary of the Army on 9 August 1972. The AH-56 Cheyenne, the hyper advanced helicopter that once sat poised to revolutionise the aviation world, found itself either confined to the scrapheap, or shipped to various museums across the US, where they sit to this day gathering dust, as failed testimonies to what could have been. A truly sad end for such a phenomenal aircraft.

Closing Thoughts

We have only been able to scratch at the surface of the real story today, but fortunately the rise and fall of the AH-56 has been written about extensively by historians, and there exists a plethora of great materials available for anyone wishing to study the AH-56 further. In particular the author of today’s post would firmly recommend Interservice Rivalry and Air Power in the Vietnam War by Professor Ian Horwood, a personal acquaintance of the author of today’s post, and a fantastic academic who knows the AH-56 inside and out.

Should you in the audience find yourselves particularly enamoured with the subject of today’s post and wish to see one for yourself, up to four airframes were spared the suffering of the scrapyard, and remain on public display to this day. They can be found at the Fort Polk Military Museum, Louisiana, the Fort Rucker Army Aviation Museum, Alabama, and the Don Pratt 101st Airborne Museum, Fort Campbell, Kentucky. Fort Rucker also holds a second air frame, but I am finding clashing reports on whether or not it is still on public display, so take this one with a pinch of salt!

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