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Roland Sachsenhofer
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Innovative, superior, ahead of its time- and rejected: the Lockheed AH-56 Cheyenne

April 28, 2022 · in Aviation · · 26 · 1.3K

The original: Lockheed
Some aircraft are astonishing - and the AH-56 Cheyenne undoubtedly belongs to this category. The appearance of this aircraft is astonishing, the flight performance amazes and the short but intense development history of this "Advanced Aerial Fire System" is no less surprising. Last but not least, some people are still amazed by the fact that it was not Lockheed's AH-56 Cheyenne but Bell's AH-1 Huey Cobra that won the race and was put into mass production by the US Army.

Today's observer can only guess at the revolutionary modernity that Lockheed's design exuded at the time of its presentation; we are already too used to the arrangement of the two-headed crew in the form of a gunner in front and a pilot in an elevated position behind. The slim nose section, which at that time was only known from fast fighter planes, is no longer surprising. However, the AH-56 was to be one of the first designs with which this layout was presented, tested and finally introduced.

The designers at Lockheed had even further plans with this concept. For example, the gunner was to be placed on a swivelling seat that would automatically follow the orientation of the guns mounted in two pivoting turrets. The gunner's gaze was thus always directed at the intended target. The nose turret would carry either a 40mm grenade launcher or a 7.62mm minigun, while the lower turret would carry a 30mm automatic cannon. The Cheyenne also offered the possibility for pilot and gunner to engage two targets in different directions simultaneously with both turrets.

The T64-GE-16 turbine from General Electric, which delivers over 3900 hp and is still widely used today to power heavy helicopters, provided the Cheyenne with a truly enormous power potential. On the one hand, it powered a four-blade rotor of innovative design, whose control was based on the principle of gyroscopic precession. However, the new rotor design also represented one of the technical weak points that would ultimately bring the project down.

In March 69, the third prototype, one of ten Cheyennes built, crashed due to an uncontrollable oscillation of the rotor blades. From the escort plane, it was observed how, in a matter of seconds, the rotor blades deflected further and further until they finally hit the rear fuselage and the cockpit. The pilot, Lockheed test pilot David A. Beil, died instantly.

The high-horsepower turbine, however, not only drove the pair of main and tail rotors common to helicopters, but also a pusher propeller located at the extreme end of the tail boom. This propeller was an essential feature of the Cheyenne: driven by its thrust, the construction reached a top speed of 393 km/h, which was unattainable for conventional helicopters until then. In this speed range, the main rotor only contributed about 20 percent of the lift, 80 percent was contributed by the two wings arranged in the middle of the fuselage. The fact that the Cheyenne could not claim the speed record for helicopters at that time had one reason alone: the AH-65 Cheyenne is not a helicopter! Its special design makes it a so-called "combination helicopter". This puts it in a class of aircraft to which such well-known exotics as the Fairey Rotodyne or the currently produced V-22 Osprey belong.

However, the tail propeller also served another purpose linked to the Cheyenne's planned operational use: the US Army's tender for an "Advanced Aerial Fire Systems" required, among other things, the ability to take direct aim at a target in a steep dive while taking fire. One problem with this attack method is the rapidly building speed, which, among other things, shortens the time needed to effectively keep the ground target under fire too much. The tail propeller solved this problem because, used as a dive brake, it could keep the speed of the Cheyenne plunging towards the target constant within a range chosen by the pilot.

The wings were also used to mount various missiles and weapons. A wide range of external armament could be mounted at two stations under each wing and two more under the fuselage. BGM-71 TOW anti-tank missiles were planned as well as 70mm rockets in 7 or 19 launchers. The six weapon pylons were also prepared to use external drop tanks. In total, more than 5.4 tonnes of external ordnance could thus be carried.

The performance of the ten test aircraft is still impressive today: in addition to the aforementioned 390 km/h top speed, there is the ability to fly 37 km/h in reverse and 46 km/h sideways. The climb rate was 6.5m per second, the service ceiling being reached at an altitude of 6,100 metres. The range specification is also interesting: 1,970 kilometres. Empty, the Cheyenne weighed 5,500kg, fully loaded 13,600kg. By the way, the special features of the Cheyenne made otherwise impossible flight movements possible. For example, the Cheyenne took off in a completely straight line with respect to the ground, and it could also touch down from a hover with the tail wheel first on the ground - all things that are impossible for a conventional helicopter.

The innovations described here and the resulting impressive performance spectrum were, however, paid for with a whole bundle of technical difficulties, which in the end were to lead to a premature end for the entire project. While the Lockheed AH-56 had been selected alongside Sikorsky's S-66 as the winner of the Advanced Aerial Fire Support System (AAFSS) tender in 1965, the initial enthusiasm was quickly curbed by a series of rapidly escalating technical difficulties. Issues included unacceptable instability in flight at low altitudes, uncontrollable rotor vibrations and a maximum weight that exceeded requirements. Over time, the US Army lost confidence in Lockheed to solve these problems in a timely manner. Against the backdrop of the fatal crash of March 1969, this led to the cancellation of an original order for the production of 375 examples of the AH-56 Cheyenne.

However, development work was continued for the time being after production had been halted. In 1972, however, Lockheed's ambitious helicopter project came to a definitive end: technically, the AH-56 Cheyenne was slowly becoming obsolete, and work on it was finally discontinued in favour of two new weapons systems: the A-10 Thunderbolt II and the new AH-64 Apache attack helicopter had made the AH-56 obsolete.

About the building process
Not only the description of the original demands superlatives, these are also necessary for the building process! For example, the parts found in the box from the manufacturer certainly belong to the most hair-raising kit I have ever dared to build. In view of the few puny individual parts rattling around in the thin-walled, unstable cardboard box, my courage and motivation were about to leave me. Even the information that the underlying kit of the AH-56 Cheyenne had already been brought to market in 1968 brought no consolation - apart from the realisation that it and I share the same year of construction.

The actual build proved that a certain courage is indeed appropriate: the yawning emptiness of the clearly viewable cockpit is only disturbed by a few parts whose design has nothing to do with the prototype. Accordingly, I requisitioned a lot of etched parts from the leftover box, which would provide for an appropriate density of different detail shapes and thus hopefully also for an overall impression that is at least close to the original. The centre console with the pilot's instrument panel was shortened in height and redesigned in shape.

A massive and not to be negated problem can be found in the clear part of the cockpit canopy. A massive ejector mark is emblazoned in the middle of the windscreen! After some deliberation I decided on a "surgical intervention", the windscreen was removed by means of a drill and saw and replaced by a suitable piece of clear film. To my own surprise, this worked very well straight away.

I worked intensively on the rotor head, the kit parts are mostly pure fiction and simply not usable without massive modifications. In order not to let the description get out of hand, I will summarise a long process: The central rotor disc as well as the rotor blades were sawn to size and brought into a reasonable shape with a lot of sandpaper, almost the entire rest of the complex rotor construction was then created from "scratched" metal and plastic parts.

The undercarriage was redesigned in such a way that I cut the main undercarriage legs apart and then subsequently inserted two shiny metal rods as struts in each of the four holes I had drilled in the two halves of the undercarriage legs. The tail wheel was also completely rebuilt. I was also able to hollow out the lower part of the tail fin that accommodates this wheel, which greatly improved the overall appearance. The wheels themselves are from a resin set for the CH-53; to me they seem to fit very well in shape and size.

Time and energy was also put into repairing the panel lines, which were far too deep and also incorrect in their shape. The first act was the most important one: at the beginning I filled all the trenches with putty and after drying I sanded them over. After that, selected panel lines could be quite comfortably drawn into the plastic.

Finally, I would like to mention that the enclosed decals were surprising: they were easy to use! I only added some maintenance instructions, the material for this came again from the fundus.

The project for the AH-56 Cheyenne was one of my most exciting modelling experiences. One of the experiences is that it can be a real pleasure to lower the standards of authenticity and perfection once in a while and to dare to build a kit that confronts you with previously unknown challenges. Well, I'm glad I rode the beast!

Reader reactions:
16  Awesome

20 additional images. Click to enlarge.

26 responses

  1. Great subject! Haven’t seen that kit in almost 50 years.

  2. That’s amazing work!

  3. Great work Roland. I do believe I recognize the source of that front instrument panel. 🙂

  4. wow - that is an interesting (and until now, unknown to me) subject! Nicely done!

  5. Really stellar work on bringing an unusual and obscure subject to a beautiful finish!

  6. Amazing work, and article

  7. Nicely done Roland, I indeed remember reading about this helicopter and the Army's pursuit of a genuine attack chopper.

  8. Excellent build, Roland @rosachsenhofer
    Background story is very informative as well.

  9. Amazing result of of the (at places) simplistic/crude/inaccurate oldie of a kit, Roland!
    Equally amazing writeup!

  10. Wow! Color me impressed Roland! ?

  11. Nicely done! The videos that can be seen on the web about this craft are impressive.

  12. Man, you made that toy into a serious model! Great work!

  13. Excellent build Roland. If you haven't sent a link to Atlantis you should. They absolutely love seeing their kits build up beautifully like yours. Well done Sir.

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