Lockheed Cheyenne AH56A

A historically unique rotorcraft that also 'unloaded' its rotor

The Lockheed Cheyenne
AH-56A Prototype

Here are a variety of pictures of the Lockheed Cheyenne AH-56A, an experimental Helicopter from the 1960s-70s that partially unloaded its rotor and also used a pusher prop.


AH-56 was designed to fit the requirements of the US Army's Advanced Aerial Fire Support System (AAFSS) program (1964-71). The helicopter won the competition and the first prototype flew (unofficially) in September 1966. AH-56 was expected to enter service in 1968, but the program was terminated and the helicopter never advanced beyond prototype stage.

Lockheed created a flying model with a 5 foot diameter, two bladed main rotor with a fixed collective pitch control. Power came from a single McCoy 60 model airplane engine which turned the rotor through a combination belt & gear drive with an 8:1 reduction ratio. The model helicopter proved very stable and easy to fly. The concept had been proven and now it was time for a full scale aircraft.

Of interest to modelers is the fact that a radio-controlled model of this helicopter was flown in 1958, making it one of the very first successful model helicopters, if not the first. The model weighed 11 pounds with a 5-ft. diameter main rotor, 2-blade and constant 2 1/2" chord. A McCoy 60 drove the main rotor at 1012 rpm. The radio was a 8-channel Citizenship reed outfit with 6 channels being used - two for pitch, two for roll and two for throttle. A large "gyro" ring was later added under the rotor. The model was flown extensively at Sepulveda Basin, Los Angeles.

While the AAFSS was won by the Lockheed AH-56A Cheyenne, Bell had entered a scaled-down version of it's Iroquois Warrior and the other competitor was the Sikorsky (S-66) (1964) which looked similar to the AH-56A Cheyenne, but had a Rotorprop tail rotor which could rotate on it's axis throught 90degrees to act both as an anti-torque rotor or as a pusher, thereby transforming the S-66 into a compound aircraft in cruising flight.

Lockheed officially rolled-out the first prototype on May 3, 1967. The rigid-rotor Cheyenne, with a crew of two, featured a XM112 swiveling gunner's station linked to rotating belly and nose turrets, and a laser range-finder tied to a fire control computer. It was armed with an XM52 30mm automatic gun in the belly turret and a XM51 40mm grenade launcher or a XM53 7.62mm Gatling machine gun in the chin-turret, TOWs, and XM200 rocket launchers. Ten prototypes were completed before the program was terminated August 9, 1972 due to delayed development, rising costs, and the appearance of the two competitive company-funded initiatives by Sikorsky and Bell. The Army wanted a smaller, more agile Advanced Attack Helicopter (AAH) with a less complicated fire control and navigation system. The helicopter's mission would eventually be assumed by the Boeing (formerly McDonnell Douglas) AH-64 Series Apache attack helicopter. The Cheyenne had a single rigid four-bladed main rotor and anti-torque tail rotor, and a three-bladed pusher. The Cheyenne was powered by one General Electric T64-GE-16 3435 shp turbine engine. The AH-56A had a speed of 246 mph (214 knots).

Metric Data for AH-56A

Test Flight :

A very early test flight.

Test Flight :

Shortly after this pic was taken, one of the experimental AH56A craft experienced a rotor blade control failure. A blade sliced thru the pilot compartment (back seat) killing the pilot.

Flying Pic :

Note the normal tail rotor but note there is also the pusher prop of about the same diameter, alongside it.

The Cheyenne rotor system

The Cheyenne had a most remarkable (if not the most remarakable) rotor system. The techniques employed for tilting the rotor were (and remain) unlike anything ever devised.

If you look at the pictures you will see what amounts to a simple gyroscope just above the rotor. This gyro being the 4 'weighted arms' that were a single unit mounted on a gimbal and which spun with the rotor. This gyro was used for two purposes :-

Inside the body of the helicopter was the equivilent of a 'swash-plate', the inner ring had 4 control rods that ran up the inside of the rotor shaft - in the picture you will notice that the rotor shaft is short and quite thick - the control rods were directly coupled to the gimbal mounted gyro.

To explain this part of the control system, picture a conventional style swash-plate at the bottom of the rotor shaft. It has an inner and outer ring just like any swash-plate does. At the top of the rotor shaft, picture the gyro-gimbal. The gyro-gimbal in this example resembles a swash-plate in that it can tilt in any direction but, unlike the swash-plate, it has no seperate inner & outer rings that turn independantly. So the gyro-gimbal can tilt but will only spin with the main rotor shaft.

When spinning, the weighted arms become a gyro and of course this gimbal will now resist any tilting force at the point it is applied but will tilt 90 degrees further round in the direction of rotation, and this tilting will be in proportion to the continued force being applied at the original point.

So while the rotor was not spinning, if you were to tilt the lower swash-unit the gyro at the top would tilt in exactly the same directions, it would track the lower swash-plate exactly, but while the rotor was spinning, gyroscopic force would take over and prevent tilting both the upper gyro-gimbal and the lower swash-plate coupled to it, at the point the tilting force was being applied.

This is due to the tilting force being resisted by gyroscopic-precession from the upper gyro-gimbal. Of course this resistance is not there when the rotor (& thus gyro-gimbal) is not spinning.

The amazing part of this whole assembly then is what happened with the lower-swash unit and the upper gyro-gimbal it was coupled to, when the rotor was spinning, and also what actually happened when a tilting moment was applied to the lower swash-plate. The bit we are missing to this point is how the internal tilting control rods that go from the pilot's servos to the lower swash-plate outer ring were physically connected.

Remembering that there are 4 control rods inside the rotor shaft linking the inner race of the lower swash-plate to the upper gyro-gimbal, we now add 4 more control rods, but these go from from the pilot's cyclic servos to the lower swash-plate outer-ring and are connected to it with spring loaded rods. Because of the springs, each pair of these cyclic servo control rods could move against the swash-plate outer ring and and apply a force without the swash-plate having to tilt at the point the spring pressure was applied.

While the swash-plate would not actually tilt at the points the force was applied, the force or spring pressure was still transmitted up the rotor-shaft control rods to the gyro-gimbal at the top which was of course providing powerful resistance (due to gyroscopic-precession) to the applied force.

Being a gyro, the gyro-gimbal would tilt 90 degrees further round in rotation. So the gyro-gimbal at the top precesses and then because of the direct links back down to the lower swash-plate unit, the swash-plate unit would then be forced to tilt in the same direction as the upper gyro-gimbal and this was possible because the lower swash-plate could force itself against the springs on the spring loaded cyclic servo control rods.

Also the upper gyro-gimbal's links to the rotor blade pitch horns, would cause the blades to change pitch. As it was a rigid rotor system and taking into account gyroscopic precession forces, the change in blade pitch would cause the whole craft to tilt 90 degress later than maximum (and minimum) pitch.

So if we look again at the chain of events needed to tilt the craft forward it was ...

that the lower swash-plate would have a moment or 'force' applied at its' rear from the spring loaded rods connected from the pilot's servos to the outer, non-turning part of the lower swash-plate. Because the swash-plate was directly coupled to the upper gyro-gimbal, it would follow the behaviour of the upper gyro-gimbal and would not tilt with the applied force but resist it. The springs pushing on the swash-plate outer ring would compress as the pilot pushed the cyclic stick forward.

The spring pressure force would in turn be 'transmitted' up through the control rods that are inside the rotor-shaft and apply the force at the *back* of the upper gyro-gimbal. The upper gyro-gimbal then behaved like any gyro and precessed 90 degrees later and this of course caused the other pair of internal control rods in the rotor shaft to push down to the lower swash-unit at this later 90 degree position.

At this position the lower swash-plate would tilt. To do so the lower swash-plate outer ring would push against the springs on the other pair of servo to swash-plate control rods.

Restated, because the pilot's servo to swash-plate control rods were spring loaded, the springs that were 90 degrees further from where the original spring load or 'force' was being applied, would 'give' under the precessing push coming back down thru the internal rotor control rods from the upper gyro-gimbal unit.

The pilot, by pushing forward on the cyclic stick applies a spring loaded force up at 180 degrees & down at 0 degrees on the outer-ring of the lower swash-plate. The force is 'transmitted' up the rotor shaft through the control rods inside it that couple the swash-plate to the upper gyro-gimbal, the gyro-gimbal precesses and as it does so, does 2 things, one it directly tilits the rotor blades, and 2 pushes back down thru the control rods in the rotor shaft, to tilt the lower swash-plate in unison with itself. The lower swash-plate can tilt at this position because it is able to push against the other pair of spring loaded servo-to-swash control rods that are at the 90 & 270 degree positions.

Thus in final summary, the tilting push began 180 degrees before the actual tilt occurs as to tilt the helicopter forward, the pilot's cyclic servos apply a spring-loaded force at the 180 and 0 degree positions on the outer non-turning ring of the lower swash-plate. The swash-plate, because it is coupled to the upper gyro-gimbal physically tilts at the 90 degree (port) & 270 degree (starboard) positions. The upper gyro-gimbal which through gyroscopic precession, caused the 90/270 tilt, also applies maximum and minimum pitch to the rigid rotor blades at the 90 & 270 degree position - because the craft has a rigid rotor and due to the effects of gyroscopic precession, the whole craft tilts at the 0 degree (nose down) and 180 degree (tail-up) positions. The tilt of the craft is totally due to gyroscopic precession. This of course is not what tilts a rotor system that is not rigid, i.e. has free teetering blades.

Doug Marker Feb 2000 (with info obtained back in the 1970s from one of the original AH-56A designers)

Created: 01 Feb 1999 - Updated: 12 Feb 2000