In the 1960s and in response to a competition run by the US Army to find an Advanced Aerial
Fire Support System during the Vietnam war, Lockheed, in their first major foray into
building helicopters, created possibly the best attack helicopter of the 20th century.
Ten prototypes were built and tested, these could fly faster than any existing helicopter
and even by today's standards its still one of the fastest helicopters ever made. It could
perform manoeuvres no previous helicopter could do and had a weapons system that was
well ahead of its time. The US army liked it so much they authorised the building of
375 as part of a contract that would eventually lead to over 3,300 being supplied but in the
end, none would go into production and the whole project would end up being cancelled,
leaving just a few of the prototypes to end up in museums.
Many still consider the Lockheed AH-56 Cheyenne to have been a better helicopter than the
one that would ultimately be built over a decade later, the Boeing AH-64 Apache, so
how did something so advanced and with so much potential end up as just another museum
exhibit. The
story of the AH-64 Cheyenne starts back in the 1950s when engineers at Lockheed were
looking to make a small easily controlled helicopter, they had an idea about using a
rigid rotor which they thought would make it more stable and easy to fly. Now to understand
this we need to know how a helicopter rotor works and some of the issues they have.
In a normal helicopter rotor setup, each of the blades acts like the wing of an aircraft
to give it lift but unlike a propeller that flies at 90 degrees to the direction of travel
a helicopter rotor flies almost edge on to the direction of travel.
If we have a helicopter with 2 rotor blades for example and it's in a normal stationary
hover, both the rotor blades create an equal amount of lift but when it transitions to
forward flight the airspeed of the advancing blade increases to the speed of the rotation
plus the forward speed, this creates greater lift for that blade, however, the airspeed
for the other retreating blade now does the opposite and its airspeed is the rotational
speed minus the forward speed which lowers it lift.
This lopsided lift causes the helicopter to tip to one side but due the gyroscopic precession
effect of the rotors, the lift of the blades is shifted by 90 degrees causing the helicopter
to tip upwards at the front rather than to one side.
To get around these issues the rotor blades are often hinged at the root and allow them
to move up and down or “flap”, this offsets the differing lift of the rotor blades as
they go from an advancing blade to retreating one to stabilise the system but this additional
flexibility means that the hull of the aircraft cant follow the rotor hub easily and it has
less control power. The Lockheed engineers theorised that if the
rotor blades were fixed to a rigid rotor with no hinges and the blades were flexible and
strong enough they could be more responsive and stable. They also thought that if the
angle of the rotor blades were fixed to a control gyroscope, effectively a large ring
under the rotor and connected to the rotor control arms, it’s the gyroscopic properties
would automatically stabilize the rotor until further control inputs were received from
the pilot. Models built with a rigid gyroscopically controlled
rotor proved the concept and a larger test craft, the CL-475 was built. Because the rotor
was hingeless the hull would follow the rotor and made the CL-475 not only very easy to
fly but also to be extremely stable, even when the centre of gravity was changed dramatically
with a man perched on a boom a couple of meters to one side, something that would have made
a traditional helicopter unstable was handled with ease by the CL-475.
The US army was impressed by the ridged rotor concept and set up a new project to research
its capabilities. A new bigger design, the XH-51 was built and tested which proved the
craft had fixed-wing flying capabilities and it could handle even greater offset loads
like this with a man on a 4.8-meter boom to one side and sudden changes in the centre
of gravity were handled with ease. It was also was found to be very fast reaching over
320km/h and could perform aerobatics more associated with fixed-wing aircraft rather
than helicopters. To further expand on the speed and stability
in 1963 Lockheed added wings and jet engine to create a compound helicopter, a sort of
half helicopter, half fixed-wing aircraft. The idea behind this was to address the problem
of retreating blade stall. The faster a helicopter flies in a forward motion the less effective
the retreating blade is at making lift to the point where it no longer is effective
at all. By using small stub wings and a jet engine,
at high speed, the wings provided the lift and the jet provided the forward thrust, this
virtually removed the need for the rotor blades to produce lift. The helicopter then became
effectively a fixed-wing aircraft that was controlled by the rotor blades.
Its top speed eventually increased to over 480km/h and in very low altitude attack style
tests often just 10 meters above the ground at speeds of upto 400km/h it was found to
be very difficult to track by trained observers and showed that the compound helicopter concept
could offer almost the best of both helicopter and fixed-wing aircraft in one unique package.
This was something that the US army was very interested in, they had been looking at arming
helicopters since the 1950s and by the time of the Vietnam war they had modified various
types to carry machine guns, cannons and rocket launchers. Helicopters had became the primary
methods of carrying troops and equipment to and from the battlefields but they were vulnerable
to ground fire and even though they had armaments they weren’t designed to carry larger qualities
of men and guns, this made them slow and difficult to manoeuvre. The main battlefield helicopter
the Bell UH-1 or “Hewy” often couldn’t keep up with the troop carriers they were
assigned to protect. The Army needed a fast, heavily armed Aerial
Fire Support System and in 1966 they setup up a competition to create a new class of
attack helicopter capable of taking on this role. 11 companies entered designs, but it
was a new design from Lockheed that built on the knowledge gained from the XH-51 that
was chosen. This would become the AH-56 Cheyenne, an aircraft
four time the size XH-51. As part of the US Armies requirements it had to have a dash
speed of over 400km/h, a ferry range of 3900 km virtually the distance from New York to
Los Angeles and be able to hover out of ground effect at 6000ft or 1800 meters.
The jet engine was replaced by a pusher propeller at the rear which was also reversible so in
a dive it could be used to slow the AH-56 and enable it to do extremely quick changes
of direction, it could even fly backwards and then transition to rapid forward flight.
During forward flight, about 75% on the engine power which came from the 3900 hp General
Electric T64 turboshaft engine drove the pusher prop with about 25% going to the main rotor.
It wasn’t only the flying capabilities that were impressive but also the fully integrated
weapons system. The AH-56 had a twin cockpit for the pilot and gunner. The weapons that
could be fitted included a nose turret were either a 40mm grenade launcher or 7.62mm minigun.
A belly turret was fitted with 30mm cannon and 6 hardpoints on the wings which could
have a combination 70mm rockets and / or TOW wire-guided antitank missiles. In all, it
could carry over 3600 kg of armaments, making it a formidable weapons platform with some
likening it to a fly tank. The gunner's seat and aiming platform rotated
through 360 degrees and once the target was acquired with the laser range finder it would
remain locked on regardless of the movements of the aircraft. The pilot had an early helmet
operated aiming system, where ever he could look the guns would follow and for nighttime
operations, there was also an infrared site. A fire control computer took information from
the gunners aiming site and combined it with two gyros and a Doppler radar to keep the
target locked on even allowing for the winds at altitude and the ballistic properties of
the weapons selected. This meant that the gunner or pilot didn’t have to adjust the
aim after the initial firing. Between the pilot and gunner, they could select
separate targets and then allocate up to fire three separate weapons at different targets
all of which remained locked on target as the aircraft performed evasive manoeuvres.
The stability of the rigid rotor platform and the laser-guided computer-controlled weapons
produced exceptional accuracy with the first shot from the 30mm cannon consistently hitting
a 25cm target at 3km. This allowed the AH-56 to use the terrain
to its advantage, it could fly at treetop level, detect an enemy at a distance and lock
on with its weapons then duck down and move to a better position before making its attack
with the weapons already aimed and locked on.
There was also a reconnaissance version of the AH-56 proposed that could also use radar,
infrared imaging and electronic signal detection to home in on enemy radio transmissions. Once
the target had been located its position would automatically be transmitted back to the intelligence
centre via a data link and the pilot would relay a description of what he could see.
If the target was small enough the AH-56 could engage it with its 30mm cannon.
The information gathered could be used to direct other attack aircraft or artillery
to the target. In the case of artillery fire, the AH-56 could quickly fly to a higher altitude
to obtain metrological data with is onboard sensors which would then be automatically
sent by data link and relayed to the artillery battery to increase its accuracy.
The navigation system could also have the co-ordinates of multiple targets entered into
it at any time. The pilot could bring any of them up at will to obtain range and bearing
and with its exceptional ferry range, it could be sent to remote locations where there was
no ground support and it could wait until it was called for…. and don’t forget that
all this was in 1972. Many of the control and guidance systems developed
for the AH-56 would become the basis of weapons for decades to come and you can even see echos
of them in things like the helmet control for the Lockheed F-35.
But for all its advantages it had problems. One of the worst was that under certain circumstances
the rigid rotor control system could be made induce a half P hop, a vibration that occurs
once every two rotations of the rotor. This can cause the pilot's arm to increase the
movement of the control stick which is known as pilot induced oscillation. On one test
flight whilst investigating this issue, the oscillations became so bad that the rotor
blades hit the tail boom and the cockpit killing the pilot and crashing the aircraft.
The army stopped the program and issued a cure notice for Lockheed to fix outstanding
issues before production could start and although most of these were fixed other issues which
out of the control of Lockheed were coming to the fore.
One of these was the 1948 Key West agreement that determined what type of aircraft the
US Army could use for close aerial support and what had to be done by the US Airforce.
Generally, this meant that the Army could use helicopters for troop movement and supplying
equipment but the airforce would use fixed-wing aircraft for the close aerial support and
bombardment of enemy positions. The airforce said that exceptional abilities
of the AH-56 made it duplicate the role that the air force would have in close aerial support
and in particular the upcoming Fairchild Republic A-10 Thunderbolt which it had in development
and therefore this violated the key west agreement. Although a subsequent investigation found
there was no overlap, intense lobbying from Bell helicopters to increase the use the attack
version of the Hewy, the Bell AH-1 Cobra which had been introduced in 1969 and was eating
at the support for the AH-56, even though it had nowhere near same the capabilities
but it was significantly cheaper and therefore could be supplied in much greater numbers.
The use of the AH-1 Cobra in Vietnam also made the US army change its requirement from
high-speed to high survivability. One of the consequences of this was that a new attack
helicopter should have two engines instead of the one in the AH-56.
By 1972, virtually all of the outstanding problems had been fixed and the AH-56 Cheyanne
had proved its exceptional capabilities to combine the best of a fixed-wing aircraft
with that of a helicopter and a world-beating weapons system.
But for all that, during a firing demonstration at the Yuma proving range, a TOW missile missed
the target completely and hit the ground. Even though it was known that the TOW’s
had a failure rate of around 10% and over 130 had been fired in previous tests which
had all worked perfectly, this provided the final excuse to cancel the project once and
for all. In the end, the US Airforce got the close
air support aircraft it wanted in the A-10 Thunderbold and over a decade later the Army
got the AH-64 Apache but only after agreeing to have the speed limited to 160km/h slower
than the AH-56. Many supporters of the Cheyanne pointed out
although the cost of the AH-56 had risen to a much higher $3 million each compared to
the initial $1 million, the whole program was still much cheaper than the combined development
costs of the A-10 Thunderbolt and the AH-64 Apache and that the Cheyanne could have done
the job of both in one aircraft and been better at it.
Inter-service rivalry and the vested interests of well-embedded military suppliers like Bell
meant that the US missed out on an exceptional aircraft for decades and Lockheed which had
proved its design abilities pulled out of rotary-winged aircraft for good.
But the belief by many that the AH-56 Cheyanne was the right design all along increased in
2019 when Boeing released concept images of the Advanced AH-64 Block 2 Compound Apache,
a future attack variant with stub wings and a pusher propeller, they even admitted that
the configuration was similar to the AH-56 Cheyanne, just 50 years later on.