AH-56 Cheyenne

[tacks]

I`ve just read some articles about the Lockheed AH-56A program.

This tandem-seat helicopter was a main/ tailrotor configuration.
Additionally a pusher propeller on the tail and wings (wingspan 7.9 metres - about 26 feet) were fixed, which gave her a cruising speed of nearly 200 kts.

Obviously, at high speeds the whole thing worked out quite nicely.
But I´m thinking about her hover capabilities.
Was she efficient in stationary hover (prop plus fairly wide wings!)?
How about her behaviour at low speeds/hover (was it a feathered prop?).

[Lu Zuckerman]

To: tacks

The Cheyenne was very stable in a hover with minimal impact due to the wings. In a hover the propeller was in flat pitch and the driving power was absorbed by the tail rotor. The pilot would introduce cyclic pitch and the helicopter moved forward in a conventional manner. Once the helicopter was at speed the pilot had a beta control and he increased pitch in the propeller and reduced tail rotor pitch and now the propeller was absorbing the power. The pilot would lower collective and at this point the wings were providing the lift. In this situation the helicopter was an autogyro with flight control being provided by cyclic input.

The pilot could also place the propeller in reverse pitch and this would counter the forward thrust of the rotor system. This allowed the pilot to place the helicopter in an inclined attitude relative to the horizon making the helicopter a very stable weapons platform.

At high speeds the helicopter rotor system became unstable. This could have been caused by one of two things.

1: Autogyros (certain types) use the rotor system as a means of control and the rotor is at flat pitch. With the introduction of forward cyclic input the blades can go to negative pitch causing the blades to flap down and make contact with the fuselage. This is the situation with the Cheyenne and on two occasions there was rotor incursion and in one case the rotor disintegrated and caused a great deal of damage to a wind tunnel. In the other case the pilot was killed.

2: The rotor blades on the Cheyenne were very stiff as compared to a conventional helicopter blade. The Cheyenne blades were also different from other helicopter blades of that time period. At each blade station the aerodynamic shape of the blade was different which introduced a degree of instability. The changing of the blade cross section coupled with the blade stiffness effected the phase angle of the blade and the pilot would never know where the disc would dip down with cyclic input. This unstable blade condition may have been the cause of the two incidents mentioned above.

It took two years of design development by Parker Bertea and they finally solved the problem of variable phase angles. Although the design changes gave the Cheyenne a very stable and vibration free ride the design was too complex and it contained a large number of failure points that were catastrophic in nature.

The Army cancelled the program and opened bids for the new AAH. This led to the Apache contract won by Hughes.


:E :E

[NickLappos]

Tacks,
The whole thing really didn't really work out nicely, the helicopter had rotor instabilities at high speed that caused at least one crew to be lost during development. It was scrapped because these problems were never really solved.

The rotor used a unique control system that worked with a stabilizer bar type system, where picth changes were fed thru the gyro bar and then to the blade.

The payload was actually fairly small for its installed power, the wings caused a loss of hover performance (about 10% penalty for the vertical drag of the wing) and the tail thruster was a weight penalty as well.

[Lu Zuckerman]

To: tacks



The whole thing really didn't really work out nicely, the helicopter had rotor instabilities at high speed that caused at least one crew to be lost during development. It was scrapped because these problems were never really solved.

What Nick stated is true and mirrors what I had stated previously however the problems with the rotor instability were solved and according to test pilot reports the Cheyenne was the smoothest helicopter they had ever flown. It was that the solution to the problem was extremely complex and prone to catastrophic failure. The Army had made up their mind that they had already expended too much money for so little return. Also they had shifted their thinking relative to where the AAH would be deployed and also its’ mission. This evolved into the Apache.

The Lynx had a similar problem and the problem with correcting the phase shift was purely electronic where the system employed on the Cheyenne was mainly mechanical driving electrical sensors. The way it worked was when the pilot made a cyclic input the system would monitor which way the disc was moving relative to cyclic input and sent corrective signals to the primary servo which corrected the direction of flap.

The rotor used a unique control system that worked with a stabilizer bar type system, where picth changes were fed thru the gyro bar and then to the blade

This explanation is a bit simplistic. It is true that the system was unique and it was completely different from any other helicopter flight control system. Mounted on top of the main rotor was a gyroscope that was driven by a constant velocity joint attached to both the rotorhead and the gyroscope. The gyroscope was made of four tubes in a cruciform shape. Attached to the end of each tube was a heavy weight that looked like a large tin can. A stout cable connected the four arms and kept them rigid and restricted any in plane movement. The gyroscope was installed so that each arm extended between the blades so that they were 45-degrees from the two blades. The gyroscope arms also had a pitch horn the led the control arm by 45-degrees and attached to the blade

The gyroscope under rotation had the capability to exert a very high degree of force if it were nutated by control input. Being a gyroscope it would respond 90-degrees in the direction of rotation when a force had been applied to it. The control inputs to the gyro were pitch rods that ran through the main rotor mast. These rods were in turn attached to the swash plate which was mounted to the underside of the transmission. Pitch links ran from the stationary part of the swashplate to a servomechanism. The collective stick and cyclic stick drove the servomechanism via a mixing unit. The output of each servomechanism was connected to a horseshoe shaped spring, which was connected to the swashplate by the previously mentioned pitch link. The gyroscope had rigidity in space so when the pilot displaced the servomechanism it compressed the spring, which was resisted by the rigid gyro. This applied force causes the gyro to precess inputting a control force on the blades changing their pitch. With rigidity in space the gyro would hold the position causing the rotor system to align itself with the plane of rotation of the gyro. This allowed the Cheyenne to fly like a fixed wing aircraft and not like a helicopter.

Even though the gyro had a 90-degree phase angle the blades did not. Therein lay the problem.


:E :E

[46Driver]

Perhaps y'all can either confirm or deny this, but didn't the Air Force jumpstart the A-10 program due to their concerns that the AH-56 was greatly expanding the Army's aviation envelope, in particular CAS?

[Bronx]

http://img.photobucket.com/albums/v298/BronxNYC/AH-56Cheyennehoveringlow.jpg

http://img.photobucket.com/albums/v298/BronxNYC/AH-56Cheyenneinflight.jpg

http://img.photobucket.com/albums/v298/BronxNYC/AH-56Cheyennerear.jpg

[Lu Zuckerman]

One of the things that help kill the Cheyenne was that it had a very low degree of maintainability. It had the complexity of a Century series fighter and the Army mechanics were completely unable to maintain it. The Army did not learn its' lesson because the Apache was even more complex and it too had a very low degree of maintainability.

To: 46Driver

The Army and Air Force were constantly arguing about what types of aircraft the Army could fly and the missions performed by the Army that were normally performed by the Air Force.

:E :E

[NickLappos]

46Driver,

The Vietnam war taught us how effective a low-tech machine can be at close air support, and that jump-started the A-10. The most effective bomber for CAS was the A-1E, which carried several tons, went in slow and could hit the target, in spite of its big radial engine and oil-soaked windshield. Most fast-movers were more capable of area targets and less effective that the A-1, IMHO (I directed the CAS for a bunch of different type arcraft as a Cav pilot).

The Cheyenne was very long on PR and short on delivery of the goods. It didn't get defeated by politics, it left a few too many smoking holes.

here is a site that gets the program history right:
http://www.aiaa.org/tc/vstol/augmented.html

[46Driver]

Thanks for the info. This might, ok does, qualify as a thread hijack but do you have any info on "The CarterCopter". Saw it in the Navy Leagues "Seapower" magazine and then went to the webpage www.cartercopters.com I am curious to know y'alls perspectives on it.

[NickLappos]

46Driver,

I met Jay Carter at an FAA gathering and hear his talk. Lots of interesting stuff gong on at his shop, too novel to be assessed based on conventional helicopter methodology.

Looks like a difficult job ahead for Cartercopter as it pushes through the Mu-1 barrier (meaning when that retreating blade starts going into reverse flow). My hat is off to those like that team who push technology in new directions.

On the plus side, if it all works, high speed transport with short takeoff runs is possible. On the negative, if it works, the traditional helicopter missions (rescue, rig transport, troop lift and such) will probably not be accomplished by that machine very efficiently.

[plt_aeroeng]

Responding to Nick's comment about the A-1 in 'Nam, I recall some of us talking to a Spad driver at the time and he told us that on one occasion he was acting as a forward air controller for some fast movers, and he marked the target for their MK82 500 lb bombs with a couple of 1000 lb bombs.

On the other hand...

Same guy showed us a picture of a returned A-1 with the pilot standing in the hole in the wing caused by a 75 mm AA shell. The A-10 design philosophy clearly follows the trail of the A-1.

And yet, the A-10 loses favor because it is "slow". Although it can take a lot of punishment, speed really helps survivability. I remember looking at an operations research study that demonstrated that vulnerability decreases at the cube of the speed.

Apaches are effective weapons in a permissive environment, but unless you can really take advantage of terrain masking, at helicopter speed you are vulnerable even with a decent self defense suite.

An unnamed senior ex-military pilot. now with Sikorsky, once told an audience I was part of that an RWR is only good for telling you when to bend over and kiss goodbye.

[RDRickster]

Hmmmm. Let's not forget that the Air Force charter specifically requires them to provide air support for the Army on the ground. When the USAF was created in 1947, they had to maintain that focus. In the 1970's, the Army challenged the Air Force and they had a large exercise over several weeks just to show that the Air Force could move so many millions of tons within a short time. They almost couldn't do it and nearly lost their charter!

In my opinion, the A-10 is the only direct support aircraft around. Remember when they were thinking about pulling it from the inventory before the first Gulf War?... then it proved itself remendously. In fact, pilots would come back and ask just for bullets... no time for missles (and the bullets chewed up the tanks pretty darn good).

What else do we have? F15E Strike Eagles?
Uh... 1000 pounders are not exactly close proximity weapons.

Attack helicopters provide excellent close air support.

[Shawn Coyle]

Be cautious of 'studies' unless you know who proposed them, what questions they wanted answers to, and what position they were trying to support.
I'll be the one referred to here was proposed by the F-16 mafia who wanted their machine for ground support -
Yes, perhaps vulnerability does decrease as the cube of speed, but then again, so does the ability to hit targets precisely. And cost and complexity goes up as the cube of something, probably speed.

[tacks]

Thank you guys for the very detailed replies, especially Lu and Nick.

Found some b/w pics on a web-site of a german helicopter museum.

http://www.hubschraubermuseum.de/Hubschraubermuseum_Buckeburg/Archiv/Lockheed/Lockheed_AH-56A_Cheyenne/lockheed_ah-56a_cheyenne.html

Very interseting chopper, isn`t it?

[Lu Zuckerman]

To: tacks

The Apache would most likely never have been built if it were not for the intransigence of the US Army. Early on in the program the Army asked Lockheed to add on additional equipment increasing the gross weight. This taxed the lifting capabilities of the rotor system and Lockheed requested that the Army allow them to increase the rotor diameter along with attendant design changes. The Army refused so the aerodynamicist (Ray Prouty) had to redesign the blades to increase lift. This lead to the multiple aerodynamic shapes at the different blade stations. This coupled with the blade stiffness had a severe impact on blade stability and the shifting of the phase angle. The rest is history.


:E :E

[Aser]

The rise and fall of the Lockheed Cheyenne.
It was just too good...

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Regards
Aser

[loaysa]

Really astonishing capabilites!!. Why hasn´t this rotor hub system been used extensively??.
Thanks a lot!