Lockheed AH-56 Cheyenne and derivatives

[dragon72]

Murphy's Law of Armour:- No matter how much armour you have or how good it is something will find its way inside.

 

[Abraham Gubler]

Your analysis is fundamentally right, although if we want to be rigorous, for a given total engine power, the twin-engine bays are not twice as large as the single engine one. You would actually have to grow the single engine to match total power, but what helps is that power vs size effect is definitely more than linear. So twice the power means less than twice the size.

Looking at the AH-56 and AH-64 and measuring the sizes of the engine bays (not including filters and exhausts) and comparing them provides the following results. The AH-56’s T64 bay is 61” (L) x 29” (H) x 35” (W) and each of the AH-64’s T700 bays is 58” (L) x 26” (H) x 32” (W). So that’s a bathtub surface area (no top) of 6,688 sqin for the AH-56 and 11,564 sqnin for the AH-64 (with only a single wall between the engines). Which would mean for the same weight of armour it could be 72% thicker on the AH-56. Which doesn’t take into account the bottom of the AH-56’s engine bay being further protected by the layering of other systems within the airframe.

 

[AeroFranz]

I appreciate you took the time to do what i was to lazy to accomplish!

 

[Triton]

While we discuss the Lockheed AH-56 Cheyenne, we should also keep in mind that the Advanced Aerial Fire Support System (AAFSS) program and the US Army's choice of the Lockheed proposal came before the US Army's purchase of the Bell AH-1G Huey Cobra.

In 1965, the US Army asked for submissions from five companies to provide an interim gunship for the Vietnam War. Submissions came in for armed variants of the Boeing-Vertol ACH-47A, Kaman HH-2C Tomahawk, Piasecki 16H Pathfinder, Sikorsky S-61, and the Bell 209. In April 1966, the Model 209 won an evaluation against the other rival helicopters and the Army signed the first production contract for 110 aircraft. By June 1967, the first AH-1G Huey Cobras had been delivered. During the remainder of the Vietnam War, Huey Cobras provided fire support for ground forces, escorted transport helicopters and other roles, including aerial rocket artillery (ARA) battalions in the two Airmobile divisions. They also formed "hunter killer" teams by pairing with OH-6A scout helicopters.

By the time that the US Army asked for RFPs for the Advanced Attack Helicopter program in 1972, the US Army already had attack helicopters based on the AH-1 in its fleet. The Advanced Attack Helicopter specification did not call for a rotary-wing aircraft with 195-knot (224 mph, 361 km/h) cruise speed and a 220-knot (253 mph, 407 km/h) dash speed. Probably because at this point, the Bell AH-1G Huey Cobras and their descendants were already providing armed escort for transport helicopters and fire support for ground forces and so the AAH program could focus on the anti-armor attack role.

Probably our discussion should also compare the AH-56 to the AH-1G or take into account that the AH-64 Apache was not designed to provide armed escort for transport helicopters.

 

[yasotay]

A grand discussion. I would note that while armor as described would protect against small to medium caliber arms that were the prevalent, the appearance of MANPADS was and remains something of a game changer for rotorcraft. Even if the armor was satisfactory to preclude penetration of a proximity burst, the overpressure might well cause the failure of a turbine engine (an issue that precludes some missiles from being mounted on the current attack aircraft). Given the tactics of the day and a low inertia (?) rotor system, having time to relight the engine was likely not a serious probability. The reasons that the Apache got two engines was that the senior Cobra pilots in the Army's requirements community who were part of the definition process for the attack helicopter program demanded the second engine.

Another consideration worthy of thought is that a significant number of helicopters have been lost to enemy fire hitting the tail rotor. I doubt that going faster would have caused more actual hits to be further forward on the Cheyenne. The proximity of the dynamics on the tail (torque and thrust rotors) would likely have had a higher probability of cascading failure from the disintegration of one system or the other. I bring it up as a discussion point only.


Personally I believe the Cheyenne fell victim to funding wars and inter-service politics. Changing environments (going from asymmetric, non-linear, hybrid war to major combat operations [to use today's parlance]) became a convenient excuse for the termination of the program. I have not met an Army Aviator from the period who does not think fondly that the AH-56 would have been a great aircraft to fight in. Even today there are a lot of new attack aviators who wonder why they don't have the capabilities of the AH-56.

 

[Longshaor]

I read an article on the AH-56 in Wings of Fame several years ago that hinted that the rotor head was so diferent that it was a part of the reason the program was canned. It seemed to indicate that the amount of work required to make it 'service ready' was far in excess of Lockheed's projections and, without it, the promised speed of the Cheyenne couldn't be realized. Is there any truth to this?

 

[F-14D]

I read an article on the AH-56 in Wings of Fame several years ago that hinted that the rotor head was so diferent that it was a part of the reason the program was canned. It seemed to indicate that the amount of work required to make it 'service ready' was far in excess of Lockheed's projections and, without it, the promised speed of the Cheyenne couldn't be realized. Is there any truth to this?

My recollection was that they had finally solved the rigid rotor/control problems, although the Cheyenne would never reach its originally promised speeds. It didn't help that the S-67 (developed for 1% as much money) turned out to be slightly faster. By then, though, it was too late. The program was becoming politically untenable, requirements (at least in their estimation) had changed and Army no longer thought it was worth the effort to continue to fight for the program, especially with USAF lobbying against the program ramping up.

 

[AeroFranz]

I would note that while armor as described would protect against small to medium caliber arms that were the prevalent, the appearance of MANPADS was and remains something of a game changer for rotorcraft. Even if the armor was satisfactory to preclude penetration of a proximity burst, the overpressure might well cause the failure of a turbine engine. [...] The reasons that the Apache got two engines was that the senior Cobra pilots in the Army's requirements community who were part of the definition process for the attack helicopter program demanded the second engine.

Thanks for the explanation, Yasotay. You bring up a good point with the vulnerability of tail rotors. What good does it do to harden the entire vehicle if a "golden BB" in the tail rotor can bring the whole thing down?
I wonder what can be done, realistically. Rotor blades can be made to withstand 23mm rounds, but I doubt the same can be done with the smaller rotor blades. A fenestron might be slightly better in that regard, but the obvious solution would be to get rid of the tail rotor period. But I doubt that's what drove the design of the Hokum.

 

[F-14D]

I would note that while armor as described would protect against small to medium caliber arms that were the prevalent, the appearance of MANPADS was and remains something of a game changer for rotorcraft. Even if the armor was satisfactory to preclude penetration of a proximity burst, the overpressure might well cause the failure of a turbine engine. [...] The reasons that the Apache got two engines was that the senior Cobra pilots in the Army's requirements community who were part of the definition process for the attack helicopter program demanded the second engine.

Thanks for the explanation, Yasotay. You bring up a good point with the vulnerability of tail rotors. What good does it do to harden the entire vehicle if a "golden BB" in the tail rotor can bring the whole thing down?
I wonder what can be done, realistically. Rotor blades can be made to withstand 23mm rounds, but I doubt the same can be done with the smaller rotor blades. A fenestron might be slightly better in that regard, but the obvious solution would be to get rid of the tail rotor period. But I doubt that's what drove the design of the Hokum.

In a word, NOTAR, which was on the Bell/Hughes LHX proposal after Army defined Tilt-Rotor out of contention (another topic). Not sure how large a vehicle you can use that with, though.

 

[Orionblamblam]

In a word, NOTAR, which was on the Bell/Hughes LHX proposal after Army defined Tilt-Rotor out of contention (another topic). Not sure how large a vehicle you can use that with, though.

Oddly enough, AH-56-sized.

 

[F-14D]

In a word, NOTAR, which was on the Bell/Hughes LHX proposal after Army defined Tilt-Rotor out of contention (another topic). Not sure how large a vehicle you can use that with, though.

Oddly enough, AH-56-sized.

Interesting drawing. Was this a notional Lockheed idea (of course in this concept they've lost the pusher, whcih means at the low end at least, acceleration would be slower)? I don't recall Hughes working on anything this big, and they're the ones who go it to work.

 

[Orionblamblam]

Interesting drawing. Was this a notional Lockheed idea (of course in this concept they've lost the pusher, whcih means at the low end at least, acceleration would be slower)?

Yes. As labelled, it's the CL-1275.

 

[F-14D]

Interesting drawing. Was this a notional Lockheed idea (of course in this concept they've lost the pusher, whcih means at the low end at least, acceleration would be slower)?

Yes. As labelled, it's the CL-1275.

Wow. Now I'm even more confused than usual; I thought -1275 had something to do with a variant of their XH-51. Learn something here every day.

 

[Orionblamblam]

Wow. Now I'm even more confused than usual; I thought -1275 had something to do with a variant of their XH-51. Learn something here every day.

That would be this variant.

 

[sferrin]

Interesting. That is starting to resemble some of Sikorsky's X-wing concepts.

 

[Orionblamblam]

Interesting. That is starting to resemble some of Sikorsky's X-wing concepts.

I don't think I have any X-Wing versions of the AH-56. Lockheed went straight from helicopter, to yanking the rotor and anti-torque rotor off, driving solely the pusher prop with the turboshaft engine, and adding a big wing with two more turbojets. You know, like what normally happens with helicopters.

 

[Abraham Gubler]

(of course in this concept they've lost the pusher, which means at the low end at least, acceleration would be slower)?

The pusher is still there just buried inside the tail. It looks like the exhaust vanes can swivel enabling thrust to be directed to the rear. But without the propeller blades being in the air it wouldn't work as a decelerator or as an autorotation windmill. A couple of the benefits of the AH-56 configuration.

 

[F-14D]

(of course in this concept they've lost the pusher, which means at the low end at least, acceleration would be slower)?

The pusher is still there just buried inside the tail. It looks like the exhaust vanes can swivel enabling thrust to be directed to the rear. But without the propeller blades being in the air it wouldn't work as a decelerator or as an autorotation windmill. A couple of the benefits of the AH-56 configuration.

I wonder. What Hughes developed was using a fairly small fan inside the tailboom building a high volume of low-pressure air, creating a boundary layer exiting through slots. This seems to be a larger, and hence less efficient (from a power required viewpoint), regular fan blowing air. I believe the ability to use the Coanda effect requiring less power was Hughes' great breakthrough that made the concept practical, and this doesn't appear to do that. Since the fan is smaller than the pusher prop, I wonder if it will give anywhere near the thrust at lower speeds. As a wise man (not me) said, a prop takes a lot of air and accelerates it a little, and this may not move that much air.

The other thing they lose with this concept is that since the same device would be providing anti-torque and propulsive thrust it would not be available at the low end (one of the reasons Lockheed went with the two props, each optimized for different function). In other words, at lower speeds, the air is blasting only to the side to counter torque. For example, Sikorsky's rotorprop (rotating pusher tailrotor), part of its S-66 competitor to Lockheed proposal, envisioned that the rotoprop would only be operating in pure thrust mode above 100 knots. Below 60 knots, it had to operate as put antitorque. So, for the S-66 the primary low-speed acceleraton thrust would need to come from the main rotor, as on conventional helos. I think Lockheed went the right way with what they did, rather what was illustrated in the -1275 drawing, Of course, then you're back to the tail rotor issue.


MORE THOUGHTS, FIVE HOURS LATER:

Abraham: Looking at the drawing again, I don't think any flow from the fan is used for propulsion. Those vanes may just be to modulate the amount of anti-torque it's putting out. For simplicity, that fan probably runs at a constant speed, since when you need anti-torque you need it virtually instantly, and you don't want to wait the admittedly brief time it takes for the fan to spin up or down. Those vanes will facilitate that.

 

[elmayerle]

I've often wondered how a modified NOTAR would work if you combined it with the engine exhaust(s) to energize the flow while also suppressing the IR signature of the vehicle. There would most certainly be some materials issues and I can see some "interesting" times developing the overall engine installation but I can see advantages, too, if it could be pulled off.

 

[Trident]

Sort of like this?

http://www.secretprojects.co.uk/forum/index.php?topic=6322

 

[F-14D]

Sort of like this?

http://www.secretprojects.co.uk/forum/index.php?topic=6322

Cirstel looks like a variation of the NOTAR concept to add IR suppression, tried in South Africa. In fact CIRSTEL is an acronym for Combined Infrared Suppression and Tail Rotor ELimination, IR suppression not being that important for civil helos. NOTAR with IR suppression was part of the Bel/Hughes LHX proposal.

In all those cases, though, the unit was not being used for propulsion.

 

[Colonial-Marine]

Two questions. What would determine what weapon would be fitted in the chin turret, and how damage resistant was the cockpit area? Would they have likely gone with angled glass panels on later variants, much as we have done with our Cobras?

 

[Abraham Gubler]

Abraham: Looking at the drawing again, I don't think any flow from the fan is used for propulsion. Those vanes may just be to modulate the amount of anti-torque it's putting out. For simplicity, that fan probably runs at a constant speed, since when you need anti-torque you need it virtually instantly, and you don't want to wait the admittedly brief time it takes for the fan to spin up or down. Those vanes will facilitate that.

So basically this design with the NOTAR is an uncompounded version of the AH-56? Though it may have been a phase 1 design to see how it flies as a NOTAR and then they sling some turbofans under the wings for compound flight.

 

[yasotay]

Two questions. What would determine what weapon would be fitted in the chin turret, and how damage resistant was the cockpit area? Would they have likely gone with angled glass panels on later variants, much as we have done with our Cobras?

Actually the primary reason for the plate glass was to reduce canopy glint. The anti-armor test done in the US and Germany demonstrated that one of the primary means of detecting a hovering attack helicopter was by the glint off of the rounded canopy. I believe it was not until the AH-64 came along that they included protection between the two crew members. I can tell you that the Army Cobra did not have any hardening of the canopy.

 

[Abraham Gubler]

Two questions. What would determine what weapon would be fitted in the chin turret, and how damage resistant was the cockpit area? Would they have likely gone with angled glass panels on later variants, much as we have done with our Cobras?

The XM51 system (armed with the XM129 40mm grenade launcher) and the XM53 fire system (armed with the XM196 7.62mm minigun) were designed to be interchangeable so helicopters could be configured per need. The 40mm GL was very much a Vietnam weapon as the grenades were used to punch through the upper tree foliage and shower the ground with fragments. The 7.62mm minigun was much better as a suppression weapon for exposed targets and they are used extensively to this day in this role.

As to the basis of provisioning between the two that would probably be even with helicopters assigned to overwatch being the minigun versions and those that go in close to get the targets being the GL version (in Vietnam). Anywhere without high tree canopy coverage all minigun would be better. However because of problems in developing the weapons the minigun was dropped from the final production plan. Though would have probably re-entered at some time if AH-56 production had gone ahead. For contemporary use the 40mm GL with a muzzle programmed air burst function would be a very lethal anti-personnel weapon.

The AH-56’s canopy was made of clear acrylic plastic and the windshields (front and top) of laminate glass. The forward fuselage structure was considered resistant to 23mm HEI but not the canopy and windshield. A Lockheed vulnerability analysis towards the end of the program recommended adding 12.7mm of transparent armour to the canopy, windshield and a crew separation plate (amongst other things). This would have removed the 23mm HEI attrition loss vulnerability for the cockpit area. The basic design of the AH-56’s forward fuselage is very convenient to modifying the aircraft with two separate cockpits with flat glass panels for the gunner/co-pilot and pilot in a post SGS/TOW version.

See attached image. Which is a low resolution version of one of the many images (and data) from the AH-56 Cheyenne Technical Manual at http://www.up-ship.com/drawndoc/drawndocair.htm

 

[F-14D]

Abraham: Looking at the drawing again, I don't think any flow from the fan is used for propulsion. Those vanes may just be to modulate the amount of anti-torque it's putting out. For simplicity, that fan probably runs at a constant speed, since when you need anti-torque you need it virtually instantly, and you don't want to wait the admittedly brief time it takes for the fan to spin up or down. Those vanes will facilitate that.

So basically this design with the NOTAR is an uncompounded version of the AH-56? Though it may have been a phase 1 design to see how it flies as a NOTAR and then they sling some turbofans under the wings for compound flight.

I'm not sure exactly what was intended by that design, maybe Abraham knows. Maybe it was just a theoretical exercise in trying to get rid of both tail props and just fly like a conventional helo. It may have been decided the pusher wasn't worth it (it wasn't a feature of Lockheed's AAH proposal). After all, the non-pusher S-67 turned out to be faster in level flight (though not by all that much) than the AH-56.

There wouldn't be much value in putting turbofans under the wings. You'd lose two stations, your payload would go down, your fuel consumption would go way up, complexity would increase and you'd gain maybe 50 knots dash. Besides, USAF would never let you. I should note that there was a proposal for NASA to take one of the Cheyenne prototypes and add a jet engine to it for some research into the effects of high speed on rotors. Not much came of it, but there was a model of the proposal at the San Diego Air and Space Museum for a number of years, not sure if it's still there. I'm not sure if its the same model as in Circle-5's 9/28 post

 

[F-14D]

I suspect that had the AH-56 gone into production, although the turret had all the options Abraham pointed out, you would have seen some changes, just like with the Cobra. The AH-1 originally was conceived having multiple nose turrets. The configurations were 7.62 plus 40mm grenade launcher, two 7.62 or just one 7.62 (theoretically you could do two 40mm, but I don't believe that configuration was ever deployed). However, supportability issues set in, and it ended up with the overwhelming majority flying with just one 7.62 and that became the standard. I suspect in the short to medium term, that would have happened with the Cheyenne as well.

In the medium to long term, if Cheyenne had entered production, there would have been no need for Apache. However, the requirement for Apache's night and poor weather fighting ability would still be needed. I would wager the nose turret would have disappeared because that location for sensors is too good to waste on another guns (Cheyenne's location for its day/VFR sensors would not have been suitable.

Regarding the change on Army Cobras to flat plate glass, it was done for the reasons stated. However USMC found that the plate plate glass actually ended up producing more distortion. So, on theirs they deliberately retained the rounded canopy, even today with their new Zulu.

 

[Jemiba]

"The basic design of the AH-56’s forward fuselage is very convenient to modifying the aircraft
with two separate cockpits with flat glass panels..."

"So, on theirs they deliberately retained the rounded canopy"

Both thoughts could be combined, I think.

 

[Abraham Gubler]

Here is a concept based on the AH-56 (with crew eyeball lines) using existing structural bulkheads to build two armoured cockpits. Using flat planes makes it easier to mount the thick (and heavy!) bullet proof glass for the windshields and canopies. And no I haven’t calculated the surface areas... Maybe later...

 

[overscan (PaulMM)]

Sorry Jens, your cockpit design says WW2 bomber to me. Abe's looks much better

 

[Jemiba]

"Abe's looks much better "

Indeed, without doubt !
It's a half-breed of a north american indian and an asian big cat.
I mainly wanted to show, that a stepped cockpit wouldn't rule out
curved windows.
But the shortened forward fuselage really gives it a more "aggressive"
look and maybe makes kitbashing easier. But that's a them for the
"speculative" section ...

 

[amsci99]

Scott,

You know I'm a sucker for Cheyenne related projects but how did NOTAR got incorporated into Lockheed's designs? I thought only MD was working extensively on NOTAR unless there was some joint co-operation for the purposes of the LHX Competition.

Interesting. That is starting to resemble some of Sikorsky's X-wing concepts.

I don't think I have any X-Wing versions of the AH-56. Lockheed went straight from helicopter, to yanking the rotor and anti-torque rotor off, driving solely the pusher prop with the turboshaft engine, and adding a big wing with two more turbojets. You know, like what normally happens with helicopters.

 

[Abraham Gubler]

But the shortened forward fuselage really gives it a more "aggressive"
look and maybe makes kitbashing easier. But that's a them for the
"speculative" section ...

That's the standard length fuselage. The base picture you were using was the CSAR version with space for passengers in an enlargened forward avionics bay.

 

[Abraham Gubler]

You know I'm a sucker for Cheyenne related projects but how did NOTAR got incorporated into Lockheed's designs? I thought only MD was working extensively on NOTAR unless there was some joint co-operation for the purposes of the LHX Competition.

Hughes Helicopters started NOTAR work in 1975. Lockheed had given up on helicopters before this date. So I guess Hughes followed Lockheed in this regard.

 

[Abraham Gubler]

Here is a concept based on the AH-56 (with crew eyeball lines) using existing structural bulkheads to build two armoured cockpits. Using flat planes makes it easier to mount the thick (and heavy!) bullet proof glass for the windshields and canopies. And no I haven’t calculated the surface areas... Maybe later...

OK for the armoured bathtubs it would come to about 12,700 sqin or 1,000 lbs for 0.5" of Titanium armour alloy. For the glass plates there is 9,000 sqin (not including tops) or 1,800 lbs for 3" thick bulletproof glass plate. These dimensions include a mixed plate, glass separator between the gunner/co-pilot and the pilot.

1.2 tonnes is pretty high for cockpit armour (the bathtub in the A-10 is only 1,200 lbs and has thickness from 0.5” to 1.5”). But combined with the aluminium forward structure of the AH-56 would pretty much stop anything up to typical ground to air 23mm HEI.

Picture with a few 'cosmetic' changes.

 

[Stargazer]

I also think Jemiba's version looks better, no offense uh?

Besides the upper cockpit looks way too close to the blades...

 

[Jemiba]

"That's the standard length fuselage"

Ooops, sorry for that ! I was just looking for the cockpit and tried to maintain the
height of the cockpit section, using flat, or slightly curved window panels. :-\

 

[Abraham Gubler]

I also think Jemiba's version looks better, no offense uh?

Besides the upper cockpit looks way too close to the blades...

The upper cockpit roof of my drawing is exactly the same height as the real AH-56.

 

[Stargazer]

I was talking about Jemiba's drawing, actually...

 

[Orionblamblam]

BLATANT SELF-PROMOTION:
AH-56A Inboard Profile drawing now available.

http://up-ship.com/blog/?p=4238