Lockheed AH-56 Cheyenne and derivatives

[Abraham Gubler]

A few pages back I made some comments about the AH-56 vs AH-64 when this thread had its last period of activity (May ’09). To summarise I don’t think the AH-64 has any real design advantages over the AH-56 for its mission apart from the systems advantages brought about by 20 years of technology advance.

While the AH-56 was very fast it was also much better at hovering than a conventional helicopter; being able to change its hover pitch. This would make it a better platform for Hellfire and shoot from cover tactics. Not to mention much better at shooting rockets and guns.

Also the AH-56 carried more armour than the AH-64. While the AH-64 had separated engines for redundancy from a single hit this does not necessarily increase protection as it requires a much larger surface area to be protected increasing the weight of armor. The AH-56’s compact single or twin pack engine enables much thicker armour to be carried for the same weight as the AH-64. Despite the increased vulnerability this makes to a heavy calibre penetrating shot (57mm) the AH-56’s much better autorotation potential (thanks to the tail rotor) enables it to get to the ground far safer than other single or twin pack engine helicopters after power loss. Also the more to the rear mounting of the AH-56’s exhaust makes for better IR protection.

Then of course was all the extra money spent on upgrading AH-1s, designing and building the AH-64 that could have been spent on AH-56 production in the 1970s and 80s. This means by 1990 the US Army could have had an all AH-56 fleet with Hellfire rather than 600 AH-64 with Hellfire and 1,000 AH-1 with TOW. All in all US Army would have been much better off if they stuck to their guns and went with AH-56 production rather than launching the AAH project. Of course USAF, US Congress and the US Army’s own leadership and the best efforts of Sikorsky, Bell and Hughes stood in the way of this path.

 

[Lampshade111]

It just might be unrealistic picturing modernized AH-56s still in service in 2009.

Er, Cobras, Chinooks, CH-53s. . .

True to an extent when it comes to the CH-47 and CH-53, both have seen plenty of upgrades however.

Yet when it comes to the Cobra, how much does an AH-1Z have in common with the original AH-1G? It is pretty much a whole new bird by now. If the AH-56 was chosen, I am sure today's models would be much different and have two main engines. But I can't help but think the Army would try to get it's successor up and running by the early 2000s. I doubt they would take the same route the Marines have with the AH-1.

How much did reliability concerns factor into the programs eventual fate? There certainly seemed to be plenty of problems left to fix, and I highly doubt some features like the gunners rotating chair would have worked out at all.

 

[Abraham Gubler]

True to an extent when it comes to the CH-47 and CH-53, both have seen plenty of upgrades however.

Of course but what SORT of upgrades? The H-47 and H-53 airframes have been upgraded to massively increase their lifting power. Because they are cargo helicopters and lifting things is what they do, so the more the merrier. The AH-56 would actually benefit from a reduction in weight of its sub-systems as technology advances and its night vision, missile systems etc move from the 1960s technology to 21st century. Replacing TOW with Hellfire would hugely reduce the empty weight of the AH-56 as the comlex guidance systems would no longer be needed.

True Yet when it comes to the Cobra, how much does an AH-1Z have in common with the original AH-1G?

What’s the difference? Rotor-power systems so it can fly faster, further and carry more. Since the AH-56 is starting with a huge deficit in lift and speed why would the rotor-power system need to be changed? It would just benefit from improved mission systems and airframe materials.

How much did reliability concerns factor into the programs eventual fate? There certainly seemed to be plenty of problems left to fix, and I highly doubt some features like the gunners rotating chair would have worked out at all.

By this statement you are clearly working from a lot less of a knowledge base about the AH-56 than others here. The primarily and significant problem with the AH-56 was making the rotor system work. Which they did by 1971. Despite disgracing itself at the Congressional display TOW worked very well for the AH-56 and is a well proven system.

As to assuming the rotating chair didn’t work well what do you base that on? Its looks... Aircraft turrets are a pretty well proven item of technology even though they are now mostly obsolete. For the technology of the 1960s and 70s it was a much simpler system than an electronic TV camera based system.

Anyone who has read my posts on this forum would know how upset I get over this kind of baseless “it looks different” assumptions. The rotating gunner’s sighting system was a proven item by the end of the AH-56 program. To allege otherwise without reference to a firsthand report is just not good enough. I would normally use harsher words but the forum owner has warned me off that...

 

[Lampshade111]

What’s the difference? Rotor-power systems so it can fly faster, further and carry more. Since the AH-56 is starting with a huge deficit in lift and speed why would the rotor-power system need to be changed? It would just benefit from improved mission systems and airframe materials.

The switch to a two-engine setup would be pretty much guaranteed by the same experiences that would lead the Army to such a conclusion. Even if you could armor a single engine better than two, if it is disabled or breaks down your still out of luck, plus there is the greater ability to operate in "hot and high" environments and so forth.

By this statement you are clearly working from a lot less of a knowledge base about the AH-56 than others here. The primarily and significant problem with the AH-56 was making the rotor system work. Which they did by 1971. Despite disgracing itself at the Congressional display TOW worked very well for the AH-56 and is a well proven system.

I won't deny that I really haven't read that much about the AH-56. It just seems from my point of view to be a somewhat overcomplicated machine, not as well suited to dealing with all of the new forward air-defense systems the Soviets were getting. In terms of engineering, the closest comparison I can think of is to the MBT-70. But I know more about the MBT-70 than the AH-56, so perhaps I am mistaken. Where have you read that all of the rotor problems were worked out by 1971?

As to assuming the rotating chair didn’t work well what do you base that on? Its looks... Aircraft turrets are a pretty well proven item of technology even though they are now mostly obsolete. For the technology of the 1960s and 70s it was a much simpler system than an electronic TV camera based system.

I am basing that assumption on the fact that it is yet another mechanical system to add wight, complexity, and maintained work to a helicopter that was already a bit too heavy for it's engine. The utility of it, probably wasn't worth all of that. A few years later it would be obsolete anyway. I can't help but think there might be some motion sickness issues, but then again that is me comparing it to that rotating drivers seat scheme in the MBT-70.

Anyone who has read my posts on this forum would know how upset I get over this kind of baseless “it looks different” assumptions. The rotating gunner’s sighting system was a proven item by the end of the AH-56 program. To allege otherwise without reference to a firsthand report is just not good enough. I would normally use harsher words but the forum owner has warned me off that...

I don't care that the AH-56 looks different, the Apache is not a good looking machine either. Yet there were plenty of features on it that weren't that mature at the time, and capabilities that were questionable in later Cold War Europe. Please go ahead and say what you want to say about me, because I don't really care if my opinions on the matter offend you.

 

[overscan (PaulMM)]

Watch the tone of discussion please. Its an interesting technical debate that shouldn't get bogged down in personal stuff.

 

[saturncanuck]

Watch the tone of discussion please. Its an interesting technical debate that shouldn't get bogged down in personal stuff.

Here here!

 

[AeroFranz]

The switch to a two-engine setup would be pretty much guaranteed by the same experiences that would lead the Army to such a conclusion. Even if you could armor a single engine better than two, if it is disabled or breaks down your still out of luck, plus there is the greater ability to operate in "hot and high" environments and so forth.

The single vs twin engine has historically been more an issue for the Marines (see AH-1 Cobras, for example), who fly long distances over water. But yeah, survivability-wise, two widely separated engines (although having higher exposed areas) are seen as better than a single engine. At least that's what drove the A-10 design (the ultimate reference in the field of survivability was written by Ball, in the AIAA education series).
As for hot/high performance, all that matters is total shaft horsepower, be it one or two engines. The decrease in density takes away a pretty much fixed percentage of the max continuous horsepower.

Where have you read that all of the rotor problems were worked out by 1971?

One source is the AH-56 book in the warbird tech series. I think the problem was called the 2-P hop or something like that.

Since the AH-56 is starting with a huge deficit in lift and speed why would the rotor-power system need to be changed? It would just benefit from improved mission systems and airframe materials.

I think Abraham meant surplus here

Please go ahead and say what you want to say about me, because I don't really care if my opinions on the matter offend you.

Let's not provoke/invite certain tones! As the moderators pointed out, we've had some serious discussions about this just recently. I'm glad Abraham is behaving because he usually has some pretty interesting things to say when he's supporting his arguments. The quality of the information tends to degrade when you're wasting time on personal attacks.

 

[yasotay]

Agree that civility needs to be maintained. It is what sets this site apart from others...

My point is that the decision to terminate the AH-56 was made not due to technical issues, but due to changing priorities within the Army and potential long arguments over roles and missions. One should remember that at the end of Vietnam, attack helicopters were considered to be an infringement of Title 10 missions by the USAF. With massive conventional war and the potential for escalation becoming the focus securing an aircraft at the least cost that could kill tanks was the focus. One must remember that in the ugly days of the Cold War a division occupied no more that 20 to 30 kilometers of space and fought in mass. With those distances "speed" was not as important as the ability to sneak and snipe. The Army was transitioning from a light infantry, airmobile force, to a heavy mechanized defending force. What is not well known is that the demise of the AH-56 allowed the Army to secure from the USAF the attack helicopter mission for the Army.

As to single engine versus twin, as a former Army helicopter pilot who has done both; give me twin, no matter how reliable the single is (See attached picture). A lot of analysis went into the development of both UH-60 and AH-64 from combat. Both are far more reliable and survivable than the helicopters they replaced. As AeroFranz mentions Ball's work on survivability is a great work that makes it clear how twins are more viable than single engine.

Had AH-56 made it into service, it might well still be serving very well. Ironically the parameters that called for its development are once again in vogue. Its speed and firepower would certainly be welcomed in current fights. Perhaps this is why the USAF is considering a COIN aircraft?

 

[Abraham Gubler]

I’ll try and ignore the unsubstantiated allegations (something that sets this site apart from others and is worth fighting to retain) of mission system complexity against the AH-56 as the person making these claims has freely admitted they don’t actually know much about the aircraft and is making the strange comparison with a tank. Obviously never heard of those hundreds of thousands of turrets that have been mounted in aircraft or General Electric’s remote turret control mount fitted in the tens of thousands to aircraft starting with the B-29 and developed into swivelling seat periscopes for the B-35 and further developed before ending with the General Electric XM 112 on the AH-56, which was developed BEFORE television systems had evolved to provide reduced weight and complexity from rotating systems. Not to mention the advantage of gunner’s orientation for aircrew who grew up long before computer games and don’t have the learned ability to perceive changes in orientation via visual cues alone like most of us under the age of 40 today.

On engine survivability much of this argument is prefixed on similar levels of protection. An even benchmark, which doesn’t exist between the AH-56 and the AH-64. And also comparing aircraft that lacked the AH-56 level of protection like the UH-1 and AH-1. Talking combat survivability between a single and double, separated engine bays not single versus twin engines for single engine failure survivability.

Now I’m repeating what has been posted before but the AH-64 is designed to survive an attrition loss from a single side hit by a 23mm HEI. The AH-56 is designed to survive a mission loss from the same. The difference is if a 23mm HEI hits a single side of an AH-64 it will destroy that side’s engine but the armour plate between engines will protection the engine on the other side. The AH-64 then has to jettison its weapons load, turn around and head for home and wash out the underwear of the aircrew. The AH-56 has enough armour that the 23mm HEI hits the side it doesn’t do significant damage. So the AH-56 can continue its mission.

Considering any trained AA battery commander positions guns and missiles to catch the threat in a crossfire the one side only protection of the AH-64 is only good for standoff missions not penetration and close air support missions.

This comparison is without taking into advantage the huge weight savings of the AH-64’s mission systems compared to the AH-56. If they were both benchmarked with the same mission systems the AH-56 would have a SURPLUS (brain fart before!) of thousands of pounds of weight to increase the thickness of the armour plate around the engine bay. If built in the 1980s it could also benefit from composite materials rather than aluminium armour and have more protection for weight. It is quite reasonable to assume that a late 80s AH-56 block could have engine bay armour resistant to 30mm HEI.

Then of course the AH-64’s two exterior mounted engine bays are far more exposed to enemy fire than the single buried engine bay on the AH-56. You can’t even hit the AH-56 engine from the front without having gone through the cockpit (which is worse than losing engine power) or through multiple ammo bins and two 12.7mm resistant sides of an armoured fuel tank. Fires from behind and below have to penetrate the large weapon systems avionics bay to hit the AH-56 engine. The AH-64 only has protection of its engine by the rest of the aircraft from a narrow field of fire straight below (the stub wings) or from one side to the other.

Then add to this the much higher cruising speed, acceleration and deceleration of the AH-56 which make it much harder to hit. Harder to hit, harder to hit the engine bay and harder to penetrate the engine bay. Only after these three layers of the survivability onion does multiple redundancy come into effect.

 

[Lampshade111]

I’ll try and ignore the unsubstantiated allegations (something that sets this site apart from others and is worth fighting to retain) of mission system complexity against the AH-56 as the person making these claims has freely admitted they don’t actually know much about the aircraft and is making the strange comparison with a tank. Obviously never heard of those hundreds of thousands of turrets that have been mounted in aircraft or General Electric’s remote turret control mount fitted in the tens of thousands to aircraft starting with the B-29 and developed into swivelling seat periscopes for the B-35 and further developed before ending with the General Electric XM 112 on the AH-56, which was developed BEFORE television systems had evolved to provide reduced weight and complexity from rotating systems. Not to mention the advantage of gunner’s orientation for aircrew who grew up long before computer games and don’t have the learned ability to perceive changes in orientation via visual cues alone like most of us under the age of 40 today.

All sorts of "cutting edge" systems and concepts, even when they had been used on other vehicles before, when included into the MBT-70 made it a mess to maintain and operate. The AH-56 had no shortage of new and old technologies ad there would have been plenty to check, maintain, and worry about, it was a far more complicated machine than the AH-64. That is why I made that comparison. And I know about the use of turrets on fixed wing aircraft, yet it is on a helicopter flying at 200 mph a few meters above the ground where I doubt it's utility.

Your survivability scenario is based around the AH-64 flying just like the AH-56 would or like a AH-1 in Vietnam. That is not how it is was built to fight.

 

[circle-5]

[...] Par-Tool made five different variants of this AH-56 model: Army, Navy, Air Force, Marines and a compound project with two side-mounted turbofans. I have a photo of the latter one somewhere -- will post when I find it. [...]

I found it. Note: jet engines are in addition to pusher prop, for maximum forward thrust. Hi-viz markings and absence of armament suggest this was intended as a research aircraft. To fit the tone of this thread, my post clearly needs an unsubstantiated allegation, so here it goes: this would have been a very fast helicopter!

 

[Triton]

Could an upgrade of avionics and the weapon systems from analog/mechanical to digital given the AH-56 night and all weather capability? Did it make more financial sense to start over with a new program rather than to retrofit/upgrade the AH-56 design with these digital systems?

According to Wikipedia, another reason given by the US Army for the cancellation of the AH-56 was that it was too big. Comparing the weight and dimensions of the AH-56 to the AH-64, it seems that this objection was baseless.

Lockheed AH-56 Cheyenne

Length: 54 ft 8 in (16.66 m)
Rotor diameter: 51 ft 3 in (15.62 m)
Height: 13 ft 8.5 in (4.18 m)
Empty weight: 12,215 lb (5,540 kg)

Hughes/McDonnell Douglas/Boeing AH-64 Apache

Length: 58.17 ft (17.73 m) (with both rotors turning)
Rotor diameter: 48 ft 0 in (14.63 m)
Height: 12.7 ft (3.87 m)
Disc area: 1,809.5 ft² (168.11 m²)
Empty weight: 11,387 lb (5,165 kg)

 

[Orionblamblam]

CL-1051, crew of three, six passengers. A bit more like a Mil 24 "Hind."

 

[Rosdivan]

I’ll try and ignore the unsubstantiated allegations (something that sets this site apart from others and is worth fighting to retain) of mission system complexity against the AH-56 as the person making these claims has freely admitted they don’t actually know much about the aircraft and is making the strange comparison with a tank. Obviously never heard of those hundreds of thousands of turrets that have been mounted in aircraft or General Electric’s remote turret control mount fitted in the tens of thousands to aircraft starting with the B-29 and developed into swivelling seat periscopes for the B-35 and further developed before ending with the General Electric XM 112 on the AH-56, which was developed BEFORE television systems had evolved to provide reduced weight and complexity from rotating systems. Not to mention the advantage of gunner’s orientation for aircrew who grew up long before computer games and don’t have the learned ability to perceive changes in orientation via visual cues alone like most of us under the age of 40 today.

Your survivability scenario is based around the AH-64 flying just like the AH-56 would or like a AH-1 in Vietnam. That is not how it is was built to fight.

I don't have a dog in this fight, but I didn't get that impression at all from his description except at the very end which gave the impression of being an "even if we ignored the above" throwaway line. What does the flight profile have to do with a higher rated armor protection level and lower surface area exposed to enemy fire?

Perhaps it might be appropriate to rate how the Cheyenne would have done in the same circumstances as the failed Blackhawk Karbala raid in 2003?

 

[Abraham Gubler]

All sorts of "cutting edge" systems and concepts, even when they had been used on other vehicles before, when included into the MBT-70 made it a mess to maintain and operate.

The MBT70 had two things which didn't work that well. A gun launched missile and a counter rotating driver's position. Neither have anything to do with the AH-56 except for the later an apparent but non-existent physical similarity. On the MBT70 as the turret rotated the turret mounted driver’s station would counter rotate to keep the driver pointed forward (so they could see where the vehicle was going).

On the AH-56 the gunner had a gyro stabilised rotating periscope that swilled the gun sights to the target under the gunner’s command. This system had nothing to do with flying the helicopter. If the gunner wanted to use the emergency controls in this cockpit the seat had to be swivelled to the front and the periscope folded down before the flight controls would activate.

For the 1960s it was the simplest way to enable a sight unit including several apertures (first use of FLIR in a helicopter) to be trainable off the helicopter’s boresight. During development of the AH-56 there were no problems caused by the XM112 worthy enough to end up in all the AH-56 literature. On the other hand the Philco Ford guns had lots of problems that were finally sorted out thanks to the delays in the program caused by the rotor unit.

The AH-56 had no shortage of new and old technologies ad there would have been plenty to check, maintain, and worry about, it was a far more complicated machine than the AH-64. That is why I made that comparison.

Do you have any evidence to support this statement? The only thing the AH-56 has that the AH-64 doesn’t is a second gun (minigun or grenade launcher) and the TOW guidance system. It doesn’t have a laser designator (it has a laser rangefinder but the difference is significant), two helmet mounted sights (only one) and it doesn’t have a second engine. Going by this balance sheet the AH-56 is way out in front thanks to the engines.

To compare the AH-64 and the AH-56 on a maintainability standard is also disingenuous as one has the benefit of another 20 years of engineering in its sub systems to - hopefully - reduce the maintenance burden. Also this assessment is purely made on a very, very shallow view of the helicopters and not substanited by any evidence.

And I know about the use of turrets on fixed wing aircraft, yet it is on a helicopter flying at 200 mph a few meters above the ground where I doubt it's utility.

Ahh... there are turrets for sensors and weapons on the AH-64, the AH-1, the RAH-66, the Tiger ARH, the Rooivalk, the HIND, the HAVOC... So I guess none of these helicopters gets any “utility” out of their turrets? The HOKUM doesn’t have a gun turret (fixed forward firing guns) but it also doesn’t have a separate gunner...

Your survivability scenario is based around the AH-64 flying just like the AH-56 would or like a AH-1 in Vietnam. That is not how it is was built to fight.

Well it’s not “based” on the better flight profile of the AH-56 (I had three layers of improvement) and this has nothing to do with Vietnam. The AH-56 and the AH-64 share the same flight profiles for the same missions. There is no difference.

If the AH-56 is engaging in long range missile shots to defeat a Soviet armour thrust across the North German Plain then it will do so hovering from near cover. The only difference is in the missile guidance system (Hellfire vs TOW) which can’t be included in any fair benchmark between the two.

The only difference is when the AH-56 has to fly from A to B it does so much faster than the AH-64. It can also accelerate and decelerate much, much quicker. When hovering it can also point its nose up or down depending on the situation enabling for much firing control.

 

[yasotay]

There is no doubt that the AH-56 was very advanced for its day. Indeed it was as high tech as you could get for its day. Had the aircraft gone into service the Army would have figured out how to make it work as a superb anti-tank helicopter for the cold war era (80s-90s). While it might have grown a second engine (remember aircraft grow by 100lbs a year on average), it is equally possible that a larger single engine arrangement might have been retained. I would expect that the AH-56 would now be sporting a third iteration of targeting and pilotage sensors by now.

One of the pieces of technology I think would have been quickly been superseded was the gunners rotating seat. Two crashes where the co-pilot gunner could not take control of the aircraft because his seat was out of place would have reduced it in peacetime use. As a combat aviator having to rotate back and fold things would have been a non-starter for me with the chance that the pilot might loose his head to a 7.62 or higher round. Zipping around above 200 knots with a dead pilot and no means to immediately take control would not have passed the make sense test with most Army Aviators I think.

 

[Abraham Gubler]

One of the pieces of technology I think would have been quickly been superseded was the gunners rotating seat. Two crashes where the co-pilot gunner could not take control of the aircraft because his seat was out of place would have reduced it in peacetime use.

No doubt. Through the 1970s the application of television technology matured enough to make the rotating periscope redundant. Of course this wasn't the case when the AH-56 was designed. I don't recall any complaints about the XM112 being recorded and certainly it didn't lead to the AH-56 program demise.

The XM112 didn't need to be folded for the gunner to pilot the helicopter but it did improve instrument view. The seat did have to be straight ahead before the gunner's flight controls would work however. The seat had two electric motors and with both operational could rotate at around 120 degrees a second. Since it only had a rotation of 110 degrees to each side this would mean it would take under a second to return the seat to straight ahead from the maximum off bore sight pitch in an emergency. The gunner had a special control for this as well. If one of the rotation motors was inoperable the seat could still rotate at 50 degrees as second. A manual backup was also fitted and it was designed specifically for returning the seat to straight ahead.

 

[Abraham Gubler]

CL-1051, crew of three, six passengers. A bit more like a Mil 24 "Hind."

This would appear to be the AH-56 derivative targeted for the US Navy's CSAR requirement. In the 70s this requirement was for something fast and VTOLish with some weapons that carried a hoist operator and a diver that was able to rescue naval aviators and NFOs who had been shot down. The McDonnell Model 260 multimission VTOL has a similar response. Obviously based upon a lot of Vietnam experience. The similar in solution HIND was developed for a KGB border guard mission which also required rapid VTOL deployment of small teams under lots of suppressive fire (they took border guarding seriously!).

 

[Stargazer]

I'm a bit lost now among all this expert talk, but it's interesting nonetheless that the Cheyenne vs. Apache debate is still relevant some 35 years down the line.

Is there a place where I could find a point-to-point comparison between the AH-56 and Sikorsky's S-67? They were made to the same requirements, I believe, but the Blackhawk never got a chance to prove its worth, being crashed during flight tests. Thanks for your tips!

 

[sferrin]

I'm a bit lost now among all this expert talk, but it's interesting nonetheless that the Cheyenne vs. Apache debate is still relevant some 35 years down the line.

Is there a place where I could find a point-to-point comparison between the AH-56 and Sikorsky's S-67? They were made to the same requirements, I believe, but the Blackhawk never got a chance to prove its worth, being crashed during flight tests. Thanks for your tips!

I don't know that they were directly comperable as the S-67 could also carry troops.

 

[JohnR]

I had never given much thought to the rotating W/O seat on the AH-56, until I saw a documentary on it on TV. More poor litte luddite head cannot get round the reason for needing for the seat to rotate, and I still don't get it after reading your posts.

How were the sighting systems mounted, were they inside the cockpit, or on the airframe?

I have tried finding one of those numbered cut away images that helps me understand aircraft layout.

I would be grateful for any clarification.

Regards and thanks for your patience.

 

[Triton]

I'm a bit lost now among all this expert talk, but it's interesting nonetheless that the Cheyenne vs. Apache debate is still relevant some 35 years down the line.

Is there a place where I could find a point-to-point comparison between the AH-56 and Sikorsky's S-67? They were made to the same requirements, I believe, but the Blackhawk never got a chance to prove its worth, being crashed during flight tests. Thanks for your tips!

Don't you mean the Sikorsky S-66? Sikorsky submitted the S-66 for the US Army's Advanced Aerial Fire Support System (AAFSS) program.
http://www.aviastar.org/helicopters_eng/sik_s-66.php

In 1964, it submitted the S-66 project to the US Army for the AAFSS specification, calling for an aircraft with a maximum speed of approximately 418km/h and ten minutes' hovering capability.

The S-66 looked very much like the Lockheed AH-56A Cheyenne (which won the contest), but had a Rotorprop tail rotor which could rotate its axis through 90° to act both as a conventional anti-torque rotor in horizontal flight and as a pusher propeller, thereby transforming the S-66 into a compound aircraft in cruising flight.

It's interesting that Sikorsky also submitted a design for the AFSS program that could become a compound aircraft in cruising flight. I understand that the Army chose the Lockheed design because it offered less technical risk than the Sikorsky submission with its Rotorprop tail rotor.

The S-67 Blackhawk was an unsolicited proposal by Sikorsky when the AH-56 program was in trouble and after the S-66 had lost the AAFSS competition.

Artist impressions of the Sikorsky S-66 appear below.

 

[Abraham Gubler]

It's interesting that Sikorsky also submitted a design for the AFSS program that could become a compound aircraft in cruising flight. I understand that the Army chose the Lockheed design because it offered less technical risk than the Sikorsky submission with its Rotorprop tail rotor.

They were the conservative submissions (see Convair)! As to the technical risk of the Rotorprop that may be so but there are other issues counting against it. The primary advantage would be reduction in weight from any weight penalty of having two rotors compared to the swivelling system. By swivelling the counter torque rotor to a pusher propeller as the main rotor unloads the S-66 loses the advantages of having a separate propeller.

In the case of the AH-56 this included using the propeller to change the pitch of the helicopter in hover, using it for rapid acceleration and deceleration from and into hover and using it as an energy gathering windmill for autorotation. All very nice things to have on a helicopter that would not be available if the propeller was swivelled to the side as a counter torque rotor.

Also analysis of compounding showed that completely unloading the rotor in cruise flight lead to too much drag so it was left with a 20% lift load. Which would be generating torque so the S-66 would not be able to have the benefit of all the thrust of the propeller unless the rotor was completely unloaded as it would need to stay swivelled (to some degree) to maintain counter torque.

 

[Abraham Gubler]

I had never given much thought to the rotating W/O seat on the AH-56, until I saw a documentary on it on TV. More poor litte luddite head cannot get round the reason for needing for the seat to rotate, and I still don't get it after reading your posts.

The XM112 or Swivelling Gunner's Station (SGS) is a single assembly that rotates the various gunners’ sights with the gunner to provide a view across the hemisphere of the aircraft. It did so to avoid the limited field of view of having to fit a rotating mirror to a periscope sight (as in the AH-1) and the huge weight, cost and complexity of 1960s digital systems (as in the A-6A TRIM). Because the gunner swivelled with the sights they could be connected to the gunner’s eye sight by a simple fixed periscope (within the entire rotating assembly). Since the SGS was more than just a single optical sight having a FLIR, laser rangefinder and the target and missile tracking sights of the TOW system at the time the AH-56 was developed this was the least complex, weighty and more reliable method of providing a wide field of view.

 

[Orionblamblam]

Coming soon:

 

[Mark Nankivil]

Sweet! Mark

 

[Spring]

Some people claim the double tail rotor on the AH-64 increase it survivability against damage on the tail and loss of control due it failure, i don't see how, since both must be linked with the same local gearbox.

Anyway, about the engines, i could not tell which design was safer, but an engine can get jammed with a impact, it think the practical requirements for such aircraft was not to survive a direct impact, but to survive an indirect one, and i think the AH-64 was better suited for that.

The video is interesting, look how the AH-56 completely miss the target with the rocket volley.

How the pilot's protection could be compared between the AH-64/65

 

[Lampshade111]

The MBT70 had two things which didn't work that well. A gun launched missile and a counter rotating driver's position. Neither have anything to do with the AH-56 except for the later an apparent but non-existent physical similarity. On the MBT70 as the turret rotated the turret mounted driver’s station would counter rotate to keep the driver pointed forward (so they could see where the vehicle was going).

There were several other things wrong with the MBT-70 too, but I won't get into that.

On the AH-56 the gunner had a gyro stabilised rotating periscope that swilled the gun sights to the target under the gunner’s command. This system had nothing to do with flying the helicopter. If the gunner wanted to use the emergency controls in this cockpit the seat had to be swivelled to the front and the periscope folded down before the flight controls would activate.

I wasn't referring to the ability to fly the thing, just the maintainability, weight, and practicality of the setup.

Do you have any evidence to support this statement? The only thing the AH-56 has that the AH-64 doesn’t is a second gun (minigun or grenade launcher) and the TOW guidance system. It doesn’t have a laser designator (it has a laser rangefinder but the difference is significant), two helmet mounted sights (only one) and it doesn’t have a second engine. Going by this balance sheet the AH-56 is way out in front thanks to the engines.

By the service entry of the AH-64, most of the technologies were more mature, it didn't have two gun turrets, didn't have the whole gunners position rotate, didn't have the pusher propeller and associated systems, had a more proven rotor configuration, and the maintained requirements of two engines are outweighed by the benefits.

And I know about the use of turrets on fixed wing aircraft, yet it is on a helicopter flying at 200 mph a few meters above the ground where I doubt it's utility.

Ahh... there are turrets for sensors and weapons on the AH-64, the AH-1, the RAH-66, the Tiger ARH, the Rooivalk, the HIND, the HAVOC... So I guess none of these helicopters gets any “utility” out of their turrets? The HOKUM doesn’t have a gun turret (fixed forward firing guns) but it also doesn’t have a separate gunner...
[/quote]

You knew what I was referring to, now your just being snide. I was not talking about the gun mounting itself but having the whole damn gunner's position rotate.

Well it’s not “based” on the better flight profile of the AH-56 (I had three layers of improvement) and this has nothing to do with Vietnam. The AH-56 and the AH-64 share the same flight profiles for the same missions. There is no difference.

If the AH-56 is engaging in long range missile shots to defeat a Soviet armour thrust across the North German Plain then it will do so hovering from near cover. The only difference is in the missile guidance system (Hellfire vs TOW) which can’t be included in any fair benchmark between the two.

The only difference is when the AH-56 has to fly from A to B it does so much faster than the AH-64. It can also accelerate and decelerate much, much quicker. When hovering it can also point its nose up or down depending on the situation enabling for much firing control.

I need to pick up that book on the AH-56, yet from what I have about the development of the AH-64, it was superior in NOE flight and maneuvering to the AH-1 and other designs tested over the years. Even if it had TOWs in the place of Hellfires I believe the AH-64 would have the advantage when in it's element, popping up from cover to engage armor on the West German battlefield.

I will yield to you that the rotating seat probably was the best solution at the time, and eventually remaining reliability issues could have been worked out. However I don't buy your idea that it's single engine configuration would have been more survivable than two engines, just it is less exposed. And in the event of a engine failure not caused by enemy fire, the single engine design is worse off.

 

[Abraham Gubler]

You knew what I was referring to, now your just being snide. I was not talking about the gun mounting itself but having the whole damn gunner's position rotate.

I wasn’t being snide I was just pointing out how illogical your statement is. Rotation is the same thing be it for a gun or a sensor head or an aircrew sitting on a periscope. The change in angular relationship is only made extreme when the two points are very close. For such engagements the AH-56 had the first helmet mounted sight to enable rapid tracking of targets.

And in the event of a engine failure not caused by enemy fire, the single engine design is worse off.

Which is why I had prefixed the argument with single engine bay versus multiple, separated engine bays. What this means is the AH-56 could have a twin pack engine mounted within the single bay like the AH-1J to provide the one engine out safety in non combat situations. This is a separate issue to combat survivability.

I need to pick up that book on the AH-56, yet from what I have about the development of the AH-64, it was superior in NOE flight and maneuvering to the AH-1 and other designs tested over the years. Even if it had TOWs in the place of Hellfires I believe the AH-64 would have the advantage when in it's element, popping up from cover to engage armor on the West German battlefield.

I will yield to you that the rotating seat probably was the best solution at the time, and eventually remaining reliability issues could have been worked out. However I don't buy your idea that it's single engine configuration would have been more survivable than two engines, just it is less exposed.

This isn’t a contest. You have come online here and shouted several opinions without much of an understanding of the helicopters in questions or how they are used. Reading a book about the AH-64 is not enough to make such statements. That you have brought in with your ego only makes things worse for everyone else around this place. I don’t care what you think about the helicopter I’m not running a pro Cheyenne political party. I’m simply offering facts and analysis about its capability. Believe whatever you wish.

 

[Stargazer]

Some people claim the double tail rotor on the AH-64 increase it survivability against damage on the tail and loss of control due it failure, i don't see how, since both must be linked with the same local gearbox.

Perhaps in case of a blade being ruined by enemy artillery, the other rotor continues to do its job? Just a thought.

 

[Abraham Gubler]

Perhaps in case of a blade being ruined by enemy artillery, the other rotor continues to do its job? Just a thought.

I would suggest its the all too typical photo interpretation based analysis that passes for informed comment amongst much of the defence and aviation enthusiast community.

The AH-64 only has ONE counter torque tail rotor system. It is made up of two, two-bladed teetering rotors that have an azimuth spacing of 55°, instead of the more usual 90°. These rotors rotate as a single unit but are laterally stacked giving them the appearance of being separate. This design reduces the tail rotor flapping and supposedly noise.

It has nothing to do with survivability but the blind will lead the other blind into strange places.

 

[overscan (PaulMM)]

The four bladed main rotor on YAH-64 was said to be more resistant to damage than the 2 bladed one on the YAH-63, maybe someone confused main and tail rotors?

 

[Lampshade111]

I wasn’t being snide I was just pointing out how illogical your statement is. Rotation is the same thing be it for a gun or a sensor head or an aircrew sitting on a periscope. The change in angular relationship is only made extreme when the two points are very close. For such engagements the AH-56 had the first helmet mounted sight to enable rapid tracking of targets.

Yes but a larger/heavier setup being rotating means more power is needed, more mechanical complexity, and so forth. While it may have been the best solution with the technology of the day, by the 1980s it was a rather obsolete and overly complicated setup.

Which is why I had prefixed the argument with single engine bay versus multiple, separated engine bays. What this means is the AH-56 could have a twin pack engine mounted within the single bay like the AH-1J to provide the one engine out safety in non combat situations. This is a separate issue to combat survivability.

Ah, I see what you are referring too, however it would be a good idea to have a degree of spacing between the too, even if they are in the same "bay".

This isn’t a contest. You have come online here and shouted several opinions without much of an understanding of the helicopters in questions or how they are used. Reading a book about the AH-64 is not enough to make such statements. That you have brought in with your ego only makes things worse for everyone else around this place. I don’t care what you think about the helicopter I’m not running a pro Cheyenne political party. I’m simply offering facts and analysis about its capability. Believe whatever you wish.

I never said it was a contest, I just admitted I was wrong about that point. I see nothing to indicate the AH-56 would be superior in NOE flight and pop-up attacks, even with it's advantages at altitudes higher than this, and in more traditional attack runs, so we are going to disagree there. I am not trying to bring my or your ego into this matter, and would like to put that earlier business about "harsher words" behind us.

 

[AeroFranz]

Just to clarify somee points of contention, with easily verified facts

- The hingeless/rigid rotor is, at least today, well developped and superior to the articulated rotor in many ways, but especially maneuverability. If you have ever seen aerobatic teams flying the Bo-105 (or whatever EADS calls it these days) you know what I'm talking about.

-The widely separated twin-engine layout for helicopters is the result of the lessons learned in Vietnam and Joint Live Fire exercises - much thought went into its selection by the militaries. A quick market survey will reveal that twin-engines are desirable even for cost-conscious civilian operators, who don't have to face combat damage, and for all but the lightest helicopters. The twin-engine bias is likely to remain in place as long as helos will have dead-man's curves...

 

[JohnR]

To Abraham,

Thank you for the explanation, I finally get it. I think that reading info on the operations of the AH-64's FLIR etc and the arguments on roof mounted and rotor mounted sights, got me confused.

Regards.

 

[Abraham Gubler]

-The widely separated twin-engine layout for helicopters is the result of the lessons learned in Vietnam and Joint Live Fire exercises - much thought went into its selection by the militaries. A quick market survey will reveal that twin-engines are desirable even for cost-conscious civilian operators, who don't have to face combat damage, and for all but the lightest helicopters. The twin-engine bias is likely to remain in place as long as helos will have dead-man's curves...

The Cheyenne single vs Apache twin engine argument has got to be one of the most ridiculous cases of present vs past the world has ever seen. Might as well argue that the Spitfire was much more suited for WW1 air superiority missions than the Sopwith Camel.

To provide a choronology when the US Army cancelled AH-56 production they launched development of the AH-64 and production of the AH-1S. For the next 10-15 years the production helicopter was the SINGLE ENGINED AH-1S. Apart from having the same number of engines as the AH-56 it also lacked the weapons, armour, speed, etc making it far, far less survivable.

Come the 1980s and production of the twin engine AH-64 is launched. Which in an alternate universe populated by AH-56s could have been easily matched by a twin engine AH-56.

Further the problem with single vs twin combat survivability is much of the analysis is based on unarmoured or under armoured helicopters. When building flying tanks one should take a leaf from the tank book which is reduce surface area exposed to the enemy as much as possible. The primary driving influences of every tank since WW2. Tank designers know a lot more about armouring than helicopter designers. The case remains open.

 

[Evil Flower]

The only difference is when the AH-56 has to fly from A to B it does so much faster than the AH-64. It can also accelerate and decelerate much, much quicker. When hovering it can also point its nose up or down depending on the situation enabling for much firing control.

How does that work though? It sounds like it could be a handful to control.

 

[AeroFranz]

Come the 1980s and production of the twin engine AH-64 is launched. Which in an alternate universe populated by AH-56s could have been easily matched by a twin engine AH-56.

Further the problem with single vs twin combat survivability is much of the analysis is based on unarmoured or under armoured helicopters. When building flying tanks one should take a leaf from the tank book which is reduce surface area exposed to the enemy as much as possible.

Ok, my bad for not being clear enough. I wasn't trying to say that the AH-64 of the 1980s was better than the much earlier Cheyenne because of the twin-engine layout. The Cheyenne airframe is IMHO vastly superior. I was saying that the twin layout in general is, according to vulnerability studies, more survivable. I actually agree that in due time the Cheyenne could have easily acquired two engines, and still fly rings around an AH-64. In helicopters, the engines are not tightly integrated into the primary structure and thus can be moved/changed around quite a bit, provided the cg remains more or less constant.

Now, regarding the question of increasing surface area vs having redundancy, it's basically a matter of trading the two aspects of survivability: susceptibility and vulnerability. Susceptibility deals with the issue of "what are the chances of getting hit?". Obviously, the greater frontal area of 2 engines is a liability in this case. Vulnerability deals with the question of "If I am hit, how does that impact the outcome of the mission?". In this regard, a twin-engine design is fundamentally better, all the more if the increased separation between the engines diminishes the chances of having both knocked out.

So it's really hard to trade the two. All I can say is that since Vietnam, survivability has evolved into a science, so it's not just philosophies and guesses. On top of the Joint Live Fire tests, where airframes are put on pylons and getting the hell pounded out of them by actual AAA and Manpads, there is software out there used to analyze the various PKs based on aircraft geometry and libraries of weapons, fuzes, etc.
The result of these studies are twin-engine attack helicopters with more or less separated engines. I can't think of dedicated, modern single-engined attack helicopters. BTW, the twin engine bias could be as much a result of survivability consideration as psychological peace of mind for the crew, who knows it has a good chance of limping back to base if the s@!t hits the proverbial fan.

 

[Abraham Gubler]

Now, regarding the question of increasing surface area vs having redundancy, it's basically a matter of trading the two aspects of survivability: susceptibility and vulnerability. Susceptibility deals with the issue of "what are the chances of getting hit?". Obviously, the greater frontal area of 2 engines is a liability in this case. Vulnerability deals with the question of "If I am hit, how does that impact the outcome of the mission?". In this regard, a twin-engine design is fundamentally better, all the more if the increased separation between the engines diminishes the chances of having both knocked out.

Spoken like a true aircraft engineer!

Vulnerability is moderated by protection. Of course in aircraft where every pound is a thief of critical flight performance protection is primarily provided by design. But armour, and I mean real thick stuff not just a 12.7mm sheet of 5083, can provide significant protection. The key to armouring is reducing the surface area of the object to be armoured as this allows your armour to be thicker for the same weight margin.

Now for the sake of simplicity assuming same sized square rectangles for engine bays to have one engine bay protected from a full sphere requires 6/11 of the armour of two side by side bays (with armour between engines). Which means for the same weight your armour can be 145% thicker.

In the case of AH-56 vs AH-64 the position of the engine bays also has effect as much more of the aircraft’s other structure and systems provides layered protection from fires so even less surface area requires protection. For defence against ground fires this means heavy armour only on the sides and rear of the engine and moderate armour on the front and bottom of the engine bay. Whereas to provide similar protection the AH-64 would need two sets of heavy armour on the bottom, outer side, front and rear of its engine bays (not to mention one between the engines to avoid sympathetic destruction).

Designing a helicopter like a tank and a single engine bay can provide significantly more survivability. And not just attrition survivability but mission survivability. Much better to take a hit and still be able to fly and fight rather than to have to flee to safety.

 

[AeroFranz]

Now for the sake of simplicity assuming same sized square rectangles for engine bays to have one engine bay protected from a full sphere requires 6/11 of the armour of two side by side bays (with armour between engines). Which means for the same weight your armour can be 145% thicker.

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.
(I duly note that you prefaced your discussion with some concessions to simplicity!)
So 145% thicker armour is on the optimistic side, but probably not by a whole lot. The fact remains that in the case of single engines, you have more weight allowance for armor. If anything, the more powerful single engine will weigh less than two separate engines of same total power, so that allowance grows even more.
If we look exclusively at how to reduce exposed area and maximize armour protection, I would have to agree that a single engine would be preferable.

This being said, twin engines has been the standard for highly survivable aircraft (fixed wing, rotary..), so I can only assume that

a) Aircraft designers (both western and eastern) fell prey to a multi-engine fad
b) Other factors are considered in the final layout decision. I have only vague suspicions of what those could be.
c) ?