I spotted a really interesting post when I was googling for AH-56 specs. I wouldn't normally copy from other forums but I think you'll find this interesting;
ch1466 wrote:> retreating blade stall
RBS occurs when the blade on the retreating side of the airframe is going faster in the combinaiton of 'slipstream effect' than it's own motion can compensate for to generate lift.
In a _conventional_ helicopter, this matters. Because you effectively must twist the blade more and more to take a bigger bite out of the air until eventually you stall it completely as blade AOA stagnates all flow and the aircraft rolls incontrollably in the direction of retreating side lift loss.
On a rigid rotor system the blades 'unload' (cease to generate cyclic or collective pitch changes) as speed increases and lift is transferred to the wings and conventional aircraft controls (rudder and elevators). In this the rotor disk is nothing more or less than a spinning disk, storing energy for use as the speed decreases and it's services are required again.
> boom chop
ALL helicopters have what is known as a harmonic blade resonance frequency which typically is a fraction of the number of blades vs. the number of cycles needed to build up a flapping motion in them which eventually becomes divergent to the point where they move so far out of disk plane that they strike the cockpit or the boom. The cure for this is simply to slow down and unload the rotors until they work themselves out of the phenomenon. It is such a common occurence at it's most basic level that it called the 'half P (blade) hop'.
THE FIRST loss of the Cheyenne occured as government displeasure with the rate of testing required Lockheed to undertake some rapid FSD envelope clearance and validation. One phase of which involved 'pulsing' the main rotor to /exacerbate/ an existing .5P hop.
The test pilot involved pulled a stupid (as indeed 99.9999% of all airframe related accident issues in testing involve some degree of) in that he turnd off the friction locks on his collective so that when the blade hop started he could have more 'feel' for what was going on. This was _against the dash-1 warnings_ but like all TP's, this man thought he was more than mortal and so when the half-P hop phenomenon started, his frickin' ARM acted like a bobweight to amplify the motion through the collective and because his inner ear and visceral ballance senses were /just enough/ out of phase with the up-and-down porpoising of the airframe. He did not to realize the extent to which his own ham-fistedness was magnifying the problem. The result: A blade cycle that went so divergent, so fast, that they descended to go right through the rear canopy, beheading the bastard as he sat.
HIS FAULT ENTIRELY. In a test program already rife with politics 'pushing the envelope' of rotor/transmission system testing before the airframe was even really cleared to all corners of flight.
The /next/ incident involving the Cheyenne was in a further hurry-up CYA effort by Lockheed to prove it wasn't their machine's fault. For which purpose they fixed an AH-56 to a pole in the giant Ames-Dryden slowspeed tunnel and then induced the same phenomenon. In doing this, they forgot that, without the fuselage being able to flex with aerodynamic responses to the blade plane divergence (self dampening), the total torque on both the rotor system and the airframe would be /astronomically greater/ than it was in free flight. So all their remote-boot engine kill and collective/throttle controls meant nothing when the system again was stressed beyond all normal levels. And the airframe wrenched itself off the pylon mount and bounced around the tunnel for several seconds.
The last incident occured when some a non-pilot rated, labcoat wearing, chimpanzee decided to do some in-cockpit checks with the systems running. And yet again ignoring flight manual dictated procedurals did some minor damage.
NONE of these issues were the fault of the Cheyenne. They were the result of stupid people doing stupid things because they didn't care to think about the prerequisite 'total synergy' of their actions upon and with the mechanical systems of the helicopter.
> Helicopters have a forward speed limit. It is written in the
> aerodynamic math of the main rotor, and no amount of
> tailborn push is going to stop the headslap chinbang if
> you violate it.
Helicopters can go as fast as any forward swept wing or scissor wing airframe. Which is to say more probably more than Mach 1 if that is what you are willing to engineer the airfoils to Q+G sustain for the given density altitude. All's you have to do is stop the rotor as the X-50 CRW is indeed experimenting with (and the RSRA before that).
If you /don't/ stop the rotor there are some issues related to maximum tipspeed (at which point you BURP them) but even so, the XH-51 'Little Chief' did more than 300mph and the S-72 RSRA and S-69 ABC both came close.
It takes a conventional penny-farthing layout in which you are unrealistically asking the main rotor to both provide propulsive and lifting forces through a complex mechanical articulation system of cyclic and collective pitch modifications before you hit a real speed limit around 160-170 knots.
Some alternative views:
1. The AH-56 was a /very/ conceptually 'bleeding edge' aggressive in it's specifications. Made so by the General who rejected it's predecessor on the basis of it not being a sufficient leap ahead.
2. The AAFSS (Advanced Aerial Fire Support System) did NOT come 'after' the AH-1G/Bell 209 demonstrator but rather predated it by about three years. It was lags in the Cheyenne that made the 'interim' helicopter dominant by virtue of presence on the battlefield.
3. The AH-1 is a killer of pilots. And always has been. Because if you want /danger/ you go with a teetering blade system that cannot withstand -any- negative G 'float' before the lead-lag flapping hinges on the blade root run an imminent and exponentially increasing risk of 'mast bumping' which sheers the main rotor at the shaft. Then there is the highly loaded tail rotor (effectively a consequence of putting too much torque through the enhanced transmission to take all the weight added on the T53 engine) so that if you have a hangfire rocket OR a AAA OR a tree hit on your tail rotor, /at any speed/ you cannot maintain directional control of the airframe. Contrary to the UH-1 which has so much frontal mass running through the air that it simply will fly on.
Hundreds of class action law suits filed by families of U.S. servicemen lost for little or no damn reason to cheap engineering on the Snake (given the King Cobra was running a 4 blade rotor with full articulation in 1973 or so) have been quashed under a federal statute which immunizes companies supplying military systems on the basis of 'Necessary for the national defense' without even a corrective requirement to ensure that the VERY SAME accident doesn't happen /again/. (And again and again, some 35% of AH-1 losses are due to 'aggressive manuever in adverse portions of the envelope').
4. The Rigid Rotor harmonic controls were incredibly primitive on the original AH-56 spec. Basically flying barbell counterweights with tensioning guy wires. TWO completely new rotor/transmission systems were later tested, one in a grueling 1,200hr, 6 month campaign flown by a SINGLE test pilot who believed in this airframe. And the first one largely eliminated the problem while the electromechanical followon provided complete damping of the .5 and 4 p hop phenomenons.
5. ANY rigid rotor system is going to be superior to ANY flapping, articulated (hinged in both lead/lag and vertical planes) or composite-bearing 'flex' system in terms of blade performance. Because while you can use a variety of active, passive and mechanical systems to control blade resonance effects to and from the helicopter fuselage, you cannot prevent the blades from 'digging, chasing or gyrationally diverging' from the preceeding blade path through the air. Based on a given external vector impetus. Not unless the blade system is itself _rigid_ to prevent these kinds of behaviors, always 'cutting cleanly' through the disk plane. Which is why the AH-56 was among the first helicopters to roll and loop. However useless these maneuvers are to the majority of even the combat flight envelope, they DO demonstrate the ability to fly much more 'carefree' in the normal maneuvers ('pulled' [loaded] wing over to a FFAR dive attack no longer has stall limits for certain airspeeds or hot-high densities/loadouts as it did on the AH-1G operating in Vietnam's highlands.).
6. When you factor in the pusher propulsor to the back end, 'agility' increases at least another FOUR FOLD. Because you are no longer required to tilt the main disk to accelerate, an act which throws off pilot/weapons sighting and which adds tremendous amounts of drag to the airframe even as it drastically alters the 'cushion' footprint, changing effective lift at the worst possible moment of IGE/OGE transition. An AH-56 Cheyenne could hit 200 knots as fast as an AH-1 could hit 140 ''from a standing start' on the tarmac. Simply because it climbed straight up and out. The propellor also acted as a dive brake which meant that the aircraft could engage in VERY slow approaches, making the utmost of it's computed sighting while hauling upwards of 114 rockets. This at a time when the heavy hog loadout on the AH-1 was /useless/ beyond about 2,000ft (and for newbies often less than 800) so that they were salvoing off their grand total of 52 rockets in single pass just to hit ONE 'suspect
treeline'. And the only way they knew if they hit or missed something was when (thanks to envelope and power restrictions) they overflew it 4-7 seconds later to take 14.5mm from gun pits hidden on the opposite side.
Thus, both in TFR terrain hugging flight (what would later be called 'NOE' solely because the helicopters could not aggressively follow the terrain at speed and so were more or less using static hides and bobup maneuvers similar to an infantry sprint-to-cover system at 0 knots). And medium altitude (3,500-5,000ft takes you beyond 80% of the small arms envelope) the Cheyenne was a completely superior platform /then/ and NOW to the Snake and Indian.
The latter because the Apache still has to point it's nose at the dirt to kill something while it's weapons system's lack of range ensures that the inhabitants of said dirt will always see them do it. And thus to climb back /out/ of the threat's 'logical response' means tilting the plane of the rotor away from forward speed and back towards pure lift. Which is why the Cheyenne climbs -up and away from- the threat area at about 3,424fpm and 180 knots or roughly the equivalent of a mid-WWII fighter. And the Apache is lucky to hit 2,700fpm and 100knots, sitting right ontop of it. While carrying half the payload.
7. The speed of a helicopter is VERY important. Because it dictates how well you can transit to and from any basing mode to a given fight. 80-90% of each mission being spent in TRANSIT to and from the target. To, and you're heavy and slow with weapons and gas that makes navigating in hot/high conditions very tricky for power settings. From and you have battle damage or a limited fuel conservation profile to make RTB on less than 3,000lbs of starting fuel.
The Cheyenne (and indeed most compounds) simply /don't care/ about rated torque and power through the transmission because, again, they are flying like an airplane on the constant thrust and lift of wings and a propulsor.
In real world terms this means that you can mass 200 knot forces against armor from outside divisional area and even go /well/ beyond the FLOT to back 2E followon forces. Rather than have a 60-90knot 'NOE because of the air threat' system which is little more than a faster tank.
In SEA, it meant that instead of a Red or White team sitting on ramp alert with a 30 minute from start TOT. Or staging forward to a FARP (instantly telling Charles and the NVA about activity levels). You could be over a remote location in 10-15 minutes from fewer, centralized, bases.
8. In high intensity warfare only a /complete moron/ takes a 5 million dollar (AH-56, then) let alone a 14.5 million dollar AH-64A or a 56.5 million dollar (initial spares and weapons) AH-64D, where a drone or standoff sensor asset (SOTAS, Orchidee or Pave Mover) can better go. Attack Helicopters are weapons carriers and good for damn little else, ideally, this means you run out a datalink or fiber optic tether the 20-30km you need to handoff from the area surveillance asset to the weapon seeker with a man in the loop for target confirmation. At worst, you sit behind a hill and loft Hellfires while some utterly-replaceable moron on the front side of same points a GLLD or MULE at the tank column 3-5km downrange.
Indeed, it was the capability to use UAVs to perform the 'scouting' mission (AMUST and others) that actually killed the RAH-66 as /somebody/ finally woke up and did the math which proved that a 120-140 knot (Low, not NOE) Apache force could not be directed by a 150-160 knot Comanche lead scout team, even with realtime 'ALERT' driven sensors, if both had to obey the same rules on optical threats and LOS masking.
Conversely, a 100-110 knot Outrider/Shadow or ERMP Predator could not only make better time to distance as a _straight line_ but were effectively invisible at 10,000ft or greater while seeing further downrange so as to track and handoff multiple AREA threats (true surveillance and target **finding** as opposed to fixed-point targeting of a known threat). With this in mind, the Cheyenne was crippled by the TOW system but would not have been by the Hornet (TV guided precursor to the Hellfire = fire and forget) or the Shillelagh (RFCG hypervelocity = thruweather and fast salvo engagement) for either the bright-clear Middle East or the fog-haze-lowcloud obscenity of typical NATO Centfront meteorology.
These were 'under test' as later weapon upgrade options. Even as variants of FOG were examined for the AH-1 Cobra (and would have so vastly improved it's abilities beyond the AH-64A's as to make the Indian obvious for the money thieving hackjob it is).
It should also be noted that the AH-56 had a TFR and formation keeping radar pod at a time when the Apache PNVS and doppler-nav was just concept art (and one which works like /crap/, even today).
As to the look-down sensor orientation in the XM112/AAS-25 PINE turret and gunner's station, you need to think less like a blackhat wearing cavalry wannabe and more like a 'true gunship' along the lines of the AC-130 or OV-10D NOGS. Because the XM-51 and 52 could be AIMED downwards across more than a miles worth of slant, they could run racetrack orbits at the 5,000ft (or greater) level I mentioned which means that they are untouchable in a Mogadishu environment while carrying as much as 3 hours of fuel and 2,980 rounds of ballistic ammunition. And _free fall munitions_ (tested with Mk.81 and .82 and 117. Now we would be talking about the GBU-12 LGB and GBU-39 SDB). None of which requires it to pull a Polish Indian mistake of diving towards a target (constantly pressing the slant angle limits for VnE) only to climb up and away at half the speed.
EW would have to be upgraded to defeat the guided threat but to be honest, this is true of all modern helicopters with the possible exception of the SOCOM 'special mission' birds since we never put into production the SRFC systems (Comanche was supposed to make them unnecessary) which included an AAR-57 CMWS missile approach warning sensor, ASTE level expendable countermeasures (in more than one bucket) or particularly the ALQ-212 ATIRCM which is the _only_ system out there which can counter all three principle MANPADS bands. ALQ-144 and a single M130 on the Apache is a complete joke, not least because 'NOE' virtually /requires/ you not to carry any flares at all for fear of igniting the surroundings.
CONCLUSION:
The USAF took one look at the OV-1 Mohawk and screamed Key West. Not because it was terribly effective but becasue it was so much better than /any/ helicopter. And it was 'there'. Available for use. The USAr sighed and built the AH-56 Cheyenne and once more the Blue Suited Bullies were scared pissless by the notion of night visionics, INS/Moving Map navigation and fully stabilized weapons calc in an airframe capable of performing BAI or OBAS, fully 100-150nm further than helicopters were supposed to go into exclusively AF turf.
Thus they threw their weight around in Congress and 'in trade for greater commitment to the support of ground troops mission' (Super COIN becomes AX becomes the utterly worthless A-10), funding for the AAFSS dried up and THIS is what left the Army having to 'find reasons' for faults that either were non existent (stability in the hover transition) or which had been fixed (the rigid rotor harmonics issue). If they tried pulling that stunt in the 1980's, we would remember the AH-56 Cheyenne, not the A-12 Avenger II as the unlawful program termination 'for convenience of the government' by which Lockheed Martin won 3-5 billion dollars for work performed, /above and beyond/ specification. On a platform that performed better in 1972 than anything we have flying today. And was more appropriate to the contemporary mission set (COIN) than either the Snake in SEA. Or the Indian over some Baghdad slum.
]YouTube[/quote] video of it.
