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About a month before the Marine Osprey crashed at Marana on 09 Apr, I was in email correspondence with a well-known British VSTOL Test Pilot and a learned Professor in a German University about angle-of-attack indicators and their usefulness to the pilot. In our exchange we concluded that such an instrument would be of no use to an Osprey pilot, and I remarked that many things about the Osprey were unfathomable. Specifically, engine failure was protected against - but what about transmission failure, rotor pitch-lock and vortex ring? Following the accident I theorized, in a 9 Apr email to the Air Safety Week Editor, that it had probably been due to vortex ring. A few weeks later the Defense Dept stated that it was. This caused me to reflect further on the aerodynamics involved and the repeatability of that crash. It seems to me that the conditions for recurrence are inherent in the Osprey configuration.

 

A slight pause here to bring you up to speed on tilt-rotor theory... In 1981, using experience gained from the XV-3 and XV-15, Bell Helicopter Textron and Boeing Helicopters began developing the V-22 "Osprey". The unique feature of these aircraft is the two large, three-bladed prop-rotors mounted at the tips of the wings. For takeoff, the prop-rotors and their engine nacelles are rotated to the straight-up position where the lift developed is entirely propulsive. The V-22  climbs vertically into the air like a helicopter. Once off the ground, it can fly in one of three different modes:

(a)  as a helicopter,

(b) in the partially converted airplane mode;

Converting rapidly from the helicopter mode to the airplane mode is accomplished by continuous rotation (tilt) of the prop-rotors’ nacelles from the helicopter rotor position to the conventional airplane propeller position. During the twelve second conversion period, the aircraft speed increases and lift is transferred from the rotors to the wing. There’s a 100kt band where the V-22 can be in either mode; and

(c)  operating as a conventional airplane, when the V-22 can cruise efficiently at much higher speeds than a helo.

To land, the prop-rotors are rotated up to the helicopter rotor position and it’s flown as a helicopter to a vertical landing. In fact, there is a fourth possible mode, in which by rotating its prop-rotors and engines to different angles the V-22 can operate as a short takeoff and landing aircraft. Prop-rotor tilt angles of 60 to 70 degrees produce lift from both the prop-rotors and wings. After rolling a short distance along the runway, the V-22 is airborne. In this STOL mode, heavier payloads can be lifted than in the VTOL mode. The NASA Ames Research Center Vertical Motion Simulator (VMS) validated much of the tilt-rotor concept and so, over its 30 years of development, the Osprey seemed to have had most of its glitches ironed out…. until 9 Apr 00.

So what is vortex ring and why is it more significant in the V-22? In a conventional chopper, if your low-speed, steep angle-of-approach flight-path approximates your rotor downwash you’re likely to experience “recirculation”. Imagine a loose-fitting plunger descending into a water-filled pipe and what happens to the water spill-over around the periphery of the disc. If that disc is a rotor-disc in air, then at the tips you are generating a rotating vortex. It’s very similar to the draggy spiral vortex that peels off behind the wing-tips of a fixed-wing aircraft (FW) and causes wake turbulence. The only difference is that a descending helo that gets into its own “vortex ring” will remain within it. Increasing power to reduce the rate of descent becomes similar to trying to pull oneself up by one’s own boot-straps; in fact increasing power (collective) just increases the rate of descent alarmingly (it empowers the vortex). If you take nil further action, you will simply be strengthening the vortex and remaining in that helix of  “bad air”. The only way that a helo pilot can exit this recirculation bubble is to jam the cyclic forward (i.e. incline the rotor disc forward) and accelerate out of the situation – shedding that vortex ring and leaving it behind. To do this successfully, he must have the height to convert to speed. Those who did not see the clues, or did not understand the problem, or were hindered by darkness, all completed their downward helix – never to be any the wiser.

 So how does a pilot recognize VR? At night, wearing Night Vision Goggles, he has lost some depth perception, so realization may be (too) late. In the day-time below 1000ft AGL he will experience ground-rush and that will worsen noticeably if he simply goes “up-collective”. Normally, for a chopper pilot, if he’s on the ball it will simply be a fright. What difference is there for a V-22? All the difference in the world.

The V-22 has contra-rotating prop-rotors so it will be unlikely that, in chopper mode, both rotors will symmetrically or simultaneously enter vortex ring. In fact, once “in the ball-park”, it may take as little as a gust-induced wing-drop to cause one set of rotors to fulfill the conditions for VR. Why so? The instinctive reaction of lifting a dropped wing with opposite stick causes the blade angle to increase on that dropping wing. But remember that we said that increasing power was a “no-no”, it will simply cause that dropping wing, if the rotor is already in VR, to drop even faster. Once the attitude is through 60° angle-of-bank, the nose will be dropping fast and, just as happened at Marana, insufficient recovery height ensures a crash. How do we know this?

We have the visual evidence of Dash One’s loadmaster. He saw it all out the back of the first Osprey. Well, you may ask, why cannot the pilots simply see that it’s happening, jam the stick forward and fly out of that bubble? Unfortunately the correct reaction to a gust-induced wing drop (differential collective) is the lethally incorrect action for an asymmetric VR condition and it all happens in a split second. How to tell the difference? Is there no warning? Can there be no recognition and reaction? Unfortunately there are no instruments, software programs nor aerodynamic preambles that will warn crews of impending asymmVR. Liken it if you will to a snap or flick roll entered in a FW aircraft by stalling one wing. We call that auto-rotation (not to be confused with the helo’s engine-off forced landing by the same name). AsymmVR is the tilt-rotor version of that flick-rolling FW auto-rotative process, but at least in a FW you have buzz (stick), buffet (airframe) then judder (visual heavy buffeting) to tell you that you’re approaching the stall.

The Marines contend that the key is to avoid entering a VR state in the first place. They say that if he’d kept within the flight envelope, he would not have encountered this problem.  Is the solution really that simple?  i.e. 800 feet per minute max rate of descent at a minimum 40 knots of air speed. Well, in my opinion you cannot legislate against accidents, particularly when the corridor of safety is so small. It’s like telling a fighter-pilot not to overstress during air combat tactics and then groaning when he does, perhaps due to inadvertently flying through some-one’s wake – except that in the Osprey there are no second chances. How did the Marana accident come about? I understand that he only had about 5kts of tailwind and the night visibility was good. The accident Osprey was formating on the leader and, in chopper mode, came from a left rear 7 o’clock loose tactical formation position, through line astern to abeam on the RH side (i.e. 3 o’clock to the leader). He overshot that position slightly and was correcting back and turning slightly in order to pick up his own line to his hover termination point – when the right rotor entered VR.

He had innocently fulfilled all the handling requirements for asymmVR in the existing environmentals (i.e.5kts of tailwind). The debate about whether his low speed and high rate of descent throughout the approach were contributory continues - but in my opinion, requires further analysis – after all, he was loosely formating on his lead ship. I’d suggest that he entered VR Country at the point where he’d overshot on the right and was decelerating relative to the leader, turning slightly to maintain his visual sight picture to his own LZ point. Up to that time all was fine - and at that point his descent rate was probably within limits. My point is that VR is a sudden handling development that normally occurs on an approach when the pilot is adjusting his flight path to maintain his sight picture and then drops below translational speed. What’s that? That’s the speed where you can stop thinking about blades and consider them, because of the forward airspeed, as an integral lifting disc. You can hear and feel the rotor-slap change as you pass through translational. Maybe what you’re hearing are the blade vortices being left behind. If the 5 kt tailwind hadn’t been present, who knows? But I would suggest that any time that you’re reducing power and parabolically steepening (i.e. arcing over) in your approach, in order to make your hover point, you are in VR Country.

But he was not to know any of that, so he tried to pick it up with some opposite stick (differential collective). That was when it really bit, and 2 to 3 seconds later the aircraft impacted inverted 90° nose down. Why does it all happen so fast? Two reasons really, the instinctive corrective action (differential collective) exacerbates the VR - and because the prop-rotor is out on that long moment arm. Talk about possible remedial action, once in VR, is simply talk. Is there a solution in sight? Software patches? Not that I can think of. It will probably always be there as a trap for both the wary and unwary. How are the civil versions looking? Not good.

Why hasn’t the VR condition been encountered before in the V-22 Program? Even in a helicopter it’s difficult to reproduce. It’s never demo’d because of this. Your first experience of VR will probably be as a mature chopper pilot. The Osprey was only released from its developmental harness at a Pentagon ceremony some few months ago. Up until that point, all flying had been in accordance with very prescriptive test schedules. Test Pilots like it like that because, if they bust the bird, it’s not because they were trying something unscripted. So, because it requires quite an unhappy coincidence of circumstances, it had probably not been encountered during developmental test-flying.

So what are the Marines doing about it? Is it airworthy? Is it truly crashworthy? They have reassured us that there’s no mechanical failure and that they have full faith in the MV-22. “We have found no structural or design flaws that would preclude safe flight operations and we maintain complete faith in the safety of the V-22,'' LtGen McCorkle said. They have recommenced flying and the Test Pilots at Patuxent River have written a Flight Test Schedule and are belatedly going to check out that VR envelope – hopefully at height and with great caution. One wonders what the spin characteristics are like with those massive gyroscopes on the end of each wing? But when I heard that they’d launched into a VR Flight Test Schedule and that the simulators had never been set up for VR (or asymmVR), I was motivated to look further into it. As far as I can tell, that particular investigation of the Osprey’s (and XV-15’s?) handling was overlooked. There’s over 50 academic Osprey papers on the American Helicopter Society’s web-site and loads of them on the NASA and Boeing sites, but nowhere can I find any mention of a check into the “susceptibility to vortex ring of a tilt-rotor”……vexing to say the least. Asymmetric VR would seem to be a rapid onset (and non-survivable) condition that is peculiar to tilt-rotors. It may prove to have been an expensive oversight. I remain unconvinced that the asymmetric VR condition will not repeatedly revisit the Osprey Fleet.

``I feel confident we can put them back in the air,'' Gen. James L. Jones, the Marine Corps commandant, said in an interview. ``Everything appears to have been working normally'' at the time of the accident. 

Perhaps so.

http://www.boeing.com/rotorcraft/military/v22/Exec.PDF

* ******** holds an ATPL and a commercial helicopter license and has flown over 14,000 hours as an instructor pilot in three Air Forces.

http://www.navair.navy.mil/V22/fltcntls.htm  

Osprey Flight Controls graphics

   

http://www.navair.navy.mil/V22/convert.htm  

Osprey Conversion Helicopter - Airplane

   

http://www.navair.navy.mil/V22/propul.htm  

Osprey Propulsion

   

Chopper mode

Airplane Mode

     
     
http://www.pprune.org/ubb/NonCGI/Forum11/HTML/000289.html  MV22 Osprey Accident Theory  
     
http://www.pprune.org/ubb/NonCGI/Forum1/HTML/007825.html  "Loss of lift blamed for crash"  
     

http://www.helis.com/news/2000/v22safe.htm

Marines declare Osprey Safe

 
     

http://www.pprune.org/ubb/NonCGI/Forum1/HTML/007299.html

Previous Osprey Crashes

 
     

http://www.defenselink.mil/news/May2000/t05092000_t0509asd.html

USDoD Pronouncement on Osprey Safety (accident detail)

 
     

http://www.boeing.com/rotorcraft/military/v22/

Boeing Tilt-Rotor Site

 
     

http://www.washingtonpost.com/wp-dyn/articles/A23202-2000May7.html

Washington Post Article

 
     

http://www.navair.navy.mil/V22 

V22 Site

 
     

http://www.usmc.mil/marinelink/announce.nsf

USMC Press Releases

 
     

Aeroworld Link

http://www.angelfire.com/me/swissair111memorial/modded/HN0519.html 

http://www.angelfire.com/me/swissair111memorial/modded/ThatVexingVortexRing.html 

AeroWorld Report

Link to Helicopter News Story (19 May 00)

Osprey Related Links

 

Addenda

Checkboard
Captain

Posts: 1036
Registered: Aug 98

posted 26 May 2000 17:57     Click Here to See the Profile for Checkboard   Click Here to Email Checkboard     Edit/Delete Message
How about, instead of re-engineering a 10° inboard tilt of the rotors when transitioning to hover, you just tack on a couple of JATO rockets, with an "Oh my God! Help Me!" trigger switch on the cyclic to accelerate out of a VR situation. Gotta be cheaper - and could be used in a few other tactical situations...

IP: Logged

http://www.pprune.org/ubb/NonCGI/Forum11/HTML/000289.html

That JATO solution would give an instantaneous clearance out of the vortex ring condition and (I guess) could be triggered by an FCS control follow-up circuit.

Scenario: Aircraft inadvertently enters AsymmVR during approach

i.e. 1. roll starts (within set FCS inputted parameters of airspeed, nacelle-tilt, Rate-of-descent and RadAlt height)

     2. pilot makes normal instinctive differential-collective input to correct (as if for a gust-dropped wing)

     3. FCS detects no roll-restorative response to side-stick applied in the correct sense (i.e. a decrease of

         some order of magnitude in the rolling moment, but in fact identifies an increase - which confirms that it's

        AsymmVR)

     4. FCS decision is "Uh Oh, this is bad Kharma" and auto-fires "set 1" of emerg JATO (fuselage mounted and

         deployed together with gear extension ).

      5. Aircraft accelerates out of vortex as roll-rate approaches bank of 60° (or thereabouts), regains differential

        collective controllability and aborts or does further circuit and approach (with "set 2" armed).

        FCS = PFCS (primary flight control system) AFCS=Auto  (both FBW aka fly-by-wire).

          Flight control laws are all set by software and easily changed.

In fact, in order to gather useful empiric evidence of the AsymmVR condition, they would need to have something similar (to JATO) in order to ensure safety. i.e. it would probably be a sine qua non of any realistic trials and testing. When you think about the problems of replicating the condition in a full or quarter-scale tunnel, they are considerable. No tunnel is large enough that you can guarantee that the amount of air entrainment within a "stoked" vortex is representative of the free-flight un-encapsulated open-air circulation.

The size of any such JATO kit need be no greater than 2000lbs of thrust for 15 seconds (= about 25kgs weight penalty per rocket, or about 100kgs overall). Perhaps simply having that four-simultaneous capability may also enable them to lower the safety margins in some other areas - to make up for it.  It could also be used for emergency overweight STOVL departures (embassy bug-outs). Alternatively an under-fuselage (over-fuselage) centre-line thrusting pair may do the job (and avoid the possibility of asymmetric firings). C130 had six JATO bottles per side but two per side would probably do the trick for the Osprey, two attached to each fuel sponson and firing a set at a time in the AsymmVR scenario.

Minus factor include the presence of explosive ordnance (particularly on a carrier), as well as the weight. A properly programmed FCS should preclude any nuisance firings.

There may be another vital VR factor at work here. The original XV-15 (upon which they did all the early proof-of-concept work) only had a 90 degree nacelle tilt capability. Later, in both the XV-15 and In the V-22 (all variants AFAIK) they increased this to 95 degrees in order to be able to back it up on the ground or reverse whilst hover-taxiing (an important capability on a crowded flight-deck as well as on a crowded tarmac). It makes sense to me that they may have had the use of that rearwards thrusting capability in order to slow more quickly on the approach in helicopter mode (i.e. it's unlikely to have been "gated out" because they had no way of knowing that it could severely exacerbate any AsymmVR problem). To explain this last point, think of it as being very similar to VIFFing in the Harrier (Vectoring In Forward Flight that the Harrier uses in Air Combat Tactics in order to turn inside an adversary - or for braking on the ground). The only airborne use for it in the V-22 would be to make the plane-to-helo conversion a much later and swifter, more dynamic process (i.e. good for tactical surprise).

However, here's the rub. The use of it would make the Osprey much more likely to encounter VR, simply because it (VR) would not require a terribly steep approach angle or high rate of descent if 95 degrees of rotor-disc tilt was being utilised to kill energy (potential i.e. excess height or kinetic as in speed). Five degrees may not seem much but when you're talking about a standard 3 to 8 degree approach angle, it's a lot. It would be enough, particularly with a tailwind, to ensure that you were descending in your own "bad air" even at much higher speeds than you would expect to encounter VR.  Normally they just would not need to use it unless, say, they had a need to quickly kill excess energy - for instance if, as in the Marana case, the wingman had overshot his leader and had very little time to get back in position. The more I think about it, the more that makes sense. So on four counts it would explain why they'd not encountered VR earlier in the test & eval program (at all):

a. Firstly, as we know, no-one had expected that the Osprey was susceptible to it and so no-one had suggested testing for it.

b. Any testing that may have been done earlier (on the original XV-15) would have been reassuring, but being limited to 90 degrees of nacelle-rotor tilt, quite invalid regarding the Osprey.

c. It may be that the Osprey is vulnerable to AsymmVR only when 90 degrees tilt is exceeded) - such as when a very rapid correction is needed (as in the Marana aircraft's overshoot of the leader's abeam position).

d. Any testing done on the V-22 may have preceded the decision to go to 95 degrees (which was done for ground-handling reasons)

At the very least, I'd be asking whether there was any prohibition (or even caution) for use of >90 degrees of nacelle rotor-tilt airborne. Was there any idea as to what implications it could have? I'd guess the answer (through the "smoke and mirrors" reply that you'd get) would be quite revealing.  I may be wrong but I suspect that it is neither "locked out" nor "detented" when airborne. I'd guess that neither the PFCS or AFCS software has any inhibitors programmed in either. Whether or not they'd tell you that is debatable. It may all of a sudden be not FOI'able and even a matter of National Security. But it's worth asking these questions.

What I'm saying here is that it could simply be yet another case of "unintended consequences". Life and aviation is choc-a-bloc with those hind-sightable instances of human fallibility.

Extracts from Rotor & Wing International  August 1991

A frequent claim made for the Osprey is that 80% of the testing has been achieved in only 20% of the flight-test hours thanks to the extensive use of engineering and Flight simulation. Obviously the sort of error claimed to be behind the crash of airframe #5 (11 Jun 1991) lies outside of this test framework.

The general view seems to be that the V-22 Team may have successfully navigated their way through this latest ordeal, even though there must now be a number of embarrassing questions to face about quality control (somewhat ironic in an industry which is hitching its future to that somewhat ephemeral TQM (Total Quality Management) Strategy.

On the other hand flight-test accidents, incidents and such are part of the story of aviation. It must be a realistic assumption that progress in aviation is made at the expense of certainty. There may very well be other Osprey accidents in the future, and a well constructed flight-test program has no choice but to take this into account.

SUMMARY

The Osprey Story is looking to become one of those very unfortunate "missed opportunities". Because of the lack of wind-tunnel time much of the testing was done utilizing "Two Golf Charlie" (2GCHAS), an analytical software program that runs on high-speed computers such as the IBMSP2. The stuff that couldn't be bypassed was done in a quick visit to a Dutch wind-tunnel. Unfortunately the data available from hover and tethered tests extrapolated into these programs won't necessarily disclose the aircraft's susceptibility to such insidious helicopter ills as Vortex Ring (or in the contra-rotating Osprey's case the terminally lethal variant destined to become known as AsymmVR). So it always gets back to "garbage in / garbage out". The Computer software is only as smart as the engineer programming it.  It cannot discover new phenomena because, for that, you need empiric data from wind-tunnel or flight-test.  Contributing to this situation was the decision that they had to get to that cream on the cake ASAP after all the many years of development. I refer to the BELL 609 civil variant. 600 odd sales over five years are predicted. But one of the bugbears was the FAA's insistence that the same acoustic limitations should apply to the Bell 609 as any other airliner using the major civil airports. So all the available simulator time had to be diverted to that project, sorting out just how to minimize noise. Quite amazingly they almost resolved the Osprey's VR problem during that wind-tunnel testing - without realizing it and without knowing that there was such a problem . They discovered that a tilt-rotor's noise is all to do with the creation and break-down of blade vortices. They concluded that the more blades there were, the less pressure differentials in the vortices. The ideal balance between engineering possibilities/practicalities and acceptable noise levels was at five blades. With five blades the vortices are insignificantly small because of the greater "solidity" of the rotor disc. Strangely enough a five-bladed Osprey, for exactly the same reasons, would be nowhere as susceptible to the VR condition as the three-bladed prop-rotor on the MV-22.    They were also not to realise the great significance of a minor modification done to the XV-15 and V-22 Osprey that allowed it to taxi backwards on the ground (or in the hover). This was simply to increase the nacelle tilt from 90 degrees to 95. That extra 5 degrees makes all the difference if a pilot decides to utilise it in order to quickly slow down (as when, for instance, he's overshot his leader during the formation approach and must quickly get back into position - as in the Marana crash). The 95 degrees projects the rotor downdraft an extra 5 degrees forward (or ahead), making it much more likely that an unhappy coincidence of  flight path and downdraft would bring about asymmetric Vortex Ring. So you could say that hype and a blinkered loyalty to the project caused the Osprey Team to overlook what may well prove a great challenge to the Military models and the likely inability to certify the civil variants. Infatuation with computer technology may be also easily identified in the accident chain.   For those who have conceptual problems imaging what vortex ring is, think of it as a self-induced "wake turbulence". A fixed wing aircraft always loses its vortices by the simple expedient of leaving them behind. The penalty a fixed wing pays is greater drag, but only at the lower speeds on approach. Aircraft behind potentially pay a greater penalty, particularly if the aircraft ahead is heavy and slow. However the helicopter is vulnerable to settling into its own vortices during a steep low speed approach. The vortices reinforce until they form an encircling ring. This ring, once formed, is quite "sticky" and requires a rapid tilt of the rotor to gain forward speed and thereby "shed" the vortex. If the helicopter pilot is caught unawares or is fatigued or is at too low a height (or doesn't see the visual cues at night) he is likely to respond to any high descent rate by pulling in more collective blade angle. This accelerates his journey down the helix in a normal helicopter. In an Osprey it causes a 90 degree rapid roll into the ground.

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