The articles I keep seeing (see box below) and the direct transcripts of Marine Aviation Chief Lt General Fred McCorkle's press brief - lead me to believe that they are at risk of believing their own (and Boeing/Bell) hype about "there being nothing wrong with the design". So I have sent via two different routings, the following "heads up" to Col Dennis Bartels who is leading the investigating team. I've no doubt that he has all sorts of AMES people to hand and a wealth of technical advisors. However Bartels' own background is totally in the CH46 and as I've said, tandem overlapping rotored birds don't catch the vortex ring "flu".
http://www.usmc.mil/marinelink/announce.nsf decision to resume flights
http://www.usmc.mil/marinelink/announce.nsf MV22 mishap info
http://www.defenselink.mil/news/Apr2..._t411dasd.html LtGen Fred McCorkle's briefing
So in the interests of not seeing any more Marines "hit the dirt", I deemed it cheekily appropriate for them to have an outside opinion. Even when military crashes are involved I doubt very much that, under the "party system" of manufacturer participation in the investigation, you can expect all the potential bad news to surface. And now that you also are privy to this disclosure having been made, just stow it away so that you can ask them the hard questions when this accident is replicated at some time in the future.
To: Col Dennis Bartels (of Dept of Avn, HQMC Aviation Mishap Board (AMB) OPNAVINST 3750)
Subject: There is an INBUILT DESIGN Problem with the V-22 Osprey
I understand that V22's are shortly to be restored to flight status and that the accident profile is to be re-flown. All that I have read, by way of official releases and media reporting, leads me to believe that the phenomenon of vortex ring, as it applies to the V22, is not thoughly understood. Perhaps it would just make me feel less complicit in future accidents if you would read the following dissertation on the hazard.
The best way to visualize vortex ring (VR) is to imagine forcing a plunger slowly down a loose-fitting water-filled pipe. You get a spill-over inflow around the periphery of the disc's upper surface (about the top quarter/third outer annulus). This is analogous to what happens below translational speed in a helo. That spill-over air is the rotor-disc version of that self-same lift-dependent vortex that spills off the end of a FW's wingtips. In a FW it is greater at low alt, low IAS, high AUW, high AoA. It is greater in the Rotary Wing (RW) because of centrifugal flow. In fact if you've heard of "behind the drag curve", that is what happens in a FW, below min drag IAS, when it starts taking more power to fly more slowly. In a FW the vortices get "left behind" and above translational speed in a helo the vortices are simply shed and left behind (as in the FW). But unfortunately, in a RW, below translational speed on a steep approach, the spill-over can become entrained air (i.e. your downwards flight-path approximates your rotor downwash) . So the drop to below translational speed on a steep approach is the entry to the VR high-threat area - so to speak. The instinctive reaction to a higher Rate of Descent is to increase collective (more blade angle and more fuel to generate more power to maintain N2 RPM). The result is greater spill-over, greater entrainment and the effect sneaks further in along the rotor disc. That area of the disc is in effectively now what, in a FW, would be a negative angle of attack - i.e. power increases only drive you downwards faster. And the outer area of the disc, because of the blade's higher rotational velocity, is where most of your lift (and lift dependent drag) comes from. It is really ALL bad news - nil good. In a straight "chopper", because of ingrained pilot instincts, the situation can rapidly deteriorate - but it is not irretrievable if you act quickly enough.
The lack of rotor interaction (as in the CH46 and Chinook's contra-rotating and overlapping discs) may well be why the V22 is more prone to vortex ring than those conventional dual-rotor helo's. I've never heard of a Chinook or Sea Knight falling foul of it, probably because vortex figures-of-eight are a physically impossible non-entity. So with side-by-side non-overlapping tilt-rotors you've got a big problem; what happens when, due to environmentals or piloting factors, the vortex ring effect hits asymmetrically? (high-rate steep descent, relative wind, gust-blanking of one nacelle, tight-cored thermal, or simply localized wind effects causing you to suddenly drop below translational speed as you descend). In the Osprey, the inducement of such a condition may be due to simply picking up a gust-dropped wing with lateral stick movement. The power and blade-angle on that dropped wing's nacelle is increased - and maybe just enough to trigger the condition (i.e. enter VRing). Entry into a VR condition means that the wing will drop further/faster, requiring a larger correction (and remember we said that that was bad news). The effect, as in the MV-22, would be a quick roll past 90 deg and a nose-drop to the downward vertical. I doubt that recognition + reaction time + nacelle swivel-rate (12 secs) could possibly resolve that asymmetric case - for a life-saving "burst-out" of the recirc bubble. It’s illustrative if the MV-22’s response to an attempted “wing-levelling”, in an assy VR condition, could be likened to a FW's auto-rotative entry into a spin. Every incident of asymmetric vortex ring will be a killer because of the control system and instinctive pilot response. e.g. Right wing drops and pilot applies left stick to pick it up. This feeds in greater blade angle and power on the side locked into the Vortex Ring condition (which increases the adverse roll-rate). If you wish, liken it to trying to pick up a dropping [auto-rotating] wing with aileron in a conventional FW aircraft.
Once a recirculation bubble forms and you are IN that vortex ring condition in a conventional helo, the recognition feature is that a higher collective setting will undesirably beget a higher rate-of-descent. The only way out is to utilize available altitude as your escape path and jam that cyclic forward, breaking out of that recirc bubble by accelerating through translational speed (i.e. by doing this, the vortex ring is "shed"). Unfortunately I can think of no similar solution for the V-22 with its rotor configuration. I doubt that nacelle-tilt rates (i.e. a 12 second transition), once you take into account recognition and reaction time, would be anywhere sufficient for escape (according to the loadmaster witness, the MV22 "hit the ground two to three seconds after flipping"). The asymmetric nature of the Osprey's vulnerability would seem to make it a very conclusive condition.
Terrain can be a factor in that it can cause localised wind effects that hit you unexpectedly in the descent and create the conditions for vortex ring (i.e. you can suddenly lose "translational" lift due to tree-line or terrain shielding of the relative wind). Translational lift is where the blades aren't in true air-chopping hover mode any more, i.e. you experience a lift gain and drag loss because the rotor-disc entity is now generating the lift. As you pass through about 15-18 knots airspeed, you notice a distinct change in rotor-slap noise and an apparent surge in power (both during accel and decel to the hover). But unexpectedly drop below translational speed during a steep approach, and it can be "Hello Mr Vortex Ring". That is the problem they will face in envisaged V-22 ops - conversion to rotor-borne flight may be a direct transition straight into the VR hazard area. Relative wind could be the big bugbear for the Osprey's gust response. To explain, just think about the exposure of the RH nacelle as against the LH nacelle in a wind-gust from (say) 2:30 to 3:00 o'clock. One's part-blanked, one's not.
In a true "chopper", late realization is the big hazard because you are intentionally descending anyway - yet as ground-rush sets in, you belatedly become aware that your rate is high so you pull in a little power (to decrease the ROD). You become momentarily confused because the rate increases (rather than decreases) so you pull in a little more power etc etc. Got the idea? Very quickly you are in a world of hurt. Of course it would now appear that the MV22 accident crew wore it just on one side. That's insoluble.
The Osprey rotors are contra-rotating and that could be a complicating factor also. This (contra-rotating props) brings up another consideration. As all glider pilots know, Coriolis has its influence upon which direction thermals rotate (clockwise or counter-clockwise, dependent upon hemisphere). Might be drawing a long bow, but if you were to be unfortunate enough to fly, below translational speed on approach, through a thermal (rapidly rotating and fast rising air), would the effect upon each contra-rotating rotor be the same? Given that:
a. You may already be in the ball-park for vortex ring formation and,
b. the effect upon each V-22 rotor might be quite dissimilar, and that
c. You could fairly expect that a lateral stick input will be required (as in any gust upset)....the inference being that he lateral stick input will be altering blade angle and power on both sides - perhaps enough to trigger the condition asymmetrically.
d. Military operations (formations) may lead to VR as a function of wake turbulence from a preceding Osprey in the same flight.
I am wondering if it would do any good to mount the Osprey's engines so that, as the rotor shafts rotate upward from the forward flight mode to the vertical/hover mode, they would also tilt inward so that the rotor wash would be directed outward to some degree. The hovering tilt-inward solution? It would require the nacelle gear-boxes to be re-splined but it may be possible to go for about ten degrees at great technical cost. It would also effectively rob the aircraft of 1-2% power in the hover - where it needs it. I'm not sure that it would make a great difference, sort of similar to wing-taper, winglet end-plates or greater aspect ratio in a FW to reduce losses due to wake vortices. Moreover, you might end up with a part-disc effect and out-of-balance air-loads on the rotor (and cyclical stresses on the transmission gear-boxes).
I myself think that sucking some power out of the disc for a powerful bleed-air fed wing-tip reaction jet nozzle may be the answer. Perhaps such a jet could also be used for induced-flow empowering of the ailerons at slow speed. Short of that, perhaps an emergency JATO to achieve a rapid exit from a recirc bubble.
It will be interesting to see how they resolve this and whether they will try to optimistically avoid any redesign. I remain unconvinced that the asymmetric VR condition will not repeatedly revisit the Osprey Fleet. The fact that they haven't hit the condition before may simply be due to the fact that a specific VR investigation was never designed into any Osprey Flight Test Schedule. Now that it's in "operational development" as an operational Marine a/c, you have to expect that uninvestigated foibles may well emerge.
[This message has been edited by UNCTUOUS (edited 11 May 2000).]