V22: Chewing Over The Critics With Chief Test Pilot Tom Macdonald
As the MV-22 Osprey prepares to go its operational evaluation - with beyond it the promise of service entry only just round the corner - the aircraft seems to have beaten off the drumbeat of criticism about its some of its performance characteristics. Close observers that we are, we have honed these down to concerns expressed by various experts in about 10 main areas that have attracted the most negative attention over the years.
We asked Osprey chief test pilot Tom Macdonald, easily the pilot with the most Osprey hours and handling experience, to take us through these points one by one. We wanted to know - with a new chapter in the Osprey saga opening - what he thought of some of the things, said, asserted and projected about the way the aircraft handles and performs.
We broke the 'mission' into two parts. Part one was to 'fly' with him in the development simulator at Patuxent River where NAVAIR does the bulk of the test work.
Part two was the de-brief resulting from experiencing the manoeuvres. In each case Macdonald set up the 'case' in question, choosing the manoeuvres, setting the simulator up to replicate them, pointing out the way the aircraft behaved and felt, and drawing attention to what showed on the displays.
Tom, let's start with what people have said about the inability of the V22 to operate near the ground aggressively being as it were worse at this than a conventional helicopter.
What we did today was a lot of figure eight-type turns around a football field. You saw how 'carefree' it was. We were at a weight of 47,000 pounds - combat weight - and basically you could see were limber and agile. You saw the way basically I did that - by working the nacelle tilt. You don't change attitude all the time like you do in a helicopter, which can become disorienting at low level. The nacelle angle allows you to keep your eye on the target the whole time, something you can't do in a big helo. And you saw I had no concern - none whatsoever - about the blades stalling on me, or that I wouldn't have enough thrust to accelerate.
We listened to a lot of critics talk about blade stall, about the fact it has precious little margin when loaded up in a manoeuvre. They say it inhibits that type of manoeuvring and therefore increases vulnerability.
We did lots of engineering tests, tests that measured the blade stall margins a number of ways, through a wide range of low speed manoeuvres, rolling push-ups, sustained wind-up turns, things that are the most telling and difficult to fly precisely at low speeds because you don't have the 'Q' feel and you generate a lot of G. However, in virtually any kind of manoeuvre where we thought we could generate the stall - we couldn't do it.
I didn't see anything that looked remotely like a stall condition - not from the feel, or from the instruments. Why?
Maybe we do know some things here after all! Maybe the best answer comes from the fact the high twist rotor blades - something really quite new - are in fact more resistant to entry into the stall regime, just as those original Bell designers said. There are a couple of things going on which we see as contributory: a big one is a three dimensional flow going on across the blade that essentially flies the blades differently. That, by the way, was something that came out very close to the original prediction. This whole area is something you can design for at the beginning - you wouldn't build a blade that didn't do the job, why would you - and they got it right. Actual testing and predictive design has been very close here.
There's a relationship between such things as 'stalling' and the very high disk loading of the V-22. Would you care to explain a bit about this, and say what you think critics are implying when they talk about the problems of high disk loading in the V-22?
Sure. Most of our conventional helicopters have low disk loading - disk loading is functionally analogous to wing loading - a small lifting area for a relative large mass that has to be lifted. They have big 'wings,' we have little ones. You can compute the numbers - you have 38 ft diameter rotors - two of them - 50,000 pounds to lift, about 24 pounds per sq ft or something (don't have a calculator handy!) but what that tells you is that its higher than what you have in a CH-46, about 14-17 pounds and the CH-53 and the -47 and so on. So it's a lot higher. Why do we do this? To take advantage of something that's a bit tough to grasp, but I'll say it anyway - you get a significantly larger thrust benefit from a relatively small increases in airspeed.
I did notice that. I saw how the power comes on very quickly and then just sort of build and builds. There wasn't much of a translational lift period…
Right. In fact we go through that (translational lift) at 15 knots, they (the big helos) do it at anything up to 30. That gives you two things: you take off much better - good for survivability, right - and when you're in decel towards a hover, you need a lot less power than they do. It's not because there's a wing out there giving you lift, either. It comes from the physics of the thing. They may criticise the high disk loading from one point of view, but they're actually not seeing the very significant advantages coming from these other areas. In a high disk loaded vehicle you can be much more precise at matching the desired design performance to the installed power - and that's what happens here in the V-22. The power is much, much more 'precisely' applied for what you need to do. Much more efficient.
We did a very impressive high acceleration thing where I literally saw the V-22 go from a hover to 200 odd knots by the time you reached the airfield fence. That illustrates what you're saying?
Yes. And you can decel just as quickly, which is important in another way.
Which presumably helps in the thing people criticise - that there's a lack of helicopter type manoeuvrability and that therefore you're vulnerable.
Coming in, you can, in fact, stay in he airplane mode (fast) right up to the point you want to begin the landing approach - to do a conversion to the helicopter mode. You get to use the quietness, the power and the fact the pitch angle means you can keep your eyes on the LZ all the way down. You won't hear an Osprey coming until that last 45 seconds of the approach. I have a son flying Cobras in Iraq right now. He writes to me and says this ability to retain the sight angles is crucial and something his Cobras badly need - to keep the guns, missile firers, RPG launchers, whatever, in sight whatever manoeuvre you're doing. And he wants that speed to the LZ boundary so bad he can taste it! I told him the USMC's taking care of it.
There's a lot of chatter about single engine performance, about losing an engine to a gun or missile. We've all heard about the design and how it supposedly promotes survivability - the engines at the end of the wings and so on - but how about power? How about that great hulking 50,000 pounds 'helicopter' losing an engine close to the ground?
We showed many times that we can, in fact, do no-hover approaches in the OEI (one engine inoperative) case. I'll talk about auto-rotations in a minute. But in the airplane mode, OEI case, as you saw when I pulled the power off, it was no factor, a no brainer. With that high disk loading I told you about, we have the power to go off in wing-borne flight, with only a slight loss in airspeed (and if there's some wind, even better, increasing the acceleration rate to a conversion to airplane mode even more). We mostly would do a roll-on landing with an engine out, but we have also found it's possible - with very careful flight management - to put it on the deck from a very shallow, carefully managed approach and do a roll-on landing in helicopter mode mode. So I think we have to break this down a bit. Single-engined flight in airplane mode - no problems. Auto-rotations - that's a different situation obviously, and that's what people are talking about.
Well, you get dinged for not being able to 'autorotate.'
We have been, and I admit we have only looked at the middle part of the three parts there are to an autorotation landing - entry, steady state, and then, finally, the flare and pull up. We took it very easily, didn't do anything fancy to get into, enter, the auto, but we did examine it's capability under a lot of different airspeeds. We found generally we had to be at 110 knots in the steady state part in order to be able to have a chance of pulling off any type of reasonable flare at the bottom. We probably could go slower, but I'd say trading forward speed for sink rate has always been something we take great care over. In the end you'd probably have about 60 knots of forward airspeed left after you've completed the flare, and we also found that about a 4500 ft per min sink rate was what you would have been dealing with on the way down. A heavy CH-53, by comparison, sees about 3,500 fpm so we're not totally out of the question. But it looked just too dicey to us, so the critics probably are right on that one - an auto-rotation in a V-22 is not a characteristic we think the general V-22 pilot population should have to manage. If you trained for it we think it'd be kind of one of those synthetic goals - what's the point really.
So in the engine failure case, the best technique is to 'think airplane.'
The whole philosophy is exactly that, yes, but an auto is there as a last resort. By the way the numbers are interesting - fly at 170 knots, you can descend at about 3,500 fpm, and land no faster than say a T-38. You keep the nacelles down, the gear up, the flaps programme automatically, and you come in there at 120 knots in this very crash tolerant airframe.
You'd make it?
I think so, yes. The crash protection is a system - everything comes into play - airframe fuel systems, bulkheads, everything. You'll make it out of there in good shape.
I've heard your critics say; 'what happens if the interconnecting drive shaft - the ICDS - breaks and you've got one engine inoperable? They say the likelihood is that with an ICDS problem the Osprey would be lost. It broke once, didn't it?
Yes, in the Quantico accident. It broke where they didn't have a chance, and everyone on board was killed (1993 - ed).
But now?
Well, it's kind of an unrecoverable situation. We haven't fully evaluated what to do about an ICDS loss, but we're thinking about doing it. I think it would be a very interesting thing to explore further. If you were in airplane mode and - presuming you still have structural integrity and all that - obviously what you'd be dealing with is a huge amount of yaw and a whole lot of drag from that, which you'd have to deal with. What we found in the simulator is that there's a magic number - 170 knots - when the airplane is still actually flyable, still controllable. You have, in effect, an aircraft in a huge sideslip which we found we can keep under control, albeit in a descent. More work in there also told us we could in fact manage a reasonable approach. There was enough time to pull back the engine that was running, and then set up a glide and a landing. As I said, it's only be done as a simulator exercise - for obvious reasons, but it told us there's something there we can work with. At least we know it might be possible to land an Osprey under those conditions. I would put the loss of both an engine and the ICDS together in the realm of the possible rather than the probable. But, as you say, it happened once.
And you're thinking about a test programme to look it into more?
Yes.
Tom, in the simulator just now you made the Osprey behave like it did in the last few seconds before the vortex ring state (VRS) crash. You got into a high descent rate, low airspeed situation and the airplane rolled. I must say it was extremely realistic. I'm a pilot and my whole body was telling me 'wow, we've lost it…'
Quite a demo, wasn't it? You noticed how I rocked it around a bit before she went over. That was to show you how unsteady she is in the lateral axis just prior to departure. There's obviously a boundary in there, and now we know much more about it than before. It varies - as a function of the gross weight of the aircraft and other things, but it's a firm boundary. It's about at the point that sink rate equals induced flow through the rotor. So induced flow through the rotor is a function of the thrust setting, nothing we didn't know about the principles of settling with power and so on…But what was interesting was the relationships we found between sink rate and speed and how that affected the onset of VRS. We found 40-45 knots is about the fastest speed you can get into it - no matter what. As you go through below that speed, the boundary for the descent rate was 2600 fpm. Go down in speed it's even more deadly. 1500 fpm at 30 knots will put you into it. We found this in a neat way. Our instrumentation - the flight test instrumentation - told us what was happening. We pilots couldn't tell what was happening, where the data points really were. That was the real story behind the testing - the way the engineers put it altogether. They allowed us to find these boundaries with the sort of precision just flying the manoeuvres wouldn't have provided.
That roll-off - pretty decisive, wasn't it?
When it started to roll we could not stop it with lateral stick alone. Impossible. But when we went to moving the nacelle tilt angle, we got it stopped with less than three degrees of bank roll. The whole programme allowed us to find things - that two seconds of nacelle movement, forward 15 degrees will get you out of VRS. It's like stopping an incipient stall in a fixed wing airplane - just break it forward a little bit and the stall breaks, right? Same kind of thing. None of this was easy to establish. The key was being able to measure low airspeed. We adapted an ultra-sonic system used to test wake patterns around ships.
The warning system was really good. I heard the 'bitching Betty' warn 'sink rate, sink rate,' and I saw the VSI light up to show me the exact vertical descent rate.
You probably didn't also see the MFD message…?
No, but the warnings I did get would have told me at least to 'do something….'
Yep. She comes back very nicely once you move the nacelles. She rolls very predictably. We've actually done aileron rolls in the Osprey.
All the way around?
All the way around. We have a special test pilot, Bill Norton, we use for things like that.
I known Norton. Great pilot.
Truly amazing. Flown everything there is.
Talking of rolling and such, another line of dissent says the Osprey by its construction has excessive flap in its rotors, and that this can overpower the control system. That true?
Well all rotors flap, and if you have big rotors they can flap and break things off the airframe - refuelling probes. We don't have big rotors. We have small ones, but look, I take the point. It's a stiff in-plane rotor, so you have to be careful about flapping limits. There's not much flexure in the blades themselves - a design consideration. We reach the limit at 10.7 degrees - that is if they flap out of plane that much. You'd start causing structural damage, but I'm not sure it would be catastrophic. Bad for the aircraft, but not catastrophic and you'd probably want to come in and have things checked.
Do you control the flap angles? I mean, prevent them getting excessive?
Sure. Through the flight control system. It's measured all the time by sensors - they feel the heat caused by the friction of the blade interacting with the yoke - and then fed back into the control system. If you did violent sustained things you'd have a flapping problem, no question, but continuous sustained flapping would be another thing and something very hard to achieve - maybe hairy flying in a strong gusty crosswind would do it. But overall you're not going to hear about the things that give helicopters problems - chopping off tail booms and things like that. I don't think this is a critical issue, but I know it's a matter of comment out there. You get a lot of warnings about excesses all over this aircraft. That's what the warning system is designed to do - prevent damage.
Turning to something that's quite topical - critics denigrate the Osprey's ability to land in dust and snow because the disk loading - that issue again - leads to these huge clouds being blown into the pilot's vision.
Brown-out. Well, we did some testing on that last winter - approaches in blowing snow, powdery snow. That was encouraging in that we did have visibility throughout. Snow is particularly hard to handle - white-out - because there's a lack of shading and the light patterns are quite disorienting. A test pilot called Buddy Bianca, a Marine pilot, has been doing some dust brown-out flying, and in fact the Marines have had a test programme out at Yuma looking at the optimum angles for approach into those conditions. They found something unique to the Osprey - if you put the proprotors beyond vertical, you can blow the stuff forward of you! Of course you have to time things carefully because you'll start to fly backwards! Also we found pilots - if they break from normal habit patterns of looking forward - can actually see quite a lot of cues out of the side of an Osprey, which they can't in a regular helicopter. Very interesting. We have an Afghanistan veteran looking at this at Yuma.
What about, using the digital flight control system, what about coupled approaches?
We're looking at that, we're looking at a range of things. We're looking at using the NVG/HUD system on the goggles combined with a display down in the cockpit on the MD, a hover display. That's got little vectors on it, the speed and direction you're drifting. Once you've got all that, you can make a vertical descent kind of blind. We're looking at a lot of offsetting techniques you can use. We know and fully understand what a danger this brown-out is. We're treating it as critical evolving element of the Osprey.
Actually the aircraft's kind of strong. I'm not saying you want to drop it from a great height, but it can withstanding a heavy landing…
There are no qualms about a firm touch-down in this aircraft. You don't want to touch down with a roll angle, though, like any other helicopter. We're cleared to land on a nine degree slope, and we've done high sink rate landings at that angle. That's a neat thing. Land uphill, rotate the aircraft around the nacelles so you're looking level whatever deck angle the plane's at.
Lastly, Tom, some sceptics say the Osprey's not going to be any better at hot and high conditions in places like Afghanistan in the summer where the regular helos have had problems.
We've been studying that and done a lot of hot and high stuff again at Yuma. We looked at low speed manoeuvring under those conditions. Our concerns are that you won't have the hover capability at higher Das (density altitudes) and we expect the Osprey to be challenged about the same as anything else up there. We don't have all the data yet, and so some of the development testing scheduled for next summer will look at that in greater detail. We've met the operational specs at around the 7000 ft DA mark, but I admit they're much greater up there on the north west frontier and so on. We probably wouldn't do hover landings at those levels. A lot of this is going to come down to operational technique. What we've done is supply the charts, tell them what we know about the aircraft so far. The airplane is seductive in its own way - it's got lots of power. But I think hovering this aircraft above 10,000 ft will be a handful. We need to be careful - there are helicopter things people do that would get a tiltrotor into trouble up there.
Thank you.
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