V-22 Osprey: A Triumph of Money over Common Sense?

The US V-22 Osprey is a class of aircraft unto itself as it is the only manned tilt-rotor in service. Former Head of Future Projects at Westland Helicopters Ron Smith argues there’s a reason for that: it offers little and costs a king’s ransom.

The V-22 Osprey is a tilt rotor design that is used by the US Marine Corps and Air Force and 39 CMV-22 aircraft are being procured by the US Navy. The Japanese Self Defense Forces are acquiring five aircraft, Indonesia has ordered the type and Israel is very interested.

The V-22 has two large engines (6,150 hp), one mounted on each wing-tip, driving a 38 ft diameter three bladed propeller. The angle of the engine nacelles can be varied from in line with the wing up to ninety degrees to the wing to enable the aircraft to take-off and land vertically. In fact, the nacelles can tilt beyond the vertical by some 7.5 degrees to allow rearward flight or hovering in a tail wind.

So we have an aircraft that can operate like a conventional twin-engine turboprop aircraft in cruising flight and can hover like a helicopter and take-off and land vertically. That sounds like a great idea, doesn’t it?

Hover analysis

There are a couple of design issues, however. When hovering, the rotor diameters are smaller than one would expect for a helicopter in the same weight class. The V-22 has a maximum vertical take-off weight of 47,500 lb and can carry 24 seated troops.

The rotor diameter is restricted by the need for its tips to be clear of the fuselage when flying as a conventional aircraft in forward flight. This results in a higher disc loading (weight over rotor area) of around 21 lb/sq ft than would be expected for a helicopter in this weight class. This equates to reduced hover efficiency and a greater downwash (wake) velocity beneath the rotor.

The efficiency of the rotors in the hover is also suffers from the fact that the blade twist has to be a compromise between that required for an efficient hover and that required for efficient cruise flight. A second penalty arises because the wing blocks part of the airflow down from the rotor, creating a downward force that opposes the rotor lift.

My comparison helicopter is the relatively old Sikorsky CH-53D. This aircraft weighs 33,500 lb, has a rotor diameter of 72 ft 2.8 in and can carry 38 – 55 troops. The disc loading of the CH53D is around 9 lb/sq ft, under half that of the V-22. Now, the power required to hover depends on disc loading so that the V-22 will inevitably require significantly more power to hover at a given weight.

The installed power of the V-22 is (maximum) 2 X 6,150 hp or 12,300 hp total, the total maximum continuous power is 11,780 hp. By comparison, the installed power of the CH-53 is 2 x 3,925 hp, a total of 7,850 hp. The upshot is that the V-22 is 40% heavier than a CH-53, has 57% more installed power but carries around half the number of troops.

Forward Flight

I can hear voices shouting “but you’re missing the whole point!” The whole point being that the V-22 can fly like a fixed wing aircraft and land vertically when it arrives. The V-22 can cruise in airplane mode at 250 kt, which is 100 kt faster than the CH-53D’s 150 kt. The range of the V-22 is quoted as 879 nautical miles compared with a figure of 540 nautical miles for the CH-53D. Tactical mission profiles will be quite different, but there will still be a significant range advantage.

One big tactical benefit that accrues for expeditionary operations is that the assault can be mounted from further off-shore, allowing the fleet assets to be less vulnerable to any anti-ship missiles that the enemy may have. The speed and range of the V-22 allow the same tempo to be achieved from a greater stand-off range.

How does the V-22 compare with a medium STOL turboprop transport? My choice is the Alenia C-27J Spartan. The Spartan take-off weight is 67,200 lb, with 2 X 4,640 hp engines (9,280 hp total). It can carry 60 troops and cruise at 315 kt over a range of 950 nm. So, despite its higher weight, the Spartan is 65 kt faster than an Osprey on 75% of the power, while carrying 2.5 X the number of troops over a greater range. Roughly speaking, you could buy three C-27Js for the price of two V-22s.

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But, But, But

U.S. Army soldiers with Alpha Company, 4th Battalion, 10th Special Forces Group fast-rope from a CV-22 Osprey tiltrotor aircraft during exercise Emerald Warrior 2011 at Cannon Air Force Base, N.M., on March 1, 2011. DoD photo by Tech. Sgt. DeNoris Mickle, U.S. Air Force. (Released)

Yes – the V-22 CAN take-off vertically and it CAN fly 100 kt faster than a transport helicopter and it CAN land vertically on arrival. However, as we have seen, it makes a pretty inefficient helicopter and a pretty modest transport aircraft.

So, when do you need its capabilities? There are two clear military missions that come to mind: Combat Search and Rescue (and the closely related casualty evacuation) is the first role, where high transit speeds, long range and vertical take-off and landing are likely to be of critical importance.

The other role is insertion and extraction of Special Forces. Having no tail rotor and no Chinook blade slap the V-22 can achieve higher transit speeds with lower audible signatures than conventional helicopters in this role.

In Marine assault operations, it is doing the same job as a helicopter (albeit with a somewhat less payload for its size), but its real benefit seems to be to reduce the vulnerability of the assault fleet.

It would also be useful for Coastguard and Maritime rescue operations, but the organisations that provide these services are often not funded to a level that would support the use of such a complex platform.

In my opinion, you could use the aircraft for ASW operations, provided you used air-dropped sonobuoys (passive or active), rather than requiring active dipping sonar. This suggests deep water operations, rather than shallow water and littoral operations (Atlantic and Pacific, rather than Mediterranean or North Sea operations).

Its relatively inefficient hover performance and the associated high downwash velocities suggest that the Osprey would not be the preferred choice for underslung load operations and ship to ship operations.

The US Navy are buying the CMV-22B for this role, however, with the justification that the aircraft can deliver cargo direct to smaller vessels, whereas the current C-2 Greyhound can only operate to and from aircraft carriers. The C-2 therefore requires helicopters to perform onward distribution to smaller vessels across the fleet.

Other Considerations – Cost and Safety

One might say that the high development cost over a long period of time is a ‘sunk cost’ and is therefore not really relevant. Nevertheless, from a first flight date of 19 March 1989 it took until 13 June 2007 (18 years) before MV-22 Initial Operational Capability was achieved. The programme development cost is quoted as having been around $27 billion, with a fly-away cost (FY2015) of $72 million.

This fly-away cost is significantly higher than for a large helicopter. The larger Sikorsky CH-53E (greater than 70.000 lb take-off weight) has a quoted average unit cost of around $25 million. A civil AW101 is reputed to cost $28 million, a military example, rather more. Comparisons of published cost figures are notoriously difficult, but it is clear that the V-22 is likely to be significantly costly compared with a helicopter procurement.

The V-22 is a complex mechanism, with a high degree of automation and redundancy. As any reliability engineer knows, redundancy is a double-edged sword. The probability of a critical system continuing functioning after one or two system failures is greatly increased by having duplex, triplex or quadruplex redundancy, On the other hand, the probability of having a failure for a given inherent reliability is doubled, or tripled, or increased four-fold as a result. This means that a highly redundant system will have an increased failure rate.

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This can be mitigated by the use of intelligent health and usage monitoring systems that can provide early warning of incipient failures and can assist trouble shooting by flagging up the nature and location of the problem. So automation, redundancy and monitoring reduce crew workload, and increase safety, but add more black boxes to maintain and repair.

On the safety side, helicopters have a number of critical items, particularly in the dynamic system, where failure under static or fatigue loads is likely to be catastrophic. Examples would include loss of control to main or tail rotor; rotor head or rotor blade failure; and main gearbox failure.

In the V-22 (or other tilt rotor configurations) these issues, which drive inspection regimes and introduce life limited components are still valid concerns. A transmission cross shaft is provided to enable the good engine to power both rotors following a single engine failure. After such a failure, the aircraft would transition to airplane mode and ultimately make a rolling landing with the nacelles partially raised to keep the blade tips clear of the ground.

There were aircraft losses in development and a total of 12 V-22 aircraft have been destroyed in hull-loss accidents. In mature service, the aircraft seems to be performing as advertised, and safely, albeit with a significant maintenance overhead relating to the systems’ complexity.

Summary

The V-22 is in service and working well. In its own way, it is iconic. It’s a new configuration, it turns heads, it folds up on board ship. It passes my test that, every time I see one, I photograph it. Think of the Harrier, or Concorde, F-117, or B-2 – it’s just not every day that an entirely new configuration makes it to full operational service.

The V-22 is costly to procure and operate and its ‘stand out’ roles appear to be limited to CSAR, CASEVAC and support to Special Forces. Its speed and range in the Marine assault role primarily reduces the vulnerability of the assault fleet.

Where vertical take-off and landing is not essential, conventional medium STOL transports appear to offer a more efficient solution. Where high speed is not required, conventional helicopters may be more efficient at substantially lower costs.

Getting the aircraft from first flight to IOC required a substantial and sustained investment effort over some eighteen years. Now that it is established in operation there is some pressure for it to take on other roles.

It is hard for this author to believe that the V-22 will ever be efficient in ASW, COD or slung load operations. Its fuselage volume and cross-section also mitigate against the transport of larger troop units or heavy cargo.

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The money has been invested – use it for what it’s good for. It might be a comparatively niche solution, but it seems justified (at least for the United States armed forces) in its current roles (except, perhaps, COD).

It may be a triumph of money over common sense, but it is an undoubted triumph, nevertheless. (A bit like Concorde, really).

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21 comments

    • Dick Pilz

      Just like the fuselage is in the way of the rotor downwash.
      And, if you want a new aircraft, the “fly away cost” of a new CH-53K is $87 million. The CH-53E is not in production in this millennium.

  1. Vulcan

    Here’s why the V-22 is a good idea for assault landings. It’s cheaper to buy Ospreys to allow the fleet to stand further out to sea than to replace a fleet sunk by shore-based antiship missiles because it was in too close.

    • Ron R V Smith

      I agree completely – for assault operations the real benefit of V-22 is that the fleet can stand further off-shore and reduce the threat of anti-ship missiles. The added speed and range allow tempo to be maintained despite the longer ranges flown. This is what I say in the article.

  2. brian

    The real question is who allocated the funds which has allowed the US Marine a vertical take off & landing force? Which has its inherent extremely high operating costs. Ignoring helicopters, the first was the Harrier, which seemed simple enough. But then the Marines were instrumental in providing the funds for the lengthy development & procurement of the V22. The Marines main mission is shore assault so the design was predicated on the V22 fitting in the footprint of the Boeing Vertol, as space at a premium on assault ships. Another big screw up was letting the Marines to dictate the JSF requirements. The Marines wanted VTOL. So all F35s are hobbled performance wise in its other 2 versions by the Marines VTOL requirements. Must be some ex Marines in the Senate armed service commitee giving out $$$$ or there was in the late 1980s & 1990s.

  3. Alexko

    “On the other hand, the probability of having a failure for a given inherent reliability is doubled, or tripled, or increased four-fold as a result.”

    That’s not actually true. The odds do increase, but not by that much. It’s a Bernoulli process. The fact that the odds don’t increase by that much can be shown with a very simple example:

    Suppose you’re flipping a fair coin. The odds of getting heads are 1/2, those of getting tails are 1/2 as well. If you flip three coins, do you think the odds of getting heads at least once triple? That would mean they are now 3 × 1/2, which is 3/2 or 1.5 or 150%, which is greater than 1 or 100%, and therefore absurd. Therefore, the odds do not triple.

    If you’re curious about how to compute the actual odds, a simple way is to start from the general property of probabilities that the odds of something happening are always = (1 – the odds of that thing not happening). In this case, the odds of getting heads at least once are (1 – the odds of getting tails on all three coins). And the odds of getting tails on all three coins are the odds of getting tails on a given toss to the power of the number of tosses, or (1/2)^3 in this case, which is 1/(2^3), or 1/8.

    So the odds of getting heads at least once are 1 – 1/8 = 7/8, or 87.5%, not 150%.

    Redundancy works in very much the same way, assuming that the failures of individual components can be considered independent events—which may or may not be true, depending on the design, but ideally you want to design your system so that it is true.

    So if you have a hydraulic system with a failure probability of .01 (or 1%), then it has a probability of not failing of .99. And if you decide to go for quadruplex redundancy, then the odds of getting at least one failure are = (1 – the odds of zero failure) = 1 – .99^4 = 0.03940399. Granted, this is very close to .04, and therefore to the odds quadrupling, but still not equal to 0.04. And yes, for all practical purposes (i.e., maintenance costs in time and money, provisioning and purchasing, etc.), this doesn’t matter, but correcting misconceptions about probabilities is my sacred duty in life.

    In practice, the rule of thumb that a probability P will be multiplied by N for N independent events is never true, but it is very close to the truth when both P and N are very small. It starts to break down completely for larger values, such as the coin toss example above where you quickly end up with a 150% result that is massively different from the actual one, 87.5%.

      • Alexko

        Thank you, and for the article as well, by the way. I enjoyed it and learned a lot.

        What do you think of compound helicopters such as the Eurocopter X³ for the kind of roles the Osprey is meant to have?

  4. Duker

    The comment about blade twist not being most efficient for helicopter flight or cruise.
    The blades of all turbo propeller planes have variable twist the same as helicopters for their blades ( they have to vary for advancing or retreating in the rotation) Im sure you know this so perhaps there was a different point you were making, maybe it was ‘aspect ratio’ for the V22 stiff thick blades

    • Ronald V Smith

      There is a big difference in the amount of twist required. To hover efficiently, the blade twist on a helicopter rotor is typically around 8-10 degrees. The speed of the air coming into the rotor in the hover is relatively low. In a tilt rotor in forward flight the air comes into the propeller at 250 knots. This requires more twist to keep all parts of the blade working efficiently. This results in too much twist for optimum hover efficiency. The best that can be managed is a compromise.

  5. George

    One thing the author missed – the rotor diameter (and hence the rotor disk loading) was constrained by the Navy requirement to fit on aircraft carriers. Thus the rotors are smaller than they would be had the aircraft been designed without this requirement. Compounding this was the Navy requirement to fold the wings (they rotate about the wing centerline to align with the fuselage) for shipboard stowage which added weight, as did the requirement to fold the rotor blades. More weight requires more power. The wing location on top of the fuselage increases frontal area and adds drag, which reduces top speed.

      • Duker

        Every plane is a collection of ‘compromises in flight’, some are forgetting that any vertical flight for something that has a wing is a big compromise.

    • Ronald V Smith

      I agree – the folding requirement must add very significant weight and complexity. The rotor size is limited by the wing span and fuselage tip clearance, but the wing span itself may be determined by carrier dimensions, thus indirectly fixing the rotor diameter. In my experience there are often a few critical requirements (like carrying a vehicle in a C-130), whose effects ripple out across the whole design.
      I wrote a piece for Hush Kit about why the AW101 Merlin has three engines – there is a diagram in that piece that shows how some of the requirements interact with each other. I also agree that the wing location will increase drag and contribute to the adverse installed power, speed and range comparison with a straightforward twin turboprop design.
      My overall point is that it is a great achievement to be able to fly like a conventional aircraft and take-off and land vertically, but that capability involves compromises that reduce efficiency in both modes of flight.
      The main benefit of the V-22 capability in the assault role is the ability of the assault fleet to stand further offshore, but still maintain the tempo of the assault – this is a seriously important tactical contribution.

      • Dick Pilz

        As a former bullet sponge, I greatly appreciate the Osprey’s capabilities.
        I did my first hot LZ out of an H-34, aka HUS. Its engine was designed in 1932.

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