In our recent posts on the subject of helicopters we have discussed a couple of options for heavy lift, the 10-15 tonnes payload bracket.
At the lower end of that scale is the default choice of the Western world, the Chinook and at the higher end is the Sikorsky CH53K. The Russian Mi Mil-26 remains an intriguing option, perhaps one day many of the proposed joint ventures between western, Chinese and Russian companies might actually produce a workable and attractive proposition for western armed forces.
Helicopters are extremely complex, slow, expensive and very maintenance intensive but the crucial features which means military forces are willing to put up with all these is the simple fact that they can hover and land/take off without a runway.
Hovering, as opposed to vertical or ultra short take off an landing, provides so much battlefield utility that the downsides of helicopters are trumped by this single factor and if slinging loads, either dropping off or picking up, it is essential.
Recovery of personnel using a winch also makes the ability to hover an essential
We should separate the ability to hover with the ability to land anywhere, the two are not the same.
If we can eliminate the requirements to hover and relax the take off and landing requirements a tactical air lifter comes into favour, short take and landing aircraft like the CASA C235 can deliver Chinook/CH53K sized loads much greater distances, at a lower operating cost and quicker.
When I last looked at the possibility of the RAF having a tactical airlifter in the 10-15 tonne class it didn’t receive a favourable response; most of the commenters made the valid points about the cost of extra airframes, need for runways and marginal improvements in capacity over the Chinook.
With the RAF moving towards a 3 type strat/tac transport fleet of A400/C17/A330 I still think there is a gap in capability between the 10 tonnes of the Chinook and the 30 tonnes of the A400. In Richards post on Flying Cranes he floated the idea of CH53K or Super Merlin development to deliver vertical heavy lift but this still would need to hover when slinging, and slinging would be the default mode of operation.
One of the limiting factors of helicopter design is the need to counteract transmission torque, without some means of doing this a helicopter would tend to spin the fuselage not the rotor, so a number of designs have evolved. The tail rotor is the most widespread but this needs a complex transmission and bleeds power, the tandem rotor design of the Chinook has a much greater efficiency (one of the reasons they are so fast) but is still complex. Kaman have persevered with the offset intermeshing design and now specialise in sling heavy lift in the logging and power transmission installation market and the Russians have the coaxial rotor system which although a little ‘draggy’ produces some incredible performance figures. As an example look at the weight to lift ratios for the Kaman K-Max, Kamov KA32 and the proposed KA92, much better than any other conventional tail rotor type. Sikorsky have revisited their older coaxial rotor designs with the X2, modern design and materials technology have allowed an older concept to be resurrected.
With the V22, Bell have taken a different path, the XC-142 tilt wing and many more clearly shows the limitations of helicopters were well understood several decades ago but the V22 has shown itself to be just as complex, if not more complex. Cost and availability issues continue to dog this troubled aircraft.
All these solutions still have helicopter levels of cost and complexity.
One of the reasons heavy lift helicopters are often forced to sling loads and therefore have the need to hover is because, in general, their fuselages are too small. Simply making a fuselage bigger is not as simple as it sounds because more fuselage means more weight and every kilogram of fuselage means a kilogram less payload, so there is a fine balance to be struck. A bigger fuselage also means limitations in operating from ships, one of the reasons they are forced to operate from ships is because of small low range, its all a bit circular.
Loads typically hit internal volume/stress limits before weight limits. Height and width in particular, cause more problems that length.
Tactical air lifters like the C130, A400, C235 and many others generally offer greater range and speed at a much lower cost but their Achilles heel is an obvious need for somewhere to land. Even the smaller ones that can land in austere locations need a runway of some sort, these might be nothing more than a track or rough strip but it is still a relatively large area. This area has to be recce’d and prepared to some extent and for operations in theatres like Iraq and Afghanistan, this also means sanitising the area of IED’s. The larger this landing location becomes the more time consuming and labour intensive these clearance operations become. The recent resurgence in parachute delivery of stores has in part, been driven by this. The RAF have lost or damaged to such an extent that they needed to be explosively denied, 3 C130, in recent operations because of mines and IED’s, very expensive.
What would be valuable is an aircraft that could operate like a helicopter but had the payload and cost advantages of tactical airlifters.
Being able to move short, but strategically useful distances and deposit its cargo of personnel, stores and in particular, vehicles, would be a hugely impressive and useful capability and could provide a serious alternative to many of the amphibious capabilities we currently have.
I wouldn’t view this as a helicopter but more of a light tactical air lifter with very very impressive short field performance.
One possible solution might be to resurrect an old design, the Fairey Rotodyne.
Insane you say…
I might usually agree, these older designs have suffered at the hands of evolution, if they were so good why aren’t we using them today but like balloons, given the advances in computer aided design, simulation, materials technology, computerised flight control systems and engine power, is an old but proven design concept like the Rotodyne viable today?
The Fairey Rotodyne
In the post war period the British aeronautical industry was a fertile breeding ground for innovative designs. Out of this environment came the Fairey Aviation Rotodyne. It was designed by Dr J Bennett and Captain G Forsythe using the lessons learned from the record holding Fairey FB-1 Gyrodyne and Jet Gyrodyne. The FB-1 first flew in 1947 and the Rotodyne in 1957.
The Rotodyne featured a large 4 blade rotor with each blade having a tip mounted nozzle that channelled compressed air from the stub wing mounted turboprops. Because the tip nozzles were not directly coupled to the engine no counter rotating torque was generated, negating the need for a tail rotor. In normal flight the rotors would be unpowered and the pitch reduced to reduce drag, when landing or taking off the rotor would be powered to allow vertical landing or take off. It could also fly with one engine out.
Performance was impressive, 48 passengers, cruise speed of nearly 200mph and a range of over 500 miles.
Compare that with the Chinook still being used 50 years after the Rotodyne was cancelled, similar number of passengers, cruise speed slightly less and a range of about a hundred miles less, with more power.
If the words ‘ahead of its time’ could be used to describe any aircraft, the Rotodyne would be it.
Towards the end of the fifties, despite it demonstrating incredible performance, the need to reduce costs and reduce the number of aircraft manufacturers placed it firmly in the centre of a political and economic storm from which it did not emerge, being cancelled in 1962.
For a full description click here
What Would a Modern Rotodyne Look Like
The concept has been kept alive by a couple of US manufacturers, Carter Aviation and Groen Brothers Aviation although there are technical differences between the two approaches. The Carter Slowed Rotor Compound concept has very recently been licensed by AAI for use in unmanned designs, will be interesting to see what comes out of this.
In 2005 the US announced the Joint Heavy Lift program which envisaged an aircraft with the payload of a Hercules but with STOVL capabilities. Yet another alphabet soup of programmes has followed but has been beset with by a lack of definitions and operational concepts, is it a helicopter or is it a tactical airlifter?
The Groen Gyrolifter is the closest to the Rotodyne concept but was rejected from the programme, one wonders if it is because the design means it would be unable to operate from USMC amphibious vessels and therefore would not be very ‘joint’
The programme changed to Joint Future Theatre Lift and there was a realisation that vehicles were getting heavier and the Future Combat System was heading for the scrap heap. This coincided with concepts around future tactical airlifters (AJACS) and no surprises, its all in the PowerPoint stage. The Army wants vertical lift, the USMC want the same but it has to be able to operate from ships and the USAF just want something cheap.
The Sikorsky Quad Tilt Rotor seems to be the front runner but given the issues around the V22 and obvious budget issues there does not seem to be much appetite to pursue it. If the twin engine V22 is eye wateringly expensive a version with double the number of engines would be astronomically expensive.
Looking back to the Rotodyne its performance is comparable with a Chinook so why bother?
Clearly it must be in the 20 tonne Hercules payload class.
The Groen Gyrolifter proposed taking a C130 and modifying it, producing a C130 derived aircraft that could carry 18 tonnes a distance of 1,100miles or 1,000 nautical miles. Whilst this is impressive it fails to improve on the volume limitations of the C130 that have driven, in part, the need for larger aircraft.
If there is room in the equipment plan for such an aircraft it should be as simple and low cost as possible but it must have a large internal volume.
Aerodynamic efficiency and the need to exploit this efficiency by flying high means that the slender cylindrical tube is the preferred fuselage configuration. Cylindrical shapes make pressurisation, for more efficient high altitude flight, much easier but results is volumetrical inneficiency, look at a vehicle inside a tactical transport aircraft like the Hercules or C235 and it becomes obvious that even when fully loaded, it is transporting a lot of fresh air.
Pallets and vehicles have square cross sections, not round.
Without the need for high altitude and high efficiency flight profiles the designers of helicopters tend to make their fuselages less cylindrical and squarer. If we accept the trade off of medium altitude and unpressurised flight profiles the fuselage can be square and therefore closer matched to payloads.
This would lead to a new fuselage design with a square cross section.
mmm, this is sounding expensive!
But whilst we are all sucking our teeth and asking ‘how f**king much?
Consider this, the map shows 1,100mile radii from Gibraltar, RAF Akrotiri, Al Udeid Airbase in Qatar, the French airbase in Djibouti and Masirah Island in Oman.
Draw your own maps, here
Gibralter (36.1833, -5.3667), RAF Akrotiri (34.59027777, 32.98749999), Al Udeid (25.11722222, 51.315), Djibouti (11.547331, 43.159481), Masirah Island Oman (20.471111, 58.815278), RAF Mount Pleasant (-51.82222222, -58.44166666), Lungi Airport Sierra Leone (8.616444, -13.195489), Ascension (-7.933333, -14.416667), St Helena (-15.933333, -5.716667), Belize (17.51555555, -88.19583333), Al Udeid Qatar (25.117222, 51.314722), Turks and Caicos (21.75, -71.583333), Cayman Islands ( 19.333333, -81.4), British Virgin Islands (19.333333, -81.4), and Anguilla (18.21027777, -63.057)
Range could of course be extended with buddy refuelling or conventional airborne refuelling.