AVIATION APPLICATIONS,
CIVIL & MILITARY.
TILTROTOR AIRCRAFT

Prelude

Tiltrotor technology is a proven airframe type, with a history of development going back some 40 years and investment by industry and government, of approximately US$50Bil.

Its future production is assured, with the first 540 airframes of the current production aircraft reserved for US Defence Forces and further acquisitions are projected. On September the 8th, 1998, the 11th production airframe was delivered to Bell Helicopter Textron, for connection of the fuselage to the main wing; this airframe is expected to join the first ten aircraft at a naval research station, for training and evaluation purposes, midway through 1999.

IHD Powered Tiltrotor Aircraft Generally

With the high level of suitability of this powering technology to aviation applications and the equally significant advantages with regards cost of manufacture and operation, safety and flexibility. All notable aspects of great significance for civil applications, they are no less significant for combat and military support aviation applications and in this context, will provide unprecedented levels of improvement.

With regards tiltrotor type aircraft specifically, the cost savings inherent to the adoption of this power plant, are probably the single most significant aspect to be considered, with regards potential market acceptance. The current tiltrotor aircraft offered, the V-22 “Osprey” military and civil utility aircraft and 609 executive aircraft, are both twin gas turbine powered types and relatively expensive to manufacture. Regardless of the cost, an existing market has already been clearly identified for both aircraft. In the case of the V-22, the US military has already reserved a substantial number of initial production aircraft; whilst in the case of the 609, sales commitments are already well into double digits, even though this aircraft is yet to be built and flown. The latter appears a remarkable development for an executive type aircraft and especially so for one that represents such a radical departure from existing practice.

Both examples adequately confirm market acceptance of and demand for tiltrotor type aircraft.

Both existing twin gas turbine powered designs, have their power plants located at the wing tips and are cross-linked by a connecting shaft. The latter to provide a dual power format for redundancy purposes, addressing potential’s for an in-flight failure of either power plant. As arranged, the entire engine unit and proprotor is rotated through approximately 90 degrees, to address the operational scenario of take-off, transition, level flight, transition and landing. Hover is likewise addressed as a transitional scenario. Given the need to link the two power plants by the cross shaft, to maintain the dual power capability of these wing tip mounted power plants and accommodating wing deflections due to in-flight load variations and droop at start-up and during engine shut down, this is a necessarily complex, although apparently simple, mechanism.

Of necessity, tiltrotor aircraft are high wing types and in both existing examples, the proprotors in the hover mode are an excessive distance from the aircraft’s wheel level and as such, the advantages of ground effect can not be fully utilised. Whilst there is an obvious need to keep these large diameter proprotors well clear of the ground, to allow operation in a short take off/short-landing operational scenario, both as a normal mode of operation and to allow landing the aircraft at reduced power levels during emergencies; the acceptance of a steeper angle of departure and arrival would reduce this clearance requirement.

In these existing aircraft, as well as the cross-shaft, both cyclic and collective controls, as well as fuel, sensor and engine control systems, are all required to terminate within the engine modules and as such must rotate with these during each operational cycle, a somewhat complex system. A secondary consideration, in this regard, are the auxiliary services normally taken from the power plants for the various operational needs of an aircraft; typically, hydraulic and electrical power. Again, as these originate within the engine modules, they must likewise rotate with these during each operational cycle; it should also be apparent, if turbine bleed air is to be used for wing de-icing and/or cabin heating, this will add further complexity to the rotation mechanism.

In a tiltrotor powered by an IHD power plant. The power plant could be mounted above the passenger cabin in the wing route, with hydraulic motors mounted at the wing tips to drive the proprotors. Alternatively, the IHD power plant could be mounted below the floor of the passenger compartment and in either case, it would be a simple matter to incorporate a flow divider in the supply to the wing tip mounted hydrostatic drives of the proprotors, equalising the power supply to each proprotor. Fundamentally, this arrangement with its inboard engine position, would result in notably reduced wing deflection levels, during start-up and run-down procedures, whilst with the lighter unladen weight of the type, in-flight deflections are unlikely to exceed those of the parent form. Irrespective of deflection levels, there would be no requirement to consider the implications of the cross shaft of the existing types.

Essentially, such an approach would result in hydraulic lines and control systems alone needing to be addressed during rotational sequences of the proprotors. It should also be apparent, in the existing aircraft, hydraulic cross connections must rotate with the proprotor modules to maintain redundancy in the case of an engine failure and all auxiliary systems must be duplicated on each power plant for the same reason.

An IHD powered aircraft gains the usual advantage associated with the power plant type, of the engine speed being independent of proprotor speed. This effectively allows the engine components operation in a dual power mode. Effectively, during periods of high power demand, vertical departure or approach, the engine can operate at higher rpm levels and power settings and in level flight modes, the engine can operate at lower power settings and reduced rpm, improving engine efficiency and extending component life. Consistent with this capability, proprotor rpm and pitch can be optimised for the various flight regimes, independent of power plant rpm, with the engine component responding automatically to variations in power demand; again consistent with the alternative applications previously discussed. This degree of flexibility would also provide a reduction in engine related noise levels, during the more protracted periods of level flight.

As usual, the power plant in this case could be a variable geometry engine, providing levels of redundancy superior to the existing twin power systems. Consistent with this approach, the power plant could suffer a major component failure and still supply substantially more than the 50% of the total power available in the contemporary twin power gas turbine arrangement.

The relocation of the power plant to an inboard position, also has the advantage of eliminating the mass damping effect associated with wing tip mounted power plants, providing a far more responsive airframe, a notable advantage in both military and corporate aircraft. A good example of the latter, would be the advantages associated with landing a more responsive aircraft on a ship in a seaway or on an oil rig in turbulent conditions; both situations which corporate tiltrotor type aircraft will need to address.

It should also be apparent, as usual, use of the proposed power plants will eliminate a requirement for hydraulic pumps, providing a further simplification of the aircraft; and although these are highly reliable components, their elimination does reduce the number of potential failure points in the powering system, including the associated PTO, that would normally drive these pumps, further improving operational safety levels.
Civil Developments & Opportunities

The proposed powering technology is well suited to aviation purposes, especially helicopters and associated forms and with regards rotary winged aircraft, will provide substantial advantage.

With regards civil applications, market opportunities are extensive.

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AVIATION APPLICATIONS,

CIVIL & MILITARY.

Apart from executive type aircraft, the development of IHD powered tiltrotors would be exceptionally well suited to commercial inter-city and intra-city shuttle and commuter services, especially where a very short take off and landing could be utilised, as distinct from vertical approach and departure; although even in the latter case, such aircraft would demonstrate significant advantages over contemporary helicopters. This is especially the case in such highly developed regions as Europe, where regional cities are often situated closely together, allowing relatively short flight times.

There is also considerable scope for development of such aircraft for high value regional freight distribution purposes. Typically acting as components of existing courier and express freight networks, where they will be well suited to operations between regional depots, on scheduled services and equally well suited to door to door services, for larger corporations with existing helicopter access. The advent of these highly versatile diesel powered tiltrotor types, with their marked improvement in economy and reduced fuel load, will see them develop as an economic means of transport for such applications.

Given the projected power to weight ratios for aviation engines of the type proposed, such power plants would be well suited to such an aircraft type, especially when one considers increased carrying capacity resulting from the greatly improved fuel economy and greater safety of diesel power plants. It should also be recognised, with the increasing awareness of and interest in the environmental implications of aircraft operations, especially where these occur in densely populated areas, such as city centres, the great reduction in engine exhaust related emissions, offers an unprecedented opportunity to improve the public image and attitude to inner city flight operations. Which, without the advent of power plants of the type, may otherwise be restricted or even precluded, on environmental grounds.

By comparison with contemporary tiltrotor aircraft such as Boeing Bell’s “Osprey”, a tiltrotor aircraft powered as proposed, would be a far simpler aircraft to build, resulting in a notable reduction in the manufactured cost of the resulting aircraft and an operationally more economic and versatile airframe. Typically, an hydraulically powered tiltrotor would have a single central, dual power, diesel hydraulic engine (IHD), either mounted above the freight/passenger space in the wing root or below the freight/passenger accommodation, with hydraulic lines to hydrostatic drives at the wing tip rotors. This is a notably simpler arrangement than the wing tip mounted, twin gas turbine power plant, gearboxes and cross shaft connection of the “Osprey” and associated types.

Advanced Military Applications

Military Developments Generally

In parallel with the development of IHD powered transport versions of the type, there would also be a sound argument for development of full combat oriented version of this aircraft type and in this role, the reduced IR signature would be a significant advantage. Whilst the use of composites in the proprotor, could be used to reduce the RCS of the airframe. Consistent with the reduction in RCS, there would likewise be no turbine blade reflection to be considered.

Development of such combat types could firstly see development of tiltrotor aircraft as a rapid response anti-armour platform, acting in support of forward deployed combat helicopters. In such a role, tiltrotor aircraft would probably best be configured with a tandem cockpit configuration, consistent with contemporary anti-tank and anti-air combat helicopters, such as the Mi-28. In such an application, this would combine the advantages of rapid transit with a battle field supremacy rotary winged aircraft. When required, this would also allow a considerably higher munitions load, consistent with the advantages of a short take off aircraft.

Of necessity, tiltrotor aircraft are high wing types and in this regard, the proposal of an existing helicopter airframe has sound advantages. The existing fuselage height of the Mi-28 appears about right for a combat tiltrotor, as proposed and there appears to be adequate space within the upper airframe to house the power plant. Allowing re-development of the existing small hold for additional weapons stowage. As a tiltrotor, it would appear the aft section of the airframe could be shortened, as there is no requirement to consider either blade strike or, with the elimination of the tail rotor, provide rotor clearance for a tail rotor.

In land based tiltrotors, hard-points and pylons could be placed under the wings for external munitions and long range tanks. In naval variants, retention of the winglets and associated pylons for external munitions, would be a practical recourse, as this would simplify folding of the wings for hangering purposes. As previously mentioned, the existing hold could be re-developed for internal munitions and in this regard, a rotary magazine may prove off considerable advantage for larger rockets and missiles. Consistent with their greater lifting capacity and speed, such an aircraft would also be well suited to the deployment of anti-submarine torpedoes and marine mines.

The in-board relocation of the power plant, also has the advantage of eliminating the mass damping effect, associated with the currently wing tip mounted power plants, providing a far more responsive airframe. With the advent of fly by wire and computer augmented flight systems, the liabilities often associated with highly responsive airframes in helicopters can be eliminated, allowing the advantages of this responsiveness to be fully utilised. This improved level of responsiveness, would be of great advantage in both maritime operations and in combat scenarios in general.

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AVIATION APPLICATIONS,
CIVIL & MILITARY.

Integrated Tiltrotor & Helicopter Strike Force

In the role of a full combat version of the tiltrotor, as previously proposed, good argument could be made for using an existing helicopter airframe as the basis for such a development and with this in mind it would appear reasonable to propose the Mi-28 as a good initial airframe.

This airframe would appear of more than adequate size for the purpose and being a recently into service, tandem seat combat helicopter, it can be expected the airframe for this helicopter will be in production for some time to come. The use of an existing recent into service helicopter allows a great deal of commonality of components and systems and as a result, there would be little requirement for retraining of the air-gunner and a substantial reduction in production tooling. More-over, the degree of commonality that could be developed, would allow a greatly reduced logistics requirement and spares inventory for repair and maintenance.

Although good argument could also be made for the use of a Mi-24/5 airframe for a similar purpose and for similar reasons, using the more compact airframe of the Mi-28 would be better suited for in-parallel development of a naval version. The more compact airframe allowing deployment on smaller classes of surface combatants and fleet auxiliaries. For this reason, in the balance of this article, it is assumed we are considering the Mi-28 airframe alone. In either case, both the tiltrotor so developed and its parent helicopter, should enjoy a common power plant of the type proposed, maintaining the goal of reduced maintenance costs, allied with improved operational characteristics. Such a development will substantially improve the Mi-28 in its role as a combat helicopter, firstly, providing increased range and/or a lighter fuel load and secondly, reducing the IR signature of the helicopter, thus increasing its combat survivability. As this development would again provide an integrated strike force of an anti-tank and anti-air helicopter, supported by the versatility of its tiltrotor counterpart. These developments will likewise provide significant advantages with regards export sales opportunities for both aircraft types.

With regards battle field combat scenarios, helicopters could be treated as forward deployed defensive/offensive units, with the tiltrotors able to be held further in the rear, acting as rapid deployment support/attack units. Such a policy would allow a reduced number of tiltrotor aircraft to supply support over a relatively broad front, using their superior speed to arrive in the combat area, in support of a sectors forward deployed helicopter force; preferably arriving at roughly the same time as the helicopters in a coordinated attack.

Given that tiltrotors would be well suited to operation from roughly prepared airfields or short sections of straight road, in their short take off mode, their increased munitions would provide a substantial and disproportionate level of fire power, on joining an action. As such, they could provide an overwhelming strike force and may be able to depart an area after only a short engagement and if required, move on to respond to another incident, allowing the sectors forward deployed helicopters, taking part in the action, to carry out a mopping-up operation; which now need not be a rushed affair, given the tiltrotors could rejoin the initial combat, should unexpected resistance be encountered.

Such a policy would see a relatively small number of tiltrotors able to respond to developing situations over a broad front and with their greater speed and duration, they could also address more distant combat requirements, allowing the sector forward deployed helicopters to respond to more localised requirements. This policy would significantly improve helicopter combat efficiency, as it would substantially reduce the transit requirements for front line helicopters, by allowing them to be concentrated on local needs.

In addition, the greater speed and range of the supporting tiltrotor could become a formidable problem for advancing armour, in that the line of engagement, at which advancing armour could be attacked by anti-armour, rotary winged aircraft, could be brought much closer to their initial point. Providing the attacking tiltrotors, an opportunity to significantly reduce the numbers of an attacking tank force, long before they could be attacked by the front line helicopters and giving these same helicopters greater time to prepare a counter offensive, against a now substantially reduced level of threat.
Naval Variants

In considering naval variants, it should be apparent, the hydraulic drive line will readily accommodate the usual requirement to fold both the wing and proprotor for hangering purposes. This could be a considerable asset allowing naval variants to be hangered on relatively small combat platforms. In the case of larger platforms, the same capability would allow the hangering of a number of tiltrotors within a reasonably small hanger.

Development of the type for naval combat applications, would see such an aircraft, admirably suited to operations as firstly an anti-submarine aircraft and in such a role, it would exhibit characteristics far superior to the conventional helicopters currently addressing this requirement. With its far greater range and transit speed, such an anti-submarine variant would be able to carry out independent actions at great distances from its operational platform, whether the latter is a carrier or lesser fleet unit. In parallel with an anti-submarine role, naval variants could also be utilised as a stand-off unit in an anti-ship role. In this role, the aircraft could be used to effectively and significantly extend anti-ship missile range, far beyond ship launched munitions; allowing the ship initiating the attack, to remain undetected and beyond the range of retaliation.

Given the expectation that an IHD tiltrotor could be expected to enjoy better than three times the range of a gas turbine powered equivalent, for the same fuel load, naval variants could be expected to undertake missions requiring them to stay aloft without refuelling for as long as nine hours. Allowing the prosecution of an action far beyond the range of contemporary ship-board helicopters. It becomes quite practical to notably improve even this level of durability, with increased fuel capacity; as such, tiltrotors would also be well suited to use as over the horizon surveillance and targeting elements for shipboard munitions

In naval applications, the need to fold the aircraft’s wings and proprotors, for hangering purposes, is far more readily addressed with the IHD power plant, whether located in the wing root or lower fuselage; whereas the folding of these components in the existing aircraft will be a far more demanding exercise.

It should also be apparent, with the shift to IHD power plants for surface combatants, the advent of IHD powered tiltrotor aircraft for shipboard deployment, would allow a common fuel and technology base, for both the surface combatant and its aircraft. This would have significant operational advantages, including a reduced logistics requirement and given the ship’s engineers would be well versed in power plant maintenance procedures, there would be no need to incorporate aero-engine fitters, to maintain the aircraft’s power plant. Providing a reduction in vessel crewing levels. Consistent with the reduced logistics requirement associated with a single fuel type, a common fuel also greatly reduces the fuel stowage and handling requirements on board the vessel and during refuelling at sea procedures. Moreover, with the shift to a common fuel, that fuel being diesel, the vessels overall safety level is significantly improved, with the simplified systems and safer ignition characteristics associated with diesel fuel. It should also be recognised: consistent with alternative applications of this powering system, both helicopters and tiltrotors so powered, can use auxiliary systems to provide power substantially in excess of normal power plant levels, enabling both types to lift loads in excess of their normal capacity.

Given the Russian aviation industry’s vast experience in helicopter design and manufacture and its currently under-committed manufacturing capacity, it would appear the development and manufacture of such aircraft at this time, would be a major commercial opportunity. This is especially true, considering the development of the IHD power plants for armoured vehicle applications, will provide a mature powering technology able to be directly transferred to the aviation industry generally and especially relevant to the development of an advanced tiltrotor.

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