Design 698

The concept of the aircraft involves two primary versions: An AEW version, designed to operate with full capability with a vertical take-off on a hot day, and with a short 170 ft take-off from the foredeck of the vessel, can be loaded with a full complement of either air-to-air or air-to-surface missiles; the other primary version of the aircraft is envisioned to be an electronic warfare or jammer version which is very effective in coping with air threats. In effect then, what has happened in comparison, for instance, to a ship-based radar system is that the point sensor in the form of the ship’s radar has been supplemented by the distributed sensors carried by the aircraft. This has one very valuable fallout, that is, it allows the ship to run quiet or on EMCOM, which makes the detection of the ship, from an enemy point of view, very much more difficult.

There are other versions of the aircraft which are envisioned. In our scenario work done under contract to the Navy, a variant of the aeroplane called an ASW “Pouncer” was found to be very useful. For instance, a maritime patrol aircraft might have several submarine contacts at one time. If the patrol aircraft chose to prosecute one of these contacts, it wo of course, lose the other contacts. Under these circumstances, it would be highly desirable to call in a fast aircraft to prosecute the attack so that the patrol aircraft could hold all of its contacts while the attack was underway. Of course, this role as envisioned is in no way competitive with the LAMPS helicopter, which is a closer-in device with a much more substantial payload in the form of onboard ASW equipment.

Another version of the aircraft envisioned as being highly desirable is one carrying a system of sensors that would be useful for overland targeting. Such sensors would take the form of battlefield radar for detecting small moving targets, plus signal monitoring equipment for determining emitter locations. This version of the aeroplane would be used in combination with ship-based missilery, wherein missiles would be fired at long range and their guidance updates performed by the aircraft.

All of the roles of the combined aircraft/ship/missile system can be performed by ship-launched weaponry guided via the aircraft over the horizon. However, the existence of a missile capability in the aircraft provides an additional element, which greatly compounds the threat to the enemy in that offensive weapons against his surface units or air units can now come from many directions simultaneously. Furthermore, in the case of attacks against enemy units, be they surface ships or aircraft, there are certain occasions when one cannot afford the transit time of the ship-launched weapon to a particular target.

The ship which is currently envisioned and matched to the aircraft, we have dubbed DGV, stands for Destroyer, Guided Missile, Vertical Take-off Aircraft. Our investigations to date have been conducted with the help of Litton-Ingalls Shipbuilding and have been based upon the utilisation of variants of the DD-963 Spruance-class destroyer hull and machinery. At this time we envision a hangar capable of comfortably berthing 1 O of the 20,000 lb class Design 698 aircraft. With the 250 ft foredeck for STOL missileer overloads a 150 ft afterdeck for single-engine-out barricade arrestments, and having 120 vertical launch tubes, the basic configuration details of this ship have been fairly thoroughly investigated. The ship turns out to be in the 10,000-ton class, and more will be said about it at a later point in the paper.

The function of the jammer version of the aircraft is to provide a defensive screen for the surface ships to force the Backfire aircraft in closer to the ships, where they can be handled more easily by the aircraft and ship-launched weaponry.

In the area of mounting offensive operations against enemy surface or submarine units, the surveillance/missileer version is used for the detection and identification of those units, and either surface-launched, air-targeted Tomahawks or air launched Harpoon missiles can be used. The air-launched missiles are most valuable in high-ECM environments and for quick reaction strikes at extreme ranges where the fly-out time of the Tomahawk is excessive.

Already described is the operation of the ASW Pouncer aircraft, which is really a system complementary to the LAMPS Ill system which is presently coming into service. The Pouncer, of course, capitalises on its long range and high speed to prosecute submarine contacts coming from either air units or surface units. In order to increase the effectiveness of such an aircraft, advanced, lighter-weight ASW systems would be highly desirable. In the case of the interaction of the various aircraft types with maritime patrol aircraft, our scenarios have tended to indicate that the maritime patrol aircraft capability -requirements are dramatically reduced by the existence of the VTOL aircraft in support of the maritime patrol aircraft. The vulnerability of the maritime patrol aircraft in sea lane defence roles is also greatly reduced by the presence of the VTOL.

Certainly, if the role envisioned for the aircraft involves a lot of hovering or very heavy load-carrying capability for short ranges, the obvious choice would be a helicopter, but for the roles that we envision as being critical to the surface combatant, aside from ASW, which is the responsibility of LAMPS, the performance illustrated in the matrix is appropriate. In addition to the performance characteristics noted, other desired characteristics are that the aeroplane should be small for small-ship basing and that they should be numerous in order to provide better ocean surveillance coverage. In addition, it would be highly desirable if a substantial non-Navy acquisition base potential existed, perhaps for the other services or for foreign Navies.

Basically, the Design 698 aircraft is a twin tilt-nacelle aeroplane, which is controlled through transition and hovers primarily by means of horizontal and vertical vanes located in the turbofan slipstream. The aeroplane is of an otherwise conventional arrangement with one exception. Aircraft designers as a whole have gotten used to nose wheel-type tricycle landing gear arrangements. For this aeroplane, we have reverted to a dual forward landing gear configuration. This turned out to be overwhelmingly preferable for turnover stability on small decks wherein rolling motions of the ship could be expected. For a given landing gear tread, a much higher turnover stability could be achieved. Furthermore, since the aeroplane in its STOL mode does not require rotation for take-off with the fixed nacelle incidence, the tail wheel could be counted upon for the steering function. The keynote of the design is simplicity. Simplicity is obtained primarily through the fact that all VTOL equipment is located within the engine nacelles (that is, provisions for thrust, modulation of thrust, and control of the aircraft). If the capability of hovering in the event of a single engine failure is required, a backup cross shaft can readily be provided which is a straight shaft between two right-angle gearboxes in the turbofans. This shaft is housed in the cross-box structure which links the two engine packages together into what we have called a “dumbbell” assembly. For military applications, one could consider operations without the cross shaft in the event of an engine failure, for reasons which will be discussed shortly. The aircraft has excellent visibility due to the shape of the nose, which is not in any way compromised by the presence of VTOL-related. equipment. In fact, the entire airframe is completely free of VTOL provisions.

The major element of the aircraft, from a systems point of view, is the conformal surveillance radar which is housed in the leading and trailing edges of the wings and in the two fuselage strakes which are also provided to give positive ground effect. These six arrays provide 360-degree surveillance coverage about the aircraft and have an excellent detection capability against the spectrum of air and surface targets, as well as being flush assemblies of a very low weight. For missile guidance, a small pencil beam X-band guidance radar is provided, which works in consonance with the conformal arrays for guidance of weaponry against both air and surface targets.

Now for more detailed information on control of the aircraft in the hover mode. Addressing the first pitch control, we see that there are a pair of vanes in the turbofan exhaust located below the centre of gravity. Pitch control of the aircraft is provided by the symmetrical deflection of these vanes. Similarly, differential deflection of these vanes produces yaw control of the aircraft. For roll control, we have provided both vertical vanes in the fan blast and variable inlet guide vanes. Approximately 50 per cent of the roll control comes from each source. The major benefit of this kind of system is that the thrust modulation of the engine for roll control is reduced from roughly plus or minus 20 per cent of maximum thrust to plus or minus 10 per cent of maximum thrust by use of the vertical vanes. This results in a significant reduction in the fan diameter requirements and weight of the powerplant package.

A second function of the variable inlet guide vanes is to stabilise the aircraft in roll in the event of an engine failure, for not employing a cross-shaft. Such might be used for applications. For even extremely sudden or catastrophic fan failures, it is possible to very rapidly kill the thrust on the other side of the aircraft by shutting the variable inlet guide vanes completely; literally stuffing a cork in the live engine. This gives the crew adequate time for ejection. The subject of whether or not removal of the cross shaft is advisable for military applications has arisen fairly recently in order to reduce the cost and complexity of the aircraft. It may turn out to be a very logical thing to do, considering the fact that the overall probability of failure during a hover period of one of the two turbofans may be more operationally acceptable than the reduction in reliability introduced by the cross-shaft assembly itself.

Perhaps the most important feature of the Design 698 aircraft is that it utilises a conventional high bypass turbofan engine (bypass of approximately 6:1) used in virtually all modern commercial transports and also in the S-3 and A-1o military aircraft. The engines in this class are characterised by their excellent cruise efficiency and their high static thrust. Two of the TF34-100s are currently being used to power the full-scale model of Design 698 in the 40 ft x 80 ft full-scale tunnel test and the outdoor static testing to follow. As will be seen later, our ultimate intent is to build a technology demonstrator of Design 698 in exactly the same size as the wind tunnel model, using a pair of A-10 TF34 engines modified to accept inlet guide vanes and digital fuel control. Basically, these are the only modifications required. Utilisation of a slightly modified TF34 engine for the technology demonstrator is the key to being able to offer this program at a low cost.

It should be noted that a vigorous program of a variable-inletguide-vane technology demonstration for the TF34 engine is currently underway at NASA Lewis using a full-scale TF34 engine.

The projected performance of the surveillance version of Design 698 involves a very substantial upgrade of the TF34 engine. However, there is little disagreement about whether or not the technology levels are achievable in the time frame in which such an aircraft would become operational. The upgrade of the engine was defined as working in cooperation with General Electric. In the case of this aircraft, we used the same sort of short-lift ratings typical of the Pegasus engine in the definition of the allowable takeoff gross weight. The aircraft had a crew of two, with an overall mission load of 3220 lb and a fuel allowable of 6814 lb for a tropical day take-off. This results in a fuel fraction of 29. 2 per cent, which is very respectable and provides excellent mission performance, considering the rather good specific fuel consumption characteristic of the high-bypass turbofan maximum Mach number is 0. 8, the cruise Mach number and the aircraft has a time on station at 100 nautical miles of 3. 65 hours. The ceiling of the aircraft is 50,000 ft. It should be noted. that while we frequently hear cries of anguish about the “VTOL penalty”, such a penalty takes the form of oversized engines, which is not all bad. The sea level rate of climb of this aircraft is nearly 15,000 ft per minute, which is better than the Harrier, although, of course, at the higher Mach numbers the high bypass-ratio fan begins to fade compared to the lower-bypass class of the AV-8A.

Other applications of this 20,000 lb class, subsonic turbofan VSTOL are envisioned by Grumman. Basically, we see a large acquisition base potential for versions of the aircraft which we designated the OV-X, or the battlefield surveillance aircraft. We forecast interest developing in both the US Army and Marine Corps. Foreign navies are developing and building VSTOL carriers, and we envision a rather large market in these navies for this class of aircraft. Eventually, we feel that the benefits of a commercial or executive V1OL transport in the nine-passenger class, which is the class of this aircraft (Cessna Citation, etc. ), will be enthusiastically received. We think that the availability of an aircraft that can take off and land in confined areas like a helicopter, and still have a range of 1200 nautical miles with comfort and low vibration levels, will prove to be of great interest to this market. Oil rig transport is a big factor in helicopter operations today, and we feel that this aeroplane, with its high block speed and comfort, will eventually come to· replace a substantial part of the helo fleet currently in operation for these tasks. Another emergent role that may seem somewhat surprising is the search and rescue role. Recent discussions with experts in this field indicate that the prime requirement for a combat search and rescue aircraft may very well be to go along with raids against hostile territory, such as in Vietnam, and to be prepared to rescue downed aviators through air pickups. These became straightforward with the VTOL aircraft, as opposed to letting the aviator go down into the ocean beyond the range of helicopters, which was a big problem in Vietnam. The possibility of land pickups of downed aviators also becomes much more plausible with this class of aeroplane. In the search and rescue regard, US Air Force interest may well evolve.

One of the first considerations in shipboard operations of a new class of aeroplane is the amount of landing area required for operations aboard a ship.

It can be seen at once from the illustration that the required operating deck area is much smaller for the turbofan design, and additionally, it is probably much safer because of the absence of the possibility of the rotor tips striking the hangar or deck structure. Furthermore, the high disk-loading turbofan aircraft is inherently less gust-susceptible than a helicopter during hovering operations.

The next immediate question of interest is the size of the aircraft, not only for operations but for hanging.

Hangar compatibility is of great importance since modification of ships is often a painful and expensive proposition. Illustrated is the relative hanging of the Design 698 on the Perry FFG-7 class Frigate, soon to enter RAN service. It can be seen that the aeroplane hangars are very nice.

In our research, we have depicted a ship that we have dubbed the DGV, Guided Missile Aviation Destroyer. This ship has been arrived at as a reasonable design by the joint efforts of Litton-Ingalls and Grumman. The ship would displace about 12,000 tons full load and is 569 ft long. It is a high-speed surface combatant carrying 120 missiles in vertical launch tubes. The complement of aircraft for this ship would nominally be eight surveillance/missileers and two jammers, for a maximum of ten aircraft aboard.

The Design 698 in the 3 categories of wind tunnel, radio-control models, and simulation against a background showing one of our more interesting and dramatic tests of the configuration. This was a whirling arm transition facility where the aircraft could be taken from hovering flight through the transition to wing-borne flight and back down again on the end of a 50 ft ultra lightweight whirling arm. It was found that transitioning of the aircraft with the rudest type of pitch, RPM, and engine nacelle controls, aided only by a pitch-rate gyro, could be accomplished very easily as a manual task.

The end objective, of course, of the testing is to provide the basis for the construction of a manned flight demonstrator. Many VTOL concepts have been previously demonstrated, and yet this turbofan concept, potentially one of the most useful, has not been demonstrated. As an aside, many of the demonstrations of VTOL concepts occurred in the 1950s before technology was ready to give us production aircraft with a high level of mission capability; that is, engine thrust-to-weight ratios had not advanced sufficiently, and not have composite structures and ultra lightweight avionic systems with a high level of capability. Today, of course, the availability of these technologies changes the picture entirely. What happened in the ’50s was that the Korean War showed how useful helicopters could be. At the same time, their drawbacks In terms of speed, range and comfort was immediately obvious. It was quite natural to seek a marriage of the virtues of conventional aircraft with vertical lift and the search started with great vigour and little success, simply because technology was not ready to provide the marriage of the best facets of both worlds.

I am personally convinced that the time is right for a technology demonstrator in this class which could ultimately lead to a very useful production aircraft with both the capability to carry full for a very substantial mission capability, plus a very good mission load while operating In the vertical take-off mode.

Towards the end objective of demonstrating this technology, Grumman, in cooperation with NASA and the US Navy has recently completed a full-scale wind tunnel test of a demonstrator. configuration based upon the use of Mitsubishii-2 airframes. This full-scale tunnel test will be followed by outdoor tests of the aircraft in ground proximity to determine its characteristics in terms of stability, control, suck down, and reingestion. The full-scale tests are accompanied by a full program of small-scale tests of an aircraft having exactly the same configuration as the full-scale tunnel model. Following the completion of this broad test program, Grumman will be in a position to propose to build a technology demonstrator of this airframe. We can offer a low-cost program through the use of existing TF34 engines which is most attractive in these austere R&D times.

The approach to building a flight technology demonstrator of the Design 698 aircraft is already well documented. The nose module of a Mitsubishi-2 transport would be modified for a pair of side-by-side ejection seats. A new centre module would be added for balance. The dumbbell assembly would be an entirely new fabrication, specifically for the demonstrator aircraft. The Mitsubishi-2 aft section and wing assemblies could be used with minor modifications. A new vertical and horizontal tall assembly would have to be provided, as would a retractable forward main gear set and a retractable tailwheel assembly. The question frequently arises in technology demonstrators, “Why not use a fixed landing gear?” It was felt in the case of this aircraft for which high-speed performance Is really Its strongest suit It would be foolish to build an aircraft which would be limited by a fixed landing gear.

In summary, Grumman believes strongly, not only in our basic aircraft design concept but in the value of a flying technology demonstrator program predicated upon what we feel are solid future naval and military applications as well as civil uses of this 20,000 lb class of VTOL.