This html article is produced from an uncorrected text file through optical character recognition. Prior to 1940 articles all text has been corrected, but from 1940 to the present most still remain uncorrected. Artifacts of the scans are misspellings, out-of-context footnotes and sidebars, and other inconsistencies. Adjacent to each text file is a PDF of the article, which accurately and fully conveys the content as it appeared in the issue. The uncorrected text files have been included to enhance the searchability of our content, on our site and in search engines, for our membership, the research community and media organizations. We are working now to provide clean text files for the entire collection.
Practical jet-powered military aircraft capable of vertical or short take-off and landing (V/STOL) have been available since 1969, when Britain’s Hawker Siddeley Harrier entered service with the Royal Air Force for tactical ground attack and reconnaissance duties. However, in spite of the intervening years and this aircraft’s acceptance (as the AV-8A) by the U. S. Marine Corps, it is doubtful whether even the fundamentals of V/STOL are understood outside these two services. Nor is there any clear indication that those responsible for long-term combat aircraft development in the Free World have analyzed why some V/STOL systems have succeeded while most have failed, what V/STOL has to offer, what penalties are involved, and what avenues of engine development should best be explored if future V/STOL aircraft are to extend their usefulness beyond short- range subsonic applications.
As is the case for many important technological advances, it is difficult to credit any one person or document with the invention of V/STOL, although some credit must be attributed to the World War II provisional patent specification by von Wolff, covering the deflection of jets by cascades and bending jetpipes to improve airfield performance and maneuverability. However, at that time turbojets produced little thrust (only about one ton by 1944) and weighed too much (a typical thrust/weight ratio being 2:1 in Britain and 1.5:1 in Germany). As a result, true VTOL was out of the question, and the potential gains in deflecting jets for STOL were negligible.
There is no evidence of a direct link between the postwar publication of von Wolff’s document and a successful V/STOL project, but he had grasped the es-
sential fact that, whereas the reciprocating engine had been rigidly linked to its thrust vector, the gas turbine made it easy to point this vector in virtually whatever direction was required, simply by turning the direction of the exhaust flow.
The turbine engine thus offered the prospect of much more flexible application, but significant use of jet lift could only be made if several conditions were fulfilled. Above all, the gross thrust generated at the exhaust nozzles had to equal a high proportion of aircraft weight. This ruled out high speed V/STOL aircraft until the advent of later generation engines, which produced a thrust of seven times (or more) their own weight. It also indicated that V/STOL would be unacceptable for any aircraft category that is normally low-powered, or has a proportionally small payload (implying a high growth factor).
HAWKER SIDDELEY AVIATION. LIMITED
A Royal Air Force Harrier, left, an Americanized version of which is the Marine Corps AV-8A, right, demonstrates its spectacular climb-out from VTO. Nose-high attitudes up to 70° are possible, since the reaction control system operates for all jet deflection angles beyond 20°.
The E ntu 'icklu ngaring-Sud VJ101C-X2 was the world’s first (marginally) supersonic VSTOL aircraft. It combined four afterburning engines in tilting pods with two fixed lift engines just aft of the cockpit.
In addition, V/STOL required that the lift vector had to act approximately through the aircraft center of gravity, which called for genuine innovations in power- plant configuration. Finally, V/STOL relied on special attitude controls that would retain effectiveness at zero airspeed. In any event, the problem of control in jet- borne flight proved to be easily solved, owing to the large power offtakes available as high-pressure air-bleed, making possible reaction control systems similar in operation to those used on space vehicles.
To some extent the factors which determined the feasibility of V/STOL were thus analogous to those which had governed the success of the first conventional flying machines: the provision of forces along and normal to the flight path, commensurate in magnitude with aircraft mass; the availability of powerplants of light weight relative to their output; and adequate means of controlling aircraft attitude and flight path direction.
However, the two achievements differed fortuitously in one important respect, in that V/STOL is operationally such a transient state that normal stability criteria do not apply. The transition between jet-borne and wing-borne flight is in fact so short (a typical acceleration time being 20 seconds), that—given powerful controls on all axes—unstable longitudinal and rolling characteristics may be quite acceptable. Fortunately for
all concerned (who might otherwise have given up the struggle) this fact was established at an early stage by analogy with low-speed V/STOL machines and by NASA trials using powered free-flying models in large wind tunnels, which had been constructed in the 1930s to test full-scale aircraft. This tolerance to instability proved to be necessary, since—depending on aircraft configuration—the airflow patterns induced by intake suction and by jets inclined to the flight path can give a destabilizing downwash on the tailplane (horizontal stabilizer) and a large dihedral effect.
In practice, the constructor minimizes these effects by tailplane experiments and by refined methods of sideslip sensing and indication. Consequently, a great deal of V/STOL flying has been possible without autostabilization. For production aircraft, autostabilizers have nonetheless been provided to cater for adverse conditions, night landings on a carrier deck in heavy seas being an important case in point.
To return to the primary requirement of jet-lift V/STOL, the concept is essentially one of achieving a difficult objective by brute force. If allowance is made for thrust losses due to imperfections in the inlet and exhaust systems, hot gas recirculation to the intakes, power offtakes and air bleed, ground effect, and for sufficient net thrust for a positive unstick, then the testbed rating of the powerplant should be roughly 120% of the required VTO weight. A thrust/weight ratio of 1.2 is far higher than would be provided in most conventional aircraft; only the new air superiority fighters will have values in this region.
If the V/STOL aircraft lifts off after a short forward roll, then hot gas recirculation is eliminated • and, as unstick speed increases, wing lift becomes significant. Much higher weights are thus achieved, relative to VTO. The press sometimes repeats the myth that VTO radius is short because so much fuel is burned in take-
off. In reality, fuel consumption in both VTO and STO is small in relation to conventional reheat standards; the comparatively short radius given by VTO is almost entirely due to the absence of wing lift, restricting take-off weight.
The application of unusually high thrust levels to achieve V/STOL involves abnormal powerplant cost, and this has led to considerable efforts to develop devices which would cheaply "magnify” the thrust of a conventional-sized engine.
This again dovetails with the analogy to normal flying, since a wing is basically a dynamic device for magnifying engine thrust by a large factor. In the case of a fighter this magnification will peak at 8 to 12, whereas a subsonic bomber might reach a lift/drag ratio of 20. If the powerplant supplies adequate thrust to overcome drag, the wing will generate the much larger lift force needed to sustain aircraft weight. To obtain the same effect with the airframe stationary, a helicopter uses a large rotor as a thrust-magnification device, but current rotors are limited in forward (translational) speed by shock waves on the advancing blades, and by stalling the flow reversal on the retreating blades.
V/STOL: Stalled? 35
In aiming for high-speed V/STOL, it obviously would be an attractive proposition to contain a low-cost static thrust-magnifier in a conventional airframe, namely, a slender fuselage and a wing of high loading.
The most readily available form of thrust augmentation is afterburning, which (depending on the size of jetpipe that can be accommodated) gives a boost of about 35% for a turbojet, and 70% for a turbofan. However, in terms of 1950s’ technology, the only simple way in which a reheat engine could provide jet lift was in a tail-sitter. The tail-sitter concept was tested in several research vehicles, including the unreheated Rolls-Royce Avon-powered Ryan JC-13, which flew quite successfully.
The tail-sitter is mechanically as close to a conventional aircraft as is possible with VTOL, but it has the fundamental disadvantages of pilot orientation problems, special ground support equipment, and zero overload capability. Perhaps the most important lesson to be remembered from the tail-sitters is that any proposal which cannot exploit wing lift in improved STOL weights is doomed from the outset.
Two of the earliest schemes for thrust magnification were jet ejector augmentors and buried fans. Aug- mentors initially appeared to offer a boost of approximately 70%. Very low costs were promised, the extra lift being picked up in the form of suctions caused by free stream air induced by the jets to flow over fixed aluminum surfaces, rather than involving expensive rotating machinery. This concept was tested with large
augmentors in the fuselage of the Lockheed XV-4 Hummingbird, but little thrust boost seems to have been achieved.
General Electric’s lifting fans were driven by gases from the propulsion engines being diverted to expand through turbine blades mounted on the fan periphery. Evaluated in the wings of the Ryan XV-5, they proved heavy, and they limited wing loading. Furthermore, smooth transitions may have been ruled out by the inertia of these large fans. Performance curves showed disappointing gains from STO, presumably due to the momentum drag of the free stream air that was stopped to flow vertically through the wing. Later proposals therefore changed to retractable, vectorable fuselage- mounted fans, but the effects on the structure were unattractive.
A more promising approach was promoted by Rolls-Royce, simply adding jet lift by means of batteries of specialized engines. It was found that engines of exceptionally high thrust/weight ratio could be designed by sacrificing normal broad-band operating limits, specific fuel consumption, and engine life, all of which were irrelevant to this application. Whereas normal propulsion engines of the 1950s were producing T/W values of up to 5:1, lift engines offered the
prospect of 12:1 in the short term, and 24:1 by the 1970s.
Unfortunately, these manufacturers’ data are based on bare dry engine weight, and when applied to a practical aircraft design they are seriously degraded by high installation factors. A conventional powerplant adds up to approximately 1.4 times the engine manu
facturer’s brochure weight, but a lift engine (allowing for variable-geometry inlets and outlets) may have an installation factor as high as 1.7, even without the effects of enlarged fuselage volume.
In spite of these reservations, lift engines now give a very high thrust/weight ratio, and they have the valuable attribute that a single engine type can be
applied to a wide variety of V/STOL projects, hence R&D costs are not nailed at the door of a particular aircraft program.
In the fighter context they have achieved only a limited success, since the idea of separate engines for lift and propulsion was found too uneconomical, and since by the time that an improved means of using lift engines had been found, operational concepts had changed. Specifically, the nuclear war scenario that had motivated European V/STOL studies in the 1950s had
V/STOL: Stalled? 37
given way to the "graduated response,” with relaxed STOL demands that could be satisfied with high wing lift.
The pure Rolls-Royce concept was tested in the Dassault Mirage III-V, which had a Pratt & Whitney TF-30 for propulsion and a battery of eight RB-162 engines for jet lift. This was the first V/STOL aircraft able to land vertically after a single engine failure. Its disadvantages were that the wide fuselage produced too much wave drag, and that downwash induced by the lift engines gave a negative lift on the wing, and an unacceptable rolling moment in sideslip. As a front-line fighter, the Mirage III-V would also have involved an undesirable degree of complexity.
The French ultimately decided (as the Germans had done several years previously) that fighters could not live with the volume and weight penalties of a whole battery of engines that were used during only a few minutes of every flight. This led to the conclusion that the propulsive thrust should be deflected downwards by means of a bending jetpipe, this jet lift being augmented and balanced by lift engines in the forward fuselage.
This was fundamentally a development of a concept used in Germany’s VJ-101C, which had tilting pairs of propulsion engines on the wingtips, and two lift engines in the front fuselage. It lacked the engine-out capability of the Mirage III-V, but was the first V/STOL aircraft to go (marginally) supersonic on the level. More advanced thinking led the same design group to a variable-sweep strike fighter proposal, in which two bending reheat pipes at the rear were matched with two pairs of retractable, vectorable lift engines at the front. This AVS project must surely have been the most complicated fighter ever projected!
It might be said that the improved lift engine concept also reached fruition in the German VFW-1262 (or VAK-191B). This combined a central lift/thrust engine (essentially a smaller version of that in the Harrier) with single lift engines in the fore and aft fuselage. Although theoretically more efficient than the simple Harrier concept in pure VTO, this advantage probably disappears in STO, since it is difficult to deflect the thrust of the lift engines efficiently to assist acceleration.
In spite of this, the Soviet Union has done considerable work on supersonic STOL projects with lift engines, notably the MiG Faithless and Sukhoi Flagon B. With respectively two and three lift engines in the four to five-ton category, these aircraft must achieve roughly the same minimum flying speeds that the West has reached with variable geometry wings. The lift engine approach to STOL for supersonic fighters is
probably more straightforward than using variable sweep, but sacrifices the in-flight performance gains of the latter. Relative to a true V/STOL aircraft such as the Mirage III-V, the Soviet concept minimizes low- speed induced airflow problems, since only approximately half the weight is supported by jet lift. It may well avoid the complication of reaction controls by accepting moderate unstick and touchdown speeds.
In interpreting the lessons of early V/STOL programs, no attempt has been made to adhere strictly to the chronology of the 1950s and 1960s, since most concepts were developed (and abandoned) in parallel, without much interaction or cross-flow of information.
However, one project did act as a catalyst for others: the Bell X-14. Powered by two lightweight engines (initially Rolls-Royce Vipers and later GE-J85s) mounted horizontally in the front fuselage, the X-14 achieved V/STOL by turning the jets first through a fixed 45-degree cascade, and then through a similar but rotatable unit. The pilot could thus add the two deflections for jet lift, or cancel them out for horizontal thrust, or use any intermediate angle. After the complexity and poor STO potential of lift engines, buried fans, etc., this first practical application of thrust vectoring was a major breakthrough, which encouraged British (and later Soviet) designers to produce the first operational jet-powered V/STOL aircraft.
The significance of the Bell aircraft would be difficult to exaggerate, but the powerplant concept was very limited in direct development potential. The forward location of the engines ruled out a conventional slender fuselage, the pilot necessarily being moved aft to bring the aircraft center of gravity over the nozzles. The overall effect was a fuselage of large frontal area and consequently limited speed capability, while balance considerations left little scope for avionics in the front fuselage. Furthermore, since rotation of the cascades turned the jets through circular arcs rather than in a vertical plane, the system lost efficiency at intermediate angles and would only function with two counteracting engines.
The Yakovlev Freehand is a copy of the X-14 principle. Since its first appearance in 1967 it has evidently been developed for naval use, its main attraction being low cost. Any aircraft which uses turbojets optimized for high thrust/weight ratio suffers in regard to fuel consumption, hence Freehand is probably intended only to provide close support for amphibious landings. The conventional naval airpower tasks of reconnaissance and air defense are thus left to land-based patrol aircraft and air-surface missiles. Freehand thus appears less flexible than the Marine Corps’ AV-8A, although (assuming two five-ton engines) it may well be able to carry VTOL
38 U. S. Naval Institute Proceedings, October 1974
weapon loads of about two tons for short distances. In the event of one engine failing, it will be able to return to its parent vessel, but unable to land.
The story of the Harrier AV-8A development has been told too often to warrant anything more than a brief analysis of its basic design. The powerplant concept derives from an invention by the Frenchman Wibault, involving a turbine engine driving four fuselage-mounted centrifugal fans through shafts and gearboxes, the fan effluxes being variable in direction. The Harrier’s Pegasus engine achieves the same thrust vectoring by dividing the bypass air and turbine exhaust between four rotatable nozzles, which are linked together and controlled by a single lever that is located alongside the normal throttle. The fan and high- pressure spools are contra-rotated to eliminate gyroscopic precession effects.
This represented a considerable improvement over the Bell scheme, in both simplicity and development potential. By use of a front fan the thrust center was moved forward along the engine, which could therefore be positioned over (rather than ahead of) the center of gravity. This left the aircraft nose free for a conventional cockpit position, and permitted a relatively slender airframe. In addition, the fan engine produced an exceptionally high thrust/weight ratio (it still has the highest T/W of all unreheated engines) and simultaneously reduced the velocity and temperature of the efflux gases. Finally, total thrust vectoring, with all the
thrust available in any direction, had been achieved in one simple engine, making the Harrier more suitable than any other type of V/STOL aircraft for front-line operation with minimal support.
However, the Harrier is clearly not the end of the line for high speed V/STOL. The question therefore arises as to how the concept might be improved upon, for example by the Soviet project team which began work in 1968. Before discussing this, it is equally important to establish what operational benefits the simple vectored-thrust engine provides right now, and what stands to be lost in the quest for more speed and better warload-radius performance.
Regarded purely as an engine-airframe combination, the Harrier has low maintenance demands, flexibility of operation (providing attack, reconnaissance, interception of aircraft shadowing the fleet, etc.), probably as minor a ground erosion problem as will ever be achieved in a high-subsonic V/STOL aircraft, and an in-flight thrust vectoring capability which gives a significant edge over conventional designs in self-defense at low levels. Its low erosion jets appear to make this concept ideal for close support, wherever this involves rapid deployment in newly-occupied areas, for instance on a beachhead, or in the case of a moving battlefront or in an undeveloped region.
To take an idealist viewpoint, the main disadvantages in using the simplest form of vectored thrust turbofan are that the powerplant is significantly heavier than that for a conventional attack aircraft of similar size, and that when this big engine is throttled back for low level cruise, the specific fuel consumption deteriorates and the large intakes create spillage drag. Such an engine is reasonably economical at altitude, but not competitive for transport applications, so R&D costs must be borne by this one project. In addition, the thrust of a high bypass turbofan falls off as forward speed increases, and therefore the system is unsuitable for transonic level flight. The large intakes limit rearward view.
Turning to secondary disadvantages, the Pegasus is replaced by upwards removal through the hole left by the wing, a procedure which increases engine change time to four hours or more. Finally, the position of the nozzles in the fuselage sides imposes some limitations on inboard wing-mounted stores, and has led to a center-line main undercarriage and tip-mounted outriggers. The 22-foot track of this landing gear may be an embarrassment when maneuvering on a carrier deck, or taking off from a narrow roadway. It also rules out wing folding, and restricts the length of center-line stores. Yakovlev adopted a similar gear for Freehand, although this was not really justified by the risk of jet blast on the tires.
Summarizing developments to date, many jet- powered V/STOL concepts have been evaluated and most have proved flyable. However, with rare exceptions they were rejected because they introduced unacceptable complexity, and cost and performance penalties relative to conventional aircraft.
Attempts to "magnify” the thrust of a normal engine have so far proved unattractive, unless the Harrier’s 21,500-lb. Pegasus is regarded as a 5,000-lb. Orpheus with extra turbines driving a magnification device in the form of the front fan. The experts on lift engine applications have concluded that these should only be used to supplement the deflected thrust of the propulsion engines, but have failed to suggest any practical configuration that would land vertically after the failure of one engine. Such a failure would invariably necessitate cutting other engines (typically giving a 50% reduction in jet lift) to maintain the thrust center at the center of gravity, so multi-engine layouts have not produced the fail-safe capability that their complexity demands.
In operational terms, it has been found that a simple V/STOL aircraft can be operated effectively in the close support and reconnaissance roles from carrier decks, helicopter platforms, roadways, aluminum mats, etc. The Marine Corps has also investigated the use of thrust vectoring in flight as a defensive tactic for the AV-8A, producing decelerations that have been likened to driving into a brick wall, and "square turns” that no conventional aircraft can duplicate. Two of the writer’s flying friends have had this demonstrated: one lost a glass eye and the other lost his breakfast. It is nonetheless important to remember that the Pegasus engine gives only a mild indication of what might be produced in a vectored thrust aircraft optimized for dogfighting, and with provision for full thrust reversal in flight.
It therefore appears that V/STOL has potential to expand into the true fighter role. It might be anticipated that the "brute force” penalty of V/STOL would then be negligible, since aircraft such as the F-14B already have a high thrust/weight ratio, but (as discussed later) experience points to the conclusion that there will always be a price for arranging the thrust to be vectorable about the aircraft’s center of gravity.
Given that V/STOL is more expensive than the conventional design approach, what has it to offer in return? In the close support role, V/STOL can be justified by its flexibility of ship-to-shore operation, with rapid reaction and high sortie rates that cannot be produced by any other means. In the naval interceptor/dogfighter role, the case for V/STOL rests partly with the worldwide capability that may be had cheaply through small flat-tops without catapults and arresters, and partly with the unique air combat potential which thrust vectoring appears to offer.
In considering aircraft designs to suit these roles, the close support mission is already being fulfilled by the Harrier AV-8A. The improved variant currently under joint U. S./U. K. study (with 3,000 lb. more thrust, a supercritical wing, and raised cockpit) represents a low-cost program to further reduce take-off run, enhance maneuverability and high altitude cruise, and to overcome some of the operational limitations of the existing aircraft. The rate of climb provided by its high
40 U. S. Naval Institute Proceedings, October 1974
thrust/weight ratio suggests an interception capability against subsonic targets, although it will clearly never substitute for the F-4 in a high threat environment.
In the latter context the Harrier is fundamentally limited by the low specific energy of the turbofan efflux, giving a thrust decline with forward speed. Two projects which aim to overcome this restriction are now under study by Rockwell and General Dynamics.
The Rockwell XFV-12A is powered by a single Pratt & Whitney F401 (as developed for the F-14B), the exhaust from which can be diverted to jet augmentors in the wing and foreplane. Crediting Rockwell with some new type of augmentor which promises to avenge the disappointment of the Lockheed project, the XFV-12A still appears to have two basic disadvantages. First, it is difficult to see how the thrust-augmented wing principle could be evaluated without building the aircraft, since any large-scale trial rig operated close to the ground will give much more optimistic results than would be experienced off the end of a carrier deck. This is because jet augmentors rely on inducing free stream air to follow a high speed jet, and any jet spreading out over the ground is known to be extremely successful in inducing downwash, due to its large contact area with the surrounding air. Second, the XFV-12A system appears unsuited to thrust vectoring in high speed flight, hence a large part of the justification for V/STOL is lost.
The General Dynamics project reverts to the Franco- German concept of an articulated reheat pipe on the propulsion engine, balanced by two lift engines in the forward fuselage. This likewise appears unable to use thrust vectoring at speed, and is subject to the old objection that powerplant complexity is not offset by an engine-out vertical landing capability.
The two V/STOL concepts currently favored for U. S. naval fighter application thus introduce significant complexity, and fail to exploit the potential of jet lift as a combat aid. One proposal may not work at all, and the other threatens to increase losses due to engine failures.
If the front-running thrust-augmented wing should fail to live up to expectations, then what concept might be considered as an alternative to the General Dynamics project? In the short term, the improved Harrier might be seen as a low-cost substitute, given relaxed scenario requirements. In the longer term, it is clear that V/STOL powerplants for the close support aircraft and the pure fighter must diverge, as one demands low energy fan lift for minimum ground erosion at forward sites, while the other needs high energy jets for supersonics.
The point that appears to have been neglected by engine manufacturers throughout the Free World is that this latter demand does not rule out the single
vectored-thrust engine, provided that a completely fresh approach is taken to its design. Given a front fan of reduced bypass and increased pressure ratio (relative to the Pegasus), combustion in the fan flow could produce a really worthwhile thrust boost and move the concept into the supersonic field. Powerplant weight would be significantly reduced, spillage drag and specific fuel consumption in cruise improved through better thrust matching, and, with fan duct burning, the aircraft could have high performance and unrivalled maneuverability and speed agility. A move in this direction was attempted over ten years ago in the Hawker Siddeley P-1154 proposal, but far more dramatic improvements were possible, even at that stage.
Such a V/STOL aircraft would still be penalized, relative to a conventional design, by its aboveoptimum bypass ratio and limited space for duct burning, but there is no doubt that Mach 2 could be achieved. Carried to extreme, this improved vectored thrust engine would generate so much thrust at the forward nozzles that use of the turbine gases for jet lift could be eliminated. The resulting three-nozzle engine would be located further aft to bring the front nozzles in line with the center of gravity, giving longer (and more efficient) intake ducts and more scope for nose avionics without tail ballast. The two rotatable nozzles could be set low in the fuselage sides to bring the jets away from the wing-mounted stores and the tailplane, and to facilitate downward engine removal. The main undercarriage could be moved out to the wing, and a wing fold introduced.
The single vectored-thrust engine thus offers considerable scope for low-risk development, and it will be interesting to see how far the Soviets have exploited this concept for use on their new carriers. However, it must be emphasized that such aircraft, while revolutionizing fighter design and combat tactics, could not reproduce the Pegasus engine’s docile ground erosion characteristics, and because of this would sacrifice the Harrier’s unique flexibility of basing for the close support mission.
After graduating in mechanical engineering at Manchester University in 1954, Mr. Braybrook spent two years with Hawker Aircraft on a training course dealing with fighter design, development, and manufacture. From 1956 to 1958 he served in the Royal Air Force, was graded Scientist and sent on loan to the Royal Aircraft Establishment to assist in planning and evaluating air-air guided weapon trials in Australia. On completing national service he went as a Senior Project Engineer to Hawker Siddeley Aviation, where he was responsible under the direction of the late Sir Sydney Camm for a number of V/STOL fighter and attack aircraft projects. In 1968 he "temporarily retired” from designing to assess the future overseas market for fighters. Having travelled around the world eight times in the last six years, he concludes that the present military aviation situation is a re-run of the 1930s, with the Western Allies still designing "biplanes” instead of the modern equivalents of the F6F, P-51, and F4U.