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Aviation History
1957
1957 - 1710.PDF
FLIGHT, 22 November 1957 800-801 GNAT M k I ... PRIVATE VENTURER: Designer of the Gnat was Mr. W. E. W. Petter, C.B.E., B.A., F.R.Ae.S., who joined Folland Aircraft in October 1950 as deputy managing director. In July 1951, on Mr. Folland's retirement, Mr. Petter became managing direc- tor (he was, in effect, technical director as well). In early 1954, when Mr. F. H. Pollicutt left the company, Mr. Petter assumed the titles of managing director and chief engineer. He is seen on the left. through the intake lips. The idea of using the undercarriage doors as airbrakes was also conceived and the present Gnat wind- screen and canopy were developed. All these features are shown in the last plan view (opposite), which originates from May- September 1952. The engine there shown is the B.E. 26 Orpheus. The great blow came when, in autumn 1952, the Saturn was discontinued. Follands considered a great many alternatives, including the Turbomeca Ossau, which itself never materialized, but none really filled the bill. The Gnat was sadly in the doldrums; but the company did not despair and continued to try to persuade Bristol to continue development. In the meantime a Viper instal- lation was prepared in order to get the airframc into the air. While construction of the Viper-powered machine, then called Midge, was proceeding during 1953, a Bristol engine project was revived as a private venture and the larger and more powerful Orpheus emerged. The Gnat fuselage was therefore widened by four inches aft of the intakes and the wing area was increased from 126 to 136 sq ft. The first flight of the Midge was made by "Ted" Tennant from , Boscombe Down on August 11, 1954. It first had outboard ailerons and manual controls with a trimming tailplane, but some flying was later done with the Hobson screwjack moving the tail- plane as an "all-flying" surface in direct response to mechanical actuation of the elevator. In all, the Midge flew for 109 hr before it was destroyed at Chilbolton in a take-off accident while being flown by a Swiss pilot on September 26,1955. Altogether 21 pilots flew it. At about this time, Follands were again investigating the six per cent wing. An eight per cent surface had been adopted for the Gnat because it was then easier to build and gave better low-speed characteristics. Aeroelastic properties of the thinner wing were such that an inboard aileron offered much greater effectiveness in the high-speed, low-level case and, since Follands were interested in the NATO ground- attack fighter specification (which they had, in fact, initiated), they decided to standardize this arrangement. An eight per cent wing with inboard ailerons was accordingly made and flown on the Midge in February 1955. On July 18, 1955, S/L. Tennant first flew the Gnat prototype, G-39-2, from Chilbolton. This began the flight development of the Orpheus which was then rated at 4,000 1b thrust; and it behaved extremely well. Evaluation of the Midge showed that the angle of attack during the approach was rather high, impairing the pilot's vision. It was therefore decided to droop the ailerons some 20 deg by a direct mechanical linkage with the main undercarriage legs. The geo- metry of this system proved quite a design headache, but the result is that, not only do the ailerons droop only when the under- carriage comes fully down (virtually not at all in the intermediate airbrake setting), but the available aileron travel is considerably increased in the drooped position. The ailerons were by now fully powered, and the drooping linkage operated the jack valves. Though not specifically planned, the Gnat turned out to be J and 2, projects of early 1951, with two short-life engines and rocket armament substantially area-ruled for transonic flight and the drag rise associated with this regime occurred very late. Even with later versions, the area rule as such is not applied, but the cross- sectional area of the wing constitutes so small a proportion of the total cross-section that the rule is substantially fulfilled. Further steps could be taken but are not thought imperative. Follands consider that these might entail reducing fuselage volume around the wing trailing edge rather than adding bulges elsewhere. The latter course is felt to offer little advantage; and this was indeed noted with the experimentally-bulged Hunter. With the present Orpheus of 4,850 lb thrust, the Gnat has a very high thrust/weight ratio which gives it good load-carrying capabilities and intercep- tion performance. As for later. Gnats, referred to as the Mk 2 and Mk 4, the new six per cent wing, a section of which has been shown at Farn- borough, will allow high supersonic speeds in level flight; and some form of afterburning has also been specified. Follands are now developing the wing along current lines with extensive high- lift devices on both leading and trailing edges. Flap blowing will be included if necessary and aileron position may be changed for this reason. These developments are aimed at increasing the available lift for the purpose of operating from both small land bases and carriers. The aerofoil section of wing and tail surfaces of the Gnat Mk 1 is the R.A.E. 102 which was chosen as offering a good compromise between high lift and high Mach number capabilities without the need for special high-lift devices. Another Gnat development is the tandem-two-seat Trainer which has been chosen by the R.A.F. Because of its specialized application, considerable changes have been introduced. It is a rather more sophisticated aircraft, designed for a very long service life and is equipped, for all-weather flying, with the new advanced blind-flying instrumentation, engine de-icing and other features. The fuselage is lengthened by nine inches and, in order to make allowance for student pilots and to facilitate operation from small airfields, the wing has been considerably altered. Available lift has been increased by extending the area and by providing split flaps in conjunction with outboard ailerons. The split flaps give at least as great an increase in lift as plain flaps, but also provide drag, which is most useful in giving the pilot better control of the rate of descent on the approach. The thickness/chord ratio has also been reduced to seven per cent in order to maintain the high-speed performance of the Gnat fighter. Internal fuel load has been increased to 260 gal by fitting tanks in the wing torsion box out to half of each semi-span. The controls remain the same as those of the Gnat, but the area of the tail surfaces has been increased and particular attention has been given to providing docile stalling and spinning characteristics. In this respect the low tailplane of the Gnat has already given excellent results. The Orpheus engine for the trainer will be slightly de-rated to give 4,280 lb thrust, and will have an acceleration control. Liquid oxygen will also replace the present gaseous system. Folland have developed their own ejection seat, based on a Saab A striking view of the Gnat which has been flown with undenting stores. The load is two 66-gal tanks and 12 three-inch rockets.
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