The Gnat Mk I is a single-seat, high-wing monoplane interceptor and ground-attack fighter powered by a single Orpheus turbo-jet engine housed in the centre of the fuselage.
The fuselage is constructed of light-alloy pressed frames, extruded stringers and light alloy skin plating and is made in two portions which bolt together at frame 22. The upper portion of the nose, forward of frame 5, accommodates the radio installation. The cockpit is located between frames 5 and 8 and is pressurised. A Folland ejection seat is fitted and provision is made for an ant-G suit.
The single-skin Perspex canopy is hinged at the rear, opening upward and aft.
The engine is located in the fuselage aft of frame 18 and is attached to the port and starboard sides of frame 22. A single-strut mounting a t the forward end of the engine allows for expansion. The air-intake ducts at each side of the fuselage commence at frame 9 and converge and mate with the engine intake at frame 18.
The main planes are swept back and are constructed of front and rear spars, ribs, stringers and light-alloy skin plating. They are joined to a centre section to form one complete unit and the unit is attached to the fuselage at frames 12 and 16, each of the four attachments being effected by a single bolt.
The components of the tail unit are swept back. The tail planes are of variable incidence and are power-operated. The elevators are normally locked to the tail planes and in the event of power failure are unlocked manually from the cockpit. The rudder is manually operated.
The inboard ailerons are power-operated, with automatic reversion to manual. They are drooped automatically to act as landing flaps when the alighting gear is lowered.
The alighting gear comprises one nose-wheel and two main-wheel units, each unit retracting aft into the fuselage. Fairings, rigidly attached to the shock-absorber struts complete the fuselage contour when the units are retracted. Dive braking is effected by the partial lowering of the alighting gear.
The hydraulic system derives its power from one engine-driven pump and operates the ailerons, tail planes and alighting gear.
One 24 volt 3 KW generator supplies power for the electrical services. Two 12 volt 25 amp. lightweight batteries are connected in series. One 24 volt 4 amp. battery is fitted as standby.
Fuel is carried in seven (6, pre-mod.420} flexible fuel tanks mounted in the fuselage aft of the cockpit. Two jettisonable rigid tanks are carried externally~ one under each main plane.
Two 30 mm Aden guns are installed in the air intake duct fairings, one on each side of the fuselage below the main planes. The ammunition boxes are below the guns. Various combinations of bombs and rocket projectiles may be carried externally under the main planes.
The radio installation includes VHF, IFF-9, radar ranging, telebriefing and radio compass
Other equipment fitted includes a brake parachute, an oxygen and a fire-warning system.