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Aviation History
1960
1960 - 0047.PDF
FLIGHT, 8 January 1960 On this page are the PD-2852 droop-nose (March 7956;, and the PD-2879A, 28798 and 2879D, the last-named being dated December 1956 without mention of the effectiveness of the USAF category con-cept of testing. This is worthy of an article on its own, but, briefly, it is a joint effort between Northrop, as the system con-tractor, and the Air Force, acting for the Department of Defense. Structure The structure of the N-156F has been broken down into smallunits to facilitate a high degree of flexibility in manufacture and ease of replacement in case of damage. It has also been so de-signed that spares for both the fighter and trainer can be built on a common line, some 80 per cent of the components aft of theforward fuselage being interchangeable. Dealing with the wing first, this is a single unit from tip totip, of multi-spar construction in aluminium alloy. The main torque box is supplemented by a secondary torque box, whoserear wall is formed by the rear spar which supports the single slotted flaps and the inboard ailerons. The ailerons are of the sealed-gap type, having a piano-wiretype hinge on the top surface. They are fully powered without trim tabs, and when the landing gear is retracted have a move-ment of 18^° up and 14° down. Each leading-edge flap is of aluminium-alloy honey-comb construction, with a continuoushinge and a travel of 23° down. The wing tips, also honey- comb-filled, are readily detachable, being either plain or incor-porating the missile pylon. Provision has been made for pylon attachments at wing stations 83 and 111 (see diagram on page 48). After the wing structure has been assembled, complete withthe necessary wiring and piping, the main landing gear with strut doors is attached, rigged and checked out for functioning. Thewing is then ready for assembling to the fuselage. This ensures that the wing is manufactured as a complete entity and allowsof dispersed manufacture—so that, for example, wings made in one country could be fitted to fuselages manufactured in another.The checking out of the gear retraction cycle at this stage also simplifies trouble-shooting, to the extent that, if the wingfunctions correctly detached and docs not when assembled to the fuselage, then the fault can be sought in the latter. The wing/fuselage joint involves only one bolt from each of the three spars to the bottom fuselage longeron on either side, all fore and aft. The forward fuselage contains the fire-control and electronicequipment, nose gear, communications and navigation equip- ment, and the cockpit, ejection seat and controls. The fire-controlspace has a volume of some 40 cu ft and has been designed to accommodate a variety of systems to suit customers' require-ments. The upper longerons serve as the cockpit rails and also support the canopy, which is of singular interest. The forwardwindshield is a one-piece curved panel providing an unobscured view forward; it is hinged at its forward end to provide easy accessto the rear of the instrument panel. The one-piece canopy is novel in being raised and lowered manually by a lever on theright-hand inside wall of the cockpit, thus saving weight and maintenance time over the hydraulic system normally employed.External jettisoning is provided on both sides of the aircraft. The mid-section of the fuselage is the largest single sub-assembly, and contains the wing attachments, fuel tanks, air ducts, forward compartments and vertical fin. The basic con-struction consists of four longerons, bulkheads and a floor for the tank compartment. The unusual construction of the vertical finintegral with the main fuselage was chosen so that maintenance on the engines could be performed by removing only the easily-handled aft-fuselage and tailplane. Forward, the aft fuselage is of stiff construction to take theloads from the tailplane through the canted fuselage joint; aft, it is composed of very light titanium, frames covered externally withmagnesium skin. Drag-parachute loads are taken forward and down to the main structure, the compartment being of magnesium. The empennage of the N-156F consists of the fin and rudderand the all-movable tailplane. The latter has a single spar attached to a steel torque tube, and the entire chord serves asthe torque box, being of honeycomb construction. Its limits of travel are 17° nose down and 8° nose up. The vertical fin uses a single canted spar, attached to the main engine bulkhead, andhas integrally stiffened skins. The rudder is hinged to the fin's shear web, and is of honeycomb construction. With landing geardown it has a maximum movement of ±30°, but to avoid large fin loads during yaw normal travel is reduced to +6°. Conventional air/oil shock-struts are used in the landing gear.The nose unit retracts forward and the wide-track main gear inward, the wheels being housed above die flat undersurface ofthe fuselage. The main-gear retraction system deserves special mention owing to its clever utilization of the basic wing geometry.Designed and developed by the Cleveland Pneumatic Tool Co, the entire system including the main leg folds up into a depthmarginally over 3.25in. This was made possible by retracting the main leg sideways at 90° to the fore-and-aft axis, to lie be-hind the rear spar which is inclined forward at its inboard end. the side brace, of the on-centre link type, is attached at its lowerend to the forward side of the main leg by a universal coupling. Its upper end is held by a rotating pivot on the rear spar, runningoutboard from which is a U-beam rotating in lugs on the spar web. A hydraulic actuator inside (and rotating with) the U-beam,breaks the side brace upwards via another on-centre linkage. In so doing, the side brace is rotated upwards and folds back onitself to lie between the main leg and the spar. Originally of built-up steel construction, the one-piece outercylinder and drag-brace forging is now of high-strength alum- inium. Each shock strut carries a wheel with inboard brake and20x6.0 tubeless tyre. The independent braking system has its own reservoir, and is of the single manual pressure-generatingtype (rather than using the now usual power assistance). The nose gear uses an 18X4.4 tubeless tyre, and has a com-bined shimmy damper and steering unit, which traverses through ±50". Both the nose and main units have a manual releasesystem, which allows them to fall free and lock down following failure of the utility hydraulic system.The 15ft Irving drag parachute is stowed in a compartment so positioned that die deployment bag does not hit the ground—thus shortening its effective life. When deployed, the chute's drag passes through the mean e.g., thus avoiding undesirable pitchingmoments. The deployment and jettison system is manually operated, and does not rely on springs. It has been designed tolast the expected life span of die aircraft, widiout attention. Powerplant The heart of any aircraft is its powcrplant; and this is especiallytrue of the N-I56F, designed around the General Electric J85-5 (Flight, December 11). Development of tiiis type of engine wassparked by Rolls-Royce's development of the Soar turbojet, widi an unprecedented thrust/weight ratio. General Electric begandeveloping the J85 for missile applications in 1955, and when Northrop's interest became apparent it was modified for aircraftuse, producing 2,5001b dry and 3,8501b with afterburner. The latest announced development is the addition of water injection Every effort has been made to provide optimum access to equipment
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