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Aviation History
1960
1960 - 0046.PDF
FLIGHT, 8 January 1960 Evolution of the N-156F: above are the N-1S6TX (March 1955), PD-2706 (November 1955), PD-2B12 (January 1956) and PD-2832 (March 1956) "Freedom Fighter" was planned as an Ml.5 machine, with a potential of greaterthan M2 when sufficient engine thrust could be provided. The changes necessary on the fighter revolved around threemain areas. The wing had an extension added at the inboard leading-edge root in order to decrease the local t/c ratio andincrease the mean sweepback; this also decreased the wing wave drag. The engine intake ducts required a new inlet designed formaximum speed, and the M2 airplane will have variable intake geometry provided by an internal movable ramp. Another ex-ternal change was the addition of leading-edge flaps, to increase the maximum lift coefficient to match the fighter's higher grossweight. Leading-edge flaps achieve this without the drag and pitch-up associated with other high-lift devices, and result invery small trim changes. In order to obtain the required short landing roll, a container was added above the rear fuselage tohouse a 15ft-diameter ring slot braking parachute. In summary, it can be said that the N-156F design proceededsmoothly along a definite line of continual improvement, without some of the extreme overall changes suffered by many otheraircraft during their gestation period. To a considerable extent this is due to the firm direction by Northrop's aerodynamic de-partment, under G. C. Grogan. Credit must also be paid to General Electric's Small Aircraft Engine Department, who sur-passed the performance promised many years previously. With the T-38 Talon on the initial production lines, theNorthrop management, under the direction of the late Whitley C. Collins, gave the engineering department the go-ahead on theN-156F on May 27, 1958, and set a date of May 31, 1959, for the roll-out. Project engineer Ray Gardner's team had many trialsand tribulations in engineering the necessary changes; but close co-operation with the shops resulted in the target dates beingmet, and the roll-out was attended by representatives from over 40 "Allied" nations, at Norair's plant in Hawthorne, Cal. (Thecompany had undergone a corporate reconstruction and became known as the Northrop Corporation, with the Norair divisionresponsible for manned aircraft and missile production). Prior to this, the aircraft became unique when the US Depart-ment of Defense allocated nearly $50m to Northrop and G.E. for engineering development of the airframe and engines. Norair's$32m covered three aircraft and a static-test airframe. This unprecedented sponsorship of the N-156F is reflected inthe first example's use of an Air Force serial number (59-4987) without any Air Force insignia and markings. Thomas V. Jones,president of Northrop Corporation, termed the aircraft a symbol of real partnership, and said that for the first time the Depart-ment of Defense had applied advanced technology directly to the military airpower problems existing abroad. The fighter made its initial flight on July 30 last, going super-sonic despite the low thrust available from the early non-after- burning YJ85-1 engines. Flight testing has proceeded smoothly,considerably assisted by experience with the first two YT-38 Talons. All flying has so far been at Edwards AFB, and through-out Norair's chief test pilot has been Lewis A. Nelson, whose views are given in the next eight paragraphs. From the outset, the N-156F was designed with the pilot inmind. Throughout the design and development phases considera- tion of the pilot's inputs resulted in many of the outstandingfeatures of the aircraft, such as the spacious cockpit with its out- standing visibility through the one-piece curved windshield. Themost desirable control travel and forces were determined on a full-size flight control stand. Among the facilities utilized in perfecting the flight behaviourwere numerous tunnels, the full-size control test stand with simu- lator, actual system component parts, and the variable-stabilityF-86 aircraft of NASA. This F-86 was particularly valuable when early tunnel testing indicated a possible lateral directional-con-trol deficiency with the existing vertical tail design. Actual flight experience with it confirmed these predictions, and led to thepresent vertical tail with increased area and reduced sweep. Flight testing has advanced very rapidly, as borne out by thefact that a "three-card" test (of flight-test requirements) was conducted on the initial flight, and that the aircraft was flown forpreliminary evaluation by a US Air Force project pilot on its third flight. These facts attest to the excellence of the engineeringeffort in the design stages and to the thoroughness of pre-fiight planning. They are all the more impressive when one considersthat all flying to date has been conducted with modified proto- type YJ85-1 "missile" engines. These were utilized in lieu ofproduction J85-5s in order to start flying at the earliest possible date. As they produce only about half the thrust of the reheatengines, all flying has, in effect, simulated the one-engine-out case. Notwithstanding this, 94987 has flown carrying some1,2001b of test instrumentation (quite often with one engine out). A prime reason for the rapid progress of the test programmehas been the lack of need for any major alteration to the airframe or its systems. Flutter proof-testing has advanced rapidly tosupersonic speeds and high "q" dynamic pressures, both with and without the wing-tip missiles. Proof-test maximum speed is thesame for both the clean and missile-installed configurations, and has only been limited by the low thrust so far available. Aircraft characteristics throughout the flight envelope are "asrequested" by the pilots during the design phase. From first climbing aboard the pilot feels at home, and needs only applylight pressure to obtain the desired response. The aircraft can be flown hands-off at any speed—supersonic, transonic and rightdown to the stall. With regard to the stall: with the present low thrust the aircraft can be held completely controlled at a pointwhere the thrust will not sustain it, and in order to break the stall all the pilot need do is lower the nose and fly out. Take-off and landing tests indicate conclusively that the pre-dicted results were conservative. The N-156F combines M2 performance (only potentially as yet) with the landing speed ofthe F-86, the effectiveness of the wheel brakes and drag contri- buting decisively to the short roll necessary in forward-areas.The drag chute's deployment rate and stability are outstanding. Safety and reliability have been key words in the design ofthe N-l 56F, culminating in a reduction in emergency systems as a result of the basic duplication of powerplants and accessorysystems (a manual backup release is provided for the landing gear as an alternate to the normal utility hydraulic system). No discussion of the testing of the N-156F would be complete *. .-*,: This drawing depicts the final N-156F design. The following data apply: engines, two GE J85-5; span. 25ft 3in; length, 45ft 1in; height, 13ft 1in; tailplane span, 15ft ••in; wheelbase, 11ft 2in; track, 10ft 9in; gross weight, 12,1901b (overload limit with external stores, 16,3001b); max speed, in excess of M2 (1,320 m.p.h.); range, more than 2,000 miles with external tanks; landing distance from 50ft, approxi- mately 3,000ft
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