This article appeared in "Aviation" Magazine in December, 1930. The article provides a good technnical description of the Northrop Alpha and it's construction. All performance numbers must of course be taken as manufacturer's marketing claims.
I have converted this material to text in the interest of conserving filespace and bandwidth. Two generic pictures of the airplane have been omitted.
INTRODUCTION of the Northrop "Alpha" seven place low-wing all-metal cabin monoplane has attracted attention in the aviation industry due to the unusual type of wing structure employing flat sheet dural throughout. Aerodynamically tthe plane is similar to a number of other low-wing cantilever monoplanes already on the market. Structurally it presents a radical and promising innovation.
In the wing the flat sheet covering not only carries the drag load, but also the major portion of tension and compression loads normally carried by spar cap strips. The structural form is multicellular, the cells being of rectangular type. While the top and bottom sides of these cells, being protions of the top and bottom sides of these cells, being portions of the top and bottom surfaces of the wing, are of continuous sheeting, the vertical members are all lightened to a truss form. The average spacing of vertical rib members throughout the wing is 24 in., and the average spacing of the vertical members, or dimensions of the individual cells, varies with the load imposed, as does the gage of the metal employed. Also the flat sheet covering is suitably reinforced at various points by angle or channel members riveted directly to the skin and parallel to the vertical spar members.
As a result of its structural efficiency which permits more nearly full employment of all of the material used in the wing structure than has been obtained in more connventional structures, it has been possible to obtain ample strength with unusually light weight for a cantilever wing. The wing panels weight approximately 1.8 lb per sq.ft. Furthermore, the contiuous flat sheet covering gives the wing extreme torsional regidity, since the metal covering is heavier than in other skin stressed wing structures of conventional type.
In manufacturing a structure of this sort, material savings are effected in a number of ways in fabricating the wing. There are many fewere individual pieces in a structure of the Northrop type than there are in the ordinary wing with spars, ribs, covering and bracing. Many of the units in thhe Northrop wing are identical and all are of similar form. Every portion of the structure is formed from one type of material, flat sheet dural, and all fabricating is by one process, riveting. Furthermore, all riveting is of the exposed type as the wing is built up, facilitating rapid and effieicnt assembly. Hand measuring, forming, and fitting is largely eliminated since the metal parts in the Northrop wing are largely formed by pressing or stamping from flat sheet, and much of the hand work involving the use of patterns, rib and spar jigs, etc is eliminated.
Each wing panel is assembled complete in a master jig. The fuselage is built in two halves, divided longitudinally, and is then assembled. Complete jig and die equipmment for all processes of manufacture of the "Alpha" model have been provided by the Northrop Aircraft Corp. in its Burbank factory so thhat very real savings in manufacture can be effected if these planes are produced in quantity.
The Northrop "Alpha" is a Wasp Series C powered light transport low wing cabin monoplane. With a span of 41 ft. 10 in., length of 28 ft. 4 -1/2 in., and a height of 9 ft., the Alpha has a weight empty of 2,500 lb. and gross weight, loaded, of 4,300 lb. Total wing area is 295 sq.ft., the chord varying from 100 in. at the root to 55 in. at the wing tip taper being present in both plan form and thickness. The dihedral angle of the wings is 3-1/2 deg. Airfoil section is the Clark-Y curve, somewhat modified by thickening up toward the root for structural purposes. The engine is mounted in an N.A.C.A. cowling and the entire fuselage, which is circular in cross section, presents an almost unbroken streamline from nose to tail. Six passengers are carried in the cabin. The pilot occupies an open cockpit in rear of and above the cabin, which may be entered either through the cabin, or from the outside. Finnish may be either natural duralumin, or any desired color combination by lacuring direct on the metal.
Ailerons employ the same structure as the wing, are of narrow chord, approximately 12 in and extend two thirds of the length of each wing panel from the detachable wing tips, which serve to shield each aileron. Attached along the upper surface by a continuous piano type hinge and with the lower slot shielded by a flat metal plate, the ailerons are differentially controlled, having a travel upward of 20 deg. and downward of 10 deg. Operation is by means of cables from the cockpit to a lever arm within the wing and through a push-pull tube to an internal horn on the aileron.
Wing, aileron, annd empennage structures are treated internally with Lionoil, and are either left untreated on the outside or painted with Duco.
Of circular cross section, the fuselage incorporates a carefully developed streamline form fore and aft. Constructuion is by means of a series of longitudinal gores of flat dural, one side of which is crimped over to provide longitudinal stiffness. These gores are riveted to each other and to round dural bulkheads of U channel section, thus forming a smooth monocoque body. The center section is built into the fuselage, with certain additional bracing, and some bracing is also provided around the cockpit, door, and window cut-outs. Aluminium alloy castings are used in all fittings where loads are uniform, while engine mount and landing gear fittings are built up sheet steel. Wherever steel fittings are used a through treatment of Lionoil is provided between the steel and duralumin to which it is attached.
The engine, is carried on a welded chrome molybdenum steel tube mounting with a four point attachment to a heavy master ring of cast aluminum which is riveted into the forward end of the fuselage structure. Live rubber bushings are provided at each of these four points of attachment.
A heavy duralumin fire wall, with demountable center panel for reaching engine accessories from within the cabin, is built into the forward end of the fuselage at the point of engine mount attachment. The cowling is built in three sections and is attached with quick acting clamps. There are no inner baffles although the inner cowling is of special form to provide good cooling. Under test is has been determined that engine head temperatures never normally exceed 400-440 deg F. The engine turns a Hamilton-Standard propeller of 9 ft dia. and 19 deg. pitch at the 42 in. station. Exhaust gasses are carried to a bayonet silencer and heater beneath the fuselage by means of a collector ring under the cowling. All oil and fuel tanks are of aluminum riveted and welded construction and are manufactured in thhe Northrop plant. Fuel is carried in three banks of two tanks each carrying a total of 116 gal., located in the right and left center section wing stubs and beneath the cabin.
Empennage members are of the same type of construction as the wing and are of cantilever type.
The landing gear incorportes ring spring and also shock absorbers on all three wheels. Tread is 8 ft. 6 in. Bendix wheels and brakes are standard equipment. Braking is provided by a hand lever working on both wheels equally, and by means of individual differential conntrols on the rudder pedals. The tail wheel is ten by three in., equipped with oleo shock strut and pneumatic tire, and swivels through 360 deg.
The pilot's seat is of seat pack parachute type, aluminum construction, and is adjustable ffor height. A dural instrument panel equipped with Pioneer instruments throughout, is carried in special spring and rubber mountings which render it unusually free from vibration.
Entrance to the main cabin is gained by means of a wide walkway on the right wing stub and through a door at the right rear of the cabin. The main cabin is 110 in. long, and 52 in. high by 55-1/2 in. wide, max inside diameter. Six passengers are comfortably seated, either facing each other, or all facing front. Glass windows, shaped to the curve of the fuselage, are set rigidly in the structure in rubber socketts, and ventilation is provided by special vents. The passenger cabin is heated by means of a conntrollable hot air supply from the exhaust stove. Standard upholstery is leather over the seats and mohair on the cabin sides and ceiling.
It is of interest to note that the weight of the bare fuselage structure is but 240 lb., and the weight of the entire airplane structure, without any accessories, is but 1,300 lb, or little more than half of the weight empty when all installations have been completed.
The Northrop Aircraft Corp. is a division of United Aircraft and Transport Corp., with the vactory and experimental laboratory located on the United Airport, Burbank, Calif. Officers are: W. K. Jay, president and treasurer; John K. Northrop, vice president; Tom C. McNamee, secretary; and Don R. Berlin, chief engineer.
Specifications and performance figures as supplied by the manufacturer. follow:
Length overall.......................28 ft. 4-1/2 in. Height overall.......................9 ft. Wing span............................41 ft. 10 in. Total wing area......................295 sq.ft. Chord-at root........................100 in. Chord-at tip.........................66 in. Aileron area.........................20 sq.ft. Stabilizer area......................27 sq.ft. (exposed) Elevator area........................14.7 sq.ft. Fin area.............................8.4 sq.ft. Rudder area..........................7.6 sq.ft. Dihedral.............................3-1/2 deg. effective Weight empty.........................2,500 lb. Disposable load......................1,800 lb. Gross weight loaded..................4,300 lb. High speed...........................170 m.p.h. Landing speed........................55-60 m.p.h. Service ceiling......................19,300 ft. Range at cruising speed..............700 mi.