The outstanding records attained by the Boeing Flying Fortress in sustaining terrific beatings over enemy territory is attributed to the plane's all-metal, semi-monocoque structure, covered with a smooth aluminum alloy stressed skin, a feature which proves that damage to part of the plane does not necessarily cause failure of the entire structure.
Framework of the B-17 fuselage is made up of bulkheads and circumferential stiffeners, tied together by longerons and longitudinal stiffeners. Over this framework and permanently fastened to it with aluminum alloy rivets, is an aluminum alloy, 24ST clad skin. Thickness of this skin varies from .025 gauge to.051 gauge, depending upon the load it is required to carry.
The fuselage is divided into several compartments to house the crew, the bombs, armament and special equipment. These compartments include: the bombardier-navigator compartment in the nose of the plane; the cockpit which is back and above the nose compartments; the lower compartment, under the cockpit floor, which is reached through trapdoors; the bomb bay occupying the space between the bulkheads to which the front and rear wing spar connections are made; the radio compartment immediately behind the bomb bay; the waist gun compartment which extends aft to the tail turret; and the tail turret itself which is assembled as a separate unit of the fuselage.
The nose compartment, besides the metal section consists of a molded transparent plastic shell, and a flat plate glass bomb sight panel which provides wide vision for both the bombardier and the navigator. This Plexiglas nose section is bolted to the fuselage. Entrance into the nose compartment is provided by a door in the bulkhead which divides the nose compartment from the rest of the fuselage.
The pilot's cockpit and the top gun turret are located in an elevated compartment in the forepart of the fuselage. This compartment contains the pilot, co-pilot and top gunner positions, all flight controls and instruments, some radio remote control equipment and a portion of the oxygen supply. Entrance is made from either the door in the bulkhead which divides it form the bomb bay, or from the lower accessory compartment through a folding hatch in the floor between the pilots' stations.
The front section of the fuselage structure has three tail section extruded 24ST longerons, one at the bottom and two near the top to carry the primary load and to reinforce the pilot's compartment which is a cutout in the top of the basic monocoque structure. All three of these rail sections taper from the wing joining area, toward the front, reducing the "I" sections to "T" sections.
Supports of the cockpit floor are large, built-up beams which slope aft and are anchored to the bulkhead below the cockpit. Actual floors of the cockpit, as all other floors in the airplane, are not structural members, but are built of weight-saving plywood.
Windows of the pilot's compartment are flat glass panels. The windshield or front panels of ¼" thick transparent plastic are fixed and leakproof, while the side panels of the sliding type are shatterproof 3/16"-thick Plexiglas.
The lower accessory compartment under the pilots' cockpit contains much of the accessory equipment of the airplane, the vertical flight gyro and aileron servo unit, oxygen bottles, control cables and brackets, emergency bomb door release "dog-leg" mechanism, the generator voltage regulators and the external battery receptacle. This compartment also provides the means for personnel transition between the nose compartment and the remainder of the airplane. Exit form the airplane may be made at the forward end of the compartment through a hatch equipped with releasable hinge pins which permit quick and complete removal of the door in an emergency.
The bomb bay immediately aft of the pilots' compartment is the most highly reinforced part of the entire fuselage structure, since through the bulkheads which separate it form the front and rear sections of the fuselage, the wing loads are transferred. Reinforcement is also provided to support the bomb loads and body bending loads.
Bulkheads at each end of the bomb bay carry the upper and lower wing spar chord loads across the fuselage section. This crosswise strength is carried by four square steel tubes, two in each bulkhead, heat-treated to 180,000 psi. Steel tubes are connected trusswise by square tubing members of 24ST. These crosswise trusses have connected to them the anchor point of the wing terminals.
Another structural member, the lower rail section that runs through the front section of the fuselage, splices into the lower chord of the bomb bay beam on top of which is a catwalk. This truss which lies along the center line of the airplane between the doors in the bulkheads consists of upper and lower chords and diagonals, built up of square 24ST tubing, on top of which is the catwalk, constructed of extruded "T's" and a web, furnishing support for the center bomb racks as well as providing body bending continuity through the bomb bay. Outboard bomb rails are supported at the bottom by compression trusses on each side of the fuselage, and are attached to circumferential stiffeners at the top.
The bottom surface of the bomb bay between the spar bulkheads and between the side body compression trusses is composed of two doors. Doors are hinged at the sides and open simultaneously for bombing operations and for use as an emergency exit. Equipment installed permanently in the bomb bay includes the bomb rails, which reinforce the compression and bomb trusses, the fuel transfer pump, two bomb rack selector switches and the bomb door retracting mechanism.
Just aft of the bomb bay is the radio compartment with facilities for three crew members. A door in the forward end of this compartment provides access into the bomb bay, and a door in the rear bulkhead provides entry into the aft section of the fuselage. The camera pit, under the radio compartment floor, is covered by a hatch when not in use. Additional space under th floor is used for accessories and oxygen equipment. A top observation station, covered by a removable frame with transparent plastic panels, is incorporated in the rear streamlined portion of the enclosure. A life raft compartment is provided on each side of the enclosure, immediately forward of the removable window. Release handles for the inflatable life rafts are accessible from the radio compartment.
The fuselage load is carried in this portion of the airplane primarily by four rail section longerons which are toward the rear of the structure. The lower two longerons provide additional reinforcing for the ball turret cutout and give the fuselage the required strength. The upper two longerons provide reinforcing for the radio compartment gun hatch.
The rear fuselage section, or waist gun compartment, is connected with the radio compartment through a door in the bulkhead which divides them. This section extends aft to the tail turret and contains the lower ball turret, the main entrance door, space for some stowed cargo, the tail gear mechanism, two single .50-cal machine gun installations at the side windows, and the elevator and rudder servo units. The lower ball turret is suspended from a bracket at the top of two circumferential stiffeners at the forward end of the compartment, and revolves within a ring gear mounted on the floor. The main entrance door is located in the right rear side. The tail wheel housing and retracting motor are located aft of the main entrance door. A large window on each side of the fuselage, forward of the main entrance door, provides for mounting the side gun installations. These windows, as all cut-outs in the fuselage structure, are reinforced in such a way that loads are transmitted around them.
The rear portion of the fuselage does not have the heavy rail section longerons, but depends for its strength upon lighter extruded bulb angles and heavier circumferentials. These circumferentials are of "Z" type formed from sheet 24ST and vary in spacing from 10" to 20", being heavier where the spacing is wider. The bulkheads in this section which transfer empennage loads to the fuselage are considerably more than unusually wide circumferentials.
The tail gunner's compartment, forming the aft end of the fuselage, is reached through the rear fuselage compartment. An emergency escape hatch equipped with releasable hinge pins is provided for the tail gunner on the right side of the airplane, just below the elevator.
This section of the fuselage relies for structural strength primarily on the skin and circumferential stiffeners, having no longitudinal stiffeners at all.
The all-metal wing, involving a new concept of airfoil construction has made possible the great bomb loads that the modern bombers are able to carry. The Flying Fortress' wing with its loading of 45.6 psf, span of 103' 9.38", area of 1426 sq ft, root chord of 228" and tip chord of 94.5" is far removed from the old wire and fabric wings of the first bombers. The wing is a combination of NACA 0018 airfoil section at the root and NACA 0010 at the tip. Taper ratio is 2.4:1, angle of incidence 3½°, dihedral 4½°, and the sweepback of the leading edge is 8¾°. Wing chord angle to the ground line when the plane is in a taxi position is 10¾°.
Two truss-type spars, running from the wing root to the tip connection, provide the skeleton of the wing. Consisting of 24ST tubular spar chords connected by web members of tubing joined by gussets, these truss-type spars and their connecting compression and former ribs with the wing covering make up the center of the wing. This section carries practically all of the major wing loads. Sections fore of the front spar and aft of the rear spar carry only local air loads and attachment support loads.
In the spars themselves the inboard sections of the chord tubes have an inside taper in wall thickness of from .54" at the inboard ends to .13" at their outboard ends. The outboard sections are of several gauges of constant gauge square tubing. The front spar, located at 15% of the chord, joins the fuselage at an angle approximately 6° off of 90° from the plane's center line, while the rear spar joins at 90. The two spars are connected by means of truss type inter-spar ribs, spaced 15" to 18" apart, constructed of 24ST channel chords, and tubes acting as web members joined together and then joined to the spars themselves by means of riveted gussets. Heavier compression struts are provided at the wing root terminal attachment points, the landing gear mounting points, and the nacelle attachment points to carry the greater loads imposed on these locations. Slightly heavier ribs, used to distribute torsion loads, are also located at intervals among the lighter inter-spar ribs.
Over this basic truss structure is a layer of 24ST clad or 24SRT clad corrugated sheet which ranges in thickness from .064 gauge inboard to .016 gauge outboard, in turn covered with 24ST clad skin varying in gauge from .016 to .040. Attached to the structure with skin-type aluminum alloy rivets ranging in diameter from 3/32" to ¼", this corrugation, with the stressed skin, carries two-thirds of the wing loads and is laid with the corrugations running spanwise of the wing. The riveting of the nose skin to the leading edge structure is accomplished with flush type rivets for aerodynamic cleanliness.
Spanwise the wing is constructed in three sections: the inboard panel, the outboard panel and tip. Each of these sections is assembled as a separate unit.
The inboard wing action supports the engine nacelles which are connected to it by means of bulkheads attached to the front spar and fairing angles around the wing-nacelle intersection. Nacelles have a semi-monocoque construction similar to the fuselage. A firewall and the spar bulkhead, combined with longitudinal stiffeners and longerons, aid the skin in transferring the torque and thrust loads of the engine into the wing.
Outboard nacelles employ in their construction four extruded longerons and several extruded stiffeners, spaced circumferentially. Inboard nacelles employ two extruded longerons in the upper part and two formed sheet longerons in the lower part of its construction, as well as several extruded longitudinal stiffeners. Skin is applied over and riveted to this framework. Nacelle fairings aft of the front spar are constructed of skin material reinforced by longitudinal and transverse stiffeners, which transfer the loads carried by the nacelle members into the wing surfaces and provide smooth transition of the nacelle contours into those of the wing.
Inboard nacelles, slightly larger than the outboard nacelles to provide housing for the main wheels, are specially reinforced. This reinforcement consists of two fore-and-aft large heavy formed channels that tie into heavy steel landing gear support forgings which are in turn securely attached to the wing surface and to the compression ribs.
Contained in the inboard wing panels are the fuel tank trusses and the wing tank bays, and the bottom wing openings, which are covered with structural access doors. Also included in the wing are numerous electrical connections and mechanical devices, such as the flap retracing mechanism, for which several small access doors have been provided.
Landing flaps, an innovation that came with all-metal wing construction, are attached to and extend the full length of the inboard section of the Fortress wing. Flaps are of the slotted, split type having a span per side of 24' 4-15/16", a chord of 34-13/32", and a total area of 139.1 sq ft.
The flap is constructed of a series of hydropressed nose and tail ribs attached to a 24ST round tubular spar, 2¾" in diameter, and covered with skin on the nose and lower surfaces.
Attachment of the flap to the wing is at five hinge points and consists of two connecting rods, hinged at both the flap and wing ends, which operate to extend the flap backward as well as downward. Maximum movement is 45° which increased the maximum lift coefficient of the entire airplane from 1.53 to 2.0 and increases the drag coefficient from .1095 to .1775 at 1.2 times the stalling speed. It also increases gliding angle from 5° 39' to 7° 20' at 1.2 times the stalling speed and decreases landing speed 9 mph.
Flaps are actuated by means of irreversible screw and nut units which operate through torque tubes, located adjacent to the rear wing spar. Powered by an electric motor, the flaps can be raised or lowered at 126 mph in 15 to 20 sec.
Outer wing panel includes the aileron and, on the left wing, the aileron trim tab. The aileron is constructed of a series of hydropressed nose and tail ribs attached to a channel spar with clips and rivets. The area forward of the spar is covered with sheet and 24ST clad.
Ailerons have an area to the hinge line of 60.2 sq ft and an angular movement both up and down of 12°. The hinge line is located at 21.4% of the chord at the outboard hinge and at 27.3% of the chord at the inboard hinge. The distance from the plane of symmetry to centroid of aileron area is 38' 10". Mass balancing of the aileron, concealed entirely within the wing structure, is accomplished by a single weight on an arm extending forward from the aileron spar.
The left wing trim tab, located at the inboard end of the aileron, has an area of 2.64 sq ft, and an angular movement both up and down of 15°.
Wing tip is composed of two spars, each having terminals which attach to the outboard ends of the outer wing panel spars. Nose, center, and tail ribs of the wing tip are fastened to the tip spars with riveted clips. Longitudinal stiffeners, having the shape of "T" sections, tie the ribs together and provide the contour for the skin. The tip edge contour is maintained by the use of a hydropressed channel section.
The empennage of the Flying Fortress, of semi-monocoque construction as is the rest of the airplane structure, consists of the horizontal stabilizers with their elevators and trim tabs and the vertical stabilizer, the dorsal fin, the rudder and its trim tab.
Horizontal tail surfaces have an area of 331.1 sq ft, a span of 43', a maximum chord of 125", and a distance from the normal CG to 1/3 maximum chord point of 451.9".
The stabilizer, alone, has an area of 250.6 sq ft and is set at 0° relative to the longitudinal axis. Structurally, the stabilizer is of two spar construction with web-type spars made up of 24ST web sheet and 24ST extruded spar chords, rolled 24ST span-wise stiffeners, reinforcing the inter-spar skin, and hydropressed 24ST ribs. These spars, as those in the wing structure, form the skeleton of the structure and carry a major portion of the bending loads. The front spar extends 162" from the airplane centerline, or to within 8' of the stabilizer tip. The longer rear spar extends to the tip connection. The leading edge and this longer rear spar carry the shear forces in the outboard structure. The clad covering is made by means of flush type rivers forward of the front spar and modified brazier head skin type rivets aft of the front spar. Beams are "I" type and ribs are hydropressed.
Elevators with an area of 80.5 sq ft, including the tabs and balance, consist of a structure similar to the aileron covered with doped fabric. Controlled by a short torque tube the angular movement of the elevator is 23° up and 14° down. The mass balance of the elevator is such that the chordwise elements are 100% statically balanced about the centerline of the hinge. The dynamic balance coefficient about the body centerline is approximately zero, while the chord aerodynamic balance is 27%.
A trim tab with an area of 5.0 sq ft and an angular movement up and down of 11° is installed on each elevator. Located on the fuselage end of the elevator, the trim tabs are attached by means of piano hinges. These trim tabs, having an irreversible control in lieu of mass balance, are of aluminum alloy construction.
The dorsal fin, while part of the vertical tail surfaces, acts to carry a portion of the fin load forward to the fuselage structure, and at the same time is valuable in providing directional stability for the airplane. Structurally the fin consists of hydropressed ribs and extruded stiffeners covered with 24ST clad skin.
The vertical stabilizer is similar in construction to the horizontal stabilizer. It is of two spar, web type, and formed of 24ST sheet and has 24ST extrusions for spar chords. Spanwise stiffeners are rolled 24ST, and ribs are hydropress stampings of 24ST sheet stock.
The uniformly distributed mass balance of the rudder is such that the chordwise elements are 100% statically balanced about the hinge centerline. The dynamic balance coefficient about the body centerline is approximately zero, and chord aerodynamic balance is 25%. The trim tab with an area of 3.4 sq ft is constructed of aluminum alloy and is installed at the lower trailing edge of the rudder.
The Flying Fortress landing gear is a conventional tail wheel type. The main gear which retracts partially into the inboard nacelles is oleo-pneumatic. The oleo stroke is 9-19/32" and the wheel is 56" in diameter, using a sixteen-ply tire 56:19-23. The wheels are equipped with 20 x 2¾" dual duplex expander tube brakes.
The assemblies of the main landing gear are of the cantilever type, each consisting of an air-oil shock strut assembly, torsion link, drag and retracting struts and a wheel and brake. The oleo takes up or reduces the shock loading occurring at contact of the wheels with the ground. The strut takes any side load and most of the vertical load of the landing gear and the drag strut takes a portion of the load from the wheel. Torsion links are provided to prevent rotation of the wheel around the main strut.
Electrical retraction of the gear, controlled by a single switch in the cockpit, is accomplished simultaneously for both main landing gear assemblies and for the tail wheel gear. Emergency manual operation is provided for each wheel separately. The hand crank torque connections for the main gear are on each side of the forward wall in the bomb bay.
The tail wheel gear, equipped with a 26" smooth contour tire, retracts into the aft section of the waist gun compartment. It is of single shock absorber, cantilever type, and consists of a treadle and spindle assembly, anti-shimmy brake, shock absorber, and wheel. Provisions are made for full 360° swiveling, with cockpit control of the lock for taxiing, and electrical centering control for retracting. The starter crank is used with a connection provided aft of the retracting mechanisms for manual operation.
The airplane is powered by four 1200-hp Wright Cyclone engines, model R-1820—97. These engines are nine-cylinder radial air cooled type, with a 16:9 gear ratio from crankshaft to propeller shaft.
Engines are located in nacelles along the leading edges of the wings, and are mounted on engine mounts constructed of X4130 steel with X4130 steel forgings at the four firewall connections. Mounts, interchangeable between nacelles, are constructed of steel tubing welded together to make an integral frame for the purpose of securely attaching the power plant to the nacelle structure. Between the engine and the engine mount ring are interposed shock absorbers whose function is to isolate engine vibration from the structure.
Each engine is equipped with a Bendix Stromberg injection carburetor, model PD 12H2. This carburetor differs from other types in that it does not have a vented float chamber, but instead has a closed fuel system from fuel pump to discharge nozzle. The fuel is delivered by the engine-driven fuel pump at about 17 lb pressure int the regulator and control units. There it is metered according to the mass air flow rate, and automatic mixture control setting. Fuel is then forced into a nozzle bar which sprays the charge evenly across the engine-driven supercharger section entrance. Fuel is prevented from leaking into the engine by the spring controlled discharge nozzle which is closed when the nozzle fuel pressure is less than 4 psi.
Ignition is supplied by two American Bosch SF-9LC3 magnetos, attached to the accessory section rear cover. Magnetos are four-pole, flange mounted, polar inductor type. Breaker point cams turn at one half crankshaft speed. Both magnetos are timed to No. 1 cylinder. The lobe for firing the N0. 1 cylinder is identified by a red dot. The right magneto fires the front spark plugs and the left magneto fires the rear spark plugs.
Each engine is supplied with air under pressure by a turbosupercharger, General Electric Type B-22. Engine exhaust gas passes through the collector ring and tailstack to the nozzle box, expands to the atmosphere through the turbine nozzle and drives the bucket wheel at high speed, or passes through the waste gate. The bucket wheel and compressor impeller are mounted on one shaft. A ramming air inlet duct from the leading edge of the wing supplies air to the impeller, which increases its pressure and temperature. However, in order to avoid detonation, the air supplied to the carburetor passes through the intercooler where the temperature is reduced. The engine impeller, driven by the engine crankshaft, again increases the air pressure as it enters the intake manifold. Higher intake manifold pressure results in greater engine power output.
Lubrication system of the engine is of the full pressure, dry sump type, in which all moving parts are under oil pressure except the cylinder walls, piston pins, crankshaft roller, and ball bearings, which are lubricated by splash.
Propellers are Hamilton Standard, Hydromatic controlled, full feathering of 11' 7" diameter. Their minimum clearances are: to ground, level landing, inboard 17-9/32", outboard 30-1/8"; to fuselage, inboard, 9-3/8"; to engine cowl, fully feathered, about 11/32"; to leading edge of wing, 70"; and inboard to outboard, 25-7/16". Each propeller has its own feathering motor pump unit and draws oil for the feathering pump form a standby reserve in the engine oil tank.
The fuel system of the Flying Fortress consists of four independent fuel supplies of approximately equal capacities, each feeding one engine. Six self-sealing fuel tanks, located in the wing, three in the left inboard wing and three in the right inboard wing carry the main fuel supply. Further provisions have been made to increase the fuel supply by auxiliary installations of fuel tanks. Nine smaller self-sealing feeder tanks known as "Tokyo Tanks" are located outboard of the outboard nacelles. Two droppable leak-proof tanks can be carried, on on each side of the bomb bay, in place of the corresponding bombs.
Fuel capacity of the main fuel supply tanks is as follows: the two main tanks, located between the nacelles, hold 425 gal each; behind them are installed two tanks with a capacity of 212 gal each; between the front and rear spars of the inboard wing panel are located two 213-gal tanks.
Fuel in any tank is available to any other tank in the airplane through a fuel transfer system consisting of two selector valves and an electrical transfer pump.
To combat vapor lock at high altitudes and to provide a certain supply of fuel for take-off, fuel booster pumps are installed in the outlets of four supply tanks. An electrically controlled fuel shutoff valve is installed in the line between each fuel boost pump and the engine to prevent fuel flow through a severed line. Fuel passes from the shutoff valve through a strainer mounted on the forward side of the firewall and enters the engine-driven fuel pump.
The main wing fuel tanks are rigidly suspended in the wings through a system of cradles and straps. The cradles are fabricated from aluminum alloy channels and sheet, and are riveted to the wing surfaces and spars. The straps are strips of metal reinforced at the ends and attached to eh cradles of wing structure through adjustable linkages to allow for slight variations of tank contour. The straps are separated from the tanks by neoprene pads to prevent damage to the rubber shell from the sharp metallic edges.
The auxiliary tanks merely lay n thin, non-metallic pads which, in turn, are supported by the wing surface.
The oil system has several functions: to lubricate the wearing surfaces of the engine; to aid as a coolant in transferring heat away from the engine; to supply oil pressure to the propeller so as to maintain propeller speed control. The propeller feathering pump also receives its oil from the engine oil supply tank. Each engine has its won independent oil system. Oil tanks are located in the nacelles. Oil cooler and oil temperature regulators are installed in the leading edge of the wings. The oil pump, Cuno filter, and scavenger pump are incorporated in the engine. The prop feathering motors and pumps are mounted on the forward side of each nacelle fire-wall.
Oil flows from the tank by gravity and suction from the engine-driven oil pump, which then forces the oil under pressure through the filter to the various moving parts of the engine. The oil then drops down to the sump, where it passes through a screen and is picked up by the engine-driven scavenging pump, forced through the oil cooler and returned to the tank.
Production illustrations used in this article were prepared by the Production Illustration Unit, Tooling Division, Boeing Aircraft Company.
This design analysis article was originally published in the December, 1944, issue of Industrial Aviation magazine, vol 1, no 7, pp 17-24, 26-28, 30-31, 99.
The PDF of this article [ PDF, 24.8 MiB ] includes 5 photos, a three-view silhouette and 8 detail drawings and diagrams, and three data tables. Photos are not credited.