Rebuilding the Boeing Stratoliners

by Wellwood E Beall,
Vice-President, Engineering, Boeing Aircraft Co

Revealed here are the engineering details of revisions being made in these well-known airliners recently returned from war service — how they are getting Flying Fortress wings, engines, and many other key parts in readiness for new commercial service.

The Boeing Stratoliner, built in 1939, was the first four-engine transport with a supercharged cabin. Flown thousands of miles over transcontinental and transocean routes, it was the only long-range high altitude transport in operation.

When this country entered the war, an urgent need arose for airplanes of long range and proved reliability which the Air Transport Command could use to transport high-ranking Army officials and important cargo to battle zones. Stratoliners owned by Transcontinental & Western Air were among the types the Army was looking for and accordingly they were taken over for this important transport work.

The first Stratoliner flight made in Feb 1942 from Washington, DC, to Trinidad, Brazil, the African Gold Coast, Cairo, and return, initiated ATC's transocean service. In the next two years the five Stratoliners made 3,000 ocean crossings for a total of 7,500,000 miles, flown in 44,911 hr.

Upon the return of these planes for domestic service last year, it was found that they were in need of structural reconditioning. The hard military use to which the planes had been put left scars on parts of the structure. The body structure was in excellent condition although the repeated coats of camouflage paint were peeling badly. The wings, however, were considerably more battered, and examination revealed that a structural overhaul by disassembly and replacement of damaged parts would be necessary.

The basic structure of the Stratoliner was almost identical with that of the B-17B Flying Fortress constructed at about the same time, and it was decided that the wing of the latest model Fortress, the B-17G, could be adapted for use on the Stratoliner with a minimum of rework. In addition, there were other changes which could be made to advantage at the time of the wing change, and which would improve the flying characteristics of the airplane. These improvements would include complete new wings, horizontal tail surfaces, new engines and propellers, and landing gear.

Reconversion work on the first of the Stratoliners, incorporating, in addition to the major changes, many aerodynamic and mechanical features developed on the Flying Fortress, now has been completed and successful test flights made.

The new Stratoliner has the B-17G wing, slightly modified. This wing, full cantilever in construction with two main spars, hydropressed sheet leading edge ribs, and combination formed tubular interspar and trailing edge ribs, is similar in design and construction to the original Stratoliner wing. Whereas the initial Stratoliner wing employed 24SRT aluminum alloy for spars and compression ribs, in the new wing all structural tubing is 24ST aluminum alloy.

Attachment facilities and interchangeability features of the B-17 wings are the same as those on the initial Stratoliner. The wings were modified to the extent of having the original Stratoliner wing tips and outer wing leading edges.

Wing flaps of the revised Stratoliner are those designed for the B-17G wing. Electrically controlled and of split trailing edge type, they have a rearward as well as a downward movement. All operating mechanism and hinge supports for the flaps are contained within the wing contour.

The Frise-type counterbalanced ailerons, also B-17G parts, have the same physical characteristics as those of the original Stratoliner. The left aileron includes, at its inboard end, a metal trim tab operated by irreversible self-locking controls.

Fuselage of the Stratoliner is made up of the conventional circumferential formers and longitudinal stringers riveted to Alclad sheet to form the semimonocoque body structure. Except for changes required by the installation of the new tail landing gear and the horizontal stabilizer revision, the contour and structural design of the Stratoliner fuselage is identical to the original airplane.

Structural changes required for the installation of the new tail landing gear and for the attachment of the B-17G horizontal stabilizers were more extensive than for the attachment of the new wing. The B-17G stabilizers have considerably more area than those of the original Stratoliner and are fitted aft of the attaching point of the original stabilizers. These changes called or a revision of the body bulkhead to which the front spar of the stabilizer is attached, revision of the bulkhead which acts as an attaching point for the rear spar, and addition of a new bulkhead aft of this to support the rudder hinge support beam. Several circumferentials and longitudinal stiffeners in the body tail section were added to provide necessary strength.

A revision also was made in the body tail cone and fairing with the addition of 24 in to the monocoque structure. Skin patches around cutouts for the elevator and rudder torque tubes also were added.

The elevators, also of B-17 design, made necessary the change in the torque tube location through the body, since the larger stabilizer placed them farther aft.

Vertical tail surfaces of the revised airplane are those of the original airplane, with the exception of the rudder torque tube, which is assembled with collars at the body skin line.

Main landing gear has B-17G type oleos, braces, and retracting screws, axles from the original plane, and wheels, tires, tubes, and brakes similar to those used on the initial model. In order to accommodate the increased loads that would be imposed on the structure by the higher gross landing weight of the airplane — 54,000 lb as compared to 45,000 for the original Stratoliner — the tail gear thrust brace channel members, shear ties, and joints were strengthened.

Normal retraction of the main gear is electrical, with an emergency manual retraction system operable from the accessory compartment in the body. An emergency air brake system, including a pressure storage bottle and a metering valve located in the fuselage, is connected to shuttle valves mounted on the brake.

The tail landing gear treadle is similar to the assembly originally used, except that B-17G tires (26 in, smooth contour), tubes, wheels, knuckles, and axles are installed. The upper frame and retracting screw were strengthened, and the wheel well was increased in size to accommodate the new wheel.

The engine nacelle accessory cowling and engine mounts of the new Stratoliner are reworked from B-17G assemblies. All unused openings are closed, and the firewall is revised where necessary to accommodate differences in power plant equipment and to provide passage for additional controls. The engine cowls installed on this airplane are B-17G assemblies with hydraulically operated flaps, the latter controlled by "open-close" valves on a panel in the cockpit.

New Stratoliner engines — 9-cyl Wright Model GR-1820-G205A with rated takeoff power of 1,200 hp — are supported by B-17G Dynafocal shock mounts with accessory installations substantially the same as on the original Stratoliner. Installation of the new engines necessitated removal of all turbosupercharger installations, carburetor and intercooler ducts, air filters, and all pressure and intake ducts in the B-17G wing. One duct inboard of each inboard nacelle was left to provide air to the cabin. Carburetor air intakes and heaters, similar to those used on the original airplane, are installed.

The supercharging equipment was removed, and all plumbing and ducts as well as the controls in the wing were adjusted to conform with the engine installation. Fire protection is essentially the same as for the B-17G airplanes, except that the CO2, fire extinguishing system connects to carburetor air intake nozzle.

Propellers are three-blade Hamilton Standard hydromatic propellers of 11'-6" dia. They provide fast feathering — from full "low pitch" to the "feather" position in 20 sec or less. The propeller controls remain the same as those of the initial Stratoliner.

Oil tanks of B-17B design, with total capacity of 180 US gal, are installed in the nacelles of the new craft. The system is equipped with 11" dia B-17G-type oil cooler with integral surge valves.

An oil dilution system is provided with the oil solenoid valve located aft of the firewall and with the fuel takeoff between the booster pump and the engine pump. The rest of this system is the same as the B-17G.

Each wing of the new Stratoliner contains two engine fuel tanks and one feeder tank. These are of non-metallic design, and combined they have a minimum total capacity of 1,800 US gal.

Provisions are incorporated in each wing for a fuel-dump system serving the forward outboard (main) tank and the inboard tank. This system includes an hydraulically operated tail pipe actuator, controlled from the cockpit. The rearranged fuel supply system includes strainers, shut-off valve, two-speed booster pumps, engine pumps, flow meters, and liquidometer fuel gauges. No auxiliary fuel tanks are provided in the outboard wing panels. Fuel transfer between any tank in the airplane may be accomplished with the booster pumps and four fuel selector valves, two in each wing leading edge, which are connected by a cross line between wings.

Because of these changes, a number of new instrument installations were required. Electric tachometers and direct-reading manifold pressure gauges. were installed. Liquidometer fuel quantity indicators and fuel quantity transmitters replace Autosyn fuel level indicators. Autosyn type fuel flow meter gauges. are also installed, as well as B-17G oil temperature gauges.

Autosyn instruments originally operating from 60-cycle electrical energy are replaced by 400-cycle instruments, while other instruments remain the same as originally.

Flight controls of the new Stratoliner are substantially the same as those used in the original airplane, although the elevator system was reworked to bring the "no-load" friction down to current standards. This was done by increasing the diameter of the initial pulleys in the system and by removing the servo units. Rudder and tab controls are reinstalled in the body tail section to conform with the empennage revision. Engine and aileron controls are modified to accommodate the revised power plant installation and the B-17 wing.

The electrical system of the new Stratoliner is a 24VDC, single-wire grounded return type. The electrical power source consists of three 200A generators in parallel with adequate wiring. Generator relays, ammeters, voltmeter, and a selector switch are included in the circuit. An exterior power receptacle for ground operation is connected to the circuit through a relay. Two storage batteries of the type used in the B-17 airplane are connected to the main power bus through relays.

Power supply for the 12VDC radio equipment is derived from the general power bus, and it incorporates a voltage regulator of the carbon-pile type adjusted to the desired valve.

The starter system is identical to that of the original airplane, except that induction vibrators are used in place of the former booster coils.

Alternating current is supplied by two 400-cycle inverters operated alternately. These inverters supply AC power to the central buses, one of which is 115VAC and the other 26VAC. A voltmeter and switch are provided to indicate the voltage at both the 26V and the 115V taps.

Each engine is provided with a fuel flowmeter having a capacity of 100-1000 lb per hr. Fuel quantities passing to each engine are shown on the instrument panel by two dual indicators, operating on 26V 400-cycle AC.

The two-speed electrical fuel boost pumps installed on the main and inboard tanks are controlled by switches in the cockpit which operate from the 24VDC direct current power source.

Rewiring of the retracting motor circuits is carried out to conform with the revised landing gear operating system. Landing gear safety switches reinstalled so that retracting of either main wheel is prevented, unless both wheels are off the ground.

External lighting circuits are modified to accommodate sealed-beam landing lights, which are controlled through relays rather than by direct switching, and there is a separate courtesy light.

Leading edges of the principal flight surfaces on the new airplane are deiced in the same manner as on the original airplane, and the same equipment is employed, except that B-17G de-icer boots are used on the new surfaces. Propeller and carburetor anti-icing is provided as formerly, but tubing connections at the body-wing join are revised to suit the new wing installations. The tubing in the aft end of the fuselage is relocated to conform with the change in the tail surfaces.

With these revisions and improvements, we feel the new Boeing Stratoliners now rate with the best of the transport planes available today for commercial use. A good plane has been made even better.

Following initial test flights of the rebuilt Stratoliners, R C Loomis, TWA engineering supervisor, reported that "the stability of the Model 307B-1 is greatly improved over that of the Model 307B [the designation before re-conversion]. The static and dynamic stability is more positive throughout the CG range from 20% MAC to 26.5% MAC. This means that the pilot reaction will be more favorable, and the utility of the airplane will be greatly improved."

This article was originally published in the March, 1945, issue of Aviation magazine, vol 44, no 3, pp 148-151.
The original article includes 5 photos.
Photos are not credited, but are certainly from Boeing.