Into the Sub-stratosphere

by R J Minshall
Vice-President — Engineering
Boeing Aircraft Company
A detailed description of the Boeing Model 307, Stratoliner the first pressurized cabin ship to be delivered to the airlines.

To say that the aviation industry is making a marked advance this year with the introduction of the four-engine Stratoliner type transport is not the whole story. It should also be stated in another way: The aviation industry has advanced to the Stratoliner. The whole background of knowledge and progress through the years — not only on the part of the airplane manufacturers, but also the equipment and materials manufacturers, the air lines and allied industries — has made possible this new type transport. From results obtained, we sincerely believe that the Stratoliners and subsequent airplanes of the same general type will have a lasting place in the future of air transportation. The principles involved are sound.

When the idea of such a transport was conceived several years ago, it was based on several factors. It seemed apparent that the air transport industry would soon be ready for larger planes than the types then in operation. It seemed clear also that the larger planes would have certain advantages in addition to greater carrying capacity. The larger ship, by virtue of its size, would be more comfortable and more attractive to passengers because of additional conveniences offered. Along with this came the consideration of four engines. The Boeing Company was convinced of the desirability of four engines for such an airplane because of the outstanding success of its four-engine Army Flying Fortresses, and because of the demonstrated advantages of four-engine Flying Boats in transoceanic operation. The ability of an airplane to continue flight if one or even two of its power plants should fail appeared definitely attractive.

Then came the consideration of cabin supercharging — something entirely new. Sooner or later air transportation ts going to get away from the conventional altitude limitations, and get away from the discomforts which admittedly were present when, for one reason or another, it was found desirable to climb above a certain conventional level Should the projected Model 307 four-engine transport be the first to attack this problem? There was considerable discussion of the point. There was no doubt that it would be possible and practicable, but there would also be a good deal of expense involved, because it was a new field. The advantages of the supercharged cabin were so attractive that it was decided at the outset to make this a feature of the 307, and thereafter the name Boeing Stratoliner was created.

It was decided to use only a moderate degree of supercharging, because this simplified the process and at the same time brought within comfortable reach altitudes as high as would normally be desired for the first few years at least — up to 20,000 feet. In the range between 14,000 and 20,000 feet operators could over-ride most surface weather conditions, would have a wide margin of altitude over high terrain, and at the same time would gain higher speeds, smoother air, and steadier winds. To gain this end required supercharging of approximately 26 pounds per square inch (differential between outside and inside atmospheric pressure). The structure was designed to withstand supercharging of 6 pounds per square inch.

In developing the supercharging system and its equipment, and, for that matter, the mechanics of the whole airplane, simplicity and reliability were made the guiding principles. Thus the supercharging and pressure- regulating apparatus was made entirely automatic, with a system of control valves, check valves, safety valves, etc, all operating automatically, purely by action of the pressures involved. This leaves the operator merely the task of turning the apparatus on or off, and watching the few simple instruments that record the operation. it is possible, however, to control the equipment manually so as to continue operation in case of any malfunctioning of parts of the system. As an additional precaution, the system has been installed completely in duplicate. Either one of the two sets of apparatus can do the entire supercharging work.

The principle of practical simplicity likewise was carried out in the design of the supercharged cabin itself. It was made completely circular in cross-section from nose to tail, so that all atmospheric pressure loads are evenly distributed. This design has the additional advantage of providing excellent streamlining, and also provides a maximum cabin volume which is highly desirable for passenger accommodations.

The fuselage is a semi-monocoque structure, consisting of 24ST Alclad covering stiffened by 24ST extruded bulb angle longitudinal stiffeners and 24ST formed "J" section circumferential stiffeners. The circular fuselage skin is uninterrupted at the juncture of the wings. Skin thickness varies from .O2O to .040, depending on location. Longitudinal stiffeners are spaced 9° apart around the periphery of the fuselage, and run continuous through the circumferential frames, which are notched. Circumferential stiffeners are spaced approximately 16" apart along the body centerline.

Loads from the wings, empennage and tail gear are distributed to the semi-monocoque body structure by trussed bulkheads. The main wing spar bulkheads are made up of hat-section channels and square-tube truss members. Part of the longitudinal stiffeners run continuous through the bulkhead hat-section channels, while others are bolted to angle fittings The passenger cabin floor is supported by a system of fore-and-aft and spanwise beams. The fore-and-aft beam are built-up "I" sections made from 24ST extruded tee and 24ST Alclad sheet stiffeners with 24ST extruded rail and bulb angle stiffeners. The spanwise beams are 24ST extruded "I" and "Z" sections and are rigidly attached to the circumferential stiffeners at the sides of the body and to the fore-and-aft beams.

Remarking the rearmost end of the supercharged cabin is a hemispherical pressure bulkhead, consisting of a dome-shaped web of 24ST Alclad sheet with radial "J" section stiffeners. A removable circular hatch is installed in one side of this bulkhead to provide access to the tail section of the body. This bulkhead also contains a pressure relief valve which is set to discharge any pressure in the cabin in excess of .65 psi. This safety valve has a capacity to handle the full overload output of both cabin superchargers, to prevent excessive pressures in case of the remote possibility that all other controls should fail. The normal function of the automatic controls, of course, is to prevent pressures in excess of 2½ lbs.

The maximum diameter of the fuselage is 11½'. The floor level is located approximately one-third of the diameter from the bottom of the fuselage at its largest section. Cargo compartments and accessory compartment below the floor (all of which are accessible from within the airplane as well as through outside hatches in the bottom) are maintained under full cabin pressure. Cargo compartments have a combined capacity of 412 cubic feet, or 6590 pounds. An auxiliary entrance to the control cabin is provided through the bottom of the body forward of the front cargo hold, by way of a trap door in the cabin floor, and operating personnel may thus enter the control cabin without passing through the main passenger cabin.

Skin seams in the fuselage are sealed effectively by means of a tape impregnated with sealing compound, inserted between the laps prior to riveting. The seams include two rows of rivets diagonally spaced approximately 5/8" apart. All doors and hatches are sealed against leakage simply by soft rubber gaskets which press together when under the influence of cabin pressure. There are seventeen main passenger compartment windows each measuring 16" high and 12" wide. Like the rest of the cabin wall, they are designed to withstand 6 psi internal pressure. The windows are made of Plexiglas or Lucite (transparent plastics), curved to conform with body contour, and are sealed in rubber channels. Control cabin windshield and sliding side windows are 5/8" safety glass. The sliding windows are mounted in steel frames which are seated against a rubber seal to prevent leakage of air or water. Fittings where control cables pass through the supercharge cabin shell are designed with a special gland, the result of a long process of experimentation, which allows the to slide freely with but a nominal amount of air leakage.

The Company was able to utilize features of their other large planes and, with this background of knowledge, add to them the particular requirements of the new transport. The Stratoliner's wings, for example, are substantially the same as the wings of the B-17 type Flying Fortress. They are all-metal, tapered wings with span of 107' 3, and a symmetrical airfoil section. The root section is NACA .0018 and tip section is NACA .0010.

Front and rear spar chords are 24ST square tubes with tapered wall section. Terminals are steel, heat treated to 150,000 psi. Diagonals and verticals are aluminum alloy square-, rectangular- and barrel-section tubes. Compression ribs and intermediate ribs consist of hat channel section chords, tubular diagonals and verticals, and sheet gussets of aluminum alloy. The covering is 24ST Alclad, over 24ST Alclad corrugations. Leading edges are flush-riveted.

All fuel tanks are carried in the wings, in padded cradles. Between the two nacelle locations of each wing is a 425-gallon main tank and a 212.5-gallon auxiliary tank, while a second 212.5-gallon auxiliary tank is located in each wing between the inboard nacelle and the body. The total tankage is thus 1700 gallons,

Engine nacelles are of semimonocoque construction, providing a roomy structure for mounting of equipment and access to this equipment. The nacelles have "J"-section frames and bulb angle stiffeners, with partial longerons to back up engine mount bolts. The engine mounts proper are of welded steel tubing. The landing gear, which is of the single-oleo type, retracts by electrically-driven screw operation into the inboard engine nacelles.

Two different types of wing flaps are used on the Stratoliners. The Model 307s of Pan American Airways have split trailing edge type flaps while the Model 307-Bs of Transcontinental & Western Air, Inc, have slotted trailing edge flaps. In each case, the flaps extend the complete distance from body to ailerons. Leading edge slots near the wing tips have been incorporated for maximum control at the stall.

Other differences between the Model 307 and 307-B, aside from interior furnishings, are concentrated mainly in the power plants. The 307 has Wright GR-1820-G102 Cyclones (1100 hp each for takeoff and 900 hp normal rating), while the 307-B has GR-1820-G105A Cyclones of the same take-off and normal rating but with two-speed supercharging for high-altitude performance. Propellers are the Hamilton Standard Hydromatic full-feathering type, three-bladed, with a diameter of 11½'. Cooling systems vary in the two models, the PAA version having fixed ring cowls while the TWA airplanes are equipped with cowl flaps. The cantilever tail surfaces of the Stratoliner include a single vertical fin which extends forward into a low vane along the back of the body. This forward vane provide exceptional stability for high angle of yaw. The fin and stabilizers are all-metal, and the elevators and rudder are of metal structure with fabric covering. Hydraulic boost control are used on rudder and elevators, providing ease of control under all conditions of operation. Both elevator and rudder are provided with powerful trim tabs, with which the airplane may be trimmed through a wide range of speeds, and with one or more engines inoperative. The elevators are provided with control tabs which make landings easily possible even without the use of the hydraulic boost control.

Supercharging Equipment

The Stratoliner's cabin supercharging, heating and ventilating system is sufficiently interesting to be described in considerable detail. The plane can be operated either with or without cabin supercharging at any altitude. A comparatively low altitudes, below 8,000 feet, there is little advantage to pressure-cabin operation. Moreover hot weather conditions frequently prevail at low altitudes so that a comparatively large flow of cool air into the cabin is desirable. For these reasons, a separate cold air system is installed in addition to the supercharging system. A blast of fresh air is rammed into a scoop at the top of the nose of the cabin, then passes through a centrifugal rain separator and then into a central duct in the cabin's ceiling where it is distributed through grills. An auxiliary ventilation outlet of large capacity in the bottom of the accessory compartment below deck permits the large quantity of cool ventilation air at low flight levels to sweep through the passenger compartment and to be subsequently discharged to the outside atmosphere. This is adjustable to suit weather conditions.

At 8,000 feet the overhead auxiliary air scoop and the auxiliary outlet are closed, and all ventilation is carried on through the supercharging system with automatic pressure control and automatic temperature regulation from 8,000 feet to 14,700 feet, there is no change in the atmospheric pressure within the cabin. The cabin is still at an "apparent altitude" of 8,000 feet when the 14,700 foot level is reached. In operation above 14,700 feet, a differential of 2½ psi pressure between outside and inside atmosphere is maintained so that the "apparent altitude" in the cabin gradually increases. At an actual altitude of 16,000 feet, the "apparent altitude" in the cabin is approximately 9,000 feet. At 18,000 the "apparent altitude" is about 10,600 feet; and at 20,000 feet, it is approximately 12,300 feet.

The routing of supercharged ventilating air through either one of the two duplicate supercharging systems may be described in general as follows: Fresh air is taken in through an opening in the lower leading edge of the wing between the inboard engine nacelle and the body. It then passes through a centrifugal water separator to the supercharger "blower" unit mounted behind the engine in the inboard nacelle and mechanically driven by the engine. In this "blower" the air is increased in density. It is subsequently warmed by steam radiators and then flows through a duct to the automatic flow control valve in the control unit located in the accessory compartment below deck, which maintains a constant weight flow of air through all ranges of operation. From this point it is routed to ventilation grills at the sides of the cabin at the floor level, and, if desired, to the individual adjustable ventilator outlets. The air, after circulation, escapes through mushroom-shaped outlets in the floor to the accessory compartment below deck where it passes through the automatic pressure control valve and escapes to the outside atmosphere. The pressure control outlet valve and the inflow control valve, mentioned above, are combined into a mechanical unit known as the cabin supercharging control unit. There are two of these units, each serving identical functions for the duplicate supercharging system.

If, for any reason, the air inlets in the leading edge of the wing should become plugged, a secondary inlet valve opens automatically to furnish air for the supercharger blower. This auxiliary valve is inward-opening and is located within the protected landing gear well of the inboard nacelle, in the side of the duct leading to the supercharger, The suction of the blower causes the valve to open and here is no reduction in the cabin air supply rate as a consequence.

The cabin supercharger is a centrifugal compressor having an integral speed-increasing gear box. It is mounted on the rear of the firewall of he nacelle and is driven by a flexible shaft from the generator drive of the engine. The supercharger impeller shaft is geared at 11.625 times the engine speed. Tests have shown that with only one of the two blowers in operation, the 2½ psi internal cabin pressure can be maintained at altitudes considerably above 20,000 feet.

A blower relief or surge valve is located in the duct leading from the blower to the cabin. This opens automatically by means of the pressure built up in the duct in case the flow of air into the cabin is throttled down by the valve in the control unit, thus bypassing the air output from the supercharger and preventing surging or overheating of the blower. During low-altitude operation in warm weather, the entire air output of the superchargers may be bypassed through these relief valves to the outside atmosphere by shutting the control unit airflow valve and using ventilation from the auxiliary cold air system. Normally, however, the flow of supercharged air into the cabin is maintained even in non-supercharged operation, in which case the blowers serve merely as ventilation fans.

The air is warmed as it leaves the cabin supercharger by means of a simple steam-heating system located in the engine nacelle. Steam is generated in a boiler consisting of a finned cylinder which extracts heat from the engine exhaust. The steam from this boiler passes into banks of condenser tubes and the cabin air supply is heated as it flows past these tubes. Water for the boiler is contained in a conical-shaped sump located below it and so designed that if any of the water should freeze when the engine is stopped, it will immediately thaw upon starting the engines. A feedwater pump, driven by oil from the airplane's hydraulic system, circulates the water from the sump to the boiler. Heat output is regulated by the speed of this feedwater pump, which is automatically controlled by a thermostat located in the central part of the airplane cabin. The setting on the thermostat can be varied at will by the airplane hostess or steward.

Distributing the Air

After the supercharged air supply passes through the flow control valves in the accessory compartment below deck, it continues to a set of distribution valves, where it is directed into one of several alternate flow courses which may be selected according to weather conditions. During hot weather, it is preferable to divert the air into the auxiliary ventilation duct in the cabin ceiling, whence it is distributed through the overhead grills and individual ventilators for the most cooling effect. The second optional flow course consists of a series of ventilation grills in the side of the cabin near the floor, providing superior hot air ventilation during cold weather. It is also possible to direct part of the air into the overhead system and part into the floor duct system. The choice between these various alternatives is made by the hostess or steward who operates selector valve controls located in the forward end of the cabin. The major portion of the pressure air may also be diverted into nozzles extending along the lower edge of the pilots' compartment windshield for windshield defrosting.

The supercharging control unit located in the accessory compartment is a compact apparatus, representing many months of development work which automatically regulates the atmospheric pressure level in the cabin It contains both the air inflow valve and the spent air discharge valves The pipe through which the spent air is discharged to the outside atmosphere passes directly through the duct for incoming warm air, which prevents frosting of the air outlet. The cabin-pressure regulation is accomplished by variations in the degree of opening in the outlet valve, which is governed by pressure-sensitive mechanisms that are responsive to absolute pressure and to differential pressure.

The cabin supercharging, heating, ventilating and pressure-regulating apparatus has been described in the singular for the sake of simplicity. All units of the system, including superchargers, steam-heating apparatus, automatic control unit, etc, are installed in duplicate in identical form and the system may be operated by either or both sets of apparatus.

Operation of the System

The operation of the supercharging system is vested in the Flight Engineer, although he has little to do in this connection other than to turn controls on or off, and observe the pressure instruments mounted on his instrument board in the rear right portion of the control room. He has three master levers. Two of them control the right and left supercharged air inlet valves, respectively. These two levers are normally left in one position, which provides automatic regulation of the supercharged air inflow by the automatic control unit. In the opposite position, the levers cut off the supercharged air inflow, and in intermediate position, they provide manual regulation of the air inflow. The third master lever has three positions: One is for unsupercharged, cold air ventilation (auxiliary cold air intake valve open and auxiliary outlet valve open); the second is for unsupercharged, warm air ventilation (cold air intake valve closed and auxiliary valve open); the third position is for supercharged operation (cold air valve closed and auxiliary outlet valve closed and supercharging under automatic control.)

The Flight Engineer's instrument board has a compact group of pressure-cabin instruments, in a central location separated from surrounding instruments by a green border. Within the border are the following instruments: An indicator showing rate of pressure change within the cabin; a "cabin altimeter" showing the pressure altitude within the cabin instead of the airplane altitude; a supercharger discharge pressure gauge with 2-way selector switch for connecting it to right or left supercharger; a suction gauge with a 3-way selector switch to show cabin pressure as referred to outside pressure or to show flow pressure in the right or left air supply ducts. At the top of the panel is an outlet valve selector switch with 3 positions. With this switch in vertical position, the cabin supercharging is controlled by both of the automatic valve control units; in the left hand position, the system operates through the left outlet control valve only, and in the right position, the right outlet unit only.

Ease of Maintenance

A feature of the Stratoliner is the accessibility of controls and equipment, and the ease of maintenance. All accessories such as batteries, control boxes, compressors, autosyn dynamotors, propeller de-icer fluid pumps and tanks, etc, are installed in the accessory compartment between spars in the bottom of the fuselage. All of this equipment, as well as the supercharging control equipment, is readily accessible for servicing from an outside door, and the compartment is also accessible during flight. Control cables and lines under the control room floor are accessible by way of a door in the forward cargo compartment, which can also be reached during flight. A round door in the nose of the plane permits access to the back side of the main instrument panels. Accessibility to all control lines, etc, beneath the floor of the passenger cabin is provided through the lower cargo hatches. The tail cone of the airplane is readily removable for access to the tail section which can also be reached from within through the hatch in the rear pressure bulkhead.

The semimonocoque engine nacelles make power plant equipment readily accessible. All engine mounts and power plants, complete with all equipment except for exhaust collector rings, generators and hydraulic pumps, are interchangeable within each airplane and may be installed on any nacelle of any other Stratoliner. The engine mounts are quickly detachable.

Landing gear and tail gear retracting mechanisms and wing flaps are operated in the Stratoliner by individual directly-connected electric drives, which insure positive control of the units through all positions. Individual hand cranks are also provided for emergency operation. Wheel brakes, fuel dump valves, propeller-feathering, power boost controls, and various other items are operated by the airplaneā€™s s hydraulic system. There are three sources of hydraulic pressure, two engine-driven hydraulic pumps — either of which is adequate to perform the function — and an electrically-driven emergency pump mounted in the accessory compartment and controlled remotely from the cabin.

The interior of the Stratoliner was laid out to take full advantage of the size of the plane. The main passenger cabin is divided on the right side into four travel compartments separated by wood-paneled partitions, while along the left side there are nine individual reclining chairs. Each of the four compartments has deep-cushioned davenport-type seats for six passengers, which make up into upper and lower berths for four passengers, with berths running transversely in the cabin. Thus, the plane will carry 33 day passengers or 25 night passengers, including the nine reclining chairs.

Control Cabin

The Stratoliner control cabin was designed with special attention to convenience of arrangement and simplicity of operating procedures. Pilot's and Co-pilot's instruments are arranged in a series of compact groups to facilitate reading. Power plant controls and miscellaneous other controls are grouped in a systematic arrangement on a centrally-located stand between the two pilots. Behind the Co-pilot's station on the right side of the room is the Flight Engineer's station, with desk and instrument panel. The Flight Engineer's chair is mounted on a special, movable base which may be swung around into a position immediately behind the main control stand between the two pilots. These three flight officers, plus two cabin attendants, comprise the crew of the TWA Stratoliner. The Pan American version provides for two additional flight crew members — a radio operator, whose position is immediately behind the pilot in the left rear corner of the control cabin, and a navigation officer, for whom a special navigation compartment has been provided just aft of the main control room.

Performance figures for the Model 307 and the Model 307-B Stratoliner vary somewhat, due to the difference in power plants. Following are some approximate performance figures for the 307-B: Maximum speed at 17,300 feet is 246 mph, and cruising speed at 15,500 feet is 222 mph. Takeoff distance at sea level in still air with maximum gross weight (45,000 lbs) is 930 feet. Service ceiling at maximum gross weight with 4 engines is 26,200 feet; with three engines, it is 21,300 feet; with two engines, 6800 feet. The maximum range at 50% power is 2390 miles.

This article was originally published in the May, 1940, issue of Aviation magazine, vol 39, no 5, pp 46-49, 116, 119-120.
The original article includes 6 photos and three drawings.
Illustrations are not credited, but are certainly from Boeing.

Illustrations: