Flight, April 1938

AND NOW the STRATOLINER
Features of the Boeing 307 and 307-A Transports : Operation at 20,000ft.

   A STATEMENT by Mr. C. L. Egtvedt, president of the Boeing Company, goes far towards dispersing the haze of semi-secrecy which for some time has surrounded the Boeing 307 and 307-S four-engined transports now under construction at the company’s Seattle works. The basic design has been christened the Stratoliner, the name having been registered as a trade mark. The new machines will be the first large transports with pressure cabins for high-altitude operation. As in the Albatross the fuselage is of circular cross-section throughout its length. Thirty-three passengers will be accommodated by day and twenty-five by night, and there will be stowage for 2 tons of mail and cargo. Power plants in the standard 307 will be four G102 Wright Cyclones (1,100 h.p. for take-off); these are for operation at moderate altitudes, so presumably similar engines with two-speed blowers will be specified for the 307-S.
   The basic reasons for the Stratoliner are that as height increases not only do atmospheric pressure and density decrease, but movement of the air in prevailing winds becomes swifter and more uniform, and the temperature decreases steadily with altitude - normally about 3 1/2 deg. every 1,000ft. The thermometer drops to 65 deg. below zero Fahrenheit at approximately 35,000ft. (in the temperate zone), and thence upward it ceases to get colder. This region of stable temperature, ex­tending as far up as there is still a trace of atmosphere, is the true stratosphere.
   The Stratoliner’s flight path will, of course, be below the true stratosphere but above the heavy air belt that brews surface weather conditions. This region is in the troposphere, more commonly known as the sub-stratosphere.
   It is here that most of the benefits of the stratosphere can be obtained without becoming involved in the complex problems of flight in the extremely rare atmosphere and low temperature of the true stratosphere. At 20,000ft. there is very little difference between summer and winter. No matter what the weather is below; the thermometer always registers something like 10 deg. below zero. Although the winds are not entirely constant at this level, they are considerably more so than at lower altitudes and the air is virtually free from the turbulence that sometimes gives passengers a rough journey at lower levels.
   Storm clouds and icing conditions, except on rare occasions, he below this flight path. When thunderheads do extend into the upper level they are so localised that they may readily be avoided. Normal clouds at this height are so thin that they appear as thin smoke rather than fog when a machine passes through them.
   The air is sufficiently rarefied to allow high speeds at normal cruising power. On 2,500 h.p. at 20,000ft., the Stratoliner will cruise at 240 m.p.h.
   At sea level the atmospheric pressure is 14.7 Ib./sq. in.; at height up to 10,000ft., where the pressure is a third less, most persons are entirely comfortable. Up to 12,000ft. the human system continues to function quite normally, but at 13,000 and 14,000ft., where the atmospheric pressure is down to 8.6 lb./sq. in., most people are definitely aware that they are "up high": they lose their sea level spryness. Above this point, tests have shown, the low air pressure and resultant shortage of oxygen makes a person mentally drowsy and sluggish in his actions. The effect increases with more altitude.

Compressing and Warming

   At 18,000ft. the atmospheric pressure is half as great as at sea level, and at 20,000ft. it has decreased further to 6.7 Ib./sq. in. But a sample of the air taken at any of these levels shows that the percentage of oxygen and other gases is always' the same. When compressed and warmed, the air at high altitudes resumes its low-altitude condition.
   As the Stratoliner climbs, its mechanical superchargers and pressure regulating apparatus will, come into action to preserve automatically the proper pressure inside the cabin while the outside pressure continues to decrease. The thoroughly sealed cabin, with its heavy pressure-resistant windows, will enhance the plane’s normal soundproofing and should give passengers an exceptionally quiet trip.
   The automatic pressure-regulating system brings a new advantage during ascents and descents. By merely arranging the proper setting on pressure regulating controls, the crew can bring the Stratoliner from an altitude of 15,000ft. to sea level at the rate of 565 ft./min. without discomfort to passengers, because the cabin pressure changes only at the rate of 300 ft./min. This will mean a substantial time saving.
   The cylindrical walls of the Stratoliner’s cabin, including windows, doors, and fittings where controls pass through the shell, are constructed to withstand a pressure difference of 6 lb. /sq. in between inside and outside air. The machine will be operated, however, at a pressure, difference of only 2 1/2 lb./sq. in., which is sufficient to keep the "apparent cabin altitude" at 8,000ft. at a height of 14,700ft., and "at 12,200ft." when 20,000ft. high.
   Fresh air, drawn through the leading edge of each wing, is compressed by two engine-driven superchargers and circulated throughout the cabin after being heated by means of freeze-proof condenser-type steam radiators. Either of the two fully independent sets of supercharging and heating apparatus can handle the entire job alone. Spent air is discharged through an exhaust chamber below the floor.
   The entire system operates automatically with the same ease as a thermostatically controlled home air conditioning and heating system. Controls for hand regulation are also provided.
   An automatically controlled fresh air supply sufficient for 40 persons is furnished by the blowers. The air is conducted directly into the cabin ventilating system, so that individually controlled ventilators at passenger seats supply the same "conditioned air" as do the general circulation vents.
   The circular section of the fuselage was chosen because it is the most practical shape for a pressure cylinder and at the same time is advantageous aerodynamically. Semi-monocoque construction is used, with Alclad aluminium alloy skin covering on Z-section rings and longitudinal stiffeners.
   The low cantilever wing is of the typical Boeing truss and stressed-skin type. Wing covering consists of smooth Alclad skin over aluminium alloy corrugations. Fittings are of high strength steel. The flaps and undercarriage are electrically operated.
   A new semi-monocoque construction making for quick detachability of the engines is used for the nacelles. The four Wright GR-1820-G102 Cyclone engines (in the 307) drive three-bladed Hamilton Standard Hydromatic airscrews, of the constant-speed, full-feathering type.
   Fuel tanks with a capacity of 1,275 U.S. gallons are installed in the inner-wing sections. Additional tanks may be fitted to increase the capacity to 1,700 U.S. gallons. There are no fuel pipes within the fuselage.
   The Stratoliner has a span of 107ft. 3in.; a length of 74ft. 4in.; and an overall height of 17ft. 3in.
   Immediately aft of the cockpit in the nose is the men's dress­ing room, and behind this is the main passenger cabin, the right side of which is divided into four travel compartment - each containing Davenport-type seats for six passengers. Each compartment makes up into two sets of upper and lower berths at night. Along the left side of the cabin are nine reclining chairs.
   Aft of the passenger section is the women's dressing room on the right and the galley at the left. There is a pressure bulkhead just aft of the galley.
   Passengers enter the cabin through a side door, the crew having a separate entrance to the cockpit from below. Cargo is loaded into the hold through this lower doorway.


BOEING 307.
Four Wright GR-1820-G102 Cyclone Engines.

Maximum speed on 3,800 h.p 241 m.p.h. at 6,000ft.
Cruising speed on 2,500 h.p. (69.5% power) 215 m.p.h. at 10,000ft.
Cruising speed on 2,180 h.p. (60% power) 202 m.p.h. at 10,000ft
Cruising speed on 1,800 h.p. (50% power) 184 m.p.h. at 10,000ft
Take-off distance at sea level at 4,400 h.p. 1,040 ft
Take-off distance to clear 50ft obstacle 1,900ft.
Take-off distance to clear 50ft obstacle on any three engines after failure of one engine 1,200ft. from start 2,700 ft.
Landing distance to stop over 50ft. obstacle 2,300 ft.
Service ceiling with four engines 23,300 ft.
Service ceiling with any three engines 18,200 ft.
Absolute ceiling with any two engines 10,500 ft.
Absolute ceiling with any two engines at 36,000-lb. gross weight 14,200 ft.
Initial rate of climb with four engines at 840 h.p. each at sea level 1,250 ft./min.
Initial rate of climb with three engines at 840 h.p. each at sea level 700 ft./min.
Initial rate of climb with two engines at 840 h.p. each at sea level 200 ft./min.
Specific fuel consumption 0.44 lb./h.p./hr.
Range with 7,000-lb. (3,180-kg.) payload at maximum gross weight and with standard equipment, furnishings, de-icers, radio, food, water, bedding and 1,275 gals. fuel:
   Optimum cruising - average speed 150 m.p.h. (242 km.p.h.) 1,950 miles.
   Cruising at 50% power or 1,800 h.p.- 184 m.p.h. (290 km.p.h.) 1,750 miles
   Cruising at 60% power or 2,100 h.p. - 202 m.p.h. (325 km.p.h.) 1,000 miles.
   Cruising at 69.5% power or 2,500 h.p. - 215 m.p.h. (348 km.p.h.) 1,475 miles.
Range with 9,750-lb. (4,420-kg.) payload at maximum gross weight and with standard equipment, furnishings, de-icers, radio, food, water, bedding and 850 U.S. gals. of fuel:
   Cruising at 184 m.p.h. 1,150 miles.
   Cruising at 202 m.p.h. 1,050 miles.
   Cruising at 215 m.p.h. 975 miles.