b29

Design and development

YB-29 Superfortresses in flight
Interior photo of the rear pressurized cabin of the B-29 Superfortress, June 1944Manufacturing the B-29 was a complex task. It involved four main-assembly factories: two Boeing plants at Renton, Washington and Wichita, Kansas, a Bell plant at Marietta, Georgia ("Bell-Atlanta"), and a Martin plant at Omaha, Nebraska ("Martin-Omaha"). Thousands of subcontractors were involved in the project. Because of its highly advanced design, challenging requirements, and immense pressure for production, development was deeply troubled. On 18 February 1943 the second prototype crashed during testing due to an engine fire that spread to the wing, killing the entire 10 man crew and 20 others in the Frye meat packing plant just north of Boeing Field. Changes to the production craft came so often and so fast that in early 1944, B-29s would leave the production lines and fly directly to modification depots for extensive rebuilds to incorporate the latest changes. This "Battle of Kansas" (a troubleshooting modification program to get four groups ready for combat by 1 January 1944) nearly ended the program, which was only saved by General Hap Arnold’s direct intervention. It would still be nearly a year before the aircraft was operated with any sort of reliability.

The most common cause of maintenance headaches and catastrophic failures was the engine. Though the Wright R-3350 would later become a trustworthy workhorse in large piston-engined aircraft, early models were beset with dangerous reliability problems, many caused by demands that the B-29 be put in operation as soon as possible. It had an impressive power-to-weight ratio, but this came at a heavy cost to durability. Worse, the cowling Boeing designed for the engine was too close (out of a desire for improved aerodynamics), and the early cowl flaps caused problematic flutter and vibration when open in most of the flight envelope. The 18 radial cylinders, compactly arranged in front and rear rows, overheated because of insufficient flow of cooling air, which in turn caused exhaust valves to unseat.

These weaknesses combined to make an engine that would overheat regularly at combat weights, particularly during climbs after takeoff. Unseated valves released fuel-air mixtures during engine combustion that acted as a blowtorch against the valve stems. When these burned through the engines disintegrated and caught fire. A fire that was not immediately contained in the forward part of the engine by fire extinguishers became impossible to put out. An accessory housing manufactured of magnesium alloy in the back of the engine would often catch fire and produce heat so intense it burned through the firewall to the main wing spar in no more than 90 seconds, resulting in catastrophic failure of the wing.

This problem would not be fully cured until the aircraft was re-engined with the more powerful Pratt & Whitney R-4360 "Wasp Major" in the B-29D/B-50 program, which arrived too late for World War II. Interim measures included cuffs placed on propeller blades to divert a greater flow of cooling air into the intakes, which had baffles installed to direct a stream of air onto the exhaust valves. Oil flow to the valves was also increased, asbestos baffles installed around rubber push rod fittings to prevent oil loss, thorough pre-flight inspections made to detect unseated valves, and frequent replacement of the uppermost five cylinders (every 25 hours of engine time) and the entire engines (every 75 hours).[3]

Pilots, including the present-day pilots of the Commemorative Air Force’s Fifi, the last-remaining flying B-29, describe flight after takeoff as being an urgent struggle for airspeed (generally, flight after takeoff should consist of striving for altitude). Radial engines need airflow to keep them cool, and failure to get up to speed as soon as possible could result in an engine failure and risk of fire. One useful technique was to check the magnetos while already rolling rather than from a "braked" start. [4]

In wartime, the B-29 was capable of flight up to 40,000 feet (12,000 m), at speeds of up to 350 mph (true airspeed). This was its best defense, because Japanese fighters of that day could barely get that high, and few could catch the B-29, even if they were at altitude and waiting. Only the heaviest of anti-aircraft weapons could reach it, and since the Axis forces did not have proximity fuzes, hitting or damaging the airplane in combat was next to impossible.

With the revolutionary Central Fire Control System (CFCS), the B-29 had four remote controlled turrets, each armed with two .50 cal M2/AN machine guns.[5] Four gunners were able to control these turrets with the use of four General Electric made analog computers, one above the Norden bombsight in the nose[6] and three in a pressurized compartment in the rear fuselage which incorporated clear blown sighting blisters. The gunner manning the sight in the upper rear station was the "Central Fire Control gunner" whose job was to allocate turrets to each of the other three gunners, avoiding confusion in the heat of battle. The CFCS had (at that time) a highly advanced analog computer which corrected for the B-29's airspeed, the target's speed, target lead, gravity, temperature, barrel wear, and humidity. Because of this, the .50 caliber machine guns of the B-29 had a maximum effective range of 1,000 yards (910 m), double the range of the manually-aimed machine guns of the B-17 Flying Fortress. The tail gunner could only control his own weapons (two M2/AN Brownings plus, in early production B-29s, a 20 mm M2 cannon) and the lower rear turret. In post WWII, the tail guns eventually got their own APG-15 gun control radar sets.[7]

In early 1945, with a change of role from high altitude day bomber to low altitude night bomber, Le May reportedly ordered the removal of most of the defensive armament and remote controlled sighting equipment from his B-29s so that they could carry greater fuel and bomb loads. As a consequence of this requirement Bell, Marietta ((BM) produced a series of 311 B-29Bs which had the turrets and sighting equipment removed, except for the tail position which initially had the two .50 cal Browning machine guns and single M2 cannon with the APG-15 radar fitted as standard. This armament was quickly changed to three .50 caliber Brownings. This version also had an improved APQ-7 "Eagle" bombing-through-overcast radar fitted in an airfoil shaped radome under the fuselage. Most of these aircraft were assigned to the 315th Bomb Wing, Northwest Field, Guam. [8]

The crew enjoyed, for the first time in a bomber, full-pressurization comfort. This first-ever cabin pressure system for an Allied production bomber was developed for the B-29 by Garrett AiResearch.[9] The nose and the cockpit were pressurized, but the designers were faced with deciding whether to have bomb bays that were not pressurized, between fore and aft pressurized sections, or a fully pressurized fuselage with the need to de-pressurize to drop their loads. The decision was taken to have a long tunnel over the two bomb bays so that crews could crawl back and forth between the fore and aft sections, with both areas and the tunnel pressurized. The bomb bays were not pressurized.