The duct for the UPS727 Quiet Freighter centre engine is a special animal, and its characteristic bulge has to do with improving the airflow. Jim Finch got a patent on the design of it. He was a design engineer with Dee Howard, who did the re-engining of the 727 into the QF configuration by putting Rolls Royce Tay engines in all positions.
The duct forward end portion has a cross-section transitioning from a circular cross-section of substantially greater area than that of the original S-duct to an elliptical cross-section for the intermediate duct. The rear duct has a cross-section transitioning from the elliptical cross-section of the intermediate duct to a circular cross-section of substantially greater area than the original S-duct, thereby permitting installation of the Tay engine with a substantially larger air intake diameter;
The radius of curvature of the forward curved portion of the S-shaped intermediate duct portion was greater than the original S-duct and that led to the bulge. The leading bend had an increasing cross-sectional area in the range of about 1500 sq. in. to 1780 sq. in. and a radius of curvature in the range of about 0.5 deg./in.
The initial problem with the original S-duct was that Boeing found that not all JT8D engines would work with all S-ducts. Engines would have to be selected for a high surge factor for placement as the aft engine, supplied by a conventional S-duct. Boeing's S-duct was also limited in the volume of high energy air that would actually traverse the longitudinal length of the S-duct and appear at the engine fan.
The high energy air flow problem was addressed by Boeing in later versions of the S-duct. Boeing's S-duct technology dictated that more air flow from a larger diameter S-duct would solve the high energy air flow problem. However, to Boeing's dismay the larger inlet to the S-duct did not result in more high energy air reaching the engine fan. The principal reason for this failure is that the larger the inlet, the slower the air moves in the duct, therefore less high energy air reaches the engine fan.
The Tay series engine by Rolls Royce required 30% more air flow than the JT8D engine by Pratt Whitney. That increase in air flow, according to present S-duct technology, would dictate a 30% larger S-duct. That wouldn't work using the original S-duct concept.
Finch designed the S-duct for re-engining would have a configuration varying from an enlarged circular cross-sectional area at the forward open end of the air flow inlet to a first elliptical transitional area which has an increasingly reduced cross-sectional area as it approaches the main spar forging of the vertical fin of the aircraft. A second transitional section was provided rearwardly adjacent to the reduced cross-sectional area portion and has an increasing cross-sectional area and a circular cross-section as it approaches the engine intake fan.
The turning of the air was accomplished in the original 727 S-duct at lower velocities in an attempt to reduce boundary layer separation, but this called for a larger cross-section and resulted in more abrupt turns, and an unfavorable pressure gradient. The S-duct configuration of Finch allowed for increasing of the radii of curvature over previous designs and an area profile to accelerate the flow at the proper point in the curve to achieve the proper pressure gradient which maintains the required attached airflow, thereby maintaining a consistent velocity of high energy air presented over the entire engine fan disc.
Hence the bulge.
