Aerodynamic design features of the DC-9.

Abstract

The DC-9 aerodynamic design features discussed include the aft-engine T-tail arrangement, wing planform, airfoils, high-lift systems, nacelle-pylon design, and aerodynamic control systems. The aft-engine design is shown to have distinct advantages for the DC-9. There are two DC-9's : the Series 10, without leading-edge devices and therefore with airfoils having exceptional maximum lift capability; and the Series 30, with a longer fuselage, leading-edge devices, and airfoils more oriented toward minimum high-speed drag. The discussion concentrates on design areas associated with the aft-engine T-tail configuration such as the solution of the deep-stall problem and the design of the nacelle-pylon-fuselage area to minimize high Mach number drag. Causes of deep stall are shown to be related to the combined effects of wing and nacelle wakes and fuselage vortices on the horizontal tail. The DC-9 configuration has enough nose-down pitching moment capability to eliminate the possibility of locked-in deep stall. An analog simulator was used successfully to interpret extensive deep-stall wind-tunnel data. Flight-test results in all flight regimes verified the predictions of analytical methods and wind-tunnel tests.