Nếu dòng chảy trục đối xứng và thân máy bay dài, sau đó hàng loạt bảo tồn dẫn đến: b'2 = b2 - d2. Để kéo tối thiểu với thang máy cố định, downwash trong bối cảnh xa nên không đổi, vì vậy xoáy đánh thức giống như liên kết với một cánh elip với không có thân máy bay span, b '. | Fuselage Effect on Induced Drag One may estimate the drag associated with fuselage interference in the following manner If the flow were axially symmetric and the fuselage were long then mass conservation leads to b 2 b2 - d2. For minimum drag with fixed lift the downwash in the far wake should be constant so the wake vorticity is just like that associated with an elliptical wing with no fuselage of span b . The lift on the wing-fuselage system is computable from the far-field vorticity so the span efficiency is e 1 - d2 b2. In practice one does not achieve this much lift on the fuselage. Assuming a long circular fuselage and computing the lift based on images the resulting induced drag increment is about twice the simple theoretical value so s 1 - 2 d2 b2. Transonic Compressibility Drag This section deals with the effect of Mach number on drag from subsonic speeds through transonic speeds. We concentrate on some of the basic physics of compressible flow in order to estimate the incremental drag associated with Mach number. The chapter is divided into the following sections Introduction Predicting Mdiv and Mcc 3-D Effects and Sweep Predicting Cpc Notation for this chapter CL Airplane lift coefficient ACDc Incremental drag coefficient due to compressibility M Crest critical Mach number the flight Mach number at which the velocities at the crest of the wing cc in a direction normal to the isobars becomes sonic M0 The flight Mach number p Prandtl-Glauert Factor 1-Mo 1 2 t c Average thickness to chord ratio in the freestream direction for the exposed part of the wing V0 The flight speed AV Surface perturbation velocity Ac 4 Wing quarter-chord sweepback angle degrees Ac Sweepback angle of isobars at wing crest degrees Y Ratio of specific heats for air. Compressibility Drag Introduction The low speed drag level is often defined at a Mach number of below which the airplane drag coefficient at a given lift coefficient is generally invariant with Mach number. The .