Introduction to Transonic Aerodynamics by Roelof Vos & Saeed Farokhi

Author:Roelof Vos & Saeed Farokhi
Language: eng
Format: epub
Publisher: Springer Netherlands, Dordrecht

6.4 Fundamentals of Boundary-Layer Flow

Friction between a body and the surrounding fluid is one of the main causes of aircraft drag. On typical high-subsonic transports the friction drag is responsible for 40–60 % of the total drag of the airplane [57]. Every component (lifting or nonlifting) of the aircraft that is wetted by the flow contributes to the friction drag of the airplane. An obvious way to reduce friction drag is therefore to reduce the total wetted area. Alternatively, the friction coefficient can be reduced. To understand the causes of friction we need to take a detailed look into the physics of the boundary layer, the thin viscous layer in between the virtually inviscid flow and the body.

When the boundary layer separates from the body, it creates a wake between the body and the continuous streamlines. A large momentum deficiency exists in the wake, which can contribute to a significant amount of drag. In addition, the pressure distribution over the body is significantly altered when separation occurs. In normal operating conditions, separation of the boundary layer is therefore to be prevented on any part of the airplane, whether intended to generate lift or not. The most noticeable limitation that boundary-layer separation poses is on the stall speed of an aircraft. At stall, the lift over the wing reduces significantly while the drag rises rapidly. Via the aviation regulations it has been established that the minimum approach speed of the aircraft (which relates to the required landing length of the aircraft) should be % higher than the stall speed measured at 1-g conditions according to FAR/CS-25 regulations. This means that in practice, an airplane flies only at 2/3rd of its maximum lift coefficient. The latter is, in turn, governed by the separation of the boundary layer on the wing and is therefore an important constraint during the aerodynamic design of an aircraft.

Even though low-speed stall is arguably the most renown result of boundary-layer separation, it plays an important role in determining the maximum cruise altitude of high-subsonic aircraft. Boundary layers are prone to separate when a relatively large adverse pressure gradient is present and the boundary layer has thickened considerably. Such conditions occur for example on the upper wing surface when a shock wave is present. This instantaneous jump in local pressure can cause boundary layer separation at the foot of the shock wave. However, not only on the wing do we find areas that require the attention of the aerodynamic designer. Also on non-lifting bodies such as the rear of the fuselage that often has a considerable amount of upsweep (read: curvature) to allow for airplane rotation during take-off. Other areas where sharp adverse pressure gradients occur are also suspect: edges of cockpit windows, intersections between wing, nacelle and pylon, and the engine intake.

In the subsequent sections we discuss both qualitative and quantitative aspects of the boundary layer and their effect on friction drag. We follow the classical presentation of Abernathy [1] on the fundamentals of boundary layers.

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