Parasitic Drag - Interference Drag

Interference Drag

A characteristic that is dominant in bodies in transonic flow is the concept of interference drag. One can imagine two bodies of the aircraft (e.g. horizontal and vertical tail) that intersect at a particular point. Both bodies generate high supervelocities, possibly even supersonic. However, at the intersection there is less physical space for the flow to go and even higher supervelocities are generated resulting in much stronger local shock waves than would be expected if either one of the two bodies would be considered by itself. The stronger shock wave induces an increase in wave drag that is termed interference drag. Interference drag plays a role throughout the entire aircraft (e.g. nacelles, pylons, empennage) and its detrimental effect is always kept in mind by designers. Ideally, the pressure distributions on the intersecting bodies should complement each other’s pressure distribution. If one body locally displays a negative pressure coefficient, the intersecting body should have positive pressure coefficient. In reality, however, this is not always possible. Particular geometric characteristics on aircraft often show how designers have dealt with the issue of interference drag. A prime example is the wing-body fairing which smooths the sharp angle between the wing and the fuselage. Another example is the junction between the horizontal and vertical tailplane in a T-tail . Often, an additional fairing (acorn) is positioned to reduce the added supervelocities. The position of the nacelle with respect to the wing is a third example of how interference-drag considerations dominate this geometric feature. For nacelles that are positioned beneath the wing, the lateral and longitudinal distance from the wing is dominated by interference-drag considerations. If there is little vertical space available between the wing and the nacelle (because of ground clearance) the nacelle is usually positioned much more in front of the wing.