If you look closely at the aft fuselage by the engine nacelles of the Cessna Citation X, it’s pinched in quite considerably to conform with supersonic area ruling, the same reason supersonic fighter jets have “coke bottle waist” mid-fuselages- If you were to plot the cross sectional area of an aircraft from nose to tail, the ideal curve on such a graph would be a smooth elliptical curve. But in most aircraft, the wings, tail, or engine nacelles make that curve “bumpy”- so you can take away some fuselage area by the wings. In the Citation X, the fuselage is pinched in to compensate for the engine nacelles increasing the cross-sectional surface area which is what transonic area ruling- if you didn’t there would be a big increase in drag. The pinching also creates a more constant width channel between the nacelle and fuselage- this keeps air from speeding up locally and then slowing down, which would cause it to become turbulent, also increasing drag (that kind of drag is called interference drag). If you’re going to cruise at Mach 0.92, you’ll need every trick in the aerodynamics book!
Photo: JP Santiago