The outer curvature of the cowl nose is as important as the inner contour shape. The cowl nose contour must be designed to avoid excessive local velocities in high sped flight. Here the design philosophy is somewhat similar to the fuselage and wing approach; supercritical velocities can be permitted far forward on the cowl provided the local velocities are subsonic well forward of the location of the maximum nacelle diameter. Many tests of cowling shapes have been made by NASA and various aircraft companies to determine desirable contours. Cowls are often cambered to compensate for the high angles of attack at which aircraft operate.
Some examples of nacelle designs and wing-mounted installations are shown below.
Commonality between engine installations, left and right, wing and tail, etc. is made as complete as possible. Airlines keep spare engines in a neutral configuration, i.e., with all parts installed that are common to all engine positions. Only the uncommon parts must be added to adapt the engine to a particular position. A neutral engine for the DC-10 consists of the basic engine with all accessories installed, generator electrical leads coiled, certain hydraulic and fuel lines not installed, nose cowl not installed, and engine control system not installed.
One of the most difficult design problems is fitting all the necessary equipment within the slender pylon. Fuel lines, pneumatic lines, engine and reverser controls, electrical cables, and numerous instrumentation leads must fit closely and yet permit maintenance access. The nacelle is made as small as possible but must provide space for all accessories plus ventilation for accessory and engine cooling.
One can use some of the pictures in this section for initial nacelle sizing when the actual engine dimensions are known. The nacelle diameter tends to be roughly 10% greater than the bare engine to accommodate various engine systems. The inlet itself extends about 60% of the diameter in front of the fan face, and the actual inlet area is about 70% of the maximum area, although this varies depending on the engine type. For initial sizing, a representative engine may be selected and scaled (within reason) to the selected thrust level. One would expect the engine dimensions to vary with the square root of the thrust ratio (so that the area and mass flow are proportional to thrust). Statistically, the scaling is a bit less than the square root. The plots below show the variation in nacelle diameter and length as the thrust varies. The concept is sometimes called "rubberizing" an engine. Using the 85" diameter 38,250 lb PW2037 as a reference and scaling diameter by thrust to the 0.41 power yields reasonable diameters for engines over a very large thrust range. Somewhat more scatter is found in engine length but a 0.39 power thrust scaling is reasonable here as well. We note that the plots below show engine diameter and length, rather than nacelle dimensions. The nacelle must be scaled up as described above.