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'Fineness ratio' is a term used in aerospace engineering to describe the overall shape of a streamlined body. Specifically, it is the ratio of the length of a body to its maximum width; shapes that are "short and fat" have a low fineness ratio, those that are "long and skinny" have high fineness ratios. Aircraft that spend time at supersonic speeds generally have high fineness ratios, a canonical example being the Concorde.
At speeds below critical mach, one of the primary forms of drag is skin friction. As the name implies, this is drag caused by the interaction of the airflow with the aircraft's skinning. In order to minimize this for of drag, the aircraft should be designed to minimize the exposed skin area, or "wetted surface", which generally implies the fuselage should be somewhat "egg shaped", with a fineness ratio about 4.5. A good example of such a design is the Questair Venture. Most aircraft have fineness ratios significantly greater than this, however, consider the Lancair for instance. This is often due to the competing need to place the tail control surfaces at the end of a longer moment arm to increase their power; reducing the length of the fuselage would require larger controls, which would offset the drag savings from using the ideal fineness ratio. In other cases the designer is forced to use a non-ideal design due to outside factors such as seating arrangements or cargo pallet sizes. Modern airliners often have fineness ratios much higher than ideal, a side effect of their cylindrical cross-section which is selected for strength, as well as providing a single width to simplify seating layout.
As an aircraft approaches the speed of sound, shock waves form on areas of greater curvature. These shock waves radiate away energy that the engines have to supply, energy that does not go into making the aircraft go faster. This appears to be a new form of drag —referred to as wave drag— which peaks at about three times the drag at speeds even slightly below the critical mach. In order to minimize the magnitude of the wave drag, the curvature of the aircraft should be kept to a minimum, which implies much higher fineness ratios. This is why high-speed aircraft have long pointed noses and tails, and cockpit canopies that are mounted flush to the fuselage line. More technically, the best possible performance for a supersonic design is typified by two "perfect shapes", the von Kármán ogive which is pointed at both ends, or the Sears-Haack body, which has a blunt tail. Well known examples include the Concorde, F-104 Starfighter and XB-70 Valkyrie, although to some degree practically every post-WWII interceptor featured such a design. Missiles designers are even less interested in low-speed performance, and missiles generally have even higher fineness ratios than most aircraft.

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References

References

★ Form Factor

★ Basic Fluid Dynamics