KUCHEMANN'S CARROTS

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s we do the pre-flight walk around , and inspect  the wing underside we rarely give a second glance at the canoe shaped flap track fairings under the wings. Some are slender but many appear somewhat oversized to accommodate just  the flap fairings. Or do they serve some other function?

The physics of airflow alteres violently as it expands from subsonic to supersonic speeds. As the aircraft passes through the transonic speed range, local airflow approaches sonic speeds over the wing and body of the aircraft and leads to the formation of shock waves and consequent large increase in drag.

At transonic speeds, it was found, that the time-honoured principle that the drag of the individual elements of an airplane could be added in a linear manner to give the approximate drag of the entire configuration could no longer be relied upon.

Researches by Dietrich Kuchemann in the UK and Richard Whitcomb of NASA , in early 1950s, established that this wave drag can be minimised by a fuselage wing configuration synthesis, where the cross sectional area changed smoothly along the length of the aircraft. Known as the Area Rule, its basic tenet postulates that the wave drag of a simple equivalent body of revolution would be the same as a more complex wing body arrangements.

Initial  application of area rule designs can be seen in the   “Coke Bottle” or “Marylin Monroe” indented fuselage body shapes to reduce the effect of the presence of wings as in F-102 and F-106 aircraft. This, however had practical limitations and alternate efforts to address the local discrepancies in cross sectional areas led to the concept of attaching conical and pod shaped bodies along the wing , nacelle and fuselage. First successful application of this principle to combat wave drag effects was in Convair- 990. Following applications of the local area rule, several pylon, nacelle, and wing fairings were embodied, to smooth out the area distribution and facilitated in raising the cruise speed from 0.8M for the basic aircraft to 0.89 M for the modified airframe.

 These anti shock bodies  christened as ‘Whitcomb After-bodies’ or ‘Kuchemann’s Carrots’ are widespread in today’s designs.

Anti shock bodies were also apparently developed by the Soviet Designers during the same time , as seen in their installations in TU-16 and through subsequent designs such as the TU 154.

On most modern designs, the mechanism for deploying the wing flaps are encased in canoe shaped pods , which serve as anti-shock bodies and can be seen in A300/310, A 380 and Boeing 757 to name a few. Known to most as flap track fairings, garnering little attention, these pods nevertheless have an important role in transonic drag reduction and fuel economy.