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View Diary: From failure to massive cultural icon - the Boeing 747 (106 comments)

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  •  Extension smooths cross sectional area (1+ / 0-)
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    change along the length of the fuselage which reduces drag. Extending it further then it is on the -400 causes a sudden "bump" in the cross sectional area as the upper deck gets added to the wing's cross section and rapidly falls off as the wing area later drops off. I always think of the phenomena as any abrupt change in cross section providing an area of potentially disrupted/separated airflow which is a drag multiplier.

    I think it is an early, think lower mach number, manifestation of wave drag described by the area rule phenomenon that is caused by the extreme size of the aircraft. Area rule is usually thought of as applying to high trans sonic and supersonic speeds. A large aircraft like the 747 moves a lot of air out of the way as it moves. Movement accelerates the air to the point that parts of the aircraft experience supersonic air flow earlier than others. The bottom line is that wave drag likely has a larger effect earlier on the whale than on a smaller, more aerodynamically clean aircraft. That's my guess anyway.

    Time makes more converts than reason. Thomas Paine, Common Sense

    by VTCC73 on Wed Jul 24, 2013 at 10:46:01 AM PDT

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