Area rule states that bodies with the same crossectional area distribution have the same drag. Does that mean I could use a straight wing on mach 1.2 aircraft with performance as high as sweep winged plane as long as they have the same area distribution?
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5 Answers
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Sweep is not strictly required, but helpful to increase supersonic L/D. Just witness the length to which aircraft manufacturers went to sweep wings for Mach 2+ flight. However, sweep comes with its own set of problems.
answered Jun 4, 2018 at 21:16
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A little more on the F104 vs sweept wings. In order to optimize a design for a given mach number greater than 1.0, the wing needs to be entirely enveloped within the mach pressure wave cone. Hence, large area with sweep or small area short and straight. Conversely, a long straight wing would be partially inside the mach cone and partially outside (not optimum).
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Wing sweep is not required on supersonic aircraft. The F104 was a mach 2 interceptor with a straight, but tapered wing. It wasn't a well respected aircraft.
The Lockheed tests, however, determined that the most efficient shape for high-speed supersonic flight was a very small, straight, mid-mounted, trapezoidal wing.
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$\begingroup$ It seems that swept wing is preferred on aircraft that are expected to fly a lot at high subsonic speeds, but aircraft optimized for pure supersonic can have straight wings $\endgroup$ Jun 4, 2018 at 20:44
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$\begingroup$ The ideal supersonic airfoil is a flat sheet of paper. The 104's airfoil is "biconvex", a flat sheet with a bit of a curve on both surfaces. It depended on a blown flap - bleed vented along the front of the flap to reduce flow separation when they are down, to get the approach speed down to a mere 160-170 knots or so, only about 30-40 kts faster than an airliner. Not bad considering how small the wings are and how close they are to being flat sheets of plywood. $\endgroup$– John KJun 4, 2018 at 21:53
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$\begingroup$ The 104 is, incredibly when you think about it, the basis for the U2 spyplane. $\endgroup$– John KJun 4, 2018 at 21:56
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You will generally want to have a Swept Wing for supersonic aircraft as the swept wing delays the formation of shock waves. The type of drag that a swept wing prevents is Wave Drag and would be a major source of drag at supersonic speeds. I believe that the "Area rule" that you are quoting refers only to parasitic drag. You may want to read this Wikipedia article on swept wings to learn more about their benefit. The main downside of using swept wings is wing tip stall but that can usually be overcome with a Washout design.
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$\begingroup$ I thought area rule referred to form drag, the energy used for displacement of the air by the body. Parasitic is more like drag from bumps and projections etc no? $\endgroup$– John KJun 4, 2018 at 20:18
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$\begingroup$ Wave drag and skin friction are both a type of parasitic drag $\endgroup$ Jun 4, 2018 at 20:24
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$\begingroup$ Sweep or not, you will have wave drag from transsonic speeds upwards. $\endgroup$ Jun 4, 2018 at 21:06
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Another example of a supersonic aircraft that did not have swept wings is the Bell X1, the first aircraft to achieve supersonic speeds (and survive the experience). Given that the aircraft was pushing the unknown, the fuselage was shaped like a .50 caliber bullet, an object known to go supersonic smoothly... trial and error was a factor in pre computer aircraft design.
Note, though, that like the F104, the X1's wings are fairly short and set back on the fuselage, so that they are inside the shock wave cone coming off of the nose at supersonic speeds. As the X1 could barely go beyond Mach 1, its wings weren't as narrow as the F104, whose top speed was Mach 2.
