Regarding 747 (supersonic)
If you put a 747 into a fairly steep nose down attitude, you might be able to achieve a speed higher than mach .85 which is its cruise speed, but the wings and airframe were never designed to withstand those kinds of speeds. The airfoil is considered " subsonic" which means that it flies optimally below the speed of sound. I expect if you attempted to fly it beyond its specified design parameters, the wings would buffet violently, creating tremendous drag and possible structural deformation if it continued unabated. For an airplane to come close to the speed of sound (transonic) or beyond the speed of sound (supersonic) the airfoil must, among many other factors, have a high degree of sweep - like the Concorde. The airfoils currently on all modern jetliners, as remarkably advanced and efficient as they are, are simply not designed to deal with those kinds of speeds. They fly most efficiently between speeds of mach .74 and mach .85. Hope that helps you out!!!