I heard a rumour that the Boeing 747 was originally designed for supersonic cruising.
Is this correct? Are the airframe and wings able to cross the sound barrier without damage?
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No. The Boeing 747 was not designed for supersonic flight, though during testing, it was pushed quite close to the sonic speed:
Tom Cole, a spokesman at Boeing Commercial Airplane Co., said original flight tests of 747s conducted in 1969 and 1970 took 747-100 models to speeds of Mach 0.99.
However, the aircraft is not built for sustained supersonic speeds:
... Boeing and the FAA said the 747 is not built for sustained flight at the speed of sound. Its engines aren't powerful enough nor is it designed to deal with a destabilizing shock wave that develops around the speed of sound.
However, it is just possible that the airframe and wings can cope up with the supersonic flight, at least for a little while, though any sustained flight is not possible. There have been cases where the 747 has gone quite near the speed of sound and lived to tell the tale.
China Airlines Flight 006 went into a dive and went near supersonic speeds before the captain could recover it. As a result the horizontal stabilizers were ripped off and the wing bent permanently upwards.
Damage to China Airlines Flight 006 empennage. By NTSB - China Airlines Boeing 747-SP Accident Report by NTSB, http://www.rvs.uni-bielefeld.de/publications/Incidents/DOCS/ComAndRep/ChinaAir/AAR8603.html, Public Domain, Link
... the airplane was in a steep right-wing-down bank. The flight lost approximately 10,000 feet of altitude, and the airplane approached supersonic speeds (0.98 Mach) before the recovery could be completed.
Later, the Israelis took it close to sonic speed. In these cases, parts of the aircraft certainly experienced local supersonic flow; however, the airframe and wings are not designed for supersonic flow nor are the engines powerful enough. In one case, the aircraft suffered extensive damage (though it was repaired and returned to service).
The Boeing 747 is NOT designed for supersonic flight... However, it was designed as an interim aircraft while the SST(SuperSonic Transport) was being developed.
As you probably know, the SST never panned out, and the 747 is now one of the most influential aircraft in the world. As a book I read put it, the 747 was likened to a Mack truck of aircraft; it wasn't glamorous, and the SST was supposed to steal the show.
Some good reading on the 747 if you want more would be 747: Creating the world's first jumbo jet, by Joe Sutter Wide-Body: The triumph of the 747, by Clive Irving
It really is a fascinating story. It was developed in a very short time, and the amazing part is that they got it right.
Anyhow, maybe you heard that it was supposed to be an interim aircraft and got it confused. Hope this helped!
You can see from the design that it's not quite apt for supersonic flight. This topic is best explained in aviation textbooks, but here's a simple explanation. In subsonic flight there is a 'cushion' in the leading edges of the aircraft directing the airflow gently around the aircraft instead of having it hit the fuselage with full strength. However, in supersonic flight the cushion cannot be sustained and the impact between air and fuselage is much stronger.
Taking this into effect it can be seen from studying supersonic aircraft designs such as SR-71 and concorde that the area where the oncoming air can impact the fuselage is smaller than in conventional airliners. The bulge in 747's hull design wouldn't be optimal for supersonic flight.