Non Supersonic Airliner going Supersonic

The thread about rolling an airliner made me think of something I think I read a while back about a test pilot taking a non supersonic airliner supersonic in a dive. I think it was the Boeing 707 or a Lockheed Comet (if that is an airliner) Does anyone have any clue? After looking on the internet, who knew airliners could be so sexy and facinating.
Thanks,
Ben

I believe it was the DOUGLAS DC-8 which did the deed in a shallow dive.

I don’t know but since that plane is not designed for transsonic speeds I don’t think it could overcome the pressure wave.

Correct.

I’ve heard that at high altitudes the speed of sound is much lower than at sea level. I’ve seen shadows of shock waves on the top of jet liner wings when they’re cruising at altitude (the air moving fast across the top of the wing exceeds the speed of sound relative to the wing.) The real challenge is to break the speed of sound while at low altitude.

k2dave gravity is a great equaliser.

One problem with designing a supersonic aircraft is that a jet engine can’t handle supersonic airflow. On supersonic aircraft the airflow is slowed down to subsonic speeds prior to reaching the initial compressor stage of the engine. Subsonic aircraft don’t have any design features such as this and that is part of the reason why they are not supersonic in straight and level flight. In a dive however, the engines don’t need to provide the thrust required to exceed Mach 1 as gravity is doing some of the job for them. Provided the airframe is strong enough to cope, then it can be done.

I know the New Zealand airforce used to routinely put their subsonic A-4K Skyhawks into a supersonic dive. I’m not sure why they did this.

Yes the speed of sound drops as you gain altitude. Also engines are more efficient at altitude so from that angle it is easier the higher you are. But the main problem with going supersonic is not so much with achieving the speed, but more with the compressibility problems associated with the speed. These problems are the same regardless of what the actual speed of sound is, so the challange is similar at any altitude for level flight.

A clean A-4 (no external stores) at altitude is supersonic. Down near sea level, they top out at Mach 0.9…

A good reason for going supersonic in a dive is that “speed is life” - it’s a lot harder to hit a small target (like an A-4) moving really fast; and in an attack dive, if the enemy can’t hear you coming, they’re less likely to shoot you down on the way in.