The downside of surplus thrust
Quote from ssg:
I like to joke it would be nice to fly a plane with more engine then wing...
[Unquote]
Hope I’m not misunderstanding your remark, but that is precisely what enables you to go into “coffin corner”. As 411A has reminded us, the turbojets of the 1950s and 60s were at their most efficient at high altitude (unlike turbofans); and capable of powering the wings of DC-8s, B707/720s, CV880/990s, and Comets well up into potential trouble.
When later variants of many of these aeroplanes were fitted with by-pass engines and even turbofans − but retained roughly similar wings − the situation changed. Instead of maximum cruise altitude being dictated by the wing, it was thrust that limited it; i.e., you’re at climb thrust and no longer climbing…
In my experience, the two opposing situations were illustrated by the by-pass-powered VC10 and the turbofan-powered B707-320B/C. On the VC10, when looking for a step climb, we merely looked at the low-speed buffet graphs. There was always enough power to get us up to an altitude where 1·35G would cause low-speed buffet at the current weight and cruise Mach; and even higher, where 1·25G would be enough. Up there, the safe range of speed between low-speed buffet and Mmo was interestingly narrow. 1·25G is equivalent to about 35deg bank in smooth air. This leaves little margin for turbulence in turns. Most crews would avoid it unless the ATC alternative was a fuel-costly descent, and reports indicated it was smooth at the higher level.
On the turbofan 707-320B/C, the situation was reversed. Although the graph included an indication of the buffet margins, these were usually 1·4G or more at attainable altitudes. Thrust was invariably what stopped us climbing, particularly when temperatures were above ISA. So, on that type of 707, there was little risk of reaching the coffin corner of the flight envelope.