Comment by CGMthrowaway
7 hours ago
IDK if "bigger and slower aircraft" is what he meant, but rather "bigger and slower engines." Jets cruise @ mach .85 because that's the economic optimum set by wave drag, compressibility and passenger time costs. Hasn't changed in 50 years.
The relevant metrics are amount of air moved and speed the air is accelerated to, aka efficiency gains from propulsive efficiency- e.g. increasing bypass ratio, larger fan diameter, lower jet exhaust velocity
I agree with your points but they had literally stated that passenger aircraft were getting slower and provided the example of the 747 vs 787. So it was clear to me what he meant.