Why did airlines slow down after the 707?


#1

I was looking through one of my Jane’s Aircraft Recognition books I had as a child (1984 edition) and I realized that the 707 had a max cruising speed of 605 mph compared to the following:

757 - 533 mph
767 - 533 mph
777 - 560 mph
747 - 570 mph

727 - 570 mph!!

I’m curious why things had to slow down for Boeing after the 707? Seems rapid transit (sub-mach) would be the way to go to get people there faster rather than slower. Is this due to the exponential increase in air planes in the sky or just fuel efficiency/aerodynamics playing more of a role?

.t


#2

While these are the maximum speeds, you won’t find any of these aircraft routinely doing these speeds. The main reason is fuel. After the Arab oil embargo in the 1970s the 707’s flew at slower speeds. It’s doubtful that prior to the embargo the 707s flew at the max speed.

By the way, the maximum speed for the DC-8, during a test run, was just a tad over Mach 1.0.

What’s funny is that the time between airports that are relatively close to each other is about the same as it was in the 1950’s and 1960’s. A DC-6 would be scheduled for about 1:20 from SFO to LAX; today, using jets, it’s about 1:15. When oil was cheaper the average flight time (block to block) was about one hour.


#3

The one DC-8 that “supposedly” exceeded mach 1 was specially modified. It was not documented as a record, and therefore is hanger talk fodder.

The 707’s were routinely flown at Mach .82 even after the oil embargo. I flew for Ports of Call one of the last B707 passenger operators and we did that often. It was more fuel efficient than slowing it down to Mach .78, as office managers wanted, because it “fell off the step” and as the nose went up, the drag went up, thereby creating a need for just as much fuel to maintain that slower speed. Faster was more efficient.

From a different undocumented source:
The CP DC-8 that supposedly exceeded Mach 1 was, according to undocumented sources, specially modified with a 4% increased chord and 5,000lbs of ballast water that could be pumped forward or aft to shift the CG. The story says a F-104 flew chase to confirm the speed as the DC-*'s indicators were not designed for super sonic. (Come-on? You can modify an airframe to exceed Mach 1 but can’t include speed recorders? Hello?) It’s worth noting this supposed record is not recorded anywhere.


#4

Just a guess but doesn’t the frontal area of a high bypass fan have some detrimental effect?