Ever wonder why newer airliners are almost all slower than the old Boeing 707? This explains why: MIT Team Designs Airliner That Uses 70 Percent Less Fuel
At the time, airplanes like the iconic Boeing 707 were designed to fly at speeds where the early jet engines were most efficient.
“They really favored high speed, they liked to go fast,” says Drela of the early jet engines. “The faster they go, the less fuel they burn to go a certain distance.”
...
“The 707 look originated from the demands of the low-bypass engine,” says Drela, referring to the types of engines used by airliners in the ’60s and ’70s. The original 707 flew for a few years with what is known as a turbojet engine that was also used on fighter jets of the time and was even less efficient.
On modern high-bypass engines we see at the airport today, those big fan blades in the front push most of the air past the engine to provide thrust. And since most of the air bypasses the engine, only a small amount is actually is being used for combustion inside the engine which means less fuel is used. These high-bypass engines are dramatically more efficient and quieter than the older turbojet and low-bypass engines that started on airplanes such as the 707 and Douglas DC-8.
“The new engines are actually much closer to propeller engines than the old jet engines, paradoxically,” says Drela. “If you look at a propeller-driven airplane, the optimum wing sweep is zero.”
So as some passengers lament over the fact that a modern airliner is 70 to 100 miles per hour slower than the original 707, the optimum design for fuel efficiency is to fly even a little bit slower, about another 50 miles per hour.
eta: Early jet engines were twice as efficient at 600 mph than at 500 mph. Which is why the early 707 was so much more efficient than the Comet.