Why Hasn't Commercial Air Travel Gotten Any Faster Since the 1960s?
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Guest Post by Peter Dunn from the Ask an Engineer series, published by MIT’s School of Engineering. Caption: "Your link to faraway continents in hours less time: the new, fabulously swift Boeing 707." Credit: 1950s unlimited
In an era when everything else is accelerating, airplanes are actually flying at slower speeds than they used to…
Specified cruising speeds for commercial airliners today range between about 480 and 510 knots, compared to 525 knots for the Boeing 707, a mainstay of 1960s jet travel. Why? “The main issue is fuel economy,” says Aeronautics and Astronautics professor Mark Drela. “Going faster eats more fuel per passenger-mile. This is especially true with the newer ‘high-bypass’ jet engines with their large-diameter front fans.”
Observant fliers can easily spot these engines, with air intakes nearly 10 feet across, especially on newer long-range two-engine jetliners. Older engines had intakes that were less than half as wide and moved less air at higher speeds; high-bypass engines achieve the same thrust with more air at lower speed by routing most of the air (up to 93 percent in the newest designs) around the engine’s turbine instead of through it. “Their efficiency peaks are at lower speeds, which causes airplane builders to favor a somewhat slower aircraft,” says Drela. “A slower airplane can also have less wing sweep, which makes it smaller, lighter and hence less expensive.” The 707’s wing sweep was 35 degrees, while the current 777’s is 31.6 degrees.
There was, of course, one big exception: the Concorde flew primarily trans-Atlantic passenger routes at just over twice the speed of sound from 1976 until 2003. Product of a treaty between the British and French governments, the Concorde served a small high-end market and was severely constrained in where it could fly. An aircraft surpassing the speed of sound generates a shock wave that produces a loud booming sound as it passes overhead; fine, perhaps, over the Atlantic Ocean, but many countries banned supersonic flights over their land. The sonic-boom problem “was pretty much a show-stopper for supersonic transports,” says Drela.
Some hope for future supersonic travel remains, at least for those able to afford private aircraft. Several companies are currently developing supersonic business jets. Their smaller size and creative new “boom-shaping” designs could reduce or eliminate the noise, and Drela notes that supersonic flight’s higher fuel burn per passenger-mile will be less of an issue for private operators than airlines. “But whether they are politically feasible is another question,” he notes.
For now, it seems, travelers will have to appreciate the virtues of high-bypass engines, and perhaps bring along a good book.
Visit the MIT School of Engineering’s Ask an Engineer site for answers to more of your questions.
Comments
Marcelo Pacheco
Thu, 04/14/2016 10:55am
Actually, a very large part of higher turbofan efficiency is due to higher combustion temperatures, a result of better materials that can withstand higher temperatures and far more precise control over airflow inside the combustion chamber that reduces heating of the combustion chamber wall. Since those improvements are invisible, they're often ignored.
Of course the large fan does play a very large (probably 50-50) part, as it converts more of the combustion heat into mass * flow acceleration instead of loosing that extra heat.
The first twin engine wide body (767-200) cruises at Mach 0.8, while its modern equivalent (787-800) cruises at Mach 0.84-0.855, and slightly faster cruise is still possible (cancelled sonic cruiser went as fast as Mach 0.89 for normal cruise).
The other often ignored aspect of airliner cruising speed is the variability of Mach speed vs temperature (and altitude). Speed of sound is higher when hotter, slower when colder, and in the cruising range altitude for all civilian aircraft that means higher altitude = slower Mach 1 is, that also means that as more modern aircraft becomes lighter and have more powerful engines, they tend to cruise higher, which means slower true speed for the same Mach number.
All of those aircraft could cruise some 10 knots faster, by burning more fuel, but airlines nowadays like to cruise strictly at the lowest cost index (a little slower). A 787-800 and A350-900 normal cruise is Mach 0.85, but could cruise at Mach 0.87 by using more fuel.
thomas paul
Thu, 12/31/2015 2:06pm
faster air travel has less to do with 5% variation in cruising speeds than with airport parking, security, cut-off times, tarmac delays, loading strategies, etc.
the FAA insists on routing flights along VOR based victor airways instead of using modern GPS; if jets were simply allowed to fly point to point and use collision awareness/avoidance, reserved times on runways, and tighter spacing, overall travel time (and fuel costs) would drop, say 15%?
safely transitioning a complex system of such magnitude i suppose is viewed as an insurmountable task.
Danil
Wed, 03/19/2014 2:15pm
There were not only Concorde supersonic civil aircraft, but also Tupolev Tu-144.
Phil Cassady
Sat, 05/06/2017 9:06am
The "sound barrier" is the problem. Aerodynamic drag rises very rapidly as the speed of sound is approached. Required thrust and fuel burn rise with aerodynamic drag so that it is more expensive to fly closer to (or above) Mach 1. Boeing proposed the Sonic Cruiser that would fly faster, but the airlines preferred the 787 that would fly more economically.
Marcelo Pacheco
Thu, 04/14/2016 11:01am
Actually, there are quite a few GPS / RNAV defined routes, and as all aircraft become SBAS (GPS+WAAS/EGNOS) equipped, the FAA plans to migrate from VOR to RNAV cruising routes. We're talking jet aircraft, so that's Jet routes (above FL180).
But they're already there, the "Q" airways.
The other problem that us pilots don't quite get is often times routing can't be the way we want, otherwise it would become a nightmare for ATC.
chad w
Wed, 04/12/2017 4:01pm
So a more efficient engine, areo etc means we still can be at speeds in the 80s? I feel there r limits to this so we can slowly stop the exploration of the "earth" controlling the population by supposed cost etc. The fastest manes plane is still 60 yrs old. The fact this has not been broken says it all
marci
Wed, 04/27/2016 9:12pm
I liked your comment but I wish I understood more.
Phil
Wed, 05/27/2015 10:39pm
True, but concorde was the only one that wasn't a stolen design, inherently unsafe, and financially inviable.
Assad
Thu, 01/19/2017 12:24am
Plane travel is still as slow and painful as it's ever been, now that most airlines have been privatised.
rkb555
Tue, 11/29/2016 10:46am
Odd how at the turn of the 19th century, the world was still horse and buggy. 60 yrs later we can jet through the air on planes yet 60 yrs to the present, no advances have been made to improve air travel time and according to the article, we've regressed. Why has technology become so stagnant in this area ?
Jay Rei
Wed, 09/07/2016 12:49am
It's fuel-efficiency versus parasite drag. Because parasite drag rises with the square of velocity, the faster you go the more fuel (exponential increase) is required to sustain a slower accelerated increase in airspeed. See https://en.wikipedia.org/wiki/Parasitic_drag
E=MC^2. Even light (non-physical object) from the sun traveling though the Earth's atmosphere is significantly decelerated creating heat (friction) at the Earth's surface; at sea-level atmosphere (and heat) is most concentrated. Light = Heat.
Therefore it is impossible for a physical object to travel at the speed of light on Earth because the amount of fuel required to an accelerate the mass of the object traveling at the speed of light is inversely proportional to the mass of the object e.g., the amount of fuel required to accelerate a particle of sand to the speed of light requires a fuel reserve the entire size of the planet.
francis palissery
Wed, 12/02/2015 12:28pm
are there any airlines attempting to fly faster not necessarily supersonic
Munawar Memom
Sat, 10/04/2014 9:30am
Will the efficiency of airline improve if weight is reduced ? Will the aircraft move faster
Munawar Memon
Sat, 10/04/2014 8:50am
Will lighter weight help speed of airplane?
Munawar Memom
Sat, 10/04/2014 8:43am
Will the efficiency of airline improve if weight is reduced ?
JohnConnor| Ch…
Sat, 05/10/2014 4:18am
Well, just for people who are in a hurry really want to be in the place as fast as it could, probably. But I guess you're right. We must accept the fact of having such high-bypass engines and we probably expect some inflight entertainments rather than having a good book.
rowlandw
Sat, 03/22/2014 8:46am
We also haven't gone back to the Moon since then either. Wasn't that the fastest space flight speed, to attain escape vs orbital velocity?