Why Planes Don’t Fly Faster

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This is a Wendover Productions video made
possible by Audible. So I recently went down the rabbit hole of
looking at old flight schedules. Airlines used to publish these physical brochures
with their fares and flight times, and when looking at this American Airlines one from
1967 I noticed something interesting. This flight between New York and LA was scheduled
as 5 hours and 43 minutes long—that didn’t seem right. American flight 3 was scheduled to leave JFK
airport everyday at 12 noon and arrive at LAX at 2:43 pm—there’s 3 hours time difference. It turns out that nowadays American Airlines
flight 3 still leaves JFK daily at noon, but the difference is that today, flight 3 is
scheduled to arrive at LAX at 3:27 pm—44 minutes later than in 1967. This has happened across the board—almost
every flight today takes longer than it did back in the 60s. In general the actual flight times—the time
in the air—is the same but with all the congestion and delays at airports the scheduled
times now account for things going wrong. What this means though is that, overall, flying
has slowed down. In 1967 we hadn’t been to the moon and computers
looked like this but we were flying everywhere just as fast or even faster than we do today. What happened that caused this immense lack
of progress in the last 50 years? There are three main types of aircraft engine—the
turboprop, turbofan, and turbojet—and each of them has a range of speeds when they’re
most efficient. The turboprop is the kind of engine you see
on most propeller aircraft. Almost all of the thrust with turboprop engines
comes from the propeller. The turbine which spins the propeller does
intake and speed up some air, but the exhaust air is not at a very high speed so it only
accounts for less than 10% of the overall thrust. These engines are generally inexpensive both
to buy and operate so a lot of smaller commuter planes use turboprop engines. Of course there’s a trade-off—they’re
not as fast. They’re most efficient between about 325
and 375 mph. Any faster than that its better to use a turbofan. Now, these are the engines that you see everywhere. Almost every commercial aircraft is turbofan
driven. With turbofans, the air is initially sped
up by a fan—that’s what you see when you look at an engine from the front. Then, some of the air goes into the interior
combustion chamber where the actual turbine that drives the fan is and the rest of it
goes around the turbine. While air that bypasses the turbine is also
sped up, the majority of the thrust comes from the air that passes through the turbine. Turbofans are most efficient at the speeds
you see most aircraft fly today—400-620 mph. If you want to go supersonic—above 767 mph—you
need a turbojet. Turbojets are very similar to turbofans except
all the air goes through the turbine—no air is bypassed. This lets them achieve extremely high speeds
but they also require an immense amount of fuel. These engines are really only efficient between
about 1,300-1,400 mph. What really determine the efficiency of engines
is something called the bypass ratio. That’s the ratio of the amount of air that
passes through the bypass duct to the amount that passes through the engine core. The thing is, it really doesn’t take that
much more energy to spin a larger fan—what requires a lot more fuel is to put more air
through the engine core. That means that engines that accelerate more
air through the bypass duct can get more thrust for the same amount of energy so, as a rule,
the higher the bypass ratio the more efficient the engine. Take a look at this General Electric GEnx
engine. This is a relatively new super-efficient engine
used on both the 787 Dreamliner and the 747-8i. You can see the fan is much larger than the
turbine itself. That’s because this engine has a bypass
ratio of 10:1—10 times more air goes around the turbine than through it. Compare that to the CFM International CFM56—an
older and less efficient engine. You can see there’s much less fan relative
to the turbine so this engine only has a bypass ratio of 5.9:1, but that’s still considered
to be high. The difference is most striking when comparing
either of those engines to the Pratt & Whitney JT8D. This engine has a bypass ratio of only .96:1
so it’s astoundingly inefficient but its still significantly more efficient than the
Rolls-Royce/Snecma Olympus 593. This engine is a turbojet. As I mentioned, these drive all the air the
fan picks up through the turbine so that means no air is bypassed. Therefore, it has a bypass ratio of 0:1 and
is what’s known as a zero-bypass engine. Since 100% of the air passing through the
engine goes through the turbine, the fuel consumption is significantly higher than that
of the GEnx, CFM56, or even the JT8D. The Concorde, using the zero-bypass Rolls-Royce/Snecma
Olympus 593, burned 46.85 pounds of fuel per mile flown, while the 787 Dreamliner, using
the 10:1 bypass ratio General Electric GEnx, uses 18.7 pounds per mile but the Concorde
was a tiny airplane even compared to the modestly sized 787. It seated only 100 passengers compared to
291 on the Dreamliner. That means that the per-person fuel economy
on the Concorde was just about 14 miles per gallon compared to 104 miles per gallon on
the Dreamliner. In the end, Air France and British Airways,
the only two Concorde operators, could not afford to keep flying the plane. Less than 1/3 of the seats were actually occupied
by paying customers. Others were filled by those using miles or
those upgraded from first class on normal flights. After all, it cost at a minimum $7,500 of
today’s dollars to fly one way from London to New York in those three hours and that
was to fly in seats that looked like this—not all that different than the economy seats
of today. When the Concorde started flying, first class
on other planes looked like this. While nice, these seats were just larger economy
class seats and it wasn’t incredibly easy to sleep in them. By the time the Concorde stopped flying in
2003, first class looked like this and the seats went fully flat into a bed. Many chose to spend a little less to pass
7 hours in this rather than spending 3 hours in these cramped seats. British Airways even introduced the first
fully-flat business class seat in 2000 so for significantly less money than the Concorde,
travelers could cross the Atlantic sleeping horizontally. This just wasn’t luxury anymore. The whole idea of the Concorde was to create
the most efficient way to cross the Atlantic for the business traveller, but with fully-flat
beds, those traveling towards Europe could leave the US in the evening, get their nights
sleep on the plane and wake up in Europe—essentially wasting no time. No longer luxurious or efficient, the Concorde
flew its final commercial fight on October 24, 2003 thereby ending the era of commercial
supersonic flight. Here’s the thing about flying—speed really
doesn’t matter to airlines. It really only exists as a selling point for
the consumer. The cost of the airplane is a relatively small
part of the overall cost to fly so you’ll never see an airline fly faster so they can
use their planes more. Planes lifespans are generally rated in cycles—the
number of times the plane takes off and lands. The Dreamliner is rated for 44,000 cycles
and has a list price, which is often much higher than the actual sale price, of $224.6
million dollars. That means that the cost of the airplane per
flight is barely over $5,000 while the fuel cost for a flight from New York to London
is well over $15,000. Therefore, airlines always just fly their
airplanes at the most fuel efficient speed. It turns out that that speed is just about
always between 500-550 miles per hour. What’s weird about this is that its well
below the speed of sound—767 miles per hour. Why don’t planes fly just below the speed
of sound? Well, this graph shows the drag on airplanes
at different speeds. Between Mach 0.8 and mach 1.2 is what’s
known as the transonic range. At these speeds, the airflow around an airplane
is not fully subsonic or supersonic. Essentially, some air is subsonic and some
is supersonic. So beginning at mach 0.8, some airflow becomes
supersonic which increases drag exponentially and destabilizes the aircraft so its actually
quite dangerous to fly right around the speed of sound—you either have to fly well above
or below. You can actually see when an airplane is flying
transonic. Its hard to see, but there are these lines
that look like scratches on the camera lens but actually are mini supersonic shock waves. Because of the disturbances in the airflow,
flying between mach .8 and mach 1.2 actually requires more fuel than flying above mach
1.2 so that’s why we have this number—613.8 mph—that’s the speed limit for commercially
viable subsonic jets. While supersonic flight and crossing the Atlantic
in 3 hours is flashy and exciting, what’s truly impressive is hopping the pond for $100
or $200 on an airline that’s actually making a profit, and that’s becoming more of a
reality today. With current speeds, airplanes are able to
fly anywhere on earth in 24 hours and that’s fast enough for almost everyone. The barrier to travel for most people is cost,
not speed, so manufacturers and airlines will continue to focus their efforts on driving
down the cost of travel, not the time. In the end, time is the enemy of the privileged
few, cost is the enemy of the masses. This video was made possible by Audible. Audible is the leading online audiobook provider
but I’m sure you already know that. What I’ll tell you is how I use audible. I love reading but there are a lot of times
when I just can’t read a paper book—when working out, doing dishes, even when editing
a video—so I download tons of audiobooks through audible so I can learn no matter what
I’m doing. If you listen when doing tasks like me, you’re
turning what is normally boring into something fun. I recently started listening to Skyfaring—a
book by 747 pilot Mark Vanhoenacker. It gives a glimpse into the pilot’s side
of flying, but its written much more like a novel or memoir than a heavy-duty non-fiction
book. The great thing is that you can listen to
this book for free thanks to Audible with the 30 day free trial you get by signing up
using the link Audible.com/Wendover. It’s a fantastic book and Audible is a fantastic
site so please do at least give them a look over at Audible.com/Wendover. Other than that, you can support Wendover
Productions at patreon.com/wendoverproductions, follow me on Twitter @WendoverPro, watch my
last video about how the president travels here, check out my fan-moderated subreddit
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they come out. Thanks again for watching, and I’ll see
you in two weeks for another Wendover Productions video.

 

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