Faster
than a speeding bullet
How fast do airplanes fly?
Certainly this is a general question as the answer depends on the type
of aircraft.
Aircraft engineers set goals during the design phase as to
the desired cruise speed, load capability, and range of the aircraft. These
goals have a direct impact on how the aircraft will be built.
There are three types of speeds important to the pilot of an
aircraft: Indicated airspeed, true
airspeed, and groundspeed.
Indicated airspeed is the speed that the instruments display
on the airspeed indicator on the instrument panel of the aircraft. This airspeed is important to the pilot
because the aircraft will perform in certain ways based on the indicated
airspeed. The aircraft will always stall
at a given indicated airspeed in level flight.
The best rate of climb airspeed will be determined by using the
indications on the airspeed indicator and is affected by aircraft load,
temperature, and altitude.
True airspeed is the actual speed of the aircraft through
the air when corrected for air density and temperature. Aircraft performance is based on a standard
atmosphere. A standard day is 59º
Fahrenheit (15º centigrade) and 29.92 inches of barometric pressure at sea
level. As altitude increases or
temperature increases the air becomes less dense. As the density of the air decreases it has
less effect on the airspeed indicator of the aircraft.
The pilot may compute the true airspeed of the aircraft
using an E6-B computer (a simple hand held circular slide rule with additional
aviation functions). Using the E6-B
computer, the pilot enters the outside air temperature and the pressure
altitude of the aircraft. The true airspeed can then be read next to the
indicated airspeed on the face of the computer.
Groundspeed is the speed of the aircraft across the
ground. After finding the true airspeed
the pilot adjusts for any headwind or tailwind.
This speed is needed during flight planning to determine the time and
fuel necessary for the flight. Actual
groundspeed is computed during flight by using checkpoints and the amount of
time elapsed over known distances.
In the United States, pilots generally use knots as a
measure of speed. One knot is equal to
1.15 miles per hour. Therefore an aircraft
traveling at 200 knots true air speed with a twenty-knot tailwind has a ground
speed of 220 knots or 253 miles per hour.
Most small single engine aircraft fly between 100 and 200
knots true airspeed. Small twin-engine
aircraft fly between 150 knots and 250 knots.
Larger corporate twin-engine turboprop aircraft fly up to 350 knots.
So, when Cousin Elmo gets on an airliner to visit Aunt
Elvira in Squeedeldunk, how fast does the airliner
fly? Most commercial jet airliners fly
at cruise airspeeds of about 550 miles per hour -- near 80 percent of the speed
of sound. The speed of sound is about
760 miles per hour at sea level and drops to 660 miles per hour at an altitude
of 35,000 feet.
Some military aircraft have the capability of exceeding the
speed of sound. These aircraft must be
designed for flight at these higher speeds.
The fastest military fighters, such as the F-15 Eagle, may exceed Mach
2.5 (2.5 times the speed of sound) in short bursts in level flight. At this speed (1,600 miles per hour at 35,000
feet) the fuel burn is enormous. These aircraft have fuel lines the size of
fire hoses to carry enough jet fuel to the engines.
The world’s fastest aircraft is the SR-71 Blackbird spy
plane. Designed during the late 1950’s
as the brainchild of legendary aircraft designer Kelly Johnson at Lockheed
Aircraft Corporation’s “skunk works” in Burbank, California, the SR-71 could
fly at speeds of 2,500 miles per hour and at altitudes above the highest reach
of surrface to air missiles -- classified, but above
100,000 feet. Considering that a bullet
from a .50 caliber machine gun travels at about 2,000 feet per second, the
SR-71 could catch the bullet and outrun it by over 800 miles per hour!
The SR-71 was designed as a replacement for the U-2 spy plane
(also a Kelly Johnson design). The U-2
was not fast but could fly at very high altitudes (over 100,000 feet). At higher altitudes, the indicated airspeed
of the aircraft is much lower than the true airspeed. The difference between
the stall speed and the maximum speed at the altitudes the U-2 flew at was only
a few knots. If the pilot allowed the
aircraft to slow the aircraft would enter a dangerous stall. If the nose of the
aircraft dropped the aircraft could exceed the speed of sound, even though the
U-2 was not designed for supersonic flight, and the airframe could tear
apart. This small speed range was known
as the “coffin corner” of the flight envelope.
Correction:
Willy Chamberlin, chairman of the Santa Ynez
Airport Authority, was kind enough to email, with a couple of corrections, the
story reporting on the authority’s annual membership meeting.
Due to a misplaced comma, it appeared that board member
Garth Carrier was the airport manager.
In fact, Jim Kunkle, board president, also
holds the position of airport manager.
Keegan Bailey is airport operations manager.
The story additionally referred to Kim Joos,
whose name had been misspelled. Ms. Joos is a paid consultant to the airport authority and is
responsible for many of the authority’s administrative functions, including
keeping track of airport grants, board minutes and the board agenda. Ms. Joos is not, by
definition, an ex-officio member of the Authority.