traveling is just as important, if not
more so, than knowing how fast a car
is moving. However, the measurement
of an aircraft' s speed is a bit more complex than the speedometer on an
automobile. In this article we will
discuss the various types of air speed
and how they are measured. Pilots speak of several types of air
speed. When read directly off the air
speed indicator, the value is called
indicated air speed (IAS). Indicated air
speed has several factors that must be
corrected in order to determine the actual speed of an aircraft over the
ground. To determine the aircraft' s indicated air speed, two pressures are
measured. A pitot tube is positioned
on the exterior of the aircraft so that
the air molecules of the atmosphere
" ram" into it. The faster the aircraft is traveling, the greater the ram
pressure. As an aircraft climbs, the
atmospheric air pressure decreases, as
does the ram pressure. To account for
this, the aircraft has a static air
pressure port that is also connected to the air speed indicator. The greater the
difference between the ram and static
pressures, the greater the indicated air
speed. As an aircraft changes its air speed
and configuration, such as occurs
when slowing down and lowering
flaps and landing gear, the airflow
pattern over the fuselage changes.
This change of airflow affects the pressure in the pitot tube and static
port. To account for this, the pilot
refers to an air speed calibration chart
to read the calibrated air speed (CAS).
Each type of aircraft has its own
calibration chart because the airflow pattern depends on the aircraft itself. With some aircraft, such as the TB-9
Tampico, the IAS is approximately
equal to CAS at all air speeds. This is
true for relatively slow-moving aircraft
because they have an air speed
envelope (the difference between the maximum and minimum speeds) of
less than 50 knots. This small
envelope means that the airflow
pattern does not significantly change
from slow to fast air speeds. A jet
transport aircraft has a speed envelope of more than several
hundred knots.
The airflow pattern over a Boeing 737
cruising at 400 knots with flaps and
gear up is significantly different than
when the aircraft is landing at 150 knots with gear and flaps down. When flying faster than 200 knots, the
air ahead of the aircraft becomes
compressed. This air compression
increases the air density and the
pressure in the pitot tube. To account
for compressibility, the pilot refers to an air speed compressibility chart. The
greater the CAS and the higher the
altitude, the more the pilot must
subtract to attain the equivalent air
speed (EAS). As an illustration, imagine being in a
speedboat and sticking your hand out
into the wind. The wind pushes your
hand back with a particular dynamic
force. Now imagine sticking your hand
into the rushing water. The dynamic pressure exerted on your hand by the
water is greater than that of the air
because of the higher density of
water. As air is compressed into a pitot
tube, the increased density of
compression increases the dynamic pressure and therefore the air speed
that is read on the air speed indicator.
The more the air is compressed, the
greater the error. The air will be more compressed the
faster the aircraft is traveling and the
higher the pressure altitude. Think of
the air at lower pressure altitudes as
being pre-compressed by the force of
the air pressure. Think of this air as pre-compressed concrete block. Think
of the air at higher pressure altitudes
as not being compressed, and
therefore like a sponge. When the
same force is applied to a concrete
block and a sponge, the sponge will be more compressed, as is the case
with air. So for the same CAS as the
pressure altitude increases, so does
the amount that must be subtracted
from the CAS to determine the EAS.
Equivalency charts are used to make this correction. The pilot enters the CAS
and pressure altitude into the chart
and determines how much to subtract.
Equivalency charts are not airplane-
specific. It is the EAS that the aircraft feels. EAS is
a measure of the dynamic pressure
exerted on the aircraft. This dynamic
pressure plays a key role in the lift and
drag created by the aircraft. For a
given EAS the aircraft feels the same dynamic pressure, and therefore lift
and drag, regardless of altitude. The
higher the density altitude, the thinner
the air, and the faster an aircraft must
travel through the air mass to obtain
the same EAS. The actual speed of the aircraft through the air mass is called
the true air speed (TAS). A pilot flying at high altitudes must
account for reduced air density.
Imagine the space shuttle in orbit -
even though the orbital speed is more
than 17,000 knots, there is virtually no
atmosphere to ram into a pitot tube. The EAS would be nearly zero. By
knowing the air density, the pilot can
calculate the actual speed through the
air mass, or true air speed. The only
time that EAS is equal to true air speed
is when an aircraft is flying at standard sea level (SSL) conditions. It is to the
TAS that the velocity of the wind is
applied, to determine the speed over
the ground. The presence of a tailwind
or headwind will increase or decrease
the ground speed. Dynamic pressure represents the
kinetic energy of the relative wind. We
recall that the formula for kinetic
energy is one-half the mass multiplied
by the square of the velocity or KE = ½ m v2. Because air is a fluid, its mass is
represented by its density or r (rho).
The symbol r0 represents the air density at standard sea level. Here are
the equations for dynamic pressure (q): ½ r V TAS 2 = q = ½ r0 VEAS 2. This means that the dynamic pressure
can be determined by taking one-half
the air density multiplied by the squared velocity in TAS (V TAS 2). The same value will be obtained by taking
one-half the SSL air density multiplied
by the squared velocity in EAS (V EAS 2). Rearranging the previous equation,
the following equation is obtained: VTAS 2 (r /r 0) = V EAS 2. The term r /r 0 represents the ratio of the air density at some altitude to that
of SSL. This density ratio is referred to
as sigma or s. Making this substitution, the equation becomes V TAS 2 s = VEAS 2. Taking the square root of all terms
yields V TAS = V EAS. Solving for V TAS, the equation becomes: V TAS = V EAS (1/ ). The term 1/ is referred to as the
standard means of evaluation, or
SMOE. This means that in order to
determine the TAS, the EAS is
multiplied by SMOE: V TAS = V EAS x (SMOE). At approximately 40,000 feet the SMOE
value is 2. This means that if the EAS,
what the aircraft feels, is 200 knots,
then the TAS, or actual speed through
the air mass, would be 200 knots x 2,
or 400 KTAS. Mach refers to the ratio of an aircraft' s speed to that of the local speed of
sound (a). The phrase " local speed of sound" is used because the speed of sound' s propagation through the air is a function of the velocity of the air
molecules themselves. Recall that the
temperature of the air reflects the
average molecular velocity, so the
speed of sound is a function of air
temperature. At the SSL temperature of 15°C the speed of sound is
approximately 662 knots (a 0 = 661.74 knots). As the temperature
decreases with altitude, the local
speed of sound (a) also decreases. The formula for determining the local
speed of sound is a = a0 x , where a 0 represents the speed of sound at SSL.
The term T0 represents the SSL temperature of 15°C in Kelvin (288 K).
Kelvin is an absolute temperature
scale. For example, zero degrees
Celsius is equal to 273 K. The ratio of
the local temperature to that of SSL is
called theta or q. Making this substitution, the equation
becomes a = a0 x or a = 661.74 knots x . So the determination of an aircraft' s Mach number would be Mach = TAS /
(661.74 knots x ). Pilots of high-altitude fast-moving
aircraft are more concerned with
exceeding their maximum safe Mach
number than they are with dynamic
pressure limitations such as never-
exceed red lines. As an aircraft approaches the speed of sound, the
accelerated airflow over the top of the
wings will exceed the speed of sound
before the aircraft' s speed through the air exceeds the speed of sound.
The speed range in which both
subsonic and supersonic airflows
exist over an aircraft is called
transonic. This is the speed range in
which commercial jetliners cruise. If a jetliner were to exceed a safe Mach
number, the excessive area of sonic
airflow could result in a dangerous
buffeting similar to that of a stall. If this
high-speed buffet increases, it can
result in aircraft-control prob-lems. Most commercial airliners cruise at
around Mach 0.85. Because of the importance of
maintaining a cruise speed below a
maximum operating Mach number
(Mmo), high-speed aircraft must have
a Mach-indicating device. This is
typically represented by a red-and- white striped hand on the air speed
indicator that moves as the speed of
sound changes. This " barber pole" indicates the fastest speed at which
the aircraft should cruise. It is
interesting that a Mach meter does not
require a temperature input to
determine the highest speed that can
be attained without exceeding the Mmo. It seems contradictory that the
speed of sound is a function of
temperature and that the Mach meter
does not need to know what the
temperature is. Here is why: Remember that the formula for Mach is
the ratio of TAS to the local speed of
sound, or Mach = TAS / a. Because TAS is EAS multiplied by SMOE,
and the local speed of sound is the SSL
speed of sound multiplied by the
temperature ratio, the previous
equation can be rewritten as Mach =
{EAS x (SMOE)}/ (a 0 x ) or Mach = {EAS x (1/ )}/( a 0 x ). Moving the SMOE term to the
denominator yields Mach = EAS / (a 0 ). Combining the temperature ratio and
density ratio under a common radical
becomes Mach = EAS / (a 0 ). Now the magic. Certain gasses behave
in such a way that they can be said to
be ideal gasses. These gasses obey the
ideal gas equation, which states that
the temperature ratio (theta or q)
when multiplied by the density ratio (sigma or s) equals the pressure ratio
(delta or d), or d = q x s. The pressure
ratio represents the ratio of the
ambient pressure divided by the SSL
pressure of 29.921 inches of mercury,
or d = P /P 0. Substituting d for q s under the square
root radical becomes Mach = EAS / (a 0 x ) or Mach = EAS / (661.74 x ) or
Mach = EAS / (661.74 x ). So, the moral of the story is that a Mach
meter needs only pitot static inputs to
determine the Mach number of an
aircraft. That' s because the gasses that make up our atmosphere behave in a
manner consistent with the ideal gas
equation. Keeping track of all the different types
of air speed is a bit complicated, but
pilots do have one thing going their
way - there aren' t nearly as many speed limits in the air. So, the next time
a police officer asks if you know how
fast you were going, you might
respond by asking, " Do you mean indicated, calibrated, equivalent, true,
ground speed, or Mach?" That will just about guarantee that you' ll get a ticket.
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