Introduction
Several factors affect aircraft performance including the
atmosphere, aerodynamics, and aircraft icing. Pilots need an
understanding of these factors for a sound basis for prediction
of aircraft response to control inputs, especially with regard
to instrument approaches, while holding, and when operating
at reduced airspeed in instrument meteorological conditions
(IMC). Although these factors are important to the pilot flying
visual flight rules (VFR), they must be even more thoroughly
understood by the pilot operating under instrument flight
rules (IFR). Instrument pilots rely strictly on instrument
indications to precisely control the aircraft; therefore, they
must have a solid understanding of basic aerodynamic
principles in order to make accurate judgments regarding
aircraft control inputs.
Aerodynamic
Factors
Chapter 4
Leading edge
Mean camber line
Mean chord line Lower camber
Upper camber
Trailing edge
Figure 4-1. The airfoil.
Relative wind
Chord line
C/4
MC/4
Figure 4-2. Angle of attack and relative wind.
The Wing
To understand aerodynamic forces, a pilot needs to
understand basic terminology associated with airfoils.
Figure 4-1 illustrates a typical airfoil.
The chord line is the straight line intersecting the leading
and trailing edges of the airfoil, and the term chord refers
to the chord line longitudinal length (length as viewed from
the side).
The mean camber is a line located halfway between the
upper and lower surfaces. Viewing the wing edgewise, the
mean camber connects with the chord line at each end. The
mean camber is important because it assists in determining
aerodynamic qualities of an airfoil. The measurement of
the maximum camber; inclusive of both the displacement
of the mean camber line and its linear measurement from
the end of the chord line, provide properties useful in
evaluating airfoils.
Review of Basic Aerodynamics
The instrument pilot must understand the relationship
and differences between several factors that affect the
performance of an aircraft in flight. Also, it is crucial to
understand how the aircraft reacts to various control and
power changes, because the environment in which instrument
pilots fly has inherent hazards not found in visual flying. The
basis for this understanding is found in the four forces acting
on an aircraft and Newton’s Three Laws of Motion.
Relative Wind is the direction of the airflow with respect to
an airfoil.
Angle of Attack (AOA) is the acute angle measured between
the relative wind, or flightpath and the chord of the airfoil.
[Figure 4-2]
Flightpath is the course or track along which the aircraft is
flying or is intended to be flown.
The Four Forces
The four basic forces [Figure 4-3] acting upon an aircraft in
flight are lift, weight, thrust, and drag.
Lift
Lift is a component of the total aerodynamic force on an
airfoil and acts perpendicular to the relative wind. Relative
wind is the direction of the airflow with respect to an airfoil.
This force acts straight up from the average (called mean)
center of pressure (CP), which is called the center of lift. It
should be noted that it is a point along the chord line of an
airfoil through which all aerodynamic forces are considered
to act. The magnitude of lift varies proportionately with
speed, air density, shape and size of the airfoil, and AOA.
During straight-and-level flight, lift and weight are equal.
Roll
Pitch
Yaw
z y
Left
aileron
Right aileron
Rudder
Vertical
stabilizer
Horizontal
stabilizer
Elevator
Wing
LiftWeight
Drag
Thrust
Figure 4-3. The four forces and three axes of rotation.
Weight
Weight is the force exerted by an aircraft from the pull of
gravity. It acts on an aircraft through its center of gravity (CG)
and is straight down. This should not be confused with the
center of lift, which can be significantly different from the
CG. As an aircraft is descending, weight is greater than lift.
Thrust
Thrust is the forward force produced by the powerplant/
propeller or rotor. It opposes or overcomes the force of drag.
As a general rule, it acts parallel to the longitudinal axis.
Drag
Drag is the net aerodynamic force parallel to the relative
wind and is generally a sum of two components: induced
drag and parasite drag.
Induced Drag
Induced drag is caused from the creation of lift and increases
with AOA. Therefore, if the wing is not producing lift, induced
drag is zero. Conversely, induced drag decreases with airspeed.
Parasite Drag
Parasite drag is all drag not caused from the production of
lift. Parasite drag is created by displacement of air by the
aircraft, turbulence generated by the airfoil, and the hindrance
of airflow as it passes over the surface of the aircraft or
components. All of these forces create drag not from the
production of lift but the movement of an object through an
air mass. Parasite drag increases with speed and includes skin
friction drag, interference drag, and form drag.
• Skin Friction Drag
Covering the entire “wetted” surface of the aircraft is a thin
layer of air called a boundary layer. The air molecules on the
surface have zero velocity in relation to the surface; however,
the layer just above moves over the stagnant molecules
below because it is pulled along by a third layer close to
the free stream of air. The velocities of the layers increase
as the distance from the surface increases until free stream
velocity is reached, but all are affected by the free stream.
The distance (total) between the skin surface and where free
stream velocity is reached is called the boundary layer. At
subsonic levels the cumulative layers are about the thickness
of a playing card, yet their motion sliding over one another
creates a drag force. This force retards motion due to the
viscosity of the air and is called skin friction drag. Because
skin friction drag is related to a large surface area its affect
on smaller aircraft is small versus large transport aircraft
where skin friction drag may be considerable.
• Interference Drag
Interference drag is generated by the collision of airstreams
creating eddy currents, turbulence, or restrictions to smooth
flow. For instance, the airflow around a fuselage and around
the wing meet at some point, usually near the wing’s root.
These airflows interfere with each other causing a greater drag
than the individual values. This is often the case when external
items are placed on an aircraft. That is, the drag of each item
individually, added to that of the aircraft, are less than that
of the two items when allowed to interfere with one another.
Outside
force
Net
forces
Path
Apply down
elevator
Path
Net forces
Figure 4-4. Newton’s First Law of Motion: the Law of Inertia.
Time
2,000 lb300 hp
2,000 lb150 hp
= Acceleration Force
Mass
Figure 4-5. Newton’s Second Law of Motion: the Law of Momentum.
• Form Drag
Form drag is the drag created because of the shape of a
component or the aircraft. If one were to place a circular
disk in an air stream, the pressure on both the top and bottom
would be equal. However, the airflow starts to break down
as the air flows around the back of the disk. This creates
turbulence and hence a lower pressure results. Because the
total pressure is affected by this reduced pressure, it creates
a drag. Newer aircraft are generally made with consideration
to this by fairing parts along the fuselage (teardrop) so that
turbulence and form drag is reduced.
Total lift must overcome the total weight of the aircraft, which
is comprised of the actual weight and the tail-down force used
to control the aircraft’s pitch attitude. Thrust must overcome
total drag in order to provide forward speed with which to
produce lift. Understanding how the aircraft’s relationship
between these elements and the environment provide proper
interpretation of the aircraft’s instruments.
Newton’s First Law, the Law of Inertia
Newton’s First Law of Motion is the Law of Inertia. It states
that a body at rest will remain at rest, and a body in motion
will remain in motion, at the same speed and in the same
direction until affected by an outside force. The force with
which a body offers resistance to change is called the force of
inertia. Two outside forces are always present on an aircraft
in flight: gravity and drag. The pilot uses pitch and thrust
controls to counter or change these forces to maintain the
desired flightpath. If a pilot reduces power while in straight-
and-level flight, the aircraft will slow due to drag. However,
as the aircraft slows there is a reduction of lift, which causes
the aircraft to begin a descent due to gravity. [Figure 4-4]
Newton’s Second Law, the Law of Momentum
Newton’s Second Law of Motion is the Law of Momentum,
which states that a body will accelerate in the same direction
as the force acting upon that body, and the acceleration
will be directly proportional to the net force and inversely
proportional to the mass of the body. Acceleration refers
either to an increase or decrease in velocity, although
deceleration is commonly used to indicate a decrease. This
law governs the aircraft’s ability to change flightpath and
speed, which are controlled by attitude (both pitch and bank)
and thrust inputs. Speeding up, slowing down, entering
climbs or descents, and turning are examples of accelerations
that the pilot controls in everyday flight. [Figure 4-5]
Newton’s Third Law, the Law of Reaction
Newton’s Third Law of Motion is the Law of Reaction,
which states that for every action there is an equal and
opposite reaction. As shown in Figure 4-6 , the action of
the jet engine’s thrust or the pull of the propeller lead to the
reaction of the aircraft’s forward motion. This law is also
responsible for a portion of the lift that is produced by a wing,
from the downward deflection of the airflow around it. This
downward force of the relative wind results in an equal but
opposite (upward) lifting force created by the airflow over
the wing. [Figure 4-6]
Atmosphere
The atmosphere is the envelope of air which surrounds the
Earth. A given volume of dry air contains about 78 percent
nitrogen, 21 percent oxygen, and about 1 percent other gases
such as argon, carbon dioxide, and others to a lesser degree.
Action Reaction
Action
Reaction
Figure 4-6. Newton’s Third Law of Motion: the Law of Reaction.
Although seemingly light, air does have weight and a one
square inch column of the atmosphere at sea level weighs
approximately 14.7 pounds. About one-half of the air by
weight is within the first 18,000 feet. The remainder of the
air is spread over a vertical distance in excess of 1,000 miles.
Air density is a result of the relationship between temperature
and pressure. Air density is inversely related to temperature
and directly related to pressure. For a constant pressure to be
maintained as temperature increases, density must decrease,
and vice versa. For a constant temperature to be maintained
as pressure increases, density must increase, and vice versa.
These relationships provide a basis for understanding
instrument indications and aircraft performance.
Layers of the Atmosphere
There are several layers to the atmosphere with the
troposphere being closest to the Earth’s surface extending to
about 60,000 feet at the equator. Following is the stratosphere,
mesosphere, ionosphere, thermosphere, and finally the
exosphere. The tropopause is the thin layer between the
troposphere and the stratosphere. It varies in both thickness
and altitude but is generally defined where the standard
lapse (generally accepted at 2 °C per 1,000 feet) decreases
significantly (usually down to 1 °C or less).
International Standard Atmosphere (ISA)
The International Civil Aviation Organization (ICAO)
established the ICAO Standard Atmosphere as a way
of creating an international standard for reference and
performance computations. Instrument indications and
aircraft performance specifications are derived using this
standard as a reference. Because the standard atmosphere is
a derived set of conditions that rarely exist in reality, pilots
need to understand how deviations from the standard affect
both instrument indications and aircraft performance.
In the standard atmosphere, sea level pressure is 29.92 inches
of mercury ("Hg) and the temperature is 15 °C (59 °F). The
standard lapse rate for pressure is approximately a 1 "Hg
decrease per 1,000 feet increase in altitude. The standard
lapse rate for temperature is a 2 °C (3.6 °F) decrease per
1,000 feet increase, up to the top of the stratosphere. Since
all aircraft performance is compared and evaluated in
the environment of the standard atmosphere, all aircraft
performance instrumentation is calibrated for the standard
atmosphere. Because the actual operating conditions rarely,
if ever, fit the standard atmosphere, certain corrections must
apply to the instrumentation and aircraft performance. For
instance, at 10,000 ISA predicts that the air pressure should be
19.92 "Hg (29.92 "Hg – 10 "Hg = 19.92 "Hg) and the outside
temperature at –5 °C (15 °C – 20 °C). If the temperature
or the pressure is different than the International Standard
Atmosphere (ISA) prediction an adjustment must be made to
performance predictions and various instrument indications.
Pressure Altitude
Pressure altitude is the height above the standard datum
plane (SDP). The aircraft altimeter is essentially a sensitive
barometer calibrated to indicate altitude in the standard
atmosphere. If the altimeter is set for 29.92 "Hg SDP, the
altitude indicated is the pressure altitude-the altitude in the
standard atmosphere corresponding to the sensed pressure.
The SDP is a theoretical level where the pressure of the
atmosphere is 29.92 "Hg and the weight of air is 14.7 psi.
As atmospheric pressure changes, the SDP may be below,
at, or above sea level. Pressure altitude is important as a
basis for determining aircraft performance, as well as for
assigning flight levels to aircraft operating at or above 18,000
feet. The pressure altitude can be determined by either of
two methods: (1) by setting the barometric scale of the
altimeter to 29.92 "Hg and reading the indicated altitude, or
(2) by applying a correction factor to the indicated altitude
according to the reported altimeter setting.
Density Altitude
Density altitude is pressure altitude corrected for nonstandard
temperature. As the density of the air increases (lower
density altitude), aircraft performance increases. Conversely,
as air density decreases (higher density altitude), aircraft
performance decreases. A decrease in air density means a
high density altitude; an increase in air density means a lower
density altitude. Density altitude is used in calculating aircraft
performance. Under standard atmospheric conditions, air at
each level in the atmosphere has a specific density; under
standard conditions, pressure altitude and density altitude
identify the same level. Density altitude, then, is the vertical
distance above sea level in the standard atmosphere at which
a given density is to be found. It can be computed using
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0 2 4 6 8 10 12 14 16 18 20 22 24
CL-MAX
2 11 20
Angle of Attack (degrees)
Lift Coefficient - CL
Stall
1.5
Figure 4-8. Relationship of lift to AOA.
Altitude and Temperature Effects
TO FIND the effect of altitude temperature
CONNECT the temperature and airport altitude by straight line
READ the increase in take-off distance and the decrease in rate of
climb from standard sea level values here
−10
−20
−30
−40
−2
Airport pressure altitude—Thousand of feet
(read your altmeter set to 29.92 "Hg)
Percent decrease
in rate of climb
Add this percent to your
normal take off distance
Figure 4-7. Koch chart sample.
a Koch Chart or a flight computer with a density altitude
function. [Figure 4-7]
If a chart is not available, the density altitude can be estimated
by adding 120 feet for every degree Celsius above the ISA. For
example, at 3,000 feet PA, the ISA prediction is 9 °C (15 °C –
[lapse rate of 2 °C per 1,000 feet x 3 = 6 °C]). However, if the
actual temperature is 20 °C (11 °C more than that predicted
by ISA) then the difference of 11 °C is multiplied by 120 feet
equaling 1,320. Adding this figure to the original 3,000 feet
provides a density altitude of 4,320 feet (3,000 feet + 1,320 feet).
Lift
Lift always acts in a direction perpendicular to the relative
wind and to the lateral axis of the aircraft. The fact that lift is
referenced to the wing, not to the Earth’s surface, is the source
of many errors in learning flight control. Lift is not always
“up.” Its direction relative to the Earth’s surface changes as
the pilot maneuvers the aircraft.
The magnitude of the force of lift is directly proportional to
the density of the air, the area of the wings, and the airspeed.
It also depends upon the type of wing and the AOA. Lift
increases with an increase in AOA up to the stalling angle,
at which point it decreases with any further increase in AOA.
In conventional aircraft, lift is therefore controlled by varying
the AOA and speed.
Pitch/Power Relationship
An examination of Figure 4-8 illustrates the relationship
between pitch and power while controlling flightpath and
airspeed. In order to maintain a constant lift, as airspeed is
reduced, pitch must be increased. The pilot controls pitch
through the elevators, which control the AOA. When back
pressure is applied on the elevator control, the tail lowers
and the nose rises, thus increasing the wing’s AOA and lift.
Under most conditions the elevator is placing downward
pressure on the tail. This pressure requires energy that is
taken from aircraft performance (speed). Therefore, when
the CG is closer to the aft portion of the aircraft the elevator
downward forces are less. This results in less energy used for
downward forces, in turn resulting in more energy applied
to aircraft performance.
Thrust is controlled by using the throttle to establish or
maintain desired airspeeds. The most precise method
of controlling flightpath is to use pitch control while
simultaneously using power (thrust) to control airspeed. In
order to maintain a constant lift, a change in pitch requires a
change in power, and vice versa.
If the pilot wants the aircraft to accelerate while maintaining
altitude, thrust must be increased to overcome drag. As
the aircraft speeds up, lift is increased. To prevent gaining
altitude, the pitch angle must be lowered to reduce the AOA
and maintain altitude. To decelerate while maintaining
altitude, thrust must be decreased to less than the value of
drag. As the aircraft slows down, lift is reduced. To prevent
losing altitude, the pitch angle must be increased in order to
increase the AOA and maintain altitude.
Drag Curves
When induced drag and parasite drag are plotted on a graph,
the total drag on the aircraft appears in the form of a “drag
curve.” Graph A of Figure 4-9 shows a curve based on thrust
versus drag, which is primarily used for jet aircraft. Graph B