Forces Acting on the Aircraft
Thrust, drag, lift, and weight are forces that act upon all
aircraft in flight. Understanding how these forces work and
knowing how to control them with the use of power and
flight controls are essential to flight. This chapter discusses
the aerodynamics of flight—how design, weight, load factors,
and gravity affect an aircraft during flight maneuvers.
The four forces acting on an aircraft in straight-and-level,
unaccelerated flight are thrust, drag, lift, and weight. They
are defined as follows:
• Thrust—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. However, this is not always the case,
as explained later.
• Drag—a rearward, retarding force caused by disruption
of airflow by the wing, rotor, fuselage, and other
protruding objects. As a general rule, drag opposes
thrust and acts rearward parallel to the relative wind.
• Lift—is a force that is produced by the dynamic effect
of the air acting on the airfoil, and acts perpendicular
to the flight path through the center of lift (CL) and
perpendicular to the lateral axis. In level flight, lift
opposes the downward force of weight.
Aerodynamics
of Flight
Chapter 5
Lift
Weight
Thrust Drag
Figure 5-1. Relationship of forces acting on an aircraft.
Rearward component of weight
Thrust
Drag
Flight path
Relative wind
Component of weight
opposed to lift
Lift
CL
CG
Figure 5-2. Force vectors during a stabilized climb.
• Weight—the combined load of the aircraft itself, the
crew, the fuel, and the cargo or baggage. Weight is
a force that pulls the aircraft downward because of
the force of gravity. It opposes lift and acts vertically
downward through the aircraft’s center of gravity (CG).
In steady flight, the sum of these opposing forces is always
zero. There can be no unbalanced forces in steady, straight
flight based upon Newton’s Third Law, which states that for
every action or force there is an equal, but opposite, reaction
or force. This is true whether flying level or when climbing
or descending.
It does not mean the four forces are equal. It means the
opposing forces are equal to, and thereby cancel, the effects of
each other. In Figure 5-1, the force vectors of thrust, drag, lift,
and weight appear to be equal in value. The usual explanation
states (without stipulating that thrust and drag do not equal
weight and lift) that thrust equals drag and lift equals weight.
Although true, this statement can be misleading. It should be
understood that in straight, level, unaccelerated flight, it is
true that the opposing lift/weight forces are equal. They are
also greater than the opposing forces of thrust/drag that are
equal only to each other. Therefore, in steady flight:
• The sum of all upward components of forces (not just
lift) equals the sum of all downward components of
forces (not just weight)
• T he sum of all forward components of forces (not just
thrust) equals the sum of all backward components of
forces (not just drag)
This refinement of the old “thrust equals drag; lift equals
weight” formula explains that a portion of thrust is directed
upward in climbs and slow flight and acts as if it were lift
while a portion of weight is directed backward opposite to the
direction of flight and acts as if it were drag. In slow flight,
thrust has an upward component. But because the aircraft is in
level flight, weight does not contribute to drag. [Figure 5-2]
In glides, a portion of the weight vector is directed along
the forward flight path and, therefore, acts as thrust. In other
words, any time the flight path of the aircraft is not horizontal,
lift, weight, thrust, and drag vectors must each be broken down
into two components.
Another important concept to understand is angle of attack
(AOA). Since the early days of flight, AOA is fundamental to
understanding many aspects of airplane performance, stability,
and control. The AOA is defined as the acute angle between the
chord line of the airfoil and the direction of the relative wind.
Discussions of the preceding concepts are frequently omitted
in aeronautical texts/handbooks/manuals. The reason is
not that they are inconsequential, but because the main
ideas with respect to the aerodynamic forces acting upon
an aircraft in flight can be presented in their most essential
elements without being involved in the technicalities of the
aerodynamicist. In point of fact, considering only level flight,
and normal climbs and glides in a steady state, it is still true
that lift provided by the wing or rotor is the primary upward
force, and weight is the primary downward force.
By using the aerodynamic forces of thrust, drag, lift, and
weight, pilots can fly a controlled, safe flight. A more detailed
discussion of these forces follows.
Thrust
For an aircraft to start moving, thrust must be exerted and be
greater than drag. The aircraft continues to move and gain
speed until thrust and drag are equal. In order to maintain a
Level high speed Level cruise speed Level low speed
Flight path
Relative wind
Flight path
Relative wind
Flight path
Relative wind
3°
6°
12°
Level high speed Level cruise speed Level low speed
Figure 5-3. Angle of attack at various speeds.
constant airspeed, thrust and drag must remain equal, just as
lift and weight must be equal to maintain a constant altitude.
If in level flight, the engine power is reduced, the thrust is
lessened, and the aircraft slows down. As long as the thrust
is less than the drag, the aircraft continues to decelerate. To
a point, as the aircraft slows down, the drag force will also
decrease. The aircraft will continue to slow down until thrust
again equals drag at which point the airspeed will stabilize.
Likewise, if the engine power is increased, thrust becomes
greater than drag and the airspeed increases. As long as
the thrust continues to be greater than the drag, the aircraft
continues to accelerate. When drag equals thrust, the aircraft
flies at a constant airspeed.
Straight-and-level flight may be sustained at a wide range
of speeds. The pilot coordinates AOA and thrust in all
speed regimes if the aircraft is to be held in level flight. An
important fact related to the principal of lift (for a given
airfoil shape) is that lift varies with the AOA and airspeed.
Therefore, a large AOA at low airspeeds produces an equal
amount of lift at high airspeeds with a low AOA. The speed
regimes of flight can be grouped in three categories: low-
speed flight, cruising flight, and high-speed flight.
When the airspeed is low, the AOA must be relatively high
if the balance between lift and weight is to be maintained.
[Figure 5-3] If thrust decreases and airspeed decreases, lift
will become less than weight and the aircraft will start to
descend. To maintain level flight, the pilot can increase the
AOA an amount that generates a lift force again equal to the
weight of the aircraft. While the aircraft will be flying more
slowly, it will still maintain level flight. The AOA is adjusted
to maintain lift equal weight. The airspeed will naturally
adjust until drag equals thrust and then maintain that airspeed
(assumes the pilot is not trying to hold an exact speed).
Straight-and-level flight in the slow-speed regime provides
some interesting conditions relative to the equilibrium of
forces. With the aircraft in a nose-high attitude, there is a
vertical component of thrust that helps support it. For one
thing, wing loading tends to be less than would be expected.
In level flight, when thrust is increased, the aircraft speeds
up and the lift increases. The aircraft will start to climb
unless the AOA is decreased just enough to maintain the
relationship between lift and weight. The timing of this
decrease in AOA needs to be coordinated with the increase
in thrust and airspeed. Otherwise, if the AOA is decreased too
fast, the aircraft will descend, and if the AOA is decreased
too slowly, the aircraft will climb.
As the airspeed varies due to thrust, the AOA must also vary
to maintain level flight. At very high speeds and level flight,
it is even possible to have a slightly negative AOA. As thrust
is reduced and airspeed decreases, the AOA must increase
in order to maintain altitude. If speed decreases enough, the
required AOA will increase to the critical AOA. Any further
increase in the AOA will result in the wing stalling. Therefore,
extra vigilance is required at reduced thrust settings and low
speeds so as not to exceed the critical angle of attack. If the
airplane is equipped with an AOA indicator, it should be
referenced to help monitor the proximity to the critical AOA.
Some aircraft have the ability to change the direction of the
thrust rather than changing the AOA. This is accomplished
either by pivoting the engines or by vectoring the exhaust
gases. [Figure 5-4]
Lift
The pilot can control the lift. Any time the control yoke
or stick is moved fore or aft, the AOA is changed. As the
AOA increases, lift increases (all other factors being equal).
When the aircraft reaches the maximum AOA, lift begins
to diminish rapidly. This is the stalling AOA, known as
CL-MAX critical AOA. Examine Figure 5-5, noting how the
CL increases until the critical AOA is reached, then decreases
rapidly with any further increase in the AOA.
Before proceeding further with the topic of lift and how it
can be controlled, velocity must be discussed. The shape of
the wing or rotor cannot be effective unless it continually
keeps “attacking” new air. If an aircraft is to keep flying, the
lift-producing airfoil must keep moving. In a helicopter or
gyroplane, this is accomplished by the rotation of the rotor
blades. For other types of aircraft, such as airplanes, weight-
Figure 5-4. Some aircraft have the ability to change the direction of thrust.
Stall
CL
CLMAX
CD
L/D
L/DMAX
18
16
14
12
10
8
6
4
2
0
.2000
.1800
.1600
.1400
.1200
.1000
.0800
.0600
.0400
.0200
0
Coefficient of drag (CD)
Lift/drag
Angle of attack
0° 2° 4° 6° 8° 10° 12° 14° 16° 18° 20° 22°
CL
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
Figure 5-5. Coefficients of lift and drag at various angles of attack.
shift control, or gliders, air must be moving across the lifting
surface. This is accomplished by the forward speed of the
aircraft. Lift is proportional to the square of the aircraft’s
velocity. For example, an airplane traveling at 200 knots has
four times the lift as the same airplane traveling at 100 knots,
if the AOA and other factors remain constant.
L = CL . ρ . V2 . S
2
The above lift equation exemplifies this mathematically
and supports that doubling of the airspeed will result in four
times the lift. As a result, one can see that velocity is an
important component to the production of lift, which itself
can be affected through varying AOA. When examining the
equation, lift (L) is determined through the relationship of the
air density (ρ), the airfoil velocity (V), the surface area of the
wing (S) and the coefficient of lift (CL) for a given airfoil.
Taking the equation further, one can see an aircraft could
not continue to travel in level flight at a constant altitude and
maintain the same AOA if the velocity is increased. The lift
would increase and the aircraft would climb as a result of
the increased lift force or speed up. Therefore, to keep the
aircraft straight and level (not accelerating upward) and in a
state of equilibrium, as velocity is increased, lift must be kept
constant. This is normally accomplished by reducing the AOA
by lowering the nose. Conversely, as the aircraft is slowed, the
decreasing velocity requires increasing the AOA to maintain
lift sufficient to maintain flight. There is, of course, a limit to
how far the AOA can be increased, if a stall is to be avoided.
All other factors being constant, for every AOA there is
a corresponding airspeed required to maintain altitude in
steady, unaccelerated flight (true only if maintaining level
flight). Since an airfoil always stalls at the same AOA, if
increasing weight, lift must also be increased. The only
Drag
Airspeed
Minimum
drag
Total drag
Induced drag
parasite drag
Figure 5-6. Drag versus speed.
method of increasing lift is by increasing velocity if the AOA
is held constant just short of the “critical,” or stalling, AOA
(assuming no flaps or other high lift devices).
Lift and drag also vary directly with the density of the air.
Density is affected by several factors: pressure, temperature,
and humidity. At an altitude of 18,000 feet, the density of
the air has one-half the density of air at sea level. In order to
maintain its lift at a higher altitude, an aircraft must fly at a
greater true airspeed for any given AOA.
Warm air is less dense than cool air, and moist air is less
dense than dry air. Thus, on a hot humid day, an aircraft
must be flown at a greater true airspeed for any given AOA
than on a cool, dry day.
If the density factor is decreased and the total lift must equal
the total weight to remain in flight, it follows that one of the
other factors must be increased. The factor usually increased
is the airspeed or the AOA because these are controlled
directly by the pilot.
Lift varies directly with the wing area, provided there is no
change in the wing’s planform. If the wings have the same
proportion and airfoil sections, a wing with a planform area
of 200 square feet lifts twice as much at the same AOA as a
wing with an area of 100 square feet.
Two major aerodynamic factors from the pilot’s viewpoint
are lift and airspeed because they can be controlled readily
and accurately. Of course, the pilot can also control density by
adjusting the altitude and can control wing area if the aircraft
happens to have flaps of the type that enlarge wing area.
However, for most situations, the pilot controls lift and airspeed
to maneuver an aircraft. For instance, in straight-and-level flight,
cruising along at a constant altitude, altitude is maintained by
adjusting lift to match the aircraft’s velocity or cruise airspeed,
while maintaining a state of equilibrium in which lift equals
weight. In an approach to landing, when the pilot wishes to
land as slowly as practical, it is necessary to increase AOA near
maximum to maintain lift equal to the weight of the aircraft.
Lift/Drag Ratio
The lift-to-drag ratio (L/D) is the amount of lift generated by
a wing or airfoil compared to its drag. A ratio of L/D indicates
airfoil efficiency. Aircraft with higher L/D ratios are more
efficient than those with lower L/D ratios. In unaccelerated
flight with the lift and drag data steady, the proportions of
the coefficient of lift (C L) and coefficient of drag (C D) can
be calculated for specific AOA. [Figure 5-5]
The coefficient of lift is dimensionless and relates the lift
generated by a lifting body, the dynamic pressure of the fluid
flow around the body, and a reference area associated with
the body. The coefficient of drag is also dimensionless and is
used to quantify the drag of an object in a fluid environment,
such as air, and is always associated with a particular surface
area.
The L/D ratio is determined by dividing the C L by the C D,
which is the same as dividing the lift equation by the drag
equation as all of the variables, aside from the coefficients,
cancel out. The lift and drag equations are as follows (L =
Lift in pounds; D = Drag; CL = coefficient of lift; ρ = density
(expressed in slugs per cubic feet); V = velocity (in feet per
second); q = dynamic pressure per square foot (q = 1⁄2 ρv2);
S = the area of the lifting body (in square feet); and
CD = Ratio of drag pressure to dynamic pressure):
D = CD . ρ . V2 . S
2
Typically at low AOA, the coefficient of drag is low and
small changes in AOA create only slight changes in the
coefficient of drag. At high AOA, small changes in the AOA
cause significant changes in drag. The shape of an airfoil, as
well as changes in the AOA, affects the production of lift.
Notice in Figure 5-5 that the coefficient of lift curve (red)
reaches its maximum for this particular wing section at 20°
AOA and then rapidly decreases. 20° AOA is therefore the
critical angle of attack. The coefficient of drag curve (orange)
increases very rapidly from 14° AOA and completely
overcomes the lift curve at 21° AOA. The lift/drag ratio
(green) reaches its maximum at 6° AOA, meaning that at this
angle, the most lift is obtained for the least amount of drag.
Note that the maximum lift/drag ratio (L/D MAX) occurs at
one specific CL and AOA. If the aircraft is operated in steady
flight at L/DMAX, the total drag is at a minimum. Any AOA
lower or higher than that for L/D MAX reduces the L/D and
consequently increases the total drag for a given aircraft’s
FLAT PLATE
SPHERE
SPHERE WITH
A FAIRING
SPHERE INSIDE
A HOUSING
Figure 5-7. Form drag.
Figure 5-8. A wing root can cause interference drag.
lift. Figure 5-6 depicts the L/DMAX by the lowest portion of
the blue line labeled “total drag.” The configuration of an
aircraft has a great effect on the L/D.
Drag
Drag is the force that resists movement of an aircraft through
the air. There are two basic types: parasite drag and induced
drag. The first is called parasite because it in no way functions
to aid flight, while the second, induced drag, is a result of an
airfoil developing lift.
Parasite Drag
Parasite drag is comprised of all the forces that work to slow
an aircraft’s movement. As the term parasite implies, it is the
drag that is not associated with the production of lift. This
includes the displacement of the air by the aircraft, turbulence
generated in the airstream, or a hindrance of air moving over
the surface of the aircraft and airfoil. There are three types of
parasite drag: form drag, interference drag, and skin friction.
Form Drag
Form drag is the portion of parasite drag generated by the
aircraft due to its shape and airflow around it. Examples include
the engine cowlings, antennas, and the aerodynamic shape of
other components. When the air has to separate to move around
a moving aircraft and its components, it eventually rejoins
after passing the body. How quickly and smoothly it rejoins is
representative of the resistance that it creates, which requires
additional force to overcome. [Figure 5-7]
Notice how the flat plate in Figure 5-7 causes the air to swirl
around the edges until it eventually rejoins downstream. Form
drag is the easiest to reduce when designing an aircraft. The
solution is to streamline as many of the parts as possible.
Interference Drag
Interference drag comes from the intersection of airstreams
that creates eddy currents, turbulence, or restricts smooth
airflow. For example, the intersection of the wing and the
fuselage at the wing root has significant interference drag.
Air flowing around the fuselage collides with air flowing over
the wing, merging into a current of air different from the two
original currents. The most interference drag is observed when
two surfaces meet at perpendicular angles. Fairings are used
to reduce this tendency. If a jet fighter carries two identical
wing tanks, the overall drag is greater than the sum of the
individual tanks because both of these create and generate
interference drag. Fairings and distance between lifting
surfaces and external components (such as radar antennas
hung from wings) reduce interference drag. [Figure 5-8]
Skin Friction Drag
Skin friction drag is the aerodynamic resistance due to the
contact of moving air with the surface of an aircraft. Every
surface, no matter how apparently smooth, has a rough,
ragged surface when viewed under a microscope. The air
molecules, which come in direct contact with the surface of
the wing, are virtually motionless. Each layer of molecules
above the surface moves slightly faster until the molecules
are moving at the velocity of the air moving around the
aircraft. This speed is called the free-stream velocity. The area
between the wing and the free-stream velocity level is about as
wide as a playing card and is called the boundary layer. At the
top of the boundary layer, the molecules increase velocity and
move at the same speed as the molecules outside the boundary
layer. The actual speed at which the molecules move depends
upon the shape of the wing, the viscosity (stickiness) of
the air through which the wing or airfoil is moving, and its
compressibility (how much it can be compacted).
Figure 5-9. Wingtip vortex from a crop duster.
Figure 5-10. The difference in wingtip vortex size at altitude versus
near the ground.
The airflow outside of the boundary layer reacts to the
shape of the edge of the boundary layer just as it would
to the physical surface of an object. The boundary layer
gives any object an “effective” shape that is usually slightly
different from the physical shape. The boundary layer may
also separate from the body, thus creating an effective shape
much different from the physical shape of the object. This
change in the physical shape of the boundary layer causes a
dramatic decrease in lift and an increase in drag. When this
happens, the airfoil has stalled.
In order to reduce the effect of skin friction drag, aircraft
designers utilize flush mount rivets and remove any
irregularities that may protrude above the wing surface. In
addition, a smooth and glossy finish aids in transition of
air across the surface of the wing. Since dirt on an aircraft
disrupts the free flow of air and increases drag, keep the
surfaces of an aircraft clean and waxed.
Induced Drag
The second basic type of drag is induced drag. It is an
established physical fact that no system that does work in the
mechanical sense can be 100 percent efficient. This means
that whatever the nature of the system, the required work
is obtained at the expense of certain additional work that is
dissipated or lost in the system. The more efficient the system,
the smaller this loss.
In level flight, the aerodynamic properties of a wing or rotor
produce a required lift, but this can be obtained only at the
expense of a certain penalty. The name given to this penalty
is induced drag. Induced drag is inherent whenever an airfoil
is producing lift and, in fact, this type of drag is inseparable
from the production of lift. Consequently, it is always present
if lift is produced.
An airfoil (wing or rotor blade) produces the lift force by
making use of the energy of the free airstream. Whenever
an airfoil is producing lift, the pressure on the lower surface
of it is greater than that on the upper surface (Bernoulli’s
Principle). As a result, the air tends to flow from the high
pressure area below the tip upward to the low pressure area
on the upper surface. In the vicinity of the tips, there is a
tendency for these pressures to equalize, resulting in a lateral
flow outward from the underside to the upper surface. This
lateral flow imparts a rotational velocity to the air at the tips,
creating vortices that trail behind the airfoil.
When the aircraft is viewed from the tail, these vortices
circulate counterclockwise about the right tip and clockwise
about the left tip. [Figure 5-9] As the air (and vortices) roll off
the back of your wing, they angle down, which is known as
downwash. Figure 5-10 shows the difference in downwash at
altitude versus near the ground. Bearing in mind the direction
of rotation of these vortices, it can be seen that they induce
an upward flow of air beyond the tip and a downwash flow
behind the wing’s trailing edge. This induced downwash has
nothing in common with the downwash that is necessary to
produce lift. It is, in fact, the source of induced drag.
Downwash points the relative wind downward, so the more
downwash you have, the more your relative wind points
downward. That's important for one very good reason: lift is
always perpendicular to the relative wind. In Figure 5-11, you
can see that when you have less downwash, your lift vector
is more vertical, opposing gravity. And when you have more
downwash, your lift vector points back more, causing induced
drag. On top of that, it takes energy for your wings to create
downwash and vortices, and that energy creates drag.
Figure 5-11. The difference in downwash at altitude versus near
the ground.
The greater the size and strength of the vortices and
consequent downwash component on the net airflow over
the airfoil, the greater the induced drag effect becomes. This
downwash over the top of the airfoil at the tip has the same
effect as bending the lift vector rearward; therefore, the lift
is slightly aft of perpendicular to the relative wind, creating
a rearward lift component. This is induced drag.
In order to create a greater negative pressure on the top of an
airfoil, the airfoil can be inclined to a higher AOA. If the AOA
of a symmetrical airfoil were zero, there would be no pressure
differential, and consequently, no downwash component and
no induced drag. In any case, as AOA increases, induced
drag increases proportionally. To state this another way—the
lower the airspeed, the greater the AOA required to produce
lift equal to the aircraft’s weight and, therefore, the greater
induced drag. The amount of induced drag varies inversely
with the square of the airspeed.
Conversely, parasite drag increases as the square of the
airspeed. Thus, in steady state, as airspeed decreases to
near the stalling speed, the total drag becomes greater, due
mainly to the sharp rise in induced drag. Similarly, as the
aircraft reaches its never-exceed speed (VNE), the total drag
increases rapidly due to the sharp increase of parasite drag.
As seen in Figure 5-6, at some given airspeed, total drag is
at its minimum amount. In figuring the maximum range of
aircraft, the thrust required to overcome drag is at a minimum
if drag is at a minimum. The minimum power and maximum
endurance occur at a different point.
Weight
Gravity is the pulling force that tends to draw all bodies to
the center of the earth. The CG may be considered as a point
at which all the weight of the aircraft is concentrated. If the
aircraft were supported at its exact CG, it would balance in
any attitude. It will be noted that CG is of major importance in
an aircraft, for its position has a great bearing upon stability.
The allowable location of the CG is determined by the general
design of each particular aircraft. The designers determine
how far the center of pressure (CP) will travel. It is important
to understand that an aircraft’s weight is concentrated at the
CG and the aerodynamic forces of lift occur at the CP. When
the CG is forward of the CP, there is a natural tendency for the
aircraft to want to pitch nose down. If the CP is forward of
the CG, a nose up pitching moment is created. Therefore,
designers fix the aft limit of the CG forward of the CP for the
corresponding flight speed in order to retain flight equilibrium.
Weight has a definite relationship to lift. This relationship
is simple, but important in understanding the aerodynamics
of flying. Lift is the upward force on the wing acting
perpendicular to the relative wind and perpendicular to
the aircraft’s lateral axis. Lift is required to counteract the
aircraft’s weight. In stabilized level flight, when the lift force is
equal to the weight force, the aircraft is in a state of equilibrium
and neither accelerates upward or downward. If lift becomes
less than weight, the vertical speed will decrease. When lift is
greater than weight, the vertical speed will increase.
Wingtip Vortices
Formation of Vortices
The action of the airfoil that gives an aircraft lift also causes
induced drag. When an airfoil is flown at a positive AOA,
a pressure differential exists between the upper and lower
surfaces of the airfoil. The pressure above the wing is less
than atmospheric pressure and the pressure below the wing
is equal to or greater than atmospheric pressure. Since air
always moves from high pressure toward low pressure,
and the path of least resistance is toward the airfoil’s tips,
there is a spanwise movement of air from the bottom of the
airfoil outward from the fuselage around the tips. This flow
of air results in “spillage” over the tips, thereby setting up a
whirlpool of air called a vortex. [Figure 5-12]
At the same time, the air on the upper surface has a tendency
to flow in toward the fuselage and off the trailing edge. This
air current forms a similar vortex at the inboard portion of the
trailing edge of the airfoil, but because the fuselage limits the
inward flow, the vortex is insignificant. Consequently, the
deviation in flow direction is greatest at the outer tips where
the unrestricted lateral flow is the strongest.
Vortex
Figure 5-12. Wingtip vortices.
AVOID
Nominally
500–1,000 ft
Sink rate
several hundred ft/min
Figure 5-13. Avoid following another aircraft at an altitude within 1,000 feet.
vortices lead to a particularly dangerous hazard to flight,
wake turbulence.
Avoiding Wake Turbulence
Wingtip vortices are greatest when the generating aircraft is
“heavy, clean, and slow.” This condition is most commonly
encountered during approaches or departures because an
aircraft’s AOA is at the highest to produce the lift necessary
to land or take off. To minimize the chances of flying through
an aircraft’s wake turbulence:
• Avoid flying through another aircraft’s flight path.
• Rotate prior to the point at which the preceding aircraft
rotated when taking off behind another aircraft.
• Avoid following another aircraft on a similar flight
path at an altitude within 1,000 feet. [Figure 5-13]
• Approach the runway above a preceding aircraft’s
path when landing behind another aircraft and touch
down after the point at which the other aircraft wheels
contacted the runway. [Figure 5-14]
A hovering helicopter generates a down wash from its main
rotor(s) similar to the vortices of an airplane. Pilots of small
aircraft should avoid a hovering helicopter by at least three
rotor disc diameters to avoid the effects of this down wash. In
forward flight, this energy is transformed into a pair of strong,
high-speed trailing vortices similar to wing-tip vortices of larger
fixed-wing aircraft. Helicopter vortices should be avoided
because helicopter forward flight airspeeds are often very
slow and can generate exceptionally strong wake turbulence.
Wind is an important factor in avoiding wake turbulence
because wingtip vortices drift with the wind at the speed of the
wind. For example, a wind speed of 10 knots causes the vortices
to drift at about 1,000 feet in a minute in the wind direction.
When following another aircraft, a pilot should consider wind
speed and direction when selecting an intended takeoff or
landing point. If a pilot is unsure of the other aircraft’s takeoff
or landing point, approximately 3 minutes provides a margin of
As the air curls upward around the tip, it combines with the
downwash to form a fast-spinning trailing vortex. These
vortices increase drag because of energy spent in producing
the turbulence. Whenever an airfoil is producing lift, induced
drag occurs and wingtip vortices are created.
Just as lift increases with an increase in AOA, induced
drag also increases. This occurs because as the AOA is
increased, there is a greater pressure difference between the
top and bottom of the airfoil, and a greater lateral flow of air;
consequently, this causes more violent vortices to be set up,
resulting in more turbulence and more induced drag.
In Figure 5-12, it is easy to see the formation of wingtip
vortices. The intensity or strength of the vortices is directly
proportional to the weight of the aircraft and inversely
proportional to the wingspan and speed of the aircraft. The
heavier and slower the aircraft, the greater the AOA and the
stronger the wingtip vortices. Thus, an aircraft will create
wingtip vortices with maximum strength occurring during
the takeoff, climb, and landing phases of flight. These
Vortex Movement Near Ground - with Cross Winds
Vortex Movement Near Ground - No Wind
3K Wind
6K
(3K + 3K)
3K
No Wind
3K
0 (3K - 3K)
Figure 5-15. When the vortices of larger aircraft sink close to the ground (within 100 to 200 feet), they tend to move laterally over the
ground at a speed of 2 or 3 knots (top). A crosswind will decrease the lateral movement of the upwind vortex and increase the movement
of the downwind vortex. Thus a light wind with a cross runway component of 1 to 5 knots could result in the upwind vortex remaining in
the touchdown zone for a period of time and hasten the drift of the downwind vortex toward another runway (bottom).
25
Touchdown
Rotation
Wake ends
Wake begins
Figure 5-14. Avoid turbulence from another aircraft.
Thrust required
Velocity
Lift coefficient CL
Angle of attack
Out of ground effect
In ground effect
Out of ground effect
In ground effect
Figure 5-17. Ground effect changes drag and lift.
14
Figure 5-16. Ground effect changes airflow.
safety that allows wake turbulence dissipation. [Figure 5-15]
For more information on wake turbulence, see Advisory
Circular (AC) 90-23, Aircraft Wake Turbulence.
Ground Effect
Ever since the beginning of manned flight, pilots realized
that just before touchdown it would suddenly feel like the
aircraft did not want to go lower, and it would just want to go
on and on. This is due to the air that is trapped between the
wing and the landing surface, as if there were an air cushion.
This phenomenon is called ground effect.
When an aircraft in flight comes within several feet of the
surface, ground or water, a change occurs in the three-
dimensional flow pattern around the aircraft because the
vertical component of the airflow around the wing is
restricted by the surface. This alters the wing’s upwash,
downwash, and wingtip vortices. [Figure 5-16] Ground
effect, then, is due to the interference of the ground (or water)
surface with the airflow patterns about the aircraft in flight.
While the aerodynamic characteristics of the tail surfaces
and the fuselage are altered by ground effect, the principal
effects due to proximity of the ground are the changes in
the aerodynamic characteristics of the wing. As the wing
encounters ground effect and is maintained at a constant
AOA, there is consequent reduction in the upwash,
downwash, and wingtip vortices.
Induced drag is a result of the airfoil’s work of sustaining
the aircraft, and a wing or rotor lifts the aircraft simply by
accelerating a mass of air downward. It is true that reduced
pressure on top of an airfoil is essential to lift, but that is
only one of the things contributing to the overall effect of
pushing an air mass downward. The more downwash there
is, the harder the wing pushes the mass of air down. At high
angles of attack, the amount of induced drag is high; since this
corresponds to lower airspeeds in actual flight, it can be said
that induced drag predominates at low speed. However, the
reduction of the wingtip vortices due to ground effect alters
the spanwise lift distribution and reduces the induced AOA
and induced drag. Therefore, the wing will require a lower
AOA in ground effect to produce the same CL. If a constant
AOA is maintained, an increase in CL results. [Figure 5-17]
Ground effect also alters the thrust required versus velocity.
Since induced drag predominates at low speeds, the reduction
of induced drag due to ground effect will cause a significant
reduction of thrust required (parasite plus induced drag) at low
speeds. Due to the change in upwash, downwash, and wingtip
vortices, there may be a change in position (installation) error
of the airspeed system associated with ground effect. In the
majority of cases, ground effect causes an increase in the local
pressure at the static source and produces a lower indication
of airspeed and altitude. Thus, an aircraft may be airborne at
an indicated airspeed less than that normally required.
In order for ground effect to be of significant magnitude, the
wing must be quite close to the ground. One of the direct
results of ground effect is the variation of induced drag with
wing height above the ground at a constant C L. When the
wing is at a height equal to its span, the reduction in induced
drag is only 1.4 percent. However, when the wing is at a
height equal to one-fourth its span, the reduction in induced
drag is 23.5 percent and, when the wing is at a height equal
to one-tenth its span, the reduction in induced drag is 47.6
percent. Thus, a large reduction in induced drag takes place
only when the wing is very close to the ground. Because of
this variation, ground effect is most usually recognized during
the liftoff for takeoff or just prior to touchdown when landing.