There are four forces acting on a helicopter in flight.
They are lift, weight, thrust, and drag. [Figure 2-1] Lift
is the upward force created by the effect of airflow as it
passes around an airfoil. Weight opposes lift and is
caused by the downward pull of gravity. Thrust is the
force that propels the helicopter through the air.
Opposing lift and thrust is drag, which is the retarding
force created by development of lift and the movement
of an object through the air.
AIRFOIL
Before beginning the discussion of lift, you need to be
aware of certain aerodynamic terms that describe an
airfoil and the interaction of the airflow around it.
An airfoil is any surface, such as an airplane wing or a
helicopter rotor blade, which provides aerodynamic
force when it interacts with a moving stream of air.
Although there are many different rotor blade airfoil
designs, in most helicopter flight conditions, all airfoils
perform in the same manner.
Engineers of the first helicopters designed relatively
thick airfoils for their structural characteristics.
Because the rotor blades were very long and slender, it
was necessary to incorporate more structural rigidity
into them. This prevented excessive blade droop when
the rotor system was idle, and minimized blade twist-
ing while in flight. The airfoils were also designed to
be symmetrical, which means they had the same cam-
ber (curvature) on both the upper and lower surfaces.
Symmetrical blades are very stable, which helps keep
blade twisting and flight control loads to a minimum.
[Figure 2-2] This stability is achieved by keeping the
center of pressure virtually unchanged as the angle of
attack changes. Center of pressure is the imaginary
point on the chord line where the resultant of all aero-
dynamic forces are considered to be concentrated.
Today, designers use thinner airfoils and obtain the
required rigidity by using composite materials. In addi-
tion, airfoils are asymmetrical in design, meaning the
upper and lower surface do not have the same camber.
Normally these airfoils would not be as stable, but this
can be corrected by bending the trailing edge to produce
the same characteristics as symmetrical airfoils. This is
called “reflexing.” Using this type of rotor blade allows
the rotor system to operate at higher forward speeds.
One of the reasons an asymmetrical rotor blade is not
as stable is that the center of pressure changes with
changes in angle of attack. When the center of pressure
lifting force is behind the pivot point on a rotor blade, it
tends to cause the rotor disc to pitch up. As the angle of
attack increases, the center of pressure moves forward.
If it moves ahead of the pivot point, the pitch of the
rotor disc decreases. Since the angle of attack of the
rotor blades is constantly changing during each cycle
of rotation, the blades tend to flap, feather, lead, and
lag to a greater degree.
When referring to an airfoil, the span is the distance
from the rotor hub to the blade tip. Blade twist refers to
a changing chord line from the blade root to the tip.
Figure 2-2. The upper and lower curvatures are the same on a
symmetrical airfoil and vary on an asymmetrical airfoil.
Asymmetrical
Symmetrical
Lift
Weight□
Drag
Thrust□
Figure 2-1. Four forces acting on a helicopter in forward flight.
Twisting a rotor blade causes it to produce a more even
amount of lift along its span. This is necessary because
rotational velocity increases toward the blade tip. The
leading edge is the first part of the airfoil to meet the
oncoming air. [Figure 2-3] The trailing edge is the aft
portion where the airflow over the upper surface joins
the airflow under the lower surface. The chord line is
an imaginary straight line drawn from the leading to
the trailing edge. The camber is the curvature of the air-
foil’s upper and lower surfaces. The relative wind is the
wind moving past the airfoil. The direction of this wind
is relative to the attitude, or position, of the airfoil and
is always parallel, equal, and opposite in direction to
the flight path of the airfoil. The angle of attack is the
angle between the blade chord line and the direction of
the relative wind.
RELATIVE WIND
Relative wind is created by the motion of an airfoil
through the air, by the motion of air past an airfoil, or by
a combination of the two. Relative wind may be
affected by several factors, including the rotation of the
rotor blades, horizontal movement of the helicopter,
flapping of the rotor blades, and wind speed and direction.
For a helicopter, the relative wind is the flow of air with
respect to the rotor blades. If the rotor is stopped, wind
blowing over the blades creates a relative wind. When
the helicopter is hovering in a no-wind condition, rela-
tive wind is created by the motion of the rotor blades
through the air. If the helicopter is hovering in a wind,
the relative wind is a combination of the wind and the
motion of the rotor blades through the air. When the
helicopter is in forward flight, the relative wind is a
combination of the rotation of the rotor blades and the
forward speed of the helicopter.
BLADE PITCH ANGLE
The pitch angle of a rotor blade is the angle between its
chord line and the reference plane containing the rotor
hub. [Figure 2-4] You control the pitch angle of the blades
with the flight controls. The collective pitch changes each
rotor blade an equal amount of pitch no matter where it is
located in the plane of rotation (rotor disc) and is used to
change rotor thrust. The cyclic pitch control changes the
pitch of each blade as a function of where it is in the plane
of rotation. This allows for trimming the helicopter in
pitch and roll during forward flight and for maneuvering
in all flight conditions.
ANGLE OF ATTACK
When the angle of attack is increased, air flowing over
the airfoil is diverted over a greater distance, resulting
in an increase of air velocity and more lift. As angle of
attack is increased further, it becomes more difficult for
air to flow smoothly across the top of the airfoil. At this
point the airflow begins to separate from the airfoil and
enters a burbling or turbulent pattern. The turbulence
results in a large increase in drag and loss of lift in the
area where it is taking place. Increasing the angle of
attack increases lift until the critical angle of attack is
reached. Any increase in the angle of attack beyond this
point produces a stall and a rapid decrease in lift.
[Figure 2-5]
Angle of attack should not be confused with pitch
angle. Pitch angle is determined by the direction of the
relative wind. You can, however, change the angle of
attack by changing the pitch angle through the use of
the flight controls. If the pitch angle is increased, the
angle of attack is increased, if the pitch angle is
reduced, the angle of attack is reduced. [Figure 2-6]
Axis-of-Rotation—The imaginary
line about which the rotor rotates.
It is represented by a line drawn
through the center of, and perpen-
dicular to, the tip-path plane.
Tip-Path Plane—The imaginary
circular plane outlined by the
rotor blade tips as they make a
cycle of rotation.
Aircraft Pitch—When referenced
to a helicopter, is the movement of
the helicopter about its lateral, or
side to side axis. Movement of the
cyclic forward or aft causes the
nose of the helicopter to move up
or down.
Aircraft Roll—Is the movement of
the helicopter about its longitudi-
nal, or nose to tail axis. Movement
of the cyclic right or left causes the
helicopter to tilt in that direction.
Figure 2-3. Aerodynamic terms of an airfoil.
TrailingR
Edge
Chord Line Angle of
Attack
FLIGHT PATHRELATIVE WIND
UpperR
Camber
LowerR
Camber
LeadingR
Edge
AxishofhRotation
ReferencehPlane
PitchR
Angle
Chord Line
Figure 2-4. Do not confuse the axis of rotation with the rotor
mast. The only time they coincide is when the tip-path plane
is perpendicular to the rotor mast.
LIFT
MAGNUS EFFECT
The explanation of lift can best be explained by looking
at a cylinder rotating in an airstream. The local velocity
near the cylinder is composed of the airstream velocity
and the cylinder’s rotational velocity, which decreases
with distance from the cylinder. On a cylinder, which is
rotating in such a way that the top surface area is rotating
in the same direction as the airflow, the local velocity at
the surface is high on top and low on the bottom.
As shown in figure 2-7, at point “A,” a stagnation point
exists where the airstream line that impinges on the sur-
face splits; some air goes over and some under. Another
stagnation point exists at “B,” where the two air
streams rejoin and resume at identical velocities. We
now have upwash ahead of the rotating cylinder and
downwash at the rear.
The difference in surface velocity accounts for a differ-
ence in pressure, with the pressure being lower on the
top than the bottom. This low pressure area produces
an upward force known as the “Magnus Effect.” This
mechanically induced circulation illustrates the rela-
tionship between circulation and lift.
An airfoil with a positive angle of attack develops air
circulation as its sharp trailing edge forces the rear
stagnation point to be aft of the trailing edge, while the
front stagnation point is below the leading edge.
[Figure 2-8]
BERNOULLI’S PRINCIPLE
Air flowing over the top surface accelerates. The airfoil
is now subjected to Bernoulli’s Principle or the “venturi
effect.” As air velocity increases through the constricted
portion of a venturi tube, the pressure decreases.
AxishofhRotation
ReferencehPlane
PitchR
Angle
Chord Line
Angle of
Attack
RELATIVE WIND
Figure 2-6. Angle of attack may be greater than, less than, or
the same as the pitch angle.
Figure 2-5. As the angle of attack is increased, the separation
point starts near the trailing edge of the airfoil and pro-
gresses forward. Finally, the airfoil loses its lift and a stall
condition occurs.
LIFT□
□
STALL
8ϒ
12-16ϒ Figure 2-7. Magnus Effect is a lifting force produced when a
rotating cylinder produces a pressure differential. This is the
same effect that makes a baseball curve or a golf ball slice.
B A
IncreasedhLocalhVelocityR
(Decreasedhpressure)
DecreasedhLocalhVelocityR
DownwashhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhUpwash
Figure 2-8. Air circulation around an airfoil occurs when the
front stagnation point is below the leading edge and the aft
stagnation point is beyond the trailing edge.
LeadinghEdgeR
StagnationhPoint
TrailinghEdgeR
StagnationhPoint
Steady-State Flight—A condition
when an aircraft is in straight-
and-level, unaccelerated flight,
and all forces are in balance.
ward. According to Newton’s Third Law of Motion,
“for every action there is an equal and opposite reac-
tion,” the air that is deflected downward also produces
an upward (lifting) reaction.
Since air is much like water, the explanation for this
source of lift may be compared to the planing effect of
skis on water. The lift which supports the water skis
(and the skier) is the force caused by the impact pres-
sure and the deflection of water from the lower surfaces
of the skis.
Under most flying conditions, the impact pressure and
the deflection of air from the lower surface of the rotor
blade provides a comparatively small percentage of the
total lift. The majority of lift is the result of decreased
pressure above the blade, rather than the increased
pressure below it.
WEIGHT
Normally, weight is thought of as being a known, fixed
value, such as the weight of the helicopter, fuel, and
occupants. To lift the helicopter off the ground verti-
cally, the rotor system must generate enough lift to
overcome or offset the total weight of the helicopter
and its occupants. This is accomplished by increasing
the pitch angle of the main rotor blades.
The weight of the helicopter can also be influenced by
aerodynamic loads. When you bank a helicopter while
maintaining a constant altitude, the “G” load or load
factor increases. Load factor is the ratio of the load sup-
ported by the main rotor system to the actual weight of
the helicopter and its contents. In steady-state flight ,
the helicopter has a load factor of one, which means the
main rotor system is supporting the actual total weight
of the helicopter. If you increase the bank angle to 60°,
while still maintaining a constant altitude, the load fac-
tor increases to two. In this case, the main rotor system
has to support twice the weight of the helicopter and its
contents. [Figure 2-11]
Disc loading of a helicopter is the ratio of weight to the
total main rotor disc area, and is determined by divid-
ing the total helicopter weight by the rotor disc area,
which is the area swept by the blades of a rotor. Disc
area can be found by using the span of one rotor blade
as the radius of a circle and then determining the area
the blades encompass during a complete rotation. As
the helicopter is maneuvered, disc loading changes.
The higher the loading, the more power you need to
maintain rotor speed.
LeadinghEdgeR
StagnationhPoint
Compare the upper surface of an airfoil with the con-
striction in a venturi tube that is narrower in the middle
than at the ends. [Figure 2-9]
The upper half of the venturi tube can be replaced by
layers of undisturbed air. Thus, as air flows over the
upper surface of an airfoil, the camber of the airfoil
causes an increase in the speed of the airflow. The
increased speed of airflow results in a decrease in pres-
sure on the upper surface of the airfoil. At the same
time, air flows along the lower surface of the airfoil,
building up pressure. The combination of decreased
pressure on the upper surface and increased pressure
on the lower surface results in an upward force.
[Figure 2-10]
As angle of attack is increased, the production of lift is
increased. More upwash is created ahead of the airfoil
as the leading edge stagnation point moves under the
leading edge, and more downwash is created aft of the
trailing edge. Total lift now being produced is perpen-
dicular to relative wind. In summary, the production of
lift is based upon the airfoil creating circulation in the
airstream (Magnus Effect) and creating differential
pressure on the airfoil (Bernoulli’s Principle).
NEWTON’S THIRD LAW OF MOTION
Additional lift is provided by the rotor blade’s lower
surface as air striking the underside is deflected down-
Figure 2-10. Lift is produced when there is decreased pres-
sure above and increased pressure below an airfoil.
Lift
DecreasedhPressure
IncreasedhPressure
IncreasedhVelocityR
DecreasedhPressure
Figure 2-9. The upper surface of an airfoil is similar to the
constriction in a venturi tube.
DRAG
The force that resists the movement of a helicopter
through the air and is produced when lift is developed
is called drag. Drag always acts parallel to the relative
wind. Total drag is composed of three types of drag:
profile, induced, and parasite.
PROFILE DRAG
Profile drag develops from the frictional resistance of
the blades passing through the air. It does not change
significantly with the airfoil’s angle of attack, but
increases moderately when airspeed increases. Profile
drag is composed of form drag and skin friction.
Form drag results from the turbulent wake caused by
the separation of airflow from the surface of a struc-
ture. The amount of drag is related to both the size and
shape of the structure that protrudes into the relative
wind. [Figure 2-12]
Skin friction is caused by surface roughness. Even
though the surface appears smooth, it may be quite
rough when viewed under a microscope. A thin layer of
air clings to the rough surface and creates small eddies
that contribute to drag.
INDUCED DRAG
Induced drag is generated by the airflow circulation
around the rotor blade as it creates lift. The high-pres-
sure area beneath the blade joins the low-pressure air
above the blade at the trailing edge and at the rotor tips.
This causes a spiral, or vortex, which trails behind each
blade whenever lift is being produced. These vortices
deflect the airstream downward in the vicinity of the
blade, creating an increase in downwash. Therefore,
the blade operates in an average relative wind that is
inclined downward and rearward near the blade.
Because the lift produced by the blade is perpendicular
Aircraft Yaw—The movement of
the helicopter about its vertical
axis.
THRUST
Thrust, like lift, is generated by the rotation of the
main rotor system. In a helicopter, thrust can be for-
ward, rearward, sideward, or vertical. The resultant of
lift and thrust determines the direction of movement of
the helicopter.
The solidity ratio is the ratio of the total rotor blade
area, which is the combined area of all the main rotor
blades, to the total rotor disc area. This ratio provides a
means to measure the potential for a rotor system to
provide thrust.
The tail rotor also produces thrust. The amount of
thrust is variable through the use of the antitorque ped-
als and is used to control the helicopter’s yaw.
Figure 2-11. The load factor diagram allows you to calculate
the amount of “G” loading exerted with various angle of
bank.
Load Factor - "G’s"
BankhAngleh(inhDegrees)
v 1vhhhhh2vhhhhh3vhhhh4vhhhhh5vhhhhh6vhhhhh7vhhhhh8vhhhh9v
Figure 2-12. It is easy to visualize the creation of form drag by examining the airflow around a flat plate. Streamlining decreases
form drag by reducing the airflow separation.
to the relative wind, the lift is inclined aft by the same
amount. The component of lift that is acting in a rear-
ward direction is induced drag. [Figure 2-13]
As the air pressure differential increases with an
increase in angle of attack, stronger vortices form, and
induced drag increases. Since the blade’s angle of
attack is usually lower at higher airspeeds, and higher
at low speeds, induced drag decreases as airspeed
increases and increases as airspeed decreases. Induced
drag is the major cause of drag at lower airspeeds.
PARASITE DRAG
Parasite drag is present any time the helicopter is moving
through the air. This type of drag increases with airspeed.
Nonlifting components of the helicopter, such as the
cabin, rotor mast, tail, and landing gear, contribute to par-
asite drag. Any loss of momentum by the airstream, due
to such things as openings for engine cooling, creates
additional parasite drag. Because of its rapid increase
with increasing airspeed, parasite drag is the major cause
of drag at higher airspeeds. Parasite drag varies with the
square of the velocity. Doubling the airspeed increases
the parasite drag four times.
TOTAL DRAG
Total drag for a helicopter is the sum of all three drag
forces. [Figure 2-14] As airspeed increases, parasite
drag increases, while induced drag decreases. Profile
drag remains relatively constant throughout the speed
range with some increase at higher airspeeds.
Combining all drag forces results in a total drag curve.
The low point on the total drag curve shows the air-
speed at which drag is minimized. This is the point
where the lift-to-drag ratio is greatest and is referred to
as L/Dmax. At this speed, the total lift capacity of the
helicopter, when compared to the total drag of the heli-
copter, is most favorable. This is important in helicopter
performance.
Figure 2-14. The total drag curve represents the combined
forces of parasite, profile, and induced drag; and is plotted
against airspeed.
v 25e hhhhhhhhh5ve hhhhhhhhhh75hhhhhhhh1vvhhhhhhhh125hhhhhhhh15v
Speed
Drag
ParasiteR
DragR
ProfileR
Drag
InducedR
Drag
TotalhDrag
MinimumR
DraghorR
L/Dhmax
L/Dmax—The maximum ratio
between total lift (L) and the total
drag (D). This point provides the
best glide speed. Any deviation
from best glide speed increases
drag and reduces the distance you
can glide.
InducedhDrag
Figure 2-13. The formation of induced drag is associated with
the downward deflection of the airstream near the rotor
blade.