an airplane. [Figure 1-12] Therefore, the helicopter has
very little pitch deflection up or down when the helicopter
is stable in a flight mode. The variation from absolutely
level depends on the particular helicopter and the horizontal
stabilizer function.
Increasing collective (power) while maintaining a constant
airspeed induces a climb while decreasing collective causes
a descent. Coordinating these two inputs, down collective
plus aft cyclic or up collective plus forward cyclic, results
in airspeed changes while maintaining a constant altitude.
The pedals serve the same function in both a helicopter
and a fixed-wing aircraft, to maintain balanced flight. This
is done by applying pedal input in whichever direction is
necessary to center the ball in the turn and bank indicator.
Flight maneuvers are discussed in greater detail throughout
Chapter 9, Basic Flight Maneuvers.
Chapter Summary
This chapter gives the reader an overview of the history
of the helicopter, its many uses, and how it has developed
throughout the years. The chapter also introduces basic terms
and explanations of the helicopter components, sections, and
the theory behind how the helicopter flies.
Introduction
This chapter presents aerodynamic fundamentals and
principles as they apply to helicopters. The content relates
to flight operations and performance of normal flight tasks.
It covers theory and application of aerodynamics for the
pilot, whether in flight training or general flight operations.
Aerodynamics of Flight
Chapter 2
Rotor thrustLift component of rotor thrust
Propulsive force
component of
rotor thrust
Drag
WeightResultant of drag and weight
Figure 2-3. Four forces acting on a helicopter in forward flight.
Figure 2-2. Profile of an airfoil.
20´
5´
Figure 2-1. Area of a blade.
Gravity acting on the mass (the amount of matter) of an object
creates a force called weight. The rotor blade below weighs
100 lbs. It is 20 feet long (span) and is 1 foot wide (chord).
Accordingly, its surface area is 20 square feet. [Figure 2-1]
The blade is perfectly balanced on a pinpoint stand, as you
can see in Figure 2-2 from looking at it from the end (the
airfoil view). The goal is for the blade to defy gravity and
stay exactly where it is when we remove the stand. If we do
nothing before removing the stand, the blade will simply fall
to the ground. Can we exert a force (a push or pull) opposite
gravity that equals the 100 lb. weight of the blade? Yes, for
example, electromagnetic force could be used. In helicopters,
however, we use aerodynamic force to oppose weight and
to maneuver.
Every object in the atmosphere is surrounded by a gas that
exerts a static force of 2,116 lb per square foot (a force
times a unit area, called pressure) at sea level. However, that
pressure is exerted equally all over the blade (top and bottom)
and therefore does not create any useful force on the blade.
We need only create a difference of a single pound of static
pressure differential per square foot of blade surface to have a
force equal to the blade’s weight (100 lb of upward pressure
opposite 100 lb downward weight).
Total pressure consists of static pressure and, if the air is
moving, dynamic pressure (a pressure in the direction of the
air movement). As shown in Figure 2-3, if dynamic pressure
is increased the static pressure will decrease. Due to the
design of the airfoil, the velocity of the air passing over the
upper surface will be greater than that of the lower surface,
leading to higher dynamic pressure on the upper surface than
on the lower surface. The higher dynamic pressure on the
upper surface lowers the static pressure on the upper surface.
The static pressure on the bottom will now be greater than
the static pressure on the top. The blade will experience an
upward force. With just the right amount of air passing over
the blade the upward force will equal one pound per square
foot. This upward force is equal to, and acts opposite the
blade’s weight of 100 lb. So, if we now remove the stand, the
blade will defy gravity and remain in its position (ignoring
rearward drag for the moment).
The force created by air moving over an object (or moving
an object through the air) is called aerodynamic force. Aero
means air. Dynamic means moving or motion. Accordingly,
by moving the air over an airfoil we can change the static
pressures on the top and bottom thereby generating a useful
force (an aerodynamic force). The portion of the aerodynamic
force that is usually measured perpendicular to the air flowing
around the airfoil is called lift and is used to oppose weight.
Drag is the portion of aerodynamic force that is measured
as the resistance created by an object passing through the air
(or having the air passed over it). Drag acts in a streamwise
direction with the wind passing over the airfoil and retards
forward movement.
Forces Acting on the Aircraft
Once a helicopter leaves the ground, it is acted upon by
four aerodynamic forces; thrust, drag, lift, and weight.
Understanding how these forces work and knowing how to
control them with the use of power and flight controls are
essential to flight. [Figure 2-3] They are defined as follows:
Increased air
pressure underneath
Reduced air pressure Upper camber helps
to deflect air down
Mass of air deflected down
Figure 2-4. Production of lift.
• Lift—opposes the downward force of weight, is
produced by the dynamic effect of the air acting on the
airfoil and acts perpendicular to the flightpath through
the center of lift.
• Weight—the combined load of the aircraft itself, the
crew, the fuel, and the cargo or baggage. Weight 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).
• Thrust—the force produced by the power plant/
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. Drag opposes thrust and acts
rearward parallel to the relative wind.
For a more in-depth explanation of general aerodynamics,
refer to the Pilot’s Handbook of Aeronautical Knowledge.
Lift
Lift is generated when an object changes the direction of
flow of a fluid or when the fluid is forced to move by the
object passing through it. When the object and fluid move
relative to each other and the object turns the fluid flow in
a direction perpendicular to that flow, the force required to
do this work creates an equal and opposite force that is lift.
The object may be moving through a stationary fluid, or the
fluid may be flowing past a stationary object—these two are
effectively identical as, in principle, it is only the frame of
reference of the viewer which differs. The lift generated by
an airfoil depends on such factors as:
• Speed of the airflow
• Density of the air
• Total area of the segment or airfoil
• Angle of attack (AOA) between the air and the airfoil
The AOA is the angle at which the airfoil meets the oncoming
airflow (or vice versa). In the case of a helicopter, the object
is the rotor blade (airfoil) and the fluid is the air. Lift is
produced when a mass of air is deflected, and it always acts
perpendicular to the resultant relative wind. A symmetric
airfoil must have a positive AOA to generate positive lift. At
a zero AOA, no lift is generated. At a negative AOA, negative
lift is generated. A cambered or nonsymmetrical airfoil may
produce positive lift at zero, or even small negative AOA.
The basic concept of lift is simple. However, the details of how
the relative movement of air and airfoil interact to produce
the turning action that generates lift are complex. In any case
causing lift, an angled flat plate, revolving cylinder, airfoil,
etc., the flow meeting the leading edge of the object is forced to
split over and under the object. The sudden change in direction
over the object causes an area of low pressure to form behind
the leading edge on the upper surface of the object. In turn,
due to this pressure gradient and the viscosity of the fluid,
the flow over the object is accelerated down along the upper
surface of the object. At the same time, the flow forced under
the object is rapidly slowed or stagnated causing an area of
high pressure. This also causes the flow to accelerate along
the upper surface of the object. The two sections of the fluid
each leave the trailing edge of the object with a downward
component of momentum, producing lift. [Figure 2-4]
Bernoulli’s Principle
Bernoulli’s principle describes the relationship between
internal fluid pressure and fluid velocity. It is a statement
of the law of conservation of energy and helps explain why
an airfoil develops an aerodynamic force. The concept of
conservation of energy states energy cannot be created or
destroyed and the amount of energy entering a system must
also exit. Specifically, in this case the “energy” referred
to is the dynamic pressure (the kinetic energy of the air—
more velocity, more kinetic energy) and static air pressure
(potential energy). These will change among themselves, but
the total pressure energy remains constant inside the tube.
A simple tube with a constricted portion near the center of its
length illustrates this principle. An example is running water
through a garden hose. The mass of flow per unit area (cross-
sectional area of tube) is the mass flow rate. In Figure 2-5,
the flow into the tube is constant, neither accelerating nor
decelerating; thus, the mass flow rate through the tube must
be the same at stations 1, 2, and 3. If the cross-sectional area
at any one of these stations—or any given point—in the
tube is reduced, the fluid velocity must increase to maintain
a constant mass flow rate to move the same amount of fluid
through a smaller area. The continuity of mass flow causes
the air to move faster through the venturi. In other words,
fluid speeds up in direct proportion to the reduction in area.
WATER INPUT WATER OUTPUT
Station 1
Station 2
Station 3
Velocity increased
Static pressure decreased
(compared to original)
Same mass of air
Mass of air Cross-section of cylinder
PTotal = PDynamic + PStatic
34 PSF PD 41 PSF PD
2116 PSF PS
2109 PSF PS
Point 1 Point 2
PT = 2150 PSF
PD = 34 PSF
PS = 2116 PSF
V = 100 KTS
Point 1
PT = 2150 PSF
PD = 41 PSF
PS = 2109 PSF
V = 120 KTS
Point 2
Figure 2-5. Water flow through a tube.
Figure 2-6. Venturi effect.
Bernoulli (Ptotal = Pdynamic + Pstatic) states that the increase
in velocity will increase the streamwise dynamic pressure.
Since the total pressure in the tube must remain constant,
the static pressure on the sides of the venturi will decrease.
Venturi effect is the term used to describe this phenomenon.
Figure 2-6 illustrates plates of one square foot in the dynamic
flow and on the sides of the tube indicating static pressure,
with corresponding pressure. At point 2, it is easier to
visualize the static pressure reduction on the top of the airfoil
as compared to the bottom of the airfoil, which is depicted as
outside of the tube and therefore at ambient static pressure.
Keep in mind with actual blades it is not a simple as this
example because the bottom static pressure is influenced by
blade design and blade angle, among other things. However,
the basic idea is that it is the static pressure differential
between the top and bottom multiplied by the surface area
of the blade that generates the aerodynamic force.
Venturi Flow
While the amount of total energy within a closed system (the
tube) does not change, the form of the energy may be altered.
Pressure of flowing air may be compared to energy in that the
total pressure of flowing air always remains constant unless
energy is added or removed. Fluid flow pressure has two
components—static and dynamic pressure. Static pressure
is the pressure component measured in the flow but not
moving with the flow as pressure is measured. Static pressure
is also known as the force per unit area acting on a surface.
Dynamic pressure of flow is that component existing as a
result of movement of the air. The sum of these two pressures
is total pressure. As air flows through the constriction, static
pressure decreases as velocity increases. This increases
dynamic pressure. Figure 2-7 depicts the bottom half of the
constricted area of the tube, which resembles the top half of
an airfoil. Even with the top half of the tube removed, the air
0° 10° 20° 30° 40° 50° 60° 70° 80° 90°
Load factor - (in Gs)
Bank angle (in degrees)
Station 1
Station 2
Station 3
Upper layers act to restrict flow
Figure 2-7. Venturi flow.
Figure 2-8. The load factor diagram allows a pilot to calculate
the amount of “G” loading exerted with various angles of bank.
still accelerates over the curved area because the upper air
layers restrict the flow—just as the top half of the constricted
tube did. This acceleration causes decreased static pressure
above the curved portion and creates a pressure differential
caused by the variation of static and dynamic pressures.
Newton’s Third Law of Motion
Additional lift is provided by the rotor blade’s lower surface
as air striking the underside is deflected downward. According
to Newton’s Third Law of Motion, “for every action there
is an equal and opposite reaction,” 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 that supports the water skis (and the skier) is the force
caused by the impact pressure 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 vertically, the rotor disk must
generate enough lift to overcome or offset the total weight of
the helicopter and its occupants. Newton’s First Law states:
“Every object in a state of uniform motion tends to remain
in that state of motion unless an external force is applied
to it.” In this case, the object is the helicopter whether at a
hover or on the ground and the external force applied to it
is lift, which is accomplished by increasing the pitch angle
of the main rotor blades. This action forces the helicopter
into a state of motion, without it the helicopter would either
remain on the ground or at a hover.
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. The load factor is the actual load on the rotor
blades at any time, divided by the normal load or gross
weight (weight of the helicopter and its contents). Any time
a helicopter flies in a constant altitude curved flightpath, the
load supported by the rotor blades is greater than the total
weight of the helicopter. The tighter the curved flightpath
is, the steeper the bank is; the more rapid the flare or pullout
from a dive is, the greater the load supported by the rotor.
Therefore, the greater the load factor must be. [Figure 2-8]
To overcome this additional load factor, the helicopter must
be able to produce more lift. If excess engine power is not
available, the helicopter either descends or has to decelerate in
order to maintain the same altitude. The load factor and, hence,