FLAT PLATE
SPHERE
SPHERE WITH
A FAIRING
SPHERE INSIDE
A HOUSING
Form drag
Figure 2-9. 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.
apparent gross weight increase is relatively small in banks up
to 30°. Even so, under the right set of adverse circumstances,
such as high-density altitude, turbulent air, high gross weight,
and poor pilot technique, sufficient or excess power may not
be available to maintain altitude and airspeed. Pilots must take
all of these factors into consideration throughout the entire
flight from the point of ascending to a hover to landing. Above
30° of bank, the apparent increase in gross weight soars. At
30° of bank, or pitch, the apparent increase is only 16 percent,
but at 60°, it is twice the load on the wings and rotor disk.
For example, if the weight of the helicopter is 1,600 pounds,
the weight supported by the rotor disk in a 30° bank at a
constant altitude would be 1,856 pounds (1,600 + 16 percent
(or 256)). In a 60° bank, it would be 3,200 pounds; in an 80°
bank, it would be almost six times as much, or 8,000 pounds.
It is important to note that each rotor blade must support a
percentage of the gross weight. In a two-bladed system, each
blade of the 1,600-pound helicopter as stated above would
have to lift 50 percent or 800 pounds. If this same helicopter
had three rotor blades, each blade would have to lift only 33
percent, or 533 pounds. One additional cause of large load
factors is rough or turbulent air. The severe vertical gusts
produced by turbulence can cause a sudden increase in AOA,
resulting in increased rotor blade loads that are resisted by the
inertia of the helicopter.
Each type of helicopter has its own limitations that are based
on the aircraft structure, size, and capabilities. Regardless
of how much weight one can carry or the engine power
that it may have, they are all susceptible to aerodynamic
overloading. Unfortunately, if the pilot attempts to push
the performance envelope the consequence can be fatal.
Aerodynamic forces effect every movement in a helicopter,
whether it is increasing the collective or a steep bank
angle. Anticipating results from a particular maneuver or
adjustment of a flight control is not good piloting technique.
Instead pilots need to truly understand the capabilities of the
helicopter under any and all circumstances and plan never to
exceed the flight envelope for any situation.
Thrust
Thrust, like lift, is generated by the rotation of the main
rotor disk. In a helicopter, thrust can be forward, rearward,
sideward, or vertical. The resultant 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 disk area. This ratio provides a means to measure
the potential for a rotor disk to provide thrust and lift. The
mathematical calculations needed to calculate the solidity ratio
for each helicopter may not be of importance to most pilots but
what should be are the capabilities of the rotor disk to produce
and maintain lift. Many helicopter accidents are caused from
the rotor disk being overloaded. Simply put, pilots attempt
maneuvers that require more lift than the rotor disk can
produce or more power than the helicopter’s powerplant can
provide. Trying to land with a nose high attitude along with
any other unfavorable condition (i.e., high gross weight or
wind gusts) is most likely to end in disaster.
The tail rotor also produces thrust. The amount of thrust is
variable through the use of the antitorque pedals and is used
to control the helicopter’s yaw.
Drag
The force that resists the movement of a helicopter through the
air and is produced when lift is developed is called drag. Drag
must be overcome by the engine to turn the rotor. 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 AOA 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 structure.
The amount of drag is related to both the size and shape of the
structure that protrudes into the relative wind. [Figure 2-9]
Drag
Forward speed
Total drag Parasite drag
Induced drag
Profile drag
Average relative wind
Total lift
Vertical lift
Induced drag
Figure 2-10. The formation of induced drag is associated with the
downward deflection of the airstream near the rotor blade.
Figure 2-11. The total drag curve represents the combined forces of
parasite, profile, and induced drag and is plotted against airspeed.
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-pressure area
beneath the blade joins the low-pressure area 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
to the relative wind, the lift is inclined aft by the same amount.
The component of lift that is acting in a rearward direction
is induced drag. [Figure 2-10]
As the air pressure differential increases with an increase in
AOA, stronger vortices form, and induced drag increases.
Since the blade’s AOA 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.
Non-lifting components of the helicopter, such as the cabin,
rotor mast, tail, and landing gear, contribute to parasite 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; therefore,
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 2-11] 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
airspeed 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 helicopter, is most favorable.
This is an important factor in helicopter performance.
Airfoil
Helicopters are able to fly due to aerodynamic forces
produced when air passes around the airfoil. An airfoil is
any surface producing more lift than drag when passing
through the air at a suitable angle. Airfoils are most often
associated with production of lift. Airfoils are also used for
stability (fin), control (elevator), and thrust or propulsion
(propeller or rotor). Certain airfoils, such as rotor blades,
combine some of these functions. The main and tail rotor
blades of the helicopter are airfoils, and air is forced to pass
around the blades by mechanically powered rotation. In
some conditions, parts of the fuselage, such as the vertical
and horizontal stabilizers, can become airfoils. Airfoils are
carefully structured to accommodate a specific set of flight
characteristics.
Airfoil Terminology and Definitions
• Blade span—the length of the rotor blade from center
of rotation to tip of the blade.
Camber of upper surface
Camber of lower surface
Trailing edge
Leading edge
Mean camber line
Chord line
Nonsymmetrical
Symmetrical
Figure 2-12. Aerodynamic terms of an airfoil.
Figure 2-13. The upper and lower curvatures are the same on a
symmetrical airfoil and vary on a nonsymmetrical airfoil.
• Chord line—a straight line intersecting leading and
trailing edges of the airfoil. [Figure 2-12]
• Chord—the length of the chord line from leading edge
to trailing edge; it is the characteristic longitudinal
dimension of the airfoil section.
• Mean camber line—a line drawn halfway between the
upper and lower surfaces of the airfoil. [Figure 2-12]
The chord line connects the ends of the mean camber
line. Camber refers to curvature of the airfoil and
may be considered as curvature of the mean camber
line. The shape of the mean camber is important for
determining aerodynamic characteristics of an airfoil
section. Maximum camber (displacement of the mean
camber line from the chord line) and its location help
to define the shape of the mean camber line. The
location of maximum camber and its displacement
from the chord line are expressed as fractions or
percentages of the basic chord length. By varying the
point of maximum camber, the manufacturer can tailor
an airfoil for a specific purpose. The profile thickness
and thickness distribution are important properties of
an airfoil section.
• Leading edge—the front edge of an airfoil.
[Figure 2-12]
• Flightpath velocity—the speed and direction of
the airfoil passing through the air. For airfoils on
an airplane, the flightpath velocity is equal to true
airspeed (TAS). For helicopter rotor blades, flightpath
velocity is equal to rotational velocity, plus or minus
a component of directional airspeed. The rotational
velocity of the rotor blade is lowest closer to the hub
and increases outward towards the tip of the blade
during rotation.
• Relative wind—defined as the airflow relative to
an airfoil and is created by movement of an airfoil
through the air. This is rotational relative wind for
rotary-wing aircraft and is covered in detail later. As
an induced airflow may modify flightpath velocity,
relative wind experienced by the airfoil may not be
exactly opposite its direction of travel.
• Trailing edge—the rearmost edge of an airfoil.
• Induced flow—the downward flow of air through the
rotor disk.
• Resultant relative wind—relative wind modified by
induced flow.
• AOA—the angle measured between the resultant
relative wind and chord line.
• Angle of incidence (AOI)—the angle between the
chord line of a blade and rotor hub. It is usually
referred to as blade pitch angle. For fixed airfoils,
such as vertical fins or elevators, angle of incidence
is the angle between the chord line of the airfoil and
a selected reference plane of the helicopter.
• Center of pressure—the point along the chord line of
an airfoil through which all aerodynamic forces are
considered to act. Since pressures vary on the surface
of an airfoil, an average location of pressure variation is
needed. As the AOA changes, these pressures change,
and the center of pressure moves along the chord line.
Airfoil Types
Symmetrical Airfoil
The symmetrical airfoil is distinguished by having identical
upper and lower surfaces. [Figure 2-13] The mean camber
line and chord line are the same on a symmetrical airfoil,
and it produces no lift at zero AOA. Most light helicopters
incorporate symmetrical airfoils in the main rotor blades.
Nonsymmetrical Airfoil (Cambered)
The nonsymmetrical airfoil has different upper and lower
surfaces, with a greater curvature of the airfoil above the
chord line than below. [Figure 2-13] The mean camber line
and chord line are different. The nonsymmetrical airfoil design
can produce useful lift at zero AOA. A nonsymmetrical design
Section near rootA
Section in centerB
Section near tipC
A B C
Tip
Trim tab
Root
Note: “More nose-down” tilt to blade section closer to tip
Figure 2-14. Blade twist.
has advantages and disadvantages. The advantages are more
lift production at a given AOA than a symmetrical design,
an improved lift-to-drag ratio, and better stall characteristics.
The disadvantages are center of pressure travel of up to 20
percent of the chord line (creating undesirable torque on the
airfoil structure) and greater production costs.
Blade Twist
Because of lift differential due to differing rotational relative
wind values along the blade, the blade should be designed
with a twist to alleviate internal blade stress and distribute
the lifting force more evenly along the blade. Blade twist
provides higher pitch angles at the root where velocity is
low and lower pitch angles nearer the tip where velocity
is higher. This increases the induced air velocity and blade
loading near the inboard section of the blade. [Figure 2-14]
Rotor Blade and Hub Definitions
• Hub—on the mast, the attaching point for the root of
the blade, and the axis about which the blades rotate.
[See Figure 1-7]
• Tip—the farthest outboard section of the rotor blade
• Root—the inner end of the blade and is the point that
attaches to the hub
• Twist—the change in blade incidence from the root
to the outer blade
The angular position of the main rotor blades (as viewed from
above, as they rotate about the vertical axis of the mast) is
measured from the helicopter’s longitudinal axis, and usually
from its nose. The radial position of a segment of the blade is
the distance from the hub as a fraction of the total distance.
Airflow and Reactions in the Rotor Disk
Relative Wind
Knowledge of relative wind is essential for an understanding
of aerodynamics and its practical flight application for the
pilot. Relative wind is airflow relative to an airfoil. Movement
of an airfoil through the air creates relative wind. Relative
wind moves in a direction parallel to but opposite of the
movement of the airfoil. [Figure 2-15]
There are two parts to wind passing a rotor blade:
• Horizontal part—caused by the blades turning
plus movement of the helicopter through the air
[Figure 2-16]
• Vertical part—caused by the air being forced down
through the rotor blades plus any movement of the air
relative to the blades caused by the helicopter climbing
or descending [Figures 2-17 and 2-18]
Rotational Relative Wind (Tip-Path Plane)
The rotation of rotor blades as they turn about the mast
produces rotational relative wind (tip-path plane). The
term rotational refers to the method of producing relative
wind. Rotational relative wind flows opposite the physical
flightpath of the airfoil, striking the blade at 90° to the
leading edge and parallel to the plane of rotation; and it is
constantly changing in direction during rotation. Rotational
relative wind velocity is highest at blade tips, decreasing
uniformly to zero at the axis of rotation (center of the
mast). [Figure 2-19]
Results in
Results in
Results in
Relative wind
Relative wind
Relative wind
Airfoil direction
Airfoil direction
Airfoil direction
Results in
Results in
Results in
Relative wind
Relative wind
Relative wind
Airfoil direction
Airfoil direction
Airfoil direction
Results in
Results in
Results in
Relative wind
Relative wind
Relative wind
Airfoil direction
Airfoil direction
Airfoil direction
Airspeed knots
Axis of rotation
Figure 2-15. Relative wind.
Figure 2-16. Horizontal component of relative wind.
Figure 2-17. Induced flow.
Figure 2-18. Normal induced flow velocities along the blade span
during hovering flight. Downward velocity is highest at the blade
tip where blade speed is highest. As blade speed decreases nearer
the center of the disk, downward velocity is less.
Center of pressure
Rotational relative wind
Flightpath of airfoil
Chord line
Angle of incidence
Center of pressure
Chord line
Angle of attack
Rotational relative wind
Induced flow
Resultant relative wind
10–20 knotsMore horizontal
flow of air
Downward velocity of air molecules used by aft section of rotor
Fore
Fore
Aft
Aft
Rotational relative
wind
Less induced flow
Angle of attack
Resultant relative
wind
Chord line
Rotational relative
wind
Greater induced flow
Angle of attack
Resultant relative wind
Chord line
Figure 2-19. Rotational relative wind. Figure 2-20. Resultant relative wind.
Figure 2-21. A helicopter in forward flight, or hovering with a headwind or crosswind, has more molecules of air entering the aft portion
of the rotor disk. Therefore, at the rear of the rotor disk, the angle of attack is less and the induced flow is greater.
forward velocity results in decreased induced flow velocity.
This change results in an improved efficiency (additional lift)
being produced from a given blade pitch setting.
Induced Flow (Downwash)
At flat pitch, air leaves the trailing edge of the rotor blade
in the same direction it moved across the leading edge; no
lift or induced flow is being produced. As blade pitch angle
is increased, the rotor disk induces a downward flow of air
through the rotor blades creating a downward component of
air that is added to the rotational relative wind. Because the
blades are moving horizontally, some of the air is displaced
downward. The blades travel along the same path and pass a
given point in rapid succession. Rotor blade action changes the
still air to a column of descending air. Therefore, each blade has
a decreased AOA due to the downwash. This downward flow
of air is called induced flow (downwash). It is most pronounced
at a hover under no-wind conditions. [Figure 2-21]
Resultant Relative Wind
The resultant relative wind at a hover is rotational relative
wind modified by induced flow. This is inclined downward
at some angle and opposite the effective flightpath of the
airfoil, rather than the physical flightpath (rotational relative
wind). The resultant relative wind also serves as the reference
plane for development of lift, drag, and total aerodynamic
force (TAF) vectors on the airfoil. [Figure 2-20] When the
helicopter has horizontal motion, airspeed further modifies
the resultant relative wind. The airspeed component of
relative wind results from the helicopter moving through
the air. This airspeed component is added to, or subtracted
from, the rotational relative wind depending on whether
the blade is advancing or retreating in relation to helicopter
movement. Introduction of airspeed relative wind also
modifies induced flow. Generally, the downward velocity
of induced flow is reduced. The pattern of air circulation
through the disk changes when the aircraft has horizontal
motion. As the helicopter gains airspeed, the addition of
Lift vector inclined well to rear
Lift
Induced flow velocity= 60 ft/sec
Axis of rotation
α = 10°
Large blade pitch angle
Pitch angle = 18°
Induced flow velocity = 60 ft/sec Large blade-tip vortexes
V+
P−
MAX VELOCITY =
120 ft/sec
Figure 2-22. In ground effect (IGE).
In Ground Effect (IGE)
Ground effect is the increased efficiency of the rotor
disk caused by interference of the airflow when near the
ground. The air pressure or density is increased, which
acts to decrease the downward velocity of air. Ground
effect permits relative wind to be more horizontal, lift
vector to be more vertical, and induced drag to be reduced.
These conditions allow the rotor disk to be more efficient.
Maximum ground effect is achieved when hovering over
smooth hard surfaces. When hovering over surfaces as tall
grass, trees, bushes, rough terrain, and water, maximum
ground effect is reduced. Rotor efficiency is increased
by ground effect to a height of about one rotor diameter
(measured from the ground to the rotor disk) for most
helicopters. Since the induced flow velocities are decreased,
the AOA is increased, which requires a reduced blade pitch
angle and a reduction in induced drag. This reduces the
power required to hover IGE. [Figure 2-22]
Out of Ground Effect (OGE)
The benefit of placing the helicopter near the ground is lost
above IGE altitude. Above this altitude, the power required
to hover remains nearly constant, given similar conditions
(such as wind). Induced flow velocity is increased, resulting
in a decrease in AOA and a decrease in lift. Under the correct
circumstances, this downward flow can become so localized
that the helicopter and locally disturbed air will sink at
alarming rates. This effect is called vortex ring state (formerly
referenced as settling-with-power) and is discussed at length
in Chapter 11, Helicopter Emergencies and Hazards. A higher
blade pitch angle is required to maintain the same AOA as in
IGE hover. The increased pitch angle also creates more drag.
This increased pitch angle and drag requires more power to
hover OGE than IGE. [Figure 2-23]
Rotor Blade Angles
There are two angles that enable a rotor disk to produce the
lift required for a helicopter to fly: angle of incidence and
angle of attack.
Angle of Incidence
Angle of incidence is the angle between the chord line of a
main or tail rotor blade and its rotor disk. It is a mechanical
angle rather than an aerodynamic angle and is sometimes
referred to as blade pitch angle. [Figure 2-24] In the absence
of induced flow, AOA and angle of incidence are the same.
Whenever induced flow, up flow (inflow), or airspeed modifies
the relative wind, the AOA is different from the angle of
incidence. Collective input and cyclic feathering (see page
2-12) change the angle of incidence. A change in the angle of
incidence changes the AOA, which changes the coefficient of
lift, thereby changing the lift produced by the airfoil.