Figure 2-38. To compensate for blowback, you must move the
cyclic forward.
Figure 2-37. The combined upward flapping (reduced lift) of the advancing blade and downward flapping (increased lift) of the retreating
blade equalizes lift across the main rotor disk, counteracting dissymmetry of lift.
Angle of attack at 3 o’clock positionA
Upflap velocity
Resultant relative wind
Chord line
Angle of attack over noseB
Resultant relative wind
Chord line
Angle of attack at 9 o’clock positionC
Downflap velocity
Resultant relative wind
Chord line
Angle of attack over tailD
Resultant relative wind
Chord line
Blade rotation
or surface wind enters the rotor disk, turbulence and vortices
are left behind and the flow of air becomes more horizontal.
In addition, the tail rotor becomes more aerodynamically
efficient during the transition from hover to forward flight.
Figures 2-39 and 2-40 show the different airflow patterns
at different speeds and how airflow affects the efficiency of
the tail rotor.
Figure 2-38 illustrates the tilting forward of the rotor disk,
which is the result of a change in pitch angle with forward
cyclic. At a hover, the cyclic is centered and the pitch angle on
the advancing and retreating blades is the same. At low forward
speeds, moving the cyclic forward reduces pitch angle on the
advancing blade and increases pitch angle on the retreating
blade. This causes a slight rotor disk tilt. At higher forward
speeds, the pilot must continue to move the cyclic forward.
This further reduces pitch angle on the advancing blade and
further increases pitch angle on the retreating blade. As a result,
there is even more tilt to the rotor disk than at lower speeds.
A horizontal lift component (thrust) generates higher
helicopter airspeed. The higher airspeed induces blade
flapping to maintain symmetry of lift. The combination of
flapping and cyclic feathering maintains symmetry of lift and
desired attitude on the rotor disk and helicopter.
Translational Lift
Improved rotor efficiency resulting from directional flight is
called translational lift. The efficiency of the hovering rotor
disk is greatly improved with each knot of incoming wind
gained by horizontal movement of the aircraft or surface
wind. As the incoming wind produced by aircraft movement
Downward velocity of air molecules used by aft section of rotor
1–5 knots
10–15 knots
Airflow pattern just prior to effective translational lift
Figure 2-39. The airflow pattern for 1–5 knots of forward airspeed. Note how the downwind vortex is beginning to dissipate and induced
flow down through the rear of the rotor disk is more horizontal.
Figure 2-40. An airflow pattern at a speed of 10–15 knots. At this increased airspeed, the airflow continues to become more horizontal.
The leading edge of the downwash pattern is being overrun and is well back under the nose of the helicopter.
As speed increases, translational lift becomes more effective,
nose rises or pitches up, and aircraft rolls to the right.
The combined effects of dissymmetry of lift, gyroscopic
precession, and transverse flow effect cause this tendency.
It is important to understand these effects and anticipate
correcting for them. Once the helicopter is transitioning
through ETL, the pilot needs to apply forward and left
lateral cyclic input to maintain a constant rotor-disk attitude.
[Figure 2-41]
Translational Thrust
Translational thrust occurs when the tail rotor becomes more
aerodynamically efficient during the transition from hover
to forward flight. As the tail rotor works in progressively
less turbulent air, this improved efficiency produces more
antitorque thrust, causing the nose of the aircraft to yaw left
Effective Translational Lift (ETL)
While transitioning to forward flight at about 16 to 24 knots,
the helicopter goes through effective translational lift (ETL).
As mentioned earlier in the discussion on translational lift,
the rotor blades become more efficient as forward airspeed
increases. Between 16 and 24 knots, the rotor disk completely
outruns the recirculation of old vortices and begins to work
in relatively undisturbed air. The flow of air through the rotor
disk is more horizontal, which reduces induced flow and
drag with a corresponding increase in angle of attach and lift.
The additional lift available at this speed is referred to as the
ETL, which makes the rotor disk operate more efficiently.
This increased efficiency continues with increased airspeed
until the best climb airspeed is reached, and total drag is at
its lowest point.
Helicopter movement
Thrust
Drag
Weight
Lift
Resultant
Resultant
Figure 2-42. Forces acting on the helicopter during sideward flight.
Figure 2-41. Effective translational lift is easily recognized in actual
flight by a transient induced aerodynamic vibration and increased
performance of the helicopter.
16–24 knots
No recirculation
of air
More horizontal
flow of air
Reduced induced flow
increases angle of attack
Tail rotor operates in
relatively clean air
Transverse flow effect is recognized by increased vibrations
of the helicopter at airspeeds around 12 to 15 knots and can
be produced by forward flight or from the wind while in a
hover. This vibration happens at an airspeed just below ETL
on takeoff and after passing through ETL during landing. The
vibration happens close to the same airspeed as ETL because
that is when the greatest lift differential exists between the
front and rear portions of the rotor system. As such, some
pilots confuse the vibration felt by transverse flow effect with
passing through ETL. To counteract transverse flow effect,
a cyclic input to the left may be needed.
Sideward Flight
In sideward flight, the tip-path plane is tilted in the direction
that flight is desired. This tilts the total lift-thrust vector
sideward. In this case, the vertical or lift component is still
straight up and weight straight down, but the horizontal or
thrust component now acts sideward with drag acting to the
opposite side. [Figure 2-42]
Sideward flight can be a very unstable condition due to the
parasitic drag of the fuselage combined with the lack of
horizontal stabilizer for that direction of flight. Increased
altitudes help with control and the pilot must always scan in
the direction of flight. Movement of the cyclic in the intended
direction of flight causes the helicopter to move, controls the
rate of speed, and ground track, but the collective and pedals
are key to successful sideward flight. Just as in forward flight,
the collective keeps the helicopter from contacting the ground
and the pedals help maintain the correct heading; even in
sideward flight, the tail of the helicopter should remain behind
you. Inputs to the cyclic should be smooth and controlled,
and the pilot should always be aware of the tip-path plane in
relation to the ground. [Figure 2-43]
(with a main rotor turning counterclockwise) and forces the
pilot to apply right pedal (decreasing the AOA in the tail
rotor blades) in response. In addition, during this period, the
airflow affects the horizontal components of the stabilizer
found on most helicopters which tends to bring the nose of
the helicopter to a more level attitude.
Induced Flow
As the rotor blades rotate, they generate what is called
rotational relative wind. This airflow is characterized as
flowing parallel and opposite the rotor’s plane of rotation
and striking perpendicular to the rotor blade’s leading edge.
This rotational relative wind is used to generate lift. As
rotor blades produce lift, air is accelerated over the foil and
projected downward. Anytime a helicopter is producing lift,
it moves large masses of air vertically and down through the
rotor disk. This downwash or induced flow can significantly
change the efficiency of the rotor disk. Rotational relative
wind combines with induced flow to form the resultant
relative wind. As induced flow increases, resultant relative
wind becomes less horizontal. Since AOA is determined
by measuring the difference between the chord line and the
resultant relative wind, as the resultant relative wind becomes
less horizontal, AOA decreases. [See Figure 2-21]
Transverse Flow Effect
As the helicopter accelerates in forward flight, induced flow
drops to near zero at the forward disk area and increases at the
aft disk area. These differences in lift between the fore and
aft portions of the rotor disk are called transverse flow effect.
[Figure 2-41] This increases the AOA at the front disk area
causing the rotor blade to flap up and reduces AOA at the aft
disk area causing the rotor blade to flap down. Because the
rotor acts like a gyro, maximum displacement occurs 90° in the
direction of rotation. The result is a tendency for the helicopter to
roll slightly to the right as it accelerates through approximately
20 knots or if the headwind is approximately 20 knots.
Helicopter movement
Thrust
Drag
Weight Lift
Resultant
Resultant
Downward force from
the horizontal stabilizer
Ground track required
Forward reference
Side reference
Figure 2-44. Forces acting on the helicopter during rearward flight.
Figure 2-43. Forces acting on the helicopter during sideward flight.
Contacting the ground with the skids during sideward flight
will most likely result in a dynamic rollover event before the
pilot has a chance to react. Extreme caution should be used
when maneuvering the helicopter sideways to avoid such
hazards from happening. Refer to Chapter 11, Helicopter
Hazards and Emergencies.
Rearward Flight
For rearward flight, the tip-path plane is tilted rearward,
which, in turn, tilts the lift-thrust vector rearward. Drag now
acts forward with the lift component straight up and weight
straight down. [Figure 2-44]
Pilots must be aware of the hazards of rearward flight.
Because of the position of the horizontal stabilizer, the tail
end of the helicopter tends to pitch downward in rearward
flight, causing the probability of hitting the ground to be
greater than in forward flight. Another factor to consider
in rearward flight is skid design. Most helicopter skids are
not turned upward in the back, and any contact with the
ground during rearward flight can put the helicopter in an
uncontrollable position leading to tail rotor contact with the
ground. Pilots must do a thorough scan of the area before
attempting to hover rearward, looking for obstacles and
terrain changes. Slower airspeeds can help mitigate risk and
maintain a higher-than-normal hover altitude.
Turning Flight
In forward flight, the rotor disk is tilted forward, which also
tilts the total lift-thrust force of the rotor disk forward. When
the helicopter is banked, the rotor disk is tilted sideward
resulting in lift being separated into two components. Lift
acting upward and opposing weight is called the vertical
component of lift. Lift acting horizontally and opposing
inertia (centrifugal force) is the horizontal component of lift
(centripetal force). [Figure 2-45]
As the angle of bank increases, the total lift force is tilted more
toward the horizontal, thus causing the rate of turn to increase
because more lift is acting horizontally. Since the resultant
lifting force acts more horizontally, the effect of lift acting
Normal Powered Flight Autorotation
Direction of flight
Direction of flight
Figure 2-46. During an autorotation, the upward flow of relative wind permits the main rotor blades to rotate at their normal speed. In
effect, the blades are “gliding” in their rotational plane.
Figure 2-45. Forces acting on the helicopter during turning flight.
90 80
90 80
Centrifugal force (inertia)
Weight
Resultant lift
Centripetal force
(horizontal component of lift)
Vertical
component
of lift
Bank
angle
vertically is decreased. To compensate for this decreased
vertical lift, the AOA of the rotor blades must be increased in
order to maintain altitude. The steeper the angle of bank is,
the greater the AOA of the rotor blades required to maintain
altitude. Thus, with an increase in bank and a greater AOA, the
resultant lifting force increases, and the rate of turn is higher.
Simply put, collective pitch must be increased in order to
maintain altitude and airspeed while turning. Collective pitch
controls the angle of incidence and along with other factors,
determines the overall AOA in the rotor disk.
Autorotation
Autorotation is the state of flight where the main rotor disk
of a helicopter is being turned by the action of air moving
up through the rotor rather than engine power driving
the rotor. In normal, powered flight, air is drawn into the
main rotor disk from above and exhausted downward, but
during autorotation, air moves up into the rotor disk from
below as the helicopter descends. Autorotation is permitted
mechanically by a freewheeling unit, which is a special
clutch mechanism that allows the main rotor to continue
turning even if the engine is not running. If the engine fails,
the freewheeling unit automatically disengages the engine
from the main rotor allowing the main rotor to rotate freely.
It is the means by which a helicopter can be landed safely in
the event of an engine failure; consequently, all helicopters
must demonstrate this capability in order to be certified.
[Figure 2-46] If a decision is made to attempt an engine
restart in flight (the parameters for this emergency procedure
will be different for each helicopter and must be precisely
followed) the pilot must reengage the engine starter switch to
start the engine. Once the engine is started, the freewheeling
unit will reengage the engine with the main rotor.
Vertical Autorotation
Most autorotations are performed with forward speed. For
simplicity, the following aerodynamic explanation is based
on a vertical autorotative descent (no forward speed) in still
air. Under these conditions, the forces that cause the blades
to turn are similar for all blades regardless of their position in
the plane of rotation. Therefore, dissymmetry of lift resulting
from helicopter airspeed is not a factor.
During vertical autorotation, the rotor disk is divided into
three regions (as illustrated in Figure 2-47): driven region,
Blade rotation
Blade rotation
Stall region 25%
Driving region 45%
Driven region 30%
Figure 2-47. Blade regions during autorotational descent.
driving region, and stall region. Figure 2-48 shows three
blade sections that illustrate force vectors. Part A is the
driven region, B and D are points of equilibrium, part C is
the driving region, and part E is the stall region. Force vectors
are different in each region because rotational relative wind is
slower near the blade root and increases continually toward
the blade tip. Also, blade twist gives a more positive AOA in
the driving region than in the driven region. The combination
of the inflow up through the rotor with rotational relative
wind produces different combinations of aerodynamic force
at every point along the blade.
The driven region, also called the propeller region, is nearest
the blade tips. Normally, it consists of about 30 percent of
the radius. In the driven region, part A of Figure 2-48, the
TAF acts behind the axis of rotation, resulting in an overall
drag force. The driven region produces some lift, but that lift
is offset by drag. The overall result is a deceleration in the
rotation of the blade. The size of this region varies with the
blade pitch, rate of descent, and rotor rpm. When changing
autorotative rpm blade pitch, or rate of descent, the size of
the driven region in relation to the other regions also changes.
There are two points of equilibrium on the blade—one
between the driven region and the driving region, and one
between the driving region and the stall region. At points of
equilibrium, TAF is aligned with the axis of rotation. Lift
and drag are produced, but the total effect produces neither
acceleration nor deceleration.
The driving region, or autorotative region, normally lies
between 25 to 70 percent of the blade radius. Part C of
Figure 2-48 shows the driving region of the blade, which
produces the forces needed to turn the blades during
autorotation. Total aerodynamic force in the driving region is
inclined slightly forward of the axis of rotation, producing a
continual acceleration force. This inclination supplies thrust,
which tends to accelerate the rotation of the blade. Driving
region size varies with blade pitch setting, rate of descent,
and rotor rpm.
By controlling the size of this region, a pilot can adjust
autorotative rpm. For example, if the collective pitch is raised,
the pitch angle increases in all regions. This causes the point
of equilibrium to move inboard along the blade’s span, thus
increasing the size of the driven region. The stall region also
becomes larger while the driving region becomes smaller.
Reducing the size of the driving region causes the acceleration
force of the driving region and rpm to decrease. A constant
rotor rpm is achieved by adjusting the collective pitch so blade
acceleration forces from the driving region are balanced with
the deceleration forces from the driven and stall regions.
The inner 25 percent of the rotor blade is referred to as the
stall region and operates above its maximum AOA (stall
angle), causing drag, which tends to slow rotation of the
blade. Part E of Figure 2-48 depicts the stall region.
Autorotation (Forward Flight)
Autorotative force in forward flight is produced in exactly the
same manner as when the helicopter is descending vertically
in still air. However, because forward speed changes the
inflow of air up through the rotor disk, all three regions move
outboard along the blade span on the retreating side of the
disk where AOA is larger. [Figure 2-49] With lower AOA
on the advancing side blade, more of the blade falls in the
driven region. On the retreating side, more of the blade is in
the stall region. A small section near the root experiences a
reversed flow; therefore, the size of the driven region on the
retreating side is reduced.
Prior to landing from an autorotative descent (or autorotation),
the pilot must flare the helicopter in order to decelerate. The
pilot initiates the flare by applying aft cyclic. As the helicopter
flares back, the airflow patterns change around the blades
causing the rpm to increase. Pilots must adjust the collective
as necessary to keep the rpm within operating limits.
Chapter Summary
This chapter introduced the basics of aerodynamic
fundamentals and theory and how they relate to flying a
helicopter. This chapter also explained how aerodynamics
affect helicopter flight and how important it is for pilots to
understand aerodynamic principles and be prepared to react
to these effects. For additional information on aerodynamics,
refer to the aerodynamics of flight portion of the Pilot’s
Handbook of Aeronautical Knowledge.
Figure 2-48. Force vectors in vertical autorotation descent.
Inflow up through rotor
Resultant RW
Rotational
relative
wind
Chord line
Axis of rotation
Lift
TAF
Drag
B & D
Inflow
Lift
TAF
Drag
AOA
AOA
Inflow
AOA
Lift
TAF
Drag
Inflow
Lift TAF
Drag
AOA
(blade stall)
Driven region
Drag
Point of equilibrium
Point of equilibrium
Driving region
Stall region
Drag
Autorotative force
A Driven region
Equilibrium
Driving region
Stall region
Axis of rotation
Axis of rotation
Axis of rotation
Blade rotation
Blade rotation
Stall region
Driving region
Driven region
Advancing sideRetreating side
Direction of Flight
Figure 2-49. Blade regions in forward autorotation descent.