The document summarizes the aerodynamics of helicopters. It describes how helicopters generate lift through rotating wings and discusses key concepts like torque. It also analyzes airfoil shapes, pressure distributions, and how different airfoil designs impact lift and drag properties. Additionally, the summary defines important rotor system components and terminology used in helicopter aerodynamics.

1. Aerodynamics of a rotary wing type aircraft
(Helicopter)
Darshak Bhuptani
Author affiliation: B.Tech Aerospace, Indian Institute for Aeronautical Engineering and
Information Technology
darshak2512@hotmail.com
Abstract: The main effect of the rotating wing is that
the aircraft tends to rotate in opposite
The helicopter is a rotary wing type aircraft direction that of the rotors and this effect is
which generates the main aerodynamic force known as torque. Description of torque and
by rotating the rotor which hubs the wing and methods to overcome this is entitled below.
rotates it at a very high speed. As a result of
this rotation, the lift for an aircraft is Due to the motion of any system, there is a
produced at full throttle only. vibration associated with it. This tends to
induce fatigue stress in the system which can
The blades which are used in helicopters are be fatal, so appropriate device should be
of airfoil shape. The basic terminology and incorporated with the system so that vibration
pressure distribution over an airfoil is can be minimised. Various types of blade
described in detailed. As there is lift, there is setting, ground effect, hovering, effective
drag force too. The description of the various translation lift, blade stall and its effect are
types of drag force and the amount of the discussed.
power required to overcome this is
mentioned.
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2. Chapter 1 explains at least in part why an airfoil
develops an aerodynamic force.
Introduction to basic aerodynamics:
All of the forces acting on a surface over
Aerodynamics concerns the motion of air and which there is a flow of air are the result of
other gaseous fluids and other forces acting skin friction or pressure. Friction forces are
on objects in motion through the air (gases). the result of viscosity and are confined to a
In effect, Aerodynamics is concerned with the very thin layer of air near the surface. They
object (aircraft), the movement (Relative usually are not dominant and, from the
Wind), and the air (Atmosphere). aviator's perspective, can be discounted.
Newton's Laws of Motion
Newton's three laws of motion are: As an aid in visualizing what happens to
pressure as air flows over an airfoil, it is
Newton’s first law: helpful to consider flow through a tube
Inertia - A body at rest will remain at rest. (Please see Figure above). The concept of
And a body in motion will remain in motion conservation of mass states that mass cannot
at the same speed and direction until be created or destroyed; so, what goes in one
affected by some external force. Nothing
end of the tube must come out the other end.
starts or stops without an outside force to
bring about or prevent motion. Hence, the If the flow through a tube is neither
force with which a body offers resistance to accelerating nor decelerating at the input, then
change is called the force of inertia. the mass of flow per unit of time at Station 1
must equal the mass of flow per unit of time
Newton’s second law: at Station 2, and so on through Station 3. The
Acceleration - The force required to produce mass of flow per unit area (cross-sectional
a change in motion of a body is directly
area of tube) is called the Mass Flow Rate.
proportional to its mass and the rate of
change in its velocity. Acceleration refers
either to an increase or a decrease in velocity,
although Deceleration is commonly used to
indicate a decrease.
Newton’s third law:
Action / Reaction - For every action there is
an equal and opposite reaction. If an
interaction occurs between two bodies, equal
forces in opposite directions will be imparted At low flight speeds, air experiences
to each body. relatively small changes in pressure and
negligible changes in density. This airflow is
Fluid flow and Airspeed measurement. termed incompressible since the air may
(Bernoulli’s Principle) undergo changes in pressure without apparent
changes in density. Such airflow is similar to
Daniel Bernoulli, a Swiss mathematician, the flow of water, hydraulic fluid, or any
stated a principle that describes the other incompressible fluid. This suggests that
relationship between internal fluid pressure between any two points in the tube, the
and fluid velocity. His principle, essentially a velocity varies inversely with the area.
statement of the conversation of energy, Venturi effect is the name used to describe
this phenomenon. Fluid flow speeds up
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3. through the restricted area of a venturi in because the air layers restrict the flow just as
direct proportion to the reduction in area. The did the top half of the venturi tube. As a
Figure below suggests what happens to the result, acceleration causes decreased static
speed of the flow through the tube discussed. pressure above the curved shape of the tube.
A pressure differential force is generated by
the local variation of static and dynamic
pressures on the curved surface.
The total energy in a given closed system
does not change, but the form of the energy
may be altered. The pressure of the flowing
air may be likened to energy in that the total A comparison can be made with water
pressure of flowing air will always remain flowing thru a garden hose. Water moving
constant unless energy is added or taken from through a hose of constant diameter exerts a
the flow. In the previous examples there is no uniform pressure on the hose; but if the
addition or subtraction of energy; therefore diameter of a section of the hose in increased
the total pressure will remain constant. or decreased, it is certain to change the
pressure of the water at this point. Suppose
Fluid flow pressure is made up of two we were to pinch the hose, thereby
components - Static pressure and dynamic constricting the area through which the water
pressure. The Static Pressure is that flows. Assuming that the same volume of
measured by an aneroid barometer placed in water flows through the constricted portion of
the flow but not moving with the flow. The the hose in the same period of time as before
Dynamic Pressure of the flow is that the hose was pinched, it follows that the
component of total pressure due to motion of speed of flow must increase at that point. If
the air. It is difficult to measure directly, but a we constrict a portion of the hose, we not
pitot-static tube measures it indirectly. The only increase the speed of the flow, but we
sum of these two pressures is total pressure also decrease the pressure at that point. We
and is measured by allowing the flow to could achieve like results if we were to
impact against an open-end tube which is introduce streamlined solids (airfoils) at the
Venter to an aneroid barometer. This is the same point in the hose. This principle is the
incompressible or slow-speed form of the basis for measuring airspeed (fluid flow) and
Bernoulli equation. for analyzing the airfoil's ability to produce
lift.
Static pressure decreases as the velocity
increases. This is what happens to air passing
over the curved top of an aircraft's airfoil.
Consider only the bottom half of a venturi
tube in the Figure below. Notice how the
shape of the restricted area at Station 2
resembles the top surface of an airfoil. Even
when the top half of the venturi tube is taken
away, the air still accelerates over the curved
shape of the bottom half. This happens
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4. Chapter 2
The Horizontal Hinge Pin (4) is the
Rotary wing plan forms: axis which permits up and down
movement of the blade independent of
Common terms used to describe the the other blades in the system.
helicopter rotor system are shown here.
Although there is some variation in systems The Trunnion (5) is splined to the
mast and has two bearings through
between different aircraft, the terms shown
which it is secured to the yoke. The
are generally accepted by most blades are mounted to the yoke and are
manufacturers. free to teeter (flap) around the trunnion
bearings.
The system below is an example of a Fully The Yoke (6) is the structural member
Articulated rotor system: to which the blades are attached and
which fastens the rotor blades to the
mast through the trunnion and trunnion
bearings.
The Blade Grip Retainer Bearings (7)
is the bearing which permits rotation
of the blade about its span wise axis so
blade pitch can be changed (blade
Semi rigid Rotor Systems do not have vertical feathering).
/ horizontal hinge pins. Instead, the entire
rotor is allowed to teeter or flap by a trunnion Blade Twist is a characteristic built
bearing that connects the yoke to the mast into the rotor blade so angle of
(this method is commonly used on two blades incidence is less near the tip than at the
rotor systems): root. Blade twist helps distribute the
lift evenly along the blade by an
increased angle of incidence near the
root where blade speed is slower.
Outboard portions of the blade that
travel faster normally have lower
angles of incidence, so less lift is
concentrated near the blade tip.
The Chord (1) is the longitudinal
dimension of an airfoil section,
measured from the leading edge to the
trailing edge.
The Span (2) is the length of the rotor
blade from the point of rotation to the
tip of the blade.
The Vertical Hinge Pin (3) (drag
hinge) is the axis which permits fore
and aft blade movement independent
of the other blades in the system.
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5. Chapter 3 root to tip. However, the symmetrical airfoil
produces less lift than a non symmetrical
Airfoils in general: airfoil and also has relatively undesirable stall
characteristics. The helicopter blade (airfoil)
An Airfoil is a structure, piece, or body must adapt to a wide range of airspeeds and
angles of attack during each revolution of the
designed to obtain a useful reaction upon
rotor. The symmetrical airfoil delivers
itself in its motion through the air. An airfoil acceptable performance under those
may be no more than a flat plate (those alternating conditions. Other benefits are
darned engineers!) but usually it has a cross lower cost and ease of construction as
section carefully contoured in accordance compared to the non symmetrical airfoil.
with its intended application or function.
Airfoils are applied to aircraft, missiles, or Non symmetrical (cambered) airfoils may
have a wide variety of upper and lower
other aerial vehicles for:
surface designs. The advantages of the non
Sustentation (A Wing or Rotor Blade) symmetrical airfoil are increased lift-drag
For Stability (As a Fin) ratios and more desirable stall characteristics.
For Control (A Flight Surface, such Non symmetrical airfoils were not used in
as a Rudder) earlier helicopters because the centre of
For Thrust (A Propeller or Rotor pressure location moved too much when
Blade) angle of attack was changed. When centre of
pressure moves, a twisting force is exerted on
Some airfoils combine some of these the rotor blades. Rotor system components
functions. had to be designed that would withstand the
twisting forces. Recent design processes and
A helicopter flies for the same basic reason new materials used to manufacture rotor
that any conventional aircraft flies, because systems have partially overcome the problems
aerodynamic forces necessary to keep it aloft associated with use of no symmetrical
are produced when air passes about the airfoils.
rotor blades. The rotor blade, or airfoil, is the
structure that makes flight possible. Its shape Airfoil Terminology:
produces lift when it passes through the air.
Helicopter blades have airfoil sections Rotary-wing airfoils operate under diverse
designed for a specific set of flight conditions, because their speeds are a
characteristics. Usually the designer must combination of blade rotation and forward
compromise to obtain an airfoil section that movement of the helicopter. An intelligent
has the best flight characteristics for the discussion of the aerodynamic forces
mission the aircraft will perform. affecting rotor blade lift and drag requires
knowledge of blade section geometry. Rotor
Airfoil sections are of two basic types, blades are designed with specific geometry
symmetrical and non symmetrical. that adapts them to the varying conditions of
flight. Cross-section shapes of most rotor
Symmetrical airfoils have identical upper and blades are not the same throughout the span.
lower surfaces. They are suited to rotary-wing Shapes are varied along the blade radius to
applications because they have almost no take advantage of the particular airspeed
centre of pressure travel. Travel remains range experienced at each point on the blade,
relatively constant under varying angles of and to help balance the load between the root
attack, affording the best lift-drag ratios for and tip. The blade may be built with a twist,
the full range of velocities from rotor blade
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6. so an airfoil section near the root has a larger The airfoil shown in the graphic is a Positive
pitch angle than a section near the tip. Cambered Airfoil because the mean camber
line is located above the chord line. The term
"Camber" refers to the curvature of an airfoil
to its surfaces. The mean camber of an airfoil
may be considered as the curvature of the
median line (mean camber line) of the airfoil.
Pressure patterns on the airfoil:
Distribution of pressure over an airfoil section
may be a source of an aerodynamic twisting
force as well as lift. A typical example is
The Chord Line (1) is a straight line illustrated by the pressure distribution pattern
connecting the leading and trailing
developed by this cambered (non
edges of the airfoil.
symmetrical) airfoil:
The Chord (2) is the length of the
The upper surface has pressures distributed
chord line from leading edge to trailing
which produce the upper surface lift.
edge and is the characteristic
longitudinal dimension of an airfoil. The lower surface has pressures distributed
which produce the lower surface force. Net
The Mean Camber Line (3) is a line lift produced by the airfoil is the difference
drawn halfway between the upper and between lift on the upper surface and the
lower surfaces. The chord line force on the lower surface. Net lift is
connects the ends of the mean camber effectively concentrated at a point on the
line. chord called the Centre of Pressure.
The shape of the mean camber is
important in determining the
aerodynamic characteristics of an
airfoil section. Maximum Camber (4)
(displacement of the mean camber line
from the chord line) and where it is
located (expressed as fractions or
percentages of the basic chord) help to
define the shape of the mean camber
line.
The Maximum Thickness (5) of an
airfoil and where it is located
(expressed as a percentage of the
chord) help define the airfoil shape,
and hence its performance.
The Leading Edge Radius (6) of the
airfoil is the radius of curvature given
the leading edge shape.
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7. When the angle of attack is increased:
Upper surface lift increases relative to the
lower surface force.
Since the two vectors are not located at the
same point along the chord line, a twisting
force is exerted about the centre of pressure.
Centre of pressure also moves along the chord
line when angle of attack changes, because
the two vectors are separated. This
characteristic of non symmetrical airfoils
results in undesirable control forces that must
be compensated for if the airfoil is used in When the angle of attack is increased to
rotary wing applications. develop positive lift, the vectors remain
essentially opposite each other and the
twisting force is not exerted. Centre of
pressure remains relatively constant even
when angle of attack is changed. This is a
desirable characteristic for a rotor blade,
because it changes angle of attack constantly
during each revolution.
Relative wind:
Knowledge of relative wind is particularly
essential for an understanding of
aerodynamics of rotary-wing flight because
relative wind may be composed of multiple
components. Relative wind is defined as the
The pressure patterns for symmetrical airfoils airflow relative to an airfoil:
are distributed differently than for non
symmetrical airfoils:
Relative wind is created by movement of an
airfoil through the air. As an example,
consider a person sitting in an automobile on
a no-wind day with a hand extended out the
window. There is no airflow about the hand
since the automobile is not moving. However,
if the automobile is driven at 50 miles per
Upper surface lift and lower surface lift hour, the air will flow under and over the
vectors are opposite each other instead of hand at 50 miles per hour. A relative wind has
being separated along the chord line as in the been created by moving the hand through the
cambered airfoil.
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8. air. Relative wind flows in the opposite In this graphic, angle of attack is reduced by
direction that the hand is moving. The induced flow, causing the airfoil to produce
velocity of airflow around the hand in motion less lift:
is the hand's airspeed.
When the helicopter is stationary on a no-
wind day, Resultant Relative Wind is
produced by rotation of the rotor blades.
Since the rotor is moving horizontally, the
effect is to displace some of the air
downward. The blades travel along the same
path and pass a given point in rapid
succession (a three-bladed system rotating at
320 revolutions per minute passes a given
point in the tip-path plane 16 times per
second).
When the helicopter has horizontal motion,
The graphic illustrates how still air is changed the resultant relative wind discussed above is
to a column of descending air by rotor blade further changed by the helicopter airspeed.
action: Airspeed component of relative wind results
from the helicopter moving through the air. It
is added to or subtracted from the rotational
relative wind, depending on whether the blade
is advancing or retreating in relation to the
helicopter movement. Induced flow is also
modified by introduction of airspeed relative
wind. The pattern of air circulation through
the disk changes when the aircraft has
movement. Generally the downward velocity
This flow of air is called an Induced Flow of induced flow is reduced. The helicopter
(downwash). It is most predominant at a moves continually into an undisturbed air
hover under still wind conditions. Because the mass, resulting in less time to develop a
rotor system circulates the airflow down vertical airflow pattern. As a result, additional
through the rotor disk, the rotational relative lift is produced from a given blade pitch
wind is modified by the induced flow. setting.
Airflow from rotation, modified by induced
flow, produces the Resultant Relative Wind.
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9. Chapter 4 flight. If RPM is held constant, coning
increases as gross weight and G-force
Centrifugal force: increase. If gross weight and G-forces are
constant, decreasing RPM will cause
Helicopter rotor systems depend primarily on increased coning. Excessive coning can occur
rotation to produce relative wind which if RPM gets too low, gross weight is too high,
develops the aerodynamic force required for or if excessive G-forces are experienced.
flight. Because of its rotation and weight, the Excessive coning can cause undesirable
rotor system is subject to forces and moments stresses on the blade and a decrease of total
peculiar to all rotating masses. One of the lift because of a decrease in effective disk
forces produced is Centrifugal Force. area:
It is defined as the force that tends to make
rotating bodies move away from the centre of
rotation. Another force produced in the rotor
system is Centripetal Force. It is the force
that counteracts centrifugal force by keeping
an object a certain radius from the axis of
rotation.
The rotating blades of a helicopter produce
very high centrifugal loads on the rotor head
and blade attachment assemblies. As a matter
of interest, centrifugal loads may be from 6 to
12 tons at the blade root of two to four
passenger helicopters. Larger helicopters may Notice that the effective diameter of the rotor
develop up to 40 tons of centrifugal load on disk with increased coning is less than the
each blade root. In rotary-wing aircraft, diameter of the other disk with less coning. A
centrifugal force is the dominant force smaller disk diameter has less potential to
affecting the rotor system. All other forces act produce lift.
to modify this force.
Centrifugal force and lift effects on the blade
When the rotor blades are at rest, they droop can be illustrated best by a vector. First look
due to their weight and span. In fully at a rotor shaft and blade just rotating:
articulated systems, they rest against a static
or droop stop which prevents the blade from
descending so low it will strike the aircraft (or
ground!). When the rotor system begins to
turn, the blade starts to rise from the static
position because of the centrifugal force. At
operating speed, the blades extend straight out
even though they are at flat pitch and are not Now look at the same rotor shaft and blade
producing lift. when a vertical force is pushing up on the tip
of the blade:
As the helicopter develops lift during takeoff
and flight, the blades rise above the "straight
out" position and assume a coned position.
Amount of coning depends on RPM, gross
weight, and G-Forces experienced during
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10. Forces applied to a spinning rotor disk by
control input or by wind gusts will react as
follows:
This behaviour explains some of the
fundamental effects occurring during various
helicopter manoeuvres.
The vertical force is lift produced when the
blades assume a positive angle of attack. The
horizontal force is caused by the centrifugal
force due to rotation. Since one end of the
blade is attached to the rotor shaft, it is not
free to move. The other end can move and
will assume a position that is the resultant of
the forces acting on it:
The blade position is now "coned" and its
position is a resultant of the two forces, lift
and centrifugal force, acting on it.
For example:
Gyroscopic Precession: The helicopter behaves differently when
rolling into a right turn than when rolling into
Gyroscopic precession is a phenomenon a left turn.
occurring in rotating bodies in which an During the roll into a left turn, the pilot will
applied force is manifested 90 degrees later in have to correct for a nose down tendency in
the direction of rotation from where the force order to maintain altitude. This correction is
was applied. required because precession causes a nose
Although precession is not a dominant force down tendency and because the tilted disk
in rotary-wing aerodynamics, it must be produces less vertical lift to counteract
reckoned with because turning rotor systems gravity.
exhibit some of the characteristics of a gyro. Conversely, during the roll into a right turn,
The graphic shows how precession affects precession will cause a nose up tendency
the rotor disk when force is applied at a while the tilted disk will produce less vertical
given point: lift.
A downward force applied to the disk at Pilot input required to maintain altitude is
point A results in a downward change in disk significantly different during a right turn than
attitude at point B, and an upward force during a left turn, because gyroscopic
applied at Point C results in an upward precession acts in opposite directions for
change in disk attitude at point D. each.
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11. Chapter 5
Drag forces:
Drag is simply force that opposes the motion
of an aircraft through the air. However it does
have separate components that comprise it.
Total Drag produced by an aircraft is the sum
of the Profile drag, Induced drag, and
Parasite drag. Total drag is primarily a
function of airspeed. The airspeed that
produces the lowest total drag normally
determines the aircraft best-rate-of-climb
speed, minimum rate-of-descent speed for
autorotation, and maximum endurance speed. Curve "A" shows that parasite drag is
very low at slow airspeeds and
Profile Drag is the drag incurred from increases with higher airspeeds.
frictional resistance of the blades passing Parasite drag goes up at an increasing
through the air. It does not change rate at airspeeds above the midrange.
significantly with angle of attack of the airfoil
section, but increases moderately as airspeed Curve "B" shows how induced drag
increases. decreases as aircraft airspeed
increases. At a hover, or at lower
Induced Drag is the drag incurred as a result
airspeeds, induced drag is highest. It
of production of lift. Higher angles of attack
decreases as airspeed increases and the
which produce more lift also produce
helicopter moves into undisturbed air.
increased induced drag. In rotary-wing
aircraft, induced drag decreases with
Curve "C" shows the profile drag
increased aircraft airspeed. The induced drag
curve. Profile drag remains relatively
is the portion of the Total Aerodynamic
constant throughout the speed range
Force which is oriented in the direction
with some increase at the higher
opposing the movement of the airfoil.
airspeeds.
Parasite Drag is the drag incurred from the
Curve "D" shows total drag and
non lifting portions of the aircraft. It includes
represents the sum of the other three
the form drag and skin friction associated
curves. It identifies the airspeed range,
with the fuselage, cockpit, engine cowlings,
line "E", at which total drag is lowest.
rotor hub, landing gear, and tail boom to
That airspeed is the best airspeed for
mention a few. Parasite drag increases with
maximum endurance, best rate of
airspeed.
climb, and minimum rate of descent in
The graphic illustrates the different forms of autorotation.
drag versus airspeed:
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12. Chapter 6 needed to drive the tail rotor depending on
helicopter size and design. Normally, larger
Torque: helicopters use a higher percent of engine
power to counteract torque than do smaller
In accordance with Newton's law of action aircraft. A helicopter with 9,500 horsepower
and reaction, the helicopter fuselage tends to might require 1,200 horsepower to drive the
rotate in the direction opposite to the rotor tail rotor, while a 200 horsepower aircraft
blades. This effect is called torque. Torque might require only 10 horsepower for torque
must be counteracted and or controlled before correction.
flight is possible. In tandem rotor and coaxial
helicopter designs, the rotors turn in opposite Heading Control
directions to neutralize or eliminate torque
effects. In tip-jet helicopters, power originates In addition to counteracting torque, the tail
at the blade tip and equal and opposite rotor and its control linkage also permit
reaction is against the air; there is no torque control of the helicopter heading during
between the rotor and the fuselage. However, flight. Application of more control than is
the torque problem is especially important in necessary to counteract torque will cause the
single main rotor helicopters with a fuselage nose of the helicopter to swing in the
mounted power source. The torque effect on direction of pedal movement. To maintain a
the fuselage is a direct result of the constant heading at a hover or during takeoff
work/resistance of the main rotor. Therefore or approach, the pilot must use anti-torque
torque is at the geometric centre of the main pedals to apply just enough pitch on the tail
rotor. Torque results from the rotor being rotor to neutralize torque and hold a slip if
driven by the engine power output. Any necessary (keeping the aircraft in trim, the tail
change in engine power output brings about a is not used to turn the helicopter IN forward
corresponding change in torque effect. flight. Heading control in forward trimmed
Furthermore, power varies with the flight flight is normally accomplished with cyclic
manoeuvre and results in a variable torque control, using a coordinated bank and turn to
effect that must be continually corrected. the desired heading. Application of anti-
torque pedals will be required when power
The Anti-torque Rotor changes are made.
Compensation for torque in the single main In an autorotation, some degree of right pedal
rotor helicopter is accomplished by means of is required to maintain correct trim. When
a variable pitch anti-torque rotor (tail rotor) torque is not present, mast thrust bearing
located on the end of a tail boom extension at friction tends to turn the fuselage in the same
the rear of the fuselage. Driven by the main direction as main rotor rotation. To counteract
rotor at a constant ratio, the tail rotor this friction, the tail rotor thrust is applied in
produces thrust in a horizontal plane opposite an opposite direction to counter the frictional
to torque reaction developed by the main forces.
rotor. Since torque effect varies during flight
when power changes are made, it is necessary Translating Tendency
to vary the thrust of the tail rotor. Anti-torque
pedals enable the pilot to compensate for During hovering flight, the single rotor
torque variance. A significant part of the helicopter has a tendency to drift laterally to
engine power is required to drive the tail the right due to the lateral thrust being
rotor, especially during operations when supplied by the tail rotor. The pilot may
maximum power is used. From 5 to 30 prevent right lateral drift of the helicopter by
percent of the available engine power may be tilting the main rotor disk to the left. This
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13. lateral tilt results in a main rotor force to the Angle of attack:
left that compensates for the tail rotor thrust
to the right.
c
Helicopter design usually includes one or
more features which help the pilot
compensate for translating tendency:
Flight control rigging may be designed so the
rotor disk is tilted slightly left when the
cyclic control is cantered.
ANY Airfoil's Angle Of Attack or AOA (4) is
The collective pitch control system may be an aerodynamic one.
designed so that the rotor disk tilts slightly
left as collective pitch is increased to hover It is: The angle between the airfoil chord
the aircraft. line and its direction of motion relative to
the air (the resulting Relative Wind).
The main transmission may be mounted so
that the mast is tilted slightly to the left when Several factors will affect rotor blade AOA.
the helicopter fuselage is laterally level. Some are controlled by the pilot and some
occur automatically due to the rotor system
design. Pilots are able to adjust AOA by
moving the cyclic and collective pitch
controls. However, even when these controls
are held stationary, the AOA constantly
changes as the blade moves around the
circumference of the rotor disk. Other factors
affecting AOA, over which the pilot has little
control, are:
Blade Flapping
Blade Flexing
Wind Gusts / Turbulence
AOA is one of the primary factors that
determines amount of lift and drag produced
by an airfoil.
Angle of attack should not be confused with
the Angle Of Incidence.
Angle of Incidence (or AOI) is the angle
between the blade chord line and the plane
of rotation of the rotor system.
It is a mechanical angle rather than an
aerodynamic angle:
Page | 13

14. main rotor shaft.
An extreme airspeed differential between the
blade tip and root is the result.
The lift differential between the blade root
and tip is even larger because lift varies as
the square of the speed.
In the absence of induced flow and/or aircraft Therefore, when speed is doubled, lift is
airspeed, AOA and AOI are equal. increased four times.
Whenever the relative wind is modified (by
induced flow / aircraft airspeed), then AOA This means that the lift at point "A" would be
and AOI diverge becoming unequal. only one-fourth as much as lift at the blade
tip (assuming the rotor airfoil has no blade
Rotational velocities in the rotor twist along the span).
system:
Because of the potential lift differential along
During hovering, airflow over the rotor blades the blade resulting primarily from speed
is produced by rotation of the rotor system. variation, blades are designed with a twist.
The Graphic shows a two bladed system Blade twist provides a higher pitch angle at
commonly found: the root where speed is low and lower pitch
angles nearer the tip where speed is higher.
This design helps distribute the lift more
evenly along the blade. It increases both the
induced air velocity and the blade loading
near the inboard section of the blade.
This graphic compares a twisted versus an
untwisted blades lift:
Blade speed near the main rotor shaft is
much less because the distance travelled at
the smaller radius is relatively small.
The twisted blade generates more lift near
At point "A", half way from the rotor shaft to the root and less lift at the tip than the
the blade tip, the blade speed is only 198 untwisted blade.
knots which is one-half the tip speed.
Speed at any point on the blades varies with
the radius or distance from the centre of the
Page | 14

15. Dissymmetry of lift: Since lift increases as the square of the
airspeed, a potential lift variation exists
Dissymmetry of lift is the difference in lift between the advancing and retreating sides of
that exists between the advancing half of the the rotor disk. This lift differential must be
rotor disk and the retreating half. It is caused compensated for, or the helicopter would not
by the fact that in directional flight the be controllable.
aircraft relative wind is added to the rotational
relative wind on the advancing blade, and To compare the lift of the advancing half of
subtracted on the retreating blade. The blade the disk area to the lift of the retreating half,
passing the tail and advancing around the the lift equation can be used. In forward
right side of the helicopter has an increasing flight, two factors in the lift formula, density
airspeed which reaches maximum at the 3 ratio and blade area are the same for both the
o'clock position. As the blade continues, the advancing and retreating blades. The airfoil
airspeed reduces to essentially rotational shape is fixed for a given blade. The only
airspeed over the nose of the helicopter. remaining variables are changes in blade
Leaving the nose, the blade airspeed angle of attack and blade airspeed. These two
progressively decreases and reaches variables must compensate for each other
minimum airspeed at the 9 o'clock position. during forward flight to overcome
The blade airspeed then increases dissymmetry of lift.
progressively and again reaches rotational
airspeed as it passes over the tail. Two factors, Rotor RPM and Aircraft
Airspeed, control blade airspeed during flight.
Both factors are variable to some degree, but
must remain within certain operating limits.
Angle of attack remains as the one variable
that may be used by the pilot to compensate
for dissymmetry of lift.
The pitch angle of the rotor blades can be
varied throughout their range, from flat pitch
to the stalling pitch angle, to change angle of
attack and to compensate for lift differential.
The next graphic shows the relationship
between blade pitch angle and blade airspeed
during forward flight:
Blade airspeed at the outboard edge of the
shaded circle is 0 knots. Within the reverse
flow area, the air actually moves over the
blade backwards from trailing edge to
leading edge. From the reverse flow area out
to the blade tip, the blade airspeed
progressively increases up to 294 knots.
At an aircraft airspeed of 100 knots, a 200
knot blade airspeed differential exists
between the advancing and retreating blades.
Page | 15

16. Dissymmetry of Lift and the Tail Rotor
The tail rotor also experiences dissymmetry
of lift during forward flight, because of its
own advancing and retreating blades.
Although the plane of rotation is vertical, the
effects are the same as for the main rotor in
the horizontal plane. Dissymmetry is usually
corrected for by a flapping hinge action.
Two basic types of flapping hinges, the Delta
and Offset.
Either can be found on helicopters in the fleet.
Note that the delta hinge (b) is not oriented
parallel to the blade chord, designed that way
so that flapping automatically introduces
cyclic feathering which corrects for
dissymmetry of lift.
Note that blade pitch angle is lower on the
advancing side of the disk to compensate for
increased blade airspeed on that side.
Blade pitch angle is increased on the
retreating blade side to compensate for
decreased blade airspeed on that side.
These changes in blade pitch are introduced
either through the blade feathering
mechanism or blade flapping.
When made with the blade feathering
mechanism, the changes are called Cyclic
Feathering.
Pitch changes are made to individual blades
independent of the others in the system and
are controlled by the pilot's cyclic pitch
control.
The offset hinge is located outboard from the
hub and uses centrifugal force to produce
substantial forces that act on the hub itself.
One important advantage of offset hinges is
Page | 16

17. the presence of control regardless of lift understandable that the maximum upward
condition, since centrifugal force is flapping velocity will take place directly over
independent of lift. the right side of the helicopter, and the
maximum downward flapping velocity takes
Blade flapping: place directly over the left side of the
helicopter. (This discussion assumes counter
Blade Flapping is the up and down clockwise blade rotation, for clockwise
movement of a rotor blade, which, in rotation, they are reversed)
conjunction with cyclic feathering, causes
Dissymmetry of Lift to be eliminated. The flapping velocities are at maximum
values directly over the right and left sides of
The advancing blade, upon meeting the the helicopter, because at those locations the
progressively higher airspeeds brought about airspeed differential is at its maximum.
by the addition of forward flight velocity to
the rotational airspeed (of the rotor), responds In the study of cyclic pitch, in a dynamic
to the increase of speed by producing more system such as a main rotor system with
lift. inertia, there is a phase angle between the
maximum applied force and the maximum
The blade flaps (or climbs) upward, and the displacement.
change in relative wind and angle of attack
reduces the amount that would have been The force-displacement phase is 90 degrees,
generated. and is not affected by blade mass or any kind
of air dampening. It then follows that if the
maximum upward and flapping velocity is
directly over the right side of the helicopter,
the maximum displacement or actual flapping
will take place over the nose of the aircraft.
Conversely, if the maximum downward
In the case of the retreating blade, the flapping velocity is directly over the left side
opposite is true: of the helicopter, the maximum displacement
or actual flapping will take place over the tail
of the aircraft. The following graphic
illustrates this relationship:
As it loses airspeed, reducing lift causes it to
flap down (or settle), thus changing its
relative wind and angle of attack. The
resulting larger angle of attack retains the lift
that would have been lost because of the
reduced airspeed.
Flapping Velocity
Flapping Velocity, both upward and
downward, must be of such a value as to
increase or decrease the angle of attack so
that the lift will remain constant. It is
Page | 17

18. The total result of this action is a rotor tilt to
the rear which is completely independent of
any additional cyclic stick action and which
causes an angular separation between the
control axis and the thrust axis of the rotor.
There is yet another periodic force with a
phase-displacement angular separation of 90
degrees. This one arises from periodic
longitudinal forces which result from rotor
coning while the helicopter is in directional
flight and causes the rotor to tilt to the side.
The above graphic shows that the higher
angle of attack at the front of the rotor will
cause the blade to flap up over the left side of
the helicopter. The lower angle of attack over
the rear of the rotor will cause the blade to
flap down over the right side. The rotor will
thus be tilted a little to the right. The sideward
From the above graphic it may be seen that tilt of the rotor is increased at low forward
the relative wind created by the helicopter's speeds when the induced velocities are large,
forward flight causes angle of attack because the inflow not only approaches the
differences between the front and rear of the rear of the rotor but, additionally, is bent
rotor. The blade over the nose of the downward. This increases the angle of attack
helicopter experiences an increase in angle of differences.
attack because the aircraft relative wind
approaches the blade level with or below its
span. The blade over the rear of the helicopter
experiences a reduced angle of attack because
the aircraft relative wind approaches it from
above.
Page | 18

19. Chapter 6 You can recognize transverse flow effect
because of increased vibrations of the
Transverse flow effect: helicopter at airspeeds just below effective
translational lift (ETL) on takeoff and just
In forward flight, air passing through the rear passing through ETL during landing.
portion of the rotor disk has a greater
downwash angle than air passing through the To counteract transverse flow effect, a cyclic
forward portion. This is due to that air being input will be needed to correct the rolling
accelerated for a longer period of time as it tendency.
travels to the rear of the rotor system.
The downward flow at the rear of the rotor
disk causes a reduced angle of attack,
resulting in less lift. Increased angle of attack
and more lift is produced at the front portion
of the disk because airflow is more
horizontal. These differences between the
fore and aft parts of the rotor disk are called
transverse flow effect. They cause unequal
drag in the fore and aft parts of the disk
resulting in vibrations that are easily
recognizable by the pilot. The vibrations are
more noticeable for most helicopters between
10 and 20 knots.
So, what does this mean to us pilots? Well,
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.
(Assuming a counter clockwise main rotor
rotation, reverse for a clockwise rotation).
Page | 19

20. Chapter 7
Ground effect: When operating in ground effect, the
downward and outward airflow pattern tends
Ground Effect is a condition of improved to restrict vortex generation. This makes the
performance encountered when operating outboard portion of the rotor blade more
near (within 1/2 rotor diameter) of the efficient and reduces overall system
ground. It is due to the interference of the turbulence caused by ingestion and
surface with the airflow pattern of the rotor recirculation of the vortex swirls.
system, and it is more pronounced the nearer
the ground is approached. Increased blade Rotor efficiency is increased by ground effect
efficiency while operating in ground effect is up to a height of about one rotor diameter for
due to two separate and distinct phenomena. most helicopters. This graphic displays the
The high power requirement needed to hover percent increase in rotor thrust experienced at
out of ground effect is reduced when various rotor heights:
operating in ground effect.
First and most important is the reduction of
the velocity of the induced airflow. Since the
ground interrupts the airflow under the
helicopter, the entire flow is altered. This
reduces downward velocity of the induced
flow. The result is less induced drag and a
more vertical lift vector. The lift needed to
sustain a hover can be produced with a
reduced angle of attack and less power
because of the more vertical lift vector:
At a rotor height of one-half rotor diameter,
the thrust is increased about 7 percent.
At rotor heights above one rotor diameter,
the thrust increase is small and decreases to
zero at a height of about 1 1/4 rotor
diameters.
Maximum ground effect is accomplished
The second phenomenon is a reduction of the when hovering over smooth paved surfaces.
Rotor Tip Vortex: While hovering over tall grass, rough terrain,
revetments, or water, ground effect may be
seriously reduced. This phenomenon is due to
the partial breakdown and cancellation of
ground effect and the return of large vortex
patterns with increased downwash angles.
Two identical airfoils with equal blade pitch
angles are compared graphically:
Page | 20

21. The top airfoil is out-of-ground-effect while velocity, an increase of blade pitch (angle of
the bottom airfoil is in-ground-effect. The attack) would induce the necessary lift for a
airfoil that is in-ground-effect is more hover. The forces of lift and weight reach a
efficient because it operates at a larger angle state of balance during a stationary hover.
of attack and produces a more vertical lift
vector. Its increased efficiency results from a Hovering is actually an element of vertical
smaller downward induced wind velocity flight. Assuming a no-wind condition, the tip-
which increases angle of attack. The airfoil path plane of the blades will remain
operating out-of-ground-effect is less horizontal. If the angle of attack of the blades
efficient because of increased induced wind is increased while their velocity remains
velocity which reduces angle of attack. constant, additional vertical thrust is obtained.
Thus, by upsetting the vertical balance of
forces, helicopters can climb or descend
vertically.
Airflow in the Hover
At a hover, the rotor tip vortex (air swirling
around the blade tip from above to below)
reduces the effectiveness of the outer blade
portions.
Also, the vortexes of the preceding blade
severely affect the lift of the following blades.
If the vortex made by one passing blade
remains a vicious swirl for some number of
seconds, then two blades operating at 350
RPM create 700 long lasting
If a helicopter hovering out-of-ground-effect
descends into a ground-effect hover, blade
efficiency increases because of the more
favourable induced flow. As efficiency of the
rotor system increases, the pilot reduces
blade pitch angle to remain in the ground-
effect hover. Less power is required to
maintain however in-ground-effect than for
the out-of-ground-effect hover.
The Hover:
Hovering is the term applied when a
helicopter maintains a constant position at a
selected point, usually a few feet above the Vortex patterns per minute. This continuous
ground (but not always, helicopters can hover creation of new vortexes and ingestion of
high in the air, given sufficient power). existing vortexes is a primary cause of high
power requirements for hovering.
For a helicopter to hover, the main rotor must
supply lift equal to the total weight of the During hover, the rotor blades move large
helicopter. With the blades rotating at high volumes of air in a downward direction. This
Page | 21

22. pumping process uses lots of horsepower and efficiency of the rotor system and improve
accelerates the air to relatively high aircraft performance.
velocities. Air velocity under the helicopter
may reach 60 to 100 knots, depending on the Improved rotor efficiency resulting from
size of the rotor and the gross weight of the these changes is termed Effective
helicopter. Translational Lift (or ETL). The graphic
shows an airflow pattern at airspeeds between
1-5 knots:
This is the air flow around a hovering
helicopter
(Note it is out of ground effect):
Note how the downwind vortex is beginning
to dissipate and induced flow down through
the rear of the rotor disk is more horizontal
than at a hover.
Note how the downwash (induced flow) of air
has introduced another element into the This graphic below shows the airflow pattern
relative wind which alters the angle of attack at a speed of 10-15 knots. Airflow is much
of the airfoil. When there is no induced flow, more horizontal than at a hover. The leading
the relative wind is opposite and parallel to edge of the downwash pattern is being
the flight path of the airfoil. In the hovering overrun and is well back under the helicopter
case, the downward airflow alters the relative nose. At about 16 to 24 knots (depending
wind and changes the angle of attack so less upon the size, blade area, and RPM of the
aerodynamic force is produced. This rotor system) the rotor completely outruns the
condition requires the pilot to increase recirculation of old vortexes, and begins to
collective pitch to produce enough work in relatively clean air:
aerodynamic force to sustain a hover.
Although this does increase the lift, it also
increases the induced drag, and so total power
required is higher
Effective translation lift:
The efficiency of the hovering rotor system is
improved with each knot of incoming wind
gained by horizontal movement or surface
wind. As the incoming wind enters the rotor The air passing through the rotor system is
system, turbulence and vortexes are left nearly horizontal, depending on helicopter
behind and the flow of air becomes more forward air speed.
horizontal. All of these changes improve the
Page | 22

23. As the helicopter speed increases, ETL As forward airspeed increases, the "no lift"
becomes more effective and causes the nose areas move left of centre, covering more of
to rise, or pitch up (sometimes called the retreating blade sectors:
blowback). This tendency is caused by the
combined effects of dissymmetry of lift and This requires more lift at the outer retreating
transverse flow effect. Pilots must correct for blade portions to compensate for the loss of
this tendency in order to maintain a constant lift of the inboard retreating sections. In the
rotor disk attitude that will move the area of reversed flow, the rotational velocity
helicopter through the speed range where of this blade section is slower than the aircraft
blowback occurs. If the nose is permitted to airspeed; therefore, the air flows from the
pitch up while passing through this speed trailing to leading edge of the airfoil. In the
range, the aircraft may also tend to roll to the negative stall area, the rotational velocity of
right. the airfoil is faster than the aircraft airspeed;
therefore air flows from leading to trailing
When the single main rotor helicopter
edge of the blade. However due to the relative
transitions from hover to forward flight, the
arm and induced flow, blade flapping is not
tail rotor becomes more aerodynamically
sufficient to produce a positive angle of
efficient. Efficiency increases because the tail
attack. Blade flapping and
rotor works in progressively less turbulent air
as speed increases. As tail rotor efficiency
improves, more thrust is produced. This
causes the aircraft nose to yaw left if the main
rotor turns counter clockwise. During a
takeoff where power is constant, the pilot
must apply right pedal as speed increases to
correct for the left yaw tendency.
Retreating blade stall:
A tendency for the retreating blade to stall in Rotational velocity in the negative lift area is
forward flight is inherent in all present day sufficient to produce a positive angle of
helicopters and is a major factor in limiting attack, but not to a degree that produces
their forward speed. Just as the stall of an appreciable lift.
airplane wing limits the low speed
possibilities of the airplane, the stall of a rotor This graphic depicts a rotor disk that has
blade limits the high speed potential of a reached a stall condition on the retreating
helicopter. The airspeed of the retreating side:
blade (the blade moving away from the
direction of flight) slows down as forward
speed increases. The retreating blade must,
however, produce an amount of lift equal to
that of the advancing blade. Therefore, as the
airspeed of the retreating blade decreases with
forward aircraft speed, the blade angle of
attack must be increased to equalize lift
throughout the rotor disk area. As this angle
increase is continued, the blade will stall at
some high forward speed.
Page | 23

24. The Helicopter Will Roll Into The Stalled
Side, (Dependent Upon Rotor Direction Of
Rotation.
When operating at high forward airspeeds, the
following conditions are most likely to
produce blade stall:
High Blade loading (high gross weight)
Low Rotor RPM
High Density Altitude
Steep or Abrupt Turns
It is assumed that the stall angle of attack for Turbulent Air
this rotor system is 14 degrees. Distribution
of angle of attack along the blade is shown at When flight conditions are such that blade
eight positions in the rotor disk. Although the stall is likely, extreme caution should be
blades are twisted and have less pitch at the exercised when manoeuvring. An abrupt
tip than at the root, angle of attack is higher manoeuvre such as a steep turn or pull up
at the tip because of induced airflow. may result in dangerously severe blade stall.
Aircraft control and structural limitations of
Upon entry into blade stall, the first effect is the helicopter would be threatened.
generally a noticeable vibration of the
helicopter. This is followed by a rolling Blade stall normally occurs when airspeed is
tendency and a tendency for the nose to pitch high. To prevent blade stall, the pilot must fly
up. The tendency to pitch up may be slower than normal when:
relatively insignificant for helicopters with
semi rigid rotor systems due to pendulum The Density Altitude is much Higher than
action. If the cyclic stick is held forward and Standard
collective pitch is not reduced or is increased,
this condition becomes aggravated; the Carrying Maximum Weight Loads
vibration greatly increases, and control may
be lost. By being familiar with the conditions Flying high drag configurations such as
which lead to blade stall, the pilot should floats, external stores, weapons, speakers,
realize when his is flying under such floodlights, sling loads, etc.
circumstances and should take corrective
action. The Air is Turbulent
When the pilot suspects blade stall, he can
The major warnings of approaching retreating
possibly prevent it from occurring by
blade stall conditions are:
sequentially:
Abnormal Vibration
Reducing Power (collective pitch)
Nose Pitch up
Reducing Airspeed
Page | 24

25. Reducing "G" Loads during Manoeuvring causes loss of rotor efficiency even though
power is still supplied from the engine.
Increasing Rotor RPM to Max Allowable
Limit This graphic shows induced flow along the
blade span during normal hovering flight:
Checking Pedal Trim
In severe blade stall, the pilot loses control.
The helicopter will pitch up violently and roll
to the left. The only corrective action then is
to accomplish procedures as indicated
previously to shorten the duration of the stall
and regain control.
Downward velocity is highest at the blade tip
where blade airspeed is highest.
Settling with power: As blade airspeed decreases nearer the disk
centre, downward velocity is less.
Settling with Power is a condition of powered
flight where the helicopter settles into its own This graphic show induced airflow velocity
downwash. pattern along the blade span during a descent
It is also known as Vortex Ring State. conducive to settling with power:
Conditions conducive to settling with power
are a vertical or nearly vertical descent of at
least 300 feet per minute and low forward
airspeed. The rotor system must also be using
some of the available engine power (from 20
to 100 percent) with insufficient power
available to retard the sink rate. These
conditions occur during approaches with a
The descent is so rapid that induced flow at
tailwind or during formation approaches
the inner portion of the blades is upward
when some aircraft are flying in turbulence
rather than downward.
from other aircraft.
The up flow caused by the descent has
Under the conditions described above, the
overcome the down flow produced by blade
helicopter may descend at a high rate which
rotation.
exceeds the normal downward induced flow
rate of the inner blade sections. As a result,
the airflow of the inner blade sections is If the helicopter descends under these
upward relative to the disk. This produces a conditions, with insufficient power to slow or
secondary vortex ring in addition to the stop the descent, it will enter vortex ring state:
normal tip vortex system. The secondary
vortex ring is generated about the point on the
blade where airflow changes from up to
down. The result is an unsteady turbulent
flow over a large area of the disk which
Page | 25

26. The vortex ring state can be completely excess power. During the early stages of
avoided by descending on flight paths power settling, the large amount of excess
shallower than about 30 degrees (at any power may be sufficient to overcome the up
speed). flow near the centre of the rotor. If the sink
rate reaches a higher rate, power will not be
For steeper approaches, vortex ring state can available to break this up flow, and thus alter
be avoided by using a speed either faster or the vortex ring state of flow.
slower than the area of severe turbulence and
thrust variation. Normal tendency is for pilots to recover from
a descent by application of collective pitch
At very shallow angles of descent, the vortex and power. If insufficient power is available
ring wake is shed behind the helicopter. for recovery, this action may aggravate power
settling resulting in more turbulence and a
At steep angles, the vortex ring wake is higher rate of descent. Recovery can be
below the helicopter at slow rates of descent accomplished by lowering collective pitch
and above the helicopter at high rates of and increasing forward speed. Both of these
descent. methods of recovery require altitude to be
successful.
Hazards of rotating helicopter’s rotor
blades:
It is particularly tragic that rotor blade (and
tail rotor blade) strike mishaps, along with
airmen, have included bystanders, passengers,
and children among the injured persons.
Rotor strike mishaps differ from other aircraft
mishaps in that they usually result in fatal or
serious injury. This is due to the fact that a
rotor rotating under power, even at slow
speed, has sufficient force to inflict serious
injury. It should be remembered that a
This graphic shows the horizontal speed rotating rotor is extremely dangerous and
versus vertical speed relationship for a typical should be treated with all due caution.
helicopter in a descent. Straight lines
emanating from the upper left corner are lines
of constant descent angle. Superimposed on
this grid are flow state regions for the typical
helicopter. From this, several conclusions
regarding vortex ring state can be made:
Power settling is an unstable condition. If
allowed to continue, the sink rate will reach
sufficient proportions for the flow to be
entirely up through the rotor system. If
continued, the rate of descent will reach
extremely high rates. Recovery may be
initiated during the early stages of power
settling by putting on a large amount of
Page | 26

27. Chapter 8 considered to help prevent accidents on
airport ramp areas:
Conspicuity:
1. when the possibility of passengers
The rotor is difficult to see when in operation, wandering on the ramp exists, physical
and the nonprofessional public is often not barriers should be provided such as rope
aware of its danger. Even personnel familiar stanchions from the aircraft to the terminal
with the danger of a turning rotor are likely to doors.
forget it.
2. Airport management personnel should be
1. Some manufacturers of rotor blades use on the alert to keep unauthorized persons
paint schemes to increase the conspicuity of from milling around on ramps among parked
the blades. Owners should give strong aircraft. When spectators are permitted to
consideration to maintaining the conspicuity view and move among aircraft parked on a
paint scheme of the original manufacturer. ramp, the airport management personnel
should caution those persons to stay clear of
2. In the event that the paint scheme does not all propellers and not touch or move them.
lend itself to conspicuity, the owner should
consider having the blade repainted. A 3. Helicopter landing and ramp areas should
customized paint scheme should not be used be marked and provided with safety barriers
until an evaluation is made by a person to restrict access by unauthorized persons.
qualified to determine that it will not interfere
with the pilot's visibility, promote vertigo, or 4. Tail rotor danger areas should be clearly
create an unbalanced blade condition. marked on ramp areas. Helicopters should be
parked with tail rotors within the marked
3. In August of 1978, the FAA issued Report area.
No. FAA-AM-78-29, Conspicuity
Assessment of Selected Propeller and Tail
Rotor Paint Schemes. The report summarizes
the evaluation of three paint schemes for
airplane propellers and two for helicopter tail
rotor blades. The document is available to the
public through the National Technical Aircraft Service Personnel:
Information Service, Springfield, Virginia
22161. Persons directly involved with aircraft service
are most vulnerable to injuries by rotors.
Working around aircraft places them in the
In-Flight Crew Personnel: most likely position for possible rotor strike
mishaps. Aircraft service personnel should
Persons directly involved with enplaning or develop the following safety habits:
deplaning passengers and aircraft servicing
should be instructed as to their specific duties 1. Treat all rotors as if they were turning,
through proper training, with emphasis placed remain clear of the rotor arcs.
on the dangers of rotating rotors. Ramp
attendants and passenger handling personnel 2. Remember when removing an external
should be made aware of the proper power source from an aircraft, keep the
procedures and methods of directing equipment and yourself clear of the rotor.
passengers to and from parked aircraft. The
following safety measures should be 3. Always stand clear of rotor blade paths
Page | 27

28. (rotor arc's), especially when moving the rotor blades. Safety through education is the
rotor. Particular caution should be practiced best and most positive means available for
around warm engines. reducing potential mishaps from blade strikes.
4. Ground personnel should be given 5. The prestart portion of the checklist should
recurrent rotor safety lectures to keep them include an item to make sure the rotor blades
alert to dangers when working around are clear. The proper use of the aircraft
helicopters. checklist should be taught to all student
pilots.
5. Be sure all equipment and personnel are
clear of an aircraft before giving the pilot the
signal to depart. In Summary:
In reviewing rotor blade strike mishaps, the
Flight Personnel / Flight Instructors most impressive fact is that every one of them
(CFI's): was preventable. The danger of rotor blade
strikes is universally recognized.
Prior to starting an engine, flight personnel
should make certain that all personnel are The pilot can be most effective in ensuring
clear of the rotor. that his or her passengers arrive and depart
the vicinity of the helicopter safely by
1. The engine of a helicopter should be shut stopping the engine / rotor system completely
down (and rotor stopped / rotor brake at the time of loading and unloading, or by
engaged if equipped) before boarding or providing a definite means of keeping them
deplaning passengers. This is the simplest clear of the rotors if they are left in motion.
method of avoiding mishaps.
Prominent warning signs, placed in the
2. Boarding or deplaning of passengers, with aircraft's interior near or on the inside face of
an engine running, should only be allowed the aircraft doors to alert passengers and
under close supervision. The pilot in crewmembers of rotor hazards, could be
command should have knowledge that either helpful in preventing a mishap.
the company or the airport operator has
ground attendants fully trained in their
specific duties to board or deplane passengers References:
from an aircraft with an engine(s) running /
rotors spinning. The pilot should instruct www.ultraligero.net/
passengers, before they exit an aircraft with www.dynamicflight.com/aerodynamics
an engine(s) running / rotors spinning, the
path to follow to avoid the rotor blades. www.cybercom.net/~copters/helo_aero.html
www.cambridge.org/catalogue/catalogue.asp?isb
3. When it is necessary to discharge a
n=0521858607
passenger from an aircraft on which the
engine is running / rotors spinning, never www.knovel.com/web/portal/browse/display
have the aircraft with the tail rotor in the path
of the passenger's route from the aircraft. www.helicopterpage.com
www.pruftechnik.com
4. When flight and ground instructors are
instructing their students about rotors, they www.aedie.org/11chlie-papers/217-pelaez
should emphasize the dangers of rotating
Page | 28