INTRODUCTION A helicopter is an aircraft that is lifted and
propelled by one or more horizontal rotors, each rotor consisting
of two or more rotor blades. A helicopter works by having its wings
move through air while the body stays still. The helicopter blades
are called main rotor blades. During flight there are four forces
on the helicopter and those forces are lift, drag, thrust and
weight.
Slide 3
Technical Terms Bernoulli' principle :This principle states
that as the air velocity increases, the pressure decreases; and as
the velocity decreases, the pressure increases. Airfoil : is
technically defined as any surface, such as an elevator, rudder,
wing, main rotor blades, or tail rotor blades designed to obtain
reaction from the air through which it moves
Slide 4
Angle of Attack :is the acute angle measured between the chord
of an airfoil and the relative wind. Angle of Incidence : is the
acute angle between the wing's chord line and the longitudinal axis
of the airplane. (usually manufacturer had built the aircraft with
the wing has some degrees to the horizontal plane or airplane
longitudinal axis Blades : The blades of the helicopter are
airfoils with a very high aspect ratio ( length to chord ). The
angle of incidence is adjusted by means of the control from
pilots.
Slide 5
Swash Plate Assembly : The swash plate assembly consists of two
primary elements through which the rotor mast passes. One element
is a disc, linked to the cyclic pitch control. This disc is capable
of tilting in any direction but does not rotate as the rotor
rotates. Transmission : The transmission system transmits engine
power to the main rotor, tail rotor, generator and other
accessories
Slide 6
Description of lift on an airfoil In a helicopter, the
structure making flight possible is the airfoil - a surfaced body
that responds to relative motion between itself and the air with a
useful, dynamic reaction known as lift. The term airfoil, refers to
the rotary wing, and more specifically means the curvature, or
camber, of the blade. As the diagram indicates, the thick end of
the section is known as the leading edge. The small tapering end is
the trailing edge. The distance between the leading edge and the
trailing edge is known as the chord of the airfoil.
Slide 7
The rotary wing blade in a helicopter is asymmetrical, that is
it has a curvature that changes along the entire length of the
chord. If the blade were symmetrical, then the chord line would be
a straight line from the leading to the trailing edges. Since the
curvature changes constantly in rotor blade, the result is that the
chord of the blade also changes. When computing the chord line of
this type of blade, an average or mean aerodynamic chord (MAC)
becomes apparent. When a blade (airfoil) is moved through the air,
a stream of air flows over and under it. The blade is designed so
that the flow of air will be smooth and will conform to the shape
of the moving blade. If the blade is set at the proper angle and
made to move fast enough, the airflow will support the weight of
the blade. This is the nature of the action that enables rotary
wings to furnish enough lift to sustain the helicopter in
flight.
Slide 8
Hence by applying bernoullis theorem we can observe that lift
is produced by a lower pressure created on the upper surface of the
helicopters wings compared to the pressure on the wing's lower
surfaces, causing the wing to be lifted upward. The special shape
of the rotor (airfoil) is designed so that air flowing over it will
have to travel a greater distance and faster resulting in a lower
pressure area thus lifting the wing upward.
Slide 9
Lift equation Lift depends upon: (1) shape of the airfoil (2)
the angle of attack (3) the area of the surface exposed to the
airstream (4) the square of the air speed (5) the air density.
Where, L is lift force, is air density, v is air speed over the
airfoil, A is wing area, and C L is the lift coefficient at the
desired angle of attack
Slide 10
Lift in an established flow Established flow may be considered
as steady, laminar & incompressible flow. As fluid never
crosses a streamline in a steady flow; hence mass is conserved
within each streamtube. One streamtube travels over the upper
surface, while the other travels over the lower surface; dividing
these two tubes is a dividing line that intersects the airfoil on
the lower surface, typically near to the leading edge. The
streamline leaves the airfoil at the sharp trailing edge, a feature
of the flow known as the Kutta condition.
Slide 11
This image shows the streamlines over a NACA 0012 airfoil of
the real flow. The flow approaching an airfoil can be divided into
two streamtubes, which are defined based on the area between two
streamlines.
Slide 12
The upper stream tube constricts as it flows up and around the
airfoil, a part of the so-called upwash. From the conservation of
mass, the flow speed must increase as the stream tube area
decreases. The area of the lower stream tube increases, causing the
flow inside the tube to slow down. It is typically the case that
the air parcels traveling over the upper surface will reach the
trailing edge before those traveling over the bottom.
Slide 13
From Bernoulli's principle, the pressure on the upper surface
where the flow is moving faster is lower than the pressure on the
lower surface. The pressure difference thus creates a net
aerodynamic force, pointing upward and downstream to the flow
direction. The component of the force normal to the free stream is
considered to be lift; the component parallel to the free stream is
drag. In conjunction with this force by the air on the airfoil, by
Newton's third law, the airfoil imparts an equal-and-opposite force
on the surrounding air that creates the downwash.
Slide 14
Principle of Helicopter Flight Helicopter Lift is obtained by
means of one or more power driven horizontal propellers which
called Main Rotor. When the main rotor of helicopter turns, it
produces lift and reaction torque. Reaction torque tends to make
helicopter spin. On most helicopters, a small rotor near the tail
which called tail rotor compensates for this torque. On twin rotor
helicopter the rotors rotate in opposite directions, their
reactions cancel each other.
Slide 15
MAIN ROTOR The lifting force is produced by the main rotor. As
they spin in the air and produced the lift. Each blade produces an
equal share of the lifting force. The weight of a helicopter is
divided evenly between the rotor blades on the main rotor system.
If a helicopter weight 4000 lbs and it has two blades, then each
blade must be able to support 2000 lbs. In addition to the static
weight of helicopter,each blade must be accept dynamic load as
well. For example, if a helicopter pull up in a 1.5 time the
gravity force, then the effective weight of helicopter will be 1.5
time of static helicopter weight or 6000 lbs. due to gravitational
pull.
Slide 16
The tail rotor in normally linked to the main rotor via a
system of drive shafts and gearboxes.Most helicopter have a ratio
of 3:1 to 6:1. In most helicopter the engine turns a shaft that
connected to an input quill in the transmission gearbox.
Slide 17
Torque Reaction If you spin a rotor with an engine, the rotor
will rotate,but the engine and helicopter body will tend to rotate
in opposite direction to the rotor. This is called Torque reaction.
Newton's third law of motion states, " to every action there is an
equal and opposite reaction". The tail rotor is used to compensates
for this torque and hold the helicopter straight.
Slide 18
Dissymmetry of Lift All rotor systems are subject to
Dissymmetry of Lift in forward flight. At a hover, the lift is
equal across the entire rotor disk. As the helicopter gain air
speed, the advancing blade develops greater lift because of the
increased airspeed and the retreating blade will produce less lift,
this will cause the helicopter to roll. In order to overcome this
problem blade flapping is done.
Slide 19
Dissymmetry of lift in helicopter aerodynamics refers to an
uneven amount of lift on opposite sides of the rotor disc. The
dissymmetry is caused by differences in relative airspeed between
the advancing blade and the retreating blade.
Slide 20
Blade Flapping Dissymmetry of lift is compensated by blade
flapping,because of the increased airspeed and lift on the
advancing blade will cause the blade to flap up and decreasing the
angle of attack. The decreased lift on the retreating blade will
cause the blade to flap down and increasing the angle of attack.
The combination of decreased angle of attack on the advancing blade
and increased angle of attack on the retreating blade through blade
flapping action tends to equalize the lift over the two halves of
the rotor disc.
Slide 21
Flight Control Swash plate assembly : Its primary component is
the swash plate, located below the rotor head. This swash plate
consists of one non-revolving disc and one revolving disc mounted
directly on top. The swash plate is connected to the cockpit
control sticks and can be made to tilt in any direction, according
to the cyclic stick movement made by the pilot, or moved up and
down according to the collective lever movement.
Slide 22
The Collective Control :. The collective control is made by
moving a lever that rises up from the cockpit floor to the left of
the pilot's seat, which in turn raises or lowers the swash plate on
the main rotor shaft, without tilting it. This lever only moves up
and down and corresponds directly to the desired movement of the
helicopter; lifting the lever will result in the helicopter rising
while lowering it will cause the helicopter to sink
Slide 23
The Cyclic Control : The cyclic control works by tilting the
swash plate and changing the pitch angle of a rotor blade at a
given point in the rotation. As the pitch angle changes, so the
lift generated by each blade changes and as a result the helicopter
becomes 'unbalanced', and so tips towards whichever side is
experiencing the lesser amount of lift. Thus with its help,
helicopter can move right or left, backward or forward.