The Four Forces of
Flight (HFH 2-2 thru 2-7):
Lift: Upward force
produced by the
wings/rotor.
Weight: Downward force
caused by gravity.
Thrust: Forward force
produced by the
engine/rotor.
Drag: Rearward force
caused by air resistance.
Lift opposes weight.
Thrust opposes drag
Lift is created by airfoils that take advantage of two scientific
principles. Bernoulli's Principle and Newton's Third Law of
Motion.
Bernoulli’s Principle(
HFH 2-3) describes the
relationship between
internal fluid pressure
and fluid velocity. It is a
statement of the law of
conservation of energy
and helps explain why
an airfoil develops an
aerodynamic force.
It is the same high pressure seeking low pressure and
as the airflow increases in speed above an airfoil its
pressure decrease, which in turn causes the higher
pressure air below the airfoil to “seek it”. Which creates
lift.
Newton’s Third Law of Motion:(HFH 2-5)”For every
action there is a equal and opposite reaction”. When
the rotors hit the air at high they send’s the air
downward.
Resultant Relative Wind(HFH 2-11): The wind actually
felt by the rotor blades in flight.
Angle of Incidence (HFH 2-12): The angle at which an
airfoil (wing or rotor blade ) is attached to the fuselage ;
The angle between the chord line and the rotational
relative wind . Helicopter rotor blades have a variable
AOI.
Angle of Attack (HFH 2-13 ): The angle between the
chord line and the resultant relative wind . Can be
thought of as the angle of the air to the blade.
Center of Pressure(HFH 2-8): The point where the sum Weight(HFH 2-5): Normally weight is thought of as a
fixed value.It is not. To lift the helicopter off the
of all aerodynamic forces acts on an airfoil.
ground vertically, the rotor disk must generate
Root(HFH 2-9): the inner end of the blade and is the
enough lift to overcome or offset the total weight of
point that attaches to the hub
the helicopter and its occupants. Newton’s First Law
Hub(HFH 2-9):The attaching point for the root, and
states: “Every object in a state of uniform motion
the axis about which the blades rotate.
tends to remain in that state of motion unless an
Blade Span(HFH 2-13): The length of the rotor blade
external force is applied to it.” In this case, the object
from the root to the tip.
is the helicopter whether at a hover or on the ground
Blade Twist(HFH 2-9): Since the tip of a helicopter
and the external force applied to it is lift, which is
rotor blade experiences a higher airspeed than the root accomplished by increasing the pitch angle of the
, the blade is designed with twist. This is the change in
main rotor blades. The weight also changes as the G
blade incidence from the root to the tip.
forces or load factor changes.
Thrust(HFH 2-6): Thrust, like lift, is generated by the
rotation of the main rotor disk. In a helicopter unlike a
airplane thrust can be forward, rearward, sideways,
or vertical. The resultant lift and thrust determines
the direction of the helicopter. Many helicopter
accidents are caused from the rotor disk being
overloaded. Simply put, pilots attempt maneuvers
that require more lift than the rotor disk can produce
or more power than the helicopter’s power plant can
provide(settling with power). The tail rotor also
produces thrust. The amount of thrust is variable
through the use of the anti torque pedals and is used
to control the helicopter’s yaw.
Drag(HFH 2-6, 2-7) Defined as the forces that resist the
movement of a helicopter when lift is created. Drag always acts
parallel to relative wind. Wind is composed of three types.
-Profile drag which is the friction of the blades moving through the
air. There are two parts to Profile drag, form and friction.Form is
the shape of the airfoil. Friction is how smooth the airfoil is.
-Induced drag is generated by the airflow circulation around the
rotor blade as it creates lift.The high-pressure area beneath the
blade joins the low-pressure area above the blade at the trailing
edge and at the rotor tips. This causes a spiral, or vortex, which
trails behind each blade whenever lift is being produced.
-Parasite drag is created by the things not creating lift. Which
includes things like the fuselage, cabin, antennas, etc.
In straight-and-level flight, all four forces are in balance.
In climbs and descents, lift must exceed weight to climb and must
be lower to descent.
In straight-and-level flight, all four forces are in balance.
In climbs and descents , lift must exceed weight to climb and must be
lower to descent.
In turns as angle of bank increases the total lift is tilted more horizontal
, thus to keep the same altitude lift must be increased.
Gyroscopic Precession(HFH 217): Spinning objects act like a
gyroscope. As such they have
precession. Which is the
resultant deflection of a spinning
objects when a force is applied
to it. This action occurs
approximately 90 degrees in the
direction of rotation.
Dissymmetry of lift(HFH 220): Is the unequal lift
between the advancing
and retreating blade. To
compensate for
Dissymmetry of lift the
blades flap
.
Blade Flapping(HFH 2-20): When in a hover the main rotor will be
perfectly level. The entire rotor disk AOI is the same.(disregarding
translating tendency). To compensate for the unequal lift the rotor
blade with more lift will flap up and because of the rigid in plane
rotor system of the R44 the other rotor blade will flap down which
will cause an increase in AOI.
Coning(HFH 2-16):As lift on the blades is increased (in a takeoff,
for example), two major forces are acting at the same time—
centrifugal force acting outward, and lift acting upward. The result of
these two forces is that the blades assume a conical path instead of
remaining in the plane perpendicular to the mast
Coriolis Effect (Law of Conservation of
Angular Momentum)(HFH 2-16/17): In a
rotating object the closer something is to the
center the faster it spins. This is best shown with
a ice skater. This also happens when coning
happens as the blades get slightly closer to the
mast they will increase in rotor RPM. This is
usually negated as its happening as collective is
raise which negates it.
Retreating blade stall(HFH 11-10): A stall that
begins at or near the tip of a blade in a
helicopter because of the high angles of attack
required to compensate for dissymmetry of lift
at high speed.
Torque Effect: The torque that is created by the
main rotor spinning. It is compensated with the tail
rotor.
Pendular Action(HFH 2-15): Since a single main
rotor helicopter is suspended from a single point and
has considerable mass, it is free to oscillate either
laterally or longitudinally in the same way as a
pendulum. Because of this movements with the cyclic
should be smooth and not exaggerated as they will
only compound and become more and more
uncontrollable.
Hover to forward flight
Translating Tendency(Drift)(HFH 2-15):
During hovering flight, a single main rotor
helicopter tends to drift in the direction of tail
rotor thrust. In a American style helicopter that
spins counter-clockwise this is to the right.
Rotational Relative Wind(HFH 2-23): Is the
airflow that travels parallel and opposite to the
direction of the rotor blades. This is the wind
that runs into the airfoil.
Induced Flow(HFH 2-11, 2-23): When the rotors
create lift air is accelerated over the blades
downward. Anytime a helicopter is creating lift it
moves large amounts of air downward. This is called
the down wash or induced flow.
Transverse Flow Effect(HFH 2-23): As the helicopter
accelerates in forward flight, induced flow drops to near
zero at the forward disk area and increases in the aft
disk area. This difference is called Transverse Flow
Effect. This increase in AOA at the front causes the
front to flap up and the rear to flap down. But because
of Gyroscopic Precession this isn't felt until 90 degrees
and this turns into a right roll. This happens at roughly
20 knots or right before ETL.
Translational Lift(HFH 2-21): With every know of
increase forward airspeed the rotor system becomes
more efficient. As forward airspeed is produced by
aircraft movement, turbulence and vortices are left
behind and the flow of air becomes more horizontal.
This also causes the "blow back" in combination with
dissymmetry of lift and transverse flow effect.
Effective Translational Lift(HFH 2-22): Between
16-24 knots the helicopter transitions into "
Effective Translational Lift". Which is when the
entire helicopter has completely outrun its own
vortices and is in completely undisturbed air. The
air flow is more horizontal which reduces induced
flow and drag with a corresponding increase in
angle of attack and lift. Also refereed to as ETL.