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FAA AC 61-13B Basic Helicopter Handbook




Chapter 1. GENERAL AERODYNAMICS

Unless otherwise indicated, this handbook is based on a helicopter that has the following characteristics:

1 - An unsupercharged (normally aspirated) reciprocating engine.

2 - A single main rotor rotating in a counterclockwise direction (looking downward on the rotor) (US
configuration).

3 - An antitorque (tail) rotor.

4 - Skid-type landing gear.

Information is intended to be general in nature and should apply to most helicopters having these
characteristics.

Before launching into a detailed discussion of the various forces acting on a helicopter in flight, it is first
necessary that you understand the meaning of a few basic aerodynamic terms, how the force of lift is created,
and the effect that certain factors have on lift.

Airfoil - An airfoil is any surface designed to produce lift or thrust when air passes over it. Propellers and
wings of airplanes are airfoils. Rotor blades on helicopters are airfoils. The wing of an airplane is normally an
unsymmetrical airfoil, that is, the top surface has more curvature than the lower surface.

The main rotor blades of most helicopters are symmetrical airfoils; that is, having the same curvature on both
upper and lower surfaces (see fig. 1 below). Much research, however, is being conducted in the use of
unsymmetrical airfoils for main rotor blades, and at least one currently manufactured make of helicopter is
equipped with main rotor blades that are not considered true symmetrical airfoils.

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On an unsymmetrical airfoil, the center of pressure is variable - as the angle of attack increases, the center of
pressure moves forward along the airfoil surface; as the angle of attack decreases, the center of pressure moves
rearward. On a symmetrical airfoil, center of pressure movement is very limited. A symmetrical airfoil is
preferred for rotor blades so that a relatively stable center of pressure is maintained. Improvements in control
systems may allow more latitude in blade designs in the future.

Chord line - The chord line of an airfoil is an imaginary straight line from the leading edge to the trailing edge
of the airfoil



Relative wind - Relative wind is the direction of the airflow with respect to an airfoil. If an airfoil moves
forward horizontally, the relative wind moves backward horizontally (see fig. 3 below).

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the layer of air at sea level would be denser than the layer of air at the earth's surface at Denver, Colo., at
approximately 1 mile above sea level.

The above principle may be applied in flying aircraft. At lower levels the rotor blade is cutting through more
and denser air, which offers more support (lift) and increases air resistance. The same amount of power, applied
at higher altitudes where the air is thinner and less dense, propels the helicopter faster.

Density altitude

Density altitude refers to a theoretical air density which exists under standard conditions of a given altitude.
Standard conditions at sea level are:

Atmospheric pressure - 29.92 in. of Hg (inches of mercury)

Temperature - 59° F. (15° C.)

Standard conditions at any higher altitude are based on:

Atmospheric pressure (reduced to sea level): 29.92 in. of Hg

Temperature: 59° F. (15° C.) minus 3 1/2° F. (2° C.) per 1,000 feet elevation

For example, if the atmospheric pressure (reduced to sea level) at an airport located 5,000 feet above sea level
is 29.92 inches of mercury and the temperature is 59° - (3.5° x 5) = 41.5° F. (5° C.), the air density is standard
at that altitude. (The actual barometric pressure at an elevation of 5,000 feet under these conditions would be
approximately 24.92 inches of mercury since atmospheric pressure decreases approximately 1 inch per 1,000-
foot increase in altitude. The average temperature decrease per 1,000-foot increase in altitude is 3.5° F.).

Figure 51 shows a density altitude chart. If we locate the +5° vertical line along the bottom of the chart, follow
this line up to its intersection with the 5,000-foot diagonal line, then follow the horizontal line to the left side of
the chart, we read a density altitude of 5,000 feet.

Figure 51 - Density altitude chart.

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The four factors which affect density altitude are altitude, atmospheric pressure, temperature, and moisture
content of the air.

Altitude

We have already seen the effects of altitude on air density in the first section of this chapter. The greater the
elevation of an airport or landing area, the less the atmospheric pressure and, consequently, the less dense the
air. The less dense the air, the greater the density altitude. What is the result when operating at a high density
altitude? Helicopter performance is decreased (see fig. 52, below). It can be seen from the density altitude chart
that, as altitude increases, density altitude increases.

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center of gravity - An imaginary point where the resultant of all weight forces in the body may be considered to
be concentrated for any position of the body.

center of pressure - The imaginary point on the chord line where the resultant of all aerodynamic forces of an
airfoil section may be considered to be concentrated.

centrifugal force - The force created by the tendency of a body to follow a straight-line path against the force
which causes it to move in a curve, resulting in a force which tends to pull away from the axis of rotation.

chord - An imaginary straight line between the leading and trailing edges of an airfoil.

collective pitch control - The method of control by which the pitch of all rotor blades is varied equally and
simultaneously.

coriolis effect - The tendency of a mass to increase or decrease its angular velocity when its radius of rotation is
shortened or lengthened, respectively.

cyclic pitch control - The control which changes the pitch of the rotor blades individually during a cycle of
revolution to control the tilt of the rotor disc, and therefore, the direction and velocity of horizontal flight.

delta hinge (flapping hinge) - The hinge with its axis parallel to the rotor plane of rotation, which permits the
rotor blades to flap to equalize lift between the advancing blade half and retreating blade half of the rotor disc.

density altitude - Pressure altitude corrected for temperature and humidity.

disc area - The area swept by the blades of the rotor. This is a circle with its center at the hub axis and a radius
of one blade length.

disc loading - The ratio of helicopter gross weight to rotor disc area (total helicopter weight divided by the rotor
disc area).

dissymmetry of lift - The unequal lift across the rotor disc resulting from the difference in the velocity of air
over the advancing blade half and retreating blade half of the rotor disc area.

feathering axis - The axis about which the pitch angle of a rotor blade is varied. Sometimes referred to as the
spanwise axis.

feathering action - That action which changes the pitch angle of the rotor blades periodically by rotating them
around their feathering (spanwise) axis.

flopping - The vertical movement of a blade about a delta (flapping) hinge.

freewheeling unit - A component part of the transmission or power train which automatically disconnects the
main rotor from the engine when the engine stops or slows below the equivalent of rotor RPM.

ground effect - A beneficial gain in lifting power when operating near the surface - caused by the rotor
downwash field being altered from its free air state by the presence of the surface.

gyroscopic precession - A characteristic of all rotating bodies. When a force is applied to the periphery of a
rotating body parallel to its axis of rotation, the rotating body will tilt in the direction of the applied force 90°
later in the plane of rotation.

hovering in ground effect - Operating at such an altitude (usually less than one rotor diameter above the
surface) that the influence of ground effect is realized.

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hunting - The tendency of a blade (due to coriolis effect) to seek a position ahead of or behind that which would
be determined by centrifugal force alone.

pitch angle - The angle between the chord line of the rotor blade and the reference plane of the main rotor hub
or the rotor plane of rotation.

rigid rotor - A rotor system with blades fixed to the hub in such a way that they can feather but cannot flap or
drag.

semirigid rotor - A rotor system in which the blades are fixed to the hub but are free to flap and feather.

slip - The controlled flight of a helicopter in a direction not in line with its fore and aft axis.

solidity ratio - The ratio of total rotor blade area to total rotor disc area.

stall - The stall condition on the retreating blade which occurs at high forward airspeeds.

standard atmosphere - Atmospheric conditions in which (1) the air is a dry, perfect gas; (2) the temperature at
sea level is 59° F. (15° C.); (3) the pressure at sea level (or reduced to sea level) is 29.92 inches of Hg; and (4)
the temperature gradient is approximately 3.5° F. per 1,000-foot change in altitude.

tip-path plane - The plane in which rotor blade tips travel when rotating.

tip speed - The rotative speed of the rotor at its blade tips.

torque - A force or combination of forces that tends to produce a countering rotating motion. In a single rotor
helicopter where the rotor turns counterclockwise, the fuselage tends to rotate clockwise (looking down on the
helicopter).

translational lift - The additional lift obtained through airspeed because of increased efficiency of the rotor
system, whether it be when transitioning from a hover into horizontal flight or when hovering in a wind.

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