helicopter flies for the same basic reason that any conventional aircraft flies,
because aerodynamic forces necessary to keep it aloft are produced when air
passes about the rotor blades. The rotor blade, or airfoil, is the structure
that makes flight possible. Its shape produces lift when it passes through the
air. Helicopter blades have airfoil sections designed for a specific set of
flight characteristics. Usually the designer must compromise to obtain an
airfoil section that has the best flight characteristics for the mission the
aircraft will perform.
Airfoil sections are of two
basic types, symmetrical and nonsymmetrical. Symmetrical airfoils
have identical upper and lower surfaces. They are suited to rotary-wing
applications because they have almost no centre of pressure travel. Travel
remains relatively constant under varying angles of attack, affording the best
lift-drag ratios for the full range of velocities from rotor blade root to tip.
However, the symmetrical airfoil produces less lift than a nonsymmetrical
airfoil and also has relatively undesirable stall characteristics. The
helicopter blade must adapt to a wide range of airspeeds and angles of attack
during each revolution of the rotor. The symmetrical airfoil delivers acceptable
performance under those alternating conditions. Other benefits are lower cost
and ease of construction as compared to the nonsymmetrical airfoil.
airfoils may have a wide variety of upper and lower surface designs. They are
currently used on some CH-47 and all OH-58 Army helicopters, and are
increasingly being used on newly designed aircraft. Advantages of the
nonsymmetrical airfoil are increased lift-drag ratios and more desirable stall
characteristics. Nonsymmetrical airfoils were not used in earlier helicopters
because the centre of pressure location moved too much when angle of attack was
changed. When centre of pressure moves, a twisting force is exerted on the rotor
blades. Rotor system components had to be designed that would withstand the
twisting force. Recent design processes and new materials used to manufacture
rotor systems have partially overcome the problems associated with use of
Rotary-wing airfoils operate
under diverse conditions, because their speeds are a combination of blade
rotation and forward movement of the helicopter. An intelligent discussion of
the factors affecting the magnitude of rotor blade lift and drag requires a
knowledge of blade section geometry. Blades are designed with specific geometry
that adapts them to the varying conditions of flight. Cross-section shapes of
most rotor blades are not the same throughout the span. Shapes are varied along
the blade radius to take advantage of the particular airspeed range experienced
at each point on the blade, and to help balance the load between the root and
tip. The blade may be built with a twist, so an airfoil section near the root
has a larger pitch angle than a section near the tip.
The chord line is a
straight line connecting the leading and trailing edges of the airfoil.
The chord is the length
of the chord line from leading edge to trailing edge and is the characteristic
longitudinal dimension of the airfoil.
The mean camber line is
a line drawn halfway between the upper and lower surfaces. The chord line
connects the ends of the mean camber line.
The shape of the mean camber
is important in determining the aerodynamic characteristics of an airfoil
section. Maximum camber (displacement of the mean camber line from the
chord line) and the location of maximum camber help to define the shape of the
mean camber line. These quantities are expressed as fractions or percentages
of the basic chord dimension.
Thickness and thickness
distribution of the profile are important properties of an airfoil section.
The maximum thickness and its location help define the airfoil
shape and are expressed as a percentage of the chord.
The leading edge radius
of the airfoil is the radius of curvature given the leading edge shape.