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Airfoils
A 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 center 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.
Nonsymmetrical (cambered) 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 center of pressure
location moved too much when angle of attack was changed. When center 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
nonsymmetrical airfoils.
Airfoil Sections
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.
Paul Cantrell
paul at copters.com
(replace " at " with "@" to email me - this avoids SPAMMERS I hope)
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