AIRFOIL AND GEOMETRY
SELECTION
Airfoil geometry
Airfoil Pressure Distribution
Airfoil flow field and Circulation.
Effect of camber on separation
Airfoil lift, drag, and pitching moment
Typical airfoils
Airfoil Families
• The early airfoils were developed mostly by trial and error.
• In the 1930's, the NACA developed a wldely-used family of
mathematically defined airfoils called the "four-digit" airfoils.
• In these the first digit defined the percent camber, the second
defIne the location of the maximum camber, and the last two digits
defined the maximum thickness in percent of chord.
• While rarely used for Wing design today, the uncambered four-digit
airfoils are still commonly used for tall surfaces of subsonic aircraft.
• The NACA five-digit airfoils were developed to allow shifting the
position of maximum camber forward for greater maximum lift.
• The six-series airfoils were designed for increased laminar flow,
and hence reduced drag. Six-series airfoils such as the 64A series
are still widely used as a starting point for high-speed-wing design.
• The Mach 2 F-15 fighter uses the 64A airfoil modified with camber
at the leading edge.
Airfoil Design
• In the past, the designer would select an airfoil (or airfoils) from
such atalog. This selection would consider factors such as the airfoil
drag during cruise, stall and pitching-moment characteristics, the
thickness available for structure and fuel and the ease of
manufacture.
• With today's computational airfoil design capabilities, it is becoming
common for the airfoil shapes for a wing to be custom-designed.
• Modern airfoil design is based upon inverse computational
solutions for desired pressure (or velocity) distributions on the
airfoil. Methods have been developed for designing an airfoil such
that the pressure differential between the top and bottom of the
airfoil quickly reaches a maximum value attainable without airflow
separation.
• Toward the rear of the airfoil, various pressure recovery schemes
are employed to prevent separation near the trailing edge.
Laminar airfoil
Transonic effects
Design lift coefficient
Types of Stall
Stall characteristics
• Some airfoils exhibit a gradual reduction in lift during a stall, while others
show a violent loss of lift, accompanied by a rapid change in pitching
moment.
• This difference reflects the existence of three entirely different types of
airfoil stall.
• "Fat" airoils (round leading edge and t / c greater than about 14 %) stall
from the trailing edge. The turbulent boundary layer increases with angle of
attack . At around 10 deg the boundary layer begins to separate, starting at
the trailing edge and moving forward as the angle of attack is further
increased. The loss of lift is gradual. The pitching moment changes only a
small amount.
• Thinner airfoils stall from the leading edge. If the airfoil is of moderate
thickness (about 6-14%), the flow separates near the nose at a very small
angle of attack, but Immediately reattaches itself so that little effect is felt.
• At some higher angle of attack the flow fails to reattach, which almost
immediately stalls the entire airfoil. This causes an abrupt change in lift and
Pitching moment.
• Very thin airfoils exhibit another form of stall. As before, the flow separates
from the nose at a small angle of attack and reattaches almost
Airfoil Thickness Ratio
Effects of t/c on drag
Effect of t/c on critical mach number
Effect of t/c on maximum lift.
Thickness ratio historical trend.
Effect of aspect ratio on lift
Wing Vertical Location
• The wing vertical location with respect to the
fuselage is generally set by the real-world
environment in which the aircraft will operate.
• For example, virtually all high-speed
commercial transport aircraft are of low-wing
design, yet military transport aircraft designed
to similar mission profiles and payload
weights are all of high-wing design
High Wing Configuration
• The major benefit of a high wing is that it allows placing the
fuselage closer to the ground. For military transport aircraft such as
the C-5 and C-141, this allows loading and unloading the cargo
without special ground handling gear. In fact, these aircraft place
the floor of the cargo compartment about 4-5 ft off the ground,
which is the height of the cargo area of most trucks.
• If cargo is needed at a remote field lacking ground handling gear,
the trucks can be backed right up to the aircraft for loading.
• With a high wing, jet engines or propellers will have sufficient
ground clearance without excessive landing-gear length. Also, the
wing tips of a swept high wing are not as likely to strike the ground
when in a nose-high, rolled attitude. For these reasons, landing-
gear weight is generally reduced for a high-wing aircraft.
Wing Config , Sweep & Dihedral
Wing Config
• For low-speed aircraft, external struts can be used to greatly lower
wing weight. However, external struts add substantially to the drag.
Since roughly two-thirds of the lift is contributed by the upper
surface of the wing, it follows that less drag impact will be seen if
the strut disturbs the airflow on the lower surface of the wing than
if the strut is above the wing, as would be necessary for a strut-
brace, low wing.
• Another structural benefit occurs if the wing box is carried over the
top of the fuselage rather than passing through it. When the wing
box passes through the fuselage, the fuselage must be stiffened
around the cut-out area.
• This adds weight to the fuselage. However, passing the wing box
over the fuselage will increase drag due to the increase in frontal
area.
Case of a STOL ac
• For an aircraft designed with short takeoff and landing
(STOL) requirements, a high wing offers several advantages.
• The high position allows room for the very large wing flaps
needed for a high lift coefficient. The height of the wing
above the ground tends to prevent "floating," where the
ground effect increases lift as the aircraft approaches the
ground.
• A floating tendency makes it difficult to touch down on the
desired spot.
• Finally, most STOL designs are also intended to operate
from unimproved fields. A high wing places the engines and
propellers away from flying rocks and debris.
Disadvantages
• There are several disadvantages to the high-wing arrangement.
While landing-gear weight tends to be lower than other
arrangements, the fuselage weight is usually increased because it
must be strengthened to support the landing-gear loads. In many
cases an external blister is used to house the gear in the retracted
position. This adds weight and drag.
• The fuselage is also usually flattened at the bottom to provide the
desired cargo-floor height above ground. This flattened bottom is
heavier than the optimal circular fuselage. If the top of the fuselage
is circular, a fairing is required at the wing-fuselage junction.
• For small aircraft, the high wing arrangement can block the pilot's
visibility in a turn, obscuring the direction toward which the aircraft
is turning.
• Also, the high wing can block upward visibility in a climb.
Mid Wing Configuration
Mid Wing for Fighter design
• If the fuselage is roughly circular and fairings are not used, the mid-
wing arrangement provides the lowest drag. High- and low-wing
arrangements must use fairings to attain acceptable interference drag
with a circular fuselage.
• The mid wing offers some of the ground clearance benefits of the
high wing. Many fighter aircraft are mid-winged to allow bombs and
missiles to be carried under the wing. A high-wing arrangement
would restrict the pilot's visibility to the rear-the key to survival of a
fighter in combat.
• The mid-wing arrangement is probably superior for aerobatic
maneuverability.
• The dihedral usually required for adequate handling qualities in a
low-wing design in normal flight will act in the wrong direction during
inverted flight, making smooth aerobatic manuevers difficult.
• Also, the effective-dihedral contribution of either high or low wings
will make it more difficult to perform high-sideslip maneuvers such as
the knife-edge pass.
Disadvantages
• Structural carrythrough presents the major problem with the mid
wing.
• The bending moment produced by the lift on the wing must be
carried across the fuselage either by an extension of the wing box
("wing carrythrough box") or by a set of massive ring frames built
into the fuselage.
• The carrythrough box often proves lighter, but cannot be used in a
midwing design that must carry cargo or passengers. (One
exception to this, the
• German Hansa executive jet, uses a mild forward sweep to place
the carrythrough box behind the passenger compartment.) A
carrythrough box is also difficult to incorporate in a mid-wing
fighter, in which most of the fuselage will be occupied by the jet
engines and inlet ducts.
Low Wing Design
Advantages
• The major advantage of the low-wing approach
comes in landing-gear stowage. With a low wing,
the trunnion about which the gear is retracted
can be attached directly to the wing box which,
being strong already, will not need much extra
strengthening to absorb the gear loads.
• When retracted, the gear can be stowed in the
wing itself, in the wing-fuselage fairing, or in the
nacelle. This eliminates the external bliste usually
used with the high-wing approach
• To provide adequate engine and propeller clearance, the
fuselage must be placed farther off the ground than for a
high-wing aircraft. While this adds to the landing-gear
weight, it also provides greater fuselage ground clearance.
• This reduces the aft-fuselage upsweep needed to attain the
required takeoff angle of attack. The lesser aft-fuselage
upsweep reduces drag.
• While it is true that the low-wing arrangement requires
special ground equipment for loading and unloading large
airplanes, the high-speed commercial transports are only
operated out of established airfields with a full
complement of equipment. This is the main reason why
military and commercialtransports are so different.
