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and dramatically increases as airspeed increases (the square
of the velocity). Therefore, doubling the airspeed quadruples
parasite drag. [Figure 2-15]
The WSC aircraft can be designed for the purpose of being
a slow fl ying aircraft with a large wing where drag is not a
major concern, or can be designed to be a fast fl ying aircraft
with a small wing where drag is more of a concern.
2-9
Figure 2-16. Air flow around objects.
Figure 2-17. Fast WSC aircraft with complete streamlining (top) and
slow WSC aircraft with minimum streamlining (bottom).
The aircraft has plenty of items (area) for the wind to
strike including wing, wires, struts, pilot, carriage, engine,
wheels, tubes, fuel tanks, etc. Parasitic drag can be reduced
by streamlining the items. Round tubes can be streamlined
reducing the drag to one-third, and cowlings can be used
to streamline the pilot and the carriage completely, but not
without the additional expense and additional weight of the
streamlining. Streamlining does make a noticeable difference
in the speed and gas mileage of the WSC, especially for the
faster aircraft. [Figure 2-16]
With the large speed range of WSC aircraft, weight,
complexity, amount and expense of streamlining, and
resultant drag reduction are determined by the specific
mission for the aircraft and the manufacturers’ make and
model. [Figure 2-17]
Total drag is the combination of parasite and induced drag.
Total drag = parasitic drag + induced drag
To help explain the force of drag, the mathematical equation
D = CD x q x S is used. The formula for drag is the same as
the formula for lift, except the CD is used instead of the CL.
In this equation, drag (D) is the product of the coeffi cient of
drag (CD), dynamic pressure (q) determined by the velocity
squared times the air density factor, and surface area (S) of
the carriage and wing. The overall drag coeffi cient is the ratio
of drag pressure to dynamic pressure.
Induced and parasitic drag have opposite effects as AOA
decreases and speed increases. Note the total drag in
Figure 2-18. It is high at the slowest air speeds at high angles
of attack near the stall, decreases to the lowest at the most
effi cient airspeed, and then progressively increases as the
speed increases. The WSC wing can fl y with a large range
of airspeeds.
Generally, the most effi cient speed is at the lowest total
drag providing the best rate of climb, glide ratio, and cruise
economy. However, slower speeds provide higher angles
of climb, and faster speeds provide quicker transportation.
[Figure 2-18]
2-10
Drag
High Angle of Attack–
Low Speed
Low Angle of Attack–
High Speed
Total Drag
Stall
Induced Drag
Parasite Drag
LD-MAX
Figure 2-18. Airspeed versus drag.
CG
1 5
Glideslope
Root Wing Chord
Relative Wind
Flightpath
Lift
Drag
WL
WD
Weight
Resultant force
of lift and drag
components
that support the
weight during
flight
Component
of weight
that
opposes
lift (WL)
Component of weight acting along flight path.
(some call this thrust component during gliding flight)(WD)
Resultant force
Angle of Attack
Figure 2-19. Typical forces in gliding flight with no engine thrust.
to produce level fl ight, the relative wind stream becomes
horizontal with the Earth and the AOA remains about the
same. As described for the airplane in the Pilot’s Handbook
of Aeronautical Knowledge, thrust equals total drag for level
fl ight. [Figure 2-20]
When in straight and level, unaccelerated fl ight:
Lift (L) = Weight (W)
Thrust = Total Drag (DT)
At a constant airspeed, when excess thrust is added to produce
climbing fl ight, the relative air stream becomes an inclined
plane leading upward while AOA remains about the same.
The excess thrust determines the climb rate and climb angle
of the fl ightpath. [Figure 2-21]
When in straight and climbing, unaccelerated fl ight:
Lift (L) = Component of weight that opposes lift
Weight (W) = Resultant force (FR) of lift (L) and excess
thrust to climb (TE)
Thrust = Total drag (DT) plus rearward component of
weight
Weight
Weight is a measure of the force of gravity acting upon the
mass of the WSC aircraft. Weight consists of everything
directly associated with the WSC aircraft in flight: the
combined load of the total WSC aircraft (wing, wires, engine,
carriage, fuel, oil, people, clothing, helmets, baggage, charts,
books, checklists, pencils, handheld global positioning
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Weight-Shift Control Aircraft Flying Handbook(20)
