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Beginner's
Guide to Aerodynamics
Drag Equation  Level 1
Answers
 Click on The
Drag Equation to open the appropriate slide. Study the equation
and read the explanation of this concept. Then use the information to
complete the questions below.
 In the following
table, identify each variable in the equation. State the appropriate
units for each value:
Variable

Identity

Metric
Units

English
Units

D

Drag

Newtons

Pounds

Cd

Drag Coefficient

No units

No units

r

Density of air

kg/m^{3}

slug/ft^{3}

V

Velocity

m/s

ft\s

A

Reference Area

m^{2}

ft^{2}

A. What possible reference areas (A) can be used to compute the drag?
Total surface area of aircraft, frontal
area of aircraft, or wing area.
B.
If we want to compute the L/D (lift to drag ratio), which reference
area would we want to use? Why?
Wing area, since lift is computed using
the wing area.
C.
Besides reference area, air density, and velocity, drag depends on
several other values. What variable is used to model many of the complex
dependencies of drag? What are some of the dependencies of drag incorporated
in this value?
Cd  drag coefficient. Some of the dependencies
of drag incorporated in this value are shape, inclination and some
flow conditions.
 Calculate the following
problems which involve the drag of an aircraft: (Before computing,
make sure all units agree)
A. Suppose you
are flying an aircraft with the following wing shape and dimensions.
What is the total wing area?
A = 1/2h(b_{1
}+ b_{2}) = 1/2 x 76.48(25.3 + 48.9) = 2834.44 ft^{2
}for one wing
= 5668.88 ft^{2} for both wings
B. The thrust
of the aircraft is 120,000 pounds and the air density is .00048 slugs/cu.ft.
The current cruising speed is 450 ft/sec and we need to determine the drag
coefficient of the aircraft. Find the Cd. (Hint: you may want to convert
the original equation first)
Cd = 2D/(r
x V^{2 }x A) = 2(120,000)/(.00048)(450)^{2}(5668.88)
= .436
C. Weather
conditions force the aircraft to descend to a level where the air density
is .00076 slugs/cu.ft. The engine thrust increases to 250,000 pounds.
At the new cruising altitude, what is the speed of the aircraft? (Hint:
once again, you may want to convert the equation first)
V =sqrt(2D/Cd
x r x A)=sqrt(2x(250,000)/(.436 x .00076 x 5668.88)) = 516
ft/sec

Related Pages:
Standards
Activity
Worksheet
Aerodynamics Activity Index
Aerodynamics Index


