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Midterm Exam, AEP 2640, Fall 2008
Open book and notes; time limit 90 minutes
1.
(25 points)
Fluid mechanics
(
your numerical results are important for this problem
)
In level flight an airplane is subject to 2 types of drag force.
The first of these is called
“parasitic drag”, which is the force required to move a solid object through a fluid.
The
parasitic drag accounts for viscous drag (skin friction) and form drag.
The parasitic drag
force is calculated with the relationship:
Dp=Cd S (½ρV
2
)
(1)
where Cd is the parasitic drag coefficient, S is the wing surface area, ρ is the air density,
and V is the air speed.
The airfoil deflects air downwards as it passes around the wing in order to produce lift.
This deflection of air exerts a horizontal drag force on the wing called “induced drag”.
Induced drag is given by the formula:
Di=kL
2
/(½ρV
2
SπAR)
(2)
where k=1.1, L is the lift force, and AR is the aspect ratio:
AR=b
2
/S, where b is the wing span.
(3)
The total drag force is the sum
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