AOE 3104 Homework #4 Solutions
Problem 1.
Consider an airfoil in a 50 m/s air flow at standard sea level conditions. At a point on the
airfoil, the pressure is 9
.
5
×
10
4
Pa. What is the pressure coefficient at this point? (Please use SI units for
the result of each step.)
Problem 1 Solution.
For standard sea level conditions, we have
P
∞
= 10
.
132
×
10
4
N
/
m
2
,
ρ
∞
= 1
.
226 kg
/
m
3
Then
¯
q
∞
=
1
2
ρ
∞
V
2
∞
= 1530 Pa
.
So
C
p
=
P

P
∞
¯
q
∞
=
(9
.
5

10
.
1)
×
10
4
1532
.
5
=

4
.
1
Problem 2.
Consider an airplane with a wing area of 16m
2
and a wing span of 10m. In a 100 m/s air
flow at standard sea level conditions, the wing is generating 80
,
000 N of lift.
a) Calculate the lift coefficient.
b) Assuming
e
= 0
.
85, calculate the induced drag coefficient and the induced drag force.
(Please use SI units for the result of each step.)
Problem 2 Solution. a)
¯
q
∞
=
1
2
ρ
∞
V
2
∞
=
1
2
×
parenleftBig
1
.
226 kg
/
m
3
parenrightBig
×
(100 m
/
s)
2
= 6
.
13
×
10
3
Pa
Then we have
C
L
=
L
¯
q
∞
S
=
80
,
000
(6
.
13
×
10
3
)(16)
= 0
.
816
b)
The aspect ratio is
AR
=
b
2
S
=
10
2
6
= 6
.
25. So the induced drag coefficient is:
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 Spring '08
 DR.CRAIGWOOLSEY
 Force, Aerodynamics, Lift, standard sea level

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