Solution to MAE 261 Test #1 February 9, 2006
OPEN BOOK AND NOTES
Note that units were not included in intermediate calculations
below for brevity. They were required in student’s solutions.
1.
a)
What are the two major keys to flying faster and higher?
b)
When may we consider air to be incompressible?
M<0.3 or V<100m/s
c)
What is the purpose of the dimples on the surface of golf balls?
Reduce the drag
2.
Consider a heliumfilled balloon with a volume of 0.06000 m
3
.
The balloon is at sea level on a standard day. The molecular weight of
helium is 4 and the pressure and temperature of the helium inside the
balloon is the same as that outside.
a) Calculate the density of helium inside the balloon.
R
He
=
R
/
M
He
= (8314 J/(kgmol K))
/
(4 kg/kgmol) = 2079 J/(kg K)
ρ
He
= p/(R
He
T) = (1.013E5 N/m
2
)/(2079 J/(kg K)x 288.2 K) = 0.1691 kg/m
3
b) Calculate the buoyancy force on the balloon.
B = ρ
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 Spring '06
 DeJarnette
 Mach number, Airspeed, Airspeed indicator, Pitot tube, wind tunnel. p1

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