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Multidisciplinary
Design, Analysis, and
Optimization Branch
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EDUCATIONAL ACTIVITIES: THE NASA AEROQUIZ
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Week of 8/3/98:
Q:
Near the end of his Apollo 14 moon walk, Al Shepard attached a 6-iron
club to the end of a sample collecting tool, hit two golf balls, and became
the first person to golf on the moon. Although his bulky space suit forced
him to take awkward, one-handed swings, he joked that the second ball
traveled "miles and miles." If he wasn't encumbered by a suit and was able
to get "good wood" on the ball, could he have taken advantage of the
moon's low gravity and no atmosphere and put the ball into orbit?
Assume he could hit it tangentially to the horizon, avoid mountains, put
it into a circular crater-top level orbit, and achieve a record-setting
250 foot-per-second club speed!
A:
Nah, the circular orbital velocity at the moon's surface is 5517 ft/s, so a
250 ft/s whap falls far short. I wonder if they will try to locate
Al Shepard's (NASA's?) golf balls someday! After all, what would an
alien think if it found one?! (What would they think it was?)
Congratulations to Bill Strack.
Week of 8/10/98:
Q:
What do fireworks, smoke bombs, falconry, border collies, mouse traps,
radio-controlled airplanes, sharpshooters, and propane-fueled noisemaking
cannons have to do with airport operations?
A:
They are all methods to scare away birds
from runways to prevent potentially catastrophic
bird ingestion into engines.
Congratulations to Dan Shedd.
With 11,571 bird-aircraft collisions reported to the FAA
between 1992 and 1996 (with a reporting rate estimated at only 20%), airport
birds represent no small problem. The mouse traps, by the way, are set for
mice and voles that chew the wiring of runway lights and attract birds of prey.
- The Aeroquiz Editor
Week of 8/17/98:
Q:
During cryogenic testing at NASA's National Transonic Facility wind
tunnel, operators spray cryogenic liquid nitrogen into the flow upstream
of the fan. Why?
A:
To allow for variations of Reynolds number through altering the fluid
properties (primarily density). This would allow the model size to be
smaller, and/or the velocities to be lower to achieve the necessary
transonic modeling parameters.
Congratulations to Larry Lister.
Even though the test section of the NTF is only 2.5 meters wide
(making the use of sub-scale models necessary), it is capable of modeling
real-world, full-scale flow effects around objects like aircraft.
The tunnel achieves full-scale Reynolds numbers (a measure of the
importance of a flow's inertial force relative to its frictional force
over an object) by being able to increase the flow's pressure to nine
atmospheres, and to simultaneously increase the flow's density and decrease
its viscosity by reducing the temperature with the use of cryogenic nitrogen
at about minus 300 degrees Fahrenheit. Increased density increases the
numerator of the Reynolds number formula, while decreased viscosity decreases
the denominator, together raising the Reynolds numbers achieved. By using the
cryogenic approach to generate high Reynolds numbers, the NTF achieves its
performance of full-scale conditions at lower cost than concepts based on
ambient temperature operation.
- The Aeroquiz Editor
Week of 8/24/98:
Q:
This question is a followup to last week's (8/17) cryogenic wind tunnel
question. While using a cryogenic working fluid turns out to be a good idea
for modeling aerodynamic flow effects around sub-scale wind tunnel models,
it is an even better idea for modeling free convection heat transfer from
heated objects. Why?
A:
Because the rate of the heat transfer is increased.
Congratulations to Greg Hale.
As discussed in last week's question, cryogenic temperatures are useful
for dramatically increasing the Reynolds number of the flow around an object.
Cryogenic temperature effects, however, cause an even more dramatic increase
in the Grashof number -- the dimensionless parameter that describes the
relative importance of buoyant, convective forces to viscous forces. With
everything else constant, the amount of free convection heat transfer from a
heated object can be increased by a factor of 200 simply by going to an
ambient temperature of around minus 300 degrees Fahrenheit, conveniently
achieved via the use of cryogenic liquid nitrogen.
Click here to see a chart of this effect.
Click here to see the cryogenic heat transfer tunnel the Aeroquiz
Editor built in college (circa 1985!).
- The Aeroquiz Editor
Week of 8/31/98:
Q:
Why does a Frisbee need to spin in order to fly?
A:
The frisbee needs to spin to maintain stability and level flight.
The outward torque stabilizes the flight and allows the frisbee to fly farther.
Congratulations to Matthew Caito.
Two requirements need to be satisfied for a frisbee to fly: lift
and stability. If a non-spinning frisbee is mounted in the test section
of a wind tunnel, it can be shown that it can generate more than enough lift
to support its weight. However, non-spinning frisbees are very unstable
in pitch, and soon end up tumbling end over end. Spinning has
long been known to stabilize objects that are otherwise unstable.
- The Aeroquiz Editor
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