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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 1/1/01:
Q:
January first this week marks the new millennium, or at least a new year.
Many people will no doubt have celebrated this event, but some people
will legitimately have been able to ring in the new year an
astronomical fifteen times! How could this be possible?
A:
They are the crew aboard the International Space Station Alpha.
Their orbit allowed them to be over fifteen different time zones at
each timezone's midnight.
Congratulations to Philip Stehno.
Week of 1/8/01:
Q:
"Turn off the lights when you're not using them," the NASA engineer
nagged his teenage son. "Do you think I'm made of money?"
This time, the boy was prepared for that tired old cliche and fired back
a well-reasoned reply: "But dad, it's January and cold outside.
We have an all-electric house and the heat is on. If I happen to leave
on a light, it's simply helping to heat our house. And at one hundred
percent efficiency, I might add!"
"Are you kidding?" the father replied to
his smug son. "In this house, we get nearly a three hundred percent heating
return on our electric power! You're wasting energy!"
Is that possible? Is he wrong? Does he have a secret NASA invention
hidden away in the basement? Or something ordinary he bought from the
HVAC guy down the street?
A:
Two words: Heat Pump.
Congratulations to A.R. Nies.
The house is equipped with an ordinary heat pump! Heat pumps are
popular in regions that are not too cold, or even in regions that are
indeed cold, but have no cheaper heating alternatives available. In
moderately cold climates, heat pumps typically provide much more heat energy
than the mechanical energy put into them. This is done by "pumping" what
heat may be outdoors into homes via a working fluid and heat exchangers.
On a very cold day, there may be comparatively little heat outside, which
is why they are not very popular in northern climes. The local price
of electricity relative to natural gas or other energy sources is also
a factor. But in moderately cold climates, heat pumps perform very well.
A resistance heater (like the light bulb in the question) has a so-called
"coefficient of performance" of one hundred percent, but good heat pumps
have coefficients around two or three hundred percent. No alien
technology -- just thermodynamics!
- The Aeroquiz Editor
Week of 1/22/01:
Q:
The U.S. Marine crew chief backed away from his mostly disassembled
engine and squinted up at the hot South Pacific sun. It was 1944 in
the Solomon Islands and the chief had to keep his airplanes flying.
"Whaddaya think, sarge?" asked the grease-covered kid from Nebraska.
"The fuel mixture's the problem," replied the chief. "These new
carburetors vibrate too much and the fuel foams up. At high engine
speeds, the motor's running lean."
"Guess there's nothin' we can
do about it out here in the middle of nowhere," concluded the kid.
"Naw, it'll be allright," said the chief. "Go and get me my sinkers
outta my fishin' tackle box and a roll of tape. We'll fix these
airplanes."
What does the chief have in mind?
A:
Would he be fastening
the weights to the carburetor's body so as to change
the mass, and therefore either dampen the vibration
or alter the natural frequency?
Congratulations to Dan Shedd.
The added mass of the sinkers changes the carburetor's natural
harmonic frequency to something lower than the engine's vibrational
frequency. The carburetor then can operate with significantly less
vibration and the fuel foaming problem disappears.
- The Aeroquiz Editor
Week of 1/29/01:
This week's question was submitted by Ron Laird!
Q:
Some airliners have fuel tanks in their horizontal tail structure.
Obviously this allows more fuel to be carried, but what is the main
operational benefit of having this fuel in the tail?
A:
The additional tank in the horizontal stabilizer is used as a
stability trim tank (ballast) for changing the location of
the center of gravity along the longitudinal axis of the aircraft.
The alternative of deflecting a surface (like the horizontal stabilizer),
produces additional induced drag when the stabilizer generates lift,
creating a moment about the center of gravity.
Congratulations to "Ross."
The primary reason to carry fuel in the horizontal tail is to reduce the
need for aircraft "trimming." When pilots trim their airplanes, they
are balancing them about the pitching axis so that they maintain proper
orientation. Trimming forces are generated by control surfaces on the
horizontal tail. The amount of trim required depends on the geometry
of the airplane and the locations of the aerodynamic and gravitational
centers. When these control surfaces are extended into the flow of
air around the airplane, a slight increase in drag is unavoidable. The
extra weight of the fuel in the tail of the airplane causes the center of
gravity to move aft slightly. In some situations, depending on airplane
design, fuel, and payload weight and arrangement, this can relieve the pilot
from using much trim control in a slightly nose-heavy airplane. Typically
this results in a minor improvement in drag on the order of one percent,
but anything that improves performance is welcomed.
Interestingly, the fuel arrangements and trimming procedures in the
supersonic Concorde and in the recently proposed High Speed Civil
Transport are even more complex. As the Concorde accelerates through the
sound barrier, a variety of aerodynamic changes occur, which cause its
aerodynamic center of pressure to shift rearward. Fuel is pumped to an
aft trim tank to compensate for this shift.
- The Aeroquiz Editor
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