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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 7/3/00:
Q:
Heat exchangers are devices that transfer heat from one fluid
to another. They usually are simple flow passages that bring two
fluid streams close enough together that the temperature of
one stream is lowered while the temperature of the other is raised.
They sound pretty simple to build, right? In the aircraft industry,
heat exchangers are often found in environments that are approaching
3000 degrees Fahrenheit (well above the melting temperature of most
materials) and flows that are extremely turbulent, unstable, and
corrosive due to oxidation and sulfidation. They spin at thousands
of revolutions per minute, are pulled upon by forces of several
tons, and must survive without breaking for thousands of hours.
In addition, even though they would last only minutes (or even
seconds) without it, the fluid that cools them is often well
over 1000 degrees Fahrenheit! What kind of heat exchangers are these?
A:
The heat exchangers are turbine blades.
Congratulations to Frank Brown.
Does that give you an appreciation of what jet engine designers
are up against? In the neverending quest for performance and fuel
efficiency, jet engines must operate at combustor temperatures as
high as turbine materials will permit. Each turbine blade is cooled
by compressor air so that they can survive in the hostile environment
downstream of the combustor. Each turbine blade is, in fact, a small
heat exchanger.
- The Aeroquiz Editor
Week of 7/10/00:
Q:
You're driving your new 2001 Z06 Corvette on the open road.
With your LS6 V-8 engine belting out 385 SAE net horsepower, you're
cruising at the 'Vette's top speed of 171 miles per hour (you are, of
course, the mythical "professional driver on a closed course" that you
see in advertisements). An insect is flying just above the road ahead of
you. It catches a stream tube and happily and neatly rises up over
the top of the 'Vette's roof, and avoids becoming protein matter
splatter on your windshield. Now as it happens, your 'Vette is not exactly
"factory stock." You have a number of Jet Assisted Takeoff rockets
mounted to your chassis. JATO rockets are used to significantly shorten
the required takeoff field distance of many military aircraft. You
ignite the JATO rockets and are soon traveling slightly faster than the
local speed of sound (okay, so you have a lot of JATOs). Another insect
is flying just ahead at windshield height. Is there any chance that
this bug can catch a lucky stream tube and rise over the roof?
A:
The bug could still flow over the car, but the stream will be much faster.
Congratulations to "Mark."
Although the odds of hitting the windshield certainly increase with speed,
it's still possible for the bug to get lucky -- even at supersonic
Corvette speeds. This question has a little aeronautical misdirection
involved for readers who know something about supersonic flow!
I was trying to get readers familiar with compressible flow to
remember the "law of forbidden signals," and say that the bug will inevitably
hit the windshield. People who didn't know about the law
were not encumbered by it. This "law" states that pressure changes
produced by a body moving faster than the speed of sound cannot
propagate upstream to points ahead of the body in the "zone of silence."
For a slower subsonic object, the air begins to "part" in anticipation of
the object coming. But there is no "parting" of the air ahead of a supersonic
object. This rule explains why a supersonic projectile or airplane
cannot be heard until the shock wave attached to the nose of the body passes
over the ear of the observer, and why the noise is concentrated in a
"cracking" sound. In our case, the bug cannot avoid hitting a weak shock
wave just ahead of the Corvette (because the law says so!). If it survives
the shock wave, it will enter a region of subsonic flow between the
shock and the windshield. In this region the law no longer applies, and
the bug has an opportunity to find a lucky stream tube and flow over
the car like the first bug did.
- The Aeroquiz Editor
Week of 7/17/00:
Q:
In the 1950s, the famous "Century Series" of U.S. aircraft routinely
flew at supersonic speeds. Earlier, in the 1940s, many aircraft experienced
"compressibility" effects while in high speed transonic dives. Would
it surprise you if we said airfoils on World War I aircraft routinely
traveled at transonic speeds? And not in laboratory conditions, but in
operational flight? How could that be possible in those days?
Is this a leading, trick question? You bet!
A:
No, I wouldn't be surprised because the tips of a propeller can operate at
transonic speeds. And since a propeller is nothing more than a rotating
wing made up of a variety od airfoils and lots of twist, this must be
the part of a World War I aircraft you are referring to.
Congratulations to Kevin Finke.
Many operational World War I airplanes, such as the Spad and Nieuport,
had propeller blades with tips that traveled at about the speed of sound.
And yes, propellers have cross sections that are indeed airfoils!
A propeller's thrust is really the lift of its airfoils.
In laboratory settings in 1919, British researchers were among the
first to measure the classic transonic drag rise (and loss of thrust!)
on propeller blades with tip speeds slightly above the speed of sound.
- The Aeroquiz Editor
Week of 7/24/00:
Q:
Commercial airliners are often towed or pushed from place
to place within airport boundaries. The most frequently seen
"tugs" are the little ones that push aircraft away from gates
until they can taxi away freely under their own power. But
airlines also operate an interesting class of "supertug" vehicles,
which haul large aircraft around airports over longer distances in
situations other than backing away from gates. Douglas-Kalmar,
for example, makes a 53,000-pound, 540 horsepower behemoth supertug
called the TBL-400, which is capable of towing a Boeing 747.
The TBL-400 can be yours for $667,657. Why do airlines spend so
much money on supertugs? Why don't they just taxi their airplanes
from place to place under their own power?
A:
In the long run, it would be cheaper to purchase and use the super tugs
than it would be to taxi from one end of the air field to the other.
The initial investment seems prohibitive at first glance, but in the long
run it would be cheaper than running jet engines for long taxi purposes.
Congratulations to Peter Sanz.
Continental Airlines'
supertug manager Donald Thomas was quoted in this month's Car and Driver
magazine: "Every time you move an aircraft under its own power, you
cycle its engines, which decreases the time between teardowns. If the
plane takes 15 minutes to warm up, 20 minutes to taxi to another
terminal, and 10 minutes to shut down, then that's 45 minutes off
its air life. Also, if you let, say, a 747 move itself - even if
it's only 50 yards to an adjacent gate - the fuel it'll burn would
probably pay my salary for a week." Also, taxiing aircraft requires
pilots or specially licensed mechanics to steer them, both of whom
are paid more than tug drivers.
- The Aeroquiz Editor
Week of 7/31/00:
Q:
"I call this preliminary design meeting to order," growled the
chief design engineer for the company's new high speed jet engine.
"Inlet Design Team! What are your performance predictions?"
"Well sir," replied the inlet designer. "We're looking at a four
percent total pressure loss throughout the inlet and..."
"Unacceptable! Nozzle Design Team! What are your latest data?"
"Uh, well," stammered the nozzle designer. "We're projecting a similar
total pressure drop in our nozzle duct upstream of the throat.
Uh, maybe a little more."
"Out of the question! You so-called engineers know how important it
is to maintain pressure throughout an airbreathing engine! Especially
in a supersonic application like this! Pressure losses result in less
thrust and they are intolerable!" The chief's mood then brightened
as he turned to his son, who was also an engineer at the company.
"Let's hear from the combustor team now. Junior, how does your
combustor look?"
"Well, dad. I'm measuring a 4 percent pressure drop throughout my
combustor."
"That's my boy," beamed the chief. "Nice work, Junior!"
Is the pressure drop in the combustor acceptable? Or could this be
favoritism at work?
I was on vacation this week in South Dakota! Look for the answer
to the question next week. Sorry for the inconvenience.
- The Aeroquiz Editor
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