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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 10/4/99:
Q:
Although hard to believe, one of the "quietest" airplanes in the
world is the Boeing F-15 attack/air superiority fighter taking off with full
afterburners. Yes, this is a trick question, but can you figure out
why this is true?
A:
Since the F-15 can takeoff in a short distance and
then climb nearly vertically above the airport, its noise
"footprint" over neighboring communities near the airport
would be next to nothing as compared to a 747 which will
climb out at a much shallower angle. Therefore, even though
the F-15 is converting a serious amount of dinosaurs into
noise, nobody is around to hear it.
Congratulations to Kevin Finke.
The jet noise produced by an afterburning F-15 is ear-splitting.
However, when low noise takeoffs from populated areas are important,
F-15s often perform what are known as
"viking" takeoffs. After rotation and
lift off, thanks to its thrust levels being significantly higher than
its weight, the F-15 climbs virtually straight up and does not level off until
it is high enough that little noise propagates to the ground. If the F-15
had to certify under civilian aircraft noise regulations, its community
flyover noise level (officially measured at a point 6500 meters from the
point of brake release) would be very low. This is in sharp contrast to
heavy, long haul commercial jets, where community flyover altitudes are
often barely over a thousand feet. Another civilian noise measurement point,
however, is the so-called "sideline noise." For the F-15, this measurement
would peak somewhere around lift off (at a low altitude), and would be
very high!
- The Aeroquiz Editor
Week of 10/11/99:
Q:
One method for reducing aircraft aerodynamic drag that has been
attracting attention is boundary layer suction. The thin, turbulent
"boundary layer" of air that flows naturally across aircraft wings is
ingested by a suction system through many small perforated holes in the
wing surface. The flow in the region above the wing then becomes laminar,
and aerodynamic drag is significantly reduced. Perhaps counterintuitively,
another drag reduction technique is to blow air out of small holes
in the wing. Ordinarily, blowing air into the boundary layer increases
the drag of a body. So how can this reduce drag?
A:
This technique, called microblowing, is achieved by blowing an
extremely small amount of air through porous plates with very small
holes. Care is taken not to blow too much air through the holes,
which, as stated above, can increase the drag. Microblowing reduces
drag differently than the air suction method. Whereas suction lowers
drag by reducing turbulent flow, microblowing lowers drag by fooling
the flow into thinking the surface is much smoother than it really is.
Recent microblowing experiments at NASA have demonstrated significant
reductions in roughness drag.
No one got the above intended answer, and I apologize for posting
such a vague and open ended question. Congratulations to
Kevin Finke,
Richard DeLombard, and Lael vonEggers Rudd
for describing several alternate solutions!
- The Aeroquiz Editor
Week of 10/18/99:
Q:
Many people are familiar with geostationary synchronous Earth orbits.
These orbits, first postulated by science fiction writer Arthur C. Clarke
in 1945, are circular equatorial orbits at an altitude of 22,236 miles.
One revolution about the Earth at this particular altitude takes exactly
one sidereal day (23 hours, 56 minutes, 4 seconds), such that a satellite
orbiting there appears to hover motionless over a single spot on the
equator. These orbits are well suited for communications satellites since
satellite dishes don't need to be moved once they have been properly aimed
at a target satellite in the sky. There is another class of special orbits
that is well suited for Earth photography. Satellites in these orbits are
able to appear over the same spot on Earth at regular intervals and at the
same time of day. How do they work?
A:
Sounds like a polar orbit whose period is a sidereal day or some
"integer inverse" (1/3, 1/2, etc.) of a sidereal day.
Congratulations to Mark Witt.
These are generally called sun synchronous orbits. They are near polar
orbits whose altitudes are such that a satellite will always pass over a
given location at the same local solar time. Here is an example using an
exact circular polar orbit (that is, an orbit that passes directly over the
Earth's poles): A satellite at an altitude of 547 miles requires 102.6
minutes to revolve once around the Earth. In exactly one day (one Earth
rotation), a satellite at this altitude will have revolved around the Earth
exactly 14 times and would be over the same spot at the same time as it
was the day before. In this way, the same solar illumination conditions
(except for seasonal variations) can be achieved for images of a given
location.
- The Aeroquiz Editor
Week of 10/25/99:
Q:
"My data indicate a duration of 5.2," said the NASA rocket
scientist.
"I dispute your contention," replied the NASA trajectory
analyst. "The time interval was only 5.1. That asset was jettisoned
after its malfunction allowed an elongated post-percussion
trajectory. The backup systems then were engaged for periods
of 2.0 and 1.2."
The rocket scientist thought for a moment
and said, "Yes, you are indeed correct. 5.1 plus 2.0 plus
1.2 is nine."
Hints abound in this week's question!
Is their addition correct? Or can't NASA get good help these days?
A:
The starting pitcher was in for 5 & 1/3 innings before giving
up a home run. Two relief pitchers were brought in for 2 and
1 & 2/3 innings, respectively. 5 & 1/3 + 2 + 1 & 2/3 = 9.
Congratulations to Dale Martin.
Dale always uses baseball notation math during the World Series!
- The Aeroquiz Editor
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