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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 10/5/98:
Q:
At what point in aviation history was the world record for aircraft
range bettered by over 700 percent and time aloft by 500 percent?
A:
On the very first day of powered flight.
On December 17, 1903, the Wright Flyer became the first powered,
heavier-than-air machine to achieve controlled, sustained flight with a
pilot aboard. It flew forward without losing speed and landed at a point
as high as that from which it started. Orville Wright piloted the aircraft
first. The first flight remained aloft for 12 seconds and covered a distance of
120 feet. The third and final flight, piloted by Wilbur Wright on the same day,
traveled 852 feet for a 700% improvement in range and lasted 59 seconds for
a 500% improvement in time aloft.
Congratulations to Linda Wilson and
to Daniel Shedd.
Week of 10/12/98:
Q:
Sometimes, when a commercial jet aircraft is parked outside for an
extended period, its thrust reverser buckets are intentionally set to their
open position. Why?
A:
When wind passes through the bypass duct of a turbofan engine, the
fan can easily begin to turn. Since the engine is not in operation, the
oil pump can not supply oil to the low-spool bearings. The reversers block
the flow of air through the duct and helps to prevent bearing wear.
No one got the correct answer!
- The Aeroquiz Editor.
Week of 10/19/98:
Q:
You feel unusually powerful today. You decide to dig a hole. Your hole
passes through the Earth's center and out the other side. You pump all
the air out of the hole and jump in. How is your fall through the Earth
similar to a low-altitude, circular Earth orbit?
Extra credit: You dig another hole that does not pass through the center
of the Earth, say from New York to London. The walls are, of course,
frictionless. You get in and slide. What happens?
A:
The equation of motion for an object freely falling through a hole in the Earth
can be shown to be that of an undamped harmonic oscillator.
The object would free fall through
the Earth, reach a maximum speed at the Earth's center, and slow to zero
speed at the other side. Ignoring friction, the object would "yo-yo" from
one side to the other indefinitely. The period of this motion is the same as
that of a circular Earth orbit at treetop level: 84.5 minutes!
Although many people described the motion correctly, no one really knew how it
compared to a low-altitude circular orbit. Congratulations to
Norm Worthen, who noted that the trajectory through any hole in the Earth
not passing through the poles would have the Earth's rotation messing things
up. Objects would bump along the wall as they fell, and I didn't anticipate that.
As for the trajectory of an object sliding along a straight hole from New York to
London, it can be shown to have exactly the same period as an object falling straight
through the Earth. If you could dig these holes, you could slide to anywhere
on Earth in 84.5 minutes!
- The Aeroquiz Editor.
Week of 10/26/98:
Q:
A classic aerodynamics question: why is a golf ball dimpled?
A:
The dimples create turbulent flow around the golf ball.
In this special case, the turbulent flow reduces the
form drag around the golf ball, thereby increasing
its flight distance.
Congratulations to John D. Winstel.
Due to the air's viscosity, objects in flight have a layer of slow-moving
air surrounding them. The shape and size of this "boundary layer" is a
function of several variables, such as the shape, size, and speed of the
object, among others. The properties of boundary layers affect the
aerodynamic drag of objects in flight. A smooth golf ball hit at a typical
driving speed would have a "laminar" boundary layer; that is, one without
turbulent eddies roiling around the ball. This type of boundary layer
plumes outwards from the ball and hinders its forward motion by giving it
a high level of viscous "form drag" as Mr Winstel notes. If a smooth ball
is hit fast enough (at speeds of approximately 250 feet per second or more),
the boundary layer becomes turbulent and does not plume outwards from the ball
as much as when it is laminar. And although turbulent flow has higher
drag right at the surface (a higher "skin friction"), the form drag is greatly
reduced and the overall drag is lower than a ball in laminar flow. Be careful:
this is only true of some bluff bodies like a golf ball. But not many people
can hit a smooth golf ball fast enough to send it into turbulent flow. People
long ago discovered they could artificially induce turbulent flow around a golf
ball by dimpling its surface. The dimples "trip" the laminar boundary layer,
send the flow into turbulence, and reduce its overall drag.
- The Aeroquiz Editor.
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