|
|
Multidisciplinary
Design, Analysis, and
Optimization Branch
|
|
|
| |
EDUCATIONAL ACTIVITIES: THE NASA AEROQUIZ
|
|
|
| |
|
| |
Week of 9/7/98:
Q:
Before releasing satellites from the Shuttle's cargo bay for their
eventual boost to a higher orbit, the satellites are often set into
rotation using a device called a "spin table." Why?
A:
The satellites are often set into rotation before releasing
them from the shuttle's cargo bay so as to provide spin stability
to them and also to orient them properly when they have been boosted to
a higher orbit.
Congratulations to Kunal Verma.
Spinning is also attractive for other reasons. It equalizes any small
non-axial thrust component that might exist in the rocket motor and nozzle
during the boost into higher orbit and it distrubutes the solar heating
load more evenly around the payload.
- The Aeroquiz Editor
Week of 9/14/98:
Q:
In many events, track and field athletes can take advantage of low aerodynamic
drag. The reduced drag that occurs on hot days and high altitudes, for example,
has been attributed to helping U.S. athlete Bob Beamon break, in Mexico City
in 1968, what was then the world long jump record by almost two feet.
Would an airplane also experience better performance trying to take off from
a high altitude field on a hot day?
A:
No. The high altitude would translate into a longer takeoff run due to the
thinner air. In addition, a turbine engine prefers cool, dense air.
Shortest takeoff runs occur at low, cool altitudes.
Congratulations to Anthony Lee.
Although drag is indeed less on a hot day at high altitude, so is lift!
Jet engine thrust also suffers substantially. The low-density air results in a
reduced flow rate through the engine. So, thanks to Newton, we know the
force of thrust is penalized. The nozzles also have less enthalpy difference
to work with, and, for you thermodynamic cycle analysis fans out there,
the engine's integral of TdS is smaller!
- The Aeroquiz Editor
Week of 9/21/98:
Q:
When an airplane is in motion, the apparent pressure of the air it
encounters is higher than the pressure it encounters when it is at rest.
One major concern of jet engine designers is the effectiveness of the inlet's
ability to deliver, or "ram," this apparently higher pressure air into the
engine. In aeronautical parlance, this is known as ram pressure recovery.
Pontiac offers a "Ram Air" hood scoop intake option on its high performance
Firebird automobiles. Does it help? Or is it hype?
A:
The ram air scoop on an automobile may help prevent the
ingestion of distorted flow, i.e. boundary layer air.
However, the speed range of an automobile is so small
as compared to an aircraft engine that I would think the ram scoop
would be a little more cosmetic than functional.
Congratulations to Daniel Shedd.
The scoop helps, but not because of any appreciable ram pressure effect.
At the end of a quarter mile drag race, a '98 Firebird Trans Am with a WS6
Ram Air option is traveling about 100 miles per hour. This relatively modest
speed (by aircraft standards) provides an apparent increase in pressure of
only one percent. Further, automobiles spend relatively little time at 100
miles per hour. More importantly, however, the hood scoop delivers cooler,
denser, outside air to the engine with much less friction loss than a
conventional intake for a significant performance improvement. And it
looks great.
- The Aeroquiz Editor
Week of 9/28/98:
Q:
This question is a followup to last week's "ram pressure" question.
When calculating the thrust of a jet engine, engineers must account for the
loss in thrust caused by the force of the air entering the inlet. This loss
is known as "ram drag." Would it do any good to orient a jet engine's intakes
parallel to the flow so the air would enter the engine from the side? Would
this eliminate ram drag?
A:
The magnitude of the ram drag would be unaffected
by reorientation of the inlets. Ram drag is defined as
mass flow times the velocity of the incoming flow.
Congratulations to Daniel Shedd.
A moving stream of air has momentum associated with it no matter
where it enters the engine, and it must be accounted for. In fact, if an
inlet is oriented in such a way, the engine moving at high speeds
would suffer from terrible pressure
recovery and flow quality, as mentioned last week. This, unfortunately, is
a common misconception. Once, at a boat show, I saw a racing boat with intakes
along the side of its hull. All of the other boats had large, forward-facing
intakes mounted high on their decks. The owner bragged that he had none of
the ram drag that his competitors had. With six figures invested in his boat,
he made an expensive mistake.
- The Aeroquiz Editor
|
|
|
|
