| |
Week of 5/7/01:
Q:
You settle into your seat as the tug pushes your airplane away from
its gate. Your airplane soon begins taxiing towards the runway under its
own power. The pilot announces over the intercom that it will be a
few minutes before you can take off, since there are several other
aircraft waiting in line ahead of you on the taxiway. Suddenly, you
catch a whiff of kerosene. Oh no! Is there a fuel leak in the cabin?
A:
It is most likely that the flight deck crew has shut down one or more
engines for the duration of the taxiing to conserve fuel.
A 737's CFM56 engine burns approximately 12 pounds of fuel per minute
during taxi and a general rule of thumb is that if an engine can be
shut down for more than seven minutes, the savings will be less than
the cost of starter and starter valve wear-and-tear. A similar rule
of thumb applies for shutting down both engines for more than fifteen
minutes and starting the APU.
But what does this have to do with the smell of kerosene in the cabin?
Fresh air is supplied to the cabin via bleed air from either a low or
high stage compressor on the engines or APU, passed through a heat
exchanger, and sent on its way to the cabin (and is often mixed with
recirculated air to reduce the required flow rate of bleed air). Normally,
there are no odors in the "fresh" air because there is an abundant supply
of outside air to the engine's compressor and the bleed points are upstream
of the combustor. However, before an engine is started or after it is
shut down, flow in the engine stagnates and air in the accompanying
pneumatic bleed ducts can acquire the odor of unburnt fuel and will
momentarily present itself to passengers' noses as it clears through
the cabin.
For airliners, it is recommended that an engine which was shut
down for a single engine taxi be restarted and operated at a near-idle
speeds for at least two minutes prior to takeoff.
Congratulations to "Ross L."
Although "wet starts" or contamination from airport exhaust fumes can
happen, in general aircraft bleed air contaminants are extremely low.
Concentrations of air contaminants measured in studies of commercial aircraft
have been found to be lower than those in residential or commercial
buildings. The air is, however, very low in humidity when drawn
at high altitudes.
- The Aeroquiz Editor
Week of 5/14/01:
Q:
Lightweight materials, leading edge wing slats for enhanced lift,
rudders for yaw steering, and aerodynamic camber provided by
structurally-stiffened skin. All characteristics of modern airplanes,
right? Or could they also be found over 200 million years ago?
No one got the correct answer! The question stands another week!
- The Aeroquiz Editor
Week of 5/21/01:
Q:
Lightweight materials, leading edge wing slats for enhanced lift,
rudders for yaw steering, and aerodynamic camber provided by
structurally-stiffened skin. All characteristics of modern airplanes,
right? Or could they also be found over 200 million years ago?
A:
Look no further than the cousins of the dinosaurs, the pterosaurs.
This group of creatures had all the features mentioned. Hollow bones
made for a lightweight frame, the wings had "droops" over the leading
edge that formed a slat of sorts, the long tail with a paddle on the
end formed a moment arm with aerodyamic surface for a rudder, and the
thin, leathery skin closely followed the contours of the "finger"
bones for a cambered shape. The high lift created by the larger pterosaurs
made these guys great gliders that could soar for hours on prehistoric
thermals.
Congratulations to Norm Worthen.
Pterosaurs were likely as ecologically diverse as modern birds,
and were in existence long before the first bird took wing.
Pterosaurs were remarkably lightweight, in part due to very thin-shelled,
hollow bones. Even the Quetzalcoatlus northropi, with its
nearly 40-foot wingspan, probably weighed no more than a child.
A short pteroid bone extending forward from the arm may have helped
some pterosaurs control a forewing membrane -- much like a leading
edge slat on a modern airplane wing. Many pterosaurs had cranial
crests that may have been used as rudder-like steering aids.
And although the actual wing skin rarely fossilizes, a nicely preserved
wing membrane from a Rhamphorhynchus has small corrugations
that may have added strength, stiffness, and camber to its wings.
- The Aeroquiz Editor
Week of 5/28/01:
This week's question was submitted by Peter Sanz of AAR!
Q:
An Air Force pilot climbs into his F-16 and straps himself
into the ejection seat. After performing his pre-flight
checks, he selects the JFS start switch and starts the small,
powerful, gas turbine engine, or Jet Fuel Starter (JFS).
The JFS is mounted to the aircraft's accessory drive gearbox
and provides the rotational power need to overcome the
initial moment of force needed for main engine light off via
the Power Takeoff (PTO) Shaft. It can also be used to test
aircraft systems without burning the fuel needed to run the
main engine and can restart the main engine while in flight
at altitudes up to 20,000 feet.
Once the main engine begins to turn under its own power, the
speed of the main engine quickly exceeds that of the PTO
shaft connecting the main engine to the accessory drive gearbox.
One would expect the gearbox to experience
catastrophic internal failure, as it becomes a source of
drag against the accelerating, much more powerful main engine.
This is not the case, though, and the pilot taxis toward an
uneventful takeoff. What prevented the main engine from
destroying the JFS?
A:
Once the main engine starts turning under its own power, and the main
shaft speed starts to exceed the JFS, the JFS mechanically disengages from the
main shaft, and from that point on, the main engine is on its own with no
mechanical connection to the JFS.
Congratulations to Alan Nies.
Close enough! The trick is to do it with a lightweight design
in as small a volume as possible. Peter gives us more detail here:
"The accessory drive gearbox is equipped with three overrunning
clutches that prevent the faster-turning main engine from
destroying the slower-turning JFS. The overrunning clutch has
a number of pawls that, when the shaft is turned in one direction
cause the overrunning clutch to engage, allowing the clutch spur
gear to rotate in the direction of the shaft, such as when the JFS
is providing the initial rotational force. When the shaft is turned
in the opposite direction, the pawls retract, allowing the shaft to
turn without turning the overrunning clutch gear. When the main
engine's speed exceeds that of the PTO shaft, it has the same effect
as turning the shaft in the opposite direction, allowing the shaft
to spin at a faster rate than the overrunning clutch spur gear.
This allows the engine rotation to supply rotational power to the
accessory drive gearbox as the JFS begins to deccelerate."
- The Aeroquiz Editor
|
|
