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Week of 8/6/01:
Q:
An aircraft "radome" is an external dome-shaped housing that
covers the onboard radar system. Fighter aircraft, for example,
usually have cone-shaped radomes on their noses to cover the radar
antenna and equipment mounted on the forward bulkhead. Conventional
radomes are typically made of composite materials that are transparent
to radar emissions so that the radar can "see" what's ahead. Some
so-called "bandpass" radome designs, however, are constructed of
materials that are transparent to only one frequency -- that
of the aircraft's radar. Why would aircraft designers care about
making a radome opaque to all but one radar frequency?
A:
If you're looking to make a stealthy aircraft, bandpass materials
for the radome come in handy. They prevent an enemy's radar
penetrating the radome and bouncing off your radar dish back to
the enemy receiver.
Congratulations to Norm Worthen.
As Norm points out, a conventional radome that is transparent to all
frequencies is not stealthy. An enemy radar can get an excellent return
echo if its signals bounce off an aircraft's antenna and other items inside
its radome, much like the reflection from a cat's eye in a dark room.
A bandpass radome helps to eliminate this effect.
- The Aeroquiz Editor
Week of 8/13/01:
Q:
When landing in an ordinary commercial jet, you often hear
a roaring sound coming from the engines just after touchdown.
This is the sound of the thrust reversers deploying and the engines
coming up to power. The "backwards" thrust and engine drag provided
by the thrust reversers help stop the airplane. Thrust reversers
sound like a good idea, don't they? Would you be surprised -- or
alarmed -- if you knew commercial jets of the future might not be
equipped with thrust reversers?
No one got the correct answer. The question stands another week!
- The Aeroquiz Editor
Week of 8/20/01:
Q:
When landing in an ordinary commercial jet, you often hear
a roaring sound coming from the engines just after touchdown.
This is the sound of the thrust reversers deploying and the engines
coming up to power. The "backwards" thrust and engine drag provided
by the thrust reversers help stop the airplane. Thrust reversers
sound like a good idea, don't they? Would you be surprised -- or
alarmed -- if you knew commercial jets of the future might not be
equipped with thrust reversers?
A:
Engines like Pratt & Whitney's experimental Advanced Ducted
Propulsor, which has a variable pitch geared front fan, might
use a reverse-pitch setting to provide the desired backward push,
like with the reverse pitch of normal propellers.
Congratulations to Nicolas Cousineau.
Most of the time, only a relatively small fraction of the stopping force
actually comes from the reversers -- the brakes on the landing gear do most
of the work. Reversers are also heavy, costly, and are maintenance
items. They are also the primary source of noise once the airplane
has landed on the runway. And spooling the engines up quickly for
every landing fatigues them. Indeed, for these reasons, thrust
reversers are actually fastened shut on some aircraft. As for the
future, some innovative jet engine concepts can reverse thrust in
other ways. As Nicolas notes, one version of Pratt & Whitney's proposed
Advanced Ducted Propulsor engine has variable-pitch fan blades that can
rotate and supply a backwards thrust. But it is difficult or
impossible to reverse thrust for some of the more exotic advanced
engine concepts currently on the drawing board. And since
certification regulations do not call for reverser deployment on
landing (the regulations actually forbid their use!), you may see
"reverser free" aircraft someday. But it's hard to tell. One
situation where reversers are extremely important and effective
is when brakes are ineffective -- such as when landing on a
slippery runway!
- The Aeroquiz Editor
Week of 8/27/01:
Q:
According to U.S. Federal Aviation Regulations (Part 25.733e, to be exact),
any tires mounted on braked wheels for airplanes heavier than 75,000 pounds
must be inflated with dry nitrogen or other gases shown to be "inert."
What's wrong with using ordinary air?
A:
Well, the phrase "inert" gas says it all. When you see the pictures
of the white-hot brakes at the rejected takeoff test, when you know
that the tires are made of rubber and so could burn, when you know
that the tires will let go their gas content in the process, as a
cooling measure, you sure prefer to have them inflated with something
that will not promote fire at all.
Congratulations to Nicolas Cousineau.
Although they are rare and used only during emergency landings, maximum
effort stops can result in very hot brakes. It is not impossible
for grease, oil, or the tires to smolder on such a landing, and the last
thing anyone would want would be air from a burst tire to feed the fire.
Interestingly, engineers design thermosensitive fusible plugs to be used
near the wheel spit that safely vent the tire pressure if the local
temperature reaches a predetermined level.
As stated in the regulations, the gas used to inflate the tires must be
dry, since ambient temperatures at cruising altitudes are very cold.
A chunk of ice, even a small one, inside a tire can throw the wheel off
balance during landing. Carbon dioxide, for this reason, is also a poor
choice for an inflation gas.
Dry nitrogen is desirable for other reasons as well. It does not promote
rust and it will not oxidize the rubber compounds inside the tire.
Contrary to many responses to this week's question, nitrogen offers
no real weight advantage over ordinary air. The difference in weight
between an air-filled tire and a nitrogen-filled tire is very small!
- The Aeroquiz Editor
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