飞机起落架设计(中英文对照) - 副本
澳洲留学zinch中国-实训报告范文
Toy Aircraft Landing Gear Layouts
Modeling
Most aircraft today have
three landing gear．
Two main landing gear
struts located near the middle of the
aircraft
usually support about 90% of the plane’s weight
while a
smaller nose strut supports the rest．
This layout is most often referred to as
the
gear arrangement． However， there are
numerous other designs that
have also been
used over the years， and each has its own
advantages
and disadvantages． Let’s take a
closer look at the various
undercarriage
options available to engineers．
Tail wheel
or Tail dragger Gear
Though the tricycle
arrangement may be most popular today，
that
was not always the case． The tail wheel
undercarriage
dominated aircraft design for
the first four decades of flight and is still
widely used on many small piston－engine
planes．
The taildragger arrangement
consists of two main gear units
located near
the center of gravity （CG） that support the
majority of
the plane’s weight．
A much
smaller support is also located at the rear of the
fuselage such that the plane appears to drag
its tail， hence the name．
This tail unit
is usually a very small wheel but could even be a
skid on a very simple design．
What makes this form of landing gear most
attractive is its
simplicity．
The gear
are usually relatively lightweight， and the two
main
gear can also be easily encased in
streamlined fairings to produce low
drag in
flight．
Another potential advantage results
from the fact that the plane
is already tilted
to a large angle of attack as it rolls down the
runway．
This attitude helps to generate
greater lift and reduce the
distance needed
for takeoff or landing．
This attitude is
also an advantage on propeller－driven planes
since it provides a large clearance between
the propeller tips and the
ground．
Furthermore， taildragger planes are generally
easier for
ground personnel to maneuver around
in confined spaces like a
hangar．
However， the greatest liability of this landing
gear layout is
its handling characteristics．
（handling characteristics）．
This design is inherently unstable because the
plane’s center of
gravity is located behind
the two main gear．
If the plane is landing
and one wheel touches down first， the
plane
has a tendency to veer off in the direction of
that wheel．
This behavior can cause the
aircraft to turn in an increasingly
tighter
on the ground， collapsing the gear， or veering
off the runway．
Landing a taildragger can
be difficult since the pilot must line up
his
approach very carefully while making constant
rudder
adjustments to keep the plane on a
straight path until it comes to a
stop．
Many taildragger designs alleviate these handling
problems
by fitting a tailwheel that can be
locked instead of swiveling on a
castor．
Locking the tailwheel helps keep the plane rolling
in a straight
line during landing．
Another disadvantage of the taildragger is
poor pilot visibility
during taxiing since he
is forced to peer over a nose that is tilted
upward at a steep angle．
It is also
often difficult to load or unload heavy cargos
because
of the steep slope of the cabin floor．
Similarly， pilots and passengers are
forced to walk uphill during
boarding and
downhill after arrival．
Many aircraft also
rely on gravity to bring fuel from tanks to
the engine， and some planes have been known to
have difficulty
starting the engine because it
is uphill from the fuel supply．
Good examples of taildragger aircraft include the
Spitfire and
DC－3 of World War II．
Tricycle or Nosewheel Gear
Now the most
popular landing gear arrangement， the tricycle
undercarriage includes two main gear just aft
of the center of gravity
and a smaller
auxiliary gear near the nose．
The main
advantage of this layout is that it eliminates the
ground loop problem of the taildragger．
This arrangement is instead a stable design
because of the
location of the main gear with
respect to the center of gravity．
As a
result， a pilot has more latitude to land safely
even when
he is not aligned with the runway．
Furthermore， the tricycle
arrangement is generally less
demanding on the
pilot and is easier to taxi and steer．
The
tricycle gear also offers much better visibility
over the nose as
well as a level cabin floor
to ease passenger traffic and cargo handling．
A further plus is that the aircraft is at a small
angle of attack so
that the thrust of the
engine is more parallel to the direction of
travel，
allowing faster acceleration during
takeoff．
In addition， the nosewheel makes it
impossible for the plane to
tip over on its
nose during landing， as can sometimes happen on
taildraggers．
The greatest drawback to
tricycle gear is the greater weight
and drag
incurred by adding the large nosewheel strut．
Whereas many taildraggers can afford to use
non－retracting
gear with minimal impact on
performance， planes with nosewheels
almost
always require retraction mechanisms to reduce
drag．
Some planes with tricycle gear also
have difficulty rotating
the nose up during
takeoff because the main wheels are located so
close to the elevator， and there may be
insufficient control
effectiveness．
Similarly， the closeness to the rudder reduces its
effectiveness in counteracting crosswinds．
Another critical factor when designing
tricycle gear is to
properly balance the load
carried by the main gear versus the
nosewheel．
Too little load on the main wheels reduces their
braking
effectiveness while too little on the
nosewheel reduces its steering
effectiveness．
Careful balancing of weight is also important to
prevent the plane from tipping back on its
tail while at rest on the
ground．
There are many examples of aircraft with
tricycle landing gear，
including the F－16 and
Cessna 172．
Summary
Landing gear
serves three primary purposes
to provide a
support for the plane when at rest on the ground，
to provide a stable chassis for taxiing or
rolling during takeoff
and landing，
and
to provide a shock absorbing system during
landing．
Regardless， all of these tasks are
secondary to the plane’s
primary role as an
efficient mode of travel through the air．
To aircraft designers， landing gear are nothing
more than a
necessary evil since planes are
designed primarily for their
performance in
flight rather than on the ground．
There have even been attempts over the years to
eliminate
landing gear entirely． The most
extreme case was a study done by
the Royal
Navy to see if a jet plane could make a belly
landing on the
deck of an aircraft carrier
coated with a rubberized surface．
If
successful， the method would eliminate the need
for the
very strong and heavy landing gear
used on carrier－based
aircraft． Unfortunately，
the method proved impractical， but it shows
the lengths some will go to while attempting
to eliminate the need for
landing gear！
We have seen that landing gear come in many
varieties and
each option has its own
advantages and disadvantages．
Selecting the
best arrangement for a given aircraft is a
trade－
off between these strengths and
weaknesses as they apply to the
environment
the plane is designed for．
As a result，
designers try to select the simplest， smallest，
lightest， and least expensive solution
possible to do the job while
maintaining
safety．
That is why most planes only have
three landing gear rather
than four because
fewer gear weigh less， require less structure
aboard the plane， take up less space when
retracted， and generate
less drag．