Aircraft Landing Gear Layouts

Subject: Aircraft Landing Gear Layouts Thu Jun 03, 2010 8:50 am

Why do aircraft have only three landing
gear? Why not four?
- question from Chintan

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 "tricycle" landing
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.

Tailwheel or Taildragger Gear

Though the tricycle arrangement may be most
popular today, that was not always the case. The tailwheel
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.

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Taildragger or tailwheel landing gear

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 [You must be registered and logged in to see this link.]
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. 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
"ground loop" that may eventually result in
scraping a wingtip 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.

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Stable and unstable behavior of tricycle gear vs.
taildragger gear

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.

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DC-3 Dakota airliner illustrating its taildragger
landing gear

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.

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Tricycle or nosewheel landing gear

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.

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Danger of tail sitting exemplifided by an improperly
loaded 747

There are many examples of aircraft with
tricycle landing gear, including the
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Cessna 172.

Bicycle Gear

A relatively uncommon landing gear option
is the bicycle undercarriage. Bicycle gear features two main gear along

the centerline of the aircraft, one forward and one aft of the center of
gravity. Preventing the plane from
tilting over sideways are two small outrigger gear mounted along the
wing.

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Bicycle landing gear

The only real advantage of bicycle gear is
lower weight and drag than either the taildragger or tricycle
arrangements. Bicycle gear are also useful on planes with very long and
slender fuselages where there is little
room for more traditional undercarriage arrangements. Unfortunately,
bicycle gear are very demanding on the pilot
who must maintain a very level attitude during takeoff and landing while
carefully managing airspeed. The pilot
must also compensate for any rolling motion that could cause the plane
to land unevenly on one of the outrigger
gear, and crosswinds are particularly difficult to deal with.

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Bicycle landing gear of the B-47 Stratojet

Because of these limitations, bicycle gear
are generally limited to planes with high aspect ratio wings that
generate high lift at low angles of attack. Good examples of such
planes are large bombers with a narrow fuselage
and large wingspan like the [You must be registered and logged in to see this link.].
Another common application of the
bicycle undercarriage is aboard vertical takeoff and landing designs
like the
[You must be registered and logged in to see this link.].
Here, the gear layout provides safety and stability in case
of an engine failure during landing.

Single Main Gear

A special subcategory of the bicycle
undercarriage is the single gear. This layout features a single large
gear
unit and a much smaller auxiliary tailwheel along the centerline.
Outriggers are again provided for stability.

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Single main landing gear

This design is particularly simple,
lightweight, and low drag and may even include skids rather than wheels.

This simplicity makes the gear arrangement attractive for use on light
planes like gliders and sailplanes, but
the single main gear is generally impractical for larger aircraft.

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U-2 Dragon Lady and its landing gear arrangement

Perhaps the best known application of a
single main gear arrangement is the
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reconnaissance plane. This aircraft has a single large gear unit near
the center of gravity plus a much smaller tailwheel. Two additional
outriggers called "pogos" are attached
by ground crew to keep the plane from tipping during taxi, but these are
removed prior to takeoff.

Quadricycle Gear

Quadricycle gear are also very similar to
the bicycle arrangement except there are four main gear roughly equal in

size and mounted along the fuselage.

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Quadricycle landing gear

Like bicycle gear, the quadricycle
undercarriage also requires a very flat attitude during takeoff and
landing.
This arrangement is also very sensitive to roll, crosswinds, and proper
alignment with the runway. The most
significant advantage of quadricycle gear is that the plane's floor can
be very close to the ground for easier
loading and unloading of cargo. However, this benefit comes at the
price of much higher weight and drag than
bicycle gear.

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Quadricycle landing gear of the B-52 Stratofortress

Quadricycle gear are sometimes used on
cargo planes, but probably the most well known example is the
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bomber. This aircraft employs a cross between the quadricycle and
bicycle arrangements since it has four main gear plus two small
outriggers near the wingtips.

Multi-Bogey Gear

A final variation that is worth mentioning
is the use of multiple wheels per landing gear strut. It is especially
common to place two wheels on the nose strut of the tricycle arrangment
to provide safety and steering control in
case of a tire blowout. This additional tire is particularly useful on
carrier-based aircraft where two nosewheels
are a requirement. Multiple wheels are also often used on main gear
units for added safety, especially on
commercial airliners.

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Multi-bogey landing gear

When multiple wheels are placed on the same
gear unit, they are attached together on a structural device called a
bogey. The heavier the aircraft becomes, the more wheels are typically
added to the bogey to spread the plane's
weight more evenly across the runway pavement. In general, a plane
weighing less than 50,000 lb (22,680 kg) has
only one wheel per main gear strut. Aircraft weighing up to 200,000 lb
(90,720 kg) usually carry two wheels per
strut. On planes weighing up to 400,000 lb (181,440 kg), a four-wheel
bogey is typical. Aircraft of greater
weight often carry four bogeys, each with four to six wheels.

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The many, many, many landing gear wheels of the An-225

The best examples of multi-bogey aircraft
are large mega jets like the
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This mammoth cargo plane has seven pairs of wheels on each
main gear assembly plus four nosewheels, combining for a total of 32
tires! Another good example is the
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The 747 is equipped with four main gear units, each with
four-wheel bogies, plus twin nosewheels so that the plane's weight is
spread across 18 wheels.

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.

- answer by [You must be registered and logged in to see this link.],
31 October 2004
from: Aerospaceweb.org

Advisor Posts : 1274 Join date : 2009-09-17 Location : CGK

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Subject: Re: Aircraft Landing Gear Layouts Thu Jun 03, 2010 2:41 pm

nice info om momod... [You must be registered and logged in to see this image.]

Subject: Re: Aircraft Landing Gear Layouts Tue Jul 27, 2010 12:14 pm

Jadi inget pelajaran kuliah dulu... "Beban Pesbang..." hahaha... [You must be registered and logged in to see this image.]
Good knowledge Om Momod... [You must be registered and logged in to see this image.] Thanks!

Guest

Wah..wawasan Jadi bertambah luas.......Tks Om Fly Emirate...... [You must be registered and logged in to see this image.]

Subject: Re: Aircraft Landing Gear Layouts Wed Jul 28, 2010 4:30 am

sama2 om... [You must be registered and logged in to see this image.]

Subject: Re: Aircraft Landing Gear Layouts Tue Aug 03, 2010 10:54 pm

Om Fly Emirates : Ternyata landing gear memberi peranan yg amat penting ya, makasih kepada yg telah mencipta landing gear... [You must be registered and logged in to see this image.]

Subject: Re: Aircraft Landing Gear Layouts Wed Aug 04, 2010 7:50 pm

sama2 om...eh salah..bukan sama ogut ya...maksunya sama2 makasih pada pencipta landing gear... [You must be registered and logged in to see this image.]

Subject: Re: Aircraft Landing Gear Layouts Thu Aug 05, 2010 9:24 am

Fly Emirates wrote:: sama2 om...eh salah..bukan sama ogut ya...maksunya sama2 makasih pada pencipta landing gear... [You must be registered and logged in to see this image.]

Om Fly Emirates : Om PD ya... [You must be registered and logged in to see this image.]

Subject: Re: Aircraft Landing Gear Layouts Mon Aug 09, 2010 3:51 pm

maksudnya PD om?

Subject: Re: Aircraft Landing Gear Layouts Mon Aug 09, 2010 4:01 pm

Fly Emirates wrote:: maksudnya PD om?

Om Fly Emirates : Percaya Diri... [You must be registered and logged in to see this image.] . Perasan Diri juga bisa... [You must be registered and logged in to see this image.]

Subject: Re: Aircraft Landing Gear Layouts Mon Aug 09, 2010 4:04 pm

ooo...kirain PD nya lain arti om... [You must be registered and logged in to see this image.]

Subject: Re: Aircraft Landing Gear Layouts Mon Aug 09, 2010 4:07 pm

Fly Emirates wrote:: ooo...kirain PD nya lain arti om... [You must be registered and logged in to see this image.]

Om Fly Emirates : Di sana juga ada ya Om?

Subject: Re: Aircraft Landing Gear Layouts Mon Aug 09, 2010 4:08 pm

ada om...sama artinya...saya kira kalo di Malaysia beda... [You must be registered and logged in to see this image.]

Subject: Re: Aircraft Landing Gear Layouts Mon Aug 09, 2010 4:10 pm

Fly Emirates wrote:: ada om...sama artinya...saya kira kalo di Malaysia beda... [You must be registered and logged in to see this image.]

Om Fly Emirates : Ooo... [You must be registered and logged in to see this image.]

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