Monowheel Landing Gear

Landing gear is the undercarriage of an aircraft or spacecraft that is used for takeoff or landing. For aircraft it is generally needed for both. It was also formerly called ''alighting gear'' by some manufacturers, such as the Glenn L. Martin Company. For aircraft, Stinton makes the terminology distinction ''undercarriage (British) = landing gear (US)''.

For aircraft, the landing gear supports the craft when it is not flying, allowing it to take off, land, and taxi without damage. Wheeled landing gear is the most common, with skis or floats needed to operate from snow/ice/water and skids for vertical operation on land. Faster aircraft have retractable undercarriages, which fold away during flight to reduce drag.

Some unusual landing gear have been evaluated experimentally. These include: no landing gear (to save weight), made possible by operating from a catapult cradle and flexible landing deck: air cushion (to enable operation over a wide range of ground obstacles and water/snow/ice); tracked (to reduce runway loading).

For launch vehicles and spacecraft landers, the landing gear usually only supports the vehicle on landing, and is not used for takeoff or surface movement.

Given their varied designs and applications, there exists dozens of specialized landing gear manufacturers. The three largest are Safran Landing Systems, Collins Aerospace (part of Raytheon Technologies) and Héroux-Devtek.

Aircraft

The landing gear represents 2.5 to 5% of the maximum takeoff weight (MTOW) and 1.5 to 1.75% of the aircraft cost, but 20% of the airframe direct maintenance cost. A suitably-designed wheel can support , tolerate a ground speed of 300 km/h and roll a distance of ; it has a 20,000 hours time between overhaul and a 60,000 hours or 20 year life time.

Gear arrangements

File:Piper Cub Góraszka (cropped).JPG, Conventional/taildragger Piper Cub
File:Cessna 152 PR-EJQ (8476096843).jpg, Tricycle Cessna 152
File:US Navy 080922-N-2183K-061 An AV-8B Harrier jet lands aboard the amphibious assault ship USS Peleliu (LHA 5).jpg, Bicycle AV-8B Harrier
File:XC-120 front view.jpg, Quadricycle Fairchild XC-120 Packplane

Wheeled undercarriages normally come in two types:
* '' Conventional landing gear'' or ''"taildragger"'', where there are two main wheels towards the front of the aircraft and a single, much smaller, wheel or skid at the rear. The same helicopter arrangement is called tricycle tailwheel.
* '' Tricycle undercarriage'' where there are two main wheels (or wheel assemblies) under the wings and a third smaller wheel in the nose. The same helicopter arrangement is called tricycle nosewheel.
The taildragger arrangement was common during the early propeller era, as it allows more room for propeller clearance. Most modern aircraft have tricycle undercarriages. Taildraggers are considered harder to land and take off (because the arrangement is usually ''unstable'', that is, a small deviation from straight-line travel will tend to increase rather than correct itself), and usually require special pilot training. A small tail wheel or skid/bumper may be added to a tricycle undercarriage to prevent damage to the underside of the fuselage if over-rotation occurs on take-off leading to a tail strike. Aircraft with tail-strike protection include the B-29 Superfortress, Boeing 727 trijet and Concorde. Some aircraft with retractable conventional landing gear have a fixed tailwheel. Hoerner estimated the drag of the Bf 109 fixed tailwheel and compared it with that of other protrusions such as the pilot's canopy.

A third arrangement (known as tandem or bicycle) has the main and nose gear located fore and aft of the center of gravity under the fuselage with outriggers on the wings. This is used when there is no convenient location on either side of the fuselage to attach the main undercarriage or to store it when retracted. Examples include the Lockheed U-2 spy plane and the Harrier jump jet. The Boeing B-52 uses a similar arrangement, except that the fore and aft gears each have two twin-wheel units side by side.

Quadricycle gear is similar to bicycle but with two sets of wheels displaced laterally in the fore and aft positions. Raymer classifies the B-52 gear as quadricycle. The experimental Fairchild XC-120 Packplane had quadricycle gear located in the engine nacelles to allow unrestricted access beneath the fuselage for attaching a large freight container.

Helicopters use skids, pontoons or wheels depending on their size and role.

File:R-4 AC HNS1 9 300.jpg, Tricycle tailwheel Sikorsky R-4
File:Ch-54 1army.jpg, Tricycle nosewheel Sikorsky CH-54
File:CH-47 Chinook helicopter flyby.jpg, Quadricycle Boeing CH-47 Chinook

Retractable gear

To decrease drag in flight undercarriages retract into the wings and/or fuselage with wheels flush with the surrounding surface or concealed behind flush-mounted doors; this is called ''retractable gear.'' If the wheels don't retract completely but protrude partially exposed to the airstream, it is called a semi-retractable gear.

Most retractable gear is hydraulically operated, though some is electrically operated or even manually operated on very light aircraft. The landing gear is stowed in a compartment called a wheel well.

Pilots confirming that their landing gear is down and locked refer to "three greens" or "three in the green.", a reference to the electrical indicator lights (or painted panels of mechanical indicator units) from the nosewheel/tailwheel and the two main gears. Blinking green lights or red lights indicate the gear is in transit and neither up and locked or down and locked. When the gear is fully stowed up with the up-locks secure, the lights often extinguish to follow the dark cockpit philosophy; some airplanes have gear up indicator lights.

Redundant systems are used to operate the landing gear and redundant main gear legs may also be provided so the aircraft can be landed in a satisfactory manner in a range of failure scenarios. The Boeing 747 was given four separate and independent hydraulic systems (when previous airliners had two) and four main landing gear posts (when previous airliners had two). Safe landing would be possible if two main gear legs were torn off provided they were on opposite sides of the fuselage. In the case of power failure in a light aircraft, an emergency extension system is always available. This may be a manually operated crank or pump, or a mechanical free-fall mechanism which disengages the uplocks and allows the landing gear to fall under gravity.

File:Undercarriage anim.gif, Retractable landing gear animation
File:Fahrwerkschacht B737 Flugtag Bremen 2009 002.JPG, Narrowbody aircraft have twin-wheel main landing gear legs.
File:Airbus A350-941 F-WWCF MSN002 main landing gear ILA Berlin 2016 06 (cropped).jpg, Widebody aircraft main legs have multiple-axle bogies.
File:USCG Lockheed HC-130H 1704 rear port landing gear.JPG, High-wing cargo aircraft have pontoons for the main gear.

Shock absorbers

Aircraft landing gear includes wheels equipped with solid shock absorbers on light planes, and air/oil oleo struts on larger aircraft.

Oleo torque link main landing gear.JPG, Oleo strut (pressurized gas or oil) with torque links
Oleo trailing link main landing gear.JPG, Oleo strut with trailing link
Rubber trailing link main landing gear.JPG, Rubber discs in compression with trailing link of a Mooney M20
Spring steel main landing gear.JPG, Simple spring steel bar, used on many light aircraft
Tubular main landing gear.JPG, Simple steel tube

Large aircraft

As aircraft weights have increased more wheels have been added and runway thickness has increased to keep within the runway loading limit. The Zeppelin-Staaken R.VI, a large German World War I long-range bomber of 1916, used eighteen wheels for its undercarriage, split between two wheels on its nose gear struts, and sixteen wheels on its main gear units—split into four side-by-side quartets each, two quartets of wheels per side—under each tandem engine nacelle, to support its loaded weight of almost .

Multiple "tandem wheels" on an aircraft—particularly for cargo aircraft, mounted to the fuselage lower sides as retractable main gear units on modern designs—were first seen during World War II, on the experimental German Arado Ar 232 cargo aircraft, which used a row of eleven "twinned" fixed wheel sets directly under the fuselage centerline to handle heavier loads while on the ground. Many of today's large cargo aircraft use this arrangement for their retractable main gear setups, usually mounted on the lower corners of the central fuselage structure.

The prototype Convair XB-36 had most of its weight on two main wheels, which needed runways at least thick. Production aircraft used two four-wheel bogies, allowing the aircraft to use any airfield suitable for a B-29.

A relatively light Lockheed JetStar business jet, with four wheels supporting , needed a thick flexible asphalt pavement. The Boeing 727-200 with four tires on two legs main landing gears required a thick pavement. The thickness rose to for a McDonnell Douglas DC-10-10 with supported on eight wheels on two legs. The heavier, , DC-10-30/40 were able to operate from the same thickness pavements with a third main leg for ten wheels, like the first Boeing 747-100, weighing on four legs and 16 wheels. The similar-weight Lockheed C-5, with 24 wheels, needs an pavement.

The twin-wheel unit on the fuselage centerline of the McDonnell Douglas DC-10-30/40 was retained on the MD-11 airliner and the same configuration was used on the initial Airbus A340-200/300, which evolved in a complete four-wheel undercarriage bogie for the heavier Airbus A340-500/-600. The up to Boeing 777 has twelve main wheels on two three-axles bogies, like the later Airbus A350.

The Airbus A380 has a four-wheel bogie under each wing with two sets of six-wheel bogies under the fuselage. The Antonov An-225, the largest cargo aircraft, had 4 wheels on the twin-strut nose gear units like the smaller Antonov An-124, and 28 main gear wheels.

The A321neo has a twin-wheel main gear inflated to 15.7 bar (228 psi), while the A350-900 has a four-wheel main gear inflated to 17.1 bar (248 psi).

File:Virgin.atlantic.a340-600.g-vyou.arp.jpg, The A340-600 has an additional main undercarriage on the fuselage belly
File:Undercarriage.b747.arp.jpg, Wing and fuselage undercarriages on a Boeing 747-400, shortly before landing
File:Airbus-owned A380-800 (F-WWDD) at Filton Airfield (England) in mid-2010 arp.jpg, The 20-wheeled main undercarriage of an Airbus A380
File:Antonov-225 main landing gear 2.jpg, Main landing gear of the Antonov An-225 Mriya

STOL aircraft

STOL aircraft have a higher sink-rate requirement if a carrier-type, no-flare landing technique has to be adopted to reduce touchdown scatter. For example, the Saab 37 Viggen, with landing gear designed for a 5m/sec impact, could use a carrier-type landing and HUD to reduce its scatter from 300 m to 100m.

The de Havilland Canada DHC-4 Caribou used long-stroke legs to land from a steep approach with no float.

Operation from water

A flying boat has a lower fuselage with the shape of a boat hull giving it buoyancy. Wing-mounted floats or stubby wing-like sponsons are added for stability. Sponsons are attached to the lower sides of the fuselage.

A floatplane has two or three streamlined floats. Amphibious floats have retractable wheels for land operation.

An amphibious aircraft or amphibian usually has two distinct landing gears, namely a "boat" hull/floats and retractable wheels, which allow it to operate from land or water.

Beaching gear is detachable wheeled landing gear that allows a non-amphibious floatplane or flying boat to be maneuvered on land. It is used for aircraft maintenance and storage and is either carried in the aircraft or kept at a slipway. Beaching gear may consist of individual detachable wheels or a cradle that supports the entire aircraft. In the former case, the beaching gear is manually attached or detached with the aircraft in the water; in the latter case, the aircraft is maneuvered onto the cradle.

Helicopters are able to land on water using floats or a hull and floats.

File:Cessna 208 Caravan, FlyMex JP7376008.jpg, Cessna 208 floatplane with amphibious floats
File:95siki-suitei_1929.jpg, Nakajima E8N floatplane with three floats
File:Consolidated PBY-5A Catalina in flight (cropped).jpg, Amphibious Consolidated PBY Catalina with landing gear on the side
File:Short Solent III South Pacific Air Lines (4807712668).jpg, Non-amphibious Short Solent flying boat with beaching gear
File:Bundesarchiv Bild 102-10270, Flugschiff Dornier Do X.jpg, Dornier Do X with sponsons
File:RIAS 2014 HH-3 02.jpg, Sikorsky HH-3 on water
File:Schweizer 300CBi (269C) AN1062928.jpg, Schweizer 300 with inflatable floats

For take-off a step and planing bottom are required to lift from the floating position to planing on the surface. For landing a cleaving action is required to reduce the impact with the surface of the water. A vee bottom parts the water and chines deflect the spray to prevent it damaging vulnerable parts of the aircraft. Additional spray control may be needed using spray strips or inverted gutters. A step is added to the hull, just behind the center of gravity, to stop water clinging to the afterbody so the aircraft can accelerate to flying speed. The step allows air, known as ventilation air, to break the water suction on the afterbody. Two steps were used on the Kawanishi H8K. A step increases the drag in flight. The drag contribution from the step can be reduced with a fairing. A faired step was introduced on the Short SunderlandIII.

One goal of seaplane designers was the development of an open ocean seaplane capable of routine operation from very rough water. This led to changes in seaplane hull configuration. High length/beam ratio hulls and extended afterbodies improved rough water capabilities. A hull much longer than its width also reduced drag in flight. An experimental development of the Martin Marlin, the Martin M-270, was tested with a new hull with a greater length/beam ratio of 15 obtained by adding 6 feet to both the nose and tail. Rough-sea capability can be improved with lower take-off and landing speeds because impacts with waves are reduced. The Shin Meiwa US-1A is a STOL amphibian with blown flaps and all control surfaces. The ability to land and take-off at relatively low speeds of about 45 knots and the hydrodynamic features of the hull, long length/beam ratio and inverted spray gutter for example, allow operation in wave heights of 15 feet. The inverted gutters channel spray to the rear of the propeller discs.

Low speed maneuvring is necessary between slipways and buoys and take-off and landing areas. Water rudders are used on seaplanes ranging in size from the Republic RC-3 Seabee to the Beriev A-40 Hydro flaps were used on the Martin Marlin and Martin SeaMaster. Hydroflaps, submerged at the rear of the afterbody, act as a speed brake or differentially as a rudder. A fixed fin, known as a skeg, has been used for directional stability. A skeg, was added to the second step on the Kawanishi H8K flying boat hull.

High speed impacts in rough water between the hull and wave flanks may be reduced using hydro-skis which hold the hull out of the water at higher speeds. Hydro skis replace the need for a boat hull and only require a plain fuselage which planes at the rear. Alternatively skis with wheels can be used for land-based aircraft which start and end their flight from a beach or floating barge. Hydro-skis with wheels were demonstrated as an all-purpose landing gear conversion of the Fairchild C-123, known as the Panto-base Stroukoff YC-134. A seaplane designed from the outset with hydro-skis was the Convair F2Y Sea Dart prototype fighter. The skis incorporated small wheels, with a third wheel on the fuselage, for ground handling.

In the 1950s hydro-skis were envisaged as a ditching aid for large piston-engined aircraft. Water-tank tests done using models of the Lockheed Constellation, Douglas DC-4 and Lockheed Neptune concluded that chances of survival and rescue would be greatly enhanced by preventing critical damage associated with ditching.

File:Short Sunderland MR.5 ML824 NS-Z Hendon 03.77 edited-3.jpg, Short Sunderland V showing step on wing float and faired step on hull
File:Kawanishii H8K3.svg, Kawanishi H8K showing two steps on the hull, a skeg at the second step and spray strips under the forebody
File:Martin YP6M-1 Seamaster in flight.jpg, Martin SeaMaster showing outline of hydroflaps at rear
File:YC-123E with pantobase landing gear 1955.jpg, Stroukoff YC-123E showing hydro-skis on pantobase landing gear
File:Harbin SH-5.jpg, Harbin SH-5 showing deep vee forebody
File:Us-1a 04l.jpg, Shin Meiwa US-1A showing inverted spray gutter from nose to behind propellers
File:ShinMaywa US-2 at Atsugi.jpg, Shin Meiwa US-2 showing revised spray suppressor compared to that on US-1A
File:Be-42 rear view - Fairford 96.jpg, Beriev A-40 showing water rudder
File:XF2Y-1 off San Diego 1954-55 NAN1-81.jpg, Convair F2Y Sea Dart showing twin hydro-skis

Shipboard operation

The landing gear on fixed-wing aircraft that land on aircraft carriers have a higher sink-rate requirement because the aircraft are flown onto the deck with no landing flare. Other features are related to catapult take-off requirements for specific aircraft. For example, the Blackburn Buccaneer was pulled down onto its tail-skid to set the required nose-up attitude. The naval McDonnell Douglas F-4 Phantom II in UK service needed an extending nosewheel leg to set the wing attitude at launch.

The landing gear for an aircraft using a ski-jump on take-off is subjected to loads of 0.5g which also last for much longer than a landing impact.

Helicopters may have a deck-lock harpoon to anchor them to the deck.

In-flight use

Some aircraft have a requirement to use the landing-gear as a speed brake.

Flexible mounting of the stowed main landing-gear bogies on the Tupolev Tu-22R raised the aircraft flutter speed to . The bogies oscillated within the nacelle under the control of dampers and springs as an anti-flutter device.

Gear common to different aircraft

Some experimental aircraft have used gear from existing aircraft to reduce program costs. The Martin-Marietta X-24 lifting body used the nose/main gear from the North American T-39 / Northrop T-38 and the Grumman X-29 from the Northrop F-5 / General Dynamics F-16.

Other types

Skis

When an airplane needs to land on surfaces covered by snow, the landing gear usually consists of skis or a combination of wheels and skis.

Detachable

Some aircraft use wheels for takeoff and jettison them when airborne for improved streamlining without the complexity, weight and space requirements of a retraction mechanism. The wheels are sometimes mounted onto axles that are part of a separate "dolly" (for main wheels only) or "trolley" (for a three-wheel set with a nosewheel) chassis. Landing is done on skids or similar simple devices.

Historical examples include the "dolly"-using Messerschmitt Me 163 ''Komet'' rocket fighter, the Messerschmitt Me 321 ''Gigant'' troop glider, and the first eight "trolley"-using prototypes of the Arado Ar 234 jet reconnaissance bomber. The main disadvantage to using the takeoff dolly/trolley and landing skid(s) system on German World War II aircraft – intended for a sizable number of late-war German jet and rocket-powered military aircraft designs – was that aircraft would likely be scattered all over a military airfield after they had landed from a mission, and would be unable to taxi on their own to an appropriately hidden "dispersal" location, which could easily leave them vulnerable to being shot up by attacking Allied fighters. A related contemporary example are the wingtip support wheels ("pogos") on the Lockheed U-2 reconnaissance aircraft, which fall away after take-off and drop to earth; the aircraft then relies on titanium skids on the wingtips for landing.

Rearwards and sideways retraction

Some main landing gear struts on World War II aircraft, in order to allow a single-leg main gear to more efficiently store the wheel within either the wing or an engine nacelle, rotated the single gear strut through a 90° angle during the rearwards-retraction sequence to allow the main wheel to rest "flat" above the lower end of the main gear strut, or flush within the wing or engine nacelles, when fully retracted. Examples are the Curtiss P-40, Vought F4U Corsair, Grumman F6F Hellcat, Messerschmitt Me 210 and Junkers Ju 88. The Aero Commander family of twin-engined business aircraft also shares this feature on the main gears, which retract aft into the ends of the engine nacelles. The rearward-retracting nosewheel strut on the Heinkel He 219 and the forward-retracting nose gear strut on the later Cessna Skymaster similarly rotated 90 degrees as they retracted.

On most World War II single-engined fighter aircraft (and even one German heavy bomber design) with sideways retracting main gear, the main gear that retracted into the wings was raked forward in the "down" position for better ground handling, with a retracted position that placed the main wheels at some distance aft of their position when downairframe – this led to a complex angular geometry for setting up the "pintle" angles at the top ends of the struts for the retraction mechanism's axis of rotation. with some aircraft, like the P-47 Thunderbolt and Grumman Bearcat, even mandating that the main gear struts lengthened as they were extended to give sufficient ground clearance for their large four-bladed propellers. One exception to the need for this complexity in many WW II fighter aircraft was Japan's famous Zero fighter, whose main gear stayed at a perpendicular angle to the centerline of the aircraft when extended, as seen from the side.

Variable axial position of main wheels

The main wheels on the Vought F7U Cutlass could move 20 inches between a forward and aft position. The forward position was used for take-off to give a longer lever-arm for pitch control and greater nose-up attitude. The aft position was used to reduce landing bounce and reduce risk of tip-back during ground handling.

Tandem layout

The '' tandem'' or bicycle layout is used on the Hawker Siddeley Harrier, which has two main-wheels behind a single nose-wheel under the fuselage and a smaller wheel near the tip of each wing. On second generation Harriers, the wing is extended past the outrigger wheels to allow greater wing-mounted munition loads to be carried, or to permit wing-tip extensions to be bolted on for ferry flights.

A tandem layout was evaluated by Martin using a specially-modified Martin B-26 Marauder (the XB-26H) to evaluate its use on Martin's first jet bomber, the Martin XB-48. This configuration proved so manoeuvrable that it was also selected for the B-47 Stratojet. It was also used on the U-2, Myasishchev M-4, Yakovlev Yak-25, Yak-28 and Sud Aviation Vautour. A variation of the multi tandem layout is also used on the B-52 Stratofortress which has four main wheel bogies (two forward and two aft) underneath the fuselage and a small outrigger wheel supporting each wing-tip. The B-52's landing gear is also unique in that all four pairs of main wheels can be steered. This allows the landing gear to line up with the runway and thus makes crosswind landings easier (using a technique called '' crab landing''). Since tandem aircraft cannot rotate for takeoff, the forward gear must be long enough to give the wings the correct angle of attack during takeoff. During landing, the forward gear must not touch the runway first, otherwise the rear gear will slam down and may cause the aircraft to bounce and become airborne again.

Crosswind landing accommodation

One very early undercarriage incorporating castoring for crosswind landings was pioneered on the Bleriot VIII design of 1908. It was later used in the much more famous Blériot XI Channel-crossing aircraft of 1909 and also copied in the earliest examples of the Etrich Taube. In this arrangement the main landing gear's shock absorption was taken up by a vertically sliding bungee cord-sprung upper member. The vertical post along which the upper member slid to take landing shocks also had its lower end as the rotation point for the forward end of the main wheel's suspension fork, allowing the main gear to pivot on moderate crosswind landings.

Manually-adjusted main-gear units on the B-52 can be set for crosswind take-offs. It rarely has to be used from SAC-designated airfields which have major runways in the predominant strongest wind direction. The Lockheed C-5 Galaxy has swivelling 6-wheel main units for crosswind landings and castoring rear units to prevent tire scrubbing on tight turns.

"Kneeling" gear

Both the nosegear and the wing-mounted main landing gear of the World War II German Arado Ar 232 cargo/transport aircraft were designed to kneel. This made it easier to load and unload cargo, and improved taxiing over ditches and on soft ground.

Some early U.S. Navy jet fighters were equipped with "kneeling" nose gear consisting of small steerable auxiliary wheels on short struts located forward of the primary nose gear, allowing the aircraft to be taxied tail-high with the primary nose gear retracted. This feature was intended to enhance safety aboard aircraft carriers by redirecting the hot exhaust blast upwards, and to reduce hangar space requirements by enabling the aircraft to park with its nose underneath the tail of a similarly equipped jet. Kneeling gear was used on the North American FJ-1 Fury

HOME


Content is Copyleft
Website design, code, and AI is Copyrighted (c) 2014-2017 by Stephen Payne


Consider donating to Wikimedia



As an Amazon Associate I earn from qualifying purchases