Conventional landing gear
|Conventional landing gear|
|A Cessna 150 converted to taildragger configuration by installation of an after-market modification kit.|
Conventional landing gear, or tailwheel-type landing gear, is an aircraft undercarriage consisting of two main wheels forward of the center of gravity and a small wheel or skid to support the tail. The term conventional persists for historical reasons, but nowadays most aircraft—including all jet aircraft—are configured with tricycle gear.
The term taildragger is aviation jargon for an aircraft with a conventional undercarriage, although some writers have argued that the term should refer only to an aircraft with a tailskid and not a tailwheel.
In early aircraft, a tailskid made of metal or wood was used to support the tail on the ground. In most modern aircraft with conventional landing gear, a small, articulated wheel assembly is attached to the rearmost part of the airframe in place of the skid. This wheel is steered by the pilot through a connection to the rudder pedals, allowing the rudder and tailwheel to move together.
Before aircraft commonly used tailwheels, many aircraft (such as the Sopwith Camel fighter) were equipped with steerable tailskids, which operate exactly like a tailwheel. When the pilot pressed the right rudder pedal, the skid pivoted to the right, creating more drag on that side of the plane and causing it to turn to the right. While less effective than a steerable wheel, it did give the pilot some control of the direction the craft was moving while taxiing or beginning the takeoff run, before there was enough airflow over the rudder for it to become effective.
Another form of control, which is less common now than it once was, is to steer using "differential braking", in which the tailwheel itself is a simple, freely castoring mechanism, and the plane is steered by applying brakes selectively to the mainwheels in order to turn in that direction (this is also used on some tricycle geared aircraft, only with the front wheel being the freely castoring wheel instead). Like the steerable tailwheel/skid, it is usually integrated with the rudder pedals on the craft to allow an easy transition between wheeled and aerodynamic control.
Jet aircraft generally cannot use conventional landing gear, as this orients the engines at a high angle, causing their jet blast to bounce off the ground and back into the air, preventing the elevators from functioning properly. This problem occurred with the prototype of the German, Messerschmitt Me 262 jet fighter. The design was consequently changed to use tricycle landing gear.
Rare examples of jet-powered, tailwheel aircraft that saw service include the British Supermarine Attacker and the Soviet Yakovlev Yak-15. The Attacker was a naval fighter that first flew in 1946, and served with British forces until 1957 and with Pakistani forces until 1967. This configuration was chosen to allow the Attacker to make use of the wing design of a previous, propeller aircraft, the Supermarine Spiteful, without expensive design modification or retooling. The Yak 15 was a Soviet jet fighter that owed its configuration to the fact it was largely based on the Yakovlev Yak-3 propeller fighter.
Due to its smaller size the tailwheel has less parasite drag than a nosewheel, allowing the conventional geared aircraft to cruise at a higher speed on the same power. Tailwheels are less expensive to buy and maintain than a nosewheel. If a tailwheel fails on landing, the damage to the aircraft will be minimal. This is not the case in the event of a nosewheel failure, which usually results in propeller damage. Tailwheel aircraft are easier to fit into some hangars.
Due to the increased propeller clearance on tailwheel aircraft less stone chip damage will result from operating a conventional geared aircraft on rough or gravel airstrips. Because of the way airframe loads are distributed while operating on rough ground, tailwheel aircraft are better able to sustain this type of use over a long period of time, without cumulative airframe damage occurring.
Tailwheel aircraft are also more suitable for operation on skis.
Note that this mostly describes benefits with small craft rather than large ones, which almost always are equipped with retractable gear, meaning the extra drag of the nose gear is a non-issue. Also for craft of any significant size, a tricycle gear actually diminishes the stress on the fuselage, as the wheelbase or distance between the front and rear gear is less, leaving less unsupported fuselage. The unsupported rear portion of the fuselage is much lighter, as most of the weight is centered over the wings.
The conventional landing gear arrangement does have some disadvantages, compared to nose wheel equipped aircraft.
Tailwheel aircraft are much more subject to "nose-over" accidents due to injudicious application of brakes by the pilot.
Conventional geared aircraft are much more susceptible to ground looping. A ground loop occurs when directional control is lost on the ground and the tail of the aircraft passes the nose, swapping ends, in some cases completing a full circle. This event can result in damage to the aircraft's undercarriage, tires, wingtips, propeller and engine. Ground-looping occurs because, whereas a nosewheel aircraft is steered from ahead of the center of gravity, a taildragger is steered from behind (much like driving a car backwards at high speed), so that on the ground a taildragger is inherently unstable, whereas a nosewheel aircraft will self-center if it swerves on landing. In addition, some tailwheel aircraft must transition from using the rudder to steer to using the tailwheel while passing through a speed range when neither is wholly effective due to the nose high angle of the aircraft. Avoiding ground loops requires increased pilot training and skill.
Tailwheel aircraft generally suffer from poorer forward visibility on the ground, compared to nose wheel aircraft. In some cases this necessitates "S" turning on the ground to allow the pilot to see while taxiing.
Tailwheel aircraft are more difficult to taxi during high wind conditions, due to the higher angle of attack on the wings which can then develop more lift on one side, making control difficult or impossible. They also suffer from lower crosswind capability and in some wind conditions may be unable to use crosswind runways or single-runway airports.
Also due to the nose high attitude on the ground, propeller powered taildraggers are more adversely affected by P-factor - asymmetrical thrust caused by the propeller's disk being angled to the direction of travel, which causes the blades to produce more lift when going up than when going down due to the difference in angle the blade experiences when passing through the air. The aircraft will then pull to the side of the downward blade. Some aircraft lack sufficient rudder authority in some flight regimes (particularly at higher power settings on takeoff) and the pilot must compensate before the aircraft starts to yaw. Some aircraft, particularly older, higher powered aircraft such as the P-51 Mustang, cannot use full power on takeoff and still safely control their direction of travel. On landing this is less of a factor, however opening the throttle to abort a landing can induce severe yaw unless the pilot is prepared for it.
Taildragger aircraft and helicopters take up more room on board ships, as tricycle designs can be positioned with their tails over the edge of the deck.
A variation of the taildragger layout is the monowheel landing gear.
To minimize drag, many modern gliders have a single wheel, retractable or fixed, centered under the fuselage, which is referred to as monowheel gear or monowheel landing gear. Monowheel gear is also used on some powered aircraft, where drag reduction is a priority, such as the Europa XS. Both monowheel gliders and monowheel power aircraft use retractable wingtip legs (with small castor wheels attached) to prevent the wingtips from striking the ground. A monowheel aircraft may have a tailwheel (like the Europa) or a nosewheel (like the illustrated Schleicher glider).
Taildragger aircraft require more training time for student pilots to master. This was a large factor in the 1950s switch by most manufacturers to nosewheel-equipped trainers, and for many years nosewheel aircraft have been more popular than taildraggers. As a result, most PPL pilots now learn to fly in tricycle gear aircraft (e.g. Cessna 172 or Piper Cherokee) and only later transition to taildraggers.
Landing a conventional geared aircraft can be accomplished in two ways.
Normal landings are done by touching all three wheels down at the same time in a three-point landing. This method does allow the shortest landing distance but can be difficult to carry out in crosswinds, as rudder control may be reduced severely before the tailwheel can become effective.
The alternative is the wheel landing. This requires the pilot to land the aircraft on the main wheels while maintaining the tailwheel in the air with elevator to keep the angle of attack low. Once the aircraft has slowed to a speed that can ensure control will not be lost, but above the speed at which rudder effectiveness is lost, then the tailwheel is lowered to the ground.
Examples of tailwheel aircraft include:
- Cessna 170
- de Havilland Canada DHC-2 Beaver
- Denny Kitfox
- Grumman Ag Cat
- Maule M-5
- Piper J-3 Cub
- Vans RV-4
Modifications of tricycle gear aircraft
Several after-market modification companies offer kits to convert many popular nose-wheel equipped aircraft to conventional landing gear. Aircraft for which kits are available include:
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