Hydroplaning (tires)
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Hydroplaning or aquaplaning by a road vehicle occurs when a layer of water builds between the rubber tires of the vehicle and the road surface, leading to the loss of traction and thus preventing the vehicle from responding to control inputs such as steering, braking or accelerating. It becomes, in effect, an unpowered and unsteered sled.
Hydroplaning also affects aircraft tires in contact with a wet runway.
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[edit] Causes of hydroplaning
Every vehicle function that changes direction or speed, from turning, to accelerating, to braking, places a lateral load on the tires. Control of this load relies on the friction between the tire contact points and the road surface. More friction makes for a greater resistance to slipping; if water comes between the tires and the road, it can reduce friction to such an extent that the driver may lose control.
The tread of a rubber tire is designed to remove water from beneath the tire, providing high friction with the road surface even in wet conditions. Hydroplaning occurs when a tire encounters more water than it can dissipate. Water pressure in front of the wheel forces a wedge of water under the leading edge of the tire, causing it to lift from the road. The tire then skates on a sheet of water with little, if any, direct road contact, and loss of control results.
If all four tires hydroplane, the vehicle will slide until it either collides with an obstacle, or slows enough that one or more tires contact the road again and friction is regained.
The likelihood of hydroplaning increases with the speed of the vehicle and the depth of the water, and if the tire tread is worn, naturally low profile, or hampered by underinflation. Vehicle weight is an additional factor; lighter cars hydroplane more easily.
Vehicles with round-profile tires, such as bicycles and motorcycles, virtually never suffer from hydroplaning in normal road use. The contact area with the road is a canoe-shaped patch that effectively squeezes water out of the way. However, because road friction is so much less in wet conditions, the lateral force that the tires can accommodate before sliding is greatly diminished. While a slide in a four-wheeled vehicle is correctable with practice, the same slide on a motorcycle will generally cause the rider to fall, with severe consequences. Despite the relative lack of hydroplaning danger then, motorcycle riders do well to be even more cautious than car drivers in poor conditions.
See also Traction (engineering)#Loss of traction in road vehicles for effects similar to hydroplaning.
[edit] How to deal with hydroplaning (road vehicles)
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[edit] Car response
What the driver experiences when a car hydroplanes depends on which wheels have lost traction and the direction of travel.
If the car is traveling straight, it may begin to feel slightly loose. If there was a high level of road feel in normal conditions, it may suddenly diminish. Small correctional control inputs will be ignored by the car.
If the drive wheels hydroplane, there may be a sudden audible rise in engine RPM and indicated speed as they begin to spin. In a broad highway turn, if the front wheels lose traction, the car will suddenly begin to drift into an outer lane. If the rear wheels lose traction, the back of the car will begin to slew out sideways into a skid. If all four wheels hydroplane at once, the car will slide to an outer lane. When any or all of the wheels regain traction, there may be a sudden jerk in whatever direction that wheel is pointed.
[edit] Recovery
To recover, never turn the steering wheel of the car or apply the brakes. Either action could put the car into a skid from which recovery would be difficult or impossible.
Instead, with no change in steering input, gently ease pressure off the accelerator; control should then return. If braking is unavoidable, use light pumping actions on the brakes until hydroplaning has stopped.
With a manual transmission, immediately when you notice that you are hydroplaning, press the clutch down so all wheels can roll freely; this will restore grip.[citation needed]
[edit] Prevention by the driver
The best strategy remains not to hydroplane in the first place. Check tire pressure, make sure tire tread is not worn beyond its wear indicators. The risk greatly increases above 55 MPH, and in areas with any apparent standing water. Those performance vehicles with wide tires and drain grooves should take additional precautions. Driving in a lower gear can help the driver detect if the car has begun to hydroplane.
Stability control systems like ESC, ESP, and DSC should not replace proper driving technique. They rely on the same braking mechanism at the driver's disposal, which in turn depends on road contact. While stability control may help recovery from a skid when the vehicle slows enough to regain traction, it cannot prevent hydroplaning.
[edit] Hydroplaning in aircraft
Hydroplaning may reduce the effectiveness of wheel braking in aircraft on landing or aborting a take-off, when it can cause the aircraft to run off the end of the runway. Hydroplaning was a factor in an accident to Qantas Flight 1 when it ran off the end of the runway in Bangkok in 1999 during heavy rain. Aircraft which can employ reverse thrust braking have the advantage over road vehicles in such situations, as this type of braking is not affected by hydroplaning, but it requires a considerable distance to operate as it is not as effective as wheel braking on a dry runway.
Hydroplaning is a condition that can exist when an aircraft is landed on a runway surface contaminated with standing water, slush, and/or wet snow. Hydroplaning can have serious adverse effects on ground controllability and braking efficiency. The three basic types of hydroplaning are dynamic hydroplaning, reverted rubber hydroplaning, and viscous hydroplaning. Any one of the three can render an aircraft partially or totally uncontrollable anytime during the landing roll.
[edit] Dynamic hydroplaning
Dynamic hydroplaning is a relatively high-speed phenomenon that occurs when there is a film of water on the runway that is at least one-tenth inch deep. As the speed of the aircraft and the depth of the water increase, the water layer builds up an increasing resistance to displacement, resulting in the formation of a wedge of water beneath the tire. At some speed, termed the hydroplaning speed (VP), the upward force generated by water pressure equals the weight of the aircraft and the tire is lifted off the runway surface. In this condition, the tires no longer contribute to directional control, and braking action is nil. Dynamic hydroplaning is related to tire inflation pressure. Data obtained during hydroplaning tests have shown the minimum dynamic hydroplaning speed (VP)of a tire to be 8.6 times the square root of the tire pressure in pounds per square inch (PSI). For an aircraft with a main tire pressure of 24 PSI, the calculated hydroplaning speed would be approximately 42 knots. It is important to note that the calculated speed referred to above is for the start of dynamic hydroplaning. Once hydroplaning has started, it may persist to a significantly slower speed depending on the type being experienced.
[edit] Reverted rubber hydroplaning
Reverted rubber (steam) hydroplaning occurs during heavy braking that results in a prolonged locked-wheel skid. Only a thin film of water on the runway is required to facilitate this type of hydroplaning. The tire skidding generates enough heat to cause the rubber in contact with the runway to revert to its original uncured state. The reverted rubber acts as a seal between the tire and the runway, and delays water exit from the tire footprint area. The water heats and is converted to steam which supports the tire off the runway. Reverted rubber hydroplaning frequently follows an encounter with dynamic hydroplaning, during which time the pilot may have the brakes locked in an attempt to slow the aircraft. Eventually the aircraft slows enough to where the tires make contact with the runway surface and the aircraft begins to skid. The remedy for this type of hydroplane is for the pilot to release the brakes and allow the wheels to spin up and apply moderate braking. Reverted rubber hydroplaning is insidious in that the pilot may not know when it begins, and it can persist to very slow groundspeeds (20 knots or less).
[edit] Viscous hydroplaning
Viscous hydroplaning is due to the viscous properties of water. A thin film of fluid no more than one thousandth of an inch in depth is all that is needed. The tire cannot penetrate the fluid and the tire rolls on top of the film. This can occur at a much lower speed than dynamic hydroplane, but requires a smooth or smooth acting surface such as asphalt or a touchdown area coated with the accumulated rubber of past landings. Such a surface can have the same friction coefficient as wet ice. When confronted with the possibility of hydroplaning, it is best to land on a grooved runway (if available). Touchdown speed should be as slow as possible consistent with safety. After the nosewheel is lowered to the runway, moderate braking should be applied. If deceleration is not detected and hydroplaning is suspected, the nose should be raised and aerodynamic drag utilized to decelerate to a point where the brakes do become effective. Proper braking technique is essential. The brakes should be applied firmly until reaching a point just short of a skid. At the first sign of a skid, the pilot should release brake pressure and allow the wheels to spin up. Directional control should be maintained as far as possible with the rudder. Remember that in a crosswind, if hydroplaning should occur, the crosswind will cause the aircraft to simultaneously weathervane into the wind as well as slide downwind.
[edit] Prevention of hydroplaning through pavement design
Highway and runway engineers can mitigate hydroplaning through measures which assist water to drain off the road or runway, through selection of pavement materials and preparation of the shape of the surface of the pavement. One such measure is to roughen the surface with grooves into which water can settle so that 'peaks' between the grooves may contact the tire surface and maintain adequate friction.
[edit] References
- B. N. J. Persson, U. Tartaglino, O. Albohr And E. Tosatti (2004). "Sealing is at the origin of rubber slipping on wet roads". Nature Materials 3 (7 November): 882–885.
- Smart Motorist - Driving in the Rain
- Airplane Flying Handbook, FAA Publication FAA-H-8083-3A, available for download from the Flight Standards Service Web site at http://av-info.faa.gov.da:Aquaplaning
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