Air brake
In aeronautics air brakes (sometimes called "spoilers" or "lift dumpers") are a type of flight control used on aircraft to reduce speed during landing.
Air brakes differ from spoilers in that air brakes are designed to increase drag while making little change to lift, spoilers greatly reduce lift while making little change to drag.
Often, both characteristics are desirable - most airliners for example feature combined spoiler and airbrake controls. On landing, the deployment of these spoilers causes a dramatic loss of lift and hence the weight of the aircraft is transferred from the wings to the undercarriage, allowing the wheels to be mechanically braked with much less chance of skidding. In addition, the form drag created by the spoilers directly assists the braking effect. Reverse thrust is also used to help slow the aircraft on landing.
Railways
On railways an air brake is a brake operated by compressed air. A safer air brake was patented by George Westinghouse on March 5, 1872. Westinghouse's invention revolutionized the railroad industry, making stopping reliable and thus permitting trains to travel at higher speeds. Westinghouse made many alterations to improve his invention leading to various forms of the automatic brake. The United States Congress made their use mandatory by the Safety Appliance Act. By 1905, over 2,000,000 freight, passenger, mail, baggage and express cars and 89,000 locomotives were equipped with the Westinghouse Quick-Action Automatic Brake.
In the air brake's simplest form, compressed air pushes on a piston in a cylinder. The piston is connected to a shoe which can rub on the train wheel, creating friction and stopping the train. The compressed air comes from an air compressor in the locomotive and is sent from car to car by a string of hoses and pipes. There are two problems with this approach:
- it takes a long time to get enough air to the end of the train, so the brakes apply too slowly
- if there is any opening in the string of hoses and pipes, air will leak out and the brakes will no longer work.
- In particular, if the coupler between cars comes apart, the rear section will have no brakes at all (locomotives have their own brakes) potentially causing a runaway train.
Westinghouse invented a triple valve that was installed on each railroad car. It allowed compressed air from the locomotive to fill a reservoir tank on each car. As long as there is enough air pressure in the feed line through the train, the triple valve in each car keeps the brakes off and the reservoir charged. When pressure in the train line drops, the triple valve sends air from the car's reservoir to the car's brake cylinder, applying the brakes. This system is fail safe, meaning that any failure in the feed line, including a separation of part of the train, will cause the brakes to be applied, stopping the train.
Other applications
Air brakes are also used in trucks and buses.
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