Spoiler (aeronautics)
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In aeronautics a spoiler (sometimes called a lift dumper) is a device intended to reduce lift in an aircraft. Spoilers are plates on the top surface of a wing which can be extended upward into the smooth airflow and spoiling it. By doing so, the spoiler creates a carefully controlled stallover the portion of the wing behind it, dramatically reducing lift. Spoilers differ from airbrakes in that airbrakes are designed to increase drag while making little change to lift, while spoilers greatly reduce lift while making only a moderate increase in drag.
Spoilers are used by gliders in particular to control their rate of descent, and thus achieve a controlled landing at a desired spot. Spoilers are necessary because while an increased rate of descent can be achieved by angling the nose of an aircraft downwards, this may result in a significant increase in speed, possibly exceeding safe limits. Additionally, angling the nose downward may not result in a sufficient increase in descent angle.
Airliners too are usually fitted with spoilers. Spoilers are sometimes used when descending from cruise altitudes, to assist the aircraft in descending to lower altitudes without picking up speed. Their use is often limited, however, as the stall and turbulent airflow which develops behind them causes noticeable noise and vibration, which may cause discomfort to passengers. On landing, however, the spoilers are nearly always used at full effect to assist in slowing the aircraft. The increase in form drag created by the spoilers directly assists the braking effect. However, the real gain comes as the spoilers cause 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. Reverse thrust is also often used to help slow the aircraft on landing.
In air-cooled piston engine aircraft, spoilers may be needed to avoid shock cooling the engines. In a descent without spoilers, air speed is increased and the engine will be at low power, producing less heat than normal. The engine may cool too rapidly, resulting in stuck valves, cracked cylinders or other problems. Spoilers alleviate the situation by allowing the aircraft to descend at a desired rate, while letting the engine run at a power setting that keeps it from excessively rapid cooling. (This is particularly true in turbo charged air cooled piston engines, which run at higher power than normally aspirated engines.)
[edit] Spoilers as control surfaces
Some aircraft use spoilers in combination with or in lieu of ailerons for roll control. For such spoilers the term spoileron has been coined. In the case of a spoileron, in order for it to be used as a control surface, it is raised on one wing, thus decreasing lift and speed causing both roll and yaw effects.
[edit] Incidents and Accidents
Premature deployment of the spoilers at an altitude of 40 feet contributed to the crash of Air Canada Flight 621 at Toronto on July 5, 1970.
Forgetting to deploy the spoilers contributed to the crash of American Airlines Flight 1420 at Little Rock National Airport.
Forgetting to deactivate the spoilers contributed to the crash of United Airlines Flight 553 at Chicago Midway International Airport.
