Spoiler (aeronautics)

From Freepedia

In aeronautics a spoiler (sometimes called a lift dumper) is a device intended to reduce lift in an aircraft. Commonly found in gliders, it is usually a plate which can be extended upward and/or downward from the wing, extending into the smooth airflow and spoiling it. Airliners too are usually fitted with extensive spoilers which are deployed on landing.

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 an increase in descent angle.

Spoilers differ from airbrakes in that the latter 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.

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.)

Spoilers as control surfaces

Some aircraft use spoilers in combination with or in lieu of ailerons for roll control. The B-52 Stratofortress has no ailerons as they would cause excessive twisting of the highly flexible wing. It achieves roll control entirely through spoilers mounted near the center of the wing in about the same place as most gliders. Boeing's line of jet airliners have fast-acting spoilers that assist the ailerons when the pilot commands a high roll rate. These can be readily seen in operation when the pilot is fighting gusting crosswinds upon landing. The Wren 460 had full-span flaps where the outboard sections doubled as ailerons. It also had a series of spoilers on each wing that twisted broadside to the wind to assist with roll control.