What are spoilers, and how do they work?
Spoilers consist of opening panels that extend from the upper surface of the wing and have the effect of spoiling/disturbing the airflow over the wing (drag), thereby reducing the lift. For roll control: The spoilers are raised on one wing and not the other, which creates an imbalance of lift values that produces a rolling moment. The spoilers are connected to the normal aileron controls, and they work in tandem with each other for roll control.
Spoilers are in fact a more efficient roll-control surface than ailerons. The disadvantage of roll control spoilers is that they cause an overall loss of lift, which may cause a loss of height and is particularly undesirable when flying close to the ground. As airspeed brakes: The spoilers are raised symmetrically on both wings to a flight detent position (using the speed brake lever), which causes a large increase in drag that slows down the speed of the aircraft.
Note: The Buffet is usually experienced with spoiler (speed brake) deployment. As ground lift dumpers: The spoilers are raised systematically on both wings to the ground detent position (greater angle than the flight detent position), which causes a large increase in drag that (1) decreases lift over the wing, causing the aircraft to sink to the ground, and (2) acts as ground speed brakes to slow down the speed of the aircraft.
What are the three purposes of spoilers?
1. Roll control (usually in combination with the ailerons). Note that the primary purpose of spoilers is roll control.
2. Airspeed brakes
3. Ground lift dumpers
Differential and nondifferential spoilers
The difference between differential and nondifferential spoilers is in how they provide lateral roll control when already extended as speed brakes.
Nondifferential spoilers: When already partly extended as a speed brake, the spoilers will extend further on one side but will not retract on the other side in response to a roll command. When already fully extended as a speed brake, both sides remain in the extended speed brake position, and therefore, the spoilers do not provide any roll control.
Differential spoilers: When already partly extended as a speed brake, the spoilers will extend further on one side and retract on the other side in response to a roll command. When already fully extended as a speed brake, the spoilers will remain extended on one side and retract on the other side in response to a roll command.
Spoilers are needed on aircraft are to provide a degree of roll control. This is so because the ailerons have the following inadequacies:
1. The ailerons are limited in size and therefore effectiveness.
2. On a thin swept wing, ailerons that are too large will experience a high degree of air loading/lift, resulting in the wing twisting at high speeds that can produce aileron reversal (removes aileron roll control), which is very detrimental.
3. Ailerons tend to lose effectiveness at high speeds due to the span wise diagonal airflow across the aileron, which is less effective than a perpendicular airflow.
4. High-speed swept-winged aircraft cause a strong rolling moment with yaw, known as adverse rolling moment with yaw.
Other than roll control, the spoilers are needed to counteract (brake) the aircraft’s high speed in the air and on the ground:
5. Because the aircraft has low drag and the engines have a slow lag response rate, there is a need for high-drag devices in flight to act as a brake when the aircraft is required to lose speed and/or height quickly. This is achieved by the use of the spoilers on both wings being raised simultaneously to the flight detent position, which creates a drag force opposing thrust and therefore reduces the aircraft’s speed and/or height.
6. On landing or during a rejected takeoff, there is a need to dump the lift off the wing and onto the wheels to assist in stopping the aircraft. This is achieved by the use of the spoilers on both wings being raised simultaneously to the ground or up detent position in a similar manner as the in-flight speed brake. This position has a greater angle of deployment than the flight detent and/or uses more spoiler panels, therefore creating a greater drag force.
What limits the use of spoilers, and why do spoilers blowback?
Spoilers are limited by very high speeds (VDF/MDF), which cause them to blow back. At very high speeds, the spoilers will be blown back to or near to their fully retracted position. This occurs because the high air loads experienced on the spoilers’ surfaces at high speeds are greater than their design limit. Obviously, the force experienced is a function of airspeed and angle of deflection.
How is spoiler blowback prevented?
Spoilers are designed not to blow back in the normal operating speed range of the aircraft. Therefore, correct speed management of the aircraft will prevent the spoilers from blowing back.
How do you correct for spoiler blowback?
In flight, reduce speed by reducing thrust to a speed where the spoilers will operate normally, and then recycle the speed brake lever.
Note: Spoiler blowback will only occur when the aircraft’s speed is excessive (i.e., VDF/MDF), which itself should be experienced only in a nonnormal flight condition, e.g., spiral dive, etc., when the recovery drill incorporates reducing speed by closing the throttles.