High lift device

Flaps of an Airbus A340.

Flaps of an Airbus A330.

A high lift device is an aerodynamic device designed to increase lift in certain phases of an aircraft's flight. Its purpose is to increase the aerodynamic chord and the curvature of the wing profile, modifying the geometry of the profile in such a way that the stall speed during specific phases of the flight, such as landing or takeoff, is significantly reduced, allowing a slower flight than cruising. The device is inactivated by retracting one way or another during normal cruise flight. In this way, it allows the aircraft to fly at lower speeds in the takeoff, initial climb, approach and landing phases, increasing its lift coefficient. They are also used, with low extension rates, when for some reason it is necessary to fly at low speeds.

The most common are moving planes in the airfoil that, when used, modify certain characteristics of the region of the wing where they are located, such as its camber or chord.

Keep in mind that placing a high-lift device on a wing always introduces mechanical elements and therefore adds weight to the wing. Therefore, they are, at first, undesirable elements, which is why when installing them, the simplest ones are always sought.

High lift devices can be divided into two main types:

• Passives: are devices that modify the geometry of the wing, either increasing its curvature, its surface, or generating gaps to control the flow.
• Assets: are devices that require an active energy application directly to the fluid.

Passive devices

The most popular flaps systems are those in which the high-lift planes, as they descend creating an angle (measured in degrees) with the chord of the wing, move backwards increasing the wing surface. It is for this reason that the extension rate is generally not measured in degrees but in percentage, where, for example, thirty percent could mean twenty degrees of deflection, and a seven percent increase in wing area.

A very general classification can be made into two large groups:

Flap

Flap of a Boeing 737.

View of a wing with the flaps and slats deployed.

Located at the trailing edge of the wing. It increases the lift coefficient of the wing by increasing the surface area or the lift coefficient of the profile, coming into action at appropriate moments, when flying at speeds lower than those for which the wing has been designed, retracting later and remaining idle. There are also leading edge ones. Modern trailing edge flaps are very complex structures formed by two or three series on each side, and three or four successive planes, which are staggered and leaving a slot between each two of them.

Located in the rear inner part of the wings, they are deflected downwards symmetrically (both at the same time), in one or more angles, with which they change the curvature of the wing profile (more pronounced on the extrados and less pronounced on the intrados), the wing surface (on some types of flaps) and the angle of incidence, all of which increase lift but also drag.

They are operated from the cabin, either by a lever, by an electrical system or any other system, with various degrees of setting (10°, 15º, etc.) corresponding to different positions of the lever or electrical switch, and not they are lowered or raised in all their gauge at once, but gradually. In general, flap deflections of up to about 15º increase lift with little additional drag, but larger deflections increase drag more than lift.

There are several types of flaps: simple, soffit, zap flap, fowler flap, slotted flap, Krueger flap, etc.

• Simple.. It is the most used in light aviation. It's a portion of the back of the wing.
• Intrade. Located in the lower part of the wing (intradós), its effect is lower since it only affects the curvature of the intrados.
• Zap. Similar to the one of the intrados, when deflecting, it moves towards the end of the wing, increasing the wing surface as well as the curvature.
• Fowler. Identical to the flap zap, it moves completely to the end of the wing, enormously increasing the curvature and the elar surface.
• Slot. It is distinguished from the previous ones in which when deflected it leaves one or more slots that communicate the intrados and the extrados, producing a great curvature while creating an air current that eliminates the resistance of other types of flaps.
• Krueger. Like the previous ones, but located on the edge of attack instead of the edge of exit.

The flaps should only be used during takeoff, approach and landing maneuvers, or in any other circumstance in which it is necessary to fly at lower speeds than with the "clean" aircraft.

The effects produced by the flaps are:

• Increased sustainability.
• Increased resistance.
• Some increase the lengthy surface.
• Possibility of flying at lower speeds without loss.
• Lower runway length is required in takeoffs and landings.
• The approach path becomes more pronounced.
• They create a tendency to sting.
• At the time of its deflection the plane tends to ascend and lose speed.

In commercial aircraft it is necessary to include FTFs (Flap Track Fairing); they are a kind of rails on which the flaps extend.

Slats

Located on the leading edge of the wing, they are movable devices that create a slot between the leading edge of the wing and the rest of the plane. As the angle of attack increases, high pressure air at the bottom of the wing tries to reach the top of the wing, thus giving energy to the air at the top and thus increasing the maximum angle of attack. attack that the plane can reach. It is a blowing mechanism that provides momentum to the boundary layer, helping to overcome the adverse pressure gradient; thus the detachment of the current is delayed with respect to the increase in the angle of attack.

They are high-lift surfaces that act in a similar way to flaps. Located in the front part of the wing, when deflected they channel a high speed air current towards the extrados which increases lift allowing greater angles of attack to be reached without stalling. They are generally used in large airplanes to increase lift in low-speed operations (landings and takeoffs), although there are also light airplane models that have them.

In many cases its deployment and redeployment is done automatically; while the pressure exerted on them is sufficient the slats remain retracted, but when this pressure decreases to a certain level (close to stall speed) the slats deploy automatically. Due to the sudden increase or decrease (as they are extended or retracted) of lift at speeds close to stall, extreme care must be taken when flying at low speeds in airplanes with this type of device.

Active Devices

These devices increase the aircraft's lift, not by modifying geometry but by (intelligently) introducing energy into the fluid. They usually try to modify the boundary layer to prevent its detachment by introducing energy.

Flap blown

The air bled from the compressor and under pressure passes, thanks to a series of ducts, to the slot of the flaps to inject it and increase the kinetic energy of the air and generate favorable gradients that prevent the detachment of the boundary layer. Active high-lift devices are much more effective than passive ones, taking as a consideration that the air bleeding from the engines means that the power they generate is also less. Therefore, they are usually only really used in aircraft specially designed for short takeoffs and landings.

Leading Edge Roller

It consists of a cylinder that rotates on its axis clockwise, which artificially accelerates the air that goes above and slows down the air that passes through the intrados. Increasing the speed difference results in a significant increase in the lift provided to the wing.

Other active methods

• Another method is to create a low-pressure zone by succionators, in the area that the limit layer is removed, thus attaching it to the wing surface.
• Very popular also, especially in some types of aircraft, in areas more susceptible to the limit layer, are torbellin generators, small series of vertical plates oriented in aerodynamically studied senses.
• Another method is to inject energy into the fluid by cavities with vibrant membranes than by giving energy to the limit layer (and making it more turbulent) make it more resistant to detachment.
• Another asset is the use of motor exhaust gases as a direct or indirect support generator, either by means of vectorial (direct) torches as with motor configurations in the wing (indirect); this last one can be seen very well in the Antonov An-72, plane specially designed to be used in short and unprepared tracks.

Spoilers or airbrakes

Unlike the previous ones, the objective of this surface is to destroy the plane's lift. They are mainly used in jets that develop high speeds and are used to land the plane on the runway (by dissipating lift there is nothing to lift it), during descent to increase the rate of descent, working as a speedbrake facilitating landing, helping to brake on the ground and, in some aircraft, as a complement to the ailerons for lateral control and turns in flight. They are also essential in gliders, where they come from, because with their high lift, they could not adopt a landing path without them.

In commercial aircraft, there are normally these 4 modes for this device, which are manually configured:

Down detent: spoilers are off, set this way for takeoff, climb, cruise and most descent.

Flight detent: spoilers are triggered in such a way that they don't fully extend. They are used on the descent, generally to slow down if the aircraft cannot easily do so. If they are activated they are reconfigured in "down" when the desired speed is reached.

Up: spoilers are fully extended. This configuration is activated on landing when it is necessary to break the lift of the wing, thus favoring the effectiveness of the wheel brakes.

Armed: This mode is configured prior to landing. When activating it in the air, nothing happens, however, when the plane detects that it has touched down, it automatically activates the spoilers in "up" mode, thus freeing up workload for the pilots.

Use

The secondary surfaces (flaps, slats, spoilers) always work in pairs and symmetrically (under normal conditions), that is, the activation of the corresponding control causes the same movement (up or down) of the surfaces in both wings (except in the movements of the spoilers complementing the ailerons).

By affecting the lift, the shape of the profile, and the wing surface, the fact that a surface works and not its symmetry can be a serious inconvenience. Likewise, they have a speed limit, beyond which they must not be activated under penalty of causing structural damage.

There have been commercial aircraft accidents due to the inadvertent deployment of some of these surfaces in flight, which has led to improvements in the designs, incorporating elements that prevent their actuation at inappropriate speeds.

In commercial aircraft, all these surfaces (primary and secondary) are moved by electrical and hydraulic means. The reason is obvious; its wingspan makes the control surfaces larger; they are farther away from the controls that control them, and also withstand much greater pressure than in a light aircraft. All this put together means that an extraordinary force is needed to move said surfaces, a force that is carried out by the aforementioned means.

The main objective of these elements is to allow the operation at lower speeds for takeoff, landing and slow flight of the aircraft that use them.

Birds use a system similar to slats when landing, a feather in the middle of the leading edge of the wing called an alula. This allows them to fly at high angles of attack and low speeds.