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AP4ATCO - Wing Mechanism

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References

Objectives

Gain an understanding of:

  • Lift and drag modification
  • Mechanisation failures and their effects

Wing Mechanisms

By means of appropriate technical devices, the lift and drag can be altered as desired by mechanical changes to the shape of an aerofoil. This is required in order to permit landing and take-off at lower speeds, operation at greater weights from given runway lengths, and enable greater payloads to be carried.

Leading edge slats

Slats are extendable, high lift devices on the leading edge of the wings of some fixed wing aircraft. Their purpose is to increase lift during low speed operations such as takeoff, initial climb, approach and landing. They accomplish this by increasing both the surface area and the camber of the wing by deploying outwards and drooping downwards from the leading edge. In contrast, Krueger flaps increase wing camber by extending panels forward from the lower surface of the wing. Slats normally have several possible positions and extend progressively in concert with flap extension.

Slats

Slats are most often extended and retracted using hydraulically or electrically powered actuators. In some more simplistic designs, however, they are held in the retracted position by aerodynamic forces and use springs or counterweights for automatic extension at low speeds / high angles of attack.

Flaps

Flaps are a high lift device consisting of a hinged panel or panels mounted on the trailing edge of the wing. When extended, they increase the camber and, in most cases, the chord and surface area of the wing resulting in an increase of both lift and drag and a reduction of the stall speed. These factors result in an improvement in takeoff and landing performance.

There are many different flap designs and configurations in use. Large aircraft sometimes incorporate more than one type, utilising different flap designs on the inboard and outboard sections of the wing. The following are descriptions of some of the more common flap designs:

  • Plain Flap - The rear portion of the wing aerofoil rotates downwards on a simple hinge arrangement mounted at the front of the flap.
  • Split Flap - The rear portion of the lower surface of the wing aerofoil hinges downwards from the leading edge of the flap, while the upper surface remains immobile.
  • Slotted Flap - Similar to a Plain Flap but incorporates a gap between the flap and the wing to force high pressure air from below the wing over the upper surface of the flap. This helps reduce boundary layer separation and allows the airflow over the flap to remain laminar.
  • Fowler Flap - A split flap that slides rearwards level for a distance prior to hinging downwards. It thereby first increases chord (and wing surface area) and then increases camber. This produces a flap which can optimise both takeoff (partial extension for optimal lift) and landing (full extension for optimal lift and drag) performance. This type of flap or one of its variations is found on most large aircraft.
  • Double Slotted Fowler Flap - This design improves the performance of the Fowler flap by incorporating the boundary layer energising features of the slotted flap.
Flap Types

Spoilers and Speed Brakes

Spoilers And Speedbrakes are secondary flight control surfaces that can be deployed manually by the pilot or, under certain circumstances, that extend automatically. Speedbrakes are purely drag devices while spoilers simultaneously increase drag and reduce lift. Spoilers are normally wing mounted whereas speedbrakes can be located on the fuselage.

Spoilers

Spoilers are panels mounted on the upper surface of the wing that, when extended, both increase drag and decrease lift by disrupting the airflow over the wing. Dependent upon the aircraft type, spoilers can serve as many as three distinct primary functions:

  • Ground spoilers
  • Roll spoilers
  • Flight spoilers

Some aircraft such as the AIRBUS A-320 and the EMBRAER ERJ 190-100 have all three spoiler functions whereas the BA146 only incorporates the ground spoiler function. Certain aircraft designs also utilize the spoiler panels for secondary functions such as turbulence damping.

Ground Spoilers

Virtually all spoiler equiped aircraft have a ground spoiler function. During the landing ground roll or during a rejected takeoff, all spoiler panels are extended to their maximum angle. The primary purpose of the ground spoilers is to maximise wheel brake efficiency by "spoiling" or dumping the lift generated by the wing and thus forcing the full weight of the aircraft onto the landing gear. The spoiler panels also help slow the aircraft by producing aerodynamic drag. Depending upon aircraft type, the ground spoiler extension may be fully automatic when the system is armed provided that other deployment criteria such as weight on wheels, airspeed or throttle lever positon are met. Other aircraft may require the pilot to manually select the ground spoilers after landing or in the event of a rejected takeoff.

Wing Mechanism Failures

Flight control malfunction/failure, total flap failure

The inability to extend the wing flaps will necessitate a no-flap approach and landing. There are certain factors which must be considered in the execution of this manoeuvre:

  • a no-flap landing requires substantially more runway than normal. The increase in required landing distance could be as much as 50 percent.
  • circuit speed and final approach speed are increased in a no-flap configuration and can be substantially higher in some aircraft types.
  • when flying in the traffic pattern with the wing flaps retracted, the airplane must be flown in a relatively nose-high attitude to maintain altitude, as compared to flight with flaps extended.
  • losing altitude can be more of a problem without the benefit of the drag normally provided by flaps. A wider, longer traffic pattern may be required in order to avoid the necessity of diving to lose altitude and consequently building up excessive airspeed.
  • on final approach, a nose-high attitude can make it difficult to see the runway. This situation, if not anticipated, can result in serious errors in judgment of height and distance. Approaching the runway in a relatively nose-high attitude can also cause the perception that the airplane is close to a stall. This may cause the pilot to lower the nose abruptly and risk touching down on the nosewheel.
  • with the flaps retracted and the power reduced for landing, the airplane is slightly less stable in the pitch and roll axes. Without flaps, the airplane will tend to float considerably during roundout. The pilot should avoid the temptation to force the airplane onto the runway at an excessively high speed. Neither should the pilot flare excessively, because without flaps this might cause the tail to strike the runway.

Asymmetric flap

An asymmetric or split flap condition is one in which the flap(s) on one wing extends or retracts while the one(s) on the other wing remains in position. The situation can be caused by mechanical failure or jamming.

If the situation is allowed to progress unchecked, it will result in a pronounced roll towards the wing with the lessor amount of flap extended. In this case, it is possible that the induced roll could exceed the amount of aileron authority available and could result in a spin or other loss of control situation. In all cases of asymmetric flap, the wing with the greater amount of flap extended produces more lift. As a consequence, the wing with the lessor amount of flap extended will stall first.

Many aircraft have manufacturer installed provisions for detecting and, in some cases, minimizing an asymmetric flap condition. The more simplistic approach is to have two needles in the flap position indicator. In this situation, so long as the needles overlap, extension or retraction of the flaps is progressing normally. However, should the two needles separate, an asymmetric flap situation is occurring and pilot intervention to stop the flap movement is required. More sophisticated systems incorporate flap position sensors of some description and a flap brake system. If the position sensors detect that an asymmetric flap situation is occurring, the flap brakes are automatically activated to stop further movement of the flaps, thus preventing the situation from becoming worse.


Quiz Questions:

Q1: A no-flap landing requires more runway than normal. The increase in required landing distance could be as much as:

  1. 15 percent
  2. 30 percent
  3. 50 percent
  4. 75 percent

Answer


Q2: Which of the following commonly used technical devices does not modify lift created by an aerofoil at a given angle of attack:

  1. speedbrakes
  2. flaps
  3. spoilers

Answer


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