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High-Lift Devices

Introduction:

  • Varying an airfoil's camber is accomplished through the use of high-lift devices:
  • Ultimately flaps allow for slower approach speeds and steeper approaches
  • The term, "clean configuration" refers to flaps and gear up
  • The term, "dirty configuration" refers to flaps and gear down
  • There are many airplane design and flap configurations
  • Many attempts have been made to compromise this conflicting requirement of high cruise and slow landing speeds
    • High speed requires thin, moderately cambered airfoils with a small wing area
    • The high lift needed for low speeds is obtained with thicker highly cambered airfoils with a larger wing area
  • Since an airfoil cannot have two different cambers at the same time, one of two things must be done
    • The airfoil can be a compromise
    • A cruise airfoil can be combined with a device for increasing the camber of the airfoil for low-speed flight

Function:

  • Flaps work primarily by changing the camber of the airfoil since deflection adds aft camber
  • Flap deflection does not increase the critical (stall) angle of attack, and in some cases flap deflection actually decreases the critical angle of attack
    • The aircraft stalling speed however (different from angle of attack), will lower
  • Wing flaps should not induce a roll or yaw effect, and pitch changes depend on the airplane design
    • Un-commanded roll/yaw with flaps alone could indicate a split flap condition which requires immediate action (usually just returning the flaps to a previously safe position is enough)
  • Pitch behavior depends on the aircraft's flap type, wing position, and horizontal tail location
  • The increased camber from flap deflection produces lift primarily on the rear portion of the wing
  • This produces a nose-down pitching moment; however, the change in tail load from the down-wash deflected by the flaps over the horizontal tail has a significant influence on the pitching moment
  • Flap deflection of up to 15° primarily produces lift with minimal drag
  • The tendency to balloon up with initial flap deflection is because of lift increase, but the nose-down pitching moment tends to offset the balloon
    • This requires that the pilot have a basic background knowledge of flap aerodynamics and geometry
  • Deflection beyond 15° produces a large increase in drag
  • Drag from flap deflection is parasite drag, and as such is proportional to the square of the speed
  • Also, deflection beyond 15° produces a significant nose-up pitching moment in most high-wing airplanes because the resulting down-wash increases the airflow over the horizontal tail

Operational Procedures:

  • Flap operation is used for landings and takeoffs, during which the airplane is in close proximity to the ground where the margin for error is small
    • Since the recommendations given in the AFM/POH are based on the airplane and the flap design combination, the pilot must relate the manufacturer's recommendation to aerodynamic effects of flaps
  • With this information, the pilot must make a decision as to the degree of flap deflection and time of deflection based on runway and approach conditions relative to the wind conditions
  • The time of flap extension and degree of deflection are related and affect the stability of an approach
    • Large flap deflections at one single point in the landing pattern produce large lift changes that require significant pitch and power changes in order to maintain airspeed and glide slope
    • Incremental deflection of flaps on downwind, base, and final approach allow smaller adjustment of pitch and power compared to extension of full flaps all at one time
  • A soft- or short-field landing requires minimal speed at touchdown
  • The flap deflection that results in minimal ground-speed, therefore, should be used
  • If obstacle clearance is a factor, the flap deflection that results in the steepest angle of approach should be used
  • It should be noted, however, that the flap setting that gives the minimal speed at touchdown does not necessarily give the steepest angle of approach; however, maximum flap extension gives the steepest angle of approach and minimum speed at touchdown
  • Maximum flap extension, particularly beyond 30 to 35°, results in a large amount of drag
  • This requires higher power settings than used with partial flaps
  • Because of the steep approach angle combined with power to offset drag, the flare with full flaps becomes critical
  • The drag produces a high sink rate that must be controlled with power, yet failure to reduce power at a rate so that the power is idle at touchdown allows the airplane to float down the runway
  • A reduction in power too early results in a hard landing

Crosswind Procedures:

  • Crosswind component must be considered with the degree of flap extension because the deflected flap presents a surface area for the wind to act on
  • In a crosswind, the "flapped" wing on the upwind side is more affected than the downwind wing
  • This is, however, eliminated to a slight extent in the crabbed approach since the airplane is more nearly aligned with the wind
  • When using a wing low approach, however, the lowered wing partially blankets the upwind flap, but the dihedral of the wing combined with the flap and wind make lateral control more difficult
  • Lateral control becomes more difficult as flap extension reaches maximum and the crosswind becomes perpendicular to the runway
  • Crosswind effects on the "flapped" wing become more pronounced as the airplane comes closer to the ground
  • The wing, flap, and ground form a "container" that is filled with air by the crosswind
  • With the wind striking the deflected flap and fuselage side and with the flap located behind the main gear, the upwind wing will tend to rise and the airplane will tend to turn into the wind
  • Proper control position, therefore, is essential for maintaining runway alignment
  • Also, it may be necessary to retract the flaps upon positive ground contact
  • The go-around is another factor to consider when making a decision about degree of flap deflection and about where in the landing pattern to extend flaps
  • Because of the nose-down pitching moment produced with flap extension, trim is used to offset this pitching moment
  • Application of full power in the go-around increases the airflow over the "flapped" wing
  • This produces additional lift causing the nose to pitch up
  • The pitch-up tendency does not diminish completely with flap retraction because of the trim setting
  • Expedient retraction of flaps is desirable to eliminate drag, thereby allowing rapid increase in airspeed; however, flap retraction also decreases lift so that the airplane sinks rapidly
  • The degree of flap deflection combined with design configuration of the horizontal tail relative to the wing requires that the pilot carefully monitor pitch and airspeed, carefully control flap retraction to minimize altitude loss, and properly use the rudder for coordination
  • Considering these factors, the pilot should extend the same degree of deflection at the same point in the landing pattern
  • This requires that a consistent traffic pattern be used
  • Therefore, the pilot can have a preplanned go-around sequence based on the airplane's position in the landing pattern
  • There is no single formula to determine the degree of flap deflection to be used on landing, because a landing involves variables that are dependent on each other
  • The AFM/POH for the particular airplane will contain the manufacturer's recommendations for some landing situations
  • On the other hand, AFM/POH information on flap usage for takeoff is more precise
  • The manufacturer's requirements are based on the climb performance produced by a given flap design
  • Under no circumstances should a flap setting given in the AFM/POH be exceeded for takeoff

Aircraft Plan Flaps
Figure 1: Airplane Flying Handbook,
Four Basic Types of Flaps

Plain Flap:

  • Most common but least efficient flap system [Figure 1]
  • Attached on a hinged pivot, which allows the flap to the moved downward
  • The structure and function are comparable to the other control surfaces-ailerons, rudder, and elevator
  • When extended, it increases the chord line, angle of attack, and camber of the wing, which results in an increase in both lift and drag
  • It is important to remember that control surfaces are nothing more than plain flaps themselves
    • They they call same as a wing except it will only stall one wing at a time leading to a roll

Split Flap:

  • Similar to the plain flap, but more complex [Figure 1]
  • It is only the lower or underside portion of the wing
  • The deflection of the flap leaves the trailing edge of the wing undisturbed
  • It is more effective than the hinge flap because of greater lift and less pitching moment, but there is more drag
    • More useful for landing, but the partially deflected hinge flaps have the advantage in takeoff
  • The split flap has significant drag at small deflections, whereas the hinge flap does not because airflow remains "attached" to the flap

Slotted Flap:

  • The slotted flap has greater lift than the hinge flap but less than the split flap; but, because of a higher lift-drag ratio, it gives better takeoff and climb performance [Figure 1]
    • Small deflections of the slotted flap give a higher drag than the hinge flap but less than the split
    • This allows the slotted flap to be used for takeoff
  • A slotted flap will produce proportionally more lift than drag
  • Its design allows high-pressure air below the wing to be directed through a slot to flow over the upper surface of the flap delaying the airflow separation at higher angles of attack
  • This design lowers the stall speed significantly

Fowler Flap:

  • Most efficient design [Figure 1]
  • Moves backward on first part of extension increasing lift with little drag; also utilizes a slotted design resulting in lower stall speeds and increased wing area
  • Creates the greatest change in pitching moment
  • Provides greatest increase in lift coefficient with the least change in drag
  • This flap can be multi-slotted making it the most complex of the trailing edge systems
  • Drag characteristics at small deflections are much like the slotted flap
  • Because of structural complexity and difficulty in sealing the slots, Fowler flaps are most commonly used on larger airplanes

Blown Flap:

  • An aircraft with wing-mounted propellers, exhibits a blown flap effect
  • Provides extra airflow for wings by blowing air over the surfaces
  • Prevents boundary layer from stagnating, improving lift
  • At low speeds this system can "fool" the airplane into thinking it is flying faster
  • Can improve lift 2 or 3 times; however, the bleed air off the engine causes a decrease in thrust for phases of flight such as take off


Slats:

  • Aerodynamic surfaces on the leading edge of the wings
  • When deployed, they allow the wing to operate at a higher angle of attack, so it can fly slower or take off and land in a shorter distance
  • Usually used while landing or performing maneuvers, which take the aircraft close to the stall, but are usually retracted in normal flight to minimize drag
  • Slats work by increasing the camber of the wing, and also by opening a small gap (the slot) between the slat and the wing leading edge, allowing a small amount of high-pressure air from the lower surface to reach the upper surface, where it helps postpone the stall
  • The chord of the slat is typically only a few percent of the wing chord
  • They may extend over the outer third of the wing or may cover the entire leading edge
  • The slat has a counterpart found in the wings of some birds, the Alula, a feather or group of feathers which the bird can extend under control of its "thumb"

Aircraft Leading Edge Flaps/Slats

  • 3 Types:
    • Automatic:
      • The slat lies flush with the wing leading edge until reduced aerodynamic forces allow it to extend by way of springs when needed
      • This type is typical on light aircraft
    • Fixed:
      • This slat is permanently extended
      • This is rarely used, except on special low-speed aircraft (these are referred to as slots)
    • Powered:
      • The slat extension can be controlled by the pilot
      • This is commonly used on airliners

Conclusion:

  • High-lift devices can do a few things for us such as allow for slower approach speeds, and reduced pitch while on final
    • This is especially when it comes to an emergency, high-lift devices can be your best friend, or worst enemy

References: