The flight controls are the devices and systems that govern the attitude of an aircraft and, as a result, the flight path followed by the aircraft. In the case of many conventional airplanes, the primary flight controls utilize hinged, trailing edge surfaces called elevators for pitch, ailerons for roll, and the rudder for yaw. These surfaces are operated by the pilot in the flight deck or by an automatic pilot
The inputs necessary to manipulate the aircraft by the pilot
The flight control systems will vary from aircraft to aircraft but always consists of some combination of primary and secondary flight controls
Figure 1: Cessna-172N Aileron System
Primary Flight Controls:
Deflection of trailing edge control surfaces, such as the aileron, alters both lift and drag
Ailerons:
Ailerons (French for "little wing") are control surfaces on each wing which control the aircraft about its longitudinal axis allowing the aircraft to "roll" or "bank"
This action results in the airplane turning in the direction of the roll/bank
With aileron deflection, there is asymmetrical lift (rolling moment) about the longitudinal axis and drag (adverse yaw)
They are located on the trailing (rear) edge of each wing near the outer tips
They extend from about the midpoint of each wing outward toward the tip, and move in opposite directions to create aerodynamic forces that cause the airplane to roll
The yoke manipulates the airfoil through a system of cables and pulleys and act in an opposing manor
Yoke "turns" left: left aileron rises, decreasing camber and angle of attack on the right wing which creates downward lift
At the same time, the right aileron lowers, increasing camber (curvature) and angle of attack which increases upward lift and causes the aircraft to turn left
Yoke "turns" right: right aileron rises decreasing camber and angle of attack on the right wing which creates downward lift
At the same time, the left aileron lowers, increasing camber and angle of attack on the left wing which creates upward lift and causes the aircraft to turn right
Figure 2: Cessna-172N Rudder System
Rudder:
Rudders are used to control the direction (left or right) of "yaw" about an airplane's vertical axis
Like the other primary control surfaces, the rudder is a movable surface hinged to a fixed surface that, in this case, is the vertical stabilizer, or fin
Its action is very much like that of the elevators, except that it swings in a different plane - from side to side instead of up and down
It is not used to make the airplane turn, as is often erroneously believed
In practice, both aileron and rudder control input are used together to turn an aircraft, the ailerons imparting roll
This relationship is critical in maintaining coordination or creating a slip
Improperly ruddered turns at low speed can precipitate a spin
Rudders are controlled by the pilot with his/her feet through a system of cables and pulleys:
"Step" on the right rudder pedal: rudder moves right creating a yaw to the right
"Step" on the left rudder pedal: rudder moves left creating a yaw to the left
Figure 3: Cessna-172N Elevator System
Elevators/Stabilators:
Elevators and stabilators are both control surfaces which control the aircraft about its lateral axis allowing the aircraft to pitch
Elevators are attached to the trailing edge of the horizontal stabilizer
A stabilator is a combination of both the horizontal stabilizer and the elevator (the entire surface moves)
Used to pitch the aircraft up and down by creating a load on the tail
The elevators control the angle of attack of the wings
The yoke manipulates the airfoil through a system of cables and pulleys:
Yoke "pulls" back: elevator raises, creating downward lift, raising the nose, increasing the wing's angle of attack
Yoke "pushes" forward: elevator lowers creating upward lift, lowering the nose, decreasing the wing's angle of attack
Flaps allow for the varying of an airfoil's camber
The term, "clean configuration" refers to flaps and gear up
The term, "dirty configuration" refers to flaps and gear down
Many attempts have been made to compromise the conflicting requirement of high speed 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 devices for increasing the camber of the airfoil for low-speed flight (i.e., flaps)
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
Pitch behavior depends on the aircraft's flap type, wing position, and horizontal tail location
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
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
Trailing Edge Flaps:
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
The increased camber from flap deflection produces lift primarily on the rear portion of the wing allowing for decreased approach speed and steeper approach paths
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
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
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 Considerations:
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 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
Figure 5: Airplane Flying Handbook, Four Basic Types of FlapsFigure 5: Airplane Flying Handbook, Four Basic Types of Flaps
Types of Trailing Edge Flaps:
Plain Flaps:
Plain flaps are the 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
Leading Edge Flaps:
Leading edge flaps increase stall margin
There are several types:
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"
Types of Slat Systems:
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
Control Surface Tabs:
Tabs are small, adjustable aerodynamic devices on the trailing edge of the control surface
These movable surfaces reduce pressures on the controls
Trim controls a neutral point, like balancing the aircraft on a pin with unsymmetrical weights
This is done either by trim tabs (small movable surfaces on the control surface) or by moving the neutral position of the entire control surface all together
These tabs may be installed on the ailerons, the rudder, and/or the elevator
Trim Tabs:
The force of the airflow striking the tab causes the main control surface to be deflected to a position that corrects the unbalanced condition of the aircraft
An aircraft properly trimmed will, when disturbed, try to return to its previous state due to aircraft stability
Trimming is a constant task required after any power setting, airspeed, altitude, or configuration change
Proper trimming decreases pilot workload allowing for attention to be diverted elsewhere, especially important for instrument flying
Trim tabs are controlled through a system of cables and pulleys
Trim tab adjusted up: trim tab lowers creating positive lift, lowering the nose
This movement is very slight
Trim tab adjusted down: trim tab raises creating positive lift, raising the nose
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
You can remember how ailerons deflect by using your thumbs
Place your hands on the yoke with your thumbs facing straight up, if you turn left your thumbs are then pointing left, and you will notice the left aileron up, and vice versa if right
Some aircraft may have gust locks which must be removed prior to manipulating the controls or risk damage [Figure 5]
