Airplane Flying Handbook (FAA-H-8083-3C)
Chapter 12: Transition to Complex Airplanes
Introduction
A high-performance airplane is defined as an airplane with an engine capable of developing more than 200 horsepower (see 14 CFR
part 61, section 61.31(f)(1)). A complex airplane (see 14 CFR part 61, section 61.1) means an airplane that has a retractable landing
gear, flaps, and a controllable pitch propeller, including airplanes equipped with an engine control system consisting of a digital
computer and associated accessories for controlling the engine and propeller, such as a full authority digital engine control; or, in the
case of a seaplane, flaps and a controllable pitch propeller, including seaplanes equipped with an engine control system consisting of a
digital computer and associated accessories for controlling the engine and propeller, such as a full authority digital engine control.
Transition to a complex airplane, or a high-performance airplane, can be demanding for many pilots. Both increased performance and
complexity require additional planning, judgment, and piloting skills. Transition to these types of airplanes, therefore, should be
accomplished in a systematic manner through a structured course of training administered by a qualified flight instructor.
Airplanes can be designed to fly through a wide range of airspeeds. High speed flight requires smaller wing areas and
moderately cambered airfoils whereas low speed flight is obtained with airfoils with a greater camber and larger wing area. [Figure
12-1] Many compromises are often made by designers to provide for higher speed cruise flight and low speeds for landing. Flaps are a
common design effort to increase an airfoil’s camber and surface area for lower-speed flight. [Figure 12-2]
Figure 12-1. Airfoil types.
Since an airfoil cannot have two different cambers at the same time, designers and engineers deliver the desired performance
characteristics using two different methods. Either the airfoil can be a compromise, or a cruise airfoil can be combined with a device
for increasing the camber of the airfoil for low-speed flight. Camber is the asymmetry between the top and the bottom surfaces of an
airfoil. One method for varying an airfoil’s camber is the addition of trailing-edge flaps. Engineers call these devices a high-lift
system.
Function of Flaps
Flaps work primarily by changing the camber of the airfoil, which increases the wing’s lift coefficient. With some flap designs,
the surface area of the wing is also increased. Flap deflection does not increase the critical (stall) angle of attack (AOA). In some
cases, flap deflection actually decreases the critical AOA. Deflection of a wing’s control surfaces, such as ailerons and flaps, alt ers
both lift and drag. With aileron deflection, there is asymmetrical lift which imparts a rolling moment about the airplane’s
longitudinal axis. Wing flaps act symmetrically about the longitudinal axis producing no rolling moment; however, both lift and drag
increase as well as a pitching moment about the lateral axis. Lift is a function of several variables including air density, velocity,
surface area, and lift coefficient. Since flaps increase an airfoil’s lift coefficient, lift is increased. [Figure 12-3]
Figure 12-2. Coefficient of lift comparison for flap extended and retracted positions.
Figure 12-3. Lift equation.
As flaps are deflected, the aircraft may pitch nose-up, nose-down, or have minimal changes in pitch attitude. Pitching moment is
caused by the rearward movement of the wing’s center of pressure; however, that pitching behavior depends on several variable s
including flap type, wing position, downwash behavior, and horizontal tail location. Consequently, pitch behavior depends on the
design features of the particular airplane.
Flap deflection of up to 15° primarily produces lift with minimal increases in 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 downwash changes the airflow
over the horizontal tail.
Flap Effectiveness
Flap effectiveness depends on a number of factors, but the most noticeable are size and type. For the purpose of this chapter, trailing
edge flaps are classified as four basic types: plain (hinge), split, slotted, and Fowler. [Figure 12-4]
Figure 12-4. Four basic types of flaps.
The plain or hinge flap is a hinged section of the wing. The structure and function are comparable to the other control surfa ces—
ailerons, rudder, and elevator. The split flap is more complex. It is the lower or underside portion of the wing; deflection of the flap
leaves the upper trailing edge of the wing undisturbed. It is, however, more effective than the hinge flap because of greater lift and
less pitching moment, but there is more drag. Split flaps are 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.
The slotted flap has a gap between the wing and the leading edge of the flap. The slot allows high-pressure airflow on the wing
undersurface to energize the lower pressure over the top, thereby delaying flow separation. 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. 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.
The Fowler flap deflects down and aft to increase the wing area. This flap can be multi-slotted making it the most complex of the
trailing-edge systems. This system does, however, give the maximum lift coefficient. Drag characteristics at small deflections are
much like the slotted flap. Fowler flaps are most commonly used on larger airplanes because of their structural complexity an d
difficulty in sealing the slots.
Operational Procedures
It would be impossible to discuss all the many airplane design and flap combinations. Pilots should refer to the Federal Aviation
Administration (FAA) approved Airplane Flight Manual and/or Pilot’s Operating Handbook (AFM/POH) for a given airplane.
However, while some AFM/POHs are specific as to operational use of flaps, others leave the use of flaps to pilot discretion. Since
laps are often used for landings and takeoffs, when the airplane is close to the ground, pilot judgment and error avoidance are of
critical importance.
Since the recommendations given in the AFM/POH are based on the airplane and the flap design, the pilot should relate the
manufacturer’s recommendation to aerodynamic effects of flaps. This requires basic background knowledge of flap aerodynamics and
geometry. With this information, a decision as to the degree of flap deflection and time of deflection based on runway and approach
conditions relative to the wind conditions can be made.
The time of flap extension and the degree of deflection are related. Large changes in flap deflection at one single point in the landing
pattern can produce large lift changes that require significant pitch and power changes in order to maintain airspeed and descent
angle. Consequently, there is an advantage to extending flaps in increments while in the landing pattern. Incremental deflection of
flaps on downwind, base leg, and final approach allow smaller adjustments of pitch and power and support a stabilized approach.
While normal, soft-field, or short-field landings require minimal speed at touchdown, a short-field obstacle approach requires
minimum speed and a steep approach angle. Flap extension, particularly beyond 30°, results in significant levels of drag. The drag
can produce a high sink rate that the pilot needs to control with power. When a pilot uses power during a steep approach or short-field
approach to offset the drag produced by the flaps, the landing flare becomes critical. A reduction in power too early can result in a
hard landing, airplane damage, or loss of control. A reduction in power too late causes the airplane to float down the runway.
Crosswind component is another factor to be considered in the degree of flap extension. The deflected flap presents a surface area for
the wind to act on. With flaps extended in a crosswind, the wing on the upwind side is more affected than the downwind wing. The
effect is reduced to a slight extent in the crabbed approach since the airplane is more nearly aligned with the wind. When using a
wing-low approach, the lowered wing partially blocks the upwind flap. 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.
With flaps extended, the crosswind effects on the wing become more pronounced as the airplane reaches the ground. The wing, flap,
and ground on the upwind side of the airplane form a “container” that is filled with air by the crosswind. Since the flap is located
behind the main landing gear, wind striking the deflected flap tends to yaw the airplane into the wind and raise the upwind wing. The
raised wing reduces the tire forces and further increases the tendency to turn into the wind. Proper control position (ailerons into the
wind) is essential for maintaining runway alignment. Depending on the amount of crosswind, it may be necessary to retract the flaps
soon after touchdown in order to maintain control of the airplane.
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 wing. This produces additional lift causing
significant changes in pitch. 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; however, the pilot should be prepared for rapid changes in pitch forces as
the result of trim and the increase in airflow over the control surfaces. [Figure 12-5]
Figure 12-5. Flaps extended pitching moment.
During a go-around, the pilot should carefully monitor pitch and airspeed and expect that the degree of flap deflection and the design
configuration of the horizontal tail relative to the wing will affect go-around characteristics. The pilot should carefully monitor pitch
and airspeed, control flap retraction to minimize altitude loss, and use rudder for coordination. Considering these factors, it is good
practice to extend the same degree of flaps at the same point in the landing pattern for each landing. Consistent use of flaps in the
traffic pattern allows for a preplanned and familiar 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 contains 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
iven in the AFM/POH be exceeded for takeoff.
Controllable-Pitch Propeller
Fixed-pitch propellers are designed for best efficiency at one particular revolutions per minute (rpm) setting and one airspeed. A
fixed-pitch propeller provides suitable performance in a narrow range of airspeeds. However, fixed-pitch efficiency suffers
considerably when operating outside of this range. To provide improved propeller efficiency through a wide range of operation, the
propeller blade angle needs to be controllable.
Constant-Speed Propeller
A constant-speed propeller keeps the blade angle adjusted for maximum efficiency during most flight conditions. The pilot controls
the engine rpm indirectly by means of a propeller control, which is connected to the propeller governor. For maximum takeoff power,
the propeller control is moved all the way forward to the low pitch/high rpm position, and the throttle is moved forward to the
maximum allowable manifold pressure position. [Figure 12- 6] To reduce power for climb or cruise, the pilot reduces manifold
pressure to the desired value with the throttle, and then reduces engine rpm by moving the propeller control back toward the high
pitch/low rpm position. The pilot sets the rpm accurately using the tachometer.
Figure 12-6. Controllable-pitch propeller pitch angles.
When an airplane engine runs at a constant governed speed, the torque (force) exerted by the engine at the propeller shaft equals the
force resisting the moving blades. The pilot uses the propeller control to change engine rpm by adjusting the propeller blade pitch,
which increases or decreases the air resistance on the rotating propeller. For example, pulling back on the propeller control moves the
propeller blades to a higher pitch. This increases the air resistance exerted on the spinning propeller and puts an additional load on the
engine, which causes it to slow down until the forces reach equilibrium. Advancing the propeller control reduces the propeller blade
pitch. This reduces the resistance of the air against the propeller. In response, the engine rpm increases until the opposing forces
balance. In order for this system to function, a constant-speed propeller governor needs the means to sense engine rpm and a means to
control the propeller AOA. In most cases, the governor is geared to the engine crankshaft giving it a means to sense engine rpm. The
"Blade Angle Control" section of this chapter discusses the ways a propeller governor adjusts propeller blade angle.
Other factors affect constant-speed propeller blade pitch. When an airplane is nosed up into a climb from level flight, the engine tends
to slow down. Since the governor is sensitive to small changes in engine rpm, it decreases the blade angle just enough to keep the
engine speed constant. If the airplane is nosed down into a dive, the governor increases the blade angle just enough to keep the engine
speed constant. This allows the engine to maintain a constant rpm and power output. The pilot can also set engine power output by
changing rpm at a constant manifold pressure; by changing the manifold pressure at a constant rpm; or by changing both rpm an d
manifold pressure. The constant-speed propeller makes it possible to obtain an infinite number of power settings.
Takeoff, Climb, and Cruise
During takeoff, when the forward motion of the airplane is at a low speed and when maximum power and thrust are required, the
constant-speed propeller sets up a low propeller blade pitch. The low blade angle keeps the blade angle of attack, with respect to the
relative wind, small and efficient at the low speed. [Figure 12-7]
At the same time, low blade pitch allows the propeller to handle a smaller mass of air per revolution. This light propeller load allows
the engine to turn at maximum rpm and develop maximum engine power. Although the mass of air per revolution is small, the number
of rpm is high, and propeller thrust is maximized until brake release. Thrust is maximum at the beginning of the takeoff roll and then
decreases as the airplane gains speed.