Part 3: Aerodynamics of Flight | FAA PHAK Ch. 5

Level flight Medium banked turn Steeply banked turn

Centrifugal

force

Horizontal

component

Vertical

component

Resultant load

Centrifugal

force

Horizontal

component

Vertical

component

Resultant load

Lift

Total liftTotal lift

Weight

Figure 5-34. Forces during normal, coordinated turn at constant altitude.

design is necessary to obtain the best results from these two

conflicting conditions.

The second means of changing the planform is by tapering

(decreasing the length of chord from the root to the tip of the

wing). In general, tapering causes a decrease in drag (most

effective at high speeds) and an increase in lift. There is also

a structural benefit due to a saving in weight of the wing.

Most training and general aviation type airplanes are operated

at high coefficients of lift, and therefore require comparatively

high aspect ratios. Airplanes that are developed to operate at

very high speeds demand greater aerodynamic cleanness and

greater strength, which require low aspect ratios. Very low

aspect ratios result in high wing loadings and high stall speeds.

When sweepback is combined with low aspect ratio, it results

in flying qualities very different from a more conventional

high aspect ratio airplane configuration. Such airplanes

require very precise and professional flying techniques,

especially at slow speeds, while airplanes with a high aspect

ratio are usually more forgiving of improper pilot techniques.

The elliptical wing is the ideal subsonic planform since it

provides for a minimum of induced drag for a given aspect

ratio, though as we shall see, its stall characteristics in

some respects are inferior to the rectangular wing. It is also

comparatively difficult to construct. The tapered airfoil is

desirable from the standpoint of weight and stiffness, but

again is not as efficient aerodynamically as the elliptical

wing. In order to preserve the aerodynamic efficiency of the

elliptical wing, rectangular and tapered wings are sometimes

tailored through use of wing twist and variation in airfoil

sections until they provide as nearly as possible the elliptical

wing’s lift distribution. While it is true that the elliptical

wing provides the best coefficients of lift before reaching an

incipient stall, it gives little advance warning of a complete

stall, and lateral control may be difficult because of poor

aileron effectiveness.

In comparison, the rectangular wing has a tendency to stall

first at the wing root and provides adequate stall warning,

adequate aileron effectiveness, and is usually quite stable.

It is, therefore, favored in the design of low cost, low speed

airplanes.

Aerodynamic Forces in Flight Maneuvers

Forces in Turns

If an aircraft were viewed in straight-and-level flight from the

front [Figure 5-34], and if the forces acting on the aircraft

could be seen, lift and weight would be apparent: two forces.

If the aircraft were in a bank it would be apparent that lift

did not act directly opposite to the weight, rather it now acts

in the direction of the bank. A basic truth about turns is that

when the aircraft banks, lift acts inward toward the center of

the turn, perpendicular to the lateral axis as well as upward.

Newton’s First Law of Motion, the Law of Inertia, states that

an object at rest or moving in a straight line remains at rest

or continues to move in a straight line until acted on by some

other force. An aircraft, like any moving object, requires a

sideward force to make it turn. In a normal turn, this force

is supplied by banking the aircraft so that lift is exerted

inward, as well as upward. The force of lift during a turn is

separated into two components at right angles to each other.

One component, which acts vertically and opposite to the

weight (gravity), is called the “vertical component of lift.”

The other, which acts horizontally toward the center of the

turn, is called the “horizontal component of lift” or centripetal

force. The horizontal component of lift is the force that

pulls the aircraft from a straight flight path to make it turn.

Centrifugal force is the “equal and opposite reaction” of the

aircraft to the change in direction and acts equal and opposite

to the horizontal component of lift. This explains why, in a

correctly executed turn, the force that turns the aircraft is

not supplied by the rudder. The rudder is used to correct any

deviation between the straight track of the nose and tail of the

aircraft into the relative wind. A good turn is one in which the

nose and tail of the aircraft track along the same path. If no

rudder is used in a turn, the nose of the aircraft yaws to the

outside of the turn. The rudder is used rolling into the turn

to bring the nose back in line with the relative wind. Once

in the turn, the rudder should not be needed.

An aircraft is not steered like a boat or an automobile. In

order for an aircraft to turn, it must be banked. If it is not

banked, there is no force available to cause it to deviate

from a straight flight path. Conversely, when an aircraft is

banked, it turns provided it is not slipping to the inside of the

turn. Good directional control is based on the fact that the

aircraft attempts to turn whenever it is banked. Pilots should

keep this fact in mind when attempting to hold the aircraft

in straight-and-level flight.

Merely banking the aircraft into a turn produces no change in

the total amount of lift developed. Since the lift during the bank

is divided into vertical and horizontal components, the amount

of lift opposing gravity and supporting the aircraft’s weight

is reduced. Consequently, the aircraft loses altitude unless

additional lift is created. This is done by increasing the AOA

until the vertical component of lift is again equal to the weight.

Since the vertical component of lift decreases as the bank

angle increases, the AOA must be progressively increased

to produce sufficient vertical lift to support the aircraft’s

weight. An important fact for pilots to remember when making

constant altitude turns is that the vertical component of lift

must be equal to the weight to maintain altitude.

At a given airspeed, the rate at which an aircraft turns

depends upon the magnitude of the horizontal component

of lift. It is found that the horizontal component of lift is

proportional to the angle of bank—that is, it increases or

decreases respectively as the angle of bank increases or

decreases. As the angle of bank is increased, the horizontal

component of lift increases, thereby increasing the rate of

turn (ROT). Consequently, at any given airspeed, the ROT

can be controlled by adjusting the angle of bank.

To provide a vertical component of lift sufficient to hold

altitude in a level turn, an increase in the AOA is required.

Since the drag of the airfoil is directly proportional to its AOA,

induced drag increases as the lift is increased. This, in turn,

causes a loss of airspeed in proportion to the angle of bank.

A small angle of bank results in a small reduction in airspeed

while a large angle of bank results in a large reduction in

airspeed. Additional thrust (power) must be applied to prevent

a reduction in airspeed in level turns. The required amount of

additional thrust is proportional to the angle of bank.

To compensate for added lift, which would result if the

airspeed were increased during a turn, the AOA must be

decreased, or the angle of bank increased, if a constant

altitude is to be maintained. If the angle of bank is held

constant and the AOA decreased, the ROT decreases. In order

to maintain a constant ROT as the airspeed is increased, the

AOA must remain constant and the angle of bank increased.

An increase in airspeed results in an increase of the turn radius,

and centrifugal force is directly proportional to the radius of

the turn. In a correctly executed turn, the horizontal component

of lift must be exactly equal and opposite to the centrifugal

force. As the airspeed is increased in a constant-rate level turn,

the radius of the turn increases. This increase in the radius of

turn causes an increase in the centrifugal force, which must

be balanced by an increase in the horizontal component of lift,

which can only be increased by increasing the angle of bank.

In a slipping turn, the aircraft is not turning at the rate

appropriate to the bank being used, since the aircraft is yawed

toward the outside of the turning flight path. The aircraft is

banked too much for the ROT, so the horizontal lift component

is greater than the centrifugal force. [Figure 5-35] Equilibrium

between the horizontal lift component and centrifugal force

is reestablished by either decreasing the bank, increasing the

ROT, or a combination of the two changes.

A skidding turn results from an excess of centrifugal force

over the horizontal lift component, pulling the aircraft

toward the outside of the turn. The ROT is too great for the

angle of bank. Correction of a skidding turn thus involves a

reduction in the ROT, an increase in bank, or a combination

of the two changes.

To maintain a given ROT, the angle of bank must be varied

with the airspeed. This becomes particularly important in

high-speed aircraft. For instance, at 400 miles per hour (mph),

an aircraft must be banked approximately 44° to execute a

standard-rate turn (3° per second). At this angle of bank,

only about 79 percent of the lift of the aircraft comprises the

vertical component of the lift. This causes a loss of altitude

unless the AOA is increased sufficiently to compensate for

the loss of vertical lift.

Forces in Climbs

For all practical purposes, the wing’s lift in a steady state

normal climb is the same as it is in a steady level flight at the

same airspeed. Although the aircraft’s flight path changed

when the climb was established, the AOA of the wing with

respect to the inclined flight path reverts to practically the

same values, as does the lift. There is an initial momentary

change as shown in Figure 5-36. During the transition from

straight-and-level flight to a climb, a change in lift occurs

when back elevator pressure is first applied. Raising the

aircraft’s nose increases the AOA and momentarily increases

Normal turn Slipping turn Skidding turn

Centrifugal

force equals

horizontal lift

Centrifugal

force less than

horizontal lift

Centrifugal force

greater than

horizontal lift

Vertical lift

Load

Centrifugal

force

Horizontal

lift

Centrifugal

force

Horizontal

lift

Centrifugal force

Horizontal

lift

Lift Lift Lift

Weight

Figure 5-35. Normal, slipping, and skidding turns at a constant altitude.

Steady climb

normal lift

Climb entry

increased lift

Level flight

normal lift

Figure 5-36. Changes in lift during climb entry.

T D

Steady climb

forces balanced

constant speed

Climb entry drag

greater than thrust

speed slowing

Level flight

forces balanced

constant speed

Figure 5-37. Changes in speed during climb entry.

the lift. Lift at this moment is now greater than weight and

starts the aircraft climbing. After the flight path is stabilized

on the upward incline, the AOA and lift again revert to about

the level flight values.

If the climb is entered with no change in power setting, the

airspeed gradually diminishes because the thrust required

to maintain a given airspeed in level flight is insufficient to

maintain the same airspeed in a climb. When the flight path

is inclined upward, a component of the aircraft’s weight

acts in the same direction as, and parallel to, the total drag

of the aircraft, thereby increasing the total effective drag.

Consequently, the total effective drag is greater than the

power, and the airspeed decreases. The reduction in airspeed

gradually results in a corresponding decrease in drag until

the total drag (including the component of weight acting

in the same direction) equals the thrust. [Figure 5-37] Due

to momentum, the change in airspeed is gradual, varying

considerably with differences in aircraft size, weight, total

drag, and other factors. Consequently, the total effective drag

is greater than the thrust, and the airspeed decreases.

Generally, the forces of thrust and drag, and lift and weight,

again become balanced when the airspeed stabilizes but at

a value lower than in straight-and-level flight at the same

power setting. Since the aircraft’s weight is acting not only

downward but rearward with drag while in a climb, additional

power is required to maintain the same airspeed as in level

flight. The amount of power depends on the angle of climb.

When the climb is established steep enough that there is

insufficient power available, a slower speed results.

The thrust required for a stabilized climb equals drag plus a

percentage of weight dependent on the angle of climb. For

example, a 10° climb would require thrust to equal drag plus

17 percent of weight. To climb straight up would require

thrust to equal all of weight and drag. Therefore, the angle

of climb for climb performance is dependent on the amount

of excess thrust available to overcome a portion of weight.

Note that aircraft are able to sustain a climb due to excess

thrust. When the excess thrust is gone, the aircraft is no

longer able to climb. At this point, the aircraft has reached

its “absolute ceiling.”

Forces in Descents

As in climbs, the forces that act on the aircraft go through

definite changes when a descent is entered from straight-

and-level flight. For the following example, the aircraft

is descending at the same power as used in straight-and-

level flight.

As forward pressure is applied to the control yoke to initiate

the descent, the AOA is decreased momentarily. Initially,

the momentum of the aircraft causes the aircraft to briefly

continue along the same flight path. For this instant, the AOA

decreases causing the total lift to decrease. With weight now

being greater than lift, the aircraft begins to descend. At the

same time, the flight path goes from level to a descending

flight path. Do not confuse a reduction in lift with the inability

to generate sufficient lift to maintain level flight. The flight

path is being manipulated with available thrust in reserve

and with the elevator.

To descend at the same airspeed as used in straight-and-

level flight, the power must be reduced as the descent is

entered. Entering the descent, the component of weight

acting forward along the flight path increases as the angle

of descent increases and, conversely, when leveling off, the

component of weight acting along the flight path decreases

as the angle of descent decreases.

Stalls

An aircraft stall results from a rapid decrease in lift caused by

the separation of airflow from the wing’s surface brought on

by exceeding the critical AOA. A stall can occur at any pitch

attitude or airspeed. Stalls are one of the most misunderstood

areas of aerodynamics because pilots often believe an airfoil

stops producing lift when it stalls. In a stall, the wing does

not totally stop producing lift. Rather, it cannot generate

adequate lift to sustain level flight.

Since the C L increases with an increase in AOA, at some

point the CL peaks and then begins to drop off. This peak is

called the CL-MAX. The amount of lift the wing produces drops

dramatically after exceeding the CL-MAX or critical AOA, but

as stated above, it does not completely stop producing lift.

In most straight-wing aircraft, the wing is designed to stall

the wing root first. The wing root reaches its critical AOA

first making the stall progress outward toward the wingtip.

By having the wing root stall first, aileron effectiveness is

maintained at the wingtips, maintaining controllability of

the aircraft. Various design methods are used to achieve

the stalling of the wing root first. In one design, the wing is

“twisted” to a higher AOA at the wing root. Installing stall

strips on the first 20–25 percent of the wing’s leading edge

is another method to introduce a stall prematurely.

The wing never completely stops producing lift in a stalled

condition. If it did, the aircraft would fall to the Earth. Most

training aircraft are designed for the nose of the aircraft to

drop during a stall, reducing the AOA and “unstalling” the

wing. The nose-down tendency is due to the CL being aft of

the CG. The CG range is very important when it comes to

stall recovery characteristics. If an aircraft is allowed to be

operated outside of the CG range, the pilot may have difficulty

recovering from a stall. The most critical CG violation would

occur when operating with a CG that exceeds the rear limit.

In this situation, a pilot may not be able to generate sufficient

force with the elevator to counteract the excess weight aft of

the CG. Without the ability to decrease the AOA, the aircraft

continues in a stalled condition until it contacts the ground.

The stalling speed of a particular aircraft is not a fixed value

for all flight situations, but a given aircraft always stalls at

the same AOA regardless of airspeed, weight, load factor, or

density altitude. Each aircraft has a particular AOA where the

airflow separates from the upper surface of the wing and the

stall occurs. This critical AOA varies from approximately 16°

to 20° depending on the aircraft’s design. But each aircraft

has only one specific AOA where the stall occurs.

There are three flight situations in which the critical AOA is

most frequently exceeded: low speed, high speed, and turning.

One way the aircraft can be stalled in straight-and-level flight

by flying too slowly. As the airspeed decreases, the AOA

must be increased to retain the lift required for maintaining

altitude. The lower the airspeed becomes, the more the AOA

must be increased. Eventually, an AOA is reached that results

in the wing not producing enough lift to support the aircraft,

which then starts settling. If the airspeed is reduced further,

the aircraft stalls because the AOA has exceeded the critical

angle and the airflow over the wing is disrupted.

Low speed is not necessary to produce a stall. The wing

can be brought into an excessive AOA at any speed. For

example, an aircraft is in a dive with an airspeed of 100

knots when the pilot pulls back sharply on the elevator

control. [Figure 5-38] Gravity and centrifugal force prevent

an immediate alteration of the flight path, but the aircraft’s

AOA changes abruptly from quite low to very high. Since

the flight path of the aircraft in relation to the oncoming air

determines the direction of the relative wind, the AOA is

suddenly increased, and the aircraft would reach the stalling

angle at a speed much greater than the normal stall speed.

The stalling speed of an aircraft is also higher in a level turn

than in straight-and-level flight. [Figure 5-39] Centrifugal

force is added to the aircraft’s weight and the wing must

produce sufficient additional lift to counterbalance the load

imposed by the combination of centrifugal force and weight.

In a turn, the necessary additional lift is acquired by applying

back pressure to the elevator control. This increases the wing’s

Figure 5-38. Forces exerted when pulling out of a dive.

100

0 0° 10° 20° 30° 40° 50° 60° 70° 80° 90°

Percent increase in stall speed

Load factor or “G”

Bank Angle

Load factor

Stall speed increase

Figure 5-39. Increase in stall speed and load factor.

Figure 5-40. Inflight ice formation.

AOA and results in increased lift. The AOA must increase

as the bank angle increases to counteract the increasing load

caused by centrifugal force. If at any time during a turn the

AOA becomes excessive, the aircraft stalls.

At this point, the action of the aircraft during a stall should be

examined. To balance the aircraft aerodynamically, the CL

is normally located aft of the CG. Although this makes the

aircraft inherently nose-heavy, downwash on the horizontal

stabilizer counteracts this condition. At the point of stall,

when the upward force of the wing’s lift diminishes below

that required for sustained flight and the downward tail

force decreases to a point of ineffectiveness, or causes it to

have an upward force, an unbalanced condition exists. This

causes the aircraft to pitch down abruptly, rotating about its

CG. During this nose-down attitude, the AOA decreases and

the airspeed again increases. The smooth flow of air over

the wing begins again, lift returns, and the aircraft begins

to fly again. Considerable altitude may be lost before this

cycle is complete.

Airfoil shape and degradation of that shape must also be

considered in a discussion of stalls. For example, if ice, snow,

and frost are allowed to accumulate on the surface of an aircraft,

the smooth airflow over the wing is disrupted. This causes the

boundary layer to separate at an AOA lower than that of the

critical angle. Lift is greatly reduced, altering expected aircraft

performance. If ice is allowed to accumulate on the aircraft

during flight, the weight of the aircraft is increased while the

ability to generate lift is decreased. [Figure 5-40] As little as

0.8 millimeter of ice on the upper wing surface increases drag

and reduces aircraft lift by 25 percent.

Pilots can encounter icing in any season, anywhere in the

country, at altitudes of up to 18,000 feet and sometimes

higher. Small aircraft, including commuter planes, are most

vulnerable because they fly at lower altitudes where ice is more

prevalent. They also lack mechanisms common on jet aircraft

that prevent ice buildup by heating the front edges of wings.

Icing can occur in clouds any time the temperature drops

below freezing and super-cooled droplets build up on an

aircraft and freeze. (Super-cooled droplets are still liquid

even though the temperature is below 32 °Fahrenheit (F),

or 0 °Celsius (C).

Angle of Attack Indicators

The FAA along with the General Aviation Joint Steering

Committee (GAJSC) is promoting AOA indicators as one

of the many safety initiatives aimed at reducing the general

aviation accident rate. AOA indicators will specifically target

Loss of Control (LOC) accidents. Loss of control is the

number one root cause of fatalities in both general aviation

and commercial aviation. More than 25 percent of general

aviation fatal accidents occur during the maneuvering phase

of flight. Of those accidents, half involve stall/spin scenarios.

Technology such as AOA indicators can have a tremendous

impact on reversing this trend and are increasingly affordable

for general aviation airplanes. [Figure 5-41]

The purpose of an AOA indicator is to give the pilot better

situation awareness pertaining to the aerodynamic health

Figure 5-41. A variety of AOA indicators.

of the airfoil. This can also be referred to as stall margin

awareness. More simply explained, it is the margin that exists

between the current AOA that the airfoil is operating at, and

the AOA at which the airfoil will stall (critical AOA).

Angle of attack is taught to student pilots as theory in ground

training. When beginning flight training, students typically

rely solely on airspeed and the published 1G stall speed to

avoid stalls. This creates problems since this speed is only

valid when the following conditions are met:

• Unaccelerated flight (a 1G load factor)

• Coordinated flight (inclinometer centered)

• At one weight (typically maximum gross weight)

Speed by itself is not a reliable parameter to avoid a stall.

An airplane can stall at any speed. Angle of attack is a better

parameter to use to avoid a stall. For a given configuration,

the airplane always stalls at the same AOA, referred to as

the critical AOA. This critical AOA does not change with:

• Weight

• Bank angle

• Temperature

• Density altitude

• Center of gravity

An AOA indicator can have several benefits when installed

in general aviation aircraft, not the least of which is increased

situational awareness. Without an AOA indicator, the AOA

is “invisible” to pilots. These devices measure several

parameters simultaneously and determine the current AOA

providing a visual image to the pilot of the current AOA along

with representations of the proximity to the critical AOA.

[Figure 5-42] These devices can give a visual representation

of the energy management state of the airplane. The energy

state of an airplane is the balance between airspeed, altitude,

drag, and thrust and represents how efficiently the airfoil is

operating. The more efficiently the airfoil operates; the larger

stall margin that is present. With this increased situational

awareness pertaining to the energy condition of the airplane,

pilots will have information that they need to aid in preventing

a LOC scenario resulting from a stall/spin. Additionally, the

less energy that is utilized to maintain flight means greater

overall efficiency of the airplane, which is typically realized in

fuel savings. This equates to a lower operating cost to the pilot.

Just as training is required for any system on an aircraft,

AOA indicators have training considerations also. A more

comprehensive understanding of AOA in general should be

the goal of this training along with the specific operating

characteristics and limitations of the installed AOA indicator.

Ground and flight instructors should make every attempt

to receive training from an instructor knowledgeable about

AOA indicators prior to giving instruction pertaining to or in

airplanes equipped with AOA indicators. Pilot schools should

incorporate training on AOA indicators in their syllabi,

whether their training aircraft are equipped with them or not.

Installation of AOA indicators not required by type

certification in general aviation airplanes has recently been

streamlined by the FAA. The FAA established policy in

February 2014 pertaining to non-required AOA systems and

how they may be installed as a minor alteration, depending

upon their installation requirements and operational

utilization, and the procedures to take for certification of

these installations. For updated information, reference the

FAA website at www.faa.gov.

While AOA indicators provide a simple visual representation

of the current AOA and its proximity to the critical AOA, they

are not without their limitations. These limitations should

be understood by operators of general aviation airplanes

Alpha (angle of attack)

Direction of

Relative Wind Direction of

Relative Wind

Direction of

Relative Wind

Coefficient of Lift Curve

Calibration Set Points:

Ground (Zero Set Point)

Optimum Alpha Angle (OAA) (1.3 x V s)

Cruise Alpha (Va or Maneuvering Speed)

Direction of

Relative Wind

Figure 5-42. An AOA indicator has several benefits when installed in general aviation aircraft.

equipped with these devices. Like advanced automation,

such as autopilots and moving maps, the misunderstanding

or misuse of the equipment can have disastrous results. Some

items which may limit the effectiveness of an AOA indicator

are listed below:

• Calibration techniques

• Probes or vanes not being heated

• The type of indicator itself

• Flap setting

• Wing contamination

Pilots of general aviation airplanes equipped with AOA

indicators should contact the manufacturer for specific

limitations applicable to that installation.

Basic Propeller Principles

The aircraft propeller consists of two or more blades and a

central hub to which the blades are attached. Each blade of

an aircraft propeller is essentially a rotating wing. As a result

of their construction, the propeller blades are like airfoils

and produce forces that create the thrust to pull, or push,

the aircraft through the air. The engine furnishes the power

needed to rotate the propeller blades through the air at high

speeds, and the propeller transforms the rotary power of the

engine into forward thrust.

A cross-section of a typical propeller blade is shown in

Figure 5-43. This section or blade element is an airfoil

comparable to a cross-section of an aircraft wing. One

surface of the blade is cambered or curved, similar to the

upper surface of an aircraft wing, while the other surface is

flat like the bottom surface of a wing. The chord line is an

imaginary line drawn through the blade from its leading edge

to its trailing edge. As in a wing, the leading edge is the thick

edge of the blade that meets the air as the propeller rotates.

Blade angle, usually measured in degrees, is the angle

between the chord of the blade and the plane of rotation

and is measured at a specific point along the length of the

blade. [Figure 5-44] Because most propellers have a flat

blade “face,” the chord line is often drawn along the face

of the propeller blade. Pitch is not blade angle, but because

pitch is largely determined by blade angle, the two terms are

Figure 5-43. Airfoil sections of propeller blade.

Relative wind

Chord line

Thrust

Forward velocity

Rotational velocity

blade angle

Pitch or

attack

Angle

Figure 5-44. Propeller blade angle.

often used interchangeably. An increase or decrease in one is

usually associated with an increase or decrease in the other.

The pitch of a propeller may be designated in inches. A

propeller designated as a “74–48” would be 74 inches in

length and have an effective pitch of 48 inches. The pitch

is the distance in inches, which the propeller would screw

through the air in one revolution if there were no slippage.

When specifying a fixed-pitch propeller for a new type of

aircraft, the manufacturer usually selects one with a pitch

that operates efficiently at the expected cruising speed of the

aircraft. Every fixed-pitch propeller must be a compromise

because it can be efficient at only a given combination of

airspeed and revolutions per minute (rpm). Pilots cannot

change this combination in flight.

When the aircraft is at rest on the ground with the engine

operating, or moving slowly at the beginning of takeoff,

the propeller efficiency is very low because the propeller is

restrained from advancing with sufficient speed to permit

its fixed-pitch blades to reach their full efficiency. In this

situation, each propeller blade is turning through the air at

an AOA that produces relatively little thrust for the amount

of power required to turn it.

To understand the action of a propeller, consider first its

motion, which is both rotational and forward. As shown by

the vectors of propeller forces in Figure 5-44, each section of

a propeller blade moves downward and forward. The angle

at which this air (relative wind) strikes the propeller blade

is its AOA. The air deflection produced by this angle causes

the dynamic pressure at the engine side of the propeller blade

to be greater than atmospheric pressure, thus creating thrust.

The shape of the blade also creates thrust because it is

cambered like the airfoil shape of a wing. As the air flows

past the propeller, the pressure on one side is less than that

on the other. As in a wing, a reaction force is produced in the

direction of the lesser pressure. The airflow over the wing

has less pressure, and the force (lift) is upward. In the case

of the propeller, which is mounted in a vertical instead of a

horizontal plane, the area of decreased pressure is in front of

the propeller, and the force (thrust) is in a forward direction.

Aerodynamically, thrust is the result of the propeller shape

and the AOA of the blade.

Thrust can be considered also in terms of the mass of air

handled by the propeller. In these terms, thrust equals mass

of air handled multiplied by slipstream velocity minus

velocity of the aircraft. The power expended in producing

thrust depends on the rate of air mass movement. On average,

thrust constitutes approximately 80 percent of the torque (total

horsepower absorbed by the propeller). The other 20 percent

is lost in friction and slippage. For any speed of rotation,

the horsepower absorbed by the propeller balances the

horsepower delivered by the engine. For any single revolution

of the propeller, the amount of air handled depends on the

blade angle, which determines how big a “bite” of air the

propeller takes. Thus, the blade angle is an excellent means of

adjusting the load on the propeller to control the engine rpm.

The blade angle is also an excellent method of adjusting the

AOA of the propeller. On constant-speed propellers, the blade

angle must be adjusted to provide the most efficient AOA at

all engine and aircraft speeds. Lift versus drag curves, which

are drawn for propellers as well as wings, indicate that the

most efficient AOA is small, varying from +2° to +4°. The

actual blade angle necessary to maintain this small AOA

varies with the forward speed of the aircraft.

Fixed-pitch and ground-adjustable propellers are designed

for best efficiency at one rotation and forward speed. They

are designed for a given aircraft and engine combination. A

propeller may be used that provides the maximum efficiency

for takeoff, climb, cruise, or high-speed flight. Any change in

these conditions results in lowering the efficiency of both the

propeller and the engine. Since the efficiency of any machine

is the ratio of the useful power output to the actual power

input, propeller efficiency is the ratio of thrust horsepower

to brake horsepower. Propeller efficiency varies from 50 to

Effective pitch

Geometric pitch

Slip

Figure 5-45. Propeller slippage.

60 in.

40 in.

20 in.

Short travel distance— slow speed

—

129 knots

Moderate travel distance— moderate speed—

259 knots

Greater travel distance— very high speed— 389 knots

2,500 rpm

Figure 5-46. Propeller tips travel faster than the hub.

87 percent, depending on how much the propeller “slips.”

Propeller slip is the difference between the geometric pitch of

the propeller and its effective pitch. [Figure 5-45] Geometric

pitch is the theoretical distance a propeller should advance

in one revolution; effective pitch is the distance it actually

advances. Thus, geometric or theoretical pitch is based on

no slippage, but actual or effective pitch includes propeller

slippage in the air.

The reason a propeller is “twisted” is that the outer parts of the

propeller blades, like all things that turn about a central point,

travel faster than the portions near the hub. [Figure 5-46] If the

blades had the same geometric pitch throughout their lengths,

portions near the hub could have negative AOAs while the

propeller tips would be stalled at cruise speed. Twisting or

variations in the geometric pitch of the blades permits the

propeller to operate with a relatively constant AOA along its

length when in cruising flight. Propeller blades are twisted

to change the blade angle in proportion to the differences in

speed of rotation along the length of the propeller, keeping

thrust more nearly equalized along this length.

Usually 1° to 4° provides the most efficient lift/drag ratio,

but in flight the propeller AOA of a fixed-pitch propeller

varies—normally from 0° to 15°. This variation is caused

by changes in the relative airstream, which in turn results

from changes in aircraft speed. Thus, propeller AOA is the

product of two motions: propeller rotation about its axis and

its forward motion.

A constant-speed propeller automatically keeps the blade

angle adjusted for maximum efficiency for most conditions

encountered in flight. During takeoff, when maximum power

and thrust are required, the constant-speed propeller is at a

low propeller blade angle or pitch. The low blade angle keeps

the AOA small and efficient with respect to the relative wind.

At the same time, it allows the propeller to handle a smaller

mass of air per revolution. This light load allows the engine to

turn at high rpm and to convert the maximum amount of fuel

into heat energy in a given time. The high rpm also creates

maximum thrust because, although the mass of air handled

per revolution is small, the rpm and slipstream velocity are

high, and with the low aircraft speed, there is maximum thrust.

After liftoff, as the speed of the aircraft increases, the constant-

speed propeller automatically changes to a higher angle (or

pitch). Again, the higher blade angle keeps the AOA small

and efficient with respect to the relative wind. The higher

blade angle increases the mass of air handled per revolution.

This decreases the engine rpm, reducing fuel consumption

and engine wear, and keeps thrust at a maximum.

After the takeoff climb is established in an aircraft having a

controllable-pitch propeller, the pilot reduces the power output

of the engine to climb power by first decreasing the manifold

pressure and then increasing the blade angle to lower the rpm.

At cruising altitude, when the aircraft is in level flight and

less power is required than is used in takeoff or climb, the

pilot again reduces engine power by reducing the manifold

pressure and then increasing the blade angle to decrease the

rpm. Again, this provides a torque requirement to match the

reduced engine power. Although the mass of air handled per

revolution is greater, it is more than offset by a decrease in

slipstream velocity and an increase in airspeed. The AOA is

still small because the blade angle has been increased with

an increase in airspeed.

Torque and P-Factor

To the pilot, “ torque” (the left turning tendency of the

airplane) is made up of four elements that cause or produce

a twisting or rotating motion around at least one of the

airplane’s three axes. These four elements are:

1. Torque reaction from engine and propeller

Reaction

Action

Figure 5-47. Torque reaction.

Yaw

Force

Slipstream

Figure 5-48. Corkscrewing slipstream.

2. Corkscrewing effect of the slipstream

3. Gyroscopic action of the propeller

4. Asymmetric loading of the propeller (P-factor)

Torque Reaction

Torque reaction involves Newton’s Third Law of Physics—

for every action, there is an equal and opposite reaction. As

applied to the aircraft, this means that as the internal engine

parts and propeller are revolving in one direction, an equal

force is trying to rotate the aircraft in the opposite direction.

[Figure 5-47]

When the aircraft is airborne, this force is acting around

the longitudinal axis, tending to make the aircraft roll. To

compensate for roll tendency, some of the older aircraft are

rigged in a manner to create more lift on the wing that is being

forced downward. The more modern aircraft are designed

with the engine offset to counteract this effect of torque.

NOTE: Most United States built aircraft engines rotate the

propeller clockwise, as viewed from the pilot’s seat. The

discussion here is with reference to those engines.

Generally, the compensating factors are permanently set so

that they compensate for this force at cruising speed, since

most of the aircraft’s operating time is at that speed. However,

aileron trim tabs permit further adjustment for other speeds.

When the aircraft’s wheels are on the ground during the

takeoff roll, an additional turning moment around the vertical

axis is induced by torque reaction. As the left side of the

aircraft is being forced down by torque reaction, more weight

is being placed on the left main landing gear. This results in

more ground friction, or drag, on the left tire than on the right,

causing a further turning moment to the left. The magnitude

of this moment is dependent on many variables. Some of

these variables are:

1. Size and horsepower of engine

2. Size of propeller and the rpm

3. Size of the aircraft

4. Condition of the ground surface

This yawing moment on the takeoff roll is corrected by the

pilot’s proper use of the rudder or rudder trim.

Corkscrew Effect

The high-speed rotation of an aircraft propeller gives a

corkscrew or spiraling rotation to the slipstream. At high

propeller speeds and low forward speed (as in the takeoffs

and approaches to power-on stalls), this spiraling rotation

is very compact and exerts a strong sideward force on the

aircraft’s vertical tail surface. [Figure 5-48]

When this spiraling slipstream strikes the vertical fin, it

causes a yawing moment about the aircraft’s vertical axis.

The more compact the spiral, the more prominent this force

is. As the forward speed increases, however, the spiral

elongates and becomes less effective. The corkscrew flow

of the slipstream also causes a rolling moment around the

longitudinal axis.

Note that this rolling moment caused by the corkscrew flow

of the slipstream is to the right, while the yawing moment

caused by torque reaction is to the left—in effect one may

be counteracting the other. However, these forces vary

greatly and it is the pilot’s responsibility to apply proper

corrective action by use of the flight controls at all times.

These forces must be counteracted regardless of which is

the most prominent at the time.

Gyroscopic Action

Before the gyroscopic effects of the propeller can be

understood, it is necessary to understand the basic principle

of a gyroscope. All practical applications of the gyroscope

are based upon two fundamental properties of gyroscopic

action: rigidity in space and precession. The one of interest

for this discussion is precession.

Precession is the resultant action, or deflection, of a spinning

rotor when a deflecting force is applied to its rim. As can be

seen in Figure 5-49, when a force is applied, the resulting

force takes effect 90° ahead of and in the direction of rotation.

The rotating propeller of an airplane makes a very good

Aerodynamics of Flight — Part 3

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