Updated: Dec 8, 2020

Principles of Flight Lesson by wifiCFI

The Four Forces of Flight (PHAK C5)





Thrust (PHAK C5)

The forward force produced by the powerplant/ propeller or rotor. It opposes or overcomes the force of drag.

For an aircraft to start moving, thrust must be exerted and be greater than drag. The aircraft continues to move and gain speed until thrust and drag are equal.

Slowing Down 

If in level flight, the engine power is reduced, the thrust is lessened, and the aircraft slows down. As long as the thrust is less than the drag, the aircraft continues to decelerate. To a point, as the aircraft slows down, the drag force will also decrease. The aircraft will continue to slow down until thrust again equals drag at which point the airspeed will stabilize. 

Speeding Up 

If the engine power is increased, thrust becomes greater than drag and the airspeed increases. As long as the thrust continues to be greater than the drag, the aircraft continues to accelerate. When drag equals thrust, the aircraft flies at a constant airspeed.

Maintaining Straight and Level Flight

If thrust decreases and airspeed decreases, lift will become less than weight and the aircraft will start to descend. To maintain level flight, the pilot can increase the AOA an amount that generates a lift force again equal to the weight of the aircraft. While the aircraft will be flying more slowly, it will still maintain level flight.

In level flight, when thrust is increased, the aircraft speeds up and the lift increases. The aircraft will start to climb unless the AOA is decreased just enough to maintain the relationship between lift and weight. The timing of this decrease in AOA needs to be coordinated with the increase in thrust and airspeed. Otherwise, if the AOA is decreased too fast, the aircraft will descend, and if the AOA is decreased too slowly, the aircraft will climb.

As the airspeed varies due to thrust, the AOA must also vary to maintain level flight.

Horizontal Lift Principle

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.

They are essentially just wings that produce horizontal (instead of vertical) lift.

Propeller Types 

Fixed Pitch Propellers 

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.

Variable Pitch (Constant Speed) Propellers

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.

This is done through the use of a “prop governor.”

Propeller Twist

Why is propeller twist necessary? 

Because at any set RPM, the prop hub is spinning more slowly than the propeller tips. 

As seen on the previous slide, the tips are set at a lower AOA because they are spinning at a higher speed. Whereas, the length of the prop closer to the hub is set at a higher AOA due to its decreased speed. 

If the propeller was not twisted along its length then the tips of the propeller would be producing more thrust than those parts closer to the hub. 

For this reason, prop twist allows for uniform thrust along the entire length of the blade.

Left Turning Tendencies

Due to the design characteristics and direction of propeller rotation, there are 4 factors that contribute to the left turning tendencies of an aircraft. They are:



Gyroscopic Precession 

Spiraling Slipstream

P-Factor (Asymmetric Wing Loading) 

When an aircraft is flying with a high AOA, the “bite” of the downward moving blade is greater than the “bite” of the upward moving blade. This moves the center of thrust to the right of the prop disc area, causing a yawing moment toward the left around the vertical axis.

In essence, the downward moving propeller blade is creating more thrust than the upward rotating blade. Because the propeller rotates clockwise (as seen from the cockpit) this causes the downward propeller to be on the right. So, if the right (downward rotating) propeller is creating more thrust than the left (upward rotating propeller) then the aircraft will effectively yaw to the left. Creating a left turning tendency. 

When will P-Factor be experienced in flight? 

Only at increased AOA’s (angles of attack). 

The higher AOA combined with higher engine power will cause the effect of P-Factor to be more pronounced. 

How can we combat the effects of P-Factor? 

Proper use of the rudder.


Torque reaction involves Newton’s Third Law of Physics that states “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.

When is torque effect most pronounced in an aircraft?

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.

This is why right rudder is required during the takeoff roll to keep the airplane on the centerline of the runway.

Gyroscopic Precession

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. When a force is applied, the resulting force takes effect 90° ahead of and in the direction of rotation.

It is most pronounced in tailwheel aircraft during the takeoff roll.

As the tailwheel of the aircraft comes off the ground during the takeoff roll, a force is applied to the “top” of the propeller. This force is felt 90 degrees later in the direction of rotation. Because the propeller is rotating clockwise, the force is effectively felt on the “right” side of the propeller causing a left turning tendency.

Spiraling Slipstream

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.

Thus, resulting in a left turning tendency.

Lift (PHAK C5)

Lift is the upward vertical action that opposes weight.

The pilot can control the lift. Any time the control yoke or stick is moved fore or aft, the AOA is changed. As the AOA increases, lift increases (all other factors being equal).

Before proceeding further with the topic of lift and how it can be controlled, velocity must be discussed. The shape of the wing or rotor cannot be effective unless it continually keeps “attacking” new air. If an aircraft is to keep flying, the lift-producing airfoil must keep moving.

Air must constantly be moving across the lifting surface.

This is accomplished by the forward speed of the aircraft.

Lift is produced by airfoils. 

An airfoil is any portion of an aircraft designed to produce lift. Whether that be vertical, horizontal, or negative lift.

How does an airfoil produce lift?

An airfoil (wing or rotor blade) produces the lift force by making use of the energy of the free airstream. Whenever an airfoil is producing lift, the pressure on the lower surface of it is greater than that on the upper surface (Bernoulli’s Principle). 

Bernoulli’s Principle

In fluid dynamics, Bernoulli's principle states that an increase in the speed of a fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid’s potential energy.

The air flowing over the upper curved surface of the wing is travelling at a faster rate than the air flowing underneath the wing.

This difference in flow velocity creates a pressure differential above and below the wing.

The faster airflow above the wing is at a lower pressure than the slower, higher pressure airflow below the wing.

The low pressure above the wing is what “lifts” the wing into the air. Almost like a suction action. 

Angle of Attack

The AOA is defined as the acute angle between the chord line of the airfoil and the direction of the relative wind.

Critical Angle of Attack

When the aircraft reaches the maximum AOA, lift begins to diminish rapidly. This is the stalling AOA, known as the critical AOA.


Smooth airflow over the upper, curved surface of the wing is necessary for the aircraft to produce lift. 

If this smooth airflow is disturbed enough, the aircraft will experience a decrease in lift and “stall.”

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.

Stall Recovery

To recover from a stall, smooth airflow over the wing must be re-established. • This is done by reducing the AOA and adding power.

Downwash Principle

Explains the way the air is deflected off the trailing edge of the wing to aid in lift. Downwash explains how air is deflected in a downward motion off the trailing edge of the wing.

Drag (PHAK C5)

Drag is the force that resists movement of an aircraft through the air.

There are two basic types of drag: 

Parasite Drag

In no way functions to aid in flight.

Parasite drag is comprised of all the forces that work to slow an aircraft’s movement. As the term parasite implies, it is the drag that is not associated with the production of lift.

There are 3 sub-categories of Parasite Drag:

Form Drag

Form drag is the portion of parasite drag generated by the aircraft due to its shape and airflow around it. Examples include the engine cowlings, antennas, and the aerodynamic shape of other components. 

Interference Drag 

Interference drag comes from the intersection of airstreams that creates eddy currents, turbulence, or restricts smooth airflow. For example, the intersection of the wing and the fuselage at the wing root has significant interference drag. 

Skin Friction Drag 

Skin friction drag is the aerodynamic resistance due to the contact of moving air with the surface of an aircraft. Every surface, no matter how apparently smooth, has a rough, ragged surface when viewed under a microscope.

Parasite Drag Increases with Airspeed:

Imagine putting your hand out of a moving car’s window: 

If the car is going 25 mph: You won’t feel much drag (resistance). 

However, if the car is travelling 75 mph: You will feel much more drag (resistance). 

The airplane experiences drag in the same manner.

Induced Drag

Is the result of an airfoil producing lift.

Induced Drag decreases as airspeed increases.

In level flight, the aerodynamic properties of a wing or rotor produce a required lift, but this can be obtained only at the expense of a certain penalty. The name given to this penalty is induced drag. Induced drag is inherent whenever an airfoil is producing lift and, in fact, this type of drag is inseparable from the production of lift. Consequently, it is always present if lift is produced.

Wingtip Vortices

An airfoil (wing or rotor blade) produces the lift force by making use of the energy of the free airstream. Whenever an airfoil is producing lift, the pressure on the lower surface of it is greater than that on the upper surface (Bernoulli’s Principle). As a result, the air tends to flow from the high pressure area below the tip upward to the low pressure area on the upper surface. In the vicinity of the tips, there is a tendency for these pressures to equalize, resulting in a lateral flow outward from the underside to the upper surface. This lateral flow imparts a rotational velocity to the air at the tips, creating vortices that trail behind the airfoil.

Wingtip Vortices create drag along the upper, curved surface of the wing.

The action of the airfoil that gives an aircraft lift also causes induced drag.

When an airfoil is flown at a positive AOA, a pressure differential exists between the upper and lower surfaces of the airfoil.

The pressure above the wing is less than atmospheric pressure and the pressure below the wing is equal to or greater than atmospheric pressure.

Since air always moves from high pressure toward low pressure, and the path of least resistance is toward the airfoil’s tips, there is a spanwise movement of air from the bottom of the airfoil outward from the fuselage around the tips.

This flow of air results in “spillage” over the tips, thereby setting up a whirlpool of air called a vortex.

Wake Turbulence

Wingtip vortices are greatest when the generating aircraft is “heavy, clean, and slow.” This condition is most commonly encountered during approaches or departures because an aircraft’s AOA is at the highest to produce the lift necessary to land or take off.

Ways to avoid wake turbulence: 

Avoid flying through another aircraft’s flight path.

Rotate prior to the point at which the preceding aircraft rotated when taking off behind another aircraft. 

Avoid following another aircraft on a similar flight path at an altitude within 1,000 feet. 

Approach the runway above a preceding aircraft’s path when landing behind another aircraft and touch down after the point at which the other aircraft wheels contacted the runway.

Weight (PHAK C5)

Weight has a definite relationship to lift. This relationship is simple, but important in understanding the aerodynamics of flying. 

Lift is the upward force on the wing acting perpendicular to the relative wind and perpendicular to the aircraft’s lateral axis. 

Lift is required to counteract the aircraft’s weight. In stabilized level flight, when the lift force is equal to the weight force, the aircraft is in a state of equilibrium and neither accelerates upward or downward.

If lift becomes less than weight, the vertical speed will decrease.

When lift is greater than weight, the vertical speed will increase.

Ground Effect (PHAK C5)

Ever since the beginning of manned flight, pilots realized that just before touchdown it would suddenly feel like the aircraft did not want to go lower, and it would just want to go on and on. 

This phenomenon is called ground effect. 

When an aircraft in flight comes within several feet of the surface, ground or water, a change occurs in the three dimensional flow pattern around the aircraft because the vertical component of the airflow around the wing is restricted by the surface. 

This alters the wing’s upwash, downwash, and wingtip vortices.

Ground Effect blocks wingtip vortices from creating additional induced drag on the aircraft. 

Thus resulting in: 

Increase in speed

Increase in lift 

Decrease in drag

Airplane Axes (PHAK C5)

An aircraft rotates around 3 different axes. 

They are: 

The lateral axis- pitch 

The longitudinal axis- roll 

The vertical axis- yaw

Aircraft Stability (PHAK C5)

Stability is the inherent quality of an aircraft to correct for conditions that may disturb its equilibrium and to return to or to continue on the original flight path. It is primarily an aircraft design characteristic. 

The 2 categories of stability are: 

Static Stability 

Static stability refers to the initial tendency, or direction of movement, back to equilibrium. 

Dynamic Stability 

Dynamic stability refers to the aircraft response over time. 

These 2 categories of stability also include 3 sub-categories each. 

They are: 




Forces In Turns

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

Slipping Turns

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 Rate of Turn, so the horizontal lift component is greater than the centrifugal force.

Skidding Turns

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 Rate of Turn is too great for the angle of bank.

Adverse Yaw

Adverse yaw is the natural and undesirable tendency for an aircraft to yaw in the opposite direction of a roll. This is due to drag from the deflection of the ailerons. It is corrected with correct rudder input.

Load Factor (PHAK C5)

In aerodynamics, the maximum load factor (at given bank angle) is a proportion between lift and weight and has a trigonometric relationship. The load factor is measured in Gs (acceleration of gravity), a unit of force equal to the force exerted by gravity on a body at rest and indicates the force to which a body is subjected when it is accelerated.

The Vg Diagram (PHAK C5)

The flight operating strength of an aircraft is presented on a graph whose vertical scale is based on load factor. 

The diagram is called a Vg diagram, velocity versus G loads or load factor. 

Each aircraft has its own Vg diagram that is valid at a certain weight and altitude.

Maneuvering Speed (Va)

As can be seen from the Vg Diagram: Maneuvering Speed (Va) is essentially the boundary between when an airplane will stall vs when it will experience structural damage. 

Below Maneuvering Speed (Va) an airplane will stall when subjected to increased G-Forces. 

Above Maneuvering Speed (Va) an airplane will experience structural failure when subjected to increased G-Forces.

Va Changes with Weight

Maneuvering Speed (Va) is not a set speed. It changes with changes in aircraft weight. 

Heavier Aircraft = Higher Maneuvering Speed 

Lighter Aircraft = Lower Maneuvering Speed

It all has to do with the airplane’s Critical Angle of Attack. 

An aircraft will stall at it’s Critical Angle of Attack regardless of it’s weight. 

However, airplanes with higher weight must fly at an Increased Angle of Attack to maintain altitude in straight and level flight. 

Therefore, they are already closer to the Critical Angle of Attack and do not need to travel as far before stalling. 

Aircraft at a lower weight will fly at a Decreased Angle of Attack while maintaining altitude in straight and level flight. 

Therefore, they must travel further to reach their Critical Angle of Attack and stall. 

Because of this, a lighter aircraft will have a lower Maneuvering Speed to compensate for the amount of travel required to reach the Critical Angle of Attack.

FAA Sources Used for this Lesson

Pilot’s Handbook of Aeronautical Knowledge (PHAK) Chapter 5

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