Four forces act on an airplane in flight: lift, weight, thrust, and drag.
Aerodynamics is the study of how air moves around objects. In aviation, it helps us understand how an airplane generates lift, moves through the air, and stays under control.
In this lesson, you will learn how airplane wings generate lift and how angle of attack, angle of incidence, airspeed, air density, and center of pressure affect flight. You will then explore weight, thrust, and drag, and see how the four forces work together during takeoff, climb, cruise, descent, and landing.
The aerodynamic force that acts perpendicular to the relative wind.
The force of gravity pulling the airplane toward the Earth.
The force produced by a propeller or jet engine to move the airplane.
The aerodynamic force that opposes the airplane's motion through the air.
In straight, level, unaccelerated flight, lift balances weight and thrust balances drag. During climbs, descents, and turns, the directions and balance of the forces change.
Lift: How an Airplane Wing Creates It
Theory of FlightLift is the aerodynamic force that acts perpendicular to the relative airflow. It is produced mainly by the wings and allows an airplane to support its weight.
A wing generates lift by changing the way air moves around it. This creates a difference in air pressure between the upper and lower wing surfaces. To understand lift, we must understand where this pressure difference comes from and how it produces a force.
How Lift Is Created
As the airplane moves forward, the wing meets the oncoming airflow. Its shape and angle of attack change the speed and direction of the air.
During normal positive-lift flight, air moves faster over much of the upper surface. The static pressure above the wing drops, while the pressure below remains higher.
Air flows around the wing. The air changes speed and direction as it passes the upper and lower surfaces.
Static pressure is lower above the wing than below it. The wing is now between two regions with different pressures.
Air pressure acts on the wing's surfaces. These forces combine to produce a net aerodynamic force with an upward component.
Why Does a Pressure Difference Lift the Wing?
Pressure is force acting over an area. Air pushes against every part of the wing's surface. The greater the pressure, the stronger the push.
Above the wing, the pressure is lower. This creates a weaker downward pressure force. Below the wing, the pressure is higher. This creates a stronger upward pressure force.
Because the two forces are not equal, they do not cancel each other. The result is a force directed toward the lower-pressure side.
That net aerodynamic force is lift. In straight, level flight, lift acts upward and balances the airplane's weight.
Lower pressure means a weaker downward push on the wing.
Higher pressure means a stronger upward push on the wing.
How Can a Small Pressure Difference Lift an Airplane?
The pressure difference does not need to be enormous. What matters is that it acts over the wing's surface.
Imagine a flat surface measuring one square meter. If the pressure below it is 300 pascals higher than the pressure above it, the difference produces an upward force of 300 newtons.
A real wing has a curved surface, and pressure changes from one point to another. The total lift is found by adding the effects of pressure over the entire surface.
Does Air Move from High Pressure to Low Pressure?
Yes. A pressure difference can push air toward a region of lower pressure whenever an open path exists.
But the wing is solid. Air below it cannot pass directly through the wing to reach the region above. The pressure difference therefore exerts a force on the wing.
At the wingtips, air can move around the end of the wing from the higher-pressure region below to the lower-pressure region above. This creates rotating airflow called wingtip vortices.
These vortices contribute to induced drag. We will return to them later in this chapter.
The airplane moves continuously through new air. The wing keeps changing the airflow and recreating the pressure difference. This allows it to keep producing lift.
The Scientific Principles Behind Lift
Three basic ideas help explain the forces acting on an airplane. Each describes a different part of what happens during flight.
When air accelerates along a streamline, its static pressure decreases under the usual simplified conditions used to study low-speed airflow.
This helps explain the lower pressure above a lifting wing.
The wing changes the momentum of the passing air, turning it downward.
The air produces an opposite aerodynamic force on the wing.
An airplane does not accelerate when the total force acting on it is zero.
In straight, level, unaccelerated flight, lift balances weight, and thrust balances drag.
Bernoulli's principle and Newton's third law are not competing explanations of lift. The pressure distribution around the wing and the downward movement of the airflow are two parts of the same aerodynamic process.
Airfoil Shape and Lift
The cross-sectional shape of a wing is called an airfoil. Its shape is carefully designed to produce useful lift with as little drag as practical.
Camber describes the curvature of the airfoil. A cambered airfoil has a curved mean line between its upper and lower surfaces.
Camber changes the pressure distribution and can allow an airfoil to produce positive lift even when its chord line is aligned with the relative wind.
The rounded leading edge helps guide air around the wing. The trailing edge allows the upper and lower flows to leave the airfoil.
Airfoil thickness, camber, and angle of attack all affect the pressure distribution, lift, drag, and stall behavior.
A wing does not need a curved upper surface to create lift. A symmetrical airfoil can also generate lift when it meets the relative wind at a positive angle of attack.
This is important because lift depends on the whole airflow around the wing, not simply on the upper surface being longer.
The Longer-Path Myth
A common but incorrect explanation says that two air particles separating at the leading edge must meet again at the trailing edge.
Because the upper path is longer, the explanation claims that air above the wing must travel faster.
Air particles do not have to arrive together. The equal-transit-time explanation is false.
The air's speed is determined by the flow field around the wing, including its shape and angle of attack.
Angle of Attack (AOA)
The angle of attack is the angle between the wing's chord line and the relative wind.
The chord line is an imaginary straight line joining the leading edge and trailing edge of the wing.
The relative wind is the airflow approaching the wing, opposite to its motion through the air.
Changing the angle of attack changes the pressure distribution around the wing and therefore changes the lift coefficient.
How Angle of Attack Changes Lift
At a constant airspeed and air density, increasing the angle of attack increases the lift coefficient over the normal operating range of the wing.
The pressure difference becomes stronger, and the wing turns the airflow more. Lift increases until the wing reaches its maximum lift coefficient.
There is a limit. Increasing the angle of attack beyond this point does not keep increasing lift.
The Critical Angle of Attack
The critical angle of attack is the angle at which the wing reaches its maximum lift coefficient.
Beyond this angle, airflow separation increases and the lift coefficient falls. This is an aerodynamic stall.
A stall can happen in a climb, descent, turn, or level flight. It is not limited to one pitch attitude or airspeed.
Airspeed and Angle of Attack
In straight, level, unaccelerated flight, lift balances weight.
At a higher airspeed, the wing can produce the required lift at a lower angle of attack.
As the airplane slows, the pilot increases the angle of attack to maintain the same lift. Eventually the critical angle can be reached.
Pitch attitude and angle of attack are not the same. Pitch attitude describes the position of the nose relative to the horizon. Angle of attack describes the wing's angle relative to the airflow.
For a complete explanation, visit the Stall Pre-Flight Lesson .
Angle of Incidence
The angle of incidence is the angle between the wing's chord line and the airplane's longitudinal reference axis.
On a conventional training airplane, the wing is installed at a fixed angle relative to the fuselage.
Designers choose this angle so the wing can produce the lift needed during normal flight while the fuselage remains in a suitable attitude.
The pilot changes the angle of attack during flight, but does not change the wing's fixed angle of incidence.
Do Not Confuse These Two Angles
Measured between the chord line and the relative wind.
Changes during flight.
Directly affects the lift coefficient and determines the onset of a stall.
Watch the video to see the difference between these two angles.
Measured between the chord line and the aircraft's longitudinal axis.
Fixed on conventional training airplanes.
Describes how the wing is mounted on the fuselage.
Center of Pressure
Air pressure does not act at only one point on the wing. It acts over the entire surface, and its strength changes from one location to another.
The center of pressure is the point where the combined aerodynamic force on an airfoil can be considered to act.
When the angle of attack changes, the pressure distribution changes too. The center of pressure can therefore move along the chord line.
This movement can change the pitching moment acting on the wing. It is one reason why an airplane needs a properly designed tail and pitch-control system.
The center of pressure describes the location of the resulting aerodynamic force on an airfoil.
The center of gravity is where the airplane's total weight is considered to act.
These are different concepts. Center of gravity limits are specified by the aircraft manufacturer and are essential for safe stability and control.
Factors That Affect Lift
The lift produced by a wing depends on four main factors: air density, airspeed, wing area, and lift coefficient.
The lift coefficient depends strongly on the wing's shape and angle of attack.
L = Lift | ρ = Air density | V = Airspeed
S = Wing area | CL = Lift coefficient
You do not need to calculate this equation to understand the basic idea. It shows how each factor changes the amount of lift a wing can produce.
Airspeed
With the angle of attack, wing area, and air density held constant, lift increases with the square of airspeed.
This means doubling airspeed produces four times as much lift when the other factors do not change.
In steady level flight, the airplane does not need four times as much lift. As speed increases, the pilot normally reduces angle of attack to keep lift equal to weight.
At a higher speed: less angle of attack is needed to support the airplane.
At a lower speed: more angle of attack is needed.
This is why slow flight brings the wing closer to its critical angle of attack.
Wing Area
Wing area is the area used in the aerodynamic lift calculation. It is normally measured in square feet or square meters.
At the same airspeed, air density, and lift coefficient, a wing with twice the area produces twice the lift.
A larger wing can support more weight at a given speed, but larger wings also affect drag, structural weight, and the airplane's handling.
Designers choose wing area according to the airplane's mission, weight, and operating speeds.
Gliders use long, narrow wings. Their high aspect ratio helps reduce induced drag and improves gliding efficiency.
Aircraft designed for high speeds may use smaller or more swept wings. Their design balances lift, drag, weight, and performance.
Air Density
Air density describes how much air mass is present in a given volume.
At the same true airspeed and angle of attack, denser air produces more lift. Less dense air produces less lift.
Air pressure falls as altitude increases. At the same temperature, lower pressure means lower air density.
Warm air is less dense than cold air at the same pressure. Hot weather therefore reduces aircraft performance.
Moist air is less dense than dry air at the same pressure and temperature. Its effect is smaller than the effect of temperature or altitude.
Density Altitude and Aircraft Performance
Density altitude tells us how the density of the air compares with standard atmospheric conditions.
High density altitude means the air behaves like air at a higher altitude. It is less dense, and aircraft performance decreases.
The airplane needs a higher true airspeed to produce the same dynamic pressure, while the propeller and engine may also deliver less performance.
The result is a longer takeoff distance and reduced climb performance.
At high density altitude, the airplane can still produce approximately the same lift at the same indicated airspeed and angle of attack.
But the true airspeed is higher. This increases takeoff and landing ground distances and affects climb performance.
Always use the aircraft's AFM/POH performance charts when planning a flight.
How Flight Controls Affect Aerodynamic Forces
The pilot controls the airplane by changing aerodynamic forces. Ailerons, elevator, and rudder change the forces acting on the wings or tail.
Flaps are different. They change the wing's lift and drag characteristics, especially during takeoff and landing.
Ailerons — Roll Control
Ailerons are located near the outer trailing edges of the wings.
When the pilot moves the control wheel or stick to the right, the right aileron moves up and the left aileron moves down on a conventional airplane.
The right wing produces less lift while the left wing produces more. The airplane rolls to the right.
Aileron deflection also changes drag. This can produce adverse yaw, which the pilot counters with coordinated rudder input.
Elevator — Pitch Control
The elevator is attached to the horizontal tail. It changes the aerodynamic force acting on the tail.
When the pilot pulls back on the control wheel, the elevator moves up on a conventional airplane.
This normally increases the downward force on the tail, or reduces its upward force. The tail moves down and the nose moves up.
Pushing the control wheel forward produces the opposite pitching movement.
Elevator movement changes pitch. The wing's angle of attack depends on both its attitude and the relative airflow.
Rudder — Yaw Control
The rudder is attached to the vertical tail. It creates a sideways aerodynamic force.
Left rudder moves the rudder to the left. The aerodynamic force moves the tail to the right, making the nose yaw left.
Right rudder produces the opposite movement.
Pilots use the rudder to maintain coordinated flight, counter adverse yaw, and control the airplane during crosswind operations.
The rudder does not directly control the main wings' lift in the way ailerons do.
Flaps — Lift and Drag
Flaps are high-lift devices located near the inboard trailing edges of many wings.
Extending flaps changes the wing's camber. Some flap designs also increase its effective area.
This increases the maximum lift coefficient and reduces the stall speed.
Flaps also increase drag. Larger flap settings are useful during approach and landing because they allow slower flight and steeper descent without excessive acceleration.
The correct flap setting depends on the airplane and the procedure specified in its AFM/POH.
Review how the airplane moves around its three axes in Aircraft Attitude: Pitch, Bank & Yaw .
Wingtip Vortices and Winglets
We have seen that pressure is higher below a lifting wing than above it.
Near a wingtip, air can move from the high-pressure region below the wing toward the low-pressure region above.
This produces rotating airflow that trails behind the wing. These rotating flows are called wingtip vortices.
They contribute to downwash and induced drag, which is the drag associated with producing lift.
Winglets are specially designed surfaces fitted to some wingtips. They can improve the airflow near the tip and reduce induced drag.
They do not eliminate vortices. Their benefit depends on their design and the airplane's operating conditions.
Ground Effect
Ground effect occurs when an airplane flies close to the ground. It is especially important during takeoff and landing.
Near the ground, the airflow around the wing changes. The ground reduces the wing's normal downwash pattern and decreases induced drag.
At the same angle of attack, the wing can also produce more lift than it would higher above the ground. The change is not simply caused by air being compressed between the wing and the runway.
Ground Effect During Landing
As the airplane enters ground effect, induced drag decreases. The airplane loses speed more slowly during the flare.
If the approach speed is too high, the airplane may float a long distance above the runway.
This can increase the landing distance.
Ground Effect During Takeoff
The reduced induced drag can allow an airplane to become airborne before it has enough speed for a safe climb.
As the airplane climbs out of ground effect, induced drag increases.
If the airspeed is too low, the airplane may be unable to climb safely.
Ground effect becomes stronger as the wing gets closer to the ground. Its influence decreases quickly as the airplane gains height.
During takeoff, follow the published airspeed and configuration procedures for the airplane you are flying.
How Pilots Use Lift During Flight
The principles of lift are not just theoretical. Pilots use them during every phase of flight.
As the airplane accelerates, dynamic pressure increases. At the appropriate speed, the pilot raises the nose to establish the angle of attack needed for liftoff.
As airspeed decreases, the wing needs a higher angle of attack to produce the lift required for level flight.
High density altitude increases the true airspeed needed for a given indicated airspeed. Takeoff distance increases, and climb performance decreases.
Flaps change the wing's lift and drag characteristics. During the flare, ground effect reduces induced drag and can cause the airplane to float above the runway.
Lift — Key Points to Remember
The wing creates a pressure difference. Higher pressure below and lower pressure above produce the net aerodynamic force called lift.
Angle of attack changes the lift coefficient. A stall occurs when the wing exceeds its critical angle of attack.
Airspeed, air density, wing area, and wing shape affect the amount of lift the airplane can produce.
Lift — Frequently Asked Questions
Review these important questions about lift, wing aerodynamics, and aircraft performance. They cover concepts every student pilot should understand before a knowledge test.
How does the pressure difference around a wing create lift?
Air pressure is usually lower above the wing and higher below it. High pressure below the wing pushes upward, while the lower pressure above creates less downward force. The combined pressure forces produce a net upward aerodynamic force called lift.
In which direction does lift act?
Lift acts perpendicular to the relative wind, which is the airflow approaching the wing. It is not always exactly opposite to gravity. In straight-and-level flight, lift acts upward and balances the airplane's weight.
What is the angle of attack?
The angle of attack (AOA) is the angle between the wing's chord line and the relative wind. Increasing AOA increases the lift coefficient until the wing reaches its critical angle of attack. Beyond that angle, airflow separation causes a stall.
What causes an aerodynamic stall?
A wing stalls when it exceeds its critical angle of attack. Airflow separates from the upper surface, and the lift coefficient decreases. A stall is caused by excessive AOA, not simply by flying below a particular speed.
Can an airplane stall at any airspeed?
Yes. An airplane can stall at different airspeeds, in different flight attitudes, and with different power settings. What matters is whether the wing reaches its critical angle of attack. High load factors can cause a stall at a higher indicated airspeed.
What is the difference between angle of attack and angle of incidence?
Angle of attack is measured between the wing's chord line and the relative wind. It changes during flight. Angle of incidence is the angle between the wing's chord line and the aircraft's longitudinal reference axis. On a conventional fixed wing, incidence is set by the aircraft design.
What happens to lift when airspeed increases?
If air density, wing configuration, and angle of attack remain constant, lift increases approximately with the square of airspeed. However, a pilot maintaining level flight normally adjusts AOA as speed changes, so lift continues to balance weight.
How does lower air density affect lift?
At the same true airspeed and angle of attack, less dense air produces less lift. To generate the same lift, the airplane needs a higher true airspeed or a higher lift coefficient. At high density altitude, takeoff and climb performance also become worse.
How do flaps affect lift and stall speed?
Extending flaps changes the wing's shape and increases its maximum lift coefficient. This allows the airplane to produce the required lift at a lower airspeed. Flaps normally reduce stall speed, but they also increase drag, especially at larger deflections.
Why does a higher load factor increase stall speed?
A higher load factor means the wings must produce more lift. To do this at the same airspeed, the wing needs a higher angle of attack. It therefore reaches the critical AOA at a higher speed. This is important during steep, level turns.
What happens to the center of pressure when angle of attack changes?
The center of pressure is the point where the wing's total aerodynamic force can be considered to act. Its position can move along the wing's chord as AOA changes. This movement affects the pitching moment produced by the wing.
Why does an airplane behave differently in ground effect?
Close to the ground, the wing's airflow changes and induced drag decreases. The wing becomes more efficient. An airplane may lift off before reaching a safe climb speed. Leaving ground effect then increases induced drag and the power required to continue climbing.
Lift depends on airspeed, air density, wing area, wing shape, and angle of attack. A stall occurs when the wing exceeds its critical angle of attack, regardless of the airplane's pitch attitude.
Weight: The Downward Force
Theory of FlightWeight is the force of gravity acting on the airplane. It always acts vertically downward, toward the center of the Earth.
Every part of the airplane contributes to its total weight. But weight is not only about how heavy the airplane is. Where that weight is located is just as important for stability, control, and flight performance.
What Makes Up Aircraft Weight?
The total weight of an airplane includes its structure, equipment, fuel, occupants, and baggage.
Some of these weights remain nearly constant. Others change between flights or during the flight itself.
The aircraft structure, engine, installed instruments, and other equipment.
Fuel adds weight at takeoff. As fuel is consumed, the airplane becomes lighter.
Every occupant contributes to the loaded weight and affects its distribution.
Bags and equipment must remain within the permitted loading and compartment limits.
An airplane must remain within its approved weight limits. But staying below the maximum permitted weight is not enough.
The airplane must also be properly balanced.
Weight and the Flight Path
Gravity always pulls the airplane vertically downward. This direction does not change when the airplane climbs, descends, or turns.
Lift is different. Lift acts perpendicular to the relative airflow. The relationship between lift and weight therefore depends on the airplane's flight path.
During straight, level, unaccelerated flight, the vertical forces balance.
With the usual simplified force arrangement, lift equals weight.
When the flight path changes, lift is no longer directly opposite to the full weight vector.
We must consider the components of the forces along and perpendicular to the flight path.
A steady climb does not require lift to be greater than weight.
In a stabilized climb, the airplane's engine supplies the energy needed to gain altitude. The forces balance along the climbing flight path.
Similarly, an airplane can maintain a steady descent without simply having lift less than weight.
Weight and Center of Gravity
The center of gravity (CG) is the point where the airplane's total weight can be considered to act.
Imagine balancing a small airplane model on one finger. The point where it balances is its center of gravity.
The CG depends on where the airplane's weight is located. Moving a load forward or backward can change its position.
For example, placing heavy baggage near the tail moves the CG aft. Moving that baggage toward the front moves the CG forward.
Two airplanes can have exactly the same total weight but different CG positions. Their stability and handling may therefore be different.
Why Must the Center of Gravity Stay Within Limits?
Every airplane has an approved CG range. This range is established during aircraft design and testing.
The CG must remain within the approved limits for the airplane's loaded weight.
If the CG moves too far forward or too far aft, the airplane may become difficult or even impossible to control safely.
Forward CG vs. Aft CG
The position of the center of gravity affects longitudinal stability, elevator control, and performance.
More Stability, More Elevator Force
A forward CG creates a greater nose-down pitching tendency.
On a conventional airplane, the horizontal tail must normally produce more downward force to balance this tendency.
The wing must then produce more lift to support both the airplane's weight and the additional tail force.
This can increase the stall speed and reduce performance.
A CG that is too far forward can also make it difficult to raise the nose during takeoff or landing.
Less Stability, Reduced Recovery Margin
An aft CG reduces the airplane's natural longitudinal stability.
Less tail-down force may be needed, which can reduce the lift required from the wing and lower some drag.
However, the airplane becomes more sensitive to pitch changes.
A CG that is too far aft can make stall recovery more difficult.
In extreme cases, the airplane may not have enough nose-down control to recover safely.
Never assume that a forward CG is always safer or that an aft CG is better because it can reduce drag.
The airplane must remain inside its approved CG envelope. Both the forward and aft limits are important.
Managing Aircraft Weight and Balance
Before a flight, the pilot must know the loaded weight of the airplane and the position of its center of gravity.
This requires more than simply adding the weights of the passengers, fuel, and baggage. The location of each load also matters.
Start with the aircraft's current empty-weight data.
Add the planned fuel, occupants, baggage, and other items.
Check the applicable maximum weight limits.
Each item acts at a particular distance from the aircraft's reference point.
This distance is called the arm. It determines how strongly the load affects balance.
Use the aircraft's weight-and-balance data or approved loading method.
Confirm that the calculated weight and CG are within the permitted limits.
Weight, Arm, and Moment
To understand aircraft balance, pilots use the idea of a moment.
A moment describes how strongly a weight tends to turn the airplane around a reference point. It depends on both the weight and its distance from that point.
Weight is the load. Arm is its distance from the reference datum.
Moment = 20 × 50 = 1,000 lb-in.
Moment = 20 × 100 = 2,000 lb-in.
The weight is the same in both examples, but the second load acts farther from the reference point. It therefore produces a greater moment.
This is why moving baggage from one compartment to another can change the CG even when the airplane's total weight stays exactly the same.
These numbers are only examples. Use the actual arms, loading data, and CG envelope supplied for the aircraft you are flying.
How Weight and Balance Change During Flight
Weight and balance are not always fixed from takeoff to landing.
Fuel consumption changes the total weight. It can also change the CG position, depending on where the fuel tanks are located.
Fuel Consumption
As fuel burns, the airplane becomes lighter.
The CG may move forward or backward depending on the location and use of the fuel tanks.
For this reason, pilots may need to check the weight-and-balance condition at both takeoff and landing.
Movement of Loads
Moving passengers or cargo can change the distribution of weight.
Loads must remain properly secured during flight.
An unexpected movement of heavy baggage or cargo can affect balance and aircraft control.
How Aircraft Weight Affects Flight Performance
A heavier airplane must produce more lift in steady level flight. This changes the wing's operating conditions and can affect drag, required power, and performance.
The effects are especially important during takeoff, climb, maneuvering, and landing.
Takeoff Performance
A heavier airplane needs a higher liftoff speed when configuration and other conditions remain the same.
More runway is normally required to accelerate to that speed.
Takeoff distance therefore increases as weight increases.
Climb Performance
Climb performance depends on the power available after accounting for the power required to maintain flight.
Increased weight raises the power required and reduces climb performance.
This is especially important on hot days and at high-elevation airports.
Cruise and Fuel Consumption
A heavier airplane requires more lift.
At the same airspeed, producing more lift normally increases induced drag.
More power may be needed to maintain the same speed, increasing fuel consumption.
Stall Speed and Maneuvering
A heavier airplane has a higher stall speed in the same configuration and load-factor conditions.
During a turn, increasing load factor also increases the lift required.
This can further increase stall speed.
Landing Performance
A heavier airplane normally requires a higher approach and landing speed.
It also carries more kinetic energy at touchdown.
This can increase the distance required to stop.
Controllability
Weight alone does not determine handling.
CG location affects the forces required for pitch control and the airplane's longitudinal stability.
This is why weight and balance must always be considered together.
Weight and Balance Before Flight
A proper loading check is an essential part of preflight planning.
Use the current weight-and-balance information for the specific aircraft.
Equipment changes can affect the airplane's empty weight and CG.
Account for occupants, fuel, baggage, and other equipment.
Respect seat, baggage-compartment, and total weight limits.
Confirm that the total weight and CG are inside the approved envelope.
Check that the aircraft can meet the required takeoff, climb, and landing performance.
An airplane can be below its maximum approved weight and still be unsafe to fly because its CG is outside the permitted range.
Always use the aircraft's actual weight-and-balance data and the applicable AFM/POH limitations.
Weight — Key Points to Remember
Weight is the force of gravity acting vertically downward through the airplane's center of gravity.
The location of the CG affects aircraft stability, pitch control, and flight performance.
Increased weight affects takeoff distance, climb performance, stall speed, and landing distance.
Weight — Frequently Asked Questions
Review these important questions about aircraft weight, center of gravity, and weight and balance calculations. These concepts are essential for safe flight and private pilot knowledge tests.
What is the useful load of an airplane?
Useful load is the difference between the airplane's maximum allowable gross weight and its basic empty weight. It includes occupants, usable fuel, baggage, and other items carried during flight.
What is the center of gravity of an airplane?
The center of gravity (CG) is the point where the airplane's total weight is considered to act. It is also the point where the airplane would balance if it were supported there. Its position depends on how the weight is distributed.
Can an airplane be within its weight limit but still be unsafe?
Yes. An airplane can be below its maximum allowable weight but have a center of gravity outside the approved limits. This can make the airplane unstable or difficult to control. Both total weight and CG position must be within the approved limits.
What is an arm in weight and balance calculations?
The arm is the horizontal distance between the aircraft's reference datum and the center of gravity of an item. It is usually measured in inches. The manufacturer establishes the datum used for weight and balance calculations.
How do you calculate the moment of an item?
Multiply the item's weight by its arm. Moment = Weight × Arm. For example, a 20 lb bag placed 100 inches from the datum has a moment of 2,000 lb-in. Moments are used to calculate the loaded center of gravity.
How is the loaded center of gravity calculated?
Add the moments of the airplane and all loaded items. Then divide the total moment by the total weight. CG = Total Moment ÷ Total Weight. The result gives the CG position relative to the reference datum.
How does a forward center of gravity affect flight?
A forward CG increases longitudinal stability but requires more elevator force to raise the nose. It can increase stall speed and make takeoff rotation or landing flare more difficult. If the CG is too far forward, elevator authority may be insufficient.
Why is an excessively aft center of gravity dangerous?
An excessively aft CG reduces longitudinal stability. The airplane becomes more sensitive to pitch-control inputs and may be harder to control. Stall and spin recovery can become more difficult or impossible when the CG is too far aft.
How does increased aircraft weight affect stall speed?
A heavier airplane needs more lift to support its weight. At the same airspeed, the wing needs a higher angle of attack. It therefore reaches its critical angle of attack sooner. Increasing aircraft weight increases stall speed.
How does aircraft weight affect takeoff and climb performance?
A heavier airplane needs a higher airspeed to produce enough lift for takeoff. It also accelerates more slowly with the same available thrust. Takeoff distance increases, while climb rate and climb performance decrease.
Can fuel consumption change the center of gravity?
Yes. As fuel is burned, the airplane becomes lighter. The CG may also move forward or backward depending on the location of the fuel tanks. The pilot must ensure that the CG remains within the approved limits throughout the flight.
What happens when baggage is moved farther aft?
Moving baggage toward the rear increases its arm when the datum is ahead of the airplane. Its moment increases, moving the airplane's CG aft. The total weight stays the same, but the balance changes. This is why baggage location matters.
Safe loading requires two checks: the airplane must remain within its maximum allowable weight, and its center of gravity must stay inside the approved CG envelope. Always use the aircraft's weight and balance data and AFM/POH.
Thrust: The Propelling Force
Theory of FlightThrust is the force that moves an airplane forward through the air. It is produced by a propeller or a jet engine. Thrust acts mainly in the forward direction, while drag acts in the opposite direction.
For an airplane to accelerate, it needs enough forward force to overcome the forces resisting its movement. During flight, the pilot adjusts engine power to control the airplane's energy, speed, and climb performance.
To understand thrust, we first need to see how a propeller or a jet engine creates this force.
How Thrust Is Produced
Thrust is created when a propulsion system accelerates air backward. The air then exerts an equal and opposite force on the propulsion system, pushing it forward.
This is an example of Newton's Third Law of Motion: for every action, there is an equal and opposite reaction.
The engine provides energy. The propeller or jet engine uses that energy to change the movement of the air.
Fuel energy is converted into mechanical power. In a piston airplane, this power turns the propeller.
The rotating propeller blades create aerodynamic forces that accelerate air toward the rear of the airplane.
The air pushes forward on the propeller blades. This force is transmitted to the airplane as thrust.
A propeller blade works like a small rotating wing. Its shape and angle create a pressure difference between its two surfaces. This pressure difference produces an aerodynamic force on the blade.
Because the blade rotates, part of this force acts forward. At the same time, the propeller accelerates air backward. These are two ways of describing the same physical process.
Thrust Generation in Different Engine Types
Aircraft use different propulsion systems, but they share the same basic principle: accelerating air backward creates a forward force.
The main difference is how each system gets its energy and how it accelerates the air.
Piston Engine with a Propeller
This is the propulsion system used by many training airplanes, including the Cessna 172 and Piper PA-28.
The piston engine burns fuel inside its cylinders. Moving pistons turn a crankshaft, which rotates the propeller.
The propeller blades act as rotating airfoils. They accelerate air backward, producing a forward force that pulls or pushes the airplane.
Important: The engine supplies the mechanical power, but the propeller produces most of the thrust.
Turboprop Engine
A turboprop also uses a propeller, but a gas turbine turns it instead of a piston engine.
Air enters the engine and is compressed. Fuel burns in the compressed air, producing hot gases that drive a turbine.
The turbine delivers power to the propeller through a shaft and reduction gearbox.
The propeller produces most of the thrust. The engine exhaust contributes a smaller amount.
Turboprops are well suited to aircraft operating at moderate speeds.
Turbojet Engine
A turbojet creates thrust by accelerating air and combustion gases through the engine.
Incoming air is compressed and mixed with fuel. The fuel burns continuously in the combustion chamber.
The hot gases expand through a turbine and leave the engine through an exhaust nozzle.
The rearward flow of exhaust produces a forward reaction force on the engine.
Turbojets are suited to high-speed flight, but they are less fuel-efficient than modern high-bypass turbofans at normal airline speeds.
Turbofan Engine
A turbofan has a gas turbine core and a large fan at the front.
Some of the air enters the engine core, where fuel burns and provides energy to drive the turbine and fan.
Another part of the air flows around the engine core. This is called bypass air.
The fan accelerates a large amount of air backward. In a high-bypass turbofan, this airflow provides most of the thrust.
Moving a large amount of air by a smaller speed increase helps improve propulsion efficiency.
Propeller aircraft and jet aircraft do not use different laws of physics. Both produce thrust by changing the momentum of air.
A propeller moves a large amount of air backward. A turbojet accelerates its exhaust to a much higher speed. A high-bypass turbofan combines both ideas.
Thrust and Aircraft Performance
The amount of thrust available affects how an airplane accelerates, climbs, and maintains its speed.
The relationship between thrust and drag is especially important. Drag resists the airplane's motion through the air, while thrust supplies energy to continue moving or to change speed and altitude.
Thrust Accelerates the Airplane
During the takeoff roll, the engine and propeller produce forward thrust.
The airplane accelerates when thrust exceeds the aerodynamic drag and rolling resistance opposing its movement.
As speed increases, the wings produce more lift. When the correct speed is reached, the pilot rotates the airplane to establish the angle of attack needed for liftoff.
Thrust provides acceleration; the wings provide lift. The engine does not directly lift the airplane off the runway.
Extra Power Allows the Airplane to Climb
During a steady climb, the airplane gains altitude while maintaining its selected airspeed.
Some of the available thrust must overcome drag. The remaining forward force helps overcome the component of weight acting down the flight path.
This is why a sustained climb needs more power than steady level flight at the same airspeed.
The amount of excess power available determines the airplane's ability to gain altitude.
Thrust Balances Drag
In straight-and-level flight at a constant speed, thrust and drag are equal.
The engine supplies just enough thrust to overcome aerodynamic resistance.
If the pilot increases power while maintaining level flight, the airplane initially accelerates. As airspeed increases, drag also changes.
A new steady speed is reached when thrust and drag are balanced again.
Power Helps Control Speed and Flight Path
During turns and other maneuvers, pilots adjust power as needed to maintain the desired airspeed.
For example, a level turn increases the lift required from the wings. This increases induced drag, so additional power may be needed to maintain speed.
During a powered descent, reducing power helps the airplane lose altitude. The correct power setting depends on the required descent path and airspeed.
Power, pitch, and drag work together. Changing one often requires adjusting the others.
Thrust does not have to be greater than drag for an airplane to keep flying.
In steady, level flight, thrust equals drag. If thrust exceeds drag, the airplane has energy available to accelerate, climb, or both.
If drag exceeds thrust, the airplane must lose speed, altitude, or some combination of the two.
Engine Power and Thrust Are Not the Same
Pilots often talk about adding power or reducing power. In everyday flying, these expressions are useful, but power and thrust are not the same thing.
Power
Power describes how quickly the engine delivers energy.
A piston engine sends mechanical power to the propeller.
Engine power is commonly expressed in horsepower or kilowatts.
Thrust
Thrust is the forward force produced by the propulsion system.
It depends on engine power, propeller or engine design, airspeed, and atmospheric conditions.
Thrust is a force, so it is measured in pounds-force or newtons.
A propeller converts engine power into thrust. Its efficiency changes with flight conditions. This means the same engine power does not always produce the same thrust.
Factors That Affect Available Thrust
Even when the airplane and engine remain the same, available thrust can change during flight.
Engine Power Setting
Increasing the throttle commands more engine power. With other conditions unchanged, the propeller can accelerate more air and produce more thrust.
Reducing power decreases the energy supplied to the propulsion system.
Airspeed and Propeller Efficiency
A propeller does not produce exactly the same thrust at every airspeed.
As the airplane moves faster, the air reaches the rotating blades at a different angle. This changes the aerodynamic forces acting on the blades and affects propeller efficiency.
Example: A fixed-pitch propeller is designed as a compromise. It cannot provide its best efficiency at every airspeed. A constant-speed propeller automatically changes blade angle to work efficiently over a wider range of conditions.
Air Density and Altitude
Air density affects both engine performance and the amount of air the propeller can accelerate.
At high density altitude, the air is less dense. A normally aspirated piston engine produces less power because less air enters its cylinders. The propeller also operates in less dense air.
As a result, less thrust and less excess power are available for takeoff and climb.
Turbocharged engines and turbine engines have different performance characteristics, but they are also affected by atmospheric conditions and operating limits.
On a hot day or at a high-elevation airport, the airplane may accelerate more slowly and climb less effectively.
Always check the takeoff and climb performance charts in the aircraft's AFM/POH for the actual conditions.
How Pilots Control Thrust
The pilot controls engine power using the cockpit engine controls. The exact method depends on the aircraft and propulsion system.
Throttle Control
In a typical training airplane with a fixed-pitch propeller, moving the throttle forward increases engine power.
Engine RPM rises, and the propeller produces more thrust under the same flight conditions.
The pilot uses the throttle to select the required power for takeoff, climb, cruise, and descent.
Throttle and Propeller Controls
In an airplane with a constant-speed propeller, the throttle controls engine power while the propeller control selects the desired RPM.
A governor automatically changes propeller blade angle to maintain the selected RPM.
This allows the propeller to operate efficiently at different airspeeds and power settings.
Turbine aircraft use power or thrust levers and engine management systems. The selected setting determines how the engine operates within its limits.
In every case, pilots must follow the procedures and limitations published for their aircraft.
What Happens If an Airplane Loses Thrust?
An airplane does not immediately stop flying if the engine fails.
The wings can continue producing lift while the airplane moves through the air. Without engine thrust, however, the airplane normally needs to descend to maintain its airspeed.
During a glide, the airplane loses altitude and uses gravitational potential energy to overcome drag.
Loss of thrust does not mean loss of lift. The airplane can continue gliding if the pilot maintains the appropriate airspeed and angle of attack.
Pilots train for this situation by practicing engine-failure procedures and forced landings .
Does Wind Change the Thrust Produced by an Engine?
Wind changes the airplane's groundspeed, but it does not directly add thrust to the engine.
For example, an airplane flying at the same true airspeed and power setting needs essentially the same thrust whether it has a headwind or a tailwind, assuming otherwise identical conditions and steady wind.
With a headwind, its groundspeed is lower. With a tailwind, its groundspeed is higher.
This distinction matters during flight planning. Wind changes travel time and distance covered over the ground, while the propulsion system produces thrust relative to the surrounding air.
Thrust: Key Points to Remember
Propellers and jet engines accelerate air backward, creating a forward reaction force.
Thrust overcomes drag and supplies energy for acceleration and climbing.
Available thrust depends on engine power, propulsion-system efficiency, airspeed, and atmospheric conditions.
Thrust — Frequently Asked Questions
Review these important questions about thrust, aircraft engines, and flight performance. They cover concepts every student pilot should understand before a knowledge test.
How is thrust produced by an airplane?
Thrust is produced when a propeller or jet engine accelerates air backward. The air exerts an equal and opposite force on the propulsion system, pushing the airplane forward. This follows Newton's Third Law of Motion.
What produces thrust in a piston-engine airplane?
The piston engine supplies mechanical power to turn the propeller. The rotating propeller blades accelerate air backward. The propeller produces the thrust that moves the airplane forward. The engine provides the power needed to turn it.
What is the difference between piston and turboprop engines?
Both use propellers to produce most of their thrust. A piston engine uses moving pistons and a crankshaft to turn the propeller. A turboprop uses a gas turbine to provide the power needed to turn it.
What is the difference between a turbojet and a turbofan?
A turbojet produces thrust mainly by accelerating hot exhaust gases through a nozzle. A turbofan also has a large fan that moves air around the engine core. In high-bypass turbofans, this bypass airflow produces most of the thrust.
What is the advantage of a constant-speed propeller?
A constant-speed propeller automatically changes its blade angle to maintain the selected RPM. This helps the propeller operate efficiently over a wider range of airspeeds and power settings. A fixed-pitch propeller cannot change its blade angle in flight.
Are engine power and thrust the same thing?
No. Power describes how quickly an engine delivers energy. Thrust is the forward force produced by the propulsion system. A propeller converts engine power into thrust. The amount of thrust also depends on airspeed, propeller efficiency, and atmospheric conditions.
What is the relationship between thrust and drag in level flight?
In straight-and-level flight at a constant airspeed, thrust equals drag. Thrust acts forward, while drag acts backward. Because these forces are balanced, the airplane maintains a constant speed.
What happens when thrust exceeds drag?
When thrust exceeds drag, the airplane gains mechanical energy. This extra energy can be used to increase airspeed, gain altitude, or both. The result depends on how the pilot controls the airplane's pitch and flight path.
Why does a steady climb require additional power?
During a steady climb, thrust must overcome drag and the component of weight acting down the flight path. The airplane therefore needs excess thrust and power to gain altitude while maintaining its airspeed.
Which forces oppose thrust during the takeoff roll?
During the takeoff roll, aerodynamic drag and rolling resistance oppose the airplane's forward movement. Thrust must exceed these resisting forces for the airplane to accelerate. As airspeed increases, the wings produce more lift.
How does high density altitude affect available thrust?
At high density altitude, the air is less dense. A normally aspirated piston engine produces less power, and propeller performance decreases. Less thrust and excess power are available, reducing takeoff acceleration and climb performance.
Can an airplane continue flying after losing engine thrust?
Yes. The wings can continue producing lift while the airplane moves through the air. Without engine thrust, the airplane normally descends to maintain airspeed. The pilot establishes the appropriate glide speed and follows engine-failure procedures.
Thrust is the forward force produced by accelerating air backward. In steady, level flight, thrust equals drag. Excess thrust provides energy for acceleration or climbing. Engine power, airspeed, propeller efficiency, and air density affect the thrust available.
Drag: The Resisting Force
Theory of FlightDrag is the aerodynamic force that opposes an airplane's movement through the air. It acts in the opposite direction to the airplane's motion relative to the surrounding air.
Drag is created by the airflow around the airplane. The aircraft's shape, surface, speed, and the lift produced by its wings all affect the amount of drag.
There are two main types: parasite drag and induced drag. Understanding how they change with airspeed helps pilots manage aircraft performance.
The Two Main Types of Drag
Drag Caused by Moving Through Air
Parasite drag comes from the airplane's shape, surface friction, and the interaction of airflow around different aircraft parts.
It does not require the wings to produce lift. Parasite drag becomes more important as airspeed increases.
Drag Caused by Producing Lift
Induced drag is a result of lift generation. The airflow around a lifting wing produces downwash and trailing vortices.
Induced drag becomes especially important at low airspeeds when the wing must produce the required lift at a higher lift coefficient.
Parasite Drag
Parasite drag is the resistance created when the airplane moves through the air. It comes from three main sources: form drag, skin friction drag, and interference drag.
All three contribute to total parasite drag. Their relative importance depends on the airplane's design and configuration.
Form Drag
Form drag, also called pressure drag, comes from pressure differences around an object moving through air.
A blunt shape creates a large, disturbed wake behind it. The pressure behind the object is lower than the pressure in front, producing a rearward force.
Streamlined aircraft shapes reduce airflow separation and form drag.
Skin Friction Drag
Skin friction drag comes from the interaction between the moving air and the aircraft's surface.
A thin layer of air next to the surface is slowed by friction. The movement within this boundary layer creates a force that resists the airplane's motion.
Dirt, ice, and rough surfaces can increase this drag.
Interference Drag
Interference drag occurs where airflow around two aircraft parts interacts.
For example, air moving around the wing meets air flowing along the fuselage. These interacting flows can create additional turbulence and resistance.
Smooth junctions and aerodynamic fairings help reduce this drag.
Why Parasite Drag Increases with Airspeed
The faster an airplane moves through the air, the greater the aerodynamic resistance caused by its shape and surfaces.
At the same air density and aircraft configuration, parasite drag increases approximately with the square of airspeed.
If an airplane doubles its airspeed, its parasite drag becomes approximately four times greater, assuming the drag coefficient remains unchanged.
This explains why flying much faster requires considerably more power.
Induced Drag
Induced drag is the aerodynamic cost of producing lift.
As explained in the Lift chapter, the pressure difference around a wing produces lift and creates a complex airflow pattern behind the airplane.
The important additional point here is why this airflow produces a force that acts backward.
How Downwash Creates Induced Drag
A lifting wing turns the airflow downward. This downward movement of air is called downwash.
Downwash changes the direction of the local airflow around the wing.
As a result, the wing's aerodynamic force is tilted slightly backward.
The backward component of this force is induced drag.
Wingtip vortices are part of the three-dimensional airflow that produces this downwash.
Why Induced Drag Increases at Low Airspeed
Consider an airplane maintaining straight-and-level flight.
When airspeed decreases, the wings must still produce enough lift to support the airplane.
The pilot increases the angle of attack to produce the required lift at the lower speed.
This requires a higher lift coefficient and produces stronger induced effects. Induced drag therefore increases as the airplane slows down in steady, level flight.
Induced drag is not high simply because the airplane is flying slowly.
It increases because the wing must produce the required lift at a higher lift coefficient. A high-load-factor maneuver can also create substantial induced drag at higher airspeeds.
Wing Design and Induced Drag
The size and shape of the wing affect induced drag. For a given wing area and lift, a longer, narrower wing can produce less induced drag than a shorter, wider wing.
Winglets can also improve efficiency by changing the airflow near the wingtips and reducing induced drag. We examined their function in the Lift chapter.
How Drag Changes with Airspeed
Parasite drag and induced drag change in opposite ways as an airplane changes speed in steady, level flight.
Induced Drag Becomes More Important
To maintain level flight at a lower speed, the wing needs a higher lift coefficient.
Induced drag increases, even though parasite drag decreases.
Parasite Drag Becomes More Important
At higher speeds, the wing needs a lower lift coefficient to maintain level flight.
Induced drag decreases, but parasite drag increases rapidly.
Total Drag and the Drag Curve
Total drag is the sum of parasite drag and induced drag.
At very low speeds, induced drag can be large. At high speeds, parasite drag becomes dominant.
Between these two regions, there is a speed where total drag is at its minimum.
In a simplified drag model, minimum total drag occurs when induced drag and parasite drag are approximately equal.
This point corresponds to the airplane's best lift-to-drag ratio for its configuration. It is important for aerodynamic efficiency and gliding performance.
The Back Side of the Power Curve
At very low airspeeds, induced drag can become so large that the airplane needs more power to maintain level flight as it slows down.
This is often called the region of reversed command.
For example, when an airplane flies very slowly with a high angle of attack, further reducing its speed can require more engine power to maintain altitude.
This relationship is important during slow flight and low-speed maneuvering.
Minimum drag and minimum power required are not the same condition.
The speed of minimum total drag is associated with the best lift-to-drag ratio. The speed of minimum power required is typically lower for a propeller-driven airplane.
How Pilots Manage Drag
Pilots cannot eliminate drag, but they can control several factors that affect it.
Flaps and Landing Gear
Extending flaps changes the wing's lift and drag characteristics. Larger flap deflections normally create substantial additional drag.
Extending retractable landing gear also increases parasite drag.
These changes help pilots control the descent path and approach speed.
Finding an Efficient Speed
Flying faster increases parasite drag. Flying too slowly can increase induced drag.
The best speed depends on the flight objective: range, endurance, climb, approach, or glide.
Pilots use the aircraft's published performance information to select suitable speeds.
Turns and Load Factor
During a level turn, the wings must produce more lift than in straight, level flight.
This increases induced drag. In a steep turn, additional engine power may be needed to maintain airspeed and altitude.
Using Drag to Reduce Speed
Drag is not always something pilots want to minimize.
During descent and landing, additional drag can help control speed and lose altitude.
Flaps, landing gear, and other approved drag-producing configurations help pilots manage the approach.
Why Drag Matters for Flight Performance
Drag affects how much power an airplane needs, how fast it can fly, and how efficiently it uses fuel.
If drag increases while the airplane maintains the same speed and altitude, more thrust and power are needed to overcome it.
Reducing unnecessary drag can improve cruise efficiency and help increase range and endurance.
During a glide, aerodynamic efficiency also affects how far the airplane can travel without engine power.
You can explore the practical relationship between airspeed, power, fuel consumption, and flight efficiency in our lesson on Flight for Range and Endurance .
Drag: Key Points to Remember
Comes from aircraft shape, surface friction, and interacting airflow. It increases rapidly with airspeed.
Results from lift generation. It becomes especially important at low speeds and high lift coefficients.
Is the sum of parasite and induced drag. Its minimum occurs between the low-speed and high-speed regions.
Drag — Frequently Asked Questions
Review these important questions about parasite drag, induced drag, and aircraft performance. They cover concepts every student pilot should understand before a knowledge test.
What is aerodynamic drag?
Drag is the aerodynamic force that opposes an airplane's movement through the air. It acts opposite to the aircraft's motion relative to the surrounding air.
What are the two main types of drag?
The two main types are parasite drag and induced drag. Parasite drag comes from the airplane's shape, surface friction, and airflow interactions. Induced drag results from producing lift.
What happens to parasite drag when airspeed doubles?
Parasite drag becomes approximately four times greater if airspeed doubles. This assumes that air density, aircraft configuration, and drag coefficient remain unchanged. Parasite drag increases approximately with the square of airspeed.
What causes form drag?
Form drag comes from pressure differences around an object moving through air. A blunt shape creates a large disturbed wake with lower pressure behind it. Streamlined shapes help reduce this resistance.
What causes skin friction drag?
Skin friction drag comes from air moving along an aircraft's surface. Air close to the surface is slowed by friction, creating resistance. Rough surfaces, dirt, and contamination can increase this drag.
Where does interference drag occur?
Interference drag occurs where airflow around different aircraft parts interacts. A common example is the wing-fuselage junction. Smooth fairings can reduce the additional resistance created in these areas.
Why does induced drag increase at low airspeed?
In steady, level flight, the wings must still support the airplane's weight. At lower airspeed, they need a higher lift coefficient. This increases the induced effects associated with lift. Induced drag rises as airspeed decreases.
Why does a steep, level turn increase induced drag?
A steep, level turn increases the airplane's load factor. The wings must produce more lift to maintain altitude. Producing more lift increases induced drag. Additional power may be needed to maintain airspeed.
How is downwash related to induced drag?
A lifting wing turns air downward, creating downwash. This changes the direction of the local airflow and tilts the wing's aerodynamic force slightly backward. The rearward component is induced drag.
At what airspeed is total drag lowest?
Total drag is lowest at an intermediate speed between the low-speed and high-speed regions. In a simplified drag model, parasite drag and induced drag are equal at this point. It corresponds to the best lift-to-drag ratio.
How do flaps and landing gear affect drag?
Extending flaps normally increases drag, especially at larger deflections. Extending retractable landing gear also increases parasite drag. Pilots use these changes to help control airspeed and descent during approach and landing.
What is the region of reversed command?
It is a low-speed region where slowing down further requires more power to maintain level flight. Induced drag becomes large because the wing needs a high lift coefficient. This is important during slow flight.
Parasite drag increases with airspeed, while induced drag becomes more important at low speeds when the wing must produce a higher lift coefficient. Total drag is the sum of both. Understanding this relationship helps pilots manage speed, power, and aerodynamic efficiency.
The Interplay of Forces During Takeoff, Flight, and Landing
Lift, weight, thrust, and drag act on an airplane throughout the flight. What changes is how these forces work together. During takeoff, the airplane must accelerate. During a climb, it gains altitude. In cruise, it maintains a steady speed and altitude. During descent and landing, it must lose altitude and eventually slow down.
Understanding these changes helps you see why a pilot adjusts power, pitch, airspeed, and aircraft configuration during each phase of flight.
Takeoff: From Ground Roll to Liftoff
At the beginning of the takeoff roll, the airplane is supported by its landing gear. The engine produces thrust, and the airplane starts moving forward. To accelerate, thrust must be greater than the forces resisting its movement: aerodynamic drag and rolling resistance from the wheels.
As airspeed increases, the wings produce more lift. This reduces the amount of weight carried by the landing gear. At the appropriate rotation speed, the pilot gently raises the nose to increase the wing's angle of attack. The wings then produce enough lift to support the airplane, allowing it to leave the runway.
Ground Roll
Thrust accelerates the airplane along the runway. Drag increases as airspeed rises. The wheels still support much of the airplane's weight, but the wings gradually produce more lift.
Rotation
At the recommended rotation speed, the pilot raises the nose. This increases the angle of attack and allows the wing to produce the lift needed for liftoff. The airplane is now supported by aerodynamic forces instead of its landing gear.
Initial Climb
The pilot establishes the recommended climb attitude and airspeed. Engine power supplies the energy needed to gain altitude while overcoming drag. The airplane must maintain sufficient airspeed and remain within its safe angle-of-attack range.
Close to the runway, ground effect reduces induced drag. This can allow an airplane to become airborne before it has enough airspeed to climb safely out of ground effect. That is why lifting off too early can be dangerous. The correct rotation and climb speeds are found in the aircraft's approved flight manual or pilot's operating handbook (AFM/POH).
Climb: How an Airplane Gains Altitude
During a climb, the airplane moves upward along an inclined flight path. The important point is that lift does not need to be greater than weight during a steady climb. Lift acts perpendicular to the flight path, while weight always acts vertically downward.
The Forces During a Steady Climb
Because the flight path is inclined, weight can be divided into two parts. One part acts perpendicular to the flight path. The other part acts backward along the climb path, opposing the airplane's upward movement.
In a simple steady-climb model, lift balances the part of weight perpendicular to the flight path. Thrust must overcome drag and the part of weight pulling the airplane back down the climb path.
This explains why an airplane needs extra power to maintain a climb at constant airspeed. The engine must provide enough energy to overcome drag and increase the airplane's altitude.
In a steady climb, the airplane is not continuously accelerating upward. Its flight path is simply inclined above the horizon. Excess thrust provides the energy needed to gain altitude. For a shallow climb, lift remains close to the airplane's weight.
If the pilot raises the nose without adding enough power, the airplane may climb briefly while losing airspeed. If the angle of attack becomes too high, the wing can stall. A safe climb therefore requires the correct combination of power, pitch, and airspeed.
Cruise: Maintaining Speed and Altitude
In straight-and-level flight at a constant airspeed, the airplane is in equilibrium. The forces are balanced, so the airplane neither accelerates nor changes altitude.
Lift = Weight
Lift supports the airplane against gravity. When lift and weight are balanced, the airplane maintains a constant altitude in straight-and-level flight.
Thrust = Drag
Thrust balances aerodynamic drag. With no net force along the flight path, the airplane maintains a constant airspeed.
What Happens When Power Changes?
Imagine that the airplane is flying straight and level at a constant speed. The pilot increases power while maintaining altitude. Thrust now exceeds drag, so the airplane accelerates.
As airspeed increases, drag also changes. The pilot adjusts pitch and trim to maintain level flight. Eventually, the airplane reaches a new steady speed where thrust and drag are balanced again.
If the pilot reduces power while keeping the same altitude, the airplane begins to slow down. It must then be adjusted to a lower steady airspeed or allowed to descend.
Lift, weight, thrust, and drag are still acting on the airplane. Their effects cancel one another, allowing steady flight. A force balance means no acceleration, not that the airplane has stopped moving.
Turns: When the Force Balance Changes
During a turn, the airplane changes direction. This requires a force directed toward the center of the turn. The pilot creates that force by banking the wings.
Lift Tilts With the Wings
When the airplane banks, the lift force tilts with it. Part of lift still supports the airplane's weight. Another part acts toward the center of the turn and changes the airplane's direction.
More Total Lift Is Needed
In a level turn, the vertical part of lift must still balance weight. Because lift is tilted, the wing must produce more total lift. This increases load factor and induced drag. Additional power may be needed to maintain airspeed.
A steeper bank requires more lift to maintain altitude. The increased load factor also raises the airplane's stall speed. This is why a steep, level turn requires careful control of airspeed, bank angle, and angle of attack.
Descent and Approach: Controlling the Flight Path
In a steady descent, the airplane follows a flight path angled downward. Gravity now helps move the airplane along that path. The pilot controls the descent by adjusting power, pitch, airspeed, and, when needed, flap configuration.
Gravity Helps Overcome Drag
During a constant-speed descent, part of the airplane's weight acts forward along the descending flight path. This helps overcome drag. As a result, an airplane can descend steadily with thrust lower than drag.
Even with the engine at idle, an airplane can maintain forward airspeed while descending because it converts altitude into energy that helps overcome drag.
Maintain a Stable Approach
On final approach, the pilot aims to maintain the correct airspeed, descent path, and aircraft configuration.
Flaps, when used according to the AFM/POH, allow the wing to produce more lift at a given airspeed and usually add drag. This helps the airplane approach at a lower speed and descend on the desired path.
Power and pitch work together during the approach. A power change affects the energy available to the airplane. A pitch change affects angle of attack and how that energy is divided between altitude and airspeed. The pilot coordinates both controls to maintain a stable approach.
An airplane can descend with its wings producing normal lift. It does not need to lose lift completely. A controlled descent is a change in flight path; a stall occurs when the wing exceeds its critical angle of attack. These are two different situations.
Landing: From the Flare to Touchdown
As the airplane approaches the runway, the pilot reduces the rate of descent during the flare. The airplane's nose is gradually raised to change the flight path and prepare for touchdown.
With power reduced, the airplane continues to lose airspeed. The pilot controls the attitude as the airplane settles toward the runway. Once the wheels touch down and the wings produce less lift, more of the airplane's weight is supported by the landing gear.
Flare
The pilot changes the airplane's flight path to reduce the descent rate. Airspeed decreases as the airplane continues toward the runway.
Touchdown
The landing gear begins supporting the airplane's weight. As lift decreases, more weight transfers to the wheels.
Landing Roll
Aerodynamic drag, rolling resistance, and wheel braking help slow the airplane. Reverse thrust is available only on aircraft equipped with that system; most basic training airplanes do not have it.
What If the Landing Cannot Be Completed Safely?
If the approach becomes unstable or a safe landing is no longer possible, the pilot performs a go-around. This changes the airplane from a descending flight path to a climb.
The pilot applies the appropriate power, establishes the correct climb attitude and airspeed, and adjusts the aircraft configuration according to the AFM/POH. The airplane must first regain enough energy to maintain a safe climb.
How the Four Forces Work Together
The four forces are present throughout the flight, but their balance changes with the airplane's speed, direction, and flight path. The following comparison summarizes the main relationships.
FAA-Style Questions: Forces During Flight
Test your understanding of how lift, weight, thrust, and drag work together during takeoff, climb, cruise, descent, and landing.
What must thrust overcome for an airplane to accelerate during takeoff?
Thrust must exceed the combined resistance of aerodynamic drag and wheel rolling resistance. This allows the airplane to accelerate along the runway.
What happens to the load on the landing gear as airspeed increases?
As airspeed increases, the wings produce more lift. Lift reduces the load carried by the wheels until the airplane becomes airborne.
Why can an airplane lift off too early in ground effect?
Ground effect reduces induced drag near the runway. The airplane may become airborne at a speed too low for a safe climb out of ground effect.
Must lift be greater than weight during a steady climb?
No. Lift balances the part of weight perpendicular to the flight path. Thrust overcomes drag and the part of weight acting backward along the climb path.
Why does a steady climb require more thrust than level flight?
During a climb, thrust must overcome drag and the backward component of weight along the rising flight path. This allows the airplane to gain altitude without losing airspeed.
How are the four forces balanced in straight-and-level flight?
At constant airspeed and altitude, lift equals weight and thrust equals drag. The airplane maintains steady flight without accelerating.
What happens if thrust becomes greater than drag in level flight?
The airplane accelerates if the pilot maintains level flight. As speed increases, drag changes until a new balance can be established.
Why does a banked turn require more lift to maintain altitude?
Banking tilts the lift force. Only part of lift acts upward. The wings must produce more total lift to keep its vertical component equal to weight.
What happens to stall speed as bank angle increases in a level turn?
Stall speed increases because the wings must produce more lift. The resulting higher load factor increases the stall speed.
Why can an airplane descend at constant airspeed with thrust below drag?
During descent, part of the airplane's weight acts forward along the descending flight path. Gravity helps overcome drag, allowing the airplane to descend steadily.
How do flaps help during an approach to landing?
Flaps increase the wing's maximum lift capability, allowing flight at lower airspeeds. They also usually increase drag, helping control the approach path.
What helps an airplane slow down after touchdown?
Wheel braking, rolling resistance, and aerodynamic drag slow the airplane. As lift decreases, more weight transfers to the wheels, improving braking effectiveness.
Conclusion and Key Takeaways
Every airplane flies under the influence of four forces: lift, weight, thrust, and drag. These forces determine how an airplane accelerates, climbs, turns, maintains altitude, and descends.
The most important lesson is that these forces work together. A change in airspeed, power, pitch, or bank angle changes how the airplane responds. Understanding these relationships is essential for safe and controlled flight.
The Four Forces at a Glance
Lift
Lift is produced by the wings as they move through the air. It results from the pressure distribution around the wing and acts perpendicular to the relative wind.
Remember: Lift depends on airspeed, air density, wing characteristics, and angle of attack.
Weight
Weight is the force of gravity acting downward through the airplane's center of gravity.
Remember: Aircraft weight and CG position affect stability, controllability, takeoff performance, and climb capability.
Thrust
Thrust is produced by the airplane's propulsion system. It moves the airplane through the air and supplies the energy needed to accelerate or climb.
Remember: Available thrust and engine power are essential to aircraft performance.
Drag
Drag acts opposite to the airplane's movement through the surrounding air. It includes parasite drag and induced drag.
Remember: Drag affects airspeed, fuel consumption, climb performance, and efficiency.
These relationships describe straight-and-level flight at constant airspeed. During a climb, descent, or turn, the forces act in different directions and must be considered in relation to the airplane's flight path.
Three Essential Principles to Remember
Forces Determine Acceleration
When forces are balanced, an airplane can maintain a constant speed and direction.
When the forces are not balanced, the airplane accelerates. This may mean changing speed, changing direction, or both.
Thrust Changes the Airplane's Energy
When thrust exceeds drag, the airplane gains total mechanical energy. It can use this extra energy to increase airspeed, gain altitude, or both.
When drag exceeds thrust, the airplane loses total mechanical energy. During a descent, it can use altitude to maintain or increase airspeed, but its total mechanical energy still decreases.
Flight Path Changes the Force Balance
During a climb, part of weight opposes the airplane's movement along the flight path. During descent, gravity helps move it forward.
During a banked turn, lift is tilted. The airplane needs more total lift to maintain altitude.
A pilot cannot change gravity, but can control power, pitch, bank, and aircraft configuration. These controls change the airplane's aerodynamic forces and how its energy is used.
Always remember that a wing can stall at any airspeed if its critical angle of attack is exceeded. Good flying requires understanding the forces, monitoring airspeed, and maintaining the correct aircraft attitude for each phase of flight.
Typical Exam Questions
Test what you have learned about lift, weight, thrust, drag, and the forces acting during flight. Choose one answer for each question. You will see a short explanation after every answer.
