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Four Forces of Flight

Four forces acting on an aircraft

Four forces acting on an aircraft (Let’s Talk Science)

Four forces acting on an aircraft

Four forces acting on an aircraft (Let’s Talk Science)

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Learn about the four forces of flight.

You’ve probably seen an airplane flying at some point. But have you ever wondered how an aircraft flies? The answer is easy - with a little physics! Flight is all about forces and movement, which can be explained using physics. 

Let’s start with the forces. There are four forces that act on things that fly. These are weightliftthrust, and drag. Each of these plays a key role in keeping an aircraft in the air and moving forward.

Weight

The first of the four forces exerted on aircraft is weight. The weight of an object is the force on the object due to gravity. Certain objects in space, including planets like the Earth, exert a force that attracts objects toward itself. In the case of the Earth, “toward itself” means “down toward the ground.” The force exerted on a body due to gravity can be expressed using the equation:

F = mg

Where F is the force in (N), m is the mass of the object in kg and g is the acceleration due to gravity. When doing this calculation, it is best to use the unit for gravity in N/kg:

g = 9.81 N/kg

In terms of the four forces acting upon an aircraft, weight is measured as the F in the above equation. However, we usually use the symbol W when specifically talking about weight. Substituting W for F above we get:

W = mg

From this equation, we can see that when we talk about ‘weight’ we are actually talking about how much force is acting on a mass due to gravity. This force, as mentioned above, also has a direction. We could call it “down”.

When we stand on the ground, we push down on the Earth and the Earth pushes up against our feet with the same amount of force in the opposite direction. This is an example of Newton’s Third Law). Newton’s Third Law states that for every action, there is an equal and opposite reaction. We usually abbreviate this upward force of the Earth as Fn.

Forces acting on people sitting and standing on the ground
Forces acting on people sitting and standing on the ground (Let’s Talk Science using an image by Flash vector via iStockphoto).
Image - Text Version

Shown is a cartoon drawing of a group of three people.

To the left is a young Caucasian man sitting in a wheelchair and reading a book. To his right is a young brown-skinned woman sitting cross-legged on a pillow on the floor and reading a book. Behind them is a tall brown-skinned man standing up and holding a ball. On the right side of the picture there are two large arrows pointing at each other. At the top is a red arrow pointing downwards beside the text "Weight DOWN = Action". Below this is a green arrow of the same shape and size pointing upwards beside the text "Normal Force UP = Reaction".

Any type of flying machine experiences weight. This weight is always in the direction of the Earth, no matter which way the aircraft is travelling. It is very important to know the weight of an aircraft before flight. Too much weight can cause an aircraft to fly poorly. Heavy aircraft may need higher takeoff speeds and longer runways. They also may not be able to fly as far or as high.

Direction of weights for three aircraft
From left to right: Weight directional arrow on a hot air balloon, United Airlines 777 airliner and CF-18 Hornet jet fighter (Sources: hot air balloon - Keith Pomakis [CC BY-SA] via Wikimedia Commons, airliner and jet from Scott Taylor. Used with permission).
Image - Text Version

Shown, from left to right, are photographs of a hot air balloon, a large commercial airliner and a fighter jet. Directional arrows and text are overlaid on the photo.

The green hot air balloon appears to be floating in a clear blue sky. Below the balloon is a red arrow pointing downward beside the word "weight", indicating that weight is a downward force, and the direction of the force is perpendicular to the ground.

The large twin-engined airliner, appears to be flying in a level position and travelling from left to right. Below the centre of the aircraft is a red arrow pointing downward beside the word "weight".

The fighter jet appears to be flying almost straight up into the sky. Below the rear part of the aircraft is once again a red arrow pointing downward with the word "weight" beside it.

Lift

If an aircraft is being pulled down toward the Earth by gravity and its own mass, how does it stay in the air? The answer is the second force, lift. Lift refers to the force that an object needs to overcome its weight

Lift is an upward force caused by air moving over a wing. A wing or blade, such as that of a propeller, rotor, or turbine as seen in cross-section has a special shape called an airfoil.

As a wing moves through air, the air splits and flows both over and under the wing. The difference in the movement of the air on top of the wing and below the wing generates lift. There are two explanations for the causes of lift: deflection and differences in pressure.

Diagram showing the movement of air above and below an airfoil and the forces it generates
Diagram showing the movement of air above and below an airfoil and the forces it generates (©2021 Let’s Talk Science).
Image - Text Version

Shown is a line drawing of how an airfoil redirects air as it moves.


The airfoil is a long teardrop shape with a thicker, rounded end to the left and a longer, thinner end to the right. It is slightly bulged on the top compared to the bottom.

The airfoil is horizontal in the middle of the picture, with the rounded edge pointing slightly upwards towards the left and the pointed edge pointing slightly downwards towards the right.

Above the airfoil is a blue arrow that points to the left to indicate the direction of motion.

There are five thin parallel blue lines, representing the airflow, that go across the image horizontally. Two blue lines go below the airfoil shape, rising up slightly towards the left and then sloping down, following the shape of the bottom of the airfoil. One blue line connects to the middle of each side of the airfoil. The other two blue lines follow shape of the top of the airfoil. The lines get closer to each other above the thickest part of the airfoil and then return to their normal spacing once they are past the airfoil shape.

Below the middle of the airfoil is a red arrow that is pointed downwards. Beside this is text indicates that air is pushed downward by the underside of the wing. This is the action.

Above the middle of the airfoil is a green arrow that is the same size and shape as the red arrow pointing upwards. Beside this is text that indicates when the wing is pushed upwards there is lift. This is the reaction.

 

Deflection

As air passes along a wing, some of the air is directed downward. This is called deflection. Once again Newton’s Third Law is in action. Here, the ACTION is air pushing downward under the wing, and the REACTION is the wing moving upwards.

When the leading edge of the wing points upward, such as when the aircraft is climbing, it creates a positive angle of attack. Angle of attack is the angle between the  and the direction of motion. Since air is being deflected downward by the wing, there is lift.

The opposite is also true. When the wings point downward (a negative angle of attack), there is less lift and the aircraft goes down.

When the leading edge points upwards, there is a positive angle of attack
When the leading edge points upwards, there is a positive angle of attack (©2021 Let’s Talk Science).
Image - Text Version

Shown is a line drawing with labels indicating the parts of an airfoil.

The airfoil is a long teardrop shape with a thicker, rounded end to the left and a longer, thinner end to the right. It is slightly bulged on the top compared to the bottom.

The airfoil is horizontal in the middle of the picture, with the rounded edge pointing slightly upwards towards the left and the pointed edge pointing slightly downwards towards the right.

Below the airfoil is a blue arrow that points to the left to indicate the direction of motion.

The rounded end of the airfoil is labelled as the leading edge.

The pointed end of the airfoil is labelled as the trailing edge.

A blue line extends from just past the leading edge to the trailing edge. This line is labelled as the chord line.

The angle between the direction of motion and the chord line is called the angle of attack. When the angle is upward, this is known as a positive angle of attack, which is the orientation of the airfoil shown in this diagram.

 

Pressure Differences

Lift can also result from differences in pressure. These differences occur above and below the wing as air moves past the wing.

Air pressure is measured by dividing the force of the air molecules by the area that the air molecules are in. When air moves over a wing, the layer of air is squeezed into a smaller area. As a result, the speed of the air increases and the pressure of the air decreases. The opposite occurs below the wing. The air is squeezed less, resulting in slower moving air that has higher pressure.

Air flow and pressure above and below an airfoil
Air flow and pressure above and below an airfoil (©2021 Let’s Talk Science)
Image - Text Version

Shown is another interpretation of how air moves around an airfoil. Above and below the airfoil shape are four thin blue parallel lines that follow the shape of the airfoil. Two go over the airfoil and two go under.

Between the airfoil and the first blue line above it, the space is shaded pink. This is to indicate that there is less force from air. There is also a small red arrow pointing downwards toward the airfoil. Beside the arrow is the phrase "fast air = low pressure".

Between the airfoil and the first blue line below it, the space is shaded pale blue. This is to indicate that there is more force from air. There is also a large green arrow pointing upwards toward the airfoil. Below the arrow is the phrase "slow air = high pressure".

 

Lift can be explained using Bernoulli’s Principle. It states that “as the speed of a moving increases, the pressure within the fluid decreases.” Since the force pushing up from the high pressure air is greater than the force pushing down from the low pressure air, there is lift in an upward direction.

Thrust

Early flew in hot air balloons. These lighter than air (LTA) vehicles could easily go up and down, but once in the air, they were at the mercy of the wind. A pilot had no way to steer the balloon.

Not long after they were invented, people began to think of ways to make balloons go in the direction they wanted. To accomplish this, they needed a way to push the balloon forward. This pushing is known as thrust. Like lift, thrust is another type of reaction force that can be explained using Newton’s Third Law.

Propellers

In 1784, Jean-Pierre Blanchard attached a hand-powered propeller to a balloon, which is the first recorded use of propulsion by a hot air balloon. People tried many other forms of propulsion in the 1700s and early 1800s, and it wasn’t until 1852 that Henri Giffard created an airship which used an engine to turn a propeller.

September 19, 1900 - Alberto Santos-Dumont demonstrating his No. 4 airship in Paris, France (2015) by Guy Jones (1:13 min.)

Propellers are rotating blades which may be found at the front or back of an aircraft. If they are on the front, they are called tractors. If they are at the back, they are called pushers.

A propeller consists of two or more blades connected together by a hub.

Propellers were initially made of wood but nowadays many are made of metal.

Early propellers were turned by hand, pedalled by foot, or powered by steam engines. Today, propellers are powered by either internal combustion engines or jet engines (see below).

Parts of a propeller
Parts of a propeller (Let’s Talk Science using an image of a P-51D Mustang from Scott Taylor. Used with permission.)
Image - Text Version

Shown is a photograph of the front end of a shiny silver World War II fighter plane sitting on the ground.

The propeller is connected to the front of the aircraft at the hub, which is painted a deep yellow. The location of the hub is labelled. Attached to the hub are four flat paddle-shaped propeller blades. These are painted black and have yellow tips. One of the blades is labelled. The engine, which is connected to the hub, is also labelled.

 

Each of the blades of a propeller is shaped like an airfoil. When they turn, they act as spinning wings. As the propeller turns, it pulls slow air towards itself and pushes fast air out behind itself. This generates a force directly behind the propeller - the action - that pushes the aircraft forward - the reaction.

Action, reaction and air speeds caused by a moving propeller
Action, reaction and air speeds caused by a moving propeller (Let’s Talk Science using an image of an F4U Corsair from Scott Taylor. Used with permission).
Image - Text Version

Shown is a photograph of a vintage World War II US navy fighter plane, in flight. It appears to be travelling to the left. Directional arrows and text are overlaid on the photo.

Four small parallel blue arrows point towards the propeller. Above these arrows is text identifying this as "slower air."

Four larger parallel pink arrows point from the propeller back towards the rear of the plane. Above these arrows in text identifying this as "faster air."

Pointing backwards, away from the propeller and below the aircraft is a red arrow and the word "Action".  Pointing forwards, away from the propeller and below the aircraft is a green arrow and the word "Reaction". The two arrows are the same size and shape.

 

Rotors 

Rather than propellers, helicopters use a set of rotary wings called the rotor. A rotor is made up of two or more rotor blades. Helicopters typically have two rotors. These are the main rotor, which is located at the top of the aircraft and the tail rotor, which is located at the back of the aircraft.

Location of rotor blades, main rotor and tail rotor on a helicopter
Location of rotor blades, main rotor and tail rotor on a helicopter (Let’s Talk Science using an image of a HH-65 Dolphin from Scott Taylor. Used with permission.)
Image - Text Version

Shown is a photograph of a bright red search and rescue helicopter with a white stripe and black rotors. A person is suspended on a wire directly below the helicopter. Directional arrows and text are overlaid on the photo.

Above the helicopter is a label indicating that the main rotor is located above the body of the aircraft. Another arrow points to one of the blades indicating that it is a rotor blade. At the back end of the helicopter is an arrow pointing to another smaller rotor embedded in the tail. The label indicates that this is the tail rotor.

 

Unlike a propeller, a rotor produces both lift and thrust. In order to fly in a particular direction, the pilot changes the  of the rotor blades. This makes the rotor tip in a given direction. The helicopter will then move in that direction.

Rotors allow helicopters to take off and land vertically, as well as hover. This makes them useful for search and rescue, firefighting, and medical transport.

Arrows showing direction of action and reaction due to moving rotor
Arrows showing direction of action and reaction due to moving rotor (Let’s Talk Science using an image of a HH-65 Dolphin from Scott Taylor. Used with permission.)
Image - Text Version

Shown is a photograph of a bright red search and rescue helicopter with a white stripe and black rotors. A person is suspended on a wire directly below the helicopter. Directional arrows and text are overlaid on the photo.

Pointing upward from two of the main rotor blades are two green arrows and the text "Reaction = Lift and Thrust".

Pointing downward from two of the main rotor blades, and directly below the green arrows are two red arrows, each of which is beside the text "Action."

 

Jet Engines

Many modern aircraft have replaced engines turning propellers with jet engines. These engines create thrust by:

  1. pulling air into the engine,
  2. mixing the air with fuel,
  3. igniting the fuel/air mixture, and
  4. pushing the hot air out of the back of the engine at high speed.

As with the propeller, the jet engine pushes out air at a higher speed than the air entering the engine. This causes the aircraft to move forwards.

How thrust is produced by a turbojet engine
How thrust is produced by a turbojet engine (Let’s Talk Science using an image by lvcandy via iStockphoto).
Image - Text Version

Shown is an illustration of a cross section of a tubojet engine.
The engine looks like a tube that narrows, then widens, then narrows again.
Labelled as step 1, cool air, identified with blue arrows, is pulled into the engine on the left side of the image.

Next, the air passes through a section that looks like seveal striped lines. Labelled as step 2, this is where the air is mixed with fuel. The interior of the engine appears blue to indicate that the air is cold in this region.

Next the air moves into the narrow part of the engine. Here the air and fuel mixture is ignited. Labelled as step 3, this is where the air and fuel mixture is ignited. The interior of the engine appears as yellow to indicate that the air is heated in this region. A red directional arrow points further into the engine.

Finally, the air passes through several more striped lines. Labelled as step 4, this is where a turbine pushes hot air out the back of the engine at high speed.

 

Drag

The fourth and final force of flight is called drag. Another term for drag is air resistance. Like other fluids, air can resist, or try to stop the movement of an object through it. This is similar to how water behaves when you try to walk or swim through it. The same is true for aircraft. Air resists the movement of aircraft through it. This resistance counteracts thrust and slows down forward motion.

There are two main types of drag: parasite drag and lift-induced drag.

Parasite Drag

Form drag is drag that is caused by the shape of an object travelling through a fluid. Some shapes, such as the airfoil shape, move fairly smoothly through air. The air moves neatly above and below the shape without creating a lot of  behind it. However, other shapes do not move smoothly through air. Shapes like the sphere and the flat plate create a lot of turbulence behind them. This turbulence slows down their movement.

Air movement around objects of different shapes
Air movement around objects of different shapes (Let’s Talk Science using an image by MikeRun [CC BY-SA 4.0] via Wikimedia Commons).
Image - Text Version

Shown are four drawings used to describe how air flows around objects of different shapes. The air flow is represented by thin blue lines with small arrows that are point to the right. In each diagram, the lines initially come towards the shape in parallel.

The top left drawing shows how the air would move around a thin rectangular object in a vertical position. The lines indicate that some air would be blocked by the object, but that most air move over and below the object. Directly behind the object are a number of curved lines. These indicate turbulent air, which occurs directly behind the object.

The top right drawing shows how the air would move around a circular object. The lines indicate that some air would be blocked by the object, but that most air move over and below the object. Directly behind the object are a few  curved lines indicating turbulent air. There are fewer turbulence lines than in the drawing of the rectangular object.

The bottom left drawing shows how the air would move around an airfoil. The lines indicate that almost all air would  move over and below the object. As the airfoil is parallel to the direction of airflow, there are no turbulence lines.

The bottom right drawing shows how the air would move around a thin rectangular object in a horizontal position. The lines indicate that almost all of the air would  move over and below the object. The airflow lines are almost undisturbed by the object and there is no turbulence.

Early aircraft such as the Curtiss 1911 Model D had a lot of form drag, especially from vertical parts such as wing struts. Over time, advances in aerodynamics and materials has led to much more streamlined designs, such as the SR-71 Blackbird.

Left: Curtiss 1911 Model D. Right: SR-71 Blackbird
Left: Curtiss 1911 Model D. Right: SR-71 Blackbird (Sources: Left: Scott Taylor. Used with permission. Right: Public domain image from NASA via Wikimedia Commons).
Image - Text Version

Shown are two photographs of aircraft in flight.

The photo on the left is of an early biplane. This early aircraft looks a lot like a box kite. It is made from wood and fabric with wires connecting the two sets of parallel wings. A pilot sits in the open near the engine.

The photo on the right is of a modern stealth aircraft. It is very flat and angular, with a long flat fuselage and a single set of sleek wings.

Surface friction drag occurs whenever an object moves through a fluid. The roughness of the surface affects how much the fluid slows down the movement of the object. This is because the rough spots cause turbulence. Surface drag is actually a form of. To reduce surface friction drag, aircraft are designed to be as smooth as possible.

This P51 Mustang has little surface friction drag
This P51 Mustang has little surface friction drag (Source: rlbowlinjr via iStockphoto).
Image - Text Version

Shown is a photograph of vintage World War II fighter aircraft in flight. The aircraft has a shiny metallic finish. It is so smooth that the runway and grass below is reflected on its side!

 

Lift-Induced Drag

The other main type of drag is lift-induced drag. This type of drag is a result of lift. The greater the lift, the greater the lift-induced drag.

When a wing is roughly parallel to the airflow the air tends to flow smoothly past the wing. As you increase the angle of attack, though, you begin to get more unstable air behind the wing. This is due to the shape of the wing becoming more like the flat plate mentioned under form drag.

The top airfoil has a small amount of lift-induced drag and the bottom airfoil has a large amount of lift-induced drag
The top airfoil has a small amount of lift-induced drag and the bottom airfoil has a large amount of lift-induced drag (©2021 Let’s Talk Science).
Image - Text Version

Shown are two drawings of how air flows around airfoils with different angles of attack.

In each drawing, the air flow is represented by four thin blue lines which initially come towards the airfoil in parallel from the left side of the drawing.

The top drawing shows how the air would move around an airfoil with a small angle of attack. Above the trailing edge of the airfoil are a few curved lines. These indicate turbulence, which is fairly minimal in this airfoil position.

The bottom drawing shows how the air would move around an airfoil with a large angle of attack. In this case, the airfoil acts al lot like the vertical rectangle shape in the diagram about form drag. Directly behind the airfoil are many curved lines indicating that extensive turbulence is generated.

 

There comes a point where the angle of attack becomes so great that the wing is no longer able to generate lift. This is known as the critical angle of attack. At this point, the aircraft stalls. Many modern aircraft have warning systems that alert the pilot if the aircraft is about to stall.

So to summarize, there are four forces that keep an aircraft in the air and moving forward: weight, lift, thrust, and drag. But if you think about it, that means an aircraft is falling, rising, moving forward, and being pulled back - all at the same time! Scientific innovations over the centuries have allowed us to keep these four forces in balance so that we can fly aircraft from one place to another.

Learn More

Fly8MA.com Ep. 5: How Airplanes Fly
This video (5:36) from Fly8MA, a private pilot education company, describes with visuals the four forces that act upon an airplane in flight.

Thinking Captain: How Do Planes Fly?
This video (2:14) provides an easy and quick breakdown on how airplanes stay in the sky and the four forces of flight.

No One Can Explain Why Planes Fly In The Air
This article (2020) in Scientific American by Ed Regis delves into the idea that, while we can clearly engineer aircraft that fly through the air, we don’t have a precise mathematical model that explains lift.

Physics Challenges For Green Aviation
This article (2020) by Brian Tillotson in Physics World tackles some of the future-looking ideas for aircraft design to help reduce fuel usage due to drag and other environmentally problematic aspects of flight.

U.S. Space and Rocket Center: Four Forces of Flight with Paper Airplanes
This video (9:27) examines the four forces of flight through both visual aids and the construction and flight of paper airplanes.

References

FAA (2016). Pilot’s Handbook of Aeronautical Knowledge. Retrieved from https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak/

Man-Vehicle Laboratory (1997). Theory of Flight. MIT Department of Aeronautics and Astronautics. Retrieved from https://web.mit.edu/16.00/www/aec/flight.html

NASA (2021). Four Forces on an Airplane. Retrieved from https://www.grc.nasa.gov/www/k-12/airplane/forces.html

NASA (2021). How does a jet engine work? Retrieved from https://www.grc.nasa.gov/WWW/k-12/UEET/StudentSite/engines.html

NASA (2021). Re-living the Wright Way. Retrieved from https://wright.nasa.gov/airplane/shortw.html

NASA (Dec. 9, 2003). The Four Forces of Flight. Retrieved from https://www.nasa.gov/audience/foreducators/k-4/features/F_Four_Forces_of_Flight.html