How Do Planes Fly — The Four Forces of Flight

Airplanes are large pieces of metals and composite materials that rarely fall from the sky. Therefore, you may wonder, ‘How do planes keep in the air and fly stably?’ Well, it’s due to four forces that are active during a flight.

The Four Forces of Flight

The four forces of flight are:

• Gravity
• Lift
• Drag
• Thrust

Let’s explain them one by one.

Gravity and Lift

Gravity is a downward force caused by an object’s mass. We might think airplanes fall from the sky with the simple perception of gravity and feel worried when we see planes in the air. The manufacturers of planes are aware of that, too, and that’s when the wings come into play.

The wings of an aircraft produce lift, which keeps the plane in the air because it provides an upward force against gravity. The design of the wing makes the airflow faster on top of the wing, leading to a lower pressure. As a result, the higher pressure under the wing pushes the structure upward. (NASA Glenn Research Center, 2014)².

If the correct amount of lift is generated, the plane will maintain a certain altitude, not falling off from the sky. However, lift is only generated when there is movement. The faster the object moves, the more lift it makes. Therefore, we’ll discuss thrust in the next section. (Glenn Research Center, 2015)¹.

Thrust and Drag

Thrust is the force causing an object to move forward. Ground speed is required for an aircraft to produce lift and prevent the plane from nosediving. To understand how an engine pushes the aircraft forward, we have to know Newton’s Third Law of Motion:

If force is applied (the equal reaction), there is always an opposing force (the opposite reaction) with the same strength pushing the object in the reverse direction.

Therefore, when an engine blasts hot and compressed air backward, another force pushes it forward. That’s how an engine works.

The more efficient an engine is, the more thrust it provides. The more thrust the engine produces, the faster the plane moves, and the more lift the airplane generates.

Like lift, there is also one force that opposes thrust — drag. The drag is the friction between the plane and the air molecules. However, it is necessary. If there is no drag, there is no air, and planes cannot fly since they can’t produce any lift.

The Forces During the Flight Procedure

However, the delicate balance between these forces only exists during the cruise. Thus, you might ask, ‘What is the distribution of these forces in different stages of the flight?’ Well, it’s easy to imagine, and we’ll talk about that here!

Takeoff

Firstly, we need more lift during takeoff because the plane is slow but needs to climb! Therefore, the flaps will deploy to a certain angle to increase the lift supplied to the airframe. Moreover, the engine will produce more thrust since it needs to maintain the speed and accelerate while it’s going up!

Because the cruise is already mentioned, we’ll skip that and jump to the descent and landing right away.

Descent and Landing

We need to slow down to descend, right? Therefore, the engine throttles have to be reduced to decelerate the airframe. This will prevent the plane from producing enough lift, and consequently, it starts to decrease its altitude.

When the plane is approaching its runway, less lift is harmful: The airplane needs more lift to ensure a smooth landing. Therefore, the flaps will deploy again near the runway. Without that, expect hard landings or even crashes on every flight you will have on a plane.

What if The Distribution of These Four Forces is Uneven?

We have to keep the lift, thrust, gravity, and drag in balance because something severe will happen if we break that stability.

For instance, if the angle of attack is like 15 degrees, the lift is optimal, and planes usually climb at about that angle of attack. However, if it’s any higher, the plane will lose lift, causing a stall. (Glenn Research Center, 2018; SKYbrary Aviation Safety, 2020)^{4, 5}.

Moreover, a stall can occur in other conditions, such as when the plane flies too high and not enough air is there. To conclude, a stall is due to the lack of lift, leading to the aircraft falling and crashing.

Furthermore, the aircraft becomes a glider when thrust disappears in the list of the four forces. That means the plane will only slow down, leaving it descending. Therefore, when an aircraft has spent all its fuel, the only way to get out of trouble is to find the closest airport to land immediately.

Conclusion

In this article, we explained how do planes fly with the four forces in balance, the distribution between these forces in different phases of a flight, and the consequences of uneven forces. If you’re interested, you can learn more and investigate deeper in the references below.

References and Credits

1. Nancy Hall. (2015, May 5). What is Lift? – Glenn Research Center | NASA. Retrieved January 29, 2021, from https://www.grc.nasa.gov/www/k-12/airplane/lift1.html
2. Dr. Robert J. Shaw. (2014, June 12). Dynamics of Flight – NASA. Retrieved January 29, 2021, from https://www.grc.nasa.gov/www/k-12/UEET/StudentSite/dynamicsofflight.html
3. Nancy Hall. (2015, May 5). What is Lift? – Glenn Research Center | NASA. Retrieved January 29, 2021, from https://www.grc.nasa.gov/www/k-12/airplane/lift1.html
4. Nancy Hall. (2018, April 5). Inclination Effects on Lift. Retrieved January 29, 2021, from https://www.grc.nasa.gov/www/k-12/airplane/incline.html
5. (2020, December 8). Stall – SKYbrary Aviation Safety. Retrieved January 29, 2021, from https://www.skybrary.aero/index.php/Stall

Image Credit: Glenn Research Center, Canva