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4 forces of flight

April 14, 2021

Benyamin Amiri

By: Benyamin Amiri

Four Forces of Flight

Reading time: 9 min

    Content List

In this lesson, we will examine the 4 forces of flight ( lift - weight - thrust - drag ), the Bernoulli’s Principle, airfoils, leading Edge, trailing edge, Chord Line, and finally wing camber

what are the four forces of flight?

During the flight, the forces acting on a plane are lift, weight, thrust, and drag. Lift is an upward force formed by the movement of air on top and under the wing of an airplane and it supports the airplane in flight.

The weight is opposite to the lift force and is formed by the force of gravity.

Thrust is generated by the rotation of the airplane propeller in the air, which propels the airplane forward. It should also be noted that the airplane propeller is rotated by the force supplied by the engine.

 

Drag is against the thrust force and is formed by the movement of the airplane, which limits the speed of the airplane.

four forces of flight

 

In the image above, the arrows that indicate the forces acting on the airplane are often called vectors. The size of the vector is indicated by the length of the vector and the direction of the vector is indicated by the direction of the arrow.

 

when are the four forces that act on an airplane in equilibrium?

In straight-and-level, unaccelerated flight, The four forces are in equilibrium. The thrust is equal to and opposite to the drag force, and the lift is equal to and opposite to the weight.

 

LIFT

As mentioned earlier, the lift force is opposite to the weight. When the airflow hits the airplane wing, this airflow is divided into two parts and passes through the upper surface of the wing and the lower surface of the wing, which due to the special shape of the wing causes low pressure at the top of the wing and high pressure at the lower surface of the wing, and eventually, due to this pressure difference, the lift force appears.

 

Now to understand why the air at the upper surface of the wing is less pressurized than at the lower surface of the wing, we need to study Bernoulli’s law to better understand the lift force.

 

Bernoulli’s Principle

Bernoulli’s principle simply stated, says, “ as the velocity of a fluid (air) increases, its internal pressure decreases.”

To better understand this law and prove it, we use the Venturi tube and test Bernoulli’s law with it.

Venturi tube

As you can see in the picture above, the amount of velocity and air pressure is shown at the inlet of the venturi pipe. But when air enters the narrow part of the pipe, the air velocity increases and its pressure decreases, which is easily seen in the picture. Then, after the air passes through the narrow part of the pipe and enters the outlet, the speed and pressure return to the initial state.

 

To apply the Bernoulli principle, air does not need to pass through a closed tube. Any surface that changes the airflow creates a venturi effect.

As you can see in the image below, the shape of the airplane wing is designed in such a way that the upper surface of the airplane wing is similar to the narrow part of the tube and the lower surface is similar to the wide part of the tube. Therefore, when the air hits the front of the airplane wing, the air velocity increases in the upper surface of the wing and its pressure decreases relative to the lower surface of the wing, which causes the lift force to be generated on the airplane wing.

The pressure difference between the upper and lower surface of the airplane wing is the main source of lift force.

Four forces of flight

Newton’s Third Law of Motion

The rest of the lift force is generated by the lower surface of the wing because the air deflects downwards when it strikes the lower surface of the wing.

 

The third law of Newton states that for “every action (force) in nature there is an equal and opposite reaction”.

 

Let’s look at an example together to better understand.

If we take our hand out of the window of a moving car, our hand will be pulled back. As we know, due to the collision of air with our hand, the air is deflected forward and according to Newton’s third law, it causes the opposite and equal force to be produced, which causes our hand to be pulled back. Watch the video for a better understanding.

 

We examined two important factors in the production of lift force, the Bernoulli principle and Newton’s third law. Many other factors determine the lifting capacity of a wing and you will learn about them in the future.

Airfoils

Airfoil is any surface or object, like a wing, fuselage, etc, that creates aerodynamic force when interacting with a moving airflow.

Leading Edge

The front part of the aircraft wing, or in other words, the part of the wing where the air first hits when the aircraft is moving.

Trailing Edge

The rear part of the aircraft wing, or in other words, the part of the aircraft wing where the two air currents above the wing surface and below the wing surface reconnect and move away from the aircraft wing.

Chord Line

The chord line is an imaginary straight line drawn between the leading edge to the trailing edge of an airfoil.

wing

Camber

The camber of an airfoil is the curved shape of their upper and lower surfaces. Usually, the upper camber is more curved, while the lower camber is comparatively flat.

 

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