# The right reason for lift force on an aerofoil

• Mohankpvk
In summary: There is no book or other sources where I could find a derivation for the expression of lift force by this method.

#### Mohankpvk

On the internet, I found two different explanations for lift force.
In one of the explanations, bernoullis equation was applied between the stream flowing under and the stream above the aerofoil(the stream above moves faster and so less pressure than the lower).
The other explanation says Bernoulis principle cannont be applied between two different streams.This explanation says that the curved path is the reason(for any mass to move along a curved path, centripetal acceleration is needed).The centripetal acceleration is given to the stream by the pressure difference (less pressure on the inside of the curve than on the outside).In this case only one of the two sides of the foil(either above or below) was analysed at a time.
Which among these would be the right explanation for lift?

I think the most comprensible explanation for the lift of an airfoil is Newton's second and third law. The air streaming along the foil is "pushed downwards" at the rear edge, which means a (downward) force must have acted on it - the (upward) reaction force lifts the airplane (or whatever).

However, the basic statement of your firstly mentioned theory is correct too. The pressure at the upside of the foil is lower than on the downside - summing up all this pressure forces must yield the same result like with the Newton-approach. The same applies for the information given by the second explanation: At curved stream lines the outside pressure is higher than the pressure at the inside. But I don't exactly see how these two statements are mutually contradictory.

I think both of the explanations are more or less correct (but neither complete nor satisfying) and you need the information of the latter to find (or understand) the pressure distribution around the foil stated by the first.

Klystron, Mohankpvk, CWatters and 1 other person
stockzahn said:
I think the most comprensible explanation for the lift of an airfoil is Newton's second and third law. The air streaming along the foil is "pushed downwards" at the rear edge, which means a (downward) force must have acted on it - the (upward) reaction force lifts the airplane (or whatever).
I think both of the explanations are more or less correct (but neither complete nor satisfying) and you need the information of the latter to find (or understand) the pressure distribution around the foil stated by the first.
I think the modern view is that the airstream is directed in a new direction, like a jet engine pointing slightly down, but it seems compatible with your explanation.

Mohankpvk said:
The other explanation says Bernoulis principle cannont be applied between two different streams...

Which among these would be the right explanation for lift?
Unfortunately, people like to argue a lot on the internet, so they take the posture that if my explanation is correct, any other explanation must be wrong. But that isn't how it works.

Both explanations (if explained correctly) are correct: the Newton's 3rd Law explanation does not include a criticism of the Bernoulli explanation, but if someone is arguing that first point, they are wrong...

...that first point doesn't even make sense, and isn't how Bernoulli's principle is applied. The baseline velocity and pressure is free stream. The pressure and velocity on the surfaces of the wing get compared with that, and then each other. Of course air doesn't flow between the top and bottom surface to create the pressure difference! (insert caveat about 3d flow)

Last edited:
Mohankpvk
russ_watters said:
Unfortunately, people like to argue a lot on the internet, so they take the posture that if my explanation is correct, any other explanation must be wrong. But that isn't how it works.

Both explanations (if explained correctly) are correct: the Newton's 3rd Law explanation does not include a criticism of the Bernoulli explanation, but if someone is arguing that first point, they are wrong...

...that first point doesn't even make sense, and isn't how Bernoulli's principle is applied. The baseline velocity and pressure is free stream. The pressure and velocity on the surfaces of the wing get compared with that, and then each other. Of course air doesn't flow between the top and bottom surface to create the pressure difference! (insert caveat about 3d flow)
Nice answer.So initially the fluid along top surface of the aerofoil should be compared with the free stream.Then the stream along the bottom surface of the aerofoil should be compared with the free stream seperately.Then the effect on the top and bottom surfaces should be combined
Is there any book or other sources where I could find a derivation for the expression of lift force by this method?

stockzahn said:
I think the most comprensible explanation for the lift of an airfoil is Newton's second and third law. The air streaming along the foil is "pushed downwards" at the rear edge, which means a (downward) force must have acted on it - the (upward) reaction force lifts the airplane (or whatever).

However, the basic statement of your firstly mentioned theory is correct too. The pressure at the upside of the foil is lower than on the downside - summing up all this pressure forces must yield the same result like with the Newton-approach. The same applies for the information given by the second explanation: At curved stream lines the outside pressure is higher than the pressure at the inside. But I don't exactly see how these two statements are mutually contradictory.

I think both of the explanations are more or less correct (but neither complete nor satisfying) and you need the information of the latter to find (or understand) the pressure distribution around the foil stated by the first.

Mohankpvk said:
The other explanation says Bernoulli principle cannot be applied between two different streams.
Bernoulli principle is applied separately for each stream, one above and one below a wing. The calculations are based on the streams just outside of the boundary layers above and below a wing. However, it's possible that this was a statement made using the air as a frame of reference as opposed to using the wing as a frame of reference. Normally the wing is used as a frame of reference, with the wing diverting (curving) the relative flow downwards. If the air is used as a frame of reference, which is common in the case of propellers, then Bernoulli is violated as the air flows downwards through the "plane" of air swept out by a wing, flowing downwards from the stream above to the stream below (this could explain the "two different streams" statement), where there is an increase in pressure corresponding to the work performed by a wing, resulting in a non-zero "exit velocity" (the velocity of the affected air when it's pressure returns to ambient) of air that is mostly downwards (lift) and somewhat forwards (drag). Nasa has a simplified description of this:

https://www.grc.nasa.gov/WWW/K-12/airplane/propanl.html

This type of analysis is not normally used for a wing, and instead the wing is used as a frame of reference.

As already mentioned, a wing produces lift by curving the relative flow downwards, "pulling" the air downwards from above the wing, and "pushing" the air downwards below the wing, resulting in curved flow. There is a pressure gradient nearly perpendicular to the surfaces of the wings, greater on the outside of curved flow, lesser on the inside of curved flow, and those pressure differences also affect the flow nearly parallel to the surface of the wings, in accordance with Bernoulli principle, and some mathematical models do "flow field" calculations to calculate the coexistent flow speeds and pressures just outside the boundary layers on the upper and lower surfaces of a wing, in order to calculate lift. Drag calculations are more complex.

Mohankpvk
Mohankpvk said:
Is there any book or other resource where I could find a derivation for the expression of lift force by this method?
I believe this is still the gold standard introductory book for aero engineering:
https://www.amazon.com/dp/0078027675/?tag=pfamazon01-20

Most of the concepts are accessible at a high school level, but actual calculations of lift and drag from geometry and airflow alone are pretty difficult.

Mohankpvk
cjl, Klystron, Mohankpvk and 1 other person

## What is the concept of lift force on an aerofoil?

The lift force on an aerofoil is the upward force generated by the wings of an aircraft as it moves through the air. It is responsible for keeping the aircraft in the air and counteracting the force of gravity.

## What is the role of the shape of an aerofoil in generating lift force?

The shape of an aerofoil plays a crucial role in generating lift force. It is designed to create a pressure difference between the upper and lower surfaces of the wings, with the upper surface having a longer distance to travel, resulting in lower pressure and creating an upward force.

## What factors affect the amount of lift force on an aerofoil?

The amount of lift force on an aerofoil is affected by several factors including the shape and angle of the wing, the speed and angle of attack of the aircraft, air density, and the viscosity of the air.

## How does the Bernoulli's principle explain the lift force on an aerofoil?

The Bernoulli's principle states that as the velocity of a fluid (in this case, air) increases, the pressure decreases. This principle explains how the shape of an aerofoil creates a pressure difference, with faster-moving air on the top surface creating lower pressure and resulting in an upward force.

## What is the correct reason for lift force on an aerofoil?

The correct reason for lift force on an aerofoil is a combination of the Bernoulli's principle and the Newton's third law of motion. The shape of the aerofoil creates a pressure difference, while the downward deflection of air molecules results in an equal and opposite reaction, known as lift force.