Aerodynamics

By: Eng. Amro Aboalhagag Ahmed 21P0146

 Lift Theory
A fluid flowing around an object exerts a force on it.
Lift is the component of this force that is perpendicular to the oncoming
flow direction.
It contrasts with the drag force, which is the component of the force
parallel to the flow direction.
Lift conventionally acts in an upward direction in order to counter the
force of gravity, but it can act in any direction at right angles to the flow.
Down below is an image of an aerofoil which are specifically designed to
increase the lift force and decrease the drag for as much as possible.
So, the question is how does this aerofoil generate enough lifting force?
As the fluid passes around this aerofoil there are two types of pressure acting
on it:
1. Shear stresses: Which act tangential to the surface of the aerofoil and
caused by frictional forces due to fluid viscosity. These stresses are not
our concern as they are the reason for drag force not the lifting

2. Pressure stresses: which act perpendicular the surface of the aerofoil

and caused by the pressure distribution. These stresses are the main
cause of the lifting force.

This leaves us with a question which is how this pressure distribution

occurs?
To understand the explanation behind this we should know first that there are
two different approaches:

1. Bernoulli’s Principle (fluid mechanics explanation)

2. Newton’s Third law (mechanical explanation(

Bernoulli's Principle

There are two common versions of this explanation, one based on

"equal transit time", and one based on "obstruction" of the airflow.

Equal transit-time :

This version of explanation depends on that two amount of particles should

arrive at the end of their different paths simultaneously at same time. And so
due to the longer path the particle on the top take sit has to move with higher
velocity creating lower pressure area on the top than the lower. This creates
pressure difference which creates lifting force.

This is completely WRONG , and there are two reasons why:

1. There is no physical law to support that two particles starting at same

position must arrive at same time.

2. Bernoulli’s equation should only be applied for particles following the

same paths and not different ones.
Obstruction of the air flow
Like the equal transit time explanation, the "obstruction" or "stream tube
pinching" explanation argues that the flow over the upper surface is
faster than the flow over the lower surface, but gives a different reason
for the difference in speed. It argues that the curved upper surface acts
as more of an obstacle to the flow, forcing the streamlines to pinch
closer together, making the stream tubes narrower. When stream tubes
become narrower, conservation of mass requires that flow speed must
increase. Reduced upper-surface pressure and upward lift follow from
the higher speed by Bernoulli's principle, just as in the equal transit time
explanation.

But there is two main flaws in this explanation also:

1. It does not explain how stream tube pinching comes about, or why
it is greater over the upper surface than the lower surface. For
conventional wings that are flat on the bottom and curved on top
this makes some intuitive sense, but it does not explain how flat
plates, symmetric aerofoils, sailboat sails, or conventional aerofoils
flying upside down can generate lift, and attempts to calculate lift
based on the amount of constriction or obstruction do not predict
experimental results.

2. Conservation of mass is not a satisfying physical reason why the

flow would speed up. Really explaining why something speeds up
requires identifying the force that makes it accelerate.
 There is still another way where we can apply the laws of physics and
fluid mechanics correctly in order to understand this.

The particle approaches the aerofoil and follows a curved path and not a
straight one.

In order for this to happen and the particle follows a curved path there must
be a higher pressure are on the outer of the curve than in the inner part. And
so the air flow is always attached to the aerofoil.

The tendency for the fluid to do this is an effect called Coandă Effect
It’s known that far around the aerofoil the pressure is atmospheric and due to
what has been explained the pressure above the curved paths of the aerofoil
must be higher.
So as we move from up to down above the aerofoil the pressure decrease and
the opposite is correct. This difference in pressure creates the lift force.
Newton’s Third Law

An aerofoil generates lift by exerting a downward force on the air

as it flows past. According to Newton's third law, the air must exert
an equal and opposite (upward) force on the aerofoil, which is lift.

As the airflow approaches the aerofoil it is curving upwards but as

it passes the aerofoil it changes direction and follows a path that is
curved downwards.

According to Newton's second law, this change in flow direction

requires a downward force applied to the air by the aerofoil. Then
Newton's third law requires the air to exert an upward force on the
aerofoil; thus a reaction force, lift, is generated opposite to the
directional change.

In the case of an airplane wing, the wing exerts a downward force

on the air and the air exerts an upward force on the wing which
leads to lift.
Aerofoil Nomenclature

4 Digit

5 Digit