DRAG CLASS NOTES FOR MECHANICAL ENGINEERING
DRAG CLASS NOTES FOR
MECHANICAL ENGINEERING
Drag is a force of resistance when a body is
moving in water or air. In air, it is
aerodynamic drag. It opposes the thrust
force. It opposes forward motion.
Direction of drag is in opposite direction to
the moving object. Firstly drag is directly
proportional to the velocity of the fluid in
a laminar flow. Secondly drag is
proportional to the square of the velocity
in a turbulent flow. At high speeds, almost
50 % power is spent in overcoming drag.
Aerofoil design has been found the best
to reduce drag. It is used in various vehicles.
Factors on Which Drag Force Depends

Viscosity of the fluid (water or air)

Upstream velocity

Density of the fluid

Shape, size and contour of the moving object in the fluid
TYPES OF DRAG
Sr. No. 
Name of drag 
Cause of drag 
How to reduce the drag? 
1. 
Parasitic drag—It is of two typesForm drag or pressure drag 
Drag depends on the shape of the body moving in a fluid. For example, A body having larger cross section and blunt shape will have larger form drag and vice versa. 
Make the shape like an aerofoil to reduce drag 
2. 
Skin friction drag 
It is due to the surface roughness of the body moving n a fluid. 
Make the surface more smooth 
3. 
Profile drag 
Sum of skin friction drag and form drag is total drag. 

4. 
Interference drag 
Drag is due mixing different stream lines between aeroplane components such as for wings and fuselage at the wing. 
Install the different parts to have minimum interference 
5. 
Lift Induced drag 
There is a lift force in a flying aircraft. It keeps the aircraft in air. During the stable flight lift is equal to the weight of the aircraft. Drag produced due to lift is called lift induced drag. 

6. 
Wave drag 
The drag produced at sonic and super sonic speeds. These high speeds produce shock waves. These shock waves produce the wave drag. 
Try to keep speed below sonic speed 
MAGNITUDE OF THE DRAG FORCE
Drag force is proportional to the square of the velocity and given by the equation
F_{D} = C_{D}ρV^{2} A/2
Where F_{D }is the drag force
C_{D} is the drag coefficient
It depends on shape size and contour of the moving object. This also depends on Reynolds number. Drag coefficient is a dimensionless quantity
ρ is the density of the fluid
V is the relative velocity of the fluid
A is the projected surface area of the object perpendicular to the motion of the fluid
Drag force always opposes motion. It consumes power which is actually a waste. This power consumed is proportional to the cube of the relative velocity. Thus drag is very high at high speeds. It should be reduced to a minimum. It is achieved by adopting an aero foil design of objects moving in water or air.
TERMINAL VELOCITY
The velocity at which drag force becomes equal to the weight is called a terminal velocity.
F_{D} = mg
mg = C_{D}ρV^{2} A/2
Where
m is the mass of the moving object
C_{D is the drag coefficient}
ρ is the density of the fluid
V is the relative velocity
A is the surface area of the object normal to the fluid.
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