Thrust calculation:
The amount of thrust produced depends upon two things:-
The MASS of air which is moved rearwards in a given time, the ACCELERATION imparted to the air.It can be expressed as:-
Thrust = Mass x Acceleration
The MASS is defined as "the quantity of matter in a body".It is expressed as :- W/g
Where:-
W = the weight of the body (in lb's or newtons) and
g = the gravitational constant (taken as 32 ft/sec/sec or 9.81 m/sec^{2} )
The ACCELERATION imparted to the air is the difference between its inlet and outlet velocity.If we let: -
V_{2} = the air velocity at exit (in ft/sec/sec or 9.81m/sec^{2} )
V_{1} = the air velocity at inlet (in ft/sec/sec or 9.81m/sec^{2} )
It may be expressed as V_{2} - V_{1}
Taking these expressions for Mass and Acceleration, the thrust produced by an engine or propeller can be calculated from the following formula:-
THRUST = W/g( V_{2 } - V_{1 } )
Example 1.
The airflow through a propeller is 256 lbs/sec, Inlet velocity 0 ft/sec, outlet velocity 700 ft/sec.Thrust developed will be:
Solution:-
THRUST = W/g( V_{2} - V_{1} )
THRUST = 256/32 x (700 - 0)
= 5600 lbs