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Also, a force of 10 pound accelerates a mass of 10 pound at the rate of 32.2 ft/sec$^{2}$. This is assuming there is no friction or other resistance to overcome. It is now apparent that the ratio of the force (in pounds) is to the mass (in pounds) as the acceleration in ft/sec$^{2}$ is to 32.2. Using M to represent the mass in pounds, the formula may be expressed thus: 

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where: 

F = force M = mass A = acceleration G = gravity 

In any formula involving work, the time factor must be considered. It is convenient to have all time factors in equivalent units (i.e., seconds, minutes, or hours). In calculating jet thrust, the term “pounds of air per second” is convenient, since the second is the same unit of time used for the force of gravity. 

## Thrust 

Using the formula below, compute the force necessary to accelerate a mass of 50 pounds by 100 ft/sec$^{2}$. 

This illustrates that if the velocity mass per second is increased by 100, the resulting thrust is 155 pounds. 

Since the turbojet engine accelerates air, the following formula can be used to determine jet thrust: 

where: 

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V$_{2}$ = jet velocity (exhaust) 

V$_{2}$ – V$_{1}$ = change in velocity; difference between inlet velocity and jet velocity 

G = Acceleration of gravity or 32.2 ft/sec 2 

As an example, to use the formula for changing the velocity of 100 pounds of mass airflow per second from 600 ft/sec to 800 ft/sec, the formula can be applied as follows: 

As shown by the formula, if the mass airflow per second and the difference in the velocity of the air from the intake to the exhaust are known, it is easy to compute the force necessary to produce the change in the velocity. Therefore, the thrust of the engine must be equal to the force required to accelerate the air mass through the engine. Then, by using the symbol “Fn” for thrust pounds, the formula becomes: 

Thrust of a gas turbine engine can be increased by two methods: increasing the mass flow of air through the engine or increasing the gas velocity. If the velocity of the turbojet engine remains constant with respect to the aircraft, the thrust decreases if the speed of the aircraft is increased. This is because V$_{1}$ increases in value. This does not present a serious problem, however, because as the aircraft speed increases, more air enters the engine, and jet velocity increases. The resultant net thrust is almost constant with increased airspeed. 

The Brayton cycle is the name given to the thermodynamic cycle of a gas turbine engine to produce thrust. This is a variable volume constant-pressure cycle of events and is commonly called the constant-pressure cycle. A more recent term is “continuous combustion cycle.” The four continuous and constant events are intake, compression, expansion (includes power), and exhaust. These cycles are discussed as they apply to a gas-turbine engine. In the intake cycle, air enters at ambient pressure and a constant volume. It leaves the intake at an increased pressure and a decrease in volume. At the compressor section, air is received from the intake at an increased pressure, slightly above ambient, and a slight decrease in volume. Air enters the compressor where it is compressed. It leaves the compressor with a large increase in 

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