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Thrust (Gaya Dorong)

The document discusses the thrust equation for jet engines. It can be written as force equals mass flow rate times the change in velocity from intake to exhaust. This equation accounts for the momentum of incoming air. Net thrust is the usable thrust produced after subtracting the momentum of the airplane's incoming air. Additional thrust can come from higher exhaust pressure than ambient pressure. Factors like air density, velocity, bleed air extraction, and power extraction also affect the amount of thrust produced.

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87 views3 pages

Thrust (Gaya Dorong)

The document discusses the thrust equation for jet engines. It can be written as force equals mass flow rate times the change in velocity from intake to exhaust. This equation accounts for the momentum of incoming air. Net thrust is the usable thrust produced after subtracting the momentum of the airplane's incoming air. Additional thrust can come from higher exhaust pressure than ambient pressure. Factors like air density, velocity, bleed air extraction, and power extraction also affect the amount of thrust produced.

Uploaded by

rickosnals
Copyright
© Attribution Non-Commercial (BY-NC)
We take content rights seriously. If you suspect this is your content, claim it here.
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THRUST ( GAYA DORONG )

The Thrust Equation From Newtons second law: F = d(mv)/dt ( F= ma , for a constant mass ) In jet engine terms, we can re-write this as:

Where: F is force in pounds w is the gas flow in pounds per second g is the gravitational constant V1 is the initial velocity of the gas, in ft/sec V2 is the final velocity of the gas, in ft/sec Note that instead of using F = ma, we are re-writing the equation as force equals mass flow per unit time multiplied by velocity change. This difference is subtle but essential: Acceleration is not the same as velocity change. Thus force is not equal to mass times velocity change; acceleration is velocity change per unit of time. Using F = mass flow per unit time multiplied by velocity change is dimensionally correct. We can re-write the thrust equation to make it more meaningful in the context of a jet engine:

This is called the net thrust, because it accounts for the momentum of the incoming air; Gross thrust is given by the first term in the equation which is the force created at the exhaust of the engine. To compute usable thrust, the gross thrust has to be reduced by the amount of the second term, which is the momentum already existing because of the airplanes speed. From the equation, you can see that net thrust is a function of the mass flow rate of the air and fuel passing through the engine, and of the exhaust velocity minus the incoming velocity. Additional Thrust Due to Internal Pressure The thrust equation as written is somewhat simplified in that it ignores one more possible component of thrust. That component is thrust due to internal pressure. Most of the internal pressure within the engine is converted to velocity of the exhaust gasses, which in turn produces thrust. At the exhaust, if the total pressure of the gasses is greater than the total pressure at the intake, this surplus of pressure will produce some additional thrust. F = A(exhaust) * { P(exhaust) - P(ambient) } This component of thrust is small compared to the thrust due to exhaust velocity, but should not be ignored. Factors Affecting Thrust: Air density, a function of temperature and pressure altitude, is a very significant component affecting thrust.

Velocity affects both the momentum and the pressure of the air entering the engine intake. Increasing aircraft speed increases the momentum of the incoming air, lowering thrust, while at the same time compressing the air at the intake (ram effect) increasing thrust by increasing density. The combined effect is show below.

Other Factors Affecting Thrust Bleed air extraction affect thrust. Power extraction for hydraulic pumps, electric generators, fuel pumps, etc., affects thrust. Humidity has a negligible effect on thrust.

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