A jet-engine thrust reverser to reduce an aircraft speed of 200 km / h after landing employs fol

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A jet-engine thrust reverser to reduce an aircraft speed of...

A jet-engine thrust reverser to reduce an aircraft speed of $200 \mathrm{km} / \mathrm{h}$ after landing employs folding vanes which deflect the exhaust gases in the direction indicated. If the engine is consuming $50 \mathrm{kg}$ of air and 0.65 kg of fuel per second, calculate the braking thrust as a fraction $n$ of the engine thrust without the deflector vanes. The exhaust gases have a velocity of $650 \mathrm{m} / \mathrm{s}$ relative to the nozzle.

Key Concepts

Thrust Reversal Mechanism

A thrust reverser, such as one using folding vanes, redirects the high-velocity exhaust gases to produce a force that opposes the forward motion of the aircraft. By deflecting the exhaust, the momentum vector is altered so that a portion of the engine's output now acts in the opposite direction, creating a braking force. Evaluating the effectiveness of this mechanism involves comparing the reversed (braking) force to the original engine thrust.

Jet Engine Thrust Generation

Jet engines produce thrust by accelerating mass of air mixed with fuel to high speeds. The generated thrust is fundamentally the product of the mass flow rate and the exhaust velocity of the gases. This principle is applied in both the standard forward thrust nozzle operation and in thrust reversal situations where the momentum is redirected to produce a braking force.

Mass Flow Rate

The mass flow rate is the amount of mass passing through a section per unit time. In jet propulsion, both the air and fuel mass flow rates combine to determine the total mass being expelled, directly influencing the generated thrust. Accurate accounting of these flow rates is critical when calculating forces, particularly when modifications like thrust reversal are applied.

Conservation of Momentum

This principle states that the momentum of a closed system remains constant if no external forces act on it. In the context of jet engines, it is used to relate the mass and velocity of the expelled gases to the force (thrust) produced. When propellant is expelled at high velocity, an equal and opposite reaction force is generated, which propels the aircraft forward or, in the case of thrust reversal, provides a braking force.

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