If the net heat rate increases, what happens to overall propulsion efficiency?

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Multiple Choice

If the net heat rate increases, what happens to overall propulsion efficiency?

Propulsion efficiency is the ratio of useful shaft power delivered to the propeller to the rate of energy input from fuel (the net heat rate). When the net heat rate increases, more energy is being supplied per unit time, but if the shaft power doesn’t increase by the same amount, the portion of energy converted into useful work shrinks. In other words, more of the fuel energy becomes heat losses rather than shaft work, so the overall propulsion efficiency falls. For example, doubling the energy input without a proportional rise in shaft power reduces the efficiency accordingly. The other options imply no change, an increase, or unpredictability, which doesn’t align with the simple ratio relationship between shaft power and energy input.

Propulsion efficiency is the ratio of useful shaft power delivered to the propeller to the rate of energy input from fuel (the net heat rate). When the net heat rate increases, more energy is being supplied per unit time, but if the shaft power doesn’t increase by the same amount, the portion of energy converted into useful work shrinks. In other words, more of the fuel energy becomes heat losses rather than shaft work, so the overall propulsion efficiency falls. For example, doubling the energy input without a proportional rise in shaft power reduces the efficiency accordingly. The other options imply no change, an increase, or unpredictability, which doesn’t align with the simple ratio relationship between shaft power and energy input.

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