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As aircraft speed increases, thrust tends to decrease somewhat; as the aircraft speed reaches a certain point, ram recovery compensates for the losses caused by the increases in speed. The inlet must be able to recover as much of the total pressure of the free airstream as possible. As air molecules are trapped and begin to be compressed in the inlet, much of the pressure loss is recovered. This added pressure at the inlet of the engine increases the pressure and airflow to the engine. This is known as “ram recovery” or “total pressure recovery.” The inlet duct must uniformly deliver air to the compressor inlet with as little turbulence and pressure variation as possible. The engine inlet duct must also hold the drag effect on the aircraft to a minimum.

Source: https://www.flight-mechanic.com/turbine-engine-inlet-systems/

But how the thrust decreases, and how Ram pressure increases?

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  • $\begingroup$ FYI, on the same site, this other page provides additional details (last third), e.g. this graph. $\endgroup$
    – mins
    Nov 10, 2021 at 12:38

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The thrust ($T$) of an engine is $T = Q(V_{out}-V_{in})$ where $Q$ is the mass flow through the engine, $V_{out}$ is the exit velocity of the exhaust gas and $V_{in}$ is the speed at which the air flow enters the engine inlet.

The mass flow $Q$ is a product of volume of the air sucked by the engine and air density.

When engine is producing considerable thrust, the exhaust speed ($V_{out}$) is the speed of sound. So for the sake of simplicity, we can consider that more or less a constant value. When aircraft is stationary, like at the beginning of the takeoff roll, $V_{in}$ is zero. As the aircraft begins to accelerate, $V_{in}$ increases.

Air is compressible gas, but at low speeds compressibility is negligible. As the forward speed increases, the $Q$ is constant for a given engine RPM, the $V_{out}$ is constant and $V_{in}$ increases, the net result is that engine begins to lose thrust.

But as the speed increases past about Mach 0.2, which equals approximately 130kts at sea level at standard day conditions, the air being packed (rammed) in the engine inlet begins to compress due to increasing pressure. That means that the density of air in the inlet increases. As density increases, and if we assume constant RPM (and thus volume) the mass of the air flowing through the engine increases.

At some point the increased mass flow $Q$ has compensated the loss of velocity change in the engine and thrust has reached its stationary engine value. This is the ram recovery point.

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In class, we haven't talked about it as a specific recovery point. But as a gradual increase in thrust based on airspeed.

Q: How does the thrust decrease?

Airspeed thrust decreases as the altitude increases. (High altitude = low pressure/less air)

less air = less thrust

Q: How does ram pressure increase?

Ram pressure increases when you fly into the air molecules fast. This happens because you push the air molecules together.

Q: What is the ram recovery point?

I had a chat about this with my turbine engine teacher, which changed my understanding on the subject.

Though you could see the ram recovery point as the moment the airspeed effect on thrust and and the ram effect on thrust are equal.

Here is a graph showing the result of ram recovery speed, and the point where the forces are equal.

(From the Dale Crane part 66 Powerplant book).

You reach the ram recovery point as soon as the inlet pressure (ram pressure) is greater than the ambient pressure.

In an idle engine run on the ground, the engine has to suck in air. As soon as the engine doesn't need to suck, and air is forced into the inlet - you have reached the ram recovery point.

Ram Recovery

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