# Tag Info

23

There are basically 3 limits that the engine faces, temperature (maximum turbine entry temperature or maximum compressor exit temperature), pressure (maximum compressor exit pressure) and stress (maximum stress in the blades as a result of spool speed). Varying the OAT for a specific engine design will hit one of these limits. When the OAT increases, the ...

12

What you see is called a flat rated engine. It means the maximum thrust from the engine is constant below the flat rated temperature (usually 30°C). Above that temperature, thrust will decrease due to the EGT (exhaust gas temperature) limit. In order to achieve a constant thrust at lower temperatures, the N1 needs to be decreased accordingly. (CFM56-5A ...

6

The gearbox is located between the front fan and the rest of the engine. It lets the front fan spin at a lower rate than the main shaft. Image source: aerospaceamerica.aiaa.org The advantage of this setup is that the front fan can have longer blades to cover a larger cross-section area, while the low-pressure compressor and turbine blades in the core can ...

5

The bypass air is accelerated by the fan at the front of the turbofan engine. This changes its velocity and therefore its momentum, which is the definition of a force (in this case: thrust): $$F = \frac{\text{d}}{\text{d}t} p = m \frac{\text{d}}{\text{d}t} v = m \cdot a$$ This thrust contributes to the total thrust of the engine. How much will depend on ...

3

Assuming that the net thrust of a turbojet is constant is not correct. It is assumed to be constant (for simplicity by the aircraft performance engineers and usually valid for low subsonic speeds), but in reality, the performance is not constant, and it also varies with altitude. This is best shown by a simple simulation of a turbojet engine. The following ...

2

A bypass fan provides thrust in the same way a propeller provides thrust: by increasing the energy content of the gas mass passing through the disk. The added energy is most effectively converted into thrust by allowing it to expand until internal pressure is equal to ambient pressure, so all added energy is converted into kinetic energy. This expansion ...

2

First, you should state whether you are designing an engine (increasing the TIT) or if you are doing an off design calculation for an existing engine. In the case of design we gas turbine performance engineers call this design point analysis, increasing the TIT indeed would increase the efficiency, but this would require an increase of the pressure (OPR, ...

2

The problem is that in bypass engines, the fan blades are much longer than the turbine blades that drive them. Both compressors and turbines should rotate as fast as possible, without shock waves occurring at the tip - so linear tip speed has an upper limit, meaning the compressor with longer blades must turn much slower than the turbine driving it. This is ...

1

From what I've read, bigger props (and fans) are more efficient, because they move more air. However, at high subsonic speeds, the tips of props start to run into the sound barrier. So, that seems to argue for more smaller props/fans, whose ends won't hit the sound barrier. As others have mentioned, extra crankshafts and gears cost weight. I think I've ...

1

There are several effects which in combination make constant thrust a good approximation at subsonic speed. Thrust is created by accelerating a working mass in opposite direction. Net thrust is the difference between the impulse of the air flowing towards the engine and the combined impulse of burnt fuel and the air exiting the engine (and propeller, if one ...

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