# Tag Info

28

The bigger the diameter, the lower the optimum speed. Generally, fan and propeller blades should run at a high dynamic pressure, but still at subsonic speeds to avoid the higher losses involved with supersonic flow. Since the tip will be the part with the highest speed, and propellers have bigger diameters than fans, propellers run at lower speeds. The high ...

26

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 ...

23

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 ...

14

The factors that are relevant here are: Increasing the bypass ratio is very desirable in order to improve the efficiency of the engine, and for the same amount of thrust, this means increasing the fan diameter. In order to keep the fan mostly subsonic, the fan rotational speed has to decrease with increasing fan diameters. Supersonic fan blade tips are ...

12

There it says "optimum rotational speed". Is that the speed of the compressor? The speed of the fan? Both. Each has its own optimal speed (that is not the same) and the gear allows them to work at that speed. Defining what "optimal" is, is part of the engineers job; usually "optimal" is what gives the maximum efficiency, but depending on the design ...

6

The purpose of the reduction gearbox is to improve engine efficiency. Normally in a turbofan engine the low speed turbine and fan are connected by a direct drive turbine shaft that requires the low turbine and fan to run at the same speed. In a geared engine, the gearbox allows both the fan and turbine to run at their optimum speeds. In this case, the ...

6

There are plans to run a turbofan-like engine with an electric motor rather than the gas generator used today. But the weight of electric engines does not promise an immediate advantage - only when the electricity is generated in a much more efficient way that what today's gas generators offer will the overall propulsion mass be lower. An example is a ...

5

The fundamental question you seem to be asking is whether adding more planets to a planetary gear improves the load carrying capability of the gearbox. The answer to that question is yes... to a point. Take a look at a picture of the PW geared turbofan gearbox. You'll note that there are five planets in there, and they are packed in pretty tightly. ...

4

To increase the propulsive efficiency (total efficiency is the product of gas turbine thermodynamic and propulsive efficiency) we need to accelerate more mass flow less fast: this requires larger fan diameters. See e.g. this answer which describes how the bypass air provides thrust. The formula:  F = \frac{\text{d}}{\text{d}t} p = m \frac{\text{d}}{\text{...

3

There are no off-centered turbofan engines in service today. However, United Technologies Corporation (UTC), has patented an engine with an angled core (Pratt & Whitney is a part of UTC). Image from US patent Gas turbine engine with separate core and propulsion unit US 8789354 B2 Air from the fan is ducted around the side and back of the core to ...

3

Grease is oil mixed with a thickener, usually a soap-type compound (an oil with a hydroxide termination that makes it solidify and also makes it hydrophilic). Adding grease to the recommended oil would change the viscosity, temperature/viscosity curve, reactivity, oxidation resistance, and so forth. If anyone noticed the change on a routine (pre-rebuild) ...

3

The premise of the question is wrong. Gearing has nothing to do with temperature limitation, rather speed limitation. Turbines are most efficient when the run fast, fans are most efficient when they run slow. In a normal engine, the fan and LPT are forced to run at the same speed. So there is always a tradeoff. Usually the LPT is forced to run much ...

2

The preceding explanations are good. But - interesting add-on is the notion that the gearing of a turbine is hardly a new concept - it's been around for decades in the form of turboprop engines. The Allison T56 is an example of a geared Turboprop engine. Simplistically, the difference between earlier turboprop designs and the more recent GTF design(...

2

The flow direction reverses only through the bypass duct pulling by blades with negative pitch. A portion of it will still be sucked by low pressure compressor making 180° turn into the core. Having to counteract the losses in turn will obviously reduce the performance. As stated in research paper by the team funded by Rolls-Royce plc: "the core mass flow ...

2

In case you're interested, US Patent 7,752,834 has what I was looking for.

2

Turbofan engines would benefit from a gearbox between the LP turbine and the fan, just like turboprops do. The problem is twofold: The scale of the gearbox. A B777 delivers over 100,000 hp of power (torque at the fan times rotational speed). The largest geared turbofan presently in production and installed on aircraft in regular service, the PW1000G, ...

1

Turboprop engines are (generally) powered by centrifugal or axial/centrifugal flow engines. These are smaller (again, generally) than an axial flow engine. This small size allows components to rotate at really, really, really ridiculous speeds, up to 50,000 rpm (maybe more). Spinning a propeller at even 5000 rpm is very inefficient for the few seconds the ...

1

In completion to @JulianHzg's anwser: Increasing the bypass ratio is very desirable in order to improve the efficiency of the engine, and for the same amount of thrust, this means increasing the fan diameter Increasing the bypass ratio is surely a way to reach a higher efficiency (and maybe the most developed topic at the moment in aircraft engines), but ...

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