42 votes

Why do airplanes use an axial flow jet engine instead of a more compact centrifugal jet engine?

Centrifugal compressors only produce a more compact engine at low mass flow, which means low thrust. The amount of thrust an engine can produce is proportional to its intake area times exhaust ...
Therac's user avatar
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24 votes
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Why do large turbofans generally have many more LP turbine stages than HP turbine stages?

Because the LP turbine extracts power for the fan, which requires the most power. The HP and IP turbine only extract power for their connected compressors - the LP turbine extracts power for the fan ...
Koyovis's user avatar
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20 votes

Why do airplanes use an axial flow jet engine instead of a more compact centrifugal jet engine?

Axial turbine engines take up a lot of space...lengthwise. Centrifugal compressors are shorter and wider, and are very often used in turboprop and turboshaft engines, for instance the Rolls Royce Dart ...
Koyovis's user avatar
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20 votes
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Do modern jet engines need igniters?

Jet fuel will not self-ignite when starting a modern turbine engine. This article from the WingMag Aviation Magazine says: As the temperature isn’t quite sufficient to initiate self-ignition (the ...
Bianfable's user avatar
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19 votes
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Compressor-Turbine: Why a shaft and not kind of a "rotating drum"?

There are a few very practical reasons why turbine engines use inner shafts/spools. First, jet engine spools are very high speed rotating parts, this means they are subjected to high amounts of ...
OSUZorba's user avatar
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15 votes
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What is the function of the protruding outlet on the Trent XWB?

That is the drain mast and breather outlet combined. There's some discussion about this in airliners.net (not the same engine, but I sure they are similar): The exhaust from the air/oil separator ...
aeroalias's user avatar
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15 votes

Why do we consider that air comes into jet engine?

Both the air and the engine are moving very, very fast. Our galaxy and everything in it is moving at a blistering speed of several hundred miles per second. Of course, we don't feel this speed. ...
Chris's user avatar
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13 votes

Compressor-Turbine: Why a shaft and not kind of a "rotating drum"?

A rotating drum will have a higher inertia. When the pilot commands a thrust increase, spooling up the drum will take longer than spooling up a shaft. Also, a big drum is harder to balance well than a ...
Peter Kämpf's user avatar
13 votes

What engine are these fan blades from?

Turbine Blade The turbine blade (part number 1475M35P01) is from the High Pressure Turbine (HPT) a CFM56-3 according to this ATSB report on performance testing of the engine. The report contains ...
Notts90's user avatar
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12 votes

Function of the compressor in a gas turbine engine

The reason is thermodynamics: without the compressor, the power output of a gas turbine would be exactly zero. It's not about being "more efficient" but about fundamentally being able to ...
Sanchises's user avatar
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11 votes
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Is this image of Compressor aerodynamics correct?

@Federico reminder has complicated a bit the answering process, but now we understand your actual problem, I can provide a focused answer immediately below, I've moved other elements at the end, you ...
mins's user avatar
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11 votes

Why do we consider that air comes into jet engine?

Because it doesn't affect the math, and because it helps with keeping relative change in perspective if we use the engine as a frame of reference instead of the airmass. Also because you are incorrect ...
Michael Hall's user avatar
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10 votes
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Why were rotary vane compressors never used in jet engines?

The most important characteristic for a jet engine compressor is high mass flow rate with light weight and if possible small size. For the same size, the flow rate can be increased by operating the ...
Jan Hudec's user avatar
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10 votes

Do modern jet engines need igniters?

The autoignition temperature of kerosene is 428F. The temperature coming out of the last stage is usually above that at moderate power settings, up to 7-800 F or more at takeoff thrust. However the ...
John K's user avatar
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10 votes

How do gas turbine engines and turbofan engines keep constant compressor side rpm?

The reason that car piston engines and turbine engines seem to behave differently is that they're used in different ways. If ran at full throttle with no load, both types of engine would accelerate to ...
HiddenWindshield's user avatar
10 votes

How does a fan in a turbofan engine suck air in?

Starting with the marine prop example you used in the comment on John's answer. It creates high pressure behind the prop and low pressure in front of the prop, so the BOAT moves forward. The water is ...
TomMcW's user avatar
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9 votes

Why do large turbofans generally have many more LP turbine stages than HP turbine stages?

The answer provided by Daniel Kiracofe is correct, I just have a couple of additional comments and I don't have enough reputation to add this as a comment to his answer. On high bypass engines, the ...
John's user avatar
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8 votes
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Why a higher compressor pressure ratio results in higher Thrust and lower TSFC?

Because the constant pressure lines are diverging with increasing enthalpy. Have a look at the right most diagram. The two lines are constant pressure lines, with the higher line indicating the ...
ROIMaison's user avatar
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8 votes
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How does an ETOPS APU start in-flight?

Some aircraft may have a special "ETOPS APU" that can be purchased, but many aircraft simply have THE APU that they come with. It is maintained to the standards required in the approved ETOPS program, ...
Ralph J's user avatar
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8 votes

What engine are these fan blades from?

The part number on the turbine blade shows it comes from a CFM56-3 series (Boeing 737 classic). No hits on the compressor blade, but it looks similar to one from the same engine.
user71659's user avatar
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8 votes

How can flow in the compressor of a jet engine go back to the inlet?

Note that the flow speed past the diffusor (which sits between the compressor and the combustion chamber) is very low in order to allow air and fuel to mix, the fuel to evaporate and the mixture to ...
Peter Kämpf's user avatar
8 votes

Function of the compressor in a gas turbine engine

The compressor raises the pressure and temperature at which the fuel is burned, thereby increasing the thermodynamic efficiency of the cycle (Brayton) on which the jet operates. Second, without the ...
niels nielsen's user avatar
8 votes
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Function of the compressor in a gas turbine engine

You don't. A (sc)ramjet doesn't have a compressor, as the forward velocity of the jet provides the necessary pressure. Most thermal engines do work by expanding a gas. In many cases, this gas expands ...
Lawnmower Man's user avatar
7 votes

Why do large turbofans generally have many more LP turbine stages than HP turbine stages?

Here's an even more interesting question. Take a look at the P&W geared turbofan cross section (e.g. this article). It only has three LPT stages! So how come the GTF has 3 and all of the other ...
Daniel K's user avatar
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6 votes

Compressor-Turbine: Why a shaft and not kind of a "rotating drum"?

You seem to be describing an Exoskeletal Turbine design. I'm no expert, only vaguely associated with the industry. They're attractive in part because your rotor materials only need to be strong in ...
cakins's user avatar
  • 61
6 votes

How are fan and compressor discs balanced?

As a 25 year gas turbine veteran across many different engines, including engine strip and assembly, it goes like this - The discs and blades are balanced independently to start with. The disc/hub ...
Swampy's user avatar
  • 121
6 votes

What is the purpose of blades on a compressor?

The statement about the rotor blades ("the rotors increase the speed of the air and descrease the pressure") is not correct, because Bernoulli's Equation does not apply in this situation. Hence, the ...
Penguin's user avatar
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6 votes
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How do the compressor stages keep spinning after start in a free turbine?

In a C20-250 engine the compressor section is driven by the high pressure turbine (Ng spool, not N1) independent of the power turbine. The power turbine drives the gearbox and subsequently the output ...
Romeo_4808N's user avatar
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6 votes

Why does the Trent XWB have intermediate compressor stages?

Jet engine designers have to find compromises all the time, and the one that counts here is between lower complexity and higher efficiency. In the 1960s, Rolls-Royce fell behind in large commercial ...
Peter Kämpf's user avatar

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