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 ...
- 22.6k
24
votes
Accepted
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 ...
- 61.2k
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 ...
- 61.2k
20
votes
Accepted
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 ...
- 53.5k
19
votes
Accepted
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 ...
- 2,599
15
votes
Accepted
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 ...
- 99.8k
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. ...
- 10.8k
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 ...
- 228k
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 ...
- 3,559
11
votes
Accepted
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 ...
- 69.7k
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 ...
- 24.1k
10
votes
Accepted
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 ...
- 56.2k
10
votes
Why do turbofan engines have low pressure compressors?
The low pressure compressor does not reduce pressure. It increases it, but operates at a lower pressure than the high pressure compressor. Some engines also have an intermediate pressure (IP) stage.
...
- 31.1k
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 ...
- 125k
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 ...
- 12.8k
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 ...
- 28.3k
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 ...
- 91
9
votes
Why do turbofan engines have low pressure compressors?
Early jet engines had only one set of compressor disks which were sitting on the same shaft and spinning at the same speed. More modern designs use two or even three concentric spools, each spinning ...
- 228k
8
votes
Accepted
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 ...
- 6,567
8
votes
Accepted
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, ...
- 48.5k
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.
- 5,890
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 ...
- 228k
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 ...
- 5,463
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 ...
- 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 ...
- 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 ...
- 4,463
6
votes
Accepted
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 ...
- 71.9k
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 ...
- 228k
6
votes
What type of bearings are used to support the shafts in jet engines?
Rolling bearings
I would like to know which types of bearings are commonly used to support the shafts in jet engines and why
Rolling bearings are mostly of two types, depending on what's rolling: ...
- 69.7k
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