59
votes
Why do new jet engines cost billions to design?
Even if we ignore the salaries of everyone involved; engineers, sales people, management, Q/A teams, manufacturing teams, more sales people, and then a few spare engineers...
I don't understand. If ...
- 100k
45
votes
Accepted
Do turbofan engine cowlings dilate during spool-up?
Now that's an interesting phenomenon! I do not believe that this is a case of an illusion of any sort, or the engine dilating. What the video probably shows, is the engine slightly turning towards the ...
- 27.2k
42
votes
Accepted
Are large jet turbofans so easy to spin?
Yes, large turbofans can be turned by hand without too much force. On smaller aircraft this is regularly done as part of the preflight walkaround. You can see an example in this YouTube video of an ...
- 54.3k
39
votes
Accepted
Why was the DC-9-80/MD-80 so successful despite being obsolete almost from birth?
It's simple. Cheap (development costs amortized decades before) and reliable. They made money for airlines. Or, you know, they wouldn't have bought them. There's more to operating costs than fuel ...
- 126k
38
votes
Accepted
Where is the generator in a large turbofan of a commercial airliner?
Image source
The generators are usually driven from the Accessory Gearbox. This CAD drawing is from the company that makes accessory gearboxes for the Rolls Royce Trent engines driving the A330 and ...
- 61.3k
36
votes
How long to clear the 'suck zone' of a turbofan after start is initiated?
A start on a TF all the way to idle is about 20-40 seconds depending on the engine, and the fan itself won't do more than creep a bit until the core actually lights off which is 5-10 seconds, so they'...
- 126k
35
votes
Accepted
Why do we refer to "power" for turboprop engines and "thrust" for turbojet/fan engines?
Ultimately what you want from all three types of engines is quantification of thrust available to push an airplane through the sky. The turbofan/jet engines are self contained and produce thrust ...
- 19k
35
votes
Why do new jet engines cost billions to design?
Jet engines are some of the most complex machines ever created. They have to be as light, efficient, safe, and reliable as possible. There's a reason that most new airliners recently have been ...
- 71.8k
34
votes
Accepted
In a turbofan what holds the spinning axis?
Stationary frames
The supporting structures of a turbofan are two stationary frames with their characteristic struts, located at the ends of the engine core: One is behind the OGV stator of the fan ...
- 71k
32
votes
Why do military turbofan engines use a low bypass ratio?
It's not military vs civilian, but subsonic vs supersonic-capable
Note that subsonic military aircraft use the same engines as civilian aircraft, even if their names might be different.
The KC-135 ...
- 230k
31
votes
Where is the generator in a large turbofan of a commercial airliner?
IDG Location: Dedicated pad on the accessory gearbox
If it's gonna be coaxial, the only place is the rear (it will be
burnt) or the front (it will get in the way of intake).
That's a good ...
- 71k
31
votes
Why are only some aircraft in this formation creating contrails?
From: Forum post - "Trails from the back of Fighter Jets", Metabunk.org
Jets practicing for air-shows (or actually in air-shows) sometimes use smoke to create trails. This is made by injecting ...
- 31.8k
31
votes
Is the bleed air passengers breathe tapped from the engines before or after it comes in contact with fuel? And why? Isn't this unhealthy?
The bleed will be tapped from one or two of the 3rd, 4th, 5th or 6th High Pressure Compressor stages. Usually there are two bleed ports. The highest pressure port will supply really hot air for ...
- 126k
31
votes
What is the difference between turbojet and turbofan engines?
Turbojets and turbofans are very similar indeed:
both are turbine engines;
both create thrust from jet exhaust;
and both have a rotating implement in front that can be called a fan. Although in the ...
- 61.3k
29
votes
What is the difference between turbojet and turbofan engines?
In a turbojet, all the air goes through the engine proper, through the combustion chamber and all the stages of compressor and post-combustion turbine blades.
In a turbofan, some of the air is just ...
- 855
28
votes
Accepted
Why are the fan blades mounted loosely?
The blades are loose in their "Fir Tree" blade mounts so that they can self balance. They are called "fir tree" because they are v shaped.
As the rotation speed and centrifugal force increases, ...
- 37.5k
27
votes
Accepted
How does a fan differ from a propeller?
The simple answer that covers the majority of engines is that a fan has a shroud. The possible exception are unducted fans or Open Rotor Engines, which are a hybrid between a turboprop and a fan ...
- 230k
27
votes
Why does the takeoff N1 limit start to decrease below 30°C OAT?
There are basically 3 limits that the engine faces, temperature (maximum turbine entry temperature or maximum combustor exit temperature or sometimes maximum first stage HPT stator exit temperature), ...
- 1,590
27
votes
Are large jet turbofans so easy to spin?
Turning the engine with one finger is not necessarily recommended, because the leading edges of the fan blades are quite sharp. But it is possible.
For some experimental vibration measurements, it is ...
- 3,010
25
votes
Can high-bypass turbofans cruise at high altitude?
Yes in principle, but some modifications are advisable.
What limits the maximum operating altitude of a jet engine (besides the thrust needed to climb up there) is the length of the combustion ...
- 230k
25
votes
Accepted
Why didn't early jetliners use turbofans?
Production turbofans just weren't available. Once they came on the market and were mass-produced, jetliners switched to them.
While turboprops did exist at the time, they were (and are) a turbine ...
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 ...
- 61.3k
24
votes
Why do we refer to "power" for turboprop engines and "thrust" for turbojet/fan engines?
Jet engines directly produce thrust by exhausting gas (and in a modern turbo fan also moving a lot of air around them), so fuel flow rate is directly related to the thrust that is generated.
In a ...
- 100k
23
votes
Accepted
Which jet engines have the highest thermal efficiency?
Thermal efficiencies are very rarely quoted for aviation gas turbines. The metrics of interest are specific fuel consumption, and power to weight ratio. While a higher thermal efficiency will increase ...
- 4,463
22
votes
Why are the fan blades mounted loosely?
I's like to expand the answers on how this could reduce vibrations, as it was mentioned in a comment.
Therefore, I'd like to look at something completely different: When computers were equipped with ...
- 7,227
20
votes
In a turbofan what holds the spinning axis?
Struts. Lots of them, and with an aerodynamic shape so they don't cause too much drag. Some of those struts double as stator vanes and reduce the swirl of the internal flow. By doing this they add a ...
- 230k
20
votes
Accepted
How does the bypass air provide thrust?
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 = \...
- 54.3k
20
votes
Can electricity be harvested from magnets in jet engines?
The power generation method you propose is used on almost all motorcycle and small lawnmower engines: magnets pressed into the rim of the flywheel, with stationary coils nearby in which current is ...
- 20.7k
Only top scored, non community-wiki answers of a minimum length are eligible