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Questions tagged [turbofan]

A type of jet engine which uses two different airflow streams (one passing through the core and another blown past it by a fan) for obtaining thrust.

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58 votes
2 answers
237k views

What is the difference between a turbofan and a turboprop engine?

I'm not an aircraft expert and I just realized that there are two different terms - turbofan and turboprop. I always had them combined in my head as a term for a large jet engine (like you would see ...
David says Reinstate Monica's user avatar
40 votes
11 answers
18k views

Why do new jet engines cost billions to design?

This question might come off as crude because it involves people's salaries, but I do not understand why new designs of large turbofans cost many billions of dollars to design. For example, the Pratt &...
DrZ214's user avatar
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37 votes
5 answers
21k views

How does a fan differ from a propeller?

Why is the fan in a turbofan different than a propeller in a small airplane? Why do propellers typically have 2 or 3 blades, but the fan has a lot more? And why is the fan shrouded but the propeller ...
Southbob's user avatar
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34 votes
3 answers
7k views

How long to clear the 'suck zone' of a turbofan after start is initiated?

I recently found this (IMO funny) picture. All funny and all. But let's make this a real situation. Let's suppose I see my crush and his BF doing this, and I come with this 'genius' idea (alright, it'...
Hugo Woesthuis's user avatar
32 votes
4 answers
13k views

Why are the fan blades mounted loosely?

In this question it is mentioned that the fan, compressor and turbine blades are mounted loosely in the disc. This is well demonstrated by this video posted by RedGrittyBrick. What is the reason for ...
TomMcW's user avatar
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30 votes
4 answers
11k views

In a turbofan what holds the spinning axis?

When the turbofan is spinning inside the housing, what holds the axis? All I know is that the axis holds the fan blades.
XTImpossible's user avatar
29 votes
2 answers
6k views

Are large jet turbofans so easy to spin?

In this question, in the accepted answer, there is a comment by alephzero that says one can actually rotate a large turbofan's blades with one finger! Is that true, ...
George Menoutis's user avatar
28 votes
8 answers
7k views

Why do we refer to "power" for turboprop engines and "thrust" for turbojet/fan engines?

In jet engines, I read that thrust is related to the fuel flow rate, whereas in turboprop engines power produced is related to the fuel flow rate. What is the reason and brief math behind this?
user5349's user avatar
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26 votes
8 answers
56k views

What is the difference between turbojet and turbofan engines?

What is the difference between a turbojet engine and a turbofan engine? I know that both of them have a fan and have turbines but how to distinguish them?
Alex Furnival Krauss's user avatar
26 votes
1 answer
25k views

What provides the greatest thrust in a high-bypass turbofan engine?

I'll try to make as much sense as I can with this question. I've been flying quite often in 2015, and I have recently wondered this question regarding high-bypass turbofan engines. I know that there ...
magicmq's user avatar
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25 votes
3 answers
4k views

Why are turbofan inlets leaned/angled forward?

(Image Source: Wikipedia) Seeing that everything is radially symmetric (apart from the nacelle mount, naturally), I wonder why the inlet (the front part of the nacelle) is usually skewed?
Pavel's user avatar
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24 votes
2 answers
5k views

Why are only some aircraft in this formation creating contrails?

The Republic Day celebrations at New Delhi, India had a series of flypasts by the Indian Air Force. What struck me the most was that in a formation with a C-17 in middle and two Su-30MKI flanking it, ...
Kanchu's user avatar
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24 votes
2 answers
15k views

Where is the generator in a large turbofan of a commercial airliner?

So, where do you put the electrical generator? 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). If it's gonna be coupled to a ...
DrZ214's user avatar
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24 votes
2 answers
26k views

How does this vortex form inside a jet engine?

I have seen this in a picture once online and thought it was very strange and wondered about the formation of this vortex and how could it happen. Source Not everyday do you see this happening. So ...
Ethan's user avatar
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20 votes
6 answers
8k views

Do turbofan engine cowlings dilate during spool-up?

I was watching this Youtube video, portraying a Fokker 70 taking off. I wouldn't have noticed if it weren't for a comment, but during spool up (~ 3:24 in the video), it appears as if the engine ...
user avatar
20 votes
2 answers
26k views

Why do military turbofan engines use a low bypass ratio?

I know that the most civilian engines use a high bypass ratio which is good for fuel economy and noise reduction. What prevents military engines from using the same technology instead of opting for ...
Anarach's user avatar
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20 votes
2 answers
3k views

How do turbofan nacelle lengths compare?

For example, compare the JT9D and the PW4000 below (both from Pratt & Whitney, both on a 747): (Left, Right) The PW4000 has a longer forward nacelle, and a covered exhaust cone. On the inside—...
user avatar
19 votes
7 answers
58k views

Why does an aircraft as big as the Airbus A400M use turboprops?

Why is it that a plane as big as the C-17 (the A400M is slightly smaller though) use turboprops instead of turbofans? Both aircraft seem to have the same purpose and are both capable of landing at ...
Madhav Sudarshan's user avatar
19 votes
1 answer
20k views

Which jet engines have the highest thermal efficiency?

Some automakers (Mercedes-Benz, Toyota) have recently been making news about the improved thermal efficiency of their engines, stating that they reached the 50% mark. I was wondering if it was the ...
clueless007's user avatar
18 votes
6 answers
9k views

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

In almost every large turbofan I see, the LP turbine stages outnumber the HP turbine stages by a factor of at least 2. Here's a photo of the RR Trent 900: The Trent 900 (like most Rolls-Royce ...
DrZ214's user avatar
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17 votes
3 answers
6k views

Can high-bypass turbofans cruise at high altitude?

They have talked for years about re-engining the B-52 with something more modern than its TF33-P-3/JT3D turbofans, which are fairly low-bypass (1.42:1 for the JT3D). Modern engines are much higher ...
Harper - Reinstate Monica's user avatar
17 votes
3 answers
2k views

Would a drone strike be more dangerous than a bird strike?

Let's say you have a seagull and a drone of a comparable size. Pretend either one was going to be ingested by large turbo fan engine, like the one found on a 737 (eg., the CFM-56) Which would cause ...
Jae Carr's user avatar
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17 votes
1 answer
4k views

How much power is drawn by a turbofan to power its own compressor?

In a modern commercial airliner how much percentage of its engine power is used to power its own compressor, the ducted fan and other ancillaries like the airconditioning etc? Maybe it can't be ...
user avatar
16 votes
7 answers
7k views

Why was the DC-9-80/MD-80 so successful despite being obsolete almost from birth?

The McDonnell Douglas DC-9-80/MD-80 series of narrowbody jetliners was the second-to-last DC-9 major version produced (from 1979 through 1999), the second-largest, the most-produced (just under half ...
Vikki's user avatar
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16 votes
3 answers
12k views

How much electrical power does one generator produce on a large turbofan?

From my related question, now I want to know just how much power does the generator produce? In other words, what is the max rating of the generator? It looks so small and that surprised me, but since ...
DrZ214's user avatar
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15 votes
2 answers
2k views

Are the A380 engines interchangeable (given they are not all equipped with reverse)?

I have noticed that the A380 only uses the two engines closest to the fuselage to reverse thrust to brake. Are all four engines the same (i.e.: could they theoretically swap engine 1 and engine 2) or ...
Sam Creamer's user avatar
14 votes
2 answers
735 views

What are these painted lines on the fan blades of the Rolls-Royce Pearl 15?

I'm not sure that they are painted but it looks like they are. What are they for? More images:
Roh's user avatar
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14 votes
2 answers
15k views

Why do Western fighters' afterburner glow is reddish orange while Eastern Bloc fighters' is blue?

Recalling what little physics I know, this could be explained by two factors: The blue color is given by the characteristic EM radiation of C-H bond breaking, C- and H- radicals forming and ...
Meatball Princess's user avatar
13 votes
3 answers
4k views

Why didn't early jetliners use turbofans?

For subsonic operation, high-bypass turbofans seem to have essentially only advantages for efficiency, noise and takeoff thrust, at least until they reach the size where ground clearance or LP/fan ...
Talisker's user avatar
  • 629
13 votes
7 answers
6k views

Why wasn't the 737 MAX's landing gear lengthened enough to mount the engines under the wings, rather than in front of them?

The reason Boeing originally added the MCAS to the 737 MAX was to compensate for a slight pitch-up moment generated by the engine nacelles at very high angles of attack. This pitch-up moment occurred ...
Vikki's user avatar
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13 votes
2 answers
5k views

Why are the engines on the MAX and neo prone to bowing?

The engines of the 737 MAX$^1$ and Airbus A320neo family$^2$ are [more] prone to bowing (shafts deforming if no debow is applied during start – as I understand it, this happens in a turnaround while ...
user avatar
13 votes
2 answers
4k views

Why is the bypass ratio of a turboprop higher than turbofan?

Whenever, I come across turboprop, the author always mentions that they have higher bypass ratio than turbofan but no one explains the reason for this. So, is the bypass ratio higher due to higher ...
Selva's user avatar
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12 votes
1 answer
1k views

What drives and/or allows the move to higher-bypass turbofans on jetliners?

Note: I'm by no means an expert on any of this, I'm just curious. There seems to be a general trend toward higher and higher bypass ratios in commercial jetliner engines. (I'm thinking here of 737/...
Flambino's user avatar
  • 747
11 votes
2 answers
3k views

Why does the takeoff N1 limit start to decrease below 30°C OAT?

It's easy to understand why the takeoff N1 limit decreases with an increase in the OAT in the chart above; the engines get less dense air at higher temperatures, and less dense air means less engine ...
lemonincider's user avatar
  • 7,565
11 votes
5 answers
4k views

Which engine type requires the shortest take-off run: a turboprop or a turbofan?

If two fixed-wing aircraft almost identical except for the engines, one turbofan and one turboprop, which one would need less runway to take off?
Tom's user avatar
  • 319
11 votes
3 answers
5k views

At what point is a turbine powered ducted propeller considered a turbofan?

Turboprop Propellers can be ducted for extra thrust. What are the criteria for determining that the engine is no longer a ducted propeller design, but a turbofan? Does there have to be a type of ...
mckenzm's user avatar
  • 373
11 votes
4 answers
2k views

If the pitch of a bypass fan blade in a turbofan is fixed how does it manage to provide thrust over such a large speed range?

A fixed pitch propeller is only good for aircraft that do around 100kts because the angle of attack on the propeller decreases as the forward airspeed of the aircraft increases, limiting the thrust ...
Ian's user avatar
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10 votes
4 answers
4k views

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?

Passengers are often surprised to hear that the air they breathe comes from inside the engines. This sounds unhealthy. Can you explain, why it is not ? Is bleed air tapped (for cabin pressurization) ...
summerrain's user avatar
  • 3,474
10 votes
5 answers
16k views

What is the bypass air in a turbofan engine actually for?

I'm planning on building a small model turbofan engine for a bit of fun but I thought I'd better get a better understanding of how they work first. I understand the majority of it at the moment but I'...
mr-matt's user avatar
  • 457
10 votes
3 answers
3k views

Why do many large turbofans have a gap (of turbine blades) between the LP turbine and HP turbine?

After browsing around a while, I noticed that many...I think almost all...large turbofans have this gap between the LP turbine and HP turbine. I took some screenshots and put a red circle around the ...
DrZ214's user avatar
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10 votes
2 answers
7k views

What are the limitations in adoptions of GTF (Geared Turbofan) technology?

The only recent engine using GTF technology is the PW1000G, however some old designs like ALF502 or TFE731 use this technology. I understand the basic concept of the benefit obtained by the engine ...
Trebia Project.'s user avatar
10 votes
2 answers
6k views

Why is there a maximum altitude to restart a jet engine?

On all the turbojets that I have flown the maximum altitude a pilot can attempt a restart is 30,000 FT MSL. What factors go in to deciding what the maximum altitude is for an engine restart / relight?...
wbeard52's user avatar
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10 votes
1 answer
4k views

If the efficiency of a turbofan engine is 35%, where does the rest of the fuel energy go?

There have been a couple of questions on this site about the efficiency and propulsive power of turbine engines (here, here, here). In a high bypass turbofan engine, what are the losses in the ...
Koyovis's user avatar
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10 votes
1 answer
158 views

Is there any version of CFM56 with can combustors?

This lamp is sold online: They claim it's made from a CFM56 combustion chamber. It looks like a can combustor, and I read everywhere that CFM56 uses an annular combustor. Is there a version of CFM56 ...
Evariste's user avatar
  • 203
9 votes
7 answers
3k views

Why don't bypass ducts in jet engines narrow down the flow to accelerate it?

Since decreasing the flow area increases flow velocity, why don't turbofan bypass ducts narrow down to produce more thrust?
Francis L.'s user avatar
  • 2,514
9 votes
3 answers
6k views

How does the bypass air provide thrust?

To my knowledge, bypass air produces 80% of total thrust. But I don't understand to how it does that. By accelerating the air, by increasing the speed or increasing the pressure of the air? Is it ...
Yakup şeker's user avatar
9 votes
2 answers
3k views

Why is it inefficient to expel high speed gas?

I'm reading the wikipedia page on turbofans, where it says turbojets are inefficient at low speeds because: energy is wasted as the propelling jet is going much faster rearwards than the aircraft is ...
CaptainCodeman's user avatar
9 votes
3 answers
1k views

Why would a jet engine use cannular combustors if the annular design is superior?

Why are cannular type burners used in aircraft engines if annular type have more advantages over it?
user14618's user avatar
  • 113
9 votes
3 answers
2k views

Why was the DC-8-70 successful, but not the 707-700?

The first high-bypass turbofan small enough to use on a narrowbody jetliner was the GE/SNECMA CFM56, which entered service in the late 1970s as an upgrade engine for the KC-135 tanker; Boeing almost ...
Vikki's user avatar
  • 28.4k
9 votes
3 answers
2k views

Why do turbofan engines not have a contrarotating second fan?

Why aren't planetary gears used to have a fan in front of the fan in a turbo fan engine. Would this not lower energy consumption etc?
SRawes's user avatar
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