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 the fan) for obtaining thrust.

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35
votes
3answers
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'...
31
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4answers
10k 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 ...
30
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5answers
10k 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 ...
27
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4answers
6k 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.
26
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6answers
4k 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?
26
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1answer
20k 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 ...
25
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3answers
2k 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?
21
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2answers
4k 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, ...
20
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8answers
13k 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?
20
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2answers
9k 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 ...
20
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6answers
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 ...
19
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2answers
14k 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 ...
19
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2answers
2k 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—...
18
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6answers
5k 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 ...
17
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7answers
39k 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 ...
17
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3answers
4k 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 ...
17
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3answers
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 ...
17
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2answers
1k 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 ...
16
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7answers
6k 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 ...
16
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3answers
8k 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 ...
16
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2answers
19k 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 ...
16
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1answer
2k 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 ...
14
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1answer
10k 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 ...
13
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3answers
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 ...
12
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1answer
748 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/...
12
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2answers
2k 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 ...
11
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2answers
2k 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 ...
11
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7answers
1k 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 ...
11
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3answers
2k 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 ...
10
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5answers
9k 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'...
10
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4answers
3k 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?
10
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2answers
3k 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?...
10
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1answer
476 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 ...
9
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3answers
3k 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) ...
9
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2answers
1k 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 ...
9
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2answers
6k 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 ...
9
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1answer
1k views

Why does the CF6/CFM56 Turbofan Fan Frame contain 12 circular holes between the primary and secondary flow?

This question is related to the answer I found on this particular thread, about the Frames in a Turbofan. In a turbofan what holds the spinning axis? I went on this thread after some research on ...
9
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4answers
989 views

How does an operator change the engine thrust rating?

If, for example, an operator has a Boeing 737-800 with CFM56-7B engines set at a 22K thrust rating, but the next operator wanted to operate the engines at 24K thrust rating, what would they need to ...
9
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4answers
885 views

Could a turbofan have two adjacent fans?

Is there any a point where it becomes possible/ideal to run two large front fans (side by side) powered from a 'single' core? Otherwise shoot this to pieces and explain the ways this is a bad idea. ...
9
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1answer
1k views

How does the efficiency of a turbofan compare between take off speeds and optimal conditions?

I know turbofans are less efficient than propellers due to accelerating a smaller mass of air faster. So my question here is not comparing turbofans vs other engines, but compare how inefficient is a ...
8
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3answers
3k 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 ...
8
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2answers
978 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?
8
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2answers
1k 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 ...
8
votes
1answer
484 views

What prevents miltiary jets and turbo-props from being refitted with a turbofan?

What prevents military jets and turbo-props (such as P-3 Orion) from being refitted with a high bypass turbofan?
8
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1answer
697 views

Why does the CF6 have a lower bypass ratio than the TF39?

The General Electric TF39, which powered the original C-5 Galaxy, was the very first high-bypass turbofan brought into mass production, with a previously unheard-of 8:1 bypass ratio. Yet, when it ...
8
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1answer
1k views

Why do some turbofan blade have forward swept tips?

I notice that in Trent700 engine fan blade there is no curve extension at the tip unlike in other engine's fan blades such as the GE90 ,T900 etc. What are the purpose of it? Trent 700 without ...
8
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3answers
456 views

Why is thrust available constant with speed for turbojet engines, when it varies with speed for turboprop engines?

Similarly, why is power available (nearly) constant with speed for a propeller engine, while it varies for a jet engine?
7
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3answers
3k views

Is it possible to have an afterburner in a turbofan with bypass ratio greater than 1:1?

Is it possible to have an afterburner in a turbofan with a bypass ratio greater than 1:1? At 1.05:1, say, can it have an afterburner?
7
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3answers
650 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?
7
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2answers
1k views

Gear reduction on large turbofans

Do engines like the GE90 and Trent 1000 use transmissions (planetary gears?) for RPM reduction on the fans, or are they welded directly to the same shaft as the low pressure turbine?