Questions tagged [jet-engine]

Engines which propel aircraft via "jet propulsion", i.e. by expelling high-velocity gasses (usually primarily air) through propelling nozzles.

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63
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10answers
74k views

Why do jet engines get better fuel efficiency at high altitudes?

I'm told that this is true, but I can't imagine why. It seems like the fact that there is less air would make the engines less efficient... But that probably just shows how little I know about jet ...
57
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4answers
27k views

Why can't jet engines operate with supersonic air and how do they slow it down?

Typically jets cannot operate when intake airflow is supersonic relative to the engine. Why is this so? Also, why are scramjets able to use supersonic air? To slow down the air to subsonic speeds, ...
35
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4answers
13k views

Why do gases in the combustion chamber only flow one direction to the gas turbine in a jet engine?

As far as I know from the working principle of jets engines, compressed air in the combustion chamber (or combustion canister) is mixed with fuel. The ignited mixture expands backwards to turn the ...
25
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3answers
12k views

Why are propeller engines uncommon on airliners?

My experience in aviation is essentially zero, but looking at Wikipedia it seems like the Tu-95 Bear offers high subsonic speed and extreme range. I assume that turboprop engines are more fuel ...
34
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4answers
25k views

How does the mounting location of a jet engine affect aircraft performance?

Different jet airplanes mount the engines in different ways. For example: Under the wing To the fuselage How does the engine location affect aircraft performance? Is one better than the other?
63
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3answers
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Are we at peak speed efficiency for jet airliners at Mach 0.85?

The cruising speed of large jet airliners has not increased in the past four decades. The 747 cruised at Mach 0.85 and the new Dreamliner 787 also cruises at Mach 0.85 even though it was designed 40 ...
40
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5answers
91k views

What are the differences between fuel types (comparing with vehicles)?

I find it funny that at gas stations, generally you can purchase "regular" gasoline (in various octane levels) or diesel. At airports, generally you can purchase 100LL avgas or Jet-A. Can you compare ...
28
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3answers
74k views

What material is used to make the hot sections of jet engines?

What metal alloy is used for the hot section of a jet engine? Specifically the exhaust nozzle? I have found mention of a titanium alloy and nickel alloy but nothing with that much detail. What sort of ...
23
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7answers
22k views

Why are adjustable area jet engine nozzles mostly limited to military use?

It seems that only jet engines with afterburners use adjustable area nozzles (this means adjusting the area of the nozzle but not necessarily the direction). This includes most fighter jets, as well ...
14
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1answer
6k views

What is the purpose of the inlet doors of the Pratt & Whitney JT3D?

I have a question about the 707 Engine Inlet Doors, according to Wikipedia the Pratt & Whitney JT3D inlet doors purpose is to provide additional air. Is the thrust higher on more air intake? And ...
60
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4answers
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Why does the Boeing 787 engine nacelle exhaust have such an unusual shape?

Why does the Boeing 787 nacelle have this unique shape at the exhaust: I haven't seen it on any other airplanes, does it serve an aerodynamic purpose or help engine efficiency or is it just to look ...
47
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3answers
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Why are the Boeing 737 nacelles odd shaped?

Why is a Boeing 737 NG engine not completely round in shape? It seems to be flat at the bottom and round at the top and sides. Earlier Boeing 737 versions (100 and 200) seem to have a more rounded ...
15
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1answer
5k views

What are the merits of an inlet cone/spike/centerbody vs. a 2D inlet?

And which is better for a scramjet? Is a 2D inlet basically a "2D cone"? Similar to how an aerospike engine is made "2D" in the linear aerospike? What's inside the cone/spike? Or is it just hollow?
23
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6answers
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In what way are the Concorde's engines considered efficient?

The Concorde is powered by a bunch of Rolls-Royce/Snecma Olympus 593 turbojet engines. Much has been said about how efficient they are at speed, and the fact that they allow long-range cruise at Mach ...
26
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6answers
67k views

How are turbine engines started?

What happens to a turbine engine from the time that a start is initiated until stable idle is achieved? What can go wrong?
26
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3answers
56k views

How does thrust reversal work on a jet engine, and when should it be used?

A two part question this time, though they are closely related enough that I figured I'd knock them out all at once. I figure the first question probably informs the second question anyway. Firstly, ...
43
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3answers
126k views

What are N1 and N2?

When I'm reading something about jet engines, it's common to see references to N1 and N2. I've never fully understood what N1 and N2 are on engine instruments and how they relate to thrust and the ...
9
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2answers
3k views

Why do engine cowl lips appear larger and more blunt on the sides than on the top and bottom?

I was looking into engine nacelles. I was not able to understand the design shape of it Why is it that in most aircraft, the engine cowl lips on the side are bigger (more blunt) than on the top and ...
12
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1answer
963 views

Why is the combustor volume of a jet engine decided by altitude?

I have read that the combustor volume of a jet engine is decided by the maximum altitude up to which that aircraft will fly. I'm getting confused by that, because the maximum mass flow rate will occur ...
67
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6answers
107k views

Why do jet engines use kerosene rather than gasoline?

Could you run a jet with gasoline? Why do all jet engines use kerosene?
29
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2answers
78k views

Which engine is more efficient between turboprop vs jet?

Are turboprops really more efficient than jets? If yes, does that mean that airlines who use jets sacrifice fuel for faster flights?
26
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5answers
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Why do we use pylons to mount the engines on jetliners?

This question focus on jet airliners. The De Havilland Comet's engine were mounted inside the wing, the Concorde's engines were mounted underwing (as many jet engines) but without pylon. It seems jet ...
38
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4answers
18k views

Do turbine engines on multi-engine aircraft rotate in opposite directions to offset torque?

I'm not an aviator, I guess I'd call myself an enthusiast. I'm also a physics nut. I've always wondered about this but I equally worried my question posed to a commercial airline crew would land me in ...
11
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2answers
4k views

Why is the compressed air (from engine compressors) taken from High Pressure (HP) stage instead of Low Pressure (LP) stage?

An aircraft that uses Turbofan or Turboprop engines would be taking the compressed air from 3rd stage and 6th stage (varies) and supplying it to the Pressurisation and Air Conditioning Kit (PACK). ...
8
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2answers
4k views

How do you choose engine placement for under wings engines?

I'd like to know how you choose the location of the engines under the wings, in the case of a 4-engines aircraft with low wings (and therefore with engines under the wings). Concerning the ...
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5answers
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How are off-wing jet engines transported?

One of the optional services in Rolls-Royce TotalCare is "Engine Transportation" This service includes routine transportation of engines between the customer’s main base (or designated station) and ...
51
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2answers
202k 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 ...
24
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4answers
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Why do turbofan blade tips exceed the speed of sound while prop tips should not?

It seems that the tips of the fan of a turbofan engine can and do break the sound barrier: The GE-90 has a fan diameter of 3124 mm and a rotational speed of 3475 RPM. Their circumferential velocity ...
12
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2answers
3k views

What prevent jet engines design with bigger by-pass ratio?

In commercial aviation, it seems jet engines grow and by-pass ratio increase with newer engines. Here is some examples: 1963: by-pass ratio: 0.96 (P&W JT8D) 1970: by-pass ratio: 5.0 (P&W JT9D)...
43
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2answers
24k views

What is this flange on the engine?

Flying home from the Denver ComiCon, on a 737, I saw an angled flange on the engine. Could you describe its purpose? Photos by CGCampbell licensed CC0
35
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4answers
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Have jet engines ever “Sheared” off?

I have read that jet engines, at least the under-wing ones, are attached to the airplane by shear nuts and bolts so that in case of unstable forces in or on the engine, the engine would fall off to ...
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4answers
3k views

How do jet engines work?

I was doing research about jet engines, and they seem really difficult to fully understand. So, can anyone explain it in a simple way? An afterburner glows on an F-15 Eagle engine, source: Wikimedia. ...
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5answers
6k views

Can a convergent nozzle be used for a supersonic jet engine?

Can a supersonic fighter incorporate a convergent nozzle or does it always have to be a convergent-divergent nozzle?
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3answers
9k views

How is combustion flame maintained in the combustion chamber after igniters are switched off?

During start phase of a jet engine, igniters in the combustion chamber create sparks to initiate combustion. As far I know, when the engine reaches self sustaining speed, igniters get turned off (...
7
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1answer
4k views

How is bleed air used to start a jet engine?

I know that most jet engines on commercial jets are started either with bleed air from an APU or bleed air from the other engine... But I'm not exactly sure how bleed air actually starts the engine? ...
14
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2answers
4k views

Why is max endurance different for jet and props?

Max endurance speed is not the same for gliders, propeller planes (also for turbo prop, right?) and jet planes. While for for jet planes max endurance is at the minimum drag speed ($V_{md}$), for ...
19
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2answers
1k 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—...
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2answers
3k views

How does contrail formation differ from turbofan to turbojet?

Air forces and intelligence agencies are indeed interested in predicting whether condensation trails (contrails) will be visible or not during aircraft operations. Photo: C-CYOW at Photobucket Among ...
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3answers
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Why are twin engine airliners more fuel efficient than three or four engine airliners?

It is said that given roughly similar aerodynamic and engine technology that twin engine airliners are more fuel efficient than 3 or 4 engine airliners. Why is this? The twin engine airliners tend ...
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3answers
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How does turbofan engine performance depend on speed and density altutude?

I am looking at a simulation engine (jsbsim). Their turbine engine simulation requires a function, given by table, of thrust at Mach number and density altitude. Engine definitions generated by their ...
36
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3answers
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How do aircraft keep fuel from freezing while in flight?

On aircraft which fly above 30,000 feet, the temperature can often can get to temperatures below Jet-A's freezing point. For example: at 36,000 feet the standard outside air temperature is -56.5°C (-...
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1answer
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Why do DC-9 and MD-82 have engines that are slightly angled upwards?

As per subject, I see that the above mentioned aircraft and some others that have engines mounted on the rear of the fuselage, have the engine intakes tilted slightly upwards. Why has this solution ...
16
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3answers
2k views

Why are modern regional jet designs shifting towards under wing engines?

Historically regional jets (eg, MD-95/B717, CRJ-700/900, ERJ-145, etc) have had their engines mounted near the rear of the fuselage. But, recently, regional jets are increasingly using an under wing ...
23
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5answers
8k views

Could most commercial jet engines today run on ethanol without any problems?

Most commercial jet engines today would be those large jet engines on large passenger craft like a Boeing 747. Can these run on straight ethanol without any problems? Would more maintenance be ...
14
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2answers
15k views

Why are turbine blades not made out of titanium, only compressor blades?

According to this video on jet engine blades, titanium is never used for the turbine, because it "melts and burns at the temperature of the flame". Only the blades of the compressor are made of this ...
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3answers
7k views

What is 2D and 3D thrust vectoring?

How do we define 2D and 3D thrust vectoring? What are the differences between the two? Is there anything called 1 D thrust vectoring?
29
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3answers
15k views

Why do turbine engines take so long to spool up?

Piston engines reach full rpm within a second or two, but turbines take much longer. Why is that?
31
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1answer
8k views

Why was there a rattling noise coming from the left engine of A319?

I recently flew on an Airbus A319. When boarding the aircraft from the front entrance we passed in front of the left engine. The fan blades were spinning and there was a rattling noise (like when a ...
24
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4answers
7k views

If jets carried pure O2 to use for combustion, would they be more efficient?

Air is around 21% Oxygen. The bulk of air is non-flammable Nitrogen. Wouldn't surrounding the jet fuel with only O2 create more efficient and clean combustion?
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2answers
8k 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 ...