Stack Exchange Network

Stack Exchange network consists of 174 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.

Visit Stack Exchange

Questions tagged [jet-engine]

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

61
votes
10answers
69k 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 ...
59
votes
6answers
94k views

Why do jet engines use kerosene rather than gasoline?

Could you run a jet with gasoline? Why do all jet engines use kerosene?
58
votes
4answers
46k views

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 ...
56
votes
3answers
11k views

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 ...
54
votes
4answers
24k 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, ...
48
votes
1answer
8k views

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 ...
47
votes
2answers
180k 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 ...
47
votes
3answers
30k views

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 ...
45
votes
2answers
8k views

How can the landing gear fit in the small room in the engine bay on the Tupolev Tu-144?

I recently went to the museum in Sinsheim, Germany, where a Tupolev Tu-144 is on display. I noticed that the landing gear of this plane is actually way wider than the space it has available when ...
42
votes
2answers
20k 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
41
votes
3answers
38k views

What are the spiral marks in the center of the engines?

When I get a chance to peek inside an engine, I noticed that many have spiral marks painted in the center. What is the purpose of these? Pictures:
41
votes
3answers
104k 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 ...
40
votes
5answers
8k views

Why is water-contaminated fuel bad, but water-injection is not?

A "wet" takeoff of a KC-135 with J57 engines—By USAF Photographer (USAF photo) [Public domain], via Wikimedia Commons Why is water-contaminated fuel bad, but water-injection is not? The way I ...
39
votes
5answers
77k 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 ...
39
votes
6answers
8k views

What is some of this extra “stuff” on jet engines?

At the risk of sounding too broad, I want to better understand the complications of jet engines. This time, I'm asking about extra "stuff", which unfortunately I have no better name for since I don't ...
39
votes
2answers
10k views

Why does NASA's B-52 008 have a smoking engine in this photograph?

This is a photograph from the launch of an X-43 from the NASA owned and operated B-52 '008'. It appears that one engine is producing a plume of smoke in the photograph. Why is this? Source
36
votes
4answers
5k views

Can supersonic conditions be replicated on the ground?

In a previous question the P&W J 58 engine for the SR-71 was discussed. The engine was a new and innovative design combining the characteristics of a turbojet and a ramjet. Before flying with ...
36
votes
4answers
16k 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 ...
36
votes
3answers
52k views

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 (-...
35
votes
2answers
12k views

Jet engines: are they owned by the airline or just rented?

For commercial jets, the engines are complex parts of the aircraft, are the most expensive components, and must be carefully maintained. Source When a company buys an aircraft, do they also own the ...
34
votes
1answer
8k views

What exactly happened in this bird strike incident?

This Boeing 737 was hit by a bird and continued all the way to its final destination, from Prague to Las Palmas. What actually happened here? Was the incident considered a bird strike or an engine ...
33
votes
3answers
12k 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 ...
32
votes
7answers
9k views

Have there been any incidents caused by an inability to shut down an engine?

I read many items on the avherald.com site where it says "engine shut down during flight" or words to similar effect. I'm wondering if there are any instances where an engine could not be shut down ...
32
votes
4answers
21k 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?
31
votes
4answers
7k views

How hazardous is it if an engine cowl falls off?

There was recently an incident where the engine cowl of the starboard engine of a United Airline's Boeing 777 fell off during flight: 'Scariest Flight of My Life.' United Airlines Made an Emergency ...
31
votes
4answers
5k views

Why isn't the APU a standard diesel generator?

Why isn't the APU a standard diesel generator? Jet-A and diesel are interchangeable (with the addition of a lubricant). So why not use a cheaper piston engine as opposed to a jet turbine? V.S.
31
votes
4answers
10k views

Why do jet engines smoke?

(YouTube video) From the video linked above, I was wondering why the engines emit so much smoke? Has something gone wrong? Poor maintenance? Isn't it dangerous?
30
votes
7answers
19k views

Why don't 747s use unreliable, but cheap, jet engines instead?

I'm a neophyte who was reading a little bit about jet engines, and the economics behind them. In particular, I was curious about what makes jet engines so expensive compared to piston engines. A ...
30
votes
4answers
7k views

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 ...
29
votes
2answers
3k views

Why should jet engine throttles be moved smoothly?

On the only jet engined aircraft I'm trained on, the Bell 206B (Allison 250 engine), I was taught to move the throttle slowly and smoothly, especially when opening. I've also see this done with Rolls ...
29
votes
2answers
4k views

Are the fastest moving pieces of a jet engine supersonic?

A jet engine is composed of fast moving pieces. Those pieces are moving into an airmass different form the ambient airmass the aircraft is flying into. The turbine pieces are spinning quite fast, ...
29
votes
2answers
10k views

Why does afterburner exhaust have “pulses”?

Instead of being consistently-dense, the afterburner exhaust looks "pulsed" in these pictures below. Is this a trick of the camera, a natural result of interaction with the air, or is there some ...
29
votes
2answers
4k views

What makes a smokeless jet engine?

I was recently reading about the MiG-29 and its many upgrades, one of which (MiG-29M) was to smokeless engines. What makes a smokeless jet engine? Is it just the fuel, or something about the way the ...
28
votes
3answers
13k 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?
28
votes
6answers
6k views

On which point(s) in a jet engine does the reaction force act?

A turbojet/turbofan engine is aimed to accelerate air or combustion gases. Sometimes these engines are named "reaction engines" to emphasize the fact the aircraft is moved forward mostly by Newton's ...
28
votes
1answer
6k 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 ...
28
votes
1answer
2k views

What would have happened if the jet engine on my aircraft had ingested caulk from the pavement?

I was recently on a flight - the aircraft was a 737-800 - where I had a good view of the front of the left engine. The situation in the title did not occur, but it seemed awfully close. After ...
27
votes
4answers
7k views

Do propeller planes give contrails like jets do?

Last weekend, while watching the second world war film "Fury" with a friend, there was a scene near the end with a large set of bombers flying overhead, leaving large contrails. That seemed wrong to ...
26
votes
4answers
13k views

Why didn’t the Captain of Cathay Pacific flight 780 shut down engine 1 and land with a more reasonable speed?

In the approach phase, engine 1 of Cathay Pacific flight 780 got stuck at about 70% N1 and it forced the crew to do an overspeed landing (230knots). Why didn't they shut it off by turning the fuel ...
26
votes
6answers
10k views

Why not just drop an engine on fire?

I am interested in the case of Qantas Flight 32 and similar incidents, where there was a fuel leak after a number two engine explosion due to the disintegration of the turbine disc. It was a very ...
26
votes
7answers
5k views

Why bother having different engine options on one aircraft?

When an airline purchases a new passenger jet, they often have a choice of engines (from different manufacturers). Why is this? I would expect that one of the engine options is "best" and that all ...
26
votes
2answers
70k 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?
25
votes
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?
25
votes
3answers
12k views

What safety features prevent jets sucking in ground staff or equipment?

There have been some gruesome deaths involving ground staff being sucked into jet engines, presumably because the engine thrust was set well above idle. Are there safety features (besides prudence on ...
25
votes
3answers
60k 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 ...
25
votes
5answers
9k views

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 ...
25
votes
3answers
4k views

What does the actual path of air within a turbojet engine look like?

While Wikipedia describes the flow path as axial, I wonder if the path could be helical instead. Wikipedia explanation is described first in section 1, section 4 is for what could be the actual flow ...
25
votes
3answers
51k 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, ...
24
votes
7answers
15k views

Are German, Japanese, and Chinese companies not able to build aircraft jet engines on their own?

As far as I know, only 4 countries can build aircraft jet engines on their own: USA {GE, P&W} Russia {Klimov, NPO Saturn} France {Safran} UK {Rolls-Royce} All other countries use jet engines ...
24
votes
4answers
18k views

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 ...