Hot answers tagged

56

Well, it does look pretty, doesn't it? Boeing wanted to reduce the noise generated because of jet blast of the engines. Many airports around the world are implementing new noise regulations. As mentioned by Boeing: To combat the sound of jet-blast from the rear of the engine, Boeing, General Electric, and NASA developed serrated edges called chevrons ...


48

I think both answers are correct, but I would like to spend some time describing the phenomena that chevrons try to reduce so that I can complement the answers already provided. So, what is happening in an engine? We have a hot gas at high speed leaving the engine core, and another gas at higher speed than the external air but much slower than the core, on ...


43

The engine in question is a CFM56 turbofan engine which is larger than the JT8D the plane was originally designed for and thus has less ground clearance. This meant that they needed to flatten the bottom by moving the accessory gearbox to the side from the bottom and shrinking the fan. To quote Wikipedia: In the early 1980s Boeing selected the CFM56-3 to ...


23

The B-52 was build in different versions (A-H) and the engines and their installation differ between these version1. The image in the question most likely shows the engine configuration of a B-52H. The following drawings suggest that this also changed the inlet design2. There seem to be different engine-designations depending on the application. For the B-...


22

Actually, there were similar tricks used on Boeing aircraft before to reduce jet noise. The Rolls-Royce Conway (as used on the Boeing 707) had a scalloped exhaust which improved jet mixing and reduced exhaust noise. Since the Conway was also the first operational bypass engine, the lower exhaust speed of this design helped to reduce noise already. At the ...


21

You are confusing the shape of the engine with the shape of the nacelle. Axial-compressor jet engines (standard on larger commercial jets) are invariably round and thin1. Engines with a centrifugal compressor are fatter in the middle and will occasionally be lumpy shapes depending on the internal airflow (some engines completely reverse the flow of air ...


20

The inlet angle is a compromise between cruise, when the aircraft has a low angle of attack, and the take-off and climb phase, when the engine runs at maximum thrust, and the angle of attack is several degrees higher than during cruise. Especially right after rotation, when the aircraft is heavy and slow, the angle of attack might reach into the lower two-...


18

Take a modern turbofan: The tips of the fan run at approximately Mach 1.5. We know from unducted props that supersonic tips make them very noisy. The same turbofan without the shroud would be extremely noisy, too. Acoustic liners on the inside of the fan shroud have major contribution in suppressing this noise source. But there is more: The nacelle also ...


15

A lot of pictures of this intake make it quite hard to determine what is the exact geometry. But the one below is by far the best - thank you @ymb1. http://www.af.mil/News/Photos/igphoto/2001513002/ Before, it was hard to tell exactly what's happening. In some pictures, it looked like the inboard engine was slightly further forward of the outboard engine. ...


10

It doesn't appear to be a patent thing. The main work on the chevron (shaped nozzles) was done by NASA. Every design decision in an aircraft is a compromise and this was no different. The addition of chevrons for noise suppression leads to a (small) increase in specific fuel consumption and Airbus considered it not worth it. Flightglobal quotes Dougie Hunter,...


10

Meeting noise regulations is the primary reason for using a long nacelle. Some engines may not need it. Since it adds weight and drag it is not used unless required. Use may be partially offset by improvement in efficiency, but this does not normally justify usage on its own. If a particular engine design does not meet the specification, the long nacelle ...


9

They aren't covers; they are fairings to integrate the flow going past the lower part of the cowl with the exhaust stream coming out the tailpipe above and the pylon below. It's because the cowl extends below the engine some distance to house various parts like accessories, plumbing, ducting etc. You have to create a nice transition for the airflow or you ...


9

The main purpose of the chevrons in the engines is to reduce the engine noise. Most of the civil airliners use high bypass turbofan engines, which produce significant amount of noise especially at high thrust conditions. Aircraft (engine) noise is especially critical during the takeoff and approach phases as it affects the people in the area around the ...


8

The nacelle is not some solid block. For the flow it looks like a hollow, slightly blocked tube. The flow ahead of the nacelle is indeed slowed down in flight, but only a little, entering the intake face at a flow speed of around Mach 0.4 to 0.5. This compresses the air ahead of the nacelle when flying in cruise at around Mach 0.8, so what enters the intake ...


7

The first turbofan, the Rolls-Royce Conway, had a bypass ratio of only 0.25 in order have a small enough diameter for placement in wing roots. It was initially planned for the V bombers and planned civilian derivatives of them. So yes, it is practical to place a turbofan engine in the wing, but comes at a cost: Buried engine placement will shift the optimum ...


7

That looks like an optical illusion due to the angle to me. A photo on this site shows a more frontal view of the outboard engines, with just the equal centre division between the two engines appearing. Close-up underneath.


6

As you have correctly mentioned, the term Nacelle generally refers to a number of sections around an aircraft engine (In the context of turbofan aircraft engines). This generally the structures formed by the Inlet, Fan Cowl, Thrust Reverser and the Exhaust. The nacelle does perform the basic function of a housing for all these subassemblies which means ...


6

Why does the B-52 outboard engine nacelle have a sharp change in shape? Not just the outboard, but all 8 engines on a B-52 have the same inlet design. The original engines had a smaller fan diameter and symmetrical round inlets. I suspect going to a larger fan diameter created flow problems which the engineers solved by modifying the inlets.


6

It is a trade-off. A long nacelle eg RR Trent 700 can give a performance (by mixing bypass and core flow) and acoustic suppression gain but it is heavier and produces more drag. see "The Jet Engine" published by Rolls-Royce, p 231.(ISBN 0 902121 2 35)


6

Yes the left (#1) reverser is engaged in this photo. G-AXDN is the British pre-production Concorde and has some differences from the final versions. G-AXDN contained internal clam shell reversers, if the GIF at the bottom of this informational page about G-AXDN is any indicator that external engine reverser may still actuate for show and in this case was ...


5

This question seems to have been worked over pretty well. My only additional comment is some background on the engine itself. Think of the SNECMA CFM-56-3 Turbofan as two parts: 1) the core gas generator designed and built by GE, and 2) the Fan/Power Turbine designed in France and mated onto the GE core. The best part of the story (you will not find this in ...


5

This inlet design was already used on the Convair B-36 D, which had twin turbojets added on the outer wing to give it a higher top speed when penetrating enemy airspace. During cruise, only the six piston engines would run and the jet engine inlets were plugged to reduce their aerodynamic drag. This plugged state is shown in the picture below (picture source)...


5

The book Synthesis of Subsonic Airplane design by E. Torenbeek contains some sections on jet engine placement. According to the writer, buried engine installation was favoured by British designers and podded engines by American designers. Figure 2-12 depicts four bomber planforms, one American and three British. The wing area of the British designs are ...


5

It's not unique to the LEAP. You can find similar grills on the CFM56-5 and Trent 700, for example. That grill is the PCE's (precooler heat exchanger) air outlet. Compressed (hot) air is taken from the engines (before combustion) for pressurization and air conditioning. This air has to pass through a PCE (cooled by air from the fan) before heading to the ...


4

This is the Fuel Gauge. Direkt auf der Triebwerksgondel sind bei der Ju 52/3m die Treibstoffanzeigen montiert The fuel guage is mounted directly on the engine pylons on the Ju 52/3m. AustrianWings It is suggested that the indicator gives a reading in hundreds of liters (Liter x 100 written underneath). Source Visible on the illustration below as ...


3

Yes, your last sentence gets it about right. Just one detail: Air does not move "forward" to flow out again once it has entered the intake - which air goes where is all settled ahead of the intake. Never forget, though, that this effect is small and spillage leaves a net drag component. When compared to a sharp-lipped intake as used on supersonic planes, ...


2

My Google foo fails me, but I did find this picture with a caption stating that it's a fuel gauge. Zooming in on the image suggests that this is plausible I also found this diagram of the wing fuel storage which shows the gauge. To read it at night, pilots would use a flashlight. I can't find any authoratative source. If I do, I'll come back and edit.


2

I'm quite suprised that this wasn't mentioned in any of the previous comments or answers, but since the inlet is quite far in front of the wing, it sees some significant upwash from the wing. To align the inlet with this local flow direction, it is slightly angled down. For the same reason the engines on Bombardier CRJ series aircraft are slightly angled ...


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