Questions tagged [engine-design]

Questions about the process and principles used when designing aircraft engines. Use "jet-engine" and "piston-engine" tags to specify a certain type of engine.

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19
votes
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—...
22
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2answers
10k 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 ...
3
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2answers
2k views

In high-altitude piston aircraft, what type of forced induction (boost) is used?

In high-altitude piston aircraft, what type of forced induction (boost) is used? Turbocharger or supercharger?
37
votes
9answers
26k views

Why increase the number of cylinders in an engine instead of increasing their volume?

I've been reading about WW2 planes lately. Some of them have 12 or even more pistons in their engines. But if your goal is to increase power, why would you add more pistons instead of simply ...
25
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3answers
3k 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?
8
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7answers
6k views

Why are contra-rotating jet engines so rare?

Why are contra-rotating jet engines so rare? Such a jet engine would have contra-rotating turbines, or contra-rotating compressors, or both. In fact I cannot think of even one aircraft that has this. ...
6
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1answer
5k views

How do exhaust cones affect the performance of turbine engines?

I've seen explanations stating that it creates uniform stream of exhaust gas towards the exhaust duct, however without the cone wouldn't it still be considered a uniform stream? Also since a ...
4
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1answer
5k views

What is the difference between single and dual annular combustor?

CFM56 uses single annular and GEnX uses dual annular combustion chamber. somebody please explain with pics.
4
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2answers
2k views

Why are turbofan inlets circular- or oval-shaped instead of square-shaped?

Source: materialsengineeringconsultant.com and initious.blogspot.com. Most of the engines are circular except for some that are oval. The oval shape provides a better ground clearance. Why aren't ...
37
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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 ...
15
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5answers
7k views

Why does the B-52 outboard engine nacelle have a sharp change in shape?

The outboard engine nacelle of a B-52 appears to have a change of shape too extreme for just blending to the center-line of the engine pod. Why is the B-52 outboard engine nacelle shaped the way it is?...
4
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1answer
280 views

Why does the spool speed increase with altitude for a turbofan engine (single spool & double spool)?

So, I want to see the effect of altitude on the compressor characteristic of a single spool & double spool turbofan engine at constant flight speed and TIT (turbine inlet temp). I am aware that ...
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) ...
4
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4answers
348 views

What changes are needed for a piston-prop aircraft to withstand saltwater environment?

I'm interested in WW2-era piston-prop aircraft. I want to know what changes are needed to make the aircraft withstand saltwater corrosion. Presumably these are different and more expensive than the ...
4
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3answers
2k views

Why does the Trent XWB have intermediate compressor stages?

I realized that the Rolls-Royce Trent XWB on the Airbus A350 has intermediate compressor stages. Why is that? Other engines like the CFM56 or IAE V2500 that power most of the A320-family have low ...
1
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1answer
132 views

What is the temperature range of engine mounts connected to the pylon?

I want to know what is the temperature of the engine mounts in modern turbofan engines for example, Rolls Royce Trent 900. Also, what are the temperature considerations you need to make while ...
8
votes
1answer
6k views

How is the central hub / shaft casing of a two-spool jet engine assembled?

This is the usual view of a two-spool turbine engine: (From source) I don't know if shaft casing or hub are correct names, this is the external skin of the volume around the N1/N2 shafts. I can't ...
8
votes
1answer
7k views

How does a labyrinth seal work when the engine isn't rotating?

I believe I understand how a labyrinth seal in a turbine engine works when a the engine is running. Any oil that makes it past the first part of the seal is thrown outwards due to the centrifugal ...
5
votes
3answers
1k views

How much weight does a thrust reverser add?

How much weight does a thrust reverser add to an engine? Let's take the CFM Leap 1B as an example, which powers the new version of the Boeing 737. This engine can produce up to 130 kN of thrust for ...
5
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2answers
758 views

Are there any ducted propeller or fans without a shaft in the centre?

Are there any propeller or fan designs that hold the propeller or fan in the duct and do not have a boss or propeller shaft? This could reduce the drag of the centre. Are there any other advantages?
1
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1answer
1k views

What would be the best fan duct design to achieve maximum thrust?

I have a few brushless motors and I am trying to get the best thrust out of each motor. I am busy doing some research but can't get a clear answer. What would be the best fan duct design to achieve ...
40
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6answers
9k 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 ...
36
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10answers
15k 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 &...
18
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6answers
6k 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 ...
4
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2answers
4k views

Why do the N1 and N2 fans have different shafts?

This question here contains a cutaway of a jet engine (which I'm putting right....here) Credit: K. Aainsqatsi, Wikipedia After reading the answer, and looking over the chart, I noticed that the ...
46
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4answers
12k views

What is this hole at the front of the engine pylon on a Boeing 707?

Images of the Boeing 707 show a hole at the front of the engine pylon just above the engine. Can someone please tell me what is the purpose? Also why do images show the hole absent on the left outer ...
18
votes
1answer
8k views

Why did Boeing remove the engine chevrons on the 777-X?

From the most recent renderings on Boeing's site, it looks like the engine chevrons found on the 787, 747-8 and 737-Max have been removed from the 777-X. Is there a good reason for this?
12
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2answers
877 views

Why don't NK-12 powered contra-rotating propellers turn in equal rpm?

The Kuznetsov NK-12 is a Soviet turboprop engine of the 1950s, designed by the Kuznetsov design bureau. The NK-12 drives two large four-bladed contra-rotating propellers, 5.6 m (18 ft) diameter (NK-...
10
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3answers
2k 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 ...
10
votes
4answers
8k views

How are turboprops and other propeller planes pressurized?

I know how most turbofan jets are pressurized: But it can't be the same with turboprops, as I don't understand how propellers can take in the air. Yet, planes like Q400s can fly above 8000', all the ...
9
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3answers
5k views

What are the advantages and disadvantages of mixing the bypass flow with the core flow?

Many Aircraft engines mix the bypass air with the core gases before exhausting to atmospheric pressure through a propelling nozzle. However, some do not mix and have separate nozzles for the flow of ...
7
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1answer
97 views

Which compressor is more like to surge and stall - axial or centrifugal?

I am studying about stall and surge in rotodynamic compressors and want to understand which one is more prone to these phenomena.
3
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2answers
197 views

What are the relative efficiencies of the various stages of a counter rotating propeller?

After going through the CFD results of a drone project I am involved in, I noticed that the data shows significantly increased (+10%) thrust generation at the second stage of a counter-rotating, 2-...
1
vote
3answers
333 views

Can a gas turbine be able to switch from torque to thrust?

Can a turbine engine be switched from providing torque (like a helicopter) to the rotor to transfer power to provide exhaust thrust (like a jet plane)? Related: BV-347 upgrade this way?
18
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5answers
6k views

Why do airplanes use an axial flow jet engine instead of a more compact centrifugal jet engine?

Axial flow jet engines take up a lot of space. Centrifugal jet engines create high pressure. Can you create a centrifugal air pump that pushes as much air as a axial jet engine. Centrifugal air ...
8
votes
1answer
901 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 ...
6
votes
2answers
352 views

Why is a turbofan engine less efficient when air leaks over fan blade tips?

Image source I understand part of the reasoning for minimal clearance between the casing and the fan blades, is to have the larger fan blade possible, which generates a larger low pressure area to ...
6
votes
1answer
2k views

In a turbofan, why are there no stators behind the fan?

Why are there no stators behind the fan? At least, I've never seen any turbofan design that has stators right behind it. (Here is the latest turbofan I've looked at, the CFM56. Cutaway part starts ...
5
votes
3answers
5k views

Why is there a pressure drop in the combustion section of a jet engine?

This image, referenced in this question, shows in green the pressure value in a jet engine: The combustion section between compressor and turbine is where fuel is injected, mixed with air, and ...
5
votes
1answer
380 views

How are rotary engines controlled?

Listen closely to the engine sound here. It seems to me all rotaries continuously sound on/off. Is that a pilot application to manage the engine's torque (with the whole block rotating), or as ...
2
votes
1answer
271 views

Who should design the nozzle of jet engines on aircraft?

There is a question in my mind that which company should design the nozzle of jet engine - the aircraft designer or the engine designer? If you watch this documentary: BBC Documentary Rolls Royce ...