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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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55 votes
6 answers
27k views

Why do most radial engines use an odd number of cylinders?

To help suppress vibration, most 'V' and 'horizontal' engines use an even number of cylinders as closely opposed as possible. Radial engines are well known for excessive vibration. For some reason ...
jwzumwalt's user avatar
  • 11.5k
49 votes
6 answers
17k views

Why are two-stroke engines nearly unheard of in aviation?

Almost all aircraft piston engines are four-stroke engines, with one power stroke per piston every other crankshaft revolution and a great deal of intricate moving parts which are only too happy to ...
Vikki's user avatar
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45 votes
4 answers
15k 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 ...
Paul Howard's user avatar
43 votes
6 answers
11k 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 ...
DrZ214's user avatar
  • 17.8k
40 votes
11 answers
18k 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 &...
DrZ214's user avatar
  • 17.8k
40 votes
9 answers
34k 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 ...
DrZ214's user avatar
  • 17.8k
38 votes
4 answers
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 ...
TomMcW's user avatar
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36 votes
1 answer
8k views

Do the Mt. Everest rescue helicopters have modified engines to operate at high altitudes?

There are so many climbers on Mt. Everest today that there is now a helicopter rescue service there. They operate rescue up to 21,000 + ft. Because this is essentially stationary service it seems that ...
blacksmith37's user avatar
30 votes
2 answers
6k views

What features enable the Su-25 Frogfoot to operate with such a wide variety of fuels?

It is widely attested* that the Su-25 Frogfoot's engines (the Turmansky R-95Sh and later the R-195) were designed to function using a wide variety of fuels, including diesel, gasoline, petrol, ...
Party Ark's user avatar
  • 14k
29 votes
2 answers
11k views

What are those hatches on the engines of a B-2 that are open during takeoff?

I just watched this drool-inducing video of Northrop Grumman B-2 Spirit, And noticed that there are a couple of hatches near each engine, which were open: I initially thought they were open for some ...
sampathsris's user avatar
26 votes
3 answers
9k 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 ...
mins's user avatar
  • 75.3k
25 votes
3 answers
4k 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?
Pavel's user avatar
  • 909
24 votes
4 answers
7k views

How did the propellers on the B-29 stop perfectly in sync?

In the photos below, the propellers on the B-29 props all stop in the same position. Is this done by hand, it is this automatic?
Boeing787's user avatar
  • 6,484
24 votes
2 answers
15k 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 ...
DrZ214's user avatar
  • 17.8k
23 votes
1 answer
3k views

How much weight does a blisk save?

A Blisk is a bladed-disk, all one monocoque component. So the blades do not have to be inserted and secured, and they are not removable. No nuts and bolts either. The downside is that any serious ...
DrZ214's user avatar
  • 17.8k
21 votes
5 answers
13k 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 ...
Useruuu's user avatar
  • 341
20 votes
6 answers
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 ...
user avatar
20 votes
1 answer
9k 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?
lchike's user avatar
  • 303
20 votes
2 answers
3k 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—...
user avatar
18 votes
2 answers
6k views

Why do new engines have a case with a triangular tiling pattern?

Take a look at the case of new engines such as the F-135: or the F-119: Why do they have a case with a triangular tiling pattern? Is this a new technology? A new material?
Roh's user avatar
  • 3,329
18 votes
6 answers
9k 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 ...
DrZ214's user avatar
  • 17.8k
16 votes
4 answers
9k views

Why hasn’t the Wankel rotary engine seen more use in aircraft?

The Wankel rotary engine seems to have several advantages over traditional reciprocating cylinder-and-piston designs. In general, they have a much higher power-to-weight ratio, less vibration, and ...
Aaron Holmes's user avatar
  • 4,857
16 votes
2 answers
5k 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-...
Jpe61's user avatar
  • 29.3k
15 votes
6 answers
9k 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?...
jwzumwalt's user avatar
  • 11.5k
14 votes
3 answers
8k views

Why are there fewer stages in the turbine than in the compressor of a turbine engine?

I'm a student interested in aviation. I'd like to understand why the turbine section has fewer stages than the compressor section in a jet engine. What would be the effects of adding stages to the ...
ts.'s user avatar
  • 141
14 votes
2 answers
2k views

What are those things on the B747 engine pylons?

I noticed some dots and lines on the Boeing 747 engine pylons. What are they? The above image is as seen in this video at 2:18.
XF-91's user avatar
  • 2,096
13 votes
4 answers
10k views

Are jet engines designed in a way to make an explosion result in the least damage possible?

I know most engines are super reliable today, but do manufacturers do anything to at least make an explosion result in the least damage possible?
George Clooney In a Mooney's user avatar
13 votes
2 answers
2k views

Are centrifugal turbojets easier to make than axial ones?

Looking at the history of jet engines, I noticed that the British first made centrifugal-types, but the Germans went straight into axial-types. Which type is easier, if any? By that I mean, which ...
DrZ214's user avatar
  • 17.8k
13 votes
2 answers
4k views

How are helicopter rotor blades balanced?

Reading this answer, I realized that the balance of a rotor is critical. When one blade is changed, the whole balance of the rotor may be affected. Moreover, I doubt 2 blades are exactly the same. For ...
Manu H's user avatar
  • 16.5k
12 votes
7 answers
11k 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. ...
DrZ214's user avatar
  • 17.8k
12 votes
3 answers
6k views

How did the sailor survive getting sucked into an engine?

I'm sure we've all seen the famous video of the Navy sailor getting sucked into an engine. Can someone explain how was this possible? I've read many comments but still don't have a visual idea how ...
Gabe's user avatar
  • 3,749
11 votes
2 answers
4k views

What is missing from a non-afterburning engine to prohibit the use of afterburning?

What components are missing in a non-afterburning engine that an afterburning engine does, so that it can't use afterburning?
anonymous's user avatar
  • 3,574
11 votes
1 answer
2k views

Which popular turboprop engine feathers its propeller blades with each shutdown?

Chapter 12 of the FAA Airplane Flying Handbook, discussing multiengine flying, goes into quite a bit of detail in the propeller section about how most feathering-propeller-equipped engines on ...
Vikki's user avatar
  • 28.4k
10 votes
4 answers
10k 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 ...
Air Canada 001's user avatar
10 votes
4 answers
5k 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) ...
summerrain's user avatar
  • 3,484
10 votes
3 answers
3k 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 ...
DrZ214's user avatar
  • 17.8k
10 votes
3 answers
3k views

is boundary-layer jet-turbine a scam?

While exploring open jet turbine designs, I've stumbled upon one CAD drawing which has made me wonder. It is based on a famous Tesla turbine and its highly-efficient way of compressing the fluid. At ...
FlegmatoidZoid's user avatar
10 votes
2 answers
1k views

Why were turbojets the first kind of turbo-engines, instead of turboprops?

During WW2, the first jet engines appeared en mass in combat aircraft. Britain and Germany made the first ones. But why were turbojets first? Why not turboprops, which are easier since the turbo-...
DrZ214's user avatar
  • 17.8k
10 votes
2 answers
1k views

How big can a pulsejet be?

Pulsejets are mostly known for propelling the V-1 flying bomb, and have some use as small remote-controlled plane engines. In either case, the pulsejets aren't particularly big, as far as jet engines ...
Eth's user avatar
  • 763
10 votes
1 answer
1k views

What is the function of the aft stator in CFM's open rotor engine design?

CFM revealed their design for a open rotor propfan engine with two stages of blades - one active set of blades rotating in the forward and an static sets of blades on the aft. According to The ...
karthikeyan's user avatar
9 votes
7 answers
3k views

Why don't bypass ducts in jet engines narrow down the flow to accelerate it?

Since decreasing the flow area increases flow velocity, why don't turbofan bypass ducts narrow down to produce more thrust?
Francis L.'s user avatar
  • 2,514
9 votes
2 answers
4k views

What is the operating temperature of a turbofan engine, and how is it kept at that temperature?

What is the operating temperature of a turbofan engine? If advancements were made to materials through material science for the internal components would increasing the operating temperature result in ...
securitydude5's user avatar
9 votes
1 answer
10k 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 ...
mins's user avatar
  • 75.3k
9 votes
3 answers
7k 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 ...
Abhishek Verma's user avatar
9 votes
2 answers
1k views

Why is the inlet cone on some subsonic engines conical (sharp)?

An inlet cone, as far as I know, is a component on an engine to guide the intake air smoothly into the compressor stage of a jet turbofan engine, it performs as a fairing(aerodynamic covering)of the ...
Richard M's user avatar
9 votes
2 answers
3k views

What is "Boost"/"Super Boost" in the PW127M/N?

This incident describes rejected take-off due to the "boost" being inoperable. The reason for the RTO is obvious: engine power lower than expected. But what is this ominous "boost" technology?
Simon Richter's user avatar
9 votes
1 answer
8k views

What is the purpose of a reverse-flow orientation for turbo-prop aircraft?

If I decide to utilize a PT6 (ignoring for now the cornucopia of variants) as the turbine engine I will be using on my newly designed turbo-prop aircraft, do I then decide how I will orient the engine ...
BigNutz's user avatar
  • 539
8 votes
2 answers
3k views

What is the purpose of a freewheeling unit in an helicopter, if a centrifugal clutch is already present?

Some helicopters such as the Bell 47 are fitted with a centrifugal clutch. Centrifugal clutch will engage only at certain engine RPM. If the engine fails, the engine RPM will reduce and the ...
Raj Arjit's user avatar
  • 268
8 votes
2 answers
950 views

Where exactly is the engine compartment, remarked in yellow?

Where exactly is the engine compartment, remarked in yellow? We know that normally the air going to the engine passes through the air filter. But when we open the alternate air valve, where does the ...
pilot162's user avatar
  • 2,227
8 votes
2 answers
444 views

What are the design principles that result in thin, long-span, narrow chord main rotor blades?

Regardless of the overall design configuration of a helicopter, it appears that helicopters have main rotor blades that all fall into the class of ultrahigh aspect-ratio wing design: that is, you ...
niels nielsen's user avatar

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