100 votes
Accepted

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

An odd number of cylinders is required by the combination of the single-crank radial design, the four-stroke (Otto) work cycle, and the desire to keep the power strokes evenly spaced in time. To keep ...
user avatar
72 votes
Accepted

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

Several issues: As you mentioned, high specific fuel consumption. About to that of turboprops, but without the reliability. If you're going to live with that SFC, you might as well as go with ...
user avatar
  • 103k
58 votes

Why do new jet engines cost billions to design?

Even if we ignore the salaries of everyone involved; engineers, sales people, management, Q/A teams, manufacturing teams, more sales people, and then a few spare engineers... I don't understand. If ...
user avatar
  • 95.9k
54 votes
Accepted

What is some of this extra "stuff" on jet engines?

Of course the basic concept of a turbine engine is relatively simple, but the devil is in the details. To make an engine efficient, reliable, powerful, and safe, a lot more components are needed. ...
user avatar
  • 69.7k
53 votes
Accepted

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

No, the helicopters are standard production versions. The Eurocopter AS350 is a common model used for these operations. In 2005, Didier Delsalle landed a Eurocopter AS350 B3 on the summit of Mt. ...
user avatar
  • 69.7k
47 votes

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

Constraints Different applications have different constraints: Aviation: very light weight, highly reliable Marine: very high endurance Automotive: moderately light weight, responsive Motorcycle: ...
user avatar
  • 10.6k
45 votes

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

This is because those engines have turbocompressors which are used to supply cabin air. The engine bleed air drives the turbine of the turbocompressor, which in turn would drive a compressor which ...
user avatar
  • 98.3k
45 votes
Accepted

Do turbofan engine cowlings dilate during spool-up?

Now that's an interesting phenomenon! I do not believe that this is a case of an illusion of any sort, or the engine dilating. What the video probably shows, is the engine slightly turning towards the ...
user avatar
  • 20.7k
43 votes
Accepted

Can supersonic conditions be replicated on the ground?

Yes, supersonic conditions can indeed be replicated on the ground for purposes of airframe and engine testing. Arnold Engineering Development Complex (AEDC) The U.S. Air Force and NASA both operate ...
user avatar
  • 18.9k
42 votes

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

Centrifugal compressors only produce a more compact engine at low mass flow, which means low thrust. The amount of thrust an engine can produce is proportional to its intake area times exhaust ...
user avatar
  • 17.6k
40 votes
Accepted

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

These are secondary intake doors which allow more air to reach the engines. They are opened by the engine control computer when the main intakes are not providing enough air for the power setting of ...
user avatar
38 votes
Accepted

Where is the generator in a large turbofan of a commercial airliner?

Image source The generators are usually driven from the Accessory Gearbox. This CAD drawing is from the company that makes accessory gearboxes for the Rolls Royce Trent engines driving the A330 and ...
user avatar
  • 57.7k
35 votes

Why do new jet engines cost billions to design?

Jet engines are some of the most complex machines ever created. They have to be as light, efficient, safe, and reliable as possible. There's a reason that most new airliners recently have been ...
user avatar
  • 69.7k
35 votes
Accepted

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

This type of structure is called isogrid structure. Solid walls are usually very heavy and therefore engineers use more weight-efficient structures such as sandwich structures or stiffened structures ...
user avatar
  • 4,098
34 votes

Can supersonic conditions be replicated on the ground?

It is a struggle, but it can be done. For the J58, Pratt&Whitney had a test facility at West Palm Beach which would simulate conditions at speed and altitude. From this site: The picture ...
user avatar
31 votes

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?

The bleed will be tapped from one or two of the 3rd, 4th, 5th or 6th High Pressure Compressor stages. Usually there are two bleed ports. The highest pressure port will supply really hot air for ...
user avatar
  • 103k
30 votes

Where is the generator in a large turbofan of a commercial airliner?

IDG Location: Dedicated pad on the accessory gearbox 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). That's a good ...
user avatar
  • 67.6k
30 votes
Accepted

How much weight does a blisk save?

Very significant. A research on blisk cost estimation with funding and data provided by Rolls-Royce states: Integrally bladed discs, commonly known as blisks, are currently found in axial-flow ...
user avatar
  • 119k
28 votes

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

Your reasoning is correct if engine mass is not important. Ships use huge engines, because increasing the number of cylinders beyond 8 will have diminishing returns in terms of smoothing out the ...
user avatar
27 votes

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

Wikipedia tells you the answer: The 707 uses engine-driven turbocompressors to supply pressurized air for cabin pressurization. On many commercial 707s, the outer port (number 1) engine mount is ...
user avatar
  • 4,343
27 votes
Accepted

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

Being a ground attack aircraft, the Su-25 has an operational ceiling of 23,000 ft and will not encounter the low temperatures to which a Boeing 777 is regularly exposed. Therefore, the risk of diesel ...
user avatar
27 votes

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

This is simply a way to achieve required rigidity/strength and vibration dampening with less material than in a solid, even thickness design. Varying the weave pattern gives a relatively easy and a ...
user avatar
  • 20.7k
26 votes
Accepted

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

As of writing this in Nov 2017, the 777X is still in development and it will be powered by the General Electric GE9X (also still in development). The current talk is that GE is working on a ...
user avatar
  • 119k
26 votes
Accepted

Why do propellors have white markings on their tips?

It is so you will see a circle when they are turning. Without contrasting bright tips turning propellers are invisible, and pose a grave risk for someone who is deaf, wearing heavy hearing protection,...
user avatar
  • 15.6k
25 votes
Accepted

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

The most (kinetic) energy is in the fan and turbine blades and disc. The engine is enclosed in a containment chamber whose purpose it is to protect the rest of the airplane from shrapnel in case of a ...
user avatar
24 votes

What is some of this extra "stuff" on jet engines?

There are many extra components, generally known as "accessories". Fuel pumps, the ignition and starter circuits and control units, turbo pumps, hydraulic pumps, stator vane angle actuators, fuel ...
user avatar
  • 30.8k
24 votes
Accepted

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

The afterburner, of course. In simplest sense, the afterburner is just an extension attached to the engine where the fuel is dumped into the exhaust, resulting in an inefficient but huge increase in ...
user avatar
  • 98.3k
24 votes
Accepted

Why do large turbofans generally have many more LP turbine stages than HP turbine stages?

Because the LP turbine extracts power for the fan, which requires the most power. The HP and IP turbine only extract power for their connected compressors - the LP turbine extracts power for the fan ...
user avatar
  • 57.7k
24 votes
Accepted

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

The B-52 was built 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. ...
user avatar
  • 1,537
24 votes

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

Doors for secondary air intakes. Buried engines need additional air during takeoff and low speed flight.
user avatar
  • 15.9k

Only top scored, non community-wiki answers of a minimum length are eligible