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Hot answers tagged turbofan

42

Yes, large turbofans can be turned by hand without too much force. On smaller aircraft this is regularly done as part of the preflight walkaround. You can see an example in this YouTube video of an A320 walkaround: Admittedly, the pilot turns the engine with four fingers here, but you can see that not much force is required. Turning it with a single finger ...

26

Turning the engine with one finger is not necessarily recommended, because the leading edges of the fan blades are quite sharp. But it is possible. For some experimental vibration measurements, it is preferable to keep the rotors turning slowly to avoid the bearings "sticking" in one position and confusing the results. A common way to do this is ...

15

if you decrease the duct cross-section, you must apply work to accelerate the flow through the smaller duct. this means the fan driving the duct has to work harder (it must absorb more shaft horsepower) against that constriction in the flow and you derive no benefit- unless you mount the fan on a more powerful engine, in which case you are burning more fuel ...

10

Mostly because there is no need to strongly accelerate the flow. In fact, thrust engine is given with the formula below: $$F_N \approx \dot{m} \cdot \left(V_{out} - V_{in} \right)$$ Which means, there are two ways to produce the same thrust: strongly accelerate few air massflow or speed up a little bit a big amount of gas. Turbofans use the second strategy,...

8

Almost all jet engines will have one or more "resonance" conditions where the combination of turbine RPM, mechanical elasticity and aeroelasticity constructively reinforces to cause mechanical or aerodynamic vibration. Think of it this way: If you have a slinky spring (mechanically elastic) toy you can hold it by one end and let the rest drop ...

7

All three engines you show are for fighter jets flying at supersonic speeds. These engines need a low bypass ratio: Why do military turbofan engines use a low bypass ratio? To achieve more thrust at lower bypass ratios, the bypass air needs to be accelerated more, which requires more than one fan stage because axial compressors can only add a limited ...

7

The simple answer is that the pressure ratio across fan is generally small, in order to maximize the propulsive efficiency, as outlined in @Asced's answer. If you have a flow that has high total pressure (compared to inlet total pressure), then you have the real-estate to accelerate it further isentropically until the nozzle pressure reaches the ambient ...

7

Both-engine-out glide performance isn't accounted for and isn't a certification requirement (of any airplane that I'm aware of). Airliners have good glide performance as a happy side effect of design optimization for cruising at high altitudes. What does have to be accounted for is single engine performance issues; things like departure performance with an ...

6

They do. It's just not by very much. Typical high bypass engine like a GE90 might narrow down the exit area by a few percent. Small enough that you probably don't even notice it when looking at it. But the air can feel it.

6

What you are describing is a characteristic of fluid dynamics known as eddies. As different portions of the fluid flow faster than others, the friction and surface adhesion between the two layers cause swirling of the fluid and reversal of the current, creating a turbulent flow region. It is this same principle that holds a ping pong ball stationary in a ...

6

Did Dassault have some magic trick up their britches It was an era of innovations. Have you seen the Vickers VC10 thrust reversers? The clamshell / target doors can be made internal within the nacelle. Literature is lacking on the Mercure, but there is a nice hint from a cutaway drawing: Source: Air Enthusiast magazine, via aviadejavu.ru The highlighted ...

6

Most jet nacelles contribute little or no lift, but there is at least one highly notable exception. The airflow inside the nacelle is dominated by the engine and any chance to create differential pressure between upper and lower leading sections is kept negligible in order not to disrupt the intake flow. The outside is more of a stubby fuselage than an ...

6

The final report notes: At the time of each TAI [thermal acoustic imaging], the inspectors attributed the indication to a defect in the paint that was used during the TAI process and allowed the blade to continue the overhaul process and be returned to service. [emphasis mine] The paint being referred to is used for the inspection. Its function is to ...

5

Lots of turbofans have convergent nozzle fan ducts although the convergence is mild. This is the Cf-34, which has a small but noticeable convergence. On a lot of engines it's barely discernible. It's because the designers are only after a small increase in velocity of the fan discharge.

5

Almost any sort of body, conveniently oriented in a fluid flow, can produce lift. A ball is one of the few exceptions... Please note that lift and drag do never cancel out, because they are –by definition– vectors at 90º... There are wingless, heavier-than-air aircraft, like this one, for example: https://en.wikipedia.org/wiki/Wainfan_Facetmobile

5

Update As @Zeus has pointed out correctly: Drag coefficient stay constant in my scenario, not drag -- I've updated the explanation accordingly. Sorry for that blunder. The conclusion does not change: Engines with constant PSFC only have an advantage when flying slowly and get worse much quicker than engines with constant TSFC -- but actual engines are mostly ...

5

Tip speeds do not actually stay the same, turbofan (and turbojet) blade tip speeds can exceed the speed of sound, up to Mach 1.3 (actually stated M1.4 in the link, my experience is M1.3 but also my professor used to mention M1.4) as seen in link. As they are enclosed, the negative effects of the shock are much smaller than if it were an open propeller. Open ...

4

The amount of current flow produced by a seebeck effect device is tiny relative to their size. You would need a plane load of them just to power the lighting system in a passenger jet.

3

This has actually been done, at least on the CFM-56 series at some point. The image is taken from here, the article is about the F108 engine which is the US military version of the CFM-56. In newer versions of the engine the blade-tip shrouds have been replaced by mid-span shrouds as below (from Wikipedia), or have been removed. As pointed out by Camille ...

3

What makes it impossible for very new turbofan designs to reach all three redlines simultaneously at sea level? The implication in your question is that GE9X was not able to reach all three redlines simultaneously but that previous engines (e.g. GE90, GEnx, LEAP, etc) could. That is incorrect. Those engines could not reach triple redline either. At least ...

3

According to this ATSB Australia Report, the Original Blade Weight of the Trent 800 Blade that failed was 11.6KG. The weight of the blade in grammes is etched into the blade root. https://www.atsb.gov.au/media/33974/tr200100445_001.pdf Page 9/10. Fan Blades of the same type all differ (slightly) in weight and are chosen specially to balance the engine when ...

3

The core is the Internal Combustion Engine - it has intake (of air), compression (of air), combustion (of the fuel/air mix), and exhaust (water vapor, CO2, air) that provides the motive force to rotate the turbine (N2) and provide perhaps 25% of the actual thrust of the whole engine when it exhausts out of the tailpipe. From How Do Jet Engines Work? on this ...

3

This is most likely some kind of a test where high speed photography (as in 5,000 - 10,000 frames/second) will be used to capture the detailed motion of the fan blades. Types of tests that might require this type of video include fan blade out testing, bird strike testing, and icing / hail testing. There may be others but those are the most common. Why the ...

3

You can't escape the tyranny of the law of conservation of energy and the second law of thermodynamics (with it's corollary limiting thermodynamic cycle efficiency). The energy taken from cooling in a reciprocating engine is truly wasted, so the thermal gradient there can be further used without affecting efficiency of the engine itself. But turbines have ...

3

When engine or wing anti-ice are needed, the required amount of bleed air increases. In order to provide enough bleed air at the required pressure, the engine idle speed must be increased. That way, the compressor runs faster, which provides more high pressure air to the bleed valves. Note that this will result in more idle thrust as well, which can have an ...

3

A fan blade at the front or turbine blade at the back of a jet engine is a small aerofoil surface which is fixed to the main shaft and spins with it. By contrast, a stator vane is a similar aerofoil surface fixed to the outer casing and remains static. Why do that? The spinning blades of a given stage cause the airflow to start spinning, and that slows the ...

2

You're asking about turbine (hot) reversers of early high-bypass turbofans. 747 flickr.com and twitter.com It had the same mechanism as the cold bypass (blocking doors), similar to what the C-17 (right image) uses nowadays. L-1011 Lockheed L-1011 TriStar - Airliner Tech Vol. 8 It had target-type, but only during development. DC-10 Design Features of the ...

2

Based on the GE website, where they used a GE90-115B as a power generation unit, called the LM9000. GE cited 65,000 horsepower, which seems to scaled more reasonably when compared to the RB211 XWB https://www.ge.com/news/reports/ge-oil-gas-just-turned-worlds-largest-jet-engine-65-megawatt-power-plant?utm_source=feedburner&utm_medium=feed&utm_campaign=...

2

Power consumption=Kinetic energy=$1/2mv^2$. Thrust=momentum change=$mv$. This is only true if the engine is perfectly efficient internally and the air is still relative to the aircraft. If the air is moving relative to the aircraft (and the engine is still perfectly efficient internally) then. Power consumption=Kinetic energy=\$\frac{1}{2}mv_e^2 - \frac{...

2

This is an order of magnitude educated guess: Based on this picture from Wikipedia: You can see that the aft mount is attached to the turbine rear frame. As such, it'll get relatively toasty. I'd say maybe 900 - 1000 F. The forward mount is attached to the fan frame just above the compressor case. As such, it will be much cooler. Maybe 200 - 300 F. ...

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