# Tag Info

39

It's simple. Cheap (development costs amortized decades before) and reliable. They made money for airlines. Or, you know, they wouldn't have bought them. There's more to operating costs than fuel burn, and in any case, fuel prices in the mid 90s were cheaper than at any time since the 20s in constant dollars. Fuel was so cheap that Air Canada was ...

34

A start on a TF all the way to idle is about 20-40 seconds depending on the engine, and the fan itself won't do more than creep a bit until the core actually lights off which is 5-10 seconds, so they'll have lots of time to get down and get away as per @ymb1's diagram, once the wheeEEEEEEEEEEtickticktickticktickticktick starts.

33

(Airbus) The suck zone ahead of a CFM56 on an A320 is less than 5 meters. The couple have enough time to take a leisurely walk toward the cockpit window. The engine start time takes upwards of a minute (the starter is limited to four 2-minute bursts, followed by a 15 min cool down). Newer engines take longer to start. On the neo with PW1000G engines it's ...

28

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 things like anti-icing. The lower pressure port farther upstream will supply air for air conditioning/pressurization and other pneumatics. Some systems combine ...

24

Turbojets and turbofans are very similar indeed: both are turbine engines; both create thrust form jet exhaust; and both have a rotating implement in front that can be called a fan. Although in the case of the turbojet, it isn't called a fan but the compressor first stage. $\$ Junkers Jumo 109-004 So what is the difference? There are four types of ...

23

There are basically 3 limits that the engine faces, temperature (maximum turbine entry temperature or maximum compressor exit temperature), pressure (maximum compressor exit pressure) and stress (maximum stress in the blades as a result of spool speed). Varying the OAT for a specific engine design will hit one of these limits. When the OAT increases, the ...

21

In a turbojet, all the air goes through the engine proper, through the combustion chamber and all the stages of compressor and post-combustion turbine blades. In a turbofan, some of the air is just pushed by a fan around the rest of the engine. This is the "bypass". As Harper points out, it's not fundamentally different from a turboprop or extracting ...

18

WHAT??? I CAN'T HEAR YOU I said, let's get away from this loud engine! Before it can even start to turn the fan, the noises the engine must make will be so deafening as to force the people in that location to flee. And even before that, there would be clunks, shakes, and vibration that would be very noticeable, and put even deaf people on notice to ...

15

Completely different design philosophies They are both turbine engines, and that is where the similarity ends. In a turbojet, the compressor-burner-turbine package is optimized to make thrust. A turbofan engine is a type of turboshaft engine. These use a compressor-burner-turbine core, but use a secondary set of turbine blades to convert its thrust ...

13

Early high-bypass engines were expensive and unreliable. The effort in developing new technology and designs, plus the cost of building a larger fan add cost. Then all of the new technology is inherently less reliable than existing proven designs. The JT9D was notoriously unreliable, causing major problems in the development of the 747. Development ...

13

The Airbus A380 comes with two engine choices: Rolls-Royce Trent 900 Engine Alliance GP7000 The thrust reversers are not part of the core engines. The reversers are developed by the company which produces the engine nacelle, in this case Safran Nacelles. They developed nacelles for both engine variants for the A380, including the first electrically ...

12

There are a couple of sources of loss throughout the process, as indicated in the figure from an old paper format uni book. I've had to translate the labelling, open to suggestions there. The percentages are valid for a high bypass turbofan manufactured in the late 80s. The total energy input starts with the fuel flow: chemical energy per second. Combustion ...

12

What you see is called a flat rated engine. It means the maximum thrust from the engine is constant below the flat rated temperature (usually 30°C). Above that temperature, thrust will decrease due to the EGT (exhaust gas temperature) limit. In order to achieve a constant thrust at lower temperatures, the N1 needs to be decreased accordingly. (CFM56-5A ...

11

Rotor bowing is not unique to this generation of engines. It was an issue with the early IAE V2500. I remember initially they would have somebody manually rotate the fan some time after shutdown. This engine still has a longer start motoring period over the comparable CFM. The root cause is simply making the shafts thinner, increasing their flexibility. ...

9

Those with gear reduction are called geared turbofans (GTFs). In general, you can check the Wikipedia article on any engine, see its classification and specifications, and also check the linked citations. The GE90 and Trent 1000 are not geared turbofans. Wikipedia lists only 5 geared turbofans, with the biggest (a recent engine; first run 2008; entry into ...

9

The rotational inertia of the fan will continue to push air through the engine after the fuel supply has been reduced. Reduce the fuel supply too quickly and the result is like holding a match in a gale: the wind blows the flame out. To prevent this, modern engines use digital controls (FADEC) to ensure that the engine is kept within its operating ...

9

First, the airframe and systems were well-engineered and had few flaws and good aerodynamic qualities. Second, the cost to re-engine it with higher bypass ratio engines would have been high, and as such needed to be included in the cost-benefit analysis and balanced against the costs of operating the plane as-is with its original engines. The expected ...

7

Airlines normally maintain what are called Engine Build Units (EBUs) in their spare engine pool. The engine will be pre-assembled up to the point where variations in position on-wing have to be accounted for. Components beyond that point are only installed at the time the engine is installed on-wing. Designers try to minimize engine "handing", mirror ...

6

No, the fan is directly connected to the outer compressor and turbine (N1). There is no direct connection to the inner compressor and turbine (N2), which could run independently, but there is no switch to somehow stop the N1 rotation in a 747 (and I am not sure, if it would run properly anyway). Some turboprops (e.g. ATR-72) can run an engine without the ...

5

I think there is a bit of a misconception in your question. The DC-8 was already out of production by the time the CFM56 was developed, so all of the DC-8 70 series are actually just retrofits of the 60 series aircraft already in use. So it’s really not correct to say “a CFM56-powered version of the DC-8 became available in 1982.” More like a CFM56 retrofit ...

5

The difference between a ducted propeller and a turbofan is mainly determined by the difference between a propeller and a fan. A propeller has relatively few blades, which are relatively long and slender. A fan has many blades, with a relatively large chord. Like a household fan. A parameter to catch blade count and chord relative to blade length, is the ...

5

Please refer to the following website, it explains the plug programming.: http://nandang-smart.blogspot.com/2014/09/cfm56-7b-identification-plug.html You may read: WHEN ENGINES ARE MOVED BETWEEN AIRCRAFT AND MAY REQUIRE A DIFFERENT THRUST INSTALLATION THIS N1 MODIFIER IS USED IN CALCULATIONS TO UPRATE OR DERATE A GIVEN ENGINE AT AN APPROPRIATE LEVEL. ...

5

Yes they can, and it has been done in the Garrett AiResearch ATF3. Although the engine was not hugely successful, it is still being used in aircraft today. From this answer: The hot exhaust stream is deflected back into the fan bypass, which cools it down and results in a low IR signature. This engine is used in the Dassault Falcon. You can see several ...

5

This was only a problem on the 1st generation of turbojets, where the fuel controller was effectively a needle valve controlled by the thrust lever and the "fuel control" was the pilot's brain. These engines also had the reverse problem; you couldn't "slam accelerate" them without overtemping and/or flaming out. Once hydromechanical fuel controllers came ...

5

The MD80 was not obsolete at the time of its first flight and the decade thereafter. The market situation for 150 pax aeroplanes around 1980: Boeing B727-200: first flight 1967. Three JT8D-7/9/11 engines. MD-80: first flight 1979. Two JT8D-200 engines. The first generation of JT8D had a bypass ratio of 0.7, the JT8D-200 engines had an increased bypass ...

4

Yes, it is possible, in fact it is pretty common. It is called a turboshaft engine or gas turbine. You can find examples of their use in: APUs An acronym for Auxiliary Power Units, these are probably the closest to what you have in mind, this is a jet engine, usually quite small, that is used to provide onboard power and sometimes hydraulic pressure. You ...

4

Not only is rotor bow not unique to this generation of engines, per @user71659's excellent answer, it's not even unique to jet engines. If you go back through the literature, you can find references to it on steam turbines, which have a very similar problem. Many steam turbines are kept rotating at 1 rpm or so while shutdown in order to keep the rotor ...

4

There was no suitable high-bypass-ratio turbofan engine available. What were the narrow bodies flying in the 1960s and 70s? Here is a list: Sud Aviation Caravelle Boeing 707 Douglas DC-8 Hawker-Siddeley Trident BAC One-Eleven Boeing 727 Tupolev 134 Tupolev 154 Douglas DC-9 Vickers VC-10 Ilyushin Il-62 Boeing 737 The thrust needed for those aircraft, about ...

4

No. You need to know one more parameter: How much fuel will be consumed per unit of time. In the end, thrust is created from heating some of the air entering that engine, and only if you know what proportion of air will be used for combustion can you make valid estimates of the thrust than can be generated. What you can calculate from the intake diameter ...

4

Bypass ratio (BPR) alone never tells the whole story. Take a low BPR engine and add to it a bigger fan, now you have higher BPR. Does that mean better efficiency? No. The reason is simple, by keeping everything the same, the same combustor/turbine do not have the right power/design to turn the now bigger fan at the optimum speed (slow fan). But if you use ...

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