28

This PDF indicates an increase by ~10 dB for an F-8K in afterburner versus the same aircraft in 100% dry thrust. This PDF indicates smaller increases: +5 dB for an F-15 +4 dB for F-22 and F-35


24

It has been around 20 years since I've been on a carrier deck, but I recall that it wasn't as dramatic of an increase as you might think. It may have gotten a little bit louder, but what I remember more is that the tone changed. The sound was more "full" when the afterburner was engaged. I realize this is a rather subjective answer.


17

It comes down to the definition of "tail pipe fire". A tail pipe fire is normally what you might call a "static fire" of unburned fuel accumulating in the tail pipe with little to no airflow through the engine, and this is normally during ground operations especially during starts. The tail pipe has an overboard drain for this purpose, but it needs time ...


14

I would say definitely yes, because of all the extra energy added to the exhaust flow and it's obvious to anyone who attended enough military airshows. Watch an F-16 depart with reheat on, then reduce thrust to military power (max thrust with reheat off) on the climb out, and it almost sounds like the engine flamed out. The flow out the nozzle may be ...


12

The answers are all very good in that they detail potential costs, but let me give a different angle for looking at these types of questions. In a highly competitive environment, companies will throw as much money at a problem as it is worth for them to have it solved. In economic speak: "marginal cost equals marginal gain". When designing a new engine, one ...


10

To explain if the afterburner makes the engine louder, you must understand what the afterburner does. In the afterburner, the exhaust gases are re-heated by injecting fuel in the afterburner duct. The left oxygen is used to burn the fuel, which results in an increased exhaust gas flow. Note that the engine itself will not spool up faster: this is done by ...


8

A performance increase after a compressor wash is expected to happen, but it does not return to the original performance as there are more degradation mechanisms at play. This answer explains that organic build-up causes a change in airfoil shape causing the recoverable performance degradation. Another source may be deposition of salts and dissolved ...


8

Some aircraft with turbofans have an engine synchronization system for exactly that reason: reducing the beat resulting from different, but similar, frequencies for the two engines. MD-80: The McDonnell Douglas MD-80 can synchronize its engines by EPR, N1 or N2 when the autothrottle is engaged: Engine Synchronizer System The engine synchronizer ...


7

Adding to other excellent answers, I'd like to focus on the nature of the research. The work involved in developing and exploring ideas that aren't just capable of resolving via computer models, are huge. As other answers note, jet engines develop at the leading edge of theory and new ideas,as well as pushing existing ones. Concrete typical example #1 ...


6

GE patented a synchrophaser system for turbofans in 1991 but I've never heard of the system being implemented. My experiences are with the CF34 family (both flying and on the technical side), both FADEC and non FADEC, and none of them had a dedicated synchrophasing system (except for N1 synching by the FADEc computers) and I can't find anything about ...


5

Yes the specific fuel consumption of the afterburner, lbs of fuel used per lb of thrust, is much higher than the core engine. This is because the fuel is being added to a part of the engine where the air is less compressed, so the energy conversion is a lot less efficient. The bright orange flame coming out the tail pipe when in reheat is pretty much all ...


5

The compressor blades accumulate a coating of organic material from the atmosphere on the blades, which in the later stages, where the temperatures get above the material's flash point, may be mostly carbon from cooked organic material like pollen and bugs (the same sort of blackened accumulated crud you'll see on the butterfly of a late stage compressor ...


5

In addition to the other answers: Jet engines are not only complex, they operate on the edge of what's physically possible. For example, modern jet engines run at internal temperatures that can be higher than the melting point of the metals used. When you design a new jet engine, in order for it to be successful in the market, it has to be better than ...


4

The problem which this gap is solving has to do with boundary layer air as @ymb1 suggested but performance is not the only complication. Boundary layer is not only slower, in some regimes of flight it can become highly turbulent compared to the air that is offset from the aircraft's surface and is freely ingested by the aircraft's intakes. Mixing the two ...


3

I think most of the answers address the points quite nicely, the teams are huge and there is a lot of expensive kit involved. I'd add three more points: There is risk involved that needs to be priced in. It's not like in Pharma, but not all engines sell equally well, so you need to manage the costs across different engines and designs. These are highly ...


3

The problem is not fundamentally about jet engines, but about building complex things in general. The reasons are the same as for building a complex software. There are only gradual differences. The question can be seen as "Why does it cost surprisingly much to create complex systems of high quality?" The main issue is complexity. The design of existing ...


2

First, you should state whether you are designing an engine (increasing the TIT) or if you are doing an off design calculation for an existing engine. In the case of design we gas turbine performance engineers call this design point analysis, increasing the TIT indeed would increase the efficiency, but this would require an increase of the pressure (OPR, ...


2

The flow direction reverses only through the bypass duct pulling by blades with negative pitch. A portion of it will still be sucked by low pressure compressor making 180° turn into the core. Having to counteract the losses in turn will obviously reduce the performance. As stated in research paper by the team funded by Rolls-Royce plc: "the core mass flow ...


1

Air needs to move through the engine for it to work. Turbine engines usually spin above 20k RPM. If there were not structures to keep the air moving through the motor then it would essentially spin in place and the motor would stall. If you cut away the side of the engine the compressor rotors would be look similar to \ and the stators would look like /. ...


1

I work within a engine test facility. During a compressor wash process we inject water and detergent. This also depends on how much work have been carried out during the overhaul on the engine. The positions are given only to provide an even flow of water through the engine. Sadly I have no experience with live aircraft, so I can not comment when their on ...


1

From an energy standpoint, the engine produces heat, thrust, and less significantly, sound. Ignore the afterburner for a second and just consider throttling up, whether a jet or your car. The engine gets louder. That's not a law of physics, that's just what happens. There's no theoretical reason why the extra waste energy can't go 101% into heat, and -1% ...


1

Yes they do use a lot of extra fuel, but the amount is depending on the operating conditions (Mach flight speed and altitude)! Comparing MIL (military) power setting (maximum power without reheat) to MAX power setting (engine will be in MIL power, but the reheat will be scheduled and the exhaust nozzle opened) you can see that it is not uncommon to have 3 ...


1

If you look at it in terms of thrust vs fuel flow, then yes, they're very inefficient. However, if you just look at the amount of fuel burnt to get an interceptor from the runway to 30,000ft, then they can be more efficient. Without afterburners the same climb would take significantly longer and could use more fuel. Without afterburners, you'd need much ...


1

This answer already answers: Are compressor maps such as this one, drawn for one blade row only or the whole compressor? The shown compressor map is indeed for an entire compressor as can be seen from the pressure ratio. If it is for one blade row, can you predict the behavior of the next blade row from the compressor map of the previous row? No you ...


1

From what I've read, bigger props (and fans) are more efficient, because they move more air. However, at high subsonic speeds, the tips of props start to run into the sound barrier. So, that seems to argue for more smaller props/fans, whose ends won't hit the sound barrier. As others have mentioned, extra crankshafts and gears cost weight. I think I've ...


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