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Jet Engines "Backfiring"?

Tue Apr 03, 2018 2:47 pm

Listening to the beautiful music that are the afterburners of some (or one?) F-16, there were a series of semi-loud bangs. Definitely not loud enough to alarm me (compared to the basic engine noise) but nevertheless powerful enough to make me feel the sound pressure in my body - some 3 km away. While the engine(s) were kept at that power level for maybe 30-60 seconds those bangs occured intermittently (the aircraft was presumably stationary, I couldn't see it). Reminded me of a back-firing piston engine or engine knocking / pinging.

What would be the cause of this? I've only experienced this on engines with afterburners. Regular high-bypass civil engines only have that deep rumble coming from behind but it's much more constant.

On another note, the sheer loudness of the engine surprised me. Going with typical descriptions, I would assume that this was around 90-100 dB, maybe more. Normal F-16 takeoffs are much quieter and their landings are basically silent.
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Re: Jet Engines "Backfiring"?

Tue Apr 03, 2018 3:59 pm

Lighting the burners I suspect will cause this.
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Re: Jet Engines "Backfiring"?

Tue Apr 03, 2018 5:39 pm

Compressor stalls during an engine run.

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Re: Jet Engines "Backfiring"?

Tue Apr 03, 2018 6:34 pm

The term 'backfire' actually refers to an engine that blows back through the carburetor, especially if the timing is too far advanced. Unfortunately the public use this expression for a miss fire, which is unburnt fuel, due to a fouled plug, that accumulates in the exhaust and subsequently explodes on the next cylinder firing. In the old days the backfire could cause engine fires in the carb and inlet manifold and is why they always had a fire extinguisher handy on start up. Also opening the throttle fully would also help to stop the fire. :)
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Re: Jet Engines "Backfiring"?

Tue Apr 03, 2018 6:54 pm

mxaxai wrote:
Regular high-bypass civil engines only have that deep rumble coming from behind

Not always --
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Re: Jet Engines "Backfiring"?

Wed Apr 04, 2018 6:04 pm

I remember USAF F106s would emit a loud bang when the afterburners were lit. Not sure if the F16 noise is similar.
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Re: Jet Engines "Backfiring"?

Wed Apr 04, 2018 9:14 pm

Sounds like a compressor stall. When the afterburners are lit the exit nozzle needs to expand to match the increase in mass flow rate. Sometimes the compressors can briefly stall as the ABs are lit before the nozzle expands fully emitting loud bangs.
I'm sure this thread will take off soon
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Re: Jet Engines "Backfiring"?

Sat Apr 07, 2018 10:45 pm

GalaxyFlyer wrote:
Compressor stalls during an engine run.


Sounds probable to me.

Compressor stalls can occur during ground runs if the airplane inlet is not directly oriented into the prevailing wind. The off axis wind vector plus the engine induced "sucking" vector causes the flow to separate and the fan face pressure to drop. The loss of mass flow causes the fan/compressor flow to reverse and the engine surges.

This is less often a problem for a subsonic airplane like a jet transport because the inlet lip can be fat and is more forgiving of off centerline airflows.

On supersonic airplanes, like the F-16, the inlets need to be sharp, making them prone to separation for off centerline ground run airflows.
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
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Re: Jet Engines "Backfiring"?

Thu Apr 12, 2018 9:02 pm

Was on an AA 727 coming into DFW some years ago that suffered a compressor stall. We heard a very loud bang from the empennage and felt the a/c drop. While recovery was quick it was definitely a pucker moment, that's for sure, given our low altitude.

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