Originally Posted by
Bullethead
G’day Gents,
I have, over the years, flown several different airliners with all the major engine brands and model of jet engines and some of them buzz or hum from light off to idle and some do not.
Now this is a memory stretch but I recall a tech instructor once explaining the hum/buzz after the engine lights off as an instability in the compressor airflow as the engine accelerates to idle.
Apparently the engine fueling schedule during start is controlled to ensure the engine is accelerating just on the stable side of the surge line and humming. Cross the surge line and you get a compressor stall and all sorts of problems, some engines are fitted with surge bleed valves to relieve compressor pressure during start to prevent a surge. Some engines modulate the engine anti ice valve to achieve the same result.
Once stable idle is achieved the hum/buzz no longer occurs. Of course there are many noises associated with engine start and operation but the above is how it was explained to me many years ago.
Mind you it could all be rubbish as I said it was a long time ago.
Cheers,
BH.
Not so much compressor instability as burner instability - otherwise known as "combustor rumble". While it's true that starting is generally done at the edge of compressor stability, if it actually goes unstable it'll result in a compressor stall (aka 'start stall')(and that's bad...).
During start, the actual pressures in the combustor are relatively low (especially as compared to the ~30 to 1 range during high power operation). This allows the flame front to 'move around' in the combustor which can give that rumbling sound. Now if that flame front instability happened at power, it would quickly fail the combustor due to localized overheating, but during start the temps are pretty low so it's not an issue.
It's really rather amazing some of the things you can hear - even during high power engine operation - if you know what to listen for. At one point in my career, I was responsible for the engine running functional tests (i.e. the engine running stuff they routinely do before the first flight of a new production aircraft). Back in the JT9D days, they needed to run to takeoff power in order to check/set the stator vane and bleed valve schedules, and JT9Ds were not exactly known for their fan and compressor stability (with FADEC, we normally limit high power static running to climb power). While we routinely tried to point the aircraft into the wind, it wasn't always practical to get it straight into the wind - especially due to jet blast concerns at high power. OTOH, running at high power with no forward speed is very challenging for the inlet with a sidewind, and an engine surge due to inlet separation during a ground run could mean a six-figure repair bill to a brand-new engine. The engine run guys had finely tuned hearing - and in the flight deck you could literally hear the sound of an inlet starting to separate - at which the guy controlling the throttle would quickly slam it to idle. I was initially skeptical that one could hear the inlet separate - until I experienced it first-hand. It was a weird, 'sucking' sound and immediately discernable from the normal 130db roar of a JT9D at TO power.