Sound of engines echoey

[bbrenham]

I've always wondered about the echoey sound we get from F-15's when coming out their shelters first thing in morning and going back late at night after their engines have cooled off outside for a few hours. Its a very echoey sound, like they are emerging from a long tunnel. Only get this when they being brought out and put back. Its nothing to do with weather or wind direction. Just been out gardening on this wet morning and heard them coming out of their hangars with this echoey sound.

My imagination has sometimes wondered if there is a long underground tunnel and those surface shelters are just the entrance. I doubt it, but it is the sound you would get of something moving along a long tunnel many hundreds of yards and not those relatively short shelters.

I would be very surprised if a base built for the cold war did not have some underground infrastructure and it would make sense to try to make it capable of withstanding a direct nuclear hit on the base. Living here in Lakenheath, I've always joked about us being ground zero and we would never know anything about it if the worst happened.

It would answer a 30 year curiosity if anyone knows.

Thank you
Brenham

[Mark]

To be fair, the base was built in 1941 after initial use as a diversion airfield in 1940 and a bombing range during the first world war.

Maybe the echoey sound is just engine noise from the extractor fans/vents at the back of each HAS?

[sschofield]

There may well be some sort of tunnel structure under the base, but it's only going to be for cables, ducting, infrastructure type things. There are a number of "vaults" in the floors of some of the HAS's that used to contain the B61s back in the day, but they're long-redundant.

I don't think anything can withstand a direct nuclear attack, that's kind of the point isn't it?

[Nighthawke]

Hi Brenham - afraid "those relatively short shelters" are just that. To be honest there wouldn't be much point in an underground nuclear-proof facility if the rest of the base is dust other than a post-apocalyptic museum!

Having lived all my life in the shadow of military airfields, like you I have always accepted that should it ever happen then that would be it in less than a blink.

Anyway back to your original question - I have no solid answer - maybe the fans idea from Mark is right - sounds reasonable. Hopefully someone who knows will reply.

[PeteHemsley]

I believe the sound you are referring to is just the starting sequence. The f-15 isn't fitted with an APU so the start sequence is initiated by a JFS (jet fuel starter) supplied with compressed air from an accumulator (the echoey sound you hear prior to ignition).

[PeteHemsley]

Here's a great video with HD start proceedure.

https://youtu.be/ZkYqDYuH4uo

[Malcolm]

There is a fairly high pitch "whoooop" sound that F-15's make twice in quick succession during their engine start-up. I understand this is the servo actuators that operate the jet intake ramps. If you look at an F-15 parked, you will see that both the intakes are parallel to the ground. If you then look at a photo of one taxying, you'll see that the intakes are tipped forwards/down. If you stand next to an F-15 as it starts up (a noisy priveledge not many get to experience nowadays) the "whooop" occurs as the transition from horizontal to droop occurs. Can't say I've noticed it during shutdown - probably does happen but you're usually deaf by then.

[bbrenham]

Thank you for all your interesting and helpful replies. We are probably never going to get to the bottom of this, but I'll explain in a little more detail.

First of all, its not the whoop whoop noise. I normally hear that shortly before take off and always thought it was the pilots testing the fan type bits at back of engine that open and close the diameter of back of engines.

This is a normal reverberation echo like you would get in a bathroom or inside a train tunnel. It only happens at start of day when bringing planes out and again at end of day. They are taxying a low very low speed and this is long before they take off. Perhaps the echoey sound is something to do with the terrain around base or some other factor or the buildings near me, but I have always found it odd its only when taking aircraft out of hangars and when its time to put them back late in evening, often around midnight. They are moving when I hear them. Sometimes even seen them from the warren slowly moving along tarmac while getting this sound. Its just a 40 year mystery for me.

[Mattaddiction]

Thank you for all your interesting and helpful replies. We are probably never going to get to the bottom of this, but I'll explain in a little more detail.

First of all, its not the whoop whoop noise. I normally hear that shortly before take off and always thought it was the pilots testing the fan type bits at back of engine that open and close the diameter of back of engines.

This is a normal reverberation echo like you would get in a bathroom or inside a train tunnel. It only happens at start of day when bringing planes out and again at end of day. They are taxying a low very low speed and this is long before they take off. Perhaps the echoey sound is something to do with the terrain around base or some other factor or the buildings near me, but I have always found it odd its only when taking aircraft out of hangars and when its time to put them back late in evening, often around midnight. They are moving when I hear them. Sometimes even seen them from the warren slowly moving along tarmac while getting this sound. Its just a 40 year mystery for me.

I've always assumed the 'eeerrryy' sound rattling around when they are starting jets up (they sound quite eeerryyy spinning up as it is) is down to the terrain, the base is sorta in a dip where it is, nature reserve on the west?? side (between the base and the village, behind the bomb stores) is higher and you can look down on the base from, and the trees of Thetford forest surround the rest, along with the buildings/accommodation to the south (06? end)? So acoustically a lot of the noise probably rattles around the open field that is the base and makes it even more sci-fi. /just my though (may or may not bee the sort of noise your talking about).

m

[Johnny]

It makes me imagine what a dinosaur in the distance sounds like, the sound on the video link when the air intake slopes down on start up I have been woundering about for ages, now I know