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Posted: 03-07-2013 23:16 Post subject: Aeroplane 'vroops'

Not sure if this is strange or something mundane.
I was walking along the other day around the outskirts of Oldham (hilly region) when I heard a plane flying over with the usual roar. However this time the roar was interrupted by a big 'vroop' as if the roar was being sucked into a small hole... then, no more roar. It sounded big and close enough to make me look up for related signs. I asked around and so far no one else heard it. A few days later it happened again but less pronounced. More than likely this is some weather phenomena due to hot air rising and valleys and I don't know what.

It was a hot clear day
The area is very hilly
There is heavy air traffic around there

anyone had anything similar happen in their ears?
I'd quite like to know as it would raise questions not least of which would be how can passing planes seemingly effect the air twenty feet over head

Posted: 03-07-2013 23:19 Post subject:

Dodgy engine?

Posted: 04-07-2013 09:13 Post subject:

I've actually heard that effect at air-shows - some manoeuvre causes it, sort of like the plane overtaking the sound of its own engines. Wish I could be more precise, but its 30 years since I've been to an air display.

Posted: 04-07-2013 10:21 Post subject:

Was this a military plane ? Was thinking maybe the loud roar was afterburners, which when closed would leave an apparent silence in comparison to the worlds ending sound of them full bore.

Posted: 04-07-2013 10:50 Post subject:

so far as I could tell it was a regular plane accompanied by that steady roaring sound them make as they travel across the field of vision before fading off. The vroop was a great big sound and rather comical I'd say the suggestion of a some manoeuvre sounds most plausible but i should really check for reports of planes entering clouds and failing to re-emerge

Posted: 05-07-2013 23:34 Post subject:

Don't know if this is relevant but I live under a flight path and quite often hear planes making a vrooping sound as they pass over. That's not to say they stop making a noise, they don't but they make this weird up slanting vroop. I live about 15 miles from the airport so those going over are well into descent and tend to be getting ready to make a turn towards the airport. Is your location anywhere near an airport?

Posted: 07-07-2013 14:00 Post subject:

I live under the approach path to an airport and often hear planes make a strange noise as they go over , a sudden drastic change in pitch. Obviously airliners don't have afterburners but perhaps there is some other component in the engines that can be adjusted or maybe just drastically reducing the throttle causes it.

Posted: 07-07-2013 15:34 Post subject:

Do you suppose it might be a doppler effect rather than an actual change in engine noise? Perhaps something about the plane making a sharpish turn relative to the person hearing the sound?

Posted: 10-07-2013 22:15 Post subject:

I'm an aircraft mechanic and I've spent a lot of time around airplanes over the past 30+ years. I'm trying to imagine the sound you're describing, but it might help to know some other crucial details. Was it a piston engine driven plane? Turboprop? Jet? How high was it and what was it doing, as in level flight at cruise altitude or descending for landing at a nearby airport? Climbing?

One thing that springs immediately to mind is the possibility that if it was a jet descending for a landing he may have deployed thrust reversers in flight. They're not supposed to do that, and the aircraft should be inhibited from allowing it to happen, but a creative pilot knows his way around that and I've seen it done. If that's what happened I could see that making a loud roar and then suddenly relative quiet. Next time you're at an airport you can hear planes landing and using their thrust reversers and it makes a sound like you describe.

There are other things going on that might cause odd sounds, like changing engine power settings, changing prop pitch, turning prop synch on or off, raising or lowering landing gear or flaps, deploying speed brakes.

Posted: 15-07-2013 10:52 Post subject:

I think I've heard the same sort of sound. For years I lived under the south east flight path for Leeds Bradford airport, about a mile and a half from the end of the runway. The area is hilly and sometimes after a plane has flown overhead there is a sudden rising tone then silence. I also often heard a of loud sishing sound at about the time the sound of the plane decelerating on the runway would have reached me. I don't recall hearing these sounds years ago, only in the last ten years or less.

I've never heard these sounds when aircraft aretaking off in either direction. However, one morning while chatting to a neighbor in the garden a plane was climbing away when the loud sound of take off was cut by an almighty bang and then complete silence. We both looked up, the plane was still rising and looked OK. The engine noise suddenly started again and the plane carried on on its sweeping turn away from Leeds. I said to the neighbor 'I'm glad I'm not on that one.'

Posted: 15-07-2013 19:54 Post subject:

Just guessing, but perhaps occasionally a jet engine may misfire slightly?
Might be a momentary fuel line blockage or glitch in the fuel pump?

Posted: 16-07-2013 18:55 Post subject:

Jet engines can suffer from compressor stalls or surges, where the airflow through the compressor blades is disrupted. They manifest themselves as loud bangs and sometimes flames from the tail pipe. These are reasonably uncommon events however.

Posted: 16-07-2013 19:19 Post subject:

I've been struggling to understand what a "vrooping sound" is....

I've often noticed the sound of a passing plane undergoing a phase shift kind of effect, which I guess happens because the sound has to travel through a turbulent atmosphere with air at different temperatures and densities. The sound will get louder and softer, different frequencies will get emplasized/de-emphasized, sometimes it will go sompletely silent for a few seconds.

Posted: 23-07-2013 15:49 Post subject:

Posted: 23-07-2013 17:17 Post subject:

The engines used on most passenger jets are high bypass ratio turbofans. There's a turbine engine, or "jet" engine that drives a large fan at the front of the engine. The air that goes through the turbine engine is called the core flow, the air that goes through the larger fan is ducted around the outside of the turbine and is called the bypass flow. Turbine engines are pretty noisy so what they do is add a scalloped duct at the outlet of the turbine which serves to break up the exhaust into a group of smaller blasts, so like a huge church organ instead of one large low pitched roar, you get multiple higher pitched tones from it. The higher pitch exhaust noise dissipates over a shorter distance than the lower pitched roar and the overall effect is a quieter sounding airplane as it flies overhead. That scalloped duct is called a mixer, or sometimes people call it a muffler in slang terms. Maybe that all factors in with a Doppler effect to create some odd acoustics....? Just guessing.

I've heard of pilots deploying thrust reversers in flight, and a lotta years ago I was aboard a 727 as we made our approach into Logan Airport over the outer Boston Harbor. They opened the thrust reversers then and I was very aware of that....you could hear the engines briefly spooling up rather hard and at the same time being thrown forward as the airplane slowed down, then they snapped the reversers closed and it got very quiet for a bit. I don't think the other passengers even noticed it or had any inkling what was going on, but that's pretty typical. I'd guess that anyone below us on a boat or island might've heard a strange vrooop sound.