Beat-up effects

[ACW599]

Filthy weather and no flying today so settled down for the annual re-read of that WW2 classic 'Night Fighter'. Amongst much else, there's a wonderful description of a Mosquito beat-up in which the author writes: "The other aircraft lifted only just clear of our roof, and for a moment I thought he had left it too late. The pressure wave struck down viciously as he flashed over the top of us".

I've often wondered about this description and whether it reflects reality. If you're lucky (?) enough to be underneath a very fast low-flying aircraft of Mosquito size, does a "pressure wave" do any such thing? A Chipmunk at about 50ft and 90-odd knots certainly doesn't cause any discernible pressure effect, but that's the closest I've been to anything vaguely similar.

Any thoughts from the learned?

[clicker]

Don't know about a/c as never been that close to a fast mud mover and certainly not a tech minded person .However on the "Myth Busters" program a couple of weeks ago they did a test with a fast moving train to prove if a person could be sucked onto the tracks by the forces generated. The results showed that the dummy didnt get sucked in but also didnt force it away either but a pushchair with a child got blown off the platform away from the train. Can't remember to trains speed but it was the fastest allowed.

[teeteringhead]

Got my para-teepee blown away on a desert survival course by a beat-up by a Shacking great F:mad:cklebomber, but that may have been the propwash ......

...... much swearing until he pulled up and dropped a Lindholme container full of cold Amstel!;)

[Captain Kirk]

The very nature of an aeroplane will create ‘pressure effects’ around it – an aerofoil stays in the air because of the high pressure region beneath the wing attempting to migrate towards the low pressure region above the wing - well, that and all the little Bernoullis that hold it up! Try holding the back of a spoon under a running tap – it will get sucked into the stream, which is a simple demonstration of the drop in pressure associated with the water speeding up as it travels over the back of the spoon; the same effect is present when air passes over a wing. If an aircraft passed low enough over a person, they would definitely experience the pressure change. The swirl of wing-tip vortices can often be heard after even a moderately low pass or from an aircraft on approach to land.

[ACW599]

>The very nature of an aeroplane will create ‘pressure effects’ around it – an aerofoil stays in the air because of the high pressure region beneath the wing attempting to migrate towards the low pressure region above the wing - well, that and all the little Bernoullis that hold it up!<

I'm familiar with Bernouilli's Theorem and the Kutta condition and so on; perhaps I didn't frame the question well enough. What Rawnsley & Wright called the 'pressure wave' is presumably downwash aided and abetted by circulatory flow. The question was really whether someone underneath would really experience that as the kind of sharp transient implied in the text or whether (as one might have intuitively thought) it would be a more diffuse pressure increase.

>The swirl of wing-tip vortices can often be heard after even a moderately a low pass or from an aircraft on approach to land.<

Interesting; I've never experienced that.

[GOLF_BRAVO_ZULU]

Stand/sit/lie on the opposite side of the runway, about 200 yards off, to where the crowd is when the Reds are displaying: you'll hear it. A very strange and erie sensation.

[clicker]

A very strange and erie sensation

Yes I'll agree with that. I was at Stansted a few years ago near the approach lights to twy 23. and quite a few seconds after each aircraft flew by I heard this noise which I was unable to id until I saw the bushes waving about and then twigged it was the vortex effect that caused them to move about.

[Wader2]

This has been mentioned before but the USAF did trials with an ultra low level F105 to see if the pressure wave could cause injury or death. They concluded that it could but the aircraft had to fly at 5 feet.

It had as much chance of knocking your head off.

[airborne_artist]

ISTR reports of Buccs taking out simulated SAM sites on Red Flag by flying low enough to build up a moving wave of sand.

[enginesuck]

Stood on the stern of illustrious after IV won a target towing bombing comp , there were some amazing beat ups, with the harriers probably only twenty feet away and at eye level going max chat, I can definately say there is a pressure wave, most people hit the deck, amazing amazing experience. Wish i had filmed it. I believe one of the pilots had an interview sans cafe after that one.:}

[aviate1138]

Not a usual hard arsed military machine but it does produce amazing vortices. The Boeing 757. The flap end vortex can be seriously vicious and long lasting under the right conditions.
Work of art this pic..... :)
http://www.airliners.net/open.file/%201008033/L/
Aviate1138

[Saintsman]

I'm sure you'd feel the pressure should you be on the receiving end of the wave created by ground effect. That really is a big bubble of air that when released must dissipate somewhere.

[dakkg651]

I remember standing on a grandstand at the end of Silverstone's remaining runway a few years ago when a couple of GR7s departed. After taking off, the lead aircraft carried out a circuit followed by a low, fast run down the runway. The aircraft passed by about 75 feet away from where I was standing. About five seconds later a tremendous half second pulse of air sent my prized (free) McLaren cap on a 50 yard flight into new ownership.
This pressure wave was strong enough to knock a certain CFS(H) chap off his feet accompanied by much merriment from his colleagues (except me. I'd lost my bl**dy cap). The wind at the time was calm, so this short term gust was without doubt caused by the aircraft. The surprising thing, apart from the strength of the pressure wave, was the five second gap between the aircraft passing us and the arrival of the wave. Anyone explain?

[ACW599]

>This pressure wave was strong enough to knock a certain CFS(H) chap off his feet accompanied by much merriment from his colleagues (except me. I'd lost my bl**dy cap). The wind at the time was calm, so this short term gust was without doubt caused by the aircraft. The surprising thing, apart from the strength of the pressure wave, was the five second gap between the aircraft passing us and the arrival of the wave.<

For this and all the other inputs, many thanks -- fascinating stuff.

Sounds as though Rawnsley & Wright's description was right on the money. If only there was the faintest chance of being able to stand underneath a passing Mosquito to verify it :-(

Thanks again to all for the enlightenment.