wv_engineer, not sure what sort of engineering you do but maybe you've heard of Bernoullie's Theory. It's what keeps the aircraft in the air in the first place and is caused by differential pressure due to the reduction of dynamic pressure in a moving fluid (the air in this case). 35,000 feet or not, the effect is relative, and enough to support the weight of a passenger liner at speed due only to the marginal difference in relative airspeed between the upper and lower aerodynamic surfaces. Clearly, the pressure difference between the high speed air outside and the static air inside the fuselage would be much greater. That difference would remain (the same way a carburettor continuously sucks fuel into the airflow of an engine) until the relative pressure equalised. If it didn't happen quickly enough through a small hole then the escaping air would expand that hole in the weakened structure to speed the process. That's where the 'explosive' effect comes in.
Not sure what kind of engineer you are mate, wiggly amps maybe?
You clearly have no idea when it is appropriate to apply Bernoulli's effect and you have only half an idea how lift on a wing is generated.
The motion of the plane does not create any pressure differential across a hole which is orientated perpendicular to the airflow, so it does not create any additional decompression effect on a hole shot through the part of the fuselage aligned with the airflow, such as near the passengers.
You are a very silly person who is not qualified to berate others about engineering knowledge.
"...the fact [sic] that they couldn't be bothered [sic] or a woeful and unforgiveable [sic][sic] technical oversight, but they neglected the most significant factor ... the relatively minor difference in static pressure ... a gigantic difference in dynamic pressure if the integrity of the structure was breached..."
You might want to look up the meanings of "static" and "dynamic". When a pressure vessel is breached, there is no longer any pressure stasis.
Not only are you making a comparison that doesn't exist and making impossible claims, but you're blaming people without cause. Shame on you!
"explosive" decompression happens when there is a violent escape of the pressurized air do to a large, major rupture, this can be caused by a explosion of some sort, or by a sudden catastrophic failure of the structural integrity of the plane.
Ask yourself this, how do you think they reduce the pressure in the cabin in the first place when they need to? If they want to reduce cabin pressure they open a valve/vent to let pressure out, this does not cause an explosive decompression, it just decompresses the cabin slowly until the desired pressure is reached, and though I'm not sure of it's exact size I would imagine the valve is larger than a bullet hole.
The balloon analogy is slightly flawed, I'll help you fix it. A balloon ruptures violently when pierced with a pin because the structural integrity of the balloon is relatively very, very weak, when the pin breaks the surface it starts a tear that continues to move around the balloon because the rubber doesn't have enough structural integrity to withstand the violence of the event. However there is no air moving rapidly across the surface of the balloon when this happens, and yet it still ruptures violently, so air moving quickly along it's surface has nothing to do with the violent rupturing of the balloon. So lets increase its structural integrity and try it again. Take a 1" piece of transparent tape (like you would wrap a present with) and place it on the inflated balloon (we have now increased its structural integrity at this location on the balloon). Now poke a pin through the tape on the balloon and remove it. The air comes out slowly and the balloon deflates slowly, no violent decompression.
A bullet hole in a plane is the same.
I guess if you had a 3 foot window on a plane that was made from tempered safety glass, so it would shatter into crumbs when struck by a bullet, you may be able to get it decompress rapidly, or the decompression would more "explosive" in nature, and if the hole was large enough, and in the right location, the air rushing in could tear the plane apart. But none of these things would happen with a hole the size of a pencil.
So unless there is something like a large defect in the integrity of the plane, like major metal fatigue, or a b@mb/explosion of some sort that would cause a large hole to appear very quickly, explosive decompression won't happen from a bullet hole.
As a pilot I know a few people who have experienced rapid decompression, one instance in particular the door seal failed at 27,000 feet. Nothing moves, just a boom and some pain as your ears try to adjust to the instant change in pressure. But that sort of thing doesnt sell movie tickets.
As for good old Bernoulli, a 747 must produce about .9psi on its wing surface to maintain flight, thats as much force as it takes to suck a milkshake through a straw. On the boundary layer of the fuselage you would not see this pressure differential as the fuselage is not designed to reduce the pressure of the air and the air speeds in the boundary layer are reduced to nil as you get closer to the fuselage.
Even if you assume the full effect of Bernoulli, then you are only changing the pressure differential by 1psi, hardly enough to drag you out of a plane.
Ask yourself this, how do you think they reduce the pressure in the cabin in the first place when they need to?
Why would "they" need to?
There are two ways to regulate cabin pressure: where the air comes in, and where it goes out.
The source of compressed air is from the compression stage of a turbine engine. It's known as "bleed air", since it's air that would otherwise be used for combustion, being "bled off" for other purposes. Reducing the bleed air to the cabin will reduce the cabin pressure.
The second place is the outflow valve(s), which exhaust stale cabin air to the atmosphere. Because the amount of bleed air can change when engine power is adjusted, automatic controls also adjust the outflow valve(s) to maintain a constant cabin pressure.
quote:
I guess if you had a 3 foot window on a plane that was made from tempered safety glass, so it would shatter into crumbs when struck by a bullet, you may be able to get it decompress rapidly, or the decompression would more "explosive" in nature, and if the hole was large enough, and in the right location, the air rushing in could tear the plane apart. But none of these things would happen with a hole the size of a pencil.
And the reason for that is that the side windows in commercial airliners aren't made to the same standards as the side windows of (unpressurized, obviously) passenger cars. Commercial passenger planes use high performance plastics in their windows. IIRC, the old 707 side windows could bulge up to 8" without failing. No doubt performance has only improved in the intervening 50 years.
Ok, I took a bit of a hammering there for my last comments. Sorry if I offended anyone, but at least it got you talking!
Looking through this whole discussion though, there are various points and analogies made, some more informed than others. There are also a lot of arguments over semantics, so I'll make my point on a few:
The term "explosive decompression" is well documented and relates to a rapid loss of pressure, often with secondary effect, that results from a breach in the structure. Two examples are the Aloha 737 where the cabin roof tore off and a stewardess was sucked out, and the BA BAC One-Eleven, where the cockpit window blew out and the Captain was partially sucked out of the hole. Both of these events did happen and were officially classified as explosive decompression incidents by the relevant investigation authorities.
Secondly, when a fluid, such as air, moves from one pressure area to another, usually taking any loose items such as seat cushions, debris or even people with it, the effect can be described as sucking or blowing, depending on perspective. Basically, it doesn't matter.
Thirdly, to realise the effect of Bernoullie's Theory, all that is required is a relative airflow. It matters not whether the object or the air are moving. That's why aerodynamic forms are tested in wind tunnels. It's a lot more practical, cheaper and safer than sending them up to fly to test. I do also have slightly more than half a clue as to how how lift on a wing is generated but it would have been a very long-winded way to set up my point if I had to explain the theory fully.
Also, a couple of analogies:
Try running water from a tap (or faucett, depending on which side of the pond you're from). Whilst it's stored in the closed piping, mains pressure is normally about 12 psi. Opening the tap releases this though, so that the pressure in the flow of water dissipates to ambient. Next, take a spoon and dangle it by the handle so that the convex side touches the flow. Although you may expect the spoon to be pushed away, it's actually drawn into the flow. This is because the total pressure (combination of static and dynamic) in the moving water stream is lower than the total pressure in the static air around it. An example of Bernoullie's effect. (I appreciate the water, having higher density, exaggerates this but it shows the effect). Now, slam the valve closed as quickly as possible and, particularly on an older system, you may hear a clunk as the flow stops and the presure builds up in the pipes again. That may seem out of place just now but I'll come back to it later.
Another example is to watch a convertible car driving at speed with its roof up. You'll see that the roof is bulged upwards. This is because the total pressure of the air inside the car is higher than the total pressure of the air with a relative airflow over the car. Even although their static pressure is equal, the lower dynamic pressure of the outside air makes the difference. This effect applies to hardtop cars too but, because the structure is rigid, there's no noticeable bulging, just like an aircraft fuselage. (On old, fabric skinned aircraft they used to add rigidity to the fabric by impregnating it with 'dope', partly because of this, but now I'm just being geeky.)
Regarding the balloon, it's designed to withstand a certain amount of pressure. Clearly, if you over inflate it though, it will burst. But if you rupture an otherwise stable balloon with a pin, it will also burst in an "explosive decompression" kind of way (this is actaully quite an accurate example as it's monocoque structure means that frames, ribs, etc. do not confuse the effect). The static pressure within will dissipate to ambient. Although this happens quickly though, it's not instant, and a pressure differential will exist until the process is complete (to counter b00mb00m's point about breaching a pressure vessel). Although the pin only made a small hole though, the balloon ended up as mere tattered remains. This is because the structure only retained its strength whilst intact. The small breach propagated because the remainder of the structure, once weakened by the hole, could not withstand the pressure difference for long enough for it to dissipate. Bear in mind that, whilst a road vehicle is engineered with an average safety margin of 10:1, aircraft, having to be lighter, are normally around 2.5:1, so they're really not very good at continuing to work with holes in them.
To prometheus6789, as a pilot you clearly know a bit about aerodynamics, but the pressure difference you considered relates only to the upper and lower surfaces of the lift planes, both of which are subjected to the relative airflow, albeit increased above and decreased below due to the shape and angle of attack of the aerofoil. You also mentioned the boundary layer. On these surfaces, it's obviously critical, so a great deal of research and expense is involved in controlling it there. On complex aircraft, techniques such as slats, slots, flaps, fences, vortex reducers and generators and even sucking and blowing are used to do this. On the fuselage it's not as critical though so it's never going to be as clean. An example is the pitot/static airspeed indicator. Even although the pitot head points as accurately into the ram air as possible, and the static vents are placed as near perpendicular as possible, the position error still means that complex calculations are required within the instruments to rectify the readings. If you've ever flown rotary, watch the ASI flutter when holding a still air hover due to the effect of the downdraught over the static vents.
Lastly, as pointed out, cabin pressure is maintained by bleed air from the engines. This usually comes from between the first and third stage of compression and, as well as maintaining cabin environment, also runs many aircraft systems. It's not just a case of pumping the plane up though. The air is circulated through, and out of the cabin (several airlines have been criticised for not circulating enough air in an attempt to save fuel as it saps power from the engines). This circulation takes place through parts of the cabin that are designed for the job though and it's strictly controlled. You are right that major parts of some aircraft have 'blown out' without catastrophic effects. Let's just say that they were the lucky ones.
Anyway, if anyone's still reading I suppose I'd better get to the point now that I've set up my argument.
In the case of the BA flight, when the Captain was sucked out of the window, on this aircraft, the window was fitted from outside. Shortly before that fateful flight the window was replaced. Due to a combination of technical errors (disasters are almost always due to a series of events) the engineer used the wrong screws to fit the window. When the aircraft pressurised (relative to outside) the force on the screws was too much and the panel blew out. Had this been all that happened, the cockpit pressure would have dissipated quickly, as it's a fairly small volume space, and he'd probably have stayed in his seat. Unfortunately though, the cockpit door, which was never designed to hold pressure, was blown forwards and into the control console by the pressure differential that now existed between the cockpit and the cabin. This caused a second explosive decompression but, as the cabin is much larger, the higher volume of air being sucked, blown, whatever, out of the open window was enough to drag him along with it. Luckily, his legs snagged on the controls, giving the First Officer enough time to grab his belt. An emergency landing was then carried out effectively, with crew still hanging on to the poor Captain, and he eventually recovered (although had some injuries including frostbite).
On the Aloha flight there were again contributing factors. It was an old aircraft and, although it had very high flying hours, what was more significant was the amount of pressure cycles it had undergone. It had been used for inter-island flights for many years. each flight time averaged only 25 minutes but the sortie involved a climb to altitude and subsequent descent, meaning that for every flying hour, the structure underwent far more pressure cycles than normal. This caused accellerated metal fatigue compared to airframes of similar age.
Whilst at 24,000 feet on that fateful flight, a portion of the forward left fuselage ruptured. This caused an initial explosive decompression that was enough to suck a stewardess out of the cabin. The weakened structure continued to disintegrate, largely due to the aerodynamic forces of the airflow peeling it away and resulted in a huge portion of the cabin roof being removed, exposing several rows of seats.
Unbelieveably, they still managed to land the thing but the subsequent board of investigation discovered that a passenger had noticed a crack in the area of the initial breach whilst boarding. Unfortunately though, that passenger did not mention it to the crew at the time. That last pressure cycle was enough to propagate the crack, causing a failure.
Just in case you're still wondering about the tap thing though, a second enquiry came up with an additional theory as to why so much damage occurred. The initial rupture caused a fairly small hole, but one big enough for the stewardess to fit through, or at least almost. Horrifically, her body actually plugged the hole for a short period. Evidence of this was the poor girl's remains splattered down the side of the panel that was recovered. This caused the same effect as slamming the tap shut and the resultant secondary damage, punching out the roof due to the pressure shock. She probably saved the rest of the passengers though, as the remaining presure then dissipated so quickly through the much larger hole that the airflow didn't last long enough to overcome anybody else's inertia and carry them out as well.
There are many more instances where explosive decompression has occurred, sometimes it's been catastrophic, in other cases the sructure has held. The question in this case though, was 'Would a bullet through a window cause it?'
I still don't think that's been tested. On its own, a small hole would not cause the explosive and damaging effect but, just like the pinhole in the balloon and the crack in the Aloha 737, it may well weaken the structure enough to cause the secondary effects. This could only be proven if the experiment was subjected to all the forces that affect an airliner flying at altitude and speed.
The myth was interesting. One thing you guys failed to aknowledge is the venturi effect that the planes movement would have on any opening. This could possibly suck something out of the plane even without any explosive decompression.
Originally posted by -rational-: You haven't a clue.
You clearly have no idea when it is appropriate to apply Bernoulli's effect and you have only half an idea how lift on a wing is generated.
The motion of the plane does not create any pressure differential across a hole which is orientated perpendicular to the airflow, so it does not create any additional decompression effect on a hole shot through the part of the fuselage aligned with the airflow, such as near the passengers.
You are a very silly person who is not qualified to berate others about engineering knowledge.
Actually this is wrong . first you have difrential pressure inside the plane vs outside the plane starting the reaction.
then you have the vertical component of lift from that difrential , high pressure inside , low pressure outside so that if you rotate the longitudal axis 90 degrees ( roll) in a normal situation it would act like lift itself.
you can actually do a small scale experiment on this by taking a sealed container with a barometer in it and make a sealed hole and take a high powered compressor and blow air across the hole . the ratio of hole size to actual contained air inside the chamber and velocity of air will all effect how much pressure drop can be seen on the barometer but you will in fact see a drop .
now add to that the difrential pressure of a plane calibrated to 8000 ft ( what we usually have our pressurization set to at altitude) and the outside pressure at flight level 320-360 ( typical altitude ) included in total outside pressure is the compensation for lapse rate and its effect on air density .
ok so now we have established that it does not matter the direction as difrential pressure and newtons laws also apply .
Originally posted by spikemus: Wouldn't the fact that the plane is moving over 100 mph also have an effect on the sudden decompression?
exactly folks. that is the whole idea. the effects of explosive decompresion that has effected planes has never happened on the ground before take off. It happens in the air. So what kind of speed are we talking about just for a ,say 747 at cruise? Its about 570 mph. use that speed as a baseline and then start testing the theories. I know that the speed will cause a suction but the effects of the explosive decompresion will be magnified. Making the "simple" bullethole that "punches a hole straight through" even worse.
This message has been edited. Last edited by: spankies,
I know that the speed will cause a suction but the effects of the explosive decompresion will be magnified.
The outside air pressure at 35,000 feet is about 3.47 psi. Just how much of that do you think you can remove ? And do you really think that the difference between 8 psi and 11.47 psi would make a huge difference on a bullet hole ?
OK the myth was about explosive decompresion Jamie and Adam both got the myth correct. Just air pressure alone is not enough to "suck" anything out of an Airplane.
What you see in movies is right also.
A bullet hole in the side of a jet moving fast enough (600 knts.) Will cause the fuselage to rip apart. That is just common phyisics.
So forget about the psi. The only thing that causes people to get "sucked" out of the airplane when it rips apart is the shear force of the air moving around. Try this get in the back of a speeding pickup stand right behind the rear glass of the truck and then when the truck is up to speed the poke your little head up and wham you get pushed back...
A bullet hole in the side of a jet moving fast enough (600 knts.) Will cause the fuselage to rip apart. That is just common phyisics.
Actually, its more like strength of materials and the skin is way to tuff to be ripped by wind. Yes, with a large enough flap hanging loose, it might rip. But the wind will not change a bullet hole into a rip.
I AM BY NO MEANS TRYING TO SAY THAT YOU GUYS SCREWED UP.BUT I WAS JUST CURIOUS WOULD THE MOVEMENT OF THE PLANE AT SAUCH A HIGH SPEED MAKE A DIFFERANCE?I MEAN IT WOULD BE LIKE A VACCUM WOULD IT NOT?THE AIR FLOWING PAST THE PLANE AT THAT ALTITUDE AND AT THAT SPEED SHOULD MAKE A HUGE DIFFERANCE.
No offense, but I found a problem with the revisit of this myth. Since you were unable to get the wind speed needed you were unable to test the damage the wind speed itself would do to the hole made by the bullet. We all know that wind speed alone has been able to rip off loose and damaged pieces on the bodies of racecars, the same would apply to broken glass thus ripping it apart.
Now I know testing this in the air is well risky but wind tunnels can get up to pretty high speeds. With the speed of the air going across the hole in the window you should be able to see the damage that one little hole can really do at 560 mph.
I thought it was the Venturi Effect, not Bernoulli. However the moving airflow outside the hole should actively suck the air out of the cabin. It won't just equalize, which is all the Mythbusters' test was testing.
