quote:

Originally posted by buster255:
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.

I welcome your comments.

quote:

Originally posted by oscardeuce:


My old "steam" altimeter has no "complex calculations" going on inside it. The altilmeter reads independantly of airspeed. In fact my fuselage (Cessna O2-A) creates more Bournelli effect than the static port. I can test this by opening the alternate static port by the pilot's seat. The altimeter jumps up about 20 or so feet.

quote:

Originally posted by avillarrealpouw:
Please, everyone that still thinks this myth is not busted, consider the following:

Even considering the worst case possible, where a perfect vacuum is achieved on the outside, and a full 1 atmosphere pressure is in the inside, the force exerted by the air gushing out of a half inch diameter hole is less than one quarter of 14.6 pounds that would be exerted on one square inch.

We are talking about just the equivalent of 4 pounds of force.

Unless the bullet can start a crack that will end up ripping a big part of the fuselage, there is no way this kind of forces can do any major damage.

Only if you can apply a part of the force caused by the sudden appearance of a big hole, can you move large objects.

quote:

Originally posted by buster255:



You're looking at this the wring way. The air escaping through your 1/2 inch hole isn't the problem, it's the rest of the air pushing against the whole fuselage, 'wanting' to escape that will do the damage. This can propagate the initial damage and allow the airflow outside to make matters worse. Read my earlier post about the Aloha 747. That was caused by a crack - there's no argument - it was! Also think of the balloon. It's only a tiny pinhole so, by your reckoning, all you should get is a slow puncture?

quote:

Originally posted by -rational-:
Utterly wrong.