Please get your attribution of quotes right…. the “Of course it does” was airborne_again, you are actually reinforcing my point.
Peter wrote:
I recall seeing a TV documentary where it said that every year some dozen(s) of freighters disappear without trace.
It’s what they call Rogue Waves: some evidence here
Airborne_Again wrote:
Cobalt wrote: Mountain or water matters very little at these speeds.Of course it does!
If you consider the density of water, 1 m3 weighs 1000 kg, you would realise that hitting that amount of weight, which obviously has a momentum would not differ much from hitting a mountain. 1 m3 isn’t much to look at, but you can understand now the power of water if you think of the weight of an average car, concentrated in 1 m3. Then think about how boats plane at only a marginal speed (20 kts) meaning the reactionary forces by the water are enough to resist a boat weighing a few 100 kgs at that speed. Then imagine an object hitting that same water at a speed 10 times greater, meaning 100 times the energy. Shattered to bits comes to mind. Aircraft are designed (and only just) to penetrate air rather than water…. a gas with a 800 times lower density.
It also occurs to me that given any asymmetry in the impact angle, (especially left-right because of the wings), the front of the aircraft would be moved sideways by the water before the rest of the aircraft had time to keep up – i.e. the fuselage would be bent open.
Even if the impact were symmetrical, long thin objects are not at all strong if stressed from the end (Euler strut failure). Again, the fuselage would be bent open.
Non-simultaneous wing impact would have the same effect, or amplify any existing effect.
Peter wrote:
I don’t think that is correct. A standard British Oxygen cylinder (like the one I have) is about 5-10mm thick and that holds the gas at 200 bar, and that is with some huge safety margin. Admittedly that is in tension, not compression… but the concave bottom is in compression for sure.
Yes, in tension, and the bottom is reinforced. It is also tiny compared with 5-6 m diameter B777. It’s the diameter over wall thickness ratio that counts. Making a submarine hull is one of the more difficult and complex things you can make.
Today I spoke to two current airline pilots and both said it would be hard to keep the speed down to anywhere near 150kt while achieving the vertical attitude. The aircraft is too slippery.
They did however think the best way to do it would be to slow down and just push the nose straight down, wings level, because there is plenty of elevator authority for that, but a limited aileron authority for doing anything too clever.
Not much. The dynamic pressure of hitting water at 130 knots is 21 bar. That is the same pressure you get when submerged 210 m below the surface. To withstand that pressure the whole nose of the aircraft has to be made of 10 cm thick or thereabout perfectly reinforced steel plates, like a submarine.
I don’t think that is correct. A standard British Oxygen cylinder (like the one I have) is about 5-10mm thick and that holds the gas at 200 bar, and that is with some huge safety margin. Admittedly that is in tension, not compression… but the concave bottom is in compression for sure.
The whole nose section of the plane will most certainly just implode at the same rate as the aircraft hits the surface.
That’s probably largely true, but who cares, if the hypothetical objective is to submerge the whole thing with minimal floating stuff escaping.
My money is on the pilot having done this as a symbolic statement, while preserving the honour of his family.
DavidS wrote:
So I am not sure what this paper tells us about real vertical impacts
Not much. The dynamic pressure of hitting water at 130 knots is 21 bar. That is the same pressure you get when submerged 210 m below the surface. To withstand that pressure the whole nose of the aircraft has to be made of 10 cm thick or thereabout perfectly reinforced steel plates, like a submarine. The whole nose section of the plane will most certainly just implode at the same rate as the aircraft hits the surface.
On p333 (middle right), they say: “We assume that the aircraft is a rigid body”. Again on p338 (top right): “Due to the limited scope of this article, we can’t delve too much into the study of impact effects.” Also if you look at the vertical entry video, the aircraft body remains intact.
So this is just an assumption they make which allows them to model the fluid dynamics with standard CFD software. They do not suggest no breakup, they just haven’t modelled it.
The vertical entry video shows up other modelling assumptions, to do with airspeed and deceleration in the water.
There is a timelog, and by taking three screen captures at 040ms, 250ms, and 500ms, we can deduce that the aircraft travels through the water at a steady 1000 pixels/second, i.e. no deceleration is modelled! As the aircraft descends roughly 30m in 460ms, this constant water-speed is about 65m/s or 130kts. Presumably this is also the modelled airspeed before impact.
So I am not sure what this paper tells us about real vertical impacts. I would expect a higher airspeed and I would also expect breakup.
The original suggestion is that the aircraft would enter the water at say 150kt, not 500kt.
Obviously that would need a live pilot in there. Otherwise it will be in a fast spiral dive (assuming it ran out of fuel).
For comparison: Space Shuttle Challenger’s remains hit the water at around 180kt. Estimated deceleration 200G.
Not exactly a nose-on vertical impact, but tumbling crew compartment wreckage impacting at terminal velocity.
NASA report here":http://history.nasa.gov/kerwin.html
AF447 impacted the ocean with a vertical speed of around 108kt. While not smashed to little bits, it definitely broke up quite badly. Again not a clean entry, but energy-wise 20-25x less than a 500kt impact.
Sure, overall destruction levels would not be the same as hitting solid ground, but there will still be loads of bits, luggage, etc.
The entire idea of an aircraft entering water when out of control and remaining in only few pieces is a bit implausible.
How about 500kt? The thread title was about a vertical impact at/above cruise speed. Nothing magical happens, but the energy is about 70x the energy to dissipate at 60kt.
