The problem is that water is MUCH, MUCH denser than air. So, as the nose strikes the waters surface, it is very rapidly slowed as much of it’s momentum is suddenly expended against the waters friction. However, the sections still in the air aren’t experiencing that level of friction, so continue moving forward at the same pace, resulting in massive, rapid compression along it’s length (something aircraft frames are not designed to handle).
As for “flying” underwater, that is quite possible. Many years ago, Popular Mechanics had an article about a type of submersible that used (some of ) the same aerodynamic effects as planes to control it’s depth in the water.
Air and water share a lot of the same flow properties, so an airplane could theoretically fly underwater if it had: [ol]
[li]some form of propulsion/power generation that could work under water (don’t know of any plane right now that meets this requirement)[/li][li] It’s airframe were robust enough to withstand the water pressure (and the transition from air to water, if you want to do that), which is a lot higher than you’d expect (just 13 feet down water pressure is too for a human to breathe without assistance (e.g. a pressurized scuba tank)).[/li][li] It’s wings could be reshaped to provide negative lift, to counter the natural buoyancy of an air-filled container (possible on many planes with flaps and slats, would be very much like flying in the air upside down).[/li][/ol]
Basically, you’d want a spaceplane with a super-sturdy airframe, and swap it’s main thrusters with propellers.
This discussion has diverted into several different scenarios (all equally very interesting), but for clarity, my original thought was about a structurally intact, dead plane, no power, no pilot, etc. I’m not aware of any plane that has ever hit either water or soil in a perfect nose down vertical position. ??? But if it did, the only force I could think of that would affect the outcome was aerodynamics & friction.
This I think is the piece I was missing. Now I understand the reason it’s impossible to enter the water intact. And it would not matter how pointy the nose is. Aerodynamics have nothing to do with it.
Because I was originally thinking only about the friction, I couldn’t reconcile how this was so. Falling flat has more surface friction than nose down. I forgot about the force of energy from momentum.
Speed is a major factor when you compare birds to planes. Think of the difference between an Olympic high diver and a suicide attempt from the Golden Gate Bridge. A high diver enters the water just right, and from not very high compared to a bridge jumper, who hits the non-compressible water faster and typically at a fatal angle.
US Airways Flight 1549 also landed on the Hudson in the direction of its flow (southerly). It had landed against the current, the damage to the plane would have been ore significant and the plane possibly would have sank.
There’s also the fact that the ocean isn’t a river. The chances of find calm ocean upon which to land would be remote,especially if there wasn’t a plan to land in the ocean in the first place. Even moderate swells would damage the aircraft and it would break up on impact.
I don’t think I’m communicating my thoughts very clearly. Let’s say we suspend a 737 a few feet underneath a barge in deep water. Every cavity in the airplane is allowed to flood completely. For argument’s sake, lets say it was out of fuel, and the fuel tanks have flooded with sea water as well. Now release that airplane. How do you think it would behave? Do you think it would plummet straight to the bottom with no forward motion at all? I imagine it descending almost vertically initially, then experiencing a number of long-period phugoid oscillations, then finally settling into a steep, stable descent, with the wings providing some amount of lift and the horizontal stab providing some amount of downforce.
Thank you, that was good information.
As mentioned before, airplane fuselages are designed to contain pressure inside the pressure vessel rather than resist pressure from outside. One of the airplanes I fly has the following limitations with regards to pressurization: +10.28psi/-0.25 psi. In this case, negative pressure means more pressure outside than inside. If you consider that the factor of safety in aerospace is typically 1.5, that might lead one to believe that my airplane would experience some sort of failure of the pressure vessel when the pressure outside is 0.375 psi greater than inside. That difference can be found less than 1’ underwater.
The -0.25 psi limitation may be imposed due to some other factor, so even if you assumed that the fuselage could resist 10.28 psi of exterior pressure (which it can’t), that’s still only 23 feet of water before Bad Things start happening.
I do not think the water would stop an airliner that is underwater with no air spaces not filled with water. We have an 100,000 pound airplane. It is underwater. It is freed from whatever tethering system used, it starts to sink.
The CG of the aircraft is where it should be. It will start to move FWD as well as down because it weighs 100,00 pounds. The things sticking out all over it, some rather large, will affect the effect of the sinking. It ain’t gonna go straight down unless you had it pointing straight down and had some way to move the control surfaces to keep it that way as the fluid moving over it & them would be shifting to some extent.
Gravity is the power needed to be able to control things somewhat on the way down. YMMV
Not all aircraft will come apart if dropped into water nose first.
How fast?
What is it’s shape.
How strong is it.
And other stuff.
Now I have gone off some pretty high stuff into water and my head has never come out my ass from the impact.
Onto the ground, prolly going to make a mess of your legs but if you are straight and perpendicular to the ground, I do not think a high speed camera would show your head crumbling, even after the lower half of your body has hit the ground.
Ergo, the tail nor the aft fuselage crumple before the wings hit.
Now I would think that an F-104, or an F-5 or T-38 Talon would be rather amazing in how fast they could hit the water going straight down.Lets line up 50 different planes and drop them from 32, 64,128 feet and on up until they are going 66 MPH on contact with the water.
Would a super Connie do better than a 747?
The F-104 vs the P-51?
With enough power you can make a brick fly, in the air or under the water.
In the unlikely event the plane lands on the water intact, then fills with water, it is going to sink. It will have no forward velocity so the tail will pull it into a nose down trajectory and it will descend vertically until it hits the bottom. That is if it doesn’t begin to spin in the water from some imbalance, which is highly likely and would negate the concept of underwater flight.
ETA: It’s probably going to go underwater nose or tail first and leave an air bubble in the other end of the plane until it reaches a depth where there’s a sudden rupture under compression, with the plane in a vertical attitude the whole time.
Lockheed’s working on one (at least they were a few years ago). It’s called the Cormorant. Last I heard they’d got the launch part working but still needed to finish recovery function.
To those who prefer to skip the link; It’s a small drone that launches from a sub, leaves the water, flies to wherever to accomplish its task, returns to the water, dives in and returns home to its sub. If they get it working it will be pretty cool, imo.
The Hudson is a tidal estuary to a point well north of where Flight 1549 landed. Depending on the state of the tide, the current could have been flowing north or south.
It would probably have been under 2 kts, so not a significant factor in damage to the plane.
I wonder if a plane like MH370 could land unbroken on the sea, then gradually fill with water in such a way that the hull does not rupture on the way down?
Only if you cut a shitton of large diameter holes in it. Else, as it loses buoyancy and starts to sink, any existing air spaces will be crushed by external pressure. It will also tend to go nose-down due to drag front the tail and the lack of forward momentum.
The old TV series “Voyage To The Bottom Of The Sea” had a flying sub. It could both take off from under the water and land in the water then submerge again.
This was, and still is, pure science fiction. Of course science fiction does tend to predict the future.
Just so we’re clear, friction between the airframe and the water isn’t the issue: it’s impact forces. When you’re riding a motorcycle at 80 MPH, raindrops hurt just as much as bugs, and for the same reason: they have mass. so imagine instead that the raindrops are a solid/contiguous mass of water (the ocean), and jack the speed up from 80 MPH to something more aircraft-like, and you can see how the ram pressure from the water could shred an aircraft.
Aerodynamics do matter. A large part of it is about reducing form drag, which scales with the density of the fluid. Since water is about 800 times as dense as air, the shape of a waterborne vehicle matters just that much more. You want a nose that’s shaped in such a way as to avoid suddenly redirecting the oncoming fluid. It wouldn’t be hard to come up with a much longer, sleeker nose that achieves this (think “Concorde”), but an aircraft designed to fly in the atmosphere will have wings shaped a certain way, including a certain degree of rounding on the wing’s leading edge, and the engines are unavoidably large with a flat face. So even if the nose survives high speed entry into the water, the rest of the plane won’t.
But a plane would have few to no significant air spaces that are sealed and thus can’t equalize their pressure to ambient.
FWIW, I knew a guy who worked at a seaplane base for about 10 years. It was reasonably common for seaplanes and float planes to sink (e.g. from a neglected slow leak). So it was a routine operation to raise these and restore them to flying condition. Since this was a fresh-water lake, the job typically involved simply drying everything out, re-lubricating, and replacing some instruments. He never told of repairing crush damage.
Leaving out the near-imposssibility of transiting from flight to submerged forward motion for any real aircraft, the potential for underwater gliding seems like something that could be tested rather easily, Mythbusters fashion, by sinking a model aircraft in a swimming pool. My guess, however, is that for just about any starting condition it would end up going vertical and, in most cases, impact the bottom nose first. I’d imagine this to be due to a combination of lack of forward momentum at the start and not being able to operate the control surfaces to obtain a horizontal attitude, i.e what Tripolar said.
