I was watching the episode “Kim” of MASH last night (BTW, if you like MASH, you’ve got to get the season sets on DVD…).
In the episode, a five year old boy ends up in the middle of a mine field. Trapper also ends up in the middle of the field in a failed attempt to rescue him.
The pair are finally rescued by a helicopter which comes to pick them up. The helicopter never actually lands, but drops a ladder, which is used to pick up the pair.
Anyway, my question was as follows: if the pressure of a foot is enough to set off a mine, would not the downward pressure exterted by the helicopter over the site (it wasn’t that high up) also set off the mines?
Well, in wartime people try all sorts of silly things. The downwash of a helicopter is less (per sqaure meter) than the pressure of a foot.
After all, we walk under chopper all the time without getting squished.
On the other hand, land mines can be tricky. The triggers can get pretty darn sensitive (espeically if they have been frozen and thawed a few dozen times). Mines often go off fo no real reason, probably air pressure changes, maybe they just loose the will to live.
In any case, the helicopter should usually work, although it would violate the manufacturer’s warantee and should not be tried at hime.
Does standing next to a helicopter feel like someone is standing on your head? No, because the energy is diffused over a much larger area.
Mines - even antipersonnel mines - aren’t that sensitive. Otherwise they’d go off during their first rain or wind storm, not to mention the fact that they’re ususally buried under a certain weight of dirt.
In the army we used to use what we called “demolition sandals”, which were 2’X3’ plastic trays with thick foam on one side and straps on the other, to strap over your boots. Supposedly, if used properly the foam would distribute your weight over a large enough area and you could safely - or nearly so - step on a mine without it going off. I’ve seen them demonstrated on dummy (smoke emitting) mines, and they seem to work pretty well, even when the wearer had a “wounded” comrade across his shoulders. Wouldn’t want to try it for real, though.
I’m not positive about this but I don’t think if you added up the total pressure of the downwash it would equal the weight of the helicopter. I thought much of the lift for the helicopter comes in a traditional wing providing lift fashion (Bernoulli Effect IIRC). That is, the blades on the rotor are essentially thin wings and provide lift the same way a conventional aircraft wing does. If you stood under a 747 that flew just above your head you wouldn’t get squished (deafened and knocked over maybe but not squished). Without doing the math I would think a 747 weighs enough to provide more than enough squishing energy (were it the case that the plane forced enough pressure down to lift the plane) even with the energy spread-out over the whole wing.
That’s all a WAG but if I’m right I think you have another reason why the downwash from a low-hovering helicopter probably wouldn’t set off land mines.
We’ve been over this misconception a few times on the SDMB. By Newton’s second and third laws, the total pressure of the downdraft of any aircraft must equal its weight. In order for a wing to provide lift, it absolutely must produce a downdraft. The key is that the pressure produced is not limited to the area directly underneath the wing: By the time it gets to the ground, it can be spread out over an arbitrarily large area.
Being lazy I was wondering if you could (briefly) explain this some more. I’m not arguing with you as you would certainly know more about this than I but I’m missing something here.
Certainly a downdraft is going to provide lift. However, I thought the Bernoulli Effect provides lift that does not include a downdraft. As a result all the pressure produced by the downdraft added up does not equal the weight of the aircraft.
As air rushes over a wing the air going over the top of the wing moves faster than the air going under it. The Bernoulli Effect states that the pressure is lower in a moving fluid than in a stationary fluid. As a result the air under the wing is at a higher pressure than the air above the wing and pushes up on the wing. In essence, the air below the wing is trying to move up into the lower pressure zone to achieve equilibrium. Once the wing move out of the way wouldn’t the higher pressure air try to move up into the lower pressure zone rather than get forced downward towards the ground?
Look, I’ll try to make this simple - ALL wings have a downwash. Trust me, I’m a pilot, I’ve flown through wing downwash (only on planes my size or smaller, though). In order for a plane to go up and/or stay up something must be pushing down. While the initial airstream over a wing is from front to back, once it’s past the airfoil, it goes down.
When my pet bird flies through the house his wings have a downwash - disturbs stuff under his flight path all the time.
Helicoptor rotors are, indeed, airfoils. The wash goes down.
If that doesn’t make you happy - when an aircraft passes through the air it displaces air. That air has to go somewhere. The wing is a device for pushing that air down so the aircraft is pushed up.
Look, I’ll try to make this simple - ALL wings have a downwash. Trust me, I’m a pilot, I’ve flown through wing downwash (only on planes my size or smaller, though). In order for a plane to go up and/or stay up something must be pushing down. While the initial airstream over a wing is from front to back, once it’s past the airfoil, it goes down.
When my pet bird flies through the house his wings have a downwash - disturbs stuff under his flight path all the time.
Helicoptor rotors are, indeed, airfoils. The wash goes down.
If that doesn’t make you happy - when an aircraft passes through the air it displaces air. That air has to go somewhere. The wing is a device for pushing that air down so the aircraft is pushed up.
I’ve avoided these discussions on the board, but I guess I’ll jump in on this one. I think I know a little about helicopters and aerodynamics.
I’d agree that a wing (rotor) must produce downdraft in order to provide lift. But I have a little trouble with your statement that the “pressure of the downdraft must equal its weight”. First, you have a dimensional problem. A pressure (like lbf/ft^2) can’t possibly equal a force (lbf) - letting that slide, though, (maybe I don’t know what you mean by “total pressure”) I can’t even think that the integrated pressure over the ground would necessarily add up to the weight. How do you account for the momentum in the air that “turns out” and flows parallel to the ground? How do you account for the momentum lost in heating up the air?
Imagine a helicopter hovering at 10,000 feet. I have to believe that all of the energy spent moving the air at the rotor will end up as heat in the atmosphere, (viscosity, turbulence, etc.) I don’t think you’d get any pressure on the ground whatsoever.
My take on the OP is that it’s entirely reasonable to expect a hovering helicopter to have such a small downwash that it wouldn’t set off mines.
The main things that determine the downwash are the gross weight of the helicopter, and the diameter of the rotor disk.
How is it that one can get the facts right, yet miss the point so badly.
Yes, the downdraft IMMEDIATELY UNDER THE AIRFOIL must exert a force equal to the weight of the flying object.
What everyone on this thread has been pointing out, yet fails to read in others’ posts, is that AIR IS NOT RIGID. It does not transfer force directly down, it dissipates force; it “squishes.” That’s already been said, repeatedly.
So the question remains. What the hell is an “lbf”?
kellymccauley has it, in part. The weight of a helicopter is spread out over quite a large area. Of course, as the experience-based among us have noted, most of that weight is directed straight down, particularly when the helicopter is close to the ground.
Not that the MASH folks probably ever bothered to research facts about mines, the helicopter rescue would probably have worked, nonetheless.
but here’s the larger picture. Landmines come in a variety of types. The variety is specifically intended so that it thwarts the most likely approach to disarming the mine. So if a mine can be disarmed by anything larger than a 100 lb force, the improved mine only is set off by a force between 100 and 150 lbs. If a mine can be disarmed by applying a force between 100 and 150 lbs, the improved mine explodes the second time the force is applied.
The helicopter rescue was liable to work, due to its slightly unusual and expensive solution to getting someone out of a minefield.
I wouldn’t bother with pillows on my feet for landmimes; I think I’d be wearing golf shoes if I knew beforehand. I don’t scuba-dive so I don’t expect watermimes to ever be much of a problem. - DougC
Thank you everyone for your answers. It helped to explain the matter. At least I know the MAS*H producers didn’t flub this one (as they flubbed other stuff)…
Oh, and another thing, if a glider does not have spoilers, speeding up will get you down, sometimes.
Id powered aircraft, sometimes you can have a power to weight to wing shape to speed ratio set up where the centerline of the wing is neg., going down, and the aircraft rises. excess lift is being produced. Kinda like that dumb ole bumble bee, he just does it, doesn’t worry about the science part.
Oh, I got 36,000,000.00 + seconds of PIC. Old, not bold.
Bawahahahaha
and that ain’t the half of what folks don’t want to believe…
