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steelerguy
01-05-2003, 03:41 PM
I have been having an off and on debate with a friend for a year now, and hopefully I can get an answer here to end it!

If a person were parasailing and the rope were to break would the persons altitude increase, even a minuscule amount, at any point after the break? I tend to think since the person will still have forward momentum they will continue to create lift for a small amount of time and rise since they no longer have a tether weighing them down. In addition to this I would imagine the energy stored in the rope would also act to increase their altitude once the rope breaks. My friend thinks gravity will overcome them immediately and they will never go above their 'cruising altitude'. Can you break down what would really happen?

Hail Ants
01-05-2003, 03:53 PM
Assuming perfectly still air, you would descend immediately. The moment the rope broke you would no longer have any forward energy to counter the huge drag the chute has, nor would you have any energy input to counter gravity.

Your forward speed & altitude would both decrease immediately.

Johnny L.A.
01-05-2003, 03:56 PM
I don't know, but this is my gut feeling: The parasail is not only lifting itself and the weight of the occupant, it is also lifting the weight of the rope. If the weight of the rope were suddenly removed, I think the existing lift would be enough to increase altitude for a moment.

Any sailplane pilots on the boards?

steelerguy
01-05-2003, 04:04 PM
I was thinking that in addition to the parasail lifting the weight of the person and rope, it is also lifting a certain amount of weight of the boat. All of this would be removed when the rope breaks. Don't know if it affects anything though.

01-05-2003, 04:07 PM
I think it would be identical to when the string breaks while you're flying a kite.

Alex_Dubinsky
01-05-2003, 04:14 PM
and what happens then?

Xema
01-05-2003, 04:17 PM
Roger, sailplane pilot reporting for duty.

If a glider (I use the term generally, to include sailplanes, hang-gliders, paragliders and parasails) is being towed at a speed greater than its stall speed and if the tow line is then released (or breaks), it can convert kinetic energy (speed) to potential energy (height) by slowing down. So the technical answer is yes.

But of these gliding devices, the parasail is much the worst at doing this. Its glide ratio is low, its speed range is narrow, and the ability of the "pilot" to control speed is small. So in practice it can do remarkably little conversion of speed to height.

So I guess what I'm saying is that the OP is correct, but he might have to be towed dangerously fast to demonstrate an actual increase of altitude, rather than just a reduction in descent rate.

By contrast, a sailplane is at the top end of this performance range. Tow one fast enough, then release the tow line, and it can gain many hundreds of feet as it slows down.

Armilla
01-05-2003, 04:23 PM
In the case of towed parasailing most of the power keeping it aloft comes from the boat. Once this is removed the system will pretty rapidly begin to fall, but there's a couple of seconds just after the breaking of the rope where I believe altitude will be gained.

The rope to the boat is maintained taut most of the time, producing forces forward and down on the parasail. This downward component of the towing force means that the lift produced by the canopy must exceed the downward forces due to gravity.

The passenger and canopy weigh a fair amount and therefore have a significant amount of forward momentum that must be dealt with.

When the rope breaks the removal of the downward component of the towing force will cause the parasail as a whole to lurch upward briefly as its greater lilfting force is no longer countered. This will only last a moment since the huge drag on the canopy will slow it down to a point where it begins falling almost immediately.

Then there's the forward momentum of the passenger. Released from the towing force the canopy will stop quickly, but the passenger, being both more massive and more aerodynamic, will continue to try to go forwards. This will cause the passenger to swing forwards and upwards under the canopy while the forward motion is neutralised by drag.

So I'd say that on release of the rope the whole thing will lurch upwards slightly, then the passenger will swing forwards and up. These things combined should cause an undeniable increase in altitude, albeit for a short and limited time.

In the cases where there are strong winds, all bets are off. Also the point at which the rope breaks will affect the outcome since it will add the unwinding of some elastic tension and weight to the scenario.

Hail Ants
01-05-2003, 06:02 PM
Again, assuming perfectly calm air, cutting the rope on a parasail would be the same as cutting the engine in an airplane. No more forward thrust = instant downward motion.

All of its upward lift is dependent on its forward thrust. And parasails are not even remotely efficient gliders. They are being dragged thru the air kicking and screaming. If rather than cutting the rope you instantly cut the throttle on the boat the parasail is not going to exert any upward force on the boat. The rope will go slack immediately as the chute begins to fall.

01-05-2003, 06:16 PM
Originally posted by Alex_Dubinsky
and what happens then?

I guess I shouldn't assume that everyone has flown a kite before in their life. :rolleyes:

Last time I flew a kite & the string broke, the kite dropped from the sky. Sure, it drifted with the wind a little, but it didn't rise or continue "flying".

The Parasail basically is a kite. It's held against the wind by a rope. Break the rope and there is insufficient wind to keep it aloft.

Cisco
01-05-2003, 06:31 PM
Originally posted by [email protected]
I guess I shouldn't assume that everyone has flown a kite before in their life. :rolleyes:




No, you shouldn't assume that. You should also note that it's an even bigger assumption that everyone has not only flown a kite but has had a string break on one while they were flying it. I know I certainly never have.



Re: the OP - I'm with the 'brief ascension before falling' camp.

steelerguy
01-05-2003, 07:02 PM
I don't think it would be the same as cutting the engines on a plane that was just flying at a steady altitude. It would be more like cutting the engines on an airplane that was tethered to the ground while trying to gain altitude.

The parasail will continue to go higher if you let more rope out. So I think it is sufficiently different than an airplane, since we don't tie airplane down, that it is hard to make an analogy between the two.

I hate this problem! :)

<<Again, assuming perfectly calm air, cutting the rope on a parasail would be the same as cutting the engine in an airplane. No more forward thrust = instant downward motion.>>

Speaker for the Dead
01-05-2003, 07:02 PM
Every time the string breaks on one of *my* kites (twice) the kite flew away, considering the string was really nothing more than a tether.

The Flying Dutchman
01-05-2003, 07:11 PM
While the parasail is connected to the end of the rope (during operation), the two bodies, parasail and end of rope are in static equilibrium with respect to each other. This equilibrium is a result of all the forces acting on the two bodies including the tension connecting the two bodies. Now let us break down the components of the tension at the connection into vertical and horizontal vectors and just consider the vertical vector. this tension or force is pulling the sail down (Whether the sail is actually going up, remains level or going down depends on the ballance of other forces.) Now if we suddenly cut off the downward force due to the rope connection, for a brief period the end of the rope and the parasail will be influenced by the other forces alone and out of " relative" static equilibrium. That means a sudden acceleration upwards for the parasail.

If the parasail was travelling at constant elevation or upwards, relative to the surface of the water, a brief rise will be most definitely be evident. However if the parasail were descending, only the rate of descent would be briefly reduced.

Of course the brief duration of the upward acceleration is dependant on reactive forces to the horizontal velocity of the parasail which immediatley deccelerates till the forward velocity is reduced to the point where the upward force due to wind resistance can no longer overcome gravity.

kiteboy
01-05-2003, 08:22 PM
I've been attached to the "kite" when the string is released... I fly hang gliders, using 2 towing methods to gain altitude:

"Aerotowing" is when a powered ultralight pulls a hang glider along behind it on a towline. The two craft maintain approximately the same altitude, so the towline and the tow forces are horizontal. This situation is most similar to sailplane towing (I think), and yes, if I release while going faster than stall speed, I can CHOOSE to kinda "zoom climb" up a little bit and convert the extra speed into altitude. But if I let the glider do what it wants, it will just slow to "trim" speed and begin to descend. If the hang glider and ultralight had some amount of upward momentum at the moment of release, the glider might use that to climb a few feet as it readjusted to the new equilibrium.

"Winch Towing" involves a vehicle mounted payout winch. A truck drives down the runway, continuously reeling out line at a specific tension, and the hang glider is pulled up into the sky, very much like a kid running with a kite and letting the string out as he goes. Again, if there is actual upward momentum at release time, the glider might gain a few feet, but otherwise immediately begins to descend.

But for parasailing, once the desired altitude is reached, don't they just tow you along at a constant speed, pretty much level with the water? To me this indicates "no upward momentum".

BTW. if a parasail line breaks, I'd bet the occupant would immediately pendulum backwards (relative to the chute). I hope the canopy doesn't experience a leading edge collapse...

Boyo Jim
01-05-2003, 08:29 PM
Originally posted by Hail Ants
Again, [b]assuming perfectly calm air, cutting the rope on a parasail would be the same as cutting the engine in an airplane. No more forward thrust = instant downward motion...

Wrong. Sure, no forward thrust, but forward velocity + mass = forward momentum. Given a number of variables, it is easily possible that the parasail would convert this energy and gain altitude at first.

I one lost a kite that had a large reel attached. The kite, whcich was almost vertical above me, with the reel acting as a ballast, sailed up and away. I chased after it for a couple of minutes, thinking I'd be able to snag the reel and save it. No luck, I just continued to gain height. When I lost control, it was already over 2,000 feet, and the wind power literally pulled it out of my hands.

don't ask
01-05-2003, 09:12 PM
I believe that you would glide forward until crashing through Alec Baldwin and Kim Basinger's skylight.

I went parasailing once. Because I told the crew how scared I was they spent much of the time amusing themselves at my expense - swinging on the cable, suddenly jerking the cable and stopping the boat. When the boat stopped I immediately began to descend, thay then crept forward at a speed that kept me about 10 feet above the water and called out "Your flight is over now walk back to the boat", before sppeding up and getting me airborn again.

Xema
01-05-2003, 10:24 PM
Originally posted by Hail Ants
assuming perfectly calm air, cutting the rope on a parasail would be the same as cutting the engine in an airplane. No more forward thrust = instant downward motion.

If you cut power in an airplane, the plane is still capable of gaining altitude, if it has some excess of speed (above its stall speed).

Xema
01-05-2003, 10:28 PM
Originally posted by [email protected]
Last time I flew a kite & the string broke, the kite dropped from the sky. Sure, it drifted with the wind a little, but it didn't rise or continue "flying".
A kite rarely is stable without the pull of its rope. A parasail is.

Pablito
01-05-2003, 11:35 PM
I went parasailing in Mexico, one of those deals where they take off not from a floating dock out in the water but with the person initially standing on the beach. There was a strong wind coming into the shore, so the parasailer would get clipped in, then the boat would take off, and up he'd go.
Anyway, the rope DID break, for the guy immediately before me in line. However, it was not the tow rope but some slipshop jury-rigged piece that held the harness together and attached to the tow rope. He was maybe 30' feet up in the air at the time and, fortunately, had cleared the initial breakers and landed in deep enough water that he wasn't injured.
He fell straight down. Not surprisingly, he didn't stick around for another try.

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