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iwasinkheson68

Large amount of twists on a skyhook deployment?

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There was a chop at my DZ the other day that resulted in a skyhook deployment of the reserve, which opened with 8-10 line twists.

The main was a sabre 2 170, wl ~1.1 with possible brake fire and definite line twists. He cutaway and his smart 175 came out with the above mentioned line twists.

All the posts suggest that a skyhook means no line twists etc, or that your main would have to be rotating at 120rpm to give you a full twist etc. I was seeing threads with "I was spinnning under my velo...no twists on the reserve" etc and just wondering if that was the accepted truth of skyhooks or the de rigueur explanation from when those posts were made.

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I have one skyhook-equipped chop... 5-6 twists but flying pretty straight. I thought at the time it was because I was rotating on more than one axis but hey, mains open in twists sometimes even when you're flat and level. Reserves aren't magic any more than the skyhook is.

I think it's possible to overthink this.
--
"I'll tell you how all skydivers are judged, . They are judged by the laws of physics." - kkeenan

"You jump out, pull the string and either live or die. What's there to be good at?

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Skyhook gets your reserve out, not necessarily twist free. It depends on many factors, one of them symmetrie during reserve inflation.

It might not even have been a skyhook deployment. Were the main and freebag found still connected?
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To absent friends

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Pobrause


It might not even have been a skyhook deployment. Were the main and freebag found still connected?



Yes, it was connected.
Don't get me wrong - it was a very good experience!!!
From my point of view there is 2 scenarios for the reserve line twists:
- 1. you have enough time to release the twists - no problem B|
- 2. you don't have the time/altitude to release the twists - be happy you have an inflated canopy over you head :D

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Advertising and other experience aside, just thinking about it I'd expect MARD deployed reserve to have line twist more often than pilot chute deployed reserves. Your violently spining main is ATTACHED to your reserve free bag and has the chance to impart some of that energy and motion to the free bag and reserve as it deploys.

When Booth introduced the Skyhook at a PIA symposium he said they were going on all tandems but weren't ready for sport rigs yet. Same day the factory was calling customers to see if they wanted it added to their Vector III order.:S Bill said they hadn't yet tested it with the jumper spinning on their back since toggles spins were face down. Half the room said hook the main up backwards.:)

While using the main as the PC has some benefit down low having an unstable violently spinning 'PC' seems not such a good idea. All those stable cutaways in the advertising back then were pretty.

Not all manufacturers think a MARD is a good idea.

I'm old for my age.
Terry Urban
D-8631
FAA DPRE

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I do not agree that a MARD deployment would have more line twists than any other kind of deployment.

1. If you are spinning under a defective canopy, you are being slung outward by centrifugal forces. This does not mean your body is spinning. When you cut away from a spinning malfunction, or a MARD deploys you are simply flung outwards from the resistance, basically in a straight line, by centrifugal force.

2. Line twists on the reserve are from something else than the MARD.

3. Spinning main impart a spin on the deployment bag? Nah. It would just twist the bridle. (OK I suppose if there were hundreds of twists it could) But a MARD would have the canopy out of the freebag long before a huge number of twists could ever develop.

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FWIW I believe that reserve line twists are caused mainly by uneven loading of the harness during deployment. A spinning main first throws you around in your harness, then you cut away and deploy before you straighten your harness out. A MARD deploys your reserve even quicker, so you could easily be even less even as the canopy inflates.

This is to say that I agree with Terry on all three points. I'm merely expanding on his point 2.
Always remember the brave children who died defending your right to bear arms. Freedom is not free.

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I was trying to avoid any dz post today but can't help it.

dpreguy

When you cut away from a spinning malfunction, or a MARD deploys you are simply flung outwards from the resistance, basically in a straight line, by centrifugal force.



I've been fighting that line of thinking for years in threads on mals. In itself, it is true. But there's more.

It is true that of you are going in a circle (approximating a downwards spiral) at 180 degrees a second, yes you fly off in a straight line. However, people forget that superimposed on that is a body rotation of 180 degrees a second that is also happening. You fling off on a straight line, tangential to the circle... but still spinning around your center of mass...(at the same speed you were spinning around your center of mass before you chopped.)

This may be soon be slowed by drag, or conscious or subconscious action on the part of the jumper, but does exist.

That doesn't change the idea that MARD deployments should have fewer twists in general, due to the speed of deployment.

I just try to fight those who over-sell whatever skydiving technology they favour. Get an RSL or get a MARD, they say, BECAUSE "RSL's don't cause line twists because you are flung out straight from a spiral!" or "MARDs work so fast you never get line twists!"

B.S.! RSL deployments can cause twists. MARD deployments can cause twists, but likely at a much reduced rate. I'm not attacking the technology. The vast majority of the time, reserve line twists are not an issue. Only very occasionally they can be serious enough to cause problems especially if low. (It has killed or injured people so shouldn't be forgotten. But reserve line twists are not considered much of a hazard overall.)

Normally though, if a MARD (or even RSL) gives you twists... well at least you've got a few hundred or quite a few hundred extra feet of altitude to deal with it! A net benefit.

(I say this as a jumper who pulls cutaway and deploys reserve on their own, and don't mind trying to get a bit stable in between, altitude permitting, but am happy to jump MARD rigs too.)

Gowler gets at the issue that twists happen for different reasons. It could be that the jumper is spinning at the point of cutaway (and the jumper and deploying canopy don't spin the same amount by the time the canopy inflates), it could be that the bag gets dragged out unevenly from the container and gets kicked into rotation, and of course once you have 1/2 a twist and some momentum, such as from loading a riser unevenly on opening due to body position... then you can keep spinning up.

I'll use Gilead's video from post #2 as an example. I'm not going frame by frame so this is really just approximate. He's spinning on his back. From the flash of cables to being upright, he continued to spin very roughly 1 turn. When he first sees the reserve, it has I think one full twist in it. Who knows exactly how much the bag rotated along with him as it came out, but somewhere about a full turn of difference happened. Who knows, maybe the was out of the bag so fast that the bag only rotated a quarter turn and then the deploying canopy would be draggy enough that it would stop rotating quickly.

But he still has momentum, so keeps spinning up. Something like 4 full twists that he has to wiggle his way out of.

So even given that the MARD was dragging the reserve out while he was spinning on his back, it doesn't look like the MARD really gave him twists. It was largely just a function of having some rotational energy left when he was past that critical 1/2 twist rotation.

It's a good example of how a "MARD gave him twists" (considering the whole reserve deployment procedure) while at the same time the "MARD didn't give him twists" (the Skyhook must have extracted the bag pretty cleanly).

So a MARD may be more likely to avoid giving a jumper twists (how often?), but it is useful to have a thread where it is pointed out that MARD's don't magically cause them to never happen.

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..."body rotation of 180 degrees per second"

This would be with or without a MRD, RSL or neither of these?

Not as an argument, but as a question: Your defective main is spinning and you are being "flung" outwards. Let's say you are being flung outwards while your main is twirling it's way in it's spin, and at the same time is going downward-yes, a spiral. If you are face to earth let's say, and cut away, then out you go in a straight line. Where is there any body rotation of 180 degrees per second? Why wouldn't you just fly away, still face to earth? What would make your body "rotate"? (At this point, I disagree with your statement that one's body would rotate. I think, if you are face to earth at the time of the release of the main risers as you are flying away in a straight line; if you started face to earth, you would stay face to earth. But I may be wrong.) What am I missing here? What force would cause your body to rotate on it's longitudinal/roll axis? = (The only axis that would produce line twists on a deploying reserve)

Once again, this is not a MARD, RSL or neither question. It's a cutaway from a spinning main question.

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dpreguy

..."body rotation of 180 degrees per second"

This would be with or without a MRD, RSL or neither of these?



Peter is right, this is basic physics of rotating bodies. If a prop blade broke while the prop was spinning at 2000 rpm, the blade would fly away in a straight line (in a vacuum) but would keep spinning about its center of mass at 2000 rpm.

As a thought experiment, imagine drawing an arrow on the prop (any direction any size). Put an imaginary camera so it hovers (and does not rotate) in front of the tip of arrow. Spin the prop at say 180 rpm. The video from the camera will show the arrow rotating around its tip at 180 rpm. This is true no matter where the arrow is, because all the points on the prop are in rotation.

If the prop breaks, the conservation of angular momentum will transfer the rotational energy to the center of mass of the pieces.
It's flare not flair, brakes not breaks, bridle not bridal, "could NOT care less" not "could care less".

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dpreguy

I think, if you are face to earth at the time of the release of the main risers as you are flying away in a straight line; if you started face to earth, you would stay face to earth. But I may be wrong.) What am I missing here? What force would cause your body to rotate on it's longitudinal/roll axis? = (The only axis that would produce line twists on a deploying reserve).



If the jumper's center of gravity is closer to the canopy than his aerodynamic center, he may also rotate on his lateral axis (forward or backward loop) or yaw axis (cartwheel).

Mark

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I'm still not buying that. The prop has no rotational motion along it's longitudinal/roll axis before it flies off, because it is fixed to the hub. After it breaks off, it flies away in the direction of it's centrifugal force, 'kind of' in a straight line; but yes-it was cutting into the air, arcing if you will, just prior, and won't fly away exactly straight. It would arc for a while. But, to postulate that it will begin rotating on it's longitudinal/roll axis....hmmmm seems like a stretch.

Easier for me to imagine a cylinder, not a prop, which flies off. Can't see how it would begin rotating on it's long axis just because it flies off.

Round parachutes were once tested on a spinning tower. A heavy dummy was spun out and the parachute was in some kind of container. It was then released. I have seen a black and white movie/video of one of those tests. Not one bit of line twist. And, they didn't deploy very quick either.

Well, I'm no physicist, so maybe I just don't get it. If there is roll axis twisting imparted on release, then why didn't the parachute on the whirling tower get line twists? As I said, they took a long time to open. Seemed like about two long seconds. If the theory is 180 degrees per second, it should have had two twists on the roll axis.

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There's no reason to fixate on the roll axis rotation. Rotation on the yaw axis would happen which would also cause line twists.
People are sick and tired of being told that ordinary and decent people are fed up in this country with being sick and tired. I’m certainly not, and I’m sick and tired of being told that I am

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dpreguy

But, to postulate that it will begin rotating on it's longitudinal/roll axis....hmmmm seems like a stretch.



You're right, that would be weird.
I think we're getting confused over axes. It's not rolling around the long axis. I'll start again with an example of my own to see if we can get the same mental model, which should make it all clearer:

Ok, so there's an ice surface, and a center point where a rope is being swung in circles. The weight at the end of the rope is a metal bar sliding on the ice. Swing the metal bar around in circles, 180 deg/sec.

If you could get a drone to hover over the bar as it goes in circles (with impressive piloting skills), without turning the drone's direction, a camera on it looking down would see it rotating 180 deg/sec. That's showing the rotational momentum it has.

If the knot at the metal bar fails and the bar goes flying, off it goes in the direction it was going at the moment it was let go. But it still has that rotational momentum, so as it flies off, it is also spinning about the vertical axis. (Axis up and down relative to the ground).

And that's the skydiver who chopped from his spinning mal, spinning away....

[inline spinning_bar.jpg]

As something of a live demonstration, I have a video on youtube,showing me intentionally cutting away one side of a square 190 F-111 canopy. (Poor video quality though.)

Even that big old canopy gave me 360 deg/sec rotation. Quite a ride, and I certainly had plenty of that rotation still, when I pulled the other cutaway cable.

I probably spun a couple times around (around the vertical axis relative to the earth), somewhere aong the way also flipping myself belly to earth before deploying an F-111 main. Guess I slowed the turn enough, as I didn't get line twists on deployment.

Video with a measly 51 views so far:
"One-sided intentional cutaway"
https://www.youtube.com/watch?v=75t0pMauATo

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So, still belly to earth. No different than being in a flat turn and pitching.

Conclusion: The shorter the time, the fewer the uncorrected turns caused by the spinning main.

My rig is too old to install a MARD, but I wish I had one.
Have an RSL and that, in most cases, shortens the time too.

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Most of the points I would have made have been covered already. Something people don't realize is how brief the time is that the RSL equipped main could impart any torsional energy. The reserve itself has a huge amount of inertia as compared to the reserve bridle and it would have to be spinning at thousands of RPM to have the option of imparting energy to cause the twist.

A more likely cause is that under a spinning mal your body is loaded up in the harness and it's easy to carry an asymmetrical body position into the reserve opening sequence.

-Michael

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That's what I think. Another thing to consider is that your reserve starts out strapped to your body. If your body is rotating your parachute should be rotating in an equal amount. That should greatly reduce line twists.
Always remember the brave children who died defending your right to bear arms. Freedom is not free.

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There are alot of gadgets out there to do your skydiving for you, or make it "safer". Try opening at a safe altitude, giving yourself enough time to stabilize and fire your reserve manually. If you want to deploy a reserve at a low altitude, wear a third, round reserve, and use it for low ones.

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gowlerk

That's what I think. Another thing to consider is that your reserve starts out strapped to your body. If your body is rotating your parachute should be rotating in an equal amount. That should greatly reduce line twists.



I know it was already mentioned, but under a malfunctioning main, nearly every time someone 'feels' that they were spinning it was centrifugal loading with the parachute at the center, not rotational relative to the jumper.

-Michael

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