what makes a round canopy squid

[alexpereira 0]

Hi everyone have this question on a test that cant find the answer... I dont have much knowledge on rounds. Does someone can help?

What makes a round parachute squid?

a) an excessive load

b)deployment at a speed lass than the critical opening speed

c) deployment at a speed greater than the critical opening speed


appreciate any help!

Alex

[RIGGER160 0]

I have seen them turn into streamers from really bad line twists and anti-inversion netting locking up.

[wolfriverjoe 1,340]

c. High speed.

NickDG covered it pretty well HERE.

Contrary to popular belief, there actually is some usable information in the Cooper thread.

"There are NO situations which do not call for a French Maid outfit." Lucky McSwervy

"~ya don't GET old by being weak & stupid!" - Airtwardo

[RiggerLee 61]

It basically means it's going too fast. The problem is that if the load is too great and the canopy in that configuration does not have enough drag to slow the load down bellow the critical speed then it may continue to squid till impact.

There are several ways to fix it. Basically you need to increase the internal preasure or decrease the external preasure to allow the skirt to expand and open. The size of the apex hole, assumeing you have one, affects this. If you can make the hole smaller or use a lower perrosity fabric at least in the upper canopy. Eather will trap more air inside the canopy. If you do some thing to disrupt the air flow up the side of the canopy such as addind tuskingurts/pocket bands or any thing to breakup the airflow holding the mouth of the canopy closed.

lee

Lee
[email protected]
www.velocitysportswear.com

[alexpereira 0]

Thanks a lot every one for the comments! Helped a lot!

Alex

[hackish 8]

Is there a way to "fix" it if you're under said configuration? For example pulling down some of the lines to offset the opening and encourage opening?

-Michael

[erdnarob 1]

By squiding I understand pulsating like a squid. A round parachute doesn't deploy like ram air canopies. For round canopies, the inflation happens first with some air going inside making the parachute looking like a pear shape with a bulge at the top while still narrow at the skirt. Then Bernouilli's forces are generated from inside to the outside because of the air going around the pear shape. Then the bulge increases in volume and finally the skirt starts to expand and the final inflation occurs. The top of the round parachute is more dense or heavy with respect to the rest of the parachute (the top of a round parachute has the seams converging and seams are more dense because of the extra fabric). That can be the cause of the top going down (because of more momentum) and sometimes the top passes under the skirt making an inversion.
I hope that answers your question.

Learn from others mistakes, you will never live long enough to make them all.

[erdnarob 1]

Hi again. In my precedent post, I tried to explain the pulsation (squiding) of a round canopy at the opening. Later I understood that you maybe meant that you wanted to know what causes the squiding during the descent.
Since I am into physics here is my explanation:
The pulsation or squiding comes from the fact that there is two forces which oppose to each other when a round parachute is descending. First, the weight of the jumper which tends to decrease the diameter of the skirt. That causes the parachute to go down faster. Since the parachute is faster, more pressure is built into the canopy which tends to expand the skirt. Wider skirt slows down the canopy and allows the extra pressure to escape from under the skirt. The result is when the extra pressure is gone, the weight of the jumper overcomes the expanding force of the pressure by decreasing the skirt diameter and the cycle starts again and again generating that squiding. The round parachutes more likely to do some squiding are genarally unmodified (no gore or part of gore removed). Just the open apex allows for some extra pressure to escape. The frequency of the squiding should increase if there is more weight suspended to the parachute but I am pretty sure that even with a small weight, there is also a slow pulsation.
When you use a round parachute with many modifications like a ParaCommander type canopy, there is so many openings including some ones oriented to the rear to give the canopy a forward speed that there is a balance between the pressure forces and the weight of the jumper which makes no pulsation or very minute ones.
Amazingly, a star behaves the same way. At a specific time of its life a star starts to pulsate when gravitational forces contracting the star oppose to nuclear forces generating heat and trying to expand the star. Contracting the star generates more heat and more heat starts nuclear reactions to increase then there is more heat and more expansion and again we have a cycle or pulsations. Fortunately right now our Sun is relatively stable and the 2 forces (gravitational and nuclear) are more or less in equilibrium. We can say that our Sun behaves like a ParaCommander right now By the way a ram air parachute is also pulsating or squiding under some circumstances. Tell me what you think about my explanation.

Learn from others mistakes, you will never live long enough to make them all.

[pchapman 261]

Yeah the term squidding refers to when the canopy is staying still stretched out with little air in it. If the deployment airspeed is above that critical airspeed for squidding, the canopy won't open at that time. If there's enough drag from the uninflated canopy (relative to the mass of the system with payload), then it will slow down, eventually drop below the critical speed, and then open.

There's a bit on it in the Irvin Recovery Systems Design Guide, free online.

Erdnarob also described the pulsating of an open canopy, which doesn't technically fall under the squidding term, although I can see how it might be described that way. Jellyfishing perhaps?

A largely unvented canopy tends to spill air in an uneven manner so it can get into those pulsating cycles or oscillate from side to side, although I'm not sure of the relationship between those two behaviours.

Quote

That can be the cause of the top going down (because of more momentum) and sometimes the top passes under the skirt making an inversion.
I hope that answers your question.

As I understand it, the post-inflation rebound (or whatever it is best called?) is mainly due to wake recontact. So it isn't so much the mass of the top of the canopy, as the mass of the air being dragged behind the opening canopy, that hits it from above when the canopy "comes to a sudden stop".

[RIGGER160 0]

http://www.youtube.com/watch?v=vA9jASQ6xtA[url]

This was determed to be the result of massive line twists. I call bullshit but everybody at the malfunction review board outranked me so...

The jumper did deploy his reserve and walked away btw

[erdnarob 1]

Thanks again Peter to add precision to my post. For sure the mass of air rushing to the top of the canopy bounces back down due both to the elasticity of the air itself and the elasticity of the fabric as well.

In this issue, I am still not sure if the question concerns the squiding during the inflation or during the descent itself.
I wonder if Jean Potvin and Gary Peek from the university of St Louis Mi have studied the inflation and descent of round parachutes. If they did, they sure have something to explain about it. They were both on a project to find out how ram air parachutes are inflating.

Learn from others mistakes, you will never live long enough to make them all.