Basejumper.com - archive

object 1500+ tower, jumper 300+ jumps..
gargoyle,bjack,42vented pc

experienced and current jumper, gainer exit stop rotation, start frontflips,
on his back 1/2way through 5th front rotation pc leaves boc, jumper unaware continues rot. reaches linestretch in a headdown position, gets fucking slammed, flys away onheading lands w/minor neck irritation..


cause?*opinion after vid review* overly sized pud and vents on pc... each succesive front rotation was increasing in speed, almost to the point of out of control, eventually airspeed combined w/rotation and vents on pc partially inflated pc , pulling from boc while jumper on back..

prevention? vid of pin check shows pud a bit larger than necessary, with most of vented portion outside boc... dont do that. especially with (almost) terminal aerials...

this could have been ugly, really easily.. be careful kids...

He didn't have a 36 or 34?

Edited to remove judgemental smiley....

42's work... ive jumped a 42 from an aircraft opening low. it hurt, but it worked...

Do you know the make of the PC?

not offhand, and prefer not to speculate...

I really hope you are just trolling as usual on this one.

In case you are not, a set of 4 PCs is less expensive than huge medical bills, months out of work. Heck, even coffins nowadays cost more than most mid-size cars.

Oh, FYI a 42" PC generates about 350 LBS of force at terminal and that is NOT considereing the snatch force.

A stadard MIL SPEC BASE bridle should have a tensile strength of 550 LBS. However most BASE bridles are built with inferior grade square weave...not to talk about the canopy PC attachment point...

What the maths?

no trolling, and i never said it was a good idea..

weve all done uneducated shit in the beginning...

i was also using the "wrap the center cell around the whole packjob" tecnique at the time...

ive learned since then. dont be scared.

You said:


and


and


and then


So this "guy" is experienced yet he is using a 42" for a near to terminal jump (way terminal to my standard) doing arials with half of his large PC sticking out

Your last quote should read:

Despite common knowledge, common sense at large, early mistakes, we still do uneducated shit all the time.

Care to elaborate on that...?

Carl and Jean Boenish tested their original BASE setups with 42" PC's out of airplanes. I believe it was at Elsinore. Maybe Nick DG knows the whole story...

A 42" PC near terminal is far from the design criteria for parachute deployment. The deployment can cause many problems since center cell stripping is so pronounced compared to using smaller pilot chutes. On a side note, even if the attachment point broke from the canopy, the canopy would/should already be deployed enough to open.

I do terminal on almost every jump. One time i was planning to go from a grand, had a 38" in. We opted to climb to the top. The snatch force was incredible. I felt like i had a canopy over my head the second i let go of the pc. I had an off heading to say the least, much less I felt as if I could have torn my aorta on opening. I know and jump with guys that use a 38" for almost everything. They seem to be comfortable with it, and it works for them. I couldn't even imagine the force of a 42" coming out at terminal. I use a 32" and it is perfect, every time. I believe that everything needs to happen in a sequence, staging if you will.. Glad everything worked out for the best under the circumstances.

I wrap my center cell around the whole pack job. I have great success with this. What is your point on that?

I'm assuming this is being taken in a literal sense, and they want to know how the bridle is routed. Maybe not.

Drag equation:

D = 1/2 rho v^2 C_w S

whereas
rho = 1.225 kg/m^3
v = 50 m/s
C_w = 1.2 (approximately)
S = pi r^2, where r = 0,53 m (approx half of 42')

results in

D = 1642 N

which is about 369 lb, ie roughly 350 lb as calculated by Nick.

I don't realyy understand what you guys are on about.
According to these calculations, a 36 doesn't even generate a pound less in drag.

I do think the difference in drag is bigger than that. Realistic drag models that take all aerodynamics into account are pretty much impossible to come up with. Get a car and a scale, and measure it on the highway. An interesting experiment.

That said, there's a secondary difference between a 42 and a 36 that is possibly more important. It's less exposed in the BOC. With the 42 (presumably without a handle), you'll need a pud sticking out. To make this sufficiently grabable, you'll want it decently sized. A 36 will most likely have a handle (internal or external). You can keep more fabric inside the BOC and still have it easily grabable. This reduces the changes of a premature during aerials.

All imho of course...

Actually, I was jumping a 38 vented pilot chute. I did have my pilot chute out of the pouch quite a bit.

My standard jump is usually one back layout, track, and pull. I decided all of a sudden to try to bust out with something I knew I could pull off without even thinking twice about my pilot chute pack job.

When my rig deployed, I had been falling for about 900 feet.

Lesson Learned! I should have talked to someone experienced with arials before trying to pull that off. I'm just lucky to walk away with only a sore body.

Don't Get Complacent, it kills!

???

According to the above calculations, a 36 generates about 70% of the drag of a 42. i.e. 260 lbs at terminal. So, what do you mean?



That is not really the problem resp. the point. The drag equation "works". What can be discussed is how you come up with the drag coefficient or which reference area you use.

For reference area I used a very simple approach and just took the area which should result if the pilot chute is flat on the ground (which it obviously is not if inflated).

Whether you come up with a value for the drag coefficient via a theoretical model (which would require a complete solution of the Navier-Stokes equations for a flow around a bluff body what even nowadays is a pretty complex problem and passes most computer capabilities) or from empirical tests does not matter for our purposes. Empirical tests for the drag coefficient of an open half sphere or a round parachute usually result in a C_w around 1.2-1.3 or so. So, as a first approach I think the drag equation scales pretty well and we have no reason to believe the drag coefficient will be drastically different for pilot-chutes of different sizes. And as mentioned above, the difference in drag is of course bigger than one pound.


Only recently (past few years) it has become possible to do meaningful calculations on such flows. A lot of research is still semi-empirical. The flow around bluff bodies has some special problems which make it usually impossible to use some very nice simplifications (Prandtl etc.) which can be used for streamlined bodies (like a wing for example).

But aerodynamics is not my specialty, maybe someone from the field can add to or correct my thoughts.

According to my "math" a 36 at terminal produces about 100 less LBS than a 42.

Maybe,
It could mean that they 'expose' the entire center cell by wrapping the outermost seams around the whole pack job.....that's what I do.
The only difference between my slider up and down packjobs is the slider, and either a tailgate(down) or masking tape(up).
KISS,
Blair

edited to add: Of course I reconfigure the brake lines you foo!

Ah what a dumass!
Sincere apologies for the BS posted previously.