Vented PC

[seekfun]

Greenmachine,

High humidity does not increase wind resistance; it actually makes the bonds between air molecules weaker and reduces the effective density of the fluid. This is why humidity contributes to density altitude considerations for aircraft load and balance. On hot 'n' humid days, you may want to pull a jumper or two out of the doghouse of that Otter...

Cheers,

Chris

Edited to add:
distance = 1/2*a*t^2
d = 1/2 (9.8 * 16)
d = 78.4 meters
78.4 meters = 257.2 feet

[themexican]

"If there is higher humidity, the air will be more dense..."

Humid air is LESS dense than dry air.

Dry air is mostly nitrogen and oxygen. A molecule of N2 has a molecular weight of approximately 28 (~14 for each nitrogen atom).
A molecule of O2 has a molecular weight of approximately 32 (~16 for each oxygen atom).
Humid air has water molecules mixed in. A molecule of H2O has a molecular weight of approximately 18 (~16 for oxygen, ~1 for each hydrogen).


Also the distance fallen calculation is incorrect.
Assuming no air resistance, the equation would be:
distance fallen = 0.5 * g * t^2
where
g = acceleration of gravity,
t = time.
So in this case with a delay of 4 seconds the distance fallen would be:
distance fallen = 0.5 * 9.81 * 4^2 = 78.48 meter (257 foot)

Or we could look at BASE Jumping Freefall Data - Johnny Utah
which says the distance fallen at 4 seconds is 242 feet.

[lifewithoutanet]

The video Tony is talking about was from YL in Moab. ~300'. Skilled jumper goes stowed, takes a standard 300' delay at 3500' MSL, and has a hesitation. As jumpers, we know what "it opened at the last possible second" means, and it opened at the last possible second. It opened just in time to surge him into the tree next to the landing area turnout.

If you want to go stowed, go stowed. I've noticed the same thing with newer jumpers, thinking that once they got their Perrine progression within three jumps, (PCA, HH, stowed), they never look back. Got to look like a badass and not a newb, so huck it stowed.

And think of these things as planned time of delay. Personally, unless I'm doing aerials, or the exit won't let me prep up, I go handheld under 1.5 seconds, most of the time. In my opinion, you lose a little by immediately cocking back to immediately pitch at low airspeeds. I also think, more pronounced at lower airspeeds, that when you throw a stowed PC out, it inflates off to the right, then swings back, instead of presenting a PC along the centerline, handheld. Might be psychological, but if you're learning low freefalls stowed, the first couple times you're going to throw that thing as far and harder than ideal. And, generally, I think the incidence of hesitations is greater when going stowed instead of handheld.

And my understanding was that venting is a solution to pilot chute oscillation at higher airspeeds, and doesn't necessarily reduce the chances of hesitations. Don't know if that's correct or not. Either way, the pilot chute, for being as simple and relatively inexpensive to the whole system, is probably the most important thing you have with you on the jump.

Its just BASE jumping, do it your style. Have fun.

+1
Well said, PK.
-C.

[tfelber]

In reviewing that video that is like 2 sec... The feet leave the object at 1s and the canopy is inflated at 3s... If you look at Tom's numbers above you'll see that the a jumper falls over twice the distance in the second two seconds as he does in the first two.

Here is the jump I'm talking about...

YouTube- PC Hesitation, 300ft, Stowed, 42" Vented PC

Hopefully everyone can see the difference...

[Blitzkrieg]

yeah, it is actually 2.5 seconds. but it's also a go and throw.

anyway, no doubt the hesi made for a scary situation, i remember that video. so maybe 4 seconds isn't totally "normal".

crazy shit.

[Blitzkrieg]

oh, and i also agree with everything Peter said...

[wormly81]

Scary video make it go away.

[CrestFallen]

Why the hell would you go stowed from that height? Does it make you more hardcore? You can take a longer delay going HH, I've never understood newbies and their thinking on this stowed business sub 300 ft. In my opinion Davo is right, you should be using a 46-48 from that height. Some will disagree and say they have used 42's for 100s of jumps from that height and lower but why not use a bigger PC. Your delays cant be more than 1.5-2 maybe.

im kinda in agreement with huck on this, i am a newbie and i find that jumping hand held gives me a chance to enjoy the gr and not think about reachin back, all i have to do is let go so i think you can suck it lower safer off subb 300 with a 48 handheld. but who knows, im just a newbie enjoyin what i find to be fun!

[Uberchris]

what a fucking nightmare that could have turned out to be.

[Pikey]

YouTube- POWER TOWER NOV 19th.mov

This video is my buddy doing a gainer from 270

Courtesy of channel 5 camera man for his own collection

Personally i use a non vented 42/46 depending what I have at hand, in this vid i was using a 46. I have had a wild PC hesi from a 350ft B with a 42 that makes me favour the 46 HH when getting down there.

[Uberchris]

well looking at this video for a second brings up a question. if you are gonna be doing 300 ft or lower, and u choose to go stowed with a 42, should you not take a slightly deeper delay to ensure that your pilot chute inflates? he took about a 1 second delay before pitching. if he were to take even a half second more of a delay do you think this still would have happened?
the guy who taught me basic rigging and how to pack, PCs, etc, said that downsizing your PC for particular jumps is fine, as long as you do not pitch even a fraction of a second prematurely. smaller PC, especially stowed = deeper delay.

any comments or anything on this? i realize that sometimes PCs hesitate just cause of bad luck sometimes too.

[wormly81]

Very interesting question uberchris.

Does the size of a pc have any relation to how quickly it will inflate or does it just affect snatch force when it inflates (based on airspeed). In the scary video it appears the whole pc remained collapsed and a bigger size pc would have made no difference.

[Greenmachine]

Yes, bigger PC's have more material
hence it takes them just little longer
to form after being all smushed up.

Yes, bigger PC's have more material
hence they create more drag and in
turn more snatch force once inflated.

If a pilot chute falls in the forest and
no one is around to video it, does it
inflate, hesitate, meditate or gyrate?

[speedphreak]

...if you are gonna be doing 300 ft or lower, and u choose to go stowed with a 42, should you not take a slightly deeper delay to ensure that your pilot chute inflates?

This has been my M.O. on the few 280-300'-ish stowed jumps I have done and also on a lot of hand held ones in the 270-300' range. Now, I personally, have not had any hesitation issues out of about 40+ jumps (yea, I know, there's nothing statistically significant about that), but I have seen a couple of slight hesitations on other people.

The original intent of my initial post, before it kinda got sidetracked on the coolness of stowed vs. hand held, was to pick other peoples brains on the merits of vented vs. unvented PCs on short delay jumps.

It was pointed out to me via PM that on a short delay that a vented PC will lose a certain amount of air through the vents until airspeed is built up enough to effectively counter act this effect. That seems logical.

I think it's a fair assumption to say you can compensate for this by taking on a bit more of a delay, but can you do the same by switching to a same sized unvented PC?

Would an unvented PC introduce a greater chance for an off heading due to the potential oscillating effect or is that not enough of a factor on a short delay jump?

[gofastmax]

When jumping stowed in the 300' range I started off with my unvented 48" with soft cap. It worked well on several jumps but I did not like having all that pc stuffed in the boc. I also stowed a vented 46'' and felt the same about it.

Next, I experimented with my vented 42'', after a few of those I did not experience any hesitations, but felt that a bit bigger pc would be better for short delays with a larger canopy (293 flik).

Lastly, I had a plain unvented / no soft cap 44'' made. I really like to way it performs and the reduced bulk compared to a 46'' or 48''.

I have never had any big hesitations on any stowed / short delay jumps (42''-48''), but after watching those two videos it is one my mind more than before.

I think I will also be paying more attention to the manner I am tossing my pc, kinda reminds me that a stronger pitch will get the pc to bridal stretch and expose the material quicker than a weak flip of the wrist..............