Terminal PCs

[LouYoung 0]

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It cannot, as best as I can tell, produce lift.

I'm no physics expert either, didn't even graduate high school. But here's one to wrap your head around....

Drag IS lift.

[Zennie 0]

From a physics point of view the two may (or may not... 460???) be the same.

Personally, when I'm referring to lift vs drag, I see drag as primarily a "pushing" counterforce as opposed to lift, which I'm seeing as a "pulling" counterforce.

- Z
"Always be yourself... unless you suck." - Joss Whedon

[Zennie 0]

And just to clarify a bit further what I'm talking about.

When I'm referring to drag I'm referring to the force created when a semi-permeable (or non-permeable) solid (i.e. canopy fabric) has velocity through a fluid (i.e. air). The fluid forced into (or through) the object produces a frictional "pushing" force that I'm referring to.

When I refer to lift, I'm referring to the upward force created when you have differential airflows over a solid, creating a vacuum (i.e. "pulling") effect.

- Z
"Always be yourself... unless you suck." - Joss Whedon

[base736 0]

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Drag IS lift.

Umm, false.

Lift, by definition, is a force perpendicular to the undisturbed airflow (http://scienceworld.wolfram.com/physics/Lift.html). Drag, by definition, is a force into the undisturbed airflow(http://scienceworld.wolfram.com/physics/Drag.html). Therefore:

(1) Drag never coincides with lift; and
(2) Lift can not be responsible for pulling a pilotchute away from the jumper (though it can be responsible for pilotchute oscillations).

Thanks for playing.

[TomAiello 25]

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When I refer to lift, I'm referring to the upward force created when you have differential airflows over a solid, creating a vacuum (i.e. "pulling") effect.

There are a lot of different explanations for lift.

I've had an actual aero professor tell me almost exactly the same thing Lou said ("drag _is_ lift"), as well as telling me that the above explanation for lift (which is the same one I have generally accepted for most of my life) was basically incorrect (according to him, the majority of "lift" in a wing is actually coming from deflection of air from the bottom skin--i.e. drag).

I think there is plenty of complexity in the subject, which includes big disagreements even between folks with a lot of high level education and a bunch of toys (like wind tunnels) to test their ideas.

Personally, I'm happy that it just works somehow. I've not got the time, energy, or brainpower to worry too much more about it, after realizing that people have spent entire academic careers trying to pin it down.

-- Tom Aiello

[email protected]
SnakeRiverBASE.com

[TomAiello 25]

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Lift, by definition, is a force perpendicular to the undisturbed airflow (http://scienceworld.wolfram.com/physics/Lift.html). Drag, by definition, is a force into the undisturbed airflow(http://scienceworld.wolfram.com/physics/Drag.html). Therefore:

(1) Drag never coincides with lift; and
(2) Lift can not be responsible for pulling a pilotchute away from the jumper (though it can be responsible for pilotchute oscillations).

If I read that correctly, though, it also implies that:

(a) Drag cannot be responsible for a PC inflating (a force perpendicular to the airflow, not back into it),

and;

(b) our most likely cause for PC inflation would be lift (which pulls the sides of the PC in a direction perpendicular to the airflow--drag would actually pull the PC back toward the jumper?).

Interesting stuff. I'm sure there's an application buried somewhere in the theory, probably having to do with Nick's drogue-built PC.

-- Tom Aiello

[email protected]
SnakeRiverBASE.com

[JaapSuter 0]

More useful information. Thanks everybody!

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"instead of the mesh part, use a solid fabric with only a tiny hole at the bottom"

...this is what's used for Relative Workshop's reserve pilot chutes. It like one big nylon bag over the spring....

Very interesting! In my infinite lack of skydiving experience I was unaware of such pilotchutes. I assume the the nylon is at least slightly permeable?

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Where you live there is no terminal object.

Actually, there is one about two hours from here, in your country. Before the end of the summer though, there will be a new local terminal wall, or my name is not Jaap Suter. And I'm not talking about the local pet-rock. That thing will unfortunately never be terminal despite its height. Stay tuned, come visit Canada!

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Are you going to Europe any time soon?

Unfortunately not. I'll be visiting North America's legal big wall, about twelve hours driving from here.

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I'm happy that it just works somehow.

Word.

Thanks everybody for the information.

[JohnnyUtah 0]

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AHhhh, the old PC debate returns to the forums Via Winkleport.

Actually it returned via Barelylegal, but I digress.
Now Winkleport is one I have never heard before.
Thanks for making me smile.But cant you at least spell it Winkelport?
Seriously though Tree, you can do better than that; with that name there are endless possibilities.

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You really must listen to yourself more often if that is the dumbest thing you have ever heard.

I didnt say it was the dumbest thing I have ever heard. I said it was two of the dumbest things I have ever heard.
You were a real contender when you said this one though Tree:

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Treejumps
Enthusiast

Nov 4, 2003, 11:16 AM
Post #5 of 8 (209 views)

Re: [Gargoyle] Cutaway toggles

Well written and logical. I think that it should also be noted that if a particiapnt in base is so concerned about a slide up line over, it might be time to re-consider jumping.

I've not had a line over, but I snagged limb of a tree one time that effectively "hooked" me inot the ground. Ibounced about a foot, and after about five minutes was able to get up. Nothing broke because I hit dirt, not rocks. But now I'm not all that worrried because I'm confident that spiralling in couild not have any greater impact than what I've already experienced.

Cya.

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2 things we know for sure. 1. A PC works on the priciple of lift. 2. A PC will inflate using f-111 in place of the mesh. (how does that air get scooped up into the pc if there are no holes?

How does the air get into the PC if there are no holes?

Im pretty sure youre talking about a Vector reserve PC (right?)
Well Tree, theres a hole there man, right at the bottom of the spring, and I would imagine air goes in that way. Maybe Bill knows.

Have you ever seen a Vector reserve PC? Ive packed loads of them.....I own one. Theyre cool man...totally groovy! I dont think I would ever BASE jump with it though.....but then again Im not that hardcore.

Have Fun, Don't Die!
Johnny Utah
My Website
email:[email protected]

[cornishe 0]

Calm down guys, before I break out the photoshop and Tom bans me again. ;-)

my beliefs =
lift is generated when the apex of the round pc is pulled down. disconnect the kill-line/bungee/whatever on your skydiving main. the pc will still inflate and open your canopy becuase it still creates drag sufficient to extract your pin,bag,etc. (yes, i did try this already), With the apex pulled in, the round will have high and low pressure areas and create lift, in addition to drag.


_

Abbie Mashaal
Skydive Idaho
Snake River Skydiving
TandemBASE

[brits17 0]

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Actually it returned via Barelylegal, but I digress.

Tree points finger --> Johnny points finger --> Brit points finger --> Jaap... hey he asked the question, I just went searchin for an answer!

Just for further debate's sake... I was thinking about a tandem drouge. Something like a 56"(?) big ass balloon piece of f-111, with a small hole at the bottom, much smaller than RWS's reserve pc. To boot, I would think that little hole is hanging out above the instructor's burble. So... why do manufacturers not prefer a large mesh bottom half if this is the most reliable inflation method?

Johnny you probably have more tandem jumps than anyone on this forum so thats why I thought of this question. Edited to add: Keep in mind the original question is regarding terminal pc inflation. With all this nitpicky, down to a more than exact science debate, just making sure all the elements are in place for the equation problem solving ;)

_______________________
aerialkinetics.com

[JohnnyUtah 0]

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Actually it returned via Barelylegal, but I digress.

Tree points finger --> Johnny points finger --> Brit points finger --> Jaap... hey he asked the question, I just went searchin for an answer!

Yea Jaap....what the hell? (and yes Im laughing my ass off)

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Just for further debate's sake... I was thinking about a tandem drouge. Something like a 56"(?) big ass balloon piece of f-111, with a small hole at the bottom, much smaller than RWS's reserve pc. To boot, I would think that little hole is hanging out above the instructor's burble. So... why do manufacturers not prefer a large mesh bottom half if this is the most reliable inflation method?

Johnny you probably have more tandem jumps than anyone on this forum so thats why I thought of this question.

1. A tandem exit is 6,000 to 13,000 feet
2. The drogue is so big that even with its current design there is drogue shock when its deployed at tandem terminal. I can only imagine what would happen if the lower half of the drogue was all mesh
3. The drogue actually collapses when the drogue release is pulled. You actually feel your fall rate speed up a bit before the chute comes out (trap door effect). The collapsed drogue has ample drag to pull out the two-man chute.

edit to add:
4. They design them to maintain a certain average fall rate.

Have Fun, Don't Die!
Johnny Utah
My Website
email:[email protected]

[TomAiello 25]

Tree, Johnny,

Please read this discussion of personal attacks.

I've reproduced the most relevant portions below:

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Personal attacks

It is important to realize that a personal attack is any attack (slurs, insults, denigrations, implications about their mothers, etc) or threat against another poster. Any such attack is not permitted here. This is true even if you are absolutely, 100% sure that the poster is stupid, or ignorant, or childish, or an ass. It doesn't matter if you think it is true or not, or even if you can prove they are an ass. You can't post such messages here. If you feel it is important to post such things, unmoderated forums like rec.skydiving do not have any such restrictions.

Clever insults that attempt to get around the above (i.e. "I like rabbits" "anyone who likes rabbits and posts here is an idiot") will result in the same sort of bans/locks/thread deletions that a personal attack will.

Consider yourselves warned. This thread has a lot of interesting technical information, and good input from both of you. Throwing in the "no, you're dumberest time infinity..." bit makes it sound like 2nd grade.

-- Tom Aiello

[email protected]
SnakeRiverBASE.com

[LouYoung 0]

Oh sure, bring actual documented definitions into it.

Lou

[brits17 0]

Yeah, I see everything you're saying. I think the point I was trying to bring up is that... it seems the evidence provided by the drogue remaining inflated throughout the freefall portion of the skydive would imply that "A pilot chute generates lift exactly as a ram air canopy does, by having the air flow over the wing (around the round canopy)" instead of air entering the pc from the bottom and hence, inflation.

_______________________
aerialkinetics.com

[Zennie 0]

I think the big problem here is that we're approaching this from a Newtonian rather than a Quantum perspective...

- Z
"Always be yourself... unless you suck." - Joss Whedon

[larsrulz 0]

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Yeah, I see everything you're saying. I think the point I was trying to bring up is that... it seems the evidence provided by the drogue remaining inflated throughout the freefall portion of the skydive would imply that "A pilot chute generates lift exactly as a ram air canopy does, by having the air flow over the wing (around the round canopy)" instead of air entering the pc from the bottom and hence, inflation.

I know nothing about BASE, but I have a fair bit of knowledge about aerodynamics given I have a graduate degree in it. It has been mentioned before on this thread, but lift is produced only perpendicular to the flow of air. So when falling straight down, lift can only be produced to the sides, where as drag is only produced parallel to the flow of air, i.e. up. The force exerted by a pilot chute (or round canopy for that matter) is soley drag by way of air deflection. When it comes down to it, a modern day parachute is actually working mostly by the same principle of air deflection, so in a way both people are correct as the pilot chute creates lift by way of air deflection, which at the same time can be referred to as drag.

I got a strong urge to fly, but I got no where to fly to. -PF

[TomAiello 25]

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The force exerted by a pilot chute (or round canopy for that matter) is solely drag by way of air deflection.

Can you comment on the inflation of the PC? Does the PC inflate through drag or through lift? Or some other mechanism?

-- Tom Aiello

[email protected]
SnakeRiverBASE.com

[Zennie 0]

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Can you comment on the inflation of the PC? Does the PC inflate through drag or through lift? Or some other mechanism?

Why do I have the sick feeling that this thread is going to result in 460 checking his references again?

- Z
"Always be yourself... unless you suck." - Joss Whedon

[JohnnyUtah 0]

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Can you comment on the inflation of the PC? Does the PC inflate through drag or through lift? Or some other mechanism?

Why do I have the sick feeling that this thread is going to result in 460 checking his references again?

Cause people just dont get it.

Tom could always refer to post#18


edit to add clicky

Have Fun, Don't Die!
Johnny Utah
My Website
email:[email protected]

[Treejumps 0]

*** "I would imagine"

It appears Johnny, that you spend a lot of time imagining. How many times do you think you will re-post the same thing?

So the air gets pushed in through the little hole? Very interesting. Keep up the good work.

Edited to add: Tom, he started it.

[larsrulz 0]

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Can you comment on the inflation of the PC? Does the PC inflate through drag or through lift? Or some other mechanism?

My academic experience is with airplanes, i.e. things that don't inflate, but the background can carry over. Without getting overly technical, lift occurs due to a pressure difference, which the case of a wing is due to difference in airspeed over the two sides of the wing. In the case of a pilot chute, prior to complete inflation the airspeed within the pilot chute will become turbulent and most likely reduce in speed, while the external speed does not decrease in speed, so there would be higher static pressure on the inside, essentially pressurizing the pilot chute during inflation, so in a way there is some lift contributing to the inflation, but by and large, the inflation (I would imagine) would be primarily due to air deflection, i.e. drag. The lift will be more useful after inflation in that it will increase the rigid pressurization of the pilot chute, as the higher static pressure air will want to leave the pilot chutes through the side, there by keeping the pilot chute inflated and rigid. Of importance to this situation is that were the pilot chute to be fully within a burble, then dynamic pressurization like this cannot occur as the air surrounding the pilot chute would not be traveling quicker, another good reason for a strong reserve pilot chute spring or big ass pilot chute for you BASErs.

I got a strong urge to fly, but I got no where to fly to. -PF

[base736 0]

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If I read that correctly, though, it also implies that:

(a) Drag cannot be responsible for a PC inflating (a force perpendicular to the airflow, not back into it),

and;

(b) our most likely cause for PC inflation would be lift (which pulls the sides of the PC in a direction perpendicular to the airflow--drag would actually pull the PC back toward the jumper?).

I think this runs the risk of rapidly degenerating into a debate along the lines of whether a dam is technically "earth" or "building". The solution, IMHO, being that it's neither. When you inflate a balloon, is the force that opens it up lift or drag? The answer, again, is "neither". Direct inflation pressure is neither lift nor drag, in any meaningful sense.

Anyway, in the spirit of being more productive...

How can we tell whether inflation pressure or lift dominates in "inflating" the PC? May I suggest the following experiment. Take an all-fabric PC a la RW and hook it up the usual way, open end down. So that inflation pressure is allowed. Make a few jumps (or, if you prefer, measure the pull force from a moving vehicle or something). Now, hook it up the opposite way, open end up. Make a few more jumps.

Lift will operate equally in both cases with respect to opening the pilot chute, since it's about the bulk of the fabric (and not about the hole, except as a way for air to get in passively). Inflation forces, however, will act to force air into the PC in the first case, and out of it in the second case.

SO... If lift dominates, the PC will work equally well both ways. If inflation pressure dominates, you'll be glad you didn't jump the second configuration (or very, very upset that you did). If lift is at all important, the second configuration will still be "okay", even if it's not quite as good as the first.

It's an easy and decisive experiment.

The force that a PC pulls back with is certainly drag almost to the exclusion of any other type of force. But that wasn't the topic of the original "lift" post.

[460 0]

It's simply enough: add a tension datalogger on the bridle for the same pilot chute with large mesh, marquisette mesh, and with and without tashengerts (sp?), in a controlled environment such as a launching system on a vehicle at deterministic speeds. ya know?

Looks like a death sandwich without the bread - Steve Deadman Morrell, BASE 174

[base736 0]

I don't see how that would isolate lift forces from inflation forces... Perhaps you could elaborate?

[TomAiello 25]

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Can you comment on the inflation of the PC? Does the PC inflate through drag or through lift? Or some other mechanism?

Why do I have the sick feeling that this thread is going to result in 460 checking his references again?

Cause people just dont get it.

Tom could always refer to post#18

So, something that you wrote on this forum is as authoritative as a textbook used by a Ph.D. physicist? I'm not doubting that you've got some good insight into these things, but I'm still going to ask as many people as I can.

I'm unlikely to accept that the answer is x, y or z simply because someone (whoever that someone is, be they you, me, or Dwain Weston and Carl Boenish speaking through mediums) proclaims that they know the answer.

-- Tom Aiello

[email protected]
SnakeRiverBASE.com