Jonsmann

Ok, I got a little carried away to bring my point across. The 150 Pa you mention seems a reasonable pressure difference between top and bottom skin when the canopy is in full flight. The absolute pressure is quite a bit lower though. Not the 0.4 bar, I will grant you that! My mind was on inflation pressures, not full flight pressures.

So we agree, thats nice! :-) You are right, this level of understanding of canopy pressurisation is adequate for most skydivers. Still the fact is that the overpressure inside the canopy is appx. +0.001 Bar, while the underpressure on the topskin is appx. -0.4 bar, both pressures relative to athmospheric pressure (appx. 1 Bar). For low speed flight it is very hard to make a high overpressure (depends only on speed), but easy to make an underpressure. So what really controls canopy pressurisation is the underpressure outside the canopy, the overpressure inside is insignificant. This is important because this thread is about openings and not canopy stability. A correct understanding of openings, require that you know that it is the underpressure that is the dominant factor. - Jacques

A quick comment on this thread. In many posts it is stated that the canopy pressurises. While this is true (very small overpressure), it is not the reason for the opening of the canopy. The pressure inside the canopy is never very high, but the pressure above the canopy is very low. In fact the canopy is sucked open rather than blown open (don't get me wrong!). The underpressure on the topskin seeks to open the canopy, but the lack of air inside the canopy prevents it (and of course the slider). In full flight the overpressure inside the canopy is indeed very low (most people would be scared to know! I think). Often I hear people say that the pressure inside the canopy is very high, but that is simply not true. What "pressurises" the canopy is the underpressure at the outside of the canopy, rather than the overpressure inside. - Jacques

*** My legstraps keep coming loose in the plane and it's getting more than a little annoying now. Does this only happen in the plane? Not under canopy? The leg straps must be under tension to be locked in place. So if your leg straps are a bit loose (and thus not under permanent tension) they will slip. Under canopy the legstraps will be under tension and thus should not slip. If they slip under canopy you have a serious problem you need to have fixed! I will suggest that you try to tighten your leg straps a bit more (0.5 to 1 inch). - Jacques

I place a knee on the slider grommets and tail while psycho packing. Keeps things in place. Also, you shouldn't roll the canopy, but rather fold it onto itself. If you roll, it will be a cylinder you put into a box shaped bag, won't fit well! When rolling the canopy the outermost part will be under tension, while the innermost part (inside the roll) will compress. This causes the tail to be pulled away from the slider grommets. If you really want to roll the canopy (instead of folding it onto itself) you should keep a knee on the slider grommets and make the roll loose. - Jacques

You know what I'm going to say. You are not ready yet! Be careful! - Jacques

They not only appear to be accelerating away, they DO accelerate away. Your energy considerations are wrong. You are confused about the frame of reference. Stop thinking about the jumpers speed relative to the ground, it has no relevance to the inertial forces involved in the deployment. - Jacques > It only appears as if the tandem pair is accelerating away from the camera because the camera is still travelling at a relatively constant velocity at that point in time.

That page hasn't been updated for 3 years! - Jacques Post: Nervures -- Come [beer adjudicator] Skydive Asia

Tight rubber bands prevent line dump, and line dump gives very hard openings! On any canopy! - Jacques

If your leg strap slips a little in the plane or during freefall, no problem. that happens because the friction adapter is not loaded. If it slips under canopy when it is loaded, DON'T JUMP IT until you get it fixed! Friend, it is not only your leg strap slipping away under you, it is your life. - Jacques

8 mph - 3.5 m/s is light winds. I am talking about 6-12 m/s, you know "normal" conditions! - Jacques

On several occations I have heard people reccommend people not to downsize, until they were able to do stand up landings in all directions, including downwind. Now I'm curious, just how many can land their present canopy "gracefully" in down wind conditions? This is for medium to high wind speeds of course. I feel confortable landing my canopy in any jumpable condition, including downwind, but standing up a medium to high speed downwind landing - no way! Can anyone do stand up downwind landings at wingloadings above 1.2 lb/ft2 in medium to high winds? - Jacques

To be able to do flat turns are an essential skill. I usually practice above 500 ft, because there are too little speed for a good flare if done low. I did it once down low, for my demo lisence exam. The examiner told me to land on a 1 m target behind a hill and in front of a few big trees. Bushes and trees on all sides about 5 meters away. The big trees was in the way for a direct approach, so I had to do two really low turns (90 degrees opposite direction), to get past the trees. The turbulence was bad, and I won't do it again unless I REALLY have to. Flat turns leaves you with little or no energy for a flare. Still it is MUCH better than being dead. - Jacques

Anyone know the normal range of pilot chute pull force? (assuming normal fall rate and deployment altitude etc.). Guesses are fine, but I would prefer to hear from people who actually know! - Jacques

I think this should do the trick: http://www.nextlink.dk/ - Jacques

I would define it: Stall: Flow separation from the top skin caused by too high an AOA. You no longer have a flying wing. Collapse: Massive deformation of the canopy, usually caused by too low cell pressurisation. A collapse is not identical to a stall, but a collapse can result from a stall. Airlocks can delay a collapse following a low speed stall. - Jacques

In the arched position I can look straight up, with protecs, mindwarps and my Oxygen A3. Still the hard part (on the A3) could be extended at the very least 1 inch! You are right that the neck is somewhat protected, between the rig and the helmet. However, when an exit funnels you will often not be in a good arched position. In my time I have been kicked in the head, beaten in the face by elbows and been taken out in formations. Trust me a high speed arm or leg can easily hit at the back of the head! My worst experience was an elbow hit in the face during exit. Everything went black for a second (I think) after that I was a bit surprised to taste blood in freefall! - Jacques

Am I the only one who think full face helmets are too open at the back of the head? The hard part of my Oxygen A3 could be extended about 2 inches further toward the neck, without restricting head movements. The back of the head is quite sensitive, it would be nice with more protection there. - Jacques

test

The burble usually refers to the low pressure region on the back of a bellyflier. However, there are also a high pressure region on the chest. It is maybe 10-20 times weaker than the low pressure on the back, but it is still there. In freefall chest mounted altimeters will therefore show a slightly lower altitude than a wrist mounted altimeter. - Jacques