Openings at Altitude

imported_Tom Aiello · June 12th, 2002, 01:03 PM

OK, here's another question I thought I knew the answer to. After some recent discussions, I'm not so sure. Can anyone help me?

Take two identical go and throw deployments. Same jumper, gear, technique, etc.

Make one deployment at 200' above sea level.

Make the other deployment at 10,000' above sea level.

Which canopy will take a greater distance to open? Why?

I've heard lots of arguments about thinner/denser air making canopies open faster/slower.

Skydiving reasoning holds that you are falling faster in thinner air, so your openings are harder. I am purposefully disregarding that (note go and throw deployment).

Anyone have any thoughts on this?

Anyone happen to have any statistical data?

--Tom Aiello
tbaiello@mac.com

TSR · June 12th, 2002, 04:35 PM

Tom,

As I'm sure that you have already been told, the canopy at 200'AMSL would open faster due to the "thicker" air pulling on the PC. You fall faster at higher altitudes due to less friction between your body and the molecules of air, the same holds true for the PC. I believe due to the low airspeed, the 10,000 throw and go would eat more altitude.

Then again, this is only my theory. I'm sure a book on basic aerodynamics would sufficiently answer your question.

-Travis

**base311** · June 12th, 2002, 06:39 PM

Well I'll stick with 'thoughts on this' since I seriously doubt I can arrive at a mathematical conclusion.

I have Theo Knacke's Parachute Recovery Systems Design Manual in front of me right now and have read sections 5.4.3 - Canopy Inflation Time, and 5.5.1 - Altitude Effects. I will be quoting mostly from this reference text to support my 'thoughts' on the matter.

But first I'll address some of what you said above regarding your conversations with others. I believe some confusion may lie with what others have read about high altitude effects on opening forces and canopy inflation times for parachutes dropped at constant indicated airspeeds and these people's (extrapolated) application of those observations to our sub-terminal opening environment.

As I see it, we have to accelerate for a bit before things start working, and most of the data published are for opening times and forces at constant airspeeds well above that with which we're dealing, so already we're comparing apples to oranges..

I realize we're not talking about opening forces, but I wonder if some people's confusion originates from knowledge of known altitude effects on openings, followed by their application of that knowledge to your question. There's no disputing that a canopy opened at a given indicated airspeed will experience forces roughly 4 times greater at 40,000 feet than at 7,000 feet, the explanation for this force increase with altitude is that the true velocity is twice as high at 40,000 feet than at sea level for parachutes dropped at constant indicated air speed. (knacke, 5-67) True airspeed can be calculated by adding 2 percent of the indicated airspeed for each thousand feet of altitude. In addition, the filling time of a canopy is only half as long at 40,000 feet as at sea level because of the 100% higher true velocity.

But WE AREN'T operating at a constant velocity, nor are we measuring our true air speed when we jump: all we really ever measure is time and/or distance. It seems to me that the acceleration we're experiencing is going to be roughly the same given the realm of our operation (negligible difference in acceleration rates between 200 feet and 10,000 feet for the few short seconds before we'll get open). Since the density of air at 10,000 feet is slightly less than 3/4 the density of air at sea level, it would take the pilot chute longer to develop the necessary drag forces for pack opening, main canopy acceleration to reach line stretch and, ultimately, opening time/distance. That, coupled with my thought that the difference in our acceleration rates would be negligible, leads me to think it would take a bit longer to get a canopy open at 10,000 feet than it would at 200 feet. I doubt that this is correct, but I'd hazard a guess that it would take about 1.3 times more distance to get open from 10,000 feet than from 200 feet in your go and throw situation. YMMV. I'm sure that it is calculable but it would take me a long while to figure it out.

later,
Gardner
p.s. knacke's book is awesome, btw. you should check it out sometime.

imported_Tom Aiello · June 12th, 2002, 09:04 PM

Thanks for the explanation. Lucid and well researched, as always.

I just ordered a copy of Knacke's book (it'll take 5-6 weeks to arrive, though, by which time I'll be out of the country--oh well).

Other than that, Poynter's manuals, and "Understanding the Sky" (currently the extent of my BASE reference section), do you have any other recommended readings?

Thanks again!

--Tom Aiello
tbaiello@mac.com