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packertom

pilot chute snatch force

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I'm hunting for some numbers on snatch force and constant pull resistance of a 26 inch ZP pilot chute.. I'm sure that someone has run this all through some tests at some point and it would save me the time of setting up a fixture.. hate to reinvent the wheel on this data..if you have it in newtons, that's cool, I can convert.

thanks,

Tom
tom@velocitysportswear.com
www.velocitysportswear.com
What's YOUR Zombie Plan?

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awesome, thanks Bill, that's about what we were figuring with the raw numbers we were running... we're going to do a few tests tomorrow to see what it's doing at lower speeds...not sure if it's scalable, but at least I know that our first whack at the numbers is close..

Tom
tom@velocitysportswear.com
www.velocitysportswear.com
What's YOUR Zombie Plan?

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The numbers quoted in the PC drag data chart .pdf are in error. While you show the time in FF you don't show the "Q" or Dynamic Pressure at the speed and altitude where the reading/calculation occurs. Further you assume all pilot chutes are equal except for size. This denies the existance of the Coefficient of Drag. Pilot chutes of the same size can have as much as a 300% differential in drag because of the design. This situation exists within our sport. The formula for Drag is Drag= Coefficient of Drag times the Plan form size, times the Dynamic pressure. D=Cd*So*Q (See: http://www.jumpshack.com/Q.htm
for a chart of Dynamic pressure.
The published Drag Coefficient of the MA-1, a U.S. Military pilot chute (36" diameter) is .65.
36 inches in diameter equates to 7.069 square feet and after the coefficient is calculated it has an effective size of 4.59 sq. ft. When you multiply the effective square footage times the Dynaminic Pressure (for this we will select a speed of 120MPH at 2000 feet) we get 35.21 pounds per square feet or 161.77 pounds total drag force. Much less than your 254 pounds after 12 seconds of FF with a 36 inch canopy.
I suggest you refer to: http://www.amazon.com/...Manual/dp/0915516853
for a manual which will explaine the entire process.
For a quicker reference see: http://en.wikipedia.org/wiki/Drag_equation
John Sherman

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$150 for a used copy? What a fucking ripoff. Amazon is a bunch of crooks. You know you can down load that whole thing for free. It's a bitch, the down load thingy doesn't seem to work very well. I had to get Bob, IT friend to down load it for me. But he was able to do it. Print it off for 35 bucks at kinkos.

You seem to have put some thought in to all this so I'll bounce some of the questions off you.

Drag coeficents and how they change relitive to renalds numbers. Normally I would think of the CD as some thing determined expearamentally or now a days in a computer. But the point being that it's normally only good for a certin range of Re numbers. Is there any way to exstapelate that data to other Re? Is there a trend as the Re change. Ideas of how great the errer is? Honstly I'd be happy if I could just bound the problem.

Thoughts any one?

Lee
Lee
lee@velocitysportswear.com
www.velocitysportswear.com

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Lee,

Reynolds numbers are generally not needed in low speed applications such as personnel delivery. Reynolds numbers below 50,000 have little or no effect on the Cd. When we did the refueling drogue tests at NASA Ames we were working as high as 250MPH and found that Reynolds numbers were of no consequence. We just don't go fast enough to affect the viscosity.

The only way a Cd can be determined is by testing. All of the Cd's I quote have been developed that way. My philosophy is to ignore the Cd and the So as single entities and relate to them collectively as "Effective Square Footage" (Cd*So). I have found that some manufacturers try to dodge the issue by declaring that their pilot chute is not the size (So) applied. If they are combined to the Effective size who cares what the physical size is.

Cd*So= Drag/Q(Dynamic pressure) You can only measure Drag and Q in the tunnel or in the air. From them you can get the Cd*So or Effective Sq. Ft. which is what matters. If pilot chutes were placarded with the Effective Sq. Ft. then they would be able to be compared and their drag could be calculated for any speed.

I agree that the price from Amazon is outrageous. I would recommend Dan Poynter or Mike Truffer as a better source. The book is apparantly out of publicatiion.

John

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Uh....

Help me out here. What units are you useing? How exactly are you calculating the Re number, What refrence length are you useing? I get...

At 5,000 ft for convenience
den 20.48 10-4 slugs/ft3
vel 176 ft/sec
length 3 ft
vis 3.637 10-7 lb*sec/ft2

I wind up with...

2,973,175

I mean, is my math just totaly fucked here or are we talking apples and oranges? I'm thinking you should at least be on the order of a million.

Lee
Lee
lee@velocitysportswear.com
www.velocitysportswear.com

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Here's a link to download the Parachute Recovery systems manual. It's a legal free download. The military approved it as public information. Just copy and paste the address into your website address bar. Once the page has fully opened just mouse over the document and right click and hit save as to download the file to your computer.

http://www.dtic.mil/cgi-bin/GetTRDoc?Location=U2&doc=GetTRDoc.pdf&AD=ADA247666

Or if you'd like a printed version for $35.00 check out this website:

http://www.sunward1.com/Parachute-Recovery-Systems-Knacke




edit to make URLs clicky.

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I would also get an Re value in the order of a few million. After all, a reference length of a few feet and 120 odd mph is like for many light aircraft wings in flight, and they're known to be flying in that few million Re number range.

I'm assuming that one would use the constructed diameter of a parachute as the reference length for the calculation? No other number is easily applicable.

As for Reynolds number effects, I know little about it for parachutes. But Hoerner's classic text on Drag showed a graph for the drag of flat plates. Re numbers had a big effect at very low numbers, but from one thousand to approaching 10 million (the highest tested) there was no real change in drag coefficient. I would think other blunt objects would behave similarly, but I couldn't rule out slightly different effects due to porosity or ribbon slot parachutes etc.


Edit: Hey, paratekk, thanks!!

I have the earlier version downloaded off the web, the 1978 Irvin manual credited to Ewing, Bixby, & Knacke. That's been mentioned a few times on dz. But the newer Knacke Recovery Systems Design manual, I've never seen that online.

Edit #2:
So Knacke's newer manual, on page 4-4 says has only a brief section on Reynolds number. It says,
"A Reynolds number effect on parachutes working in turbulent, separated flow, has not been established."

It also references a graph seen on p 5-29, which again is a short section on Re numbers. The graph doesn't show a lot of useful data (and ignore the line with the big jump in it, because that's for a sphere not a parachute). But basically from 100,000 on up past 1,000,000 (how far?) they say essentially no Re number effect.

I haven't searched the rest, but if they only devote about 2 pages out of 511 to it, Re effects aren't going to be too important in most normal conditions.

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Lee,

It has been years since I considered those calculations for Re. Thank you to Peter for his post saving me the pain. His post referencing the graph on page 5-29 in Kanacke and the subsequent comments tell the story as I know it. Many people mistaken flexible fabric bodies found on parachutes as equal to the flat ridged bodies usually found on aircraft. Those flat ridge bodies are more responsive to Reynolds Numbers while our rounded flexible bodies are not.
We had long discussions at the University of Minnesota during the AIAA Aerodynamic Decelerator Systems Conferences about this subject. I believe it was a Dr. Lee from the “Temple of Heaven Parachute Company” of China who produced the definitive paper on this subject.
Back then, I set up a spread sheet of a typical situation and substituted numbers to see the Reynolds effect. I found that 50,000 was the delineation point for my purpose, while Kanacki was more conservative with 100,000 as his low end. I learned back then that they weren’t worth the time considering and have ignored them ever since. You may need to consider them in your rocketry projects but we don’t need to for skydiving.

Thanks to “Paratekk” for the download info. While I have an original loose leaf copy of the manual, severely worn, and signed by Theo, I am reluctant to share it. Now I don’t have too.

John

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