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Parachute Measurement

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flying_silverad

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Which is the accepted method of measureing a flat, hexagional parachute, from corner to corner, or side to side?


Please take the poll. Thanks!!
 

powderburner

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(that was for all you Pat Paulson fans out there)

How about measuring total square inches (or centimeters)?
 

Micromeister

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Flyin!
it's supposed to be the deployed perimeter of the chute, but most simply use the side-side dimension. this is the reason a hemi has twice the volumn area as the same diameter flat chute.
 

shockwaveriderz

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here are some formula to determine equivalent parachute area sizes based on shape:

Equivalent surface area to circular chutes

circular chutes : Surface area =.785x diameter^2

square chutes: Surface area = length ^2 diameter = 1.13 * length where length is measured side to side

square chutes: Surface area = .50 * length ^ 2 diameter = .80 * length where length is measured as diagonal ends of the square

6 sided(hexagonal) chutes: Surface area = .65 * length ^ 2 where diameter =.91 * length where length is measured from point(tip) to point(tip)

6 sided(hexagonal) chutes : Surface area = .866 * length ^ 2 where diameter = 1.05 * length where length is measured panel side to panel side

working on other ones
 

flying_silverad

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6 sided(hexagonal) chutes: Surface area = .65 * length ^ 2 where diameter =.91 * length where length is measured from point(tip) to point(tip)

I guess what I'm asking is if someone stopped you on the street, and you talked chutes, would you assume size meant side to side or tip to tip? I a having an on going debate abou this with a friend...sort of.
 

shockwaveriderz

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flying: if somebody asked me on the street I would reply there are 2 ways to calculate the area: either side to side or tip to tip....but I don't know either of the formulas off the top of my head.....B(
 

flying_silverad

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Originally posted by shockwaveriderz
flying: if somebody asked me on the street I would reply there are 2 ways to calculate the area: either side to side or tip to tip....but I don't know either of the formulas off the top of my head.....B(
I agree. But my conversation or argument with certain bi-pod is about diameter.:D
 

Micromeister

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Flying;
That's where you can get the other Bi-pod.... Diameter based on What! Flat on the ground or inflated under load, two very different measurments. Shockwave is correct, Area is the only "consistent" way of looking at various "sized" chutes. I can't say if this will help your discussion but should make is mudder;)
 

flying_silverad

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Okay, let me put it this way. If you saw an ad for a 24" rip stop nylon chute for sale, what would anticipate the diameter to be and how would you check it when you got it?



I swear...I am not beating a dead horse
:D
 

Micromeister

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Since I make all my own chutes I really can't help Ya Flyin! Sorry.
 

shockwaveriderz

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I would expect it to be 24" in diameter...if thats what the ad said....

In small model rockets they actually use parasheets not parachutes....Ie they have no real shape......hence my diameter and area formulas work fine for them....in mid to high power...they use real parachutes.... so perhaps my diameter and areas formulas will not work....

the differemce between a parasheet and a a parachute is that a parasheet can be laid flat while a parachute cannot of course...

But still weather its a parasheet or a parchute, if some body or company advertises a chute as being 24" or 36" or 12" then it stands to reason that they should be that diameter ..I'm sure there are real world "real chute formulas to take into account the true and real shapes of real parachutes.....
 

Micromeister

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Shockie B:
Just so ya know I make an use true hemi-Nylon and plastic parachutes down to 6inch dia. which are measured to the outside of the deployed dia. which has twice the area as a flat parasheet. if measured to sides as suggested. I've done a 4inch for use with my micro's but it's seam bulk restirct it's use in anything smaller than a .375" tube:(

If I have to go out on a limb here I'd say most rocket folks will measure the Flat piece of whatever fabric or plastic as say that is the diameter, even thou it isn't even close, in operation. Which may be why the decent rate calculators are so far of;) those 24" flat pieces may only be a 20-21inch developed canopy.
 

flying_silverad

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If we consider the Handbook of Model Rocketry as a good source of info, this what I found.
On page 186- ...the discussion is about shroud line length.

"My experience indicates that the shroud lines should never be shorter than the major demension of the parachute canopy. Thus, a 12 inch parachute should have shroud lines at least 12 inches long."

To me, the major demension of a hex chute would be the distance between the two points. And as such, would assume that the 12" chute above was measured from poin to point. Also, if you look at figure 12-10, they show two options for calculating area of the same. Tip to tip and side to side (Starting to sound like famouse movie...wax on wax off)



:D
 

shockwaveriderz

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I don't know from which edition of Stine's handbook you are quoting from, but research was done in the early 70's by a doug malewicki and others that showed that in general the optimum shroud line length for a parasheet at least for duration competition is 1.5 x diameter...

But this may have something to do with the fact that parasheets are plastic, ie they are not porous and therefore there is no "bleed-thru" like there is with fabric parasheets or parachutes....
 

Micromeister

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Shockie: your correct. 1.5 x dia allows for full open canopies, whatever the material, look at the shourd length on the mars lander chutes.. about twice the dia. The shorter shroud lines the tighter the plastic wad you'll get:) This is why "Reefing in" the should lines brings the chutes down quicker by limiting the canopy blossom.
I personally use 1.25 times the dia for sport chutes, and 1.5 time the dia. for competition chutes.
 
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