Forums: Skydiving: General Skydiving Discussions:

Snowflake

Dec 19, 2002, 11:36 PM
Post #1 of 19 (1812 views)
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As has been stated in the forums before. performance under the same wingloading, but with different canopy sizes yields different results. My question, is there a formula for figuring out that x at a 1.2 wingloading on a 170 equals y at 1.0 wingloading on a 120 or is it more of a guesstimate type of thing

Moderator
Dec 19, 2002, 11:54 PM
Post #2 of 19 (1784 views)
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It's going to be a guesstimate thing.

It's a problem of scale. Specifically, canopies -don't- scale in a straight line.

billvon  (D 16479)
Moderator
Dec 20, 2002, 8:02 AM
Post #3 of 19 (1715 views)
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John LeBlanc did a seminar on this very topic at PIA one year. His conclusion - there are no formulas that work, no easy equivalencies you can use. A Spectre 170 at 1.5 to 1 is a completely different animal than a Spectre 120 at 1.5 to 1.

Anyway, what does "equal" mean? Descent speed? Glide angle? Turn rate? Stability in turbulence? Flare power? Forgiveness? All those things change with both loading and canopy size, and sometimes not in the way you'd expect. For example, a light woman might be happier with a lighter wing loading because she will be using smaller canopies, and smaller canopies tend to be more responsive in turns than larger canopies even at equal loadings.

Dec 20, 2002, 9:05 AM
Post #4 of 19 (1696 views)
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Quote:
It's a problem of scale. Specifically, canopies -don't- scale in a straight line.

I'm told that hang gliders don't scale in a straight line either, does any wing scale that way?

-
Jim

riggerrob  (D 14840)

Dec 20, 2002, 9:41 AM
Post #5 of 19 (1683 views)
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To expand on what billvon just said: few women jump heavily-loaded canopies because for light-weight women to get high wing-loadings they need to jump really tiny canopies and really tiny canopies have really short lines and those really short lines cause really fast turns, turns far faster than most sensible women want to make.

Moderator
Dec 20, 2002, 11:36 AM
Post #6 of 19 (1644 views)
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. . . turns far faster than most sensible women want to make.

Kinda sexist isn't it? I mean, I don't really want/need to make "fast" turns either.

freakflyer9999  (D 99999)

Dec 20, 2002, 12:02 PM
Post #7 of 19 (1635 views)
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I spoke to Bill Hazlet several years ago at Aerodyne (he answered his own phone) concerning scaling of parachute performance. I'm a heavy jumper, which is what initiated the conversation. We were primarily discussing the carrying capacity/strength of the larger canopies. His explanation was that many of the processes, materials, etc that go into making a canopy are not scaled when the canopy gets larger in square feet. For instance the seams and the threads used to sew those seams remain relatively the same. The height of the ribs and other features of the canopy remains relatively the same.
Bottomline his statement was that a safe wingloading under a smaller canopy may not be a safe wingloading under a larger canopy and for that reason he would not sell me a canopy without losing weight.
I'm actually glad he didn't because I ended up buying an Omega instead of a Triathlon. I think the Omega was a better performing canopy.

Jessica  (B 25202)

Dec 20, 2002, 12:03 PM
Post #8 of 19 (1634 views)
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Kinda sexist isn't it? I mean, I don't really want/need to make "fast" turns either.

You, and most other men, are also unlikely to weigh 100 lbs and be loading a 105 at 1.1:1. I think that was his point.

keskeie  (D 26254)

Dec 20, 2002, 1:11 PM
Post #9 of 19 (1616 views)
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I think Quade's point was that there was no need to say "woman" there at all. Unless that person thinks of all women as tiny and petite? Which is too bad for him....

Moderator
Dec 20, 2002, 1:19 PM
Post #10 of 19 (1603 views)
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Yes, Skeie, you're exactly right.

And ya know . . . it's interesting that Jessica, of all people, wouldn't have thought about it that way.

Additionally, if weight is the determining factor for sex, then I'm easily twice the "woman" of the one used in the example.

(This post was edited by quade on Dec 20, 2002, 1:22 PM)

keskeie  (D 26254)

Dec 20, 2002, 1:32 PM
Post #11 of 19 (1593 views)
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Oh, you are more woman than most, Quade.

Me, I'm only a woman and a half...

Moderator
Dec 20, 2002, 1:54 PM
Post #12 of 19 (1585 views)
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Either that or it takes two women to balance out one good quade.

You don't happen to have a little friend do you?

keskeie  (D 26254)

Dec 20, 2002, 2:48 PM
Post #13 of 19 (1569 views)
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I have a couple little friends, not that little, and they're already taken, sorry!

Jessica  (B 25202)

Dec 20, 2002, 4:49 PM
Post #14 of 19 (1543 views)
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And ya know . . . it's interesting that Jessica, of all people, wouldn't have thought about it that way.

Why is that?

Michele  (B 26874)

Dec 20, 2002, 5:43 PM
Post #15 of 19 (1527 views)
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You, and most other men, are also unlikely to weigh 100 lbs and be loading a 105 at 1.1:1. I think that was his point.
I think Jess has a great point. It sort of falls along the line of being taught to look for the cutaway and reserve, and not being able to see it because of breast blockage. It's something that women have to deal with on a far more regular basis than men do...(although I've seen some man boobies which rival mine... and they go braless...!

Quote:
It's a problem of scale. Specifically, canopies -don't- scale in a straight line.
Irrespective of the male/female thing, can you elaborate on why they don't scale? I understand it in a vague way (was covered in the canopy class), but it was far over my head, inasmuch as my low jump numbers and infrequent visits to the dz don't allow me the ability to observe as much I would like so as to be able to visualize it.

Ciels-
Michele

Moderator
Dec 20, 2002, 6:10 PM
Post #16 of 19 (1515 views)
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Why is that?

Correct me if I'm wrong . . . you're a writer and edit copy?

Jessica  (B 25202)

Dec 20, 2002, 6:12 PM
Post #17 of 19 (1513 views)
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Yes, I am, but on dropzone.com I'm off the clock. I'd certainly bill it if I could, though!

Moderator
Dec 20, 2002, 6:27 PM
Post #18 of 19 (1508 views)
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Irrespective of the male/female thing, can you elaborate on why they don't scale? I understand it in a vague way (was covered in the canopy class), but it was far over my head, inasmuch as my low jump numbers and infrequent visits to the dz don't allow me the ability to observe as much I would like so as to be able to visualize it.

When I was a very young lad I read an incredible book called "Me and Frumpet". I -think- I was in the 3rd grade or somewhere near there. In the book, a kid is playing with his train set that his dad bought him and his dad is furious that the kid has taken a pipe cleaner and made a little man out of it to ride on the train. The dad says, the little man isn't to scale.

Later, when dad leaves the pipe cleaner man comes to life. His name is Frumpet. Frumpet shows the kid that almost NOTHING scales.

The kid tells his dad this and the dad flips out. "The train set is perfectly to scale! Look at the engine, the track the town!"

The kid then places a bet with the dad and then proceeds to show him NINE different ways that the train set can't scale to real life.

It's a GREAT book. Too bad it's out of print.

Now -- on to canopies.

It's an engineering issue that not a lot of people think about, but in fact, very few things "scale" in a linear manner.

In the case of canopies and their performance there would be a lot of reasons but mostly, I think, because of total surface area vs. weight and drag associated with the total surface area vs. potential and kinetic energy of the weight.

A small canopy at a loading of 1:1 -should- have a lot less potential and kinetic energy vs. a larger canopy also at a loading of 1:1. Meanwhile, the larger canopy -should- have also increased it's drag, but generally speaking it won't be at the same ratio as the increase in energy. Generally, it will be slightly less.

So, you have a number of factors increasing at different rates. This should make the overall effect a curve of some sort, but that curve is going to vary quite a bit by individual canopy design -- so you can't apply any hard and fast rule to it.

crazy  (D 23767)

Dec 20, 2002, 9:28 PM
Post #19 of 19 (1477 views)
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My question, is there a formula for figuring out that x at a 1.2 wingloading on a 170 equals y at 1.0 wingloading on a 120 or is it more of a guesstimate type of thing
Everybody already said it: there are different criteria (speed, glide, reactivity...), few of them scale linearly. In addition, lots of factors can't be measured accurately, so, even for a specific criteria you can't get a formula. However, maybe it's possible, at a chosen wingloading, to estimate the variation for some criteria.

To be more specific on Quade's point, if you use a scale factor of 1.4, here are the variation of some important factors:
- chord, span and thikness: 1.4 each
- area of the canopy: 2 (1.4^2)
- area of the nose: 2
- volume of the canopy: 2.8 (1.4^3)
- length of the lines: 1.4
- weight of the pilot: 2 (to keep the same wing loading)
- height of the pilot: 1.3 (cubic root of 2 in an idealized world)
- frontal area of the pilot: 1.6

From these elements, you can get some information about the flight characteristics.

The volume of the canopy and the area of the nose gives the time needed to inflate or deflate the canopy at a given speed. The larger canopy will inflate (about 1.4 times) slower; the openings might be softer. Conversely, for a given turbulence it might be more stable.

The lift is proportional to the area, the angle of attack and the square of the air speed, so, as a first approximation, the air speed is the same if you don't change the wingloading.

The length of the lines and the frontal area of the pilot give the parasit drag of the lines and the pilot respectively. As the lift and the drag of the canopy vary with the area (at a given speed and angle of attack), the ratio lift/drag is better on the large canopy. It might have a better glide ratio, a better flare and longer swoops. As a consequence it might be easier to land.
Another consequence is that the larger canopy should allow a faster recovery from a dive. This doesn't necessarily mean a shorter natural recovery rate.

The radius of a turn (in a spiral), at a given bank angle, will be almost the same because the difference between the lines can be neglected compared to the radius.

The small canopy will be more responsive because there is less inertia (needs less time to swing the body). As a consequence it might be less forgiving on low turns (more risk to stall while digging out).

All of these can be neglected most of the time because if you take two pilots, the difference in skills, style and morphology will probably be much more significant than the difference of flight characteristics, whatever the difference in weight.

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