Different ways of measuring a canopy?

[VTmotoMike08 0]

I was reading the FAQ on the PD website that talks of the different ways that different manufacturers measure the size of their canopies. It is here

This did not really make sense to me. I am no expert on the topic, but I do know this much:

A canopy is a piece of fabric. Therefore, it DOES have an absolute area.

Assuming that ZP fabric does not stretch at all (right?) then every canopy has to have a true area. Why can't the manufactures all agree? If you laid the canopy down flat on a huge piece of grid paper and counted the squares of known size, would this not give the true area? I have heard that the size that reserves are marketed as is not really their true size as well. I, for one, would like to know how big that thing above my head really is and be able to compare it to another canopy accurately.

Also, here is another question. Is it really just a marketing gimmick when a manufacturer sells their canopies in stock sizes like 169 instead of 170? I guess it does sound a little cooler to have a 169 instead of a 170 and I bet there are a few skydivers out there who would make a purchasing decision based on that . Really, except for very small canopies, I bet there is not a jumper out there who could fly both a 169 and a 170 and tell the difference in the two. It sounds to me like they are trying to "one up" other manufacturers that sell the typical sizes.

[airtwardo 6]

Interesting point...I don't think I've ever owned a canopy that was actually the size advertised...reserves included.

~ If you choke a Smurf, what color does it turn? ~

[pilotdave 0]

Would you measure the top skin or the bottom skin if there was a difference?

Even planes don't get their wing area measured directly, since the dihedral and the imaginary part of the wing inside the fuselage affect the "effective" wing area.

Guess the answer is just because it's more complicated than that.

Dave

[darkwing 4]

As is often the case, it is more complex than you think. Do you measure the spread out area, or the inflated area? How about bottom skin, versus projected top skin? Of course it does have an area, but just like anything else, you had better define your terms. One reason people disagree is merely because of inertia. "We have always done it this way, and it is fine with us if everyone else changes to our method." It confuses customers if a mfr changes how they do it. It seems then that they have changed the canopies they make, but they just have changed the nomenclature.

One reason they change the numbers is to differentiate themselves in the market place. I doubt the manufacturers think that someone will say "I tried a 170, but I wanted something zoomier, so I bought a 169." I doubt many skydivers decide anything based on a 168 vs. 169 vs. 170.

-- Jeff
My Skydiving History

[VTmotoMike08 0]

Out of pure curiosity, I would be interested in knowing what the typical difference % in between top and bottom skin area.

edited for spelling

>> I doubt the manufacturers think that someone will say "I tried a 170, but I wanted something zoomier, so I bought a 169."

I know. I was just saying that I bet there is some skydiver out there who thinks it sounds cooler to say 169 instead of 170. But maybe not. I think you got it right the the whole differentiating themselves thing.

[RiggerLee 61]

It would be nice if they could agree and stick to a standard. but even then you have to realize that there arestill a lot of variables. The canopy itself shrinks when you sew it ands not by a little bit. It's a noticable change in the area. The inflated area is a whole nother question. There is a fairly large reduction in span width when it inflates and that can vary with the flight mode. One of the nice things about cross braces is the have less destortion in the wing. So I mean how big is it really? Look, it eather flies or it doesn't

Lee

Lee
[email protected]
www.velocitysportswear.com

[jheadley 0]

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I know. I was just saying that I bet there is some skydiver out there who thinks it sounds cooler to say 169 instead of 170. But maybe not. I think you got it right the the whole differentiating themselves thing.

You want the 169 because it may be cheaper to pack. At one place I used to pack, canopies 170 sq. ft. and up were 7 dollars. 169 and below were 6.

And 200-288 sq. ft. student canopies paid 5.

[airtwardo 6]

And 200-288 sq. ft. student canopies paid 5.

Quote

Well that makes sense, everyone knows you don't have to pack a student canopy carefully!

~ If you choke a Smurf, what color does it turn? ~

[stuntedflyer 0]

Cool topic Mike.

I'm currently working on a couple of designs for new canopies, and when figuring out sizing etc, this has probably been my biggest hurdle.

In the end, I threw up my hands and decided to apply my own measuring technique, which makes sense to me - and I believe will make sense to the customer (when I eventually have customers). Without going into too much detail, I know exactly how much bottom skin surface area there is, then add the surface area of the entire leading edge presentation profile (accounts for the angle of the leading edge between bottom and top skins).

This technique seems to me to be accurate in measuring the surface area of the canopy that is actually presented to the air.

Now ask how many nights I woke at 3am trying to decide on line lengths, and how the various manufacturers derive them!!

Cheers,
Corey.

[mark 102]

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... I know exactly how much bottom skin surface area there is, then add the surface area of the entire leading edge presentation profile (accounts for the angle of the leading edge between bottom and top skins).

This technique seems to me to be accurate in measuring the surface area of the canopy that is actually presented to the air.

This would give the surface area if the canopy were descending vertically, which is different thant the amount actually presented to the air.

However, the measurement system you describe is similar to the PIA method, which measures from the tail to the most forward portion of the nose.

Mark

[stuntedflyer 0]

Yep, I agonised over that point a fair bit, however after much thought decided to go with that method of measuring over some other methods.

You have a point with regard to the "descending vertically" comment, but it seems to me to be logical. It does represent fairly accurately (IMHO) the surface area presented to the air at the point of opening just prior to forward movement, as well as at a critical point of flare.

Maybe a function could be added to this method, which incorporates the upper portion of the canopy (fore of, and over the camber) that directly presents to the air.

More importantly, perhaps some consensus could be reached in canopy measuring, to ensure across the board standards (such as adopting the PIA method), such that when one purchases a "169", they truly get a "169". Also that the surface area of a "Joe Smith Designs 169" is the same as a "Vertical Junkies 169". (names changed to protect the guilty!)

Cheers,
Corey.

[darkwing 4]

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...You have a point with regard to the "descending vertically" comment, but it seems to me to be logical. It does represent fairly accurately (IMHO) the surface area presented to the air at the point of opening just prior to forward movement, as well as at a critical point of flare.

Maybe a function could be added to this method, which incorporates the upper portion of the canopy (fore of, and over the camber) that directly presents to the air...

I think you are too worried about "accuracy" . For example, you cite the critical point of flare, at which your method would seriously overstate the effective area. The projected area at flare would be much less, due to the pulled-down tail.

The bigger issue is consistency, and it would be nice if all the manufacturers used the same method, which does exist, but at least for reasons of advertising inertia, and product recognition, they don't want to change their method, and I understand that reluctance. They could, however, use the PIA method for each new model, and in a few years all would be well.

Let us not forget though that area is only one measure of a canopy, and it doesn't do much good to compare numbers for canopies with different airfoils, or planforms by area alone.

-- Jeff
My Skydiving History

[stuntedflyer 0]

I totally agree with what you have written, except you'll note I did not say the critical point of flare, I said a critical point of flare. Critical at least to the measurement principles I am using.

Your point about comparisons between canopies, and different airfoils and planforms is very poignant.

C.