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shadeland

PDR Size Discrepancy

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Does anyone know where the discrepancies in sizes of PDRs comes from?

For example: A PDR 143 (http://www.performancedesigns.com/products/pdreserve/ check the WL chart) is 16.27 ft wide and 8.25 ft long, multiply the two and you get 134 square feet, not 143.

However, A PD Optimum 143 is 17.33 ft wide and 8.25 ft long, giving 143 square feet.

PDR sizes 113, 126, 143, 160, 193, and 281 have these discrepancies. None of the PD Optimums do.

The Icarus Reserve and Icarus Nano was mostly accurate, but off by a few feet in some cases. Same for Aerodyne Smart and SmartLPV reserves.

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When PD reserves first came out they used a different area measuring method than other manufacturers and than the then PIA standard. It resulted in a smaller number than if measured by the PIA method. So, compared to other canopies they packed 'large' because they were larger than the labeled size. The difference in method affected the chord measurement.

As PD as always said, the Optimum is a different reserve, not just different fabric. If you notice the Aspect ration of 2.1:1 is not what you get from the numbers in the PD reserve table, but are what you get from the Optimum table. So they seem to have made the Optimums wider to make the numbers work. Or measured them wider, see below.

I used to remember the difference in the measuring systems back when the PD reserves came out. But the PIA standard has changed and I don't remember exactly. IIRC and I probably don't, the standard measurement of chord was as in the current PIA standard linked below. The PD method was to use a line from the bottom skin leading edge perpendicular to the 'horizontal line' on page 6 of the standard below. So PD's measured chord was smaller than when measured by the PIA method and the named area was less than that calculated using the PIA method. So a PD 143 as measured by PD's method was 152 or something when measured by PD's method that everyone else used. So they packed big and flew big because when measured like their competition they were bigger than 143.

I actually may have that completely wrong now that I think about it some more and the difference was where they measured the span. As noted in the standard below the span varies depending on where measured, even on a rectangular canopy. I'm old and its been a long time since I cared about this. I may have documents some where documenting PD's original measuring method but they are lost in one of my many piles.

ANYWAY.

The reality is a combination of differing opinions on measurement and perhaps marketing advantages. I don't know why they changed the span on Optimums, if they did. I haven't tried to put a tape to any of them. Just realize that the measurements from two manufacturers may not be directly comparable. Judge the canopies on performance, not a few sq. ft discrepancy.

Perhaps PD will chime in to explain but usually they don't on here.

www.pia.com/images/rocketlauncher/PDF/TECHNICAL_STANDARDS/TS100_032715.pdf
I'm old for my age.
Terry Urban
D-8631
FAA DPRE

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What I'm not sure is why the PDRs are inconsistent in that regard. Some of the models the math adds up, others they don't. And the divergence means the measured square footage (by however they measure it) is smaller than the stated size.

I even made a graph, because graphs.

The bottom axis shows the models, and the side axis shows the percent smaller the measurement math shows versus the stated size.

divergence.png

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I do not think that the answer is that simple.

But how do you / they measure the surface of a canopy?
http://www.performancedesigns.com/docs/packvol.pdf, I. canopy size


Compare it with the method used to measure the volume of your car trunk.

They put balls off x cubic inch in the trunk. If they use bigger or smaller balls, the measured volume of the car trunk changes.

So do you measure the top or the bottom surface of the canopy? The projected surface on the ground or the curved surface?
See the attached picture. Which line (distance, length) do you take to calculate the surface of your square? If the canopy is not 100 % square, but elliptical …

canopy.jpg

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jurgencamps

I do not think that the answer is that simple.

But how do you / they measure the surface of a canopy?



Well that's already reasonably well established, which if the general methods is used.

The canopy volume measurement (or car trunk) isn't as relevant here as we are discussing canopy area, not volume.

We know PD doesn't use the PIA method, but uses bottom skin measurement. Obviously for ellipticals one can integrate for the area and doesn't use just one single span or chord.

One can still get into little details -- Is the measurement as designed, or after thread takeup, or after the canopy is then tensioned? Each company could have their own little variations there.

We know it is NOT a measurement in flight. In the paragliding world, canopy areas are often quoted based on a projected area above the wing, as calculated from their 3D design tools.

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pchapman


The canopy volume measurement (or car trunk) isn't as relevant here as we are discussing canopy area, not volume.


Just wanted to explain that using a different measuring method can give you a different result. Same analogy applies to measuring the surface.

In the PIA document, PIA has measured all those canopies using their method. This allows the reader to compare the size of the surface of different brands (or types) of canopies.
PIA quotes different surfaces for a (same size type) optimum and PDR reserve

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