Forums: Skydiving: Safety and Training:
1=1 ?

 


melushell  (D 28650)

Nov 26, 2004, 3:02 PM
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1=1 ? Can't Post

ok mabe kinnda stupid subject

my question
a guy parachute with a 130 exit weight jumping a 1.2 wl has the same responsiveness and flight performance as a guy with 210 lbs at the same wl ???????

it might seem logical (or not), but i am really courious if the aerodynamics change by changing the size of the "wing"


AggieDave  (D License)

Nov 26, 2004, 3:08 PM
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Re: [melushell] 1=1 ? [In reply to] Can't Post

Nope. Same wingloading, but the smaller canopy performs a bit higher. It has to do with line length, drag and such.


melushell  (D 28650)

Nov 26, 2004, 3:19 PM
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Re: [AggieDave] 1=1 ? [In reply to] Can't Post

In reply to:
Nope. Same wingloading, but the smaller canopy performs a bit higher. It has to do with line length, drag and such.

well i guess it will be a slight difference in dragging

i am courious if the difference is alittle big bigger in the behaviour of the canopy

i imagined that it might be a factor that grows exponential with the size or something like

for me it seems perfectly logic that the canopies would react the same( well just a slight difference but nt noticable)

but i asked myself this when the difference is really big
an 120 (1.1) vs 250 (1.1)


AggieDave  (D License)

Nov 26, 2004, 3:30 PM
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Re: [melushell] 1=1 ? [In reply to] Can't Post

The canopy's responses are due to a pendulum effect (this is over simplified), generally speaking smaller canopies have shorter lineset and the pendilum swings quicker, so its "quicker" in the response.

Like I said, this is drastically over simplified.


melushell  (D 28650)

Nov 26, 2004, 3:42 PM
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Re: [AggieDave] 1=1 ? [In reply to] Can't Post

i read about pendulum efect

but if you can put a link it would be really apreciated Cool
(so you dont have to do all the explanation again)

thank you


ScratchTX  (A 37215)

Nov 26, 2004, 4:00 PM
Post #6 of 12 (1139 views)
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Re: [melushell] 1=1 ? [In reply to] Can't Post

Check out this article:

http://www.performancedesigns.com/docs/wingload.pdf


Martini  (D 23756)

Nov 26, 2004, 5:02 PM
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Re: [melushell] 1=1 ? [In reply to] Can't Post

130 lbs. @ 1.2 w.l. =108 s.f. canopy
210 lbs. @ 1.2 w.l. =175 s.f. canopy

The performance difference you are talking about here is really pretty significant. Sseveral drag factors contribute to a higher airspeed on the smaller canopy (less drag = higher airspeed): shorter lines, smaller jumper, thinner airfoil, smaller span. Also because the entire system is physically smaller and faster it turns quicker (more responsive). The combination of higher speed and increased responsiveness make the handling difference fairly substantial, possibly critical for a relatively inexperienced jumper.


Premier quade  (D 22635)
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Nov 26, 2004, 7:03 PM
Post #8 of 12 (1078 views)
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Re: [melushell] 1=1 ? [In reply to] Can't Post

Depends on exactly which performance numbers you're talking about.

Glide ratios wil be about the same.
Turn rates will be different with the smaller canopy being able to turn a -little- quicker.
Turf surf distace will slightly favor the larger canopy and jumper.

It's not only the aerodynamics that change with scale, but also the kinetic energy in the system.


UDSkyJunkie  (D 25746)

Nov 28, 2004, 7:41 PM
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Re: [melushell] 1=1 ? [In reply to] Can't Post

I won't claim to be an expert, but as an aerospace engineer, hopefully I can explain a few reasons why the smaller canopy will seem to have "more" performance...

If you are flying strait, the difference is probably very small... the canopy will fly the same speed, decend at the same rate, ect. This is because the airfoil is the same. When you introduce turns into the equation, though, there is more than the airfoil at work.

First, you've got less mass, so there is less force required to move things around. this allows things to happen more quickly. Similarly, there is less inertia (the higher the inertia, the more force it takes to make something spin). The combination of less mass and shorter lines dramatically reduces the inertia, so you can start spinning a lot faster.

The last thing, and something that I've seen a LOT of people overlook is that on a 200 sq ft canopy, you have to move the toggle a lot further than you would on a 100 sq ft canopy to get the equivalent deflection of the airfoil. So if you make a small mistake like flaring 2 inches further on one side than the other, you might not notice on a big canopy, or only think you encountered a "crosswind"... but if you flare 2 inches apart on a really small version of the same canopy, you could find yourself in a serious turn. it makes the canopy feel a lot more sensitive or "twitchy."

When you combine lower mass, lower inertia, and more sensitivity, well... it all adds up to a lot more performance. If you want a parallel, think about aircraft. Try to imagine a 747 performing like acrobatic plane. doesn't make sense, does it? if that's too outlandish for you, try to imagine even a small acrobatic plane performing like a tiny R/C plane... still never happen.


Premier billvon  (D 16479)
Moderator
Nov 28, 2004, 8:00 PM
Post #10 of 12 (805 views)
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Re: [UDSkyJunkie] 1=1 ? [In reply to] Can't Post

>If you are flying strait, the difference is probably very small... the
> canopy will fly the same speed, decend at the same rate, ect. This
> is because the airfoil is the same.

Even at the same loading, a Sabre2 120 does not fly the same as a Sabre2 230. It does not have the same forward speed, flare, glide ratio etc. John LeBlanc does an interesting presentation in which he asks about 20 questions about two hypothetical jumpers under identical canopies loaded the same (both 1:1) but one under a 230 and one under a 135. Do they flare the same? Do they have the same glide ratio? The answer, presented at the end of the talk, is always the same - no. Aerodynamics of ram-air canopies doesn't scale that way, as counterintuitive as that is.

Presentation is here:http://www.performancedesigns.com/docs/wingload.pdf


phoenixlpr  (D 3049)

Nov 28, 2004, 10:20 PM
Post #11 of 12 (790 views)
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Re: [UDSkyJunkie] 1=1 ? [In reply to] Can't Post

I think I`ve found some thing for you.

120 vs 230.

What is the drag caused by the jumper, lineset, canopy?
The lines of the 120 are shorter than the lines of the 230, arm of the jumper is about the same length. Same amount in brake input on shorter brake lines?
As you see the jumper does not scale, so any more question?


UDSkyJunkie  (D 25746)

Nov 29, 2004, 6:29 PM
Post #12 of 12 (617 views)
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Re: [billvon] 1=1 ? [In reply to] Can't Post

Quote:
It does not have the same forward speed, flare, glide ratio etc.

Sorry, I perhaps did not word what I was saying well. What I intended to say was that the difference in speed and glide ratio in level flight would be small in comparison to the difference in turning/flaring performance. Having never tested this by flying my Sabre 135 in level flight next to a guy 116 pounds heavier under a Sabre 230, I don't know how much difference there really isTongue... perhaps it is more dramatic than I believed.

I am curious if you have any input on whether the difference in performance is itself a function of WL... What I am asking is, would the hypothetical jumpers, both at 1:1 under a 135 and 230 experience as much difference as another set of hypothetical jumpers at 1.5:1 under the same 135 and 230? My guess would be not.

As a final note, something that was stressed in the article you sent a link to was that a large part of the equation is that lighter jumpers have a big problem with "percieved speed" because they start at such a light loading compared to heavier jumpers. I have to wonder how much of the speed increase is real and how much is percieved, at least in level flight (I'm not saying you or PD is wrong, just that this seems an important point to consider).



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