Wing loading and high flare

[heims 0]

I have read articles (including on PD's web site) and discussions about wing loading, but I still have a question.

Assume: Exact same wind conditions, same landing area, brand new canopies of same style and manufacturer (made correctly), same wing loading, etc (keep all variables the same except size)

Jumper 1: Canopy 170 with jumper exit weight 170
Jumper 2: Canopy 260 with jumper exit weight 260

If both jumpers flying a straight in approach flared 20 feet high (and stalled the canopy and didn't let up), would there be any difference in how each approached the ground under their stalled canopy?

[GeeeeeeFly 0]

Smaller canopy should come down sooner regardless of the fact that both canopies are at the same wingloading...

I have done a few jumps were friends and I have the same wingloaded canopies but different different sizes...

89 VX vs. 111 VX

At full flight my friend would pass me unless we both worked to keep relative to one another...

~G~

"The edge ... there is no honest way to explain it because the only people who know where it is are those that have gone over"

[Martini 0]

I'm trying to understand what you're getting at here. A canopy stalled 20 feet up is almost certain to cause serious injury to any jumper. Think about jumping off a 20 foot high roof. It seems that the heavier jumper would sustain the most serious injuries since impact speed would be approximately equal regardless of weight but the force of impact would be greater for the heavier jumper. The force of impact doesn't really change that much for a heavily loaded canopy under your conditions because the vertical drop is the most significant factor and stalling the canopy (smoothly) would eliminate most of the horizontal speed anyway. The drag of the stalled canopy overhead isn't likely to change the outcome much either. Unusual question, did you have a reason for asking?

Sometimes you eat the bear..............

[Tonto 1]

I'm having the same problem. It makes some sense if you replace the word stall, with flare... but still...
When we're talking 20 feet, are we talking jumper height, or topskin height? Many variables to considder..

t

It's the year of the Pig.

[Spizzzarko 0]

Think about it man. The examples you gave is a difference of 90' sq. Of course they are going to behave differently. The lines are longer on the bigger canopy, thus creating more drag, and making a longer moment between the jumper and the canopy, so the bigger canopy is not going to feel as responsive. Let's not forget the fact that there is 90 more square feet of fabric up there. What do you think that's going to do? It's going to slow the canopy down. So comparing different sized canopy's is like comparing apples and oranges.

[heims 0]

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It seems that the heavier jumper would sustain the most serious injuries since impact speed would be approximately equal regardless of weight but the force of impact would be greater for the heavier jumper.

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The drag of the stalled canopy overhead isn't likely to change the outcome much either.

Sorry if it seems a little strange, but as quoted above you are getting at exactly what I'm trying to understand (so you understood more than you realized ). I've had lots of discussions about wing loadings and canopy size and heard lots of arguments as to why this person or that should be jumping such and such and I'm trying to sift through some things to see if I can get at the physics involved (boy am I wishing I hadn't skipped that class ). Just call it continuing education.

[heims 0]

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It makes some sense if you replace the word stall, with flare

One step at a time (wanted to understand stall scenario first).
But go ahead and replace the word stall with flare. What then?

Think of each jumper's feet as 20 feet off the ground. (Yes I understand someone will most likely get injured from that height.) The actual height isn't what I'm trying to understand. It's what happens to the parachute and how the size of the parachute and weight of the jumper will effect what happens versus how wing loading would effect it. Does that make any more sense?

I think it's been drilled that higher wing loading would lead to higher speed and faster descent rate (still keeping parachute constant). I also have read countless discussions about same wing loading on smaller parachute = faster turns due to shorter lines, etc. But how does the same wing loading on smaller parachute effect a not-so-perfect landing (straight in approach)?

[heims 0]

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the bigger canopy is not going to feel as responsive.

Is any canopy responsive in a stall? And what does responsiveness have to do with what will happen in this scenario? Will responsiveness change how you impact the ground? I'm not following.

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making a longer moment between the jumper and the canopy,

How does this come into play here (no turns and canopy stalled or already flared)? Can you elaborate?

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there is 90 more square feet of fabric up there. . . It's going to slow the canopy down.

You seem to disagree with what a previous poster stated about the canopy not creating much drag in this situation. Can anyone talk about the physics involved here? I would really like to understand.

[flyhi 24]

Holding everything constant as you requested and assuming no one has any forward velocity, only downward velocity from gravitational attraction to the earth, then I think you get into the Newtonian thing about F=mA to determine the force upon impact. If you thing about both performing an equally efficient or inefficient landing, then the A (acceleration, or in this case deceleration) would be the same (and here we caveate it with the oft used simplification, ignore the effects of drag. From 20' it's probably negligible anyway.) and the only difference would be the m (mass). And here, as in the dating pool, the big boy loses.

You can think of acelertion as the time rate of change to go from whatever fps you are descending at to 0 fps. That would make the units feet per second squared. Obviously, cushioning the impact (really thick sox?)or doing a good plf will make the deceleration slower and reduce the force linearly.

I think. then again, they did name a cookie after that Newton guy, right?

[Edited before the physics police attack.]

Shit happens. And it usually happens because of physics.

[Spizzzarko 0]

Yes canopy's are responsive in the stall. Take a PD-300 (a common student canopy 9-cell). When it is stalled you can actually steer it in the stall. It wants to fly backwards, but you can steer it around if you are carefull with it.

The moment (Moment being defined as a distance from two items) between the wing and the jumper comes into play with responsivenes in turns and flares. It has longer lines correct? What you tell the canopy to do will take longer for it's effects to come back down the lines. IE you flare the small canopy and the big canopy at the same time, the person under the smaller canopy will pendulum out in front of the canopy before the person under the bigger canopy. The person under the bigger canopy has further to travel because the lines are longer.

As far as there being 90' more material over your head, YES it's going to slow you down. All of that stuff over your head creats more drag. go and stall a pd-300 and go and stall and stall a velocity 79. What do you think is going to hurt more? Once you are in the stall laminar airflow over the top of the canopy has deceased, Do you go back into free fall?
Why not? It's because of the drag of the material over your head. In the most basic explanation it begins to work like a round canopy by displacing air to slow you down, but unlike the round canopy a stalled square doesn't cup as much air as a round does, thus it's decent rate is higher than that of the round.

[heims 0]

Thanks. That's exactly the type of info I was looking for.

Is there any way to determine whether the drag really is negligible?

[flyhi 24]

I think what Spizzarko is saying will be the only drag you would probably have to worry about. The delta in drag between wearing lycra and cotton (skin friction) would probably not amount to much.

And I would think, reaching way back, the drag caused by an inflated canopy (creating no lift) seeing a relative wind moving from bottom to top would be defined as F = 1/2 (rho)*Vsquared where rho is the density of the medium (air) and V is the velocity of the air. If you work the units out, you will see that this F is related to the F(orce) in my post above as a force over a unit area (psi or psf). It would seem to me, then, that the larger the area of the parachute, the more drag it would have. I still think the descent caused by gravity would be considerably greater making the mass of the individual the most important aspect, but you would have to run some numbers to figure that out. And I think you could disregard the effects of the density of air since it would be a constant in this case (not varying due to altitude change or compressibility.).

Kind of interesting if you look at it in terms of, say a bicyclist. The force he is working against at 10 mph vs 20 mph is not doubled, but quadrupled because of the V squared term. One of the reasons they use the handlbars they do to reduce the projected image and thus surface area hitting their "relative wind".

Of course, if I were you, I would run this by BillVon for a sanity check. He may be able throw in some PDE's to really clear it up.

Shit happens. And it usually happens because of physics.

[heims 0]

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stall a pd-300 and go and stall and stall a velocity 79. What do you think is going to hurt more?

At the same wing loading of 1 to 1 (I'll ignore the difference in canopy styles), that's exactly what I'm asking.

So if I understand you correctly, you are saying a 300 pound (exit weight) person will be hurt less by stalling a 300 sq ft canopy than a 79 pound (exit weight person) stalling a 79 sq ft canopy (at the same height off the ground). Please help me understand why. So far, a few other posters say that the canopy will create negligible drag and the mass of the person will be more of a factor which is opposite to what you say.

So maybe the next thing for me to ask is about drag. What kind of relationship exists between the size of an object (or person) and the thing (canopy) creating the drag? Is the relationship linear? How is drag calculated (I told you I skipped Physics )?

[heims 0]

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F = 1/2 (rho)*Vsquared where rho is the density of the medium (air) and V is the velocity of the air

Forgive my ignorance, how would we know what V is?

[flyhi 24]

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how would we know what V is?

Without instrumentation, I don't think we could know exactly what V is. With enough assumptions, you could probably come up with an approximation which might be close.

Assume both individual flares and at some point reach zero forward and zero vertical speed, and assume no or negligible wind perpendicular to the bottom of the canopy other than that generated by his descent. Assume the descent is relatively vertical and no lift is generated. Assume a stable, non-moving air mass. Ignore the effects of wing tip vortices. And a key one, assume linearity (not true, but discussed later). Realize that in 20 feet you would not hit terminal velocity prior to the ground (constantly accelerating.).

With the heavy assuming out of the way, you could time it and say from 20 feet it took five seconds to impact land. you could use that and say the canopy saw wind at the rate of 4 fps. You might try from the 0-0 flare/stall down to 10 feet and find it takes 4 seconds, or 2.5 fps of velocity. And I believe that is what you would find due to the effects of acceleration due to gravity. And that would be why the linearity assumption would not be correct, but would give a place to start. To be more accurate, you might want to vid the whole thing (let me know if you find some 300 lb guy willing to do this one!) and then measure the time from say foot 18 to landing. More accurate might be from foot 19 to landing, ad nauseum.

The process itself is akin to taking the derivative of a function where you model it by analyzing smaller and smaller pieces of it until the piece is so small a linear function approximates it.

Shit happens. And it usually happens because of physics.

[Spizzzarko 0]

All right guys,

I'm going to admit that this V=h* the square root of PI shit is confusing me. So as far as the actual Physics of it, I'm not quite sure how it all works, but I'm sticking by my posts. The more canopy you have over your head = more drag in a stalled situation.

[pilotdave 0]

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F = 1/2 (rho)*Vsquared

Hmm..aren't you missing an area there? Drag depends on more than speed and air density... 1/2 rho*v^2 will give you the dynamic pressure, not a drag force. But yeah...multiply it by a coefficient of drag and a reference area, and you've got drag. Now how will the coefficients of drag compare? Probably similar. So yes, bigger area=bigger drag. But if the canopies react very differently in the stall then their coefficients of drag and areas might not be proportional to their sizes. But for 2 canopies of the same type in different sizes, I can't really imagine the bigger canopy coming down faster. Now if it was a bigger fully elliptical canopy and a smaller square canopy, who knows.

Dave