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Ronaldo

Canopy design curiosity

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Does anybody know why rectangular canopies have all upper steering lines of the same length? Since the center of canopy’s arc is on the links (actually it has 2 centers) and the USL are cascaded at a considerable distance from the links I would expect them to have different lengths in order to maintain the same arc as the canopy.
As far as I can see having the same length results in a distorted deflection of the tail which can be noticed during flare. The outer lines get pulled first which results in a very curved trailing edge. I can only speculate reasons for that like (but in the end I don’t have a clue :P):
- Reduce toggle pressure by progressively deflecting the tail?
- Simplicity of construction and packing?
- Better stall characteristics?
- ?
Engineering Law #5: The most vital dimension on any plan drawing stands the most chance of being omitted

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Maybe controllability?

If the inner part of the canopy deflected first, I would expect the canopy to want to turn one way or the other, but having the outter parts keeps the middle cells inflated longer, and stay straighter?
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Just made a drawing showing how I imagine tail deflections are with equal and proportional USL. The difference is not huge but visible
My first impression would be that the proportional lines would lead to a better flare simply by “cupping” more air.
Obviously this is a simplified 2D representation of a 3D deflection so there may be many other factors that I’m not taking into account.
The canopy is in a close scale to a Tri 160
Engineering Law #5: The most vital dimension on any plan drawing stands the most chance of being omitted

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Quote

My first impression would be that the proportional lines would lead to a better flare simply by “cupping” more air.



Nice graphic.

I don't know a thing about upper steering line design, but I'll make a comment about "cupping air".

When air is trapped underneath a surface it tries to escape by flowing out around the edges. If the edges are uniform all around, then the air escapes randomly, causing oscillations as it bursts out first one way, and then another. This is why round parachutes had vents in the apex and elsewhere. Those vents allow the trapped air to flow out smoothly in a controlled manner, eliminating the oscillations that would otherwise make the canopy swing from side to side. Likewise, it could just be possible that having these ram-air steering lines all the same length creates that bulge in the center, which allows the trapped air to flow smoothly out the rear in a controlled manner, thereby steadying the parachute. Just a wild guess on my part...

The old analogy about a falling leaf applies here. A flat leaf will tilt and rock from end to end as it falls, because the deflected air spills first from one side, and then as the leaf returns to equilibrium, tilts the other way and spills air out the other side, creating a continuous oscillation. A paper airplane, on the other hand, controls the flow of air by pushing it all smoothly out the back, thereby giving it a nice smooth glide in one direction as it falls to the ground.

So, cupping the most air is not necessarily the ultimate goal. You have to both cup the air, and control it's release flow, simultaneously, in order to have a smooth descent.

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I'm not a canopy designer or builder, but I'm going to speculate that having these lines the same length has nothing to do with flight characteristics and everything to do with making the construction simpler, cheaper, and easier. I bet the disadvantage of having the lines the same length is very small compared to the cost and potential problems of having them different lengths.
You don't have to outrun the bear.

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Hi John,
Nice thought, it makes total sense to me. I guess giving a preferred direction for the air to flow might avoid fluttering of the tail.
In a rigid wing it wouldn't be an issue but in a parachute the ramdom air spills might create alternated tensions (compression/traction) on the tail thus causing flutter.
Thanks for the round parachute info

Ronaldo
Engineering Law #5: The most vital dimension on any plan drawing stands the most chance of being omitted

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That was one of my suppositions also (simplicity). Maybe test jumping showed that performance gains were not worth manufacturing complexity.
Another valid point is that equal lines also make line trim checks easier
Engineering Law #5: The most vital dimension on any plan drawing stands the most chance of being omitted

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I believe it is more due to the taper of the trailing edge. In fact I would expect all USL to be different but it looks like PD opted to keep UST1-3 equal and UST5 slightly longer. I guess this is another example of the performanceXsimplicity choice.
Engineering Law #5: The most vital dimension on any plan drawing stands the most chance of being omitted

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