Differential line lengths?

[swilson 0]

Question for the canopy designers out there, maybe Brian St Germain?

I've noticed that the newer design high performance canopies seem to be more or less level across the top skin (side to side) with the last two cells at either end drooped at an angle of approximately 10 or 15°, give or take. Older types such as the Stilletto have an arched profile consistent with having equal length lines across the width of the canopy (I know that the A, B, C, D lengths are different).

Do these newer canopies have a differential line length to produce the flatter profile (would say the center A lines be a different length than the A lines two places out from center and the A lines at the stabilizers a different length again)?

What effect does this have on performance and handling? Would it make the canopy fly cleaner/better or would it make it more succeptible to collapsing in turbulence (or some other undesirable behaviour)? Also, what effects could this have down the road as the line set shrinks?

If someone did all the trig and modified the lines on say a Stilletto, what kind of behaviour could be expected? You could use the center lines as a reference to keep the trim the same.

I understand that the factory would prefer that all lines for a given position (A,B,C,D) be the same length because they are quicker and easier (cheaper) to make that way, but is it possibly not the ideal case performance wise?

Of course I most definitely WOULD NOT do any of this without assistance/input from a master rigger, I'm asking more out of curiosity.

[johnny1488 1]

I dont know when it first happened or who introduced it, but Icarus has built that into their canopies claiming it creates less drag and a cleaner airflow over the canopy. All I know is I love (and loved) all my Icarus canopies. Good things tend to get picked up by everyone.

Johnny
--"This ain't no book club, we're all gonna die!"
Mike Rome

[rmsmith 1]

Quote

If someone did all the trig and modified the lines on say a Stilletto, what kind of behaviour could be expected? You could use the center lines as a reference to keep the trim the same.

Actually, somebody used to do this very modification to the Stiletto canopy. I've never seen one myself, but they're out there. Why don't you just buy a used Crossfire and put it through some of your own trials; they come standard with a differential line trim.

A word of caution is in order: experimenting with line trim is best left to the professional riggers and test pilots. A problem with your canopy might not become apparent until you have something stationary to relate with, namely the ground.

[darkwing 5]

History - this is a very old notion. I believe one of the very first squares had this, the para-sled, circa 1970. I know I built a canopy in 1979 that had this feature, and I never believed that I thought of it first.

-- Jeff
My Skydiving History

[TimDave 0]

The basic idea with the trim like that is to have as much of the lifting force vector perpendicular to the weight of gravity (pointing up when in straight level flight). This would give the canopy a more powerful flare. Also the dropped end cells prevent wingtip vorticies and channel more of the air over the airfoil instead of having it spill off the sides. This improves the performance as well. I am not a canopy designer but just someone that has a small grasp of aerodynamics. If I am wrong hopefully someone could gracefully point that out to me

[markbaur 0]

Quote

The basic idea with the trim like that is to have as much of the lifting force vector perpendicular to the weight of gravity (pointing up when in straight level flight). This would give the canopy a more powerful flare.

I'll try to be graceful!

An unmodified T-10 has the lift vector exactly perpendicular to weight, but doesn't have a very good flare.

On a ram-air canopy, the landing flare converts forward speed to lift; the forward speed comes from tilting the lift vector forward.

Perhaps what you are suggesting is that making the lines progressively longer as you go from center to outside allows less anhedral, so more of the lift vector is up and less is to the side than in a conventional all-lines-the-same square canopy like a Sabre. That would make a more efficient wing, but there's a trade-off: you need some sideways "lift" to keep the canopy spread open. Otherwise instead of a little cathedraling between the lines, you'll get a lot, and the individual cells won't be as efficient.

Cross-braces (and to some extent the valves on air-locked canopies) help reduce the cathedraling of individual cells.

Mark

[TimDave 0]

See, I knew there was somebody more smarter than I are
That was what I was thinking! How much help is provided by cross ports? If there is inadequate pressurization would the cross braces be ineffective also?

[billvon 3,107]

>Perhaps what you are suggesting is that making the lines
>progressively longer as you go from center to outside allows less
> anhedral, so more of the lift vector is up and less is to the side than
> in a conventional all-lines-the-same square canopy like a Sabre.

I tried this on an old Sabre. Worst openings of my life. The problem was that the slider would no longer sit on the outer line terminations (the poker chips) and thus began deployment essentially a few inches down the lines. I've often wondered if I could rig a method of adding a few inches to the outer lines _after_ opening.

[darkwing 5]

Quote

I tried this on an old Sabre. Worst openings of my life. The problem was that the slider would no longer sit on the outer line terminations (the poker chips) and thus began deployment essentially a few inches down the lines. I've often wondered if I could rig a method of adding a few inches to the outer lines _after_ opening.

I used two approaches, either put the slider stop directly on the outboard suspension line, or add a "flare" to the stabilizer, so it becomes long enough to attach a stop.

-- Jeff
My Skydiving History