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Some canopies have their lines trimmed to put the wing at a steeper angle of attack.

1. What types of opening, flight, and flare characteristics are defined by a steeper/flatter line trim?

2. How could I measure the "steepness" of line trim between canopies?

Line trim is only one characteristic of the canopy, so I don't think you can answer your first question based solely on trim.

You can measure the steepness however you want. :)

An easy way to do it is to measure the relative lengths of the lines in different groups, but there is also the relative "height" of the cell above the line to consider.

If you took a canopy and changed the trim by 3 degrees don't you think you would see a difference in glide? How else does changing the line trim effect the opening / flight / flare of the canopy?


I'm not sure you've ever done this


Let's say I take a single canopy and measure each separate line group, then what? Steepness can be measured in grade. How can I come up with this number and compare it between different canopies of different sizes with different line lengths?


What does the height of the cell have anything to do with the length of the lines?

i guess tom was assuming comparing different canopies based on their trim, and not the same model with different trims. id be interested in change of opening and flight characteristics if one would retrim a canopy flatter or steeper.... guess i should ask one of the manufacturers...


i think few have (me neither), but i guess its a pain in the ass and inaccurate, see below....


maths!

so, get all line measurements and the distance inbetween line attachment points on bottom skin, and you got yourself plenty of of well defined triangles where you can calculate the angle of the bottom skin. (fuck, my maths teacher was right, i actually do need this stuff...) (see attached drawing, you get a head start if you got continous center AB's) this of course is while ignoring any ballooning/wrinkling/bulging effect. i dont know base canopy designs well enough to say if theres any lower camber trimmed into them, probably not, or not a lot. (edit to add: trolls dont, have a rib lying here next to me) which leads to the next point...


if you want to compare different canopies, id would advise to calculate an angle to the chord line, and not the bottom skin....
so more measuring and more triangles... yay...

i would ditch the measuring tape, and just go write the manufacturers what they have their canopies trimmed to, why this angle was chosen, and whats going on if it would be flatter or steeper...
please post what you get from there...

btw. anybody heard of rigger lee recently? where is he when one needs him? :-)

edit to add:
thinking about it, youre probably best of just ditching measuring lines and stuff, and just pin down your rig, fan out the canopy out sideways very nice, tension some marker line at your best guess for the cord line, and measure a few points from that to your 3 ring...
none of this is going to be accurate, and dont forget that dacron is elastic, so if you compare canopies, try to put the same amount of tension on them....

84n4n4,

This was very detailed and helpful. Thank you for your response.

It can get tricky and mis-leading when using the incorrect terminology to discuss wings. What you are talking about is actually known as the angle of incidence or the rigging/mounting angle.

As a general rule of thumb (when all else is identical) the "steeper" angle will obviously have a higher descent rate with less glide, and less perceived flare. It will however be more stable and I suspect that it would also have better heading performance. A "shallower" angle will result in better glide and flare but will be more prone to the effects of turbulence.
As others have mentioned though, there are many different factors in canopy performance and simply comparing a single aspect between different wings is pointless.

Thank you Fledgling, your response answered a lot.

This goes against my intuition. I would think a canopy that is trimmed steeper would have higher airspeed and therefore have a better flare.

The wing first has to overcome the increased descent rate to level off, it then still has to pitch the jumper forward to achieve a significant difference in angle of attack to increase lift, which in layman jumper terms translates to flare.
A temporarily induced increase in airspeed (ie. front risers) is not the same as a permanent change in the angle of incidence.
Again the caveat is "When all else is equal".