How to measure trim

[Dutton 0]

How can you measure the steepness of a canopy's trim?

[swoopfly 7]

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How can you measure the steepness of a canopy's trim?

i am no rigger but my initial thought is if all your lines are in trim, then the steepness should be in trim aswell.

[riggerpaul 1]

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How can you measure the steepness of a canopy's trim?

i am no rigger but my initial thought is if all your lines are in trim, then the steepness should be in trim aswell.

I don't think that's what he's asking.

I believe he wants to understand how you would measure the trim angle of canopies so as to determine which has a shallower glide or which has a steeper one.

To me, that would mean comparing the line lengths at the nose and tail, factoring in the distance between the two, and coming up with a number that tells us how much "nose down" any particular canopy has.

[DocPop 1]

Are you looking for a comparative measure of how steeply different canopies are trimmed to give some idea of how they fly?

My initial thought would be that the difference in length between A and D lines would set this angle, but I am quite sure that this is not the whole story when it comes to flight characteristics (recover arc, glide angle etc) as canopy designers manipulate many features to get the right "feel' to a canopy.

Listening to Brian Germain, John LeBlanc etc it is very much a "black art".

I guess what i am trying to say is that if you are looking to get an idea of how a canopy flies from the trim angle, you might be disappointed. I will now step aside and let someone who knows more give you a more complete answer.

"The ground does not care who you are. It will always be tougher than the human behind the controls."

~ CanuckInUSA

[Dutton 0]

That's what I'm after.

I am thinking it would be possible to measure the A lines and D lines- canopy to three ring and D to A.
Come up with a triangle and figure the trim in degrees from that.

I'd like to know how just to make comparisons out of my own curiosity.

Obviously, I'm no mathalete.

[billvon 3,120]

>I am thinking it would be possible to measure the A lines and D lines-
>canopy to three ring and D to A.

Actually you'd want:

canopy chord (distance front to back from A to D)

length of A and D lines

If you want to be accurate, cascade angle

That would give you the angle that the canopy makes from the 'centerline' of the lines, which would be proportional to (but not the same as) glide angle.

[riggerrob 643]

Half the data you need is on canopy trim charts, published by all the major manufacturers.
The next number is the length from A line to D line and you will have to apply a measuring tape to complete canopies.

The last number is derived from the length of the riser from crotch to connector link.

Then you can do a few simple trigonometry calculations to find the angle of incidence/aka trim angle.

Please note that figuring out the angle of attack (to the relative wind) is a far more complex process, best done while hanging under canopy and measuring the angle of the relative wind and maybe inventing a way to compare it with the angle of the A lines???

[pchapman 280]

I guess one can stretch everything out on the floor, canopy on its side, tension everything, and start measuring.

One issue is that we have no public standard known to normal skydivers, as to what reference points there are.

Skydiving canopies tend to have fairly flat bottomed airfoils, so the bottom skin makes for a reasonable chord reference line. (Rather than some aerospace engineering standard of nose to tail, which could work, but suffers from parachute canopies effectively having the nose of the normal airfoil cut off, at least in canopies with continuous nose openings across the span.)

If we measure the angle of the canopy, it has to be relative to some reference line -- from the confluence of the lines & risers, up to the canopy somewhere.

For the location of where the lines come together, one would have to pick some typical length risers to add to the canopy.

As for the point on the canopy, I don't know what other sources use - like Knacke or J.S. Lingard or the PIA or NASA or who ever has written something technical about ram air parachutes. The manufacturers must have some internal standard, whether or not it is industry wide. If I had to guess, how about a line up to the good old aeronautical standard of the quarter chord point.

[Dutton 0]

This is difficult. It is more than solving a triangle or the ration of a line to d line length.
I think it also has to do with the suspension point of the jumper under the canopy, which would dictate the angle the parachute wants to fly.

My brain hurts.

[pchapman 280]

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I think it also has to do with the suspension point of the jumper under the canopy, which would dictate the angle the parachute wants to fly.

Yeah, that's an additional level of detail. There's the trim angle as constructed, the geometric stuff that I talked about. Then there's the angle at which the canopy trims in flight relative to the horizon, which depends on the overall aerodynamics, glide ratio, and the drag of the jumper.