X Form Construction

I've seen two different version of attaching shroud lines to X-form chutes. One is 8 shroud lines, with one shroud line per corner of the X. That one had the shroud lines making a continuous loop along the canopy edge and seam and around to the other shroud line. The other way is making the side of the X straight for a little while and then tapering it into a triangle and attaching the shroud line there (4 shrouds). If you have a preference for one way or the other, can you let me know why? Also, if you have a different way to do it, I'd love to hear it.

On a related note, what diameter do you use in Openrocket for the size of the chute? It seems like the side of the central square would be about right, but maybe that would underpredict drag. A typical drag coefficient to go with that diameter would be great as well.

I like the 8 line type. there are two magic numbers for size determination, for an eight line chute figure that an equivalent hexagonal is 20% smaller*, in OR use a cd of .64.
*a 36" x-form is roughly equivalent to a 30" standard chute...provided that all the lines are the same length and the chute doesn't rotate, rotation adds lift(along with winding the shock cord ).
Rex

Rex R said:

I like the 8 line type. there are two magic numbers for size determination, for an eight line chute figure that an equivalent hexagonal is 20% smaller*, in OR use a cd of .64.
*a 36" x-form is roughly equivalent to a 30" standard chute...provided that all the lines are the same length and the chute doesn't rotate, rotation adds lift (along with winding the shock cord ).
Rex

Click to expand...

How does that work?

you know that a gliding chute generates lift, right? well a rotating x-form is effectively gliding while remaining stationary. I have seen the difference in descent rates using the same chute. when it rotated it came down slower, after I evened out the shrouds it came down a bit faster. helicopter recovery with soft blades.
my warthog coming down under a 36" x-form (home made) chute (thanks Russ)
Rex

Rex R said:

I like the 8 line type. there are two magic numbers for size determination, for an eight line chute figure that an equivalent hexagonal is 20% smaller*, in OR use a cd of .64.
*a 36" x-form is roughly equivalent to a 30" standard chute...provided that all the lines are the same length and the chute doesn't rotate, rotation adds lift(along with winding the shock cord ).
Rex

Click to expand...

Thank you! Since I'm a little dense right now, I wanted to check procedure to make sure I have it right. If I'm reading this right, in OR I would use an octagonal chute with Cd 0.64 to get the descent rate I want, then multiply the desired diameter by 1.2 to get the edge length of each of the 5 squares used for the X-form. Is there a nice rule of thumb for shroud line length? For round chutes, I usually use ~1 diameter between the edge of the chute and the knot bundling the shrouds together.

hmm my calculator says multiply by 1.25 to get the 'flat to flat' distance, divide that by 3 to yield the size of the squares(if you want to try your hand at making one*, Top Flight measures flat to flat). 1 diameter shrouds seem to work okay, I went with 1.5d.
*tips for making:
I use a soldering iron(40w) to 'cut' the rip stop nylon.
add a 1/2" from the center to the end of each square(well they will be square once they're hemmed).
pin your hems before you stitch the hems & use a 'zig-zig' stitch to hem the ends.
a button hole attachment makes nice line attachment points.
I use braided dacron fishing line for the shrouds some where in the 15 - 50# test range. HTH
Rex

sry little slow tonight. if you use a cd of .64 in OR then the diameter used to achieve the desired descent rate is the size you want. if you use the default cd(.8) then multiply by 1.25 to get the desired size. DO NOT use both, you'll end up with an oversize chute! an 18" x-form is about as small as I want to use, smaller than that and they start having issues about opening(for me).
Rex

OK, sounds good. I guess the next silly question is what the advantage of the X form is over other kinds of parachutes. I'm going that direction because a square rocket demands a square chute, but are there more rational reasons?

bigger chute, easier to see.
Rex

As I understand it from talking with the RocketMan, Ky, they are designed to open much quicker, hence their use on dragsters.