Constructing a Ringsail Parachute Canopy

This is the single slot. Might be a bit of a misnomer; all I really did was slice the gores and then re-hem to make up the gap. This chute is extremely stable in descent and opens very softly. I did a few other solid chutes and one parasail kite, then turned to other areas of rocketry. But I've always wanted to get back into chutes; maybe John will be the spark!

Ted Cochran said:

This is the single slot. Might be a bit of a misnomer; all I really did was slice the gores and then re-hem to make up the gap. This chute is extremely stable in descent and opens very softly. I did a few other solid chutes and one parasail kite, then turned to other areas of rocketry.

Click to expand...

That's a real decent canopy, Ted -- effectively a 'Disk-Gap-Band' (like they use to land on Mars) - and, yes, they chose that design for 1)stability and 2)gentle deployment. But, I like your color scheme on it. (I noticed on your other canopy that it matches the rocket -- such 'Fashion Sense' <g> I'll bet you show up at those BOT meetings 'Dressed to the Nine's' - no doubt.)

Ted Cochran said:

But I've always wanted to get back into chutes; maybe John will be the spark!

Click to expand...

I'll need to see if that 'spark' works for me as well -- been something like 20 years since I did any sewing on parachutes.

-- john.

Before we get any further into this topic, I want to encourage and remind folks to browse through the 'Technical References' thread in this sub-forum - particularly posts #1, 2, 3, and 11 - referring to the Knacke text - both 1978 (post 2 & 3) (Post #2 is the Irvin Industries (better quality) version of the Knacke_1978 text), and the 1991 (post 1) editions and the Edgar G. Ewing classic on 'Ringsail Parachute Design' from 1972 (post 11). The links are all there for these very relevant and comprehensive texts on recovery systems and, if you have even the most minimal and remote interest in the Ringsail, the Ewing text is a must read. I will be referring to these periodically throughout all this as Knacke_1978, Knacke_1991 (where appreciably different) and 'the Ewing text' (in regards to the ringsail) when and where appropriate. Go get them.

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Can you sew?

If we go back and look again at the title of this thread, notice the key word, "CONSTRUCTING a Ringsail Parachute Canopy." There's one piece of equipment we'll have to 'make peace' with in order to do that: a Sewing Machine. So, Can you Sew?

The short answer is: "Of COURSE you can!!!"

There may be some who say, "Real Men DON'T sew." Well, 'real' men recognize that skills in this area allow them to have things others won't have - I have a killer hunting vest (altho I don't hunt anymore) that surpasses anything I've ever seen in the marketplace. I have some killer chaps (altho I don't hunt anymore <g>) that, again, is equally unique and all this is because I learned how to sew (and, yes, I've got David Coffin's book and am gearing up for some killer golf pants too <g>). BTW, I also have some rather decent parachutes as well <g>. Note, also, that some (most) of the major fashion brands out there: Christian Dior, Ralph Lauren, et. al. (even Levi Strauss) are all men - so count yourself in good company. Just think of all the cool things you can have (like bag deployment devices, too - designed EXACTLY like you would prefer).

I have been pondering over the last few days as to how deep I wanted to get into a 'Sewing Intro' in this thread - and was initially thinking of covering most aspects of it here. However, with all the good videos online and books and webpages, I think - for the sake of brevity (is this *brief*?? <g>) - just covering the most germane aspects unique to doing 'lightweight, structural' sewing (what parachutes are) and leaving the rest to the internet. If there are particular questions, fire back here with them and we'll cover as they arise.

The main issue with sewing parachutes is that you are dealing with a lightweight fabric - which can sometimes be a little difficult (bunching up, not feeding correctly), but, truthfully, I really haven't suffered that much with these issues, so I'm not exactly sure where these people are coming from with their cautions on it (one trick in dealing with this is to inter-layer some tissue (that thin paper that your shirts come in at Christmas) to provide a little body and then ripping it out once the stitch is finished). But again, I've never had to do that.

As to the type of sewing machine needed - all these fancy electronic machines today (that *do* interest me) are not really needed and that is because of one thing: the stitches you will use. There are only two stitches that will cover (for all intents and purposes) ALL your stitching needs in recovery system construction: a straight stitch and a zig-zag stitch (and, in truth, a 'straight' stitch is simply a zig-zag stitch with zero width (or, maybe it's the other way around - a zig-zag stitch is simply a straight stitch with a width other than zero)). You don't need any kind of 'fancy' machine to do that - in fact, some of these 'small scale' sewing machines that you can find on Amazon for $50 will do most of this (heavy, multi-layer stitching of bridles, etc may be the area where these little machines have problems). Other than that caution, just about any sewing machine can handle the tasks needed for parachute construcion.

Fundamentally, sewing with a sewing machine is forming 'lock stitches' continuously thru some material (ripstop, leather, denim, etc). In order to do that, it will take two independent lengths of thread - the 'top' thread and the 'bobbin' (or bottom) thread. The miracle of the machine is that these threads can be entertwined with each stitch without having to actually cut and break the threads to get them entertwined with each other (Google "sewing machine principle" or look here (pay particular attention here to the "LOCK" stitch)):

https://home.howstuffworks.com/sewing-machine1.htm

These are some of the basic items of operation you need to pay attention to (refer to photo):

1) The Presser Foot: The presser foot is what holds the fabric down to the 'table' and allows the needle to be extracted after the stitch is formed as well as apply pressure on the fabric to allow the 'feed dogs' (underneath) to advance the fabric (ready for the next stitch). There is a lifting lever (usually on the back of the machine) that allows you to lift the foot up off the table (this is how you first position the fabric in the machine). Rotate the handwheel (by hand, obviously <g>) to pierce the fabric pieces and locate your initial stitch, get everything aligned and then lower the foot and off you go. You may have to adjust the pressure that this foot exerts to allow the 'feed dogs' to advance the fabric correctly ('feed dogs' are little ratcheting teeth UNDER the fabric that advances the fabric for the next stitch). I am not aware of any machine that doesn't have an adjustment for presser foot tension (or pressure) - so consult your owner's manual (it'll usually be right over the presser foot at the top of the machine - see photo).



2) (Top) Thread Tension: If your stitches are 'loose' and not pulled nice and tight (they look 'bulbous' on one or the other side), you'll need to adjust the thread tension. Almost always, this can be accomplished by adjusting the TOP thread tension. There are ways to adjust the bottom (bobbin) thread tension if adjusting the top just doesn't do it - but it is rare that you would have to resort to bobbin thread tension adjustments, but it is there if you need it. You should be aware that getting the stitches TOO tight will result in feeding problems (i.e. the feed dogs can't advance the fabric - as the thread tension will pull the fabric back - resulting it way too short stitch lengths, bunching and fouling etc). It's a balancing act - but, fear not, one that CAN be balanced with a little patience and perserverance. This is why you take practice runs after an adjustment.



(On the bobbin photo, the thread enters the bobbin carrier at {1} goes under the 'leaf' spring and exits at {2}. Notice the tension adjustment.)

3) Stitch length and width: Both of these are infinitely variable (generally), but I mostly use stitch counts of 8 to 12 (or 14) stitches per inch (longer stitches (i.e. lower numbers) on heavier material) and widths of '0' (for straight stitches) up to whatever seems right when you're really 'zig-zagging' (zig-zagging for, mainly, to 'lay down' material over some reinforcing tape - or to provide some stress relief (they'll 'accordian out' when needed under stretching forces without 'popping' or breaking)).

4) Reversing button (lever): You need to know how to make your machine sew backwards, because reverse stitching is how you start (and stop) each run of stitching (this locks the stitching together, preventing unravelling). Basically, start your line of stitching 1/4" or 3/8" *downstream* of where you'd like to start, then back into your starting point, then go forward into your stitch. At the end, back up again for 1/4" or so into your line of stitching, then lift your presser foot, raise the needle, and cut your threads (pull out 4" or 5" to tie off if you desire). On my machine, reversing is just a button in the center of the stitch length adjustment that I press for as long as I want to back up (see photo).

One note on starting: you should always have about 4-6" of thread 'hanging loose' out of the bobbin and top (main) thread after you cut off the previous line of stitching. Take both of these and pull (slightly) to the rear before starting your next line of stitching.

5) Stopping and turning: If you are not finished with a run of stitching - just need to turn (hard), stop the machine with the needle DOWN (into the fabric). Then lift your presser foot, rotate your fabric to the desired direction, lower the foot and continue. This way, the run of stitching is continuous and you don't have to reverse, cut or tie off the previous run of stitching. (And there's nothing preventing you from doing a little reverse stitching if this turn needs a little extra reinforcing). Notice I emphasized "DOWN" in regards to needle position when you stop. Try it the other way (with the needle UP) and you'll appreciate why I emphasized DOWN!! <g>

Some additional sewing hints:

6) It is sometimes helpful to help the sewing machine carry the fabric by grasping your layers of fabric both in front of and behind the presser foot and put a little tension on it - just stretching it out a little bit (won't take much) and then pulling it through the the presser foot/sewing needle portion (you are, in effect, taking over the job of the feed dogs - but only feeling your way to what they would naturally want to do and just helping them out a bit). Thin fabrics, like the F-111 ripstop, can sometimes slip a little and this technique helps overcome that tendency (and assures nice, even, stitches).

7) Working on parachutes in the confined quarters of a sewing machine will necessitate gathering a lot of fabric (at times, especially on the larger canopies) and 'cramming' it through the open portion of the arm (between the needle and the main pedestal of the machine). You shouldn't worry about creasing or wrinkling the fabric (I always wash mine, anyway, when finished - so that relaxes any wrinkling). The main thing is to constantly pay attention as to *where* the fabric is in regards to where the seam is to be (what I would call the 'upcoming seam'). If you get too focused on watching the 'needle area' (where the sewing is actually occurring), you may overlook the fact that the upcoming seam is caught or jammed under some of the other fabric and this will prevent a free flow of fabric into the 'needle area'. The result is the stitches get *very tiny* <g> because the feeding of the fabric is hung up and the feed dogs can't move the fabric. Going slow helps a lot, but the bottom line is "situational awareness" - pay attention.

8) Somewhat related to the advice above is to also periodically stop and look UNDER the fabric to see what the bottom side is doing. If you are not careful, some of the other fullness in the canopy can get caught up with the portion feeding into the 'needle area' and get sewn into the seam you are working on. Only option then is to "rip it out" (with apologies to J. Evans Pritchart - from 'Dead Poets Society' <g>, "I want to hear R I P P I N G!") And the tool for that is a 'seam-ripper' (something you'll surely get used to if you sew long enough):



9) Bobbins: You'll need to learn how to wind your bobbins (you'll use the same thread as your top thread). Study your sewing machine's Owner's Manual on how to do this. Most bobbin winders are a little shaft on the top of the machine adjacent to the Handwheel (over near your top thread supply).

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However, as to these machine adjustments and sewing hints - first thing is to just sit down and sew a little - because (considering adjustments) everything may be adjusted just fine (we're assuming here that you are using the 'family' sewing machine - so SOMEbody must have had things 'kind of' adjusted before you got here.) BIG HINT (for family peace): IF you make changes, MAKE A NOTE of where things were BEFORE you start - and then return them to that prior setting before turning this 'ship' over to the (real) Captain). In truth, these little 'tweaks' are really minor and something you'll easily master.

If you've never sewn before - take an hour or two on a Saturday afternoon, grab some scrap material (out of the 'rag bin') and just simply get familiar with the machine. There's no need to go like a banshee -- GO SLOW! so you can watch the machine and learn it's personality. Speed comes later. Grasping concepts comes now.

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Materials and hardware:

A few things about the materials you'll be working with:

Needles:
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There are two (broad) types of needles (with some additional special purpose needles) that are available: 'Regular' point (sharp) and 'Ball' point (as you can see in the second photo, there's a wide range of needle types that extends beyond just 'sharp' and 'ball point'). The Ball point needle has a slightly rounded tip - used for sewing double-knits and is designed to push the fabric fibers apart as it penetrates them (otherwise, being knit, it can pull the fine fibers out of the fabric and create unattractive stitching). Regular point has a sharp, near-needle point and is generally used for woven fabrics (rip-stop nylon is woven) - but I've actually used both types of needles in parachute sewing and they work fine. Given a choice, go with 'Regular' and try the Ball point when you want to see what it's like. Sizes: There are several sizing 'schemes' for needles, but what you want is the finest needle you can get -- generally a Size 9 (and they go up beyond 18-20, but that's about as big a needle that will work with home sewing machines). I use a Size 9 (where I can get away with it (i.e. if my thread is fine enough) but sometimes 11 or 12) for fabric sewing and Size 18 for bridles (with heavier thread).



Thread:
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It is unwise to sew an article such as this that will be put under stress (sometimes severe) with anything like 'normal' polyester sewing thread -- you need something heavier. Threads are sized by a couple of (legacy) sizing metrics, but the (somewhat newer) TEX sizing is the simplest and most logical (if you dig into it, you'll find some *weird* sizing systems). TEX is the weight in grams of a fiber 1000 meters (1 KM) long - so, the larger the number, the heavier the thread. Here's a reasonably good comparison chart (also shows breaking strength). You'll want to scroll down to the 'Anefil Nylon' section:

https://www.amefird.com/wp-content/uploads/2009/10/5-Thread-Size-Comparison-Chart-2-4-10.pdf

... and another really good document gets into it in a little more detail:

https://www.ylicorp.com/cpanel/document/a_thread_of_truth.pdf

Normal sewing thread is in the neighborhood of TEX 20 to TEX 25. A really nice thread (especially for radial seams on the smaller (under 36") canopies) is a TEX 35 (or Size 'A' (sometimes 'AA') in military parlance) - but probably the most universal thread for the scale of most canopies is TEX 46 (or Size 'B' (sometimes 'B/2')). The largest that most home machines can handle (generally) is TEX 69 - 70 (or Size 'E') and this is generally good for bridles and such (maybe also skirt and apex vent hems on the larger canopies). You can use a Size 9 needle for TEX 35, but you'll need to bump on up to a Size 11 or 12 needle for TEX 46 (and Size 16 or 18 needle for the TEX 69 - Size 'E' thread). Don't ask about Size 'C' or 'D' thread because it must have been obsoleted decades ago (if it ever existed), as I've never seen nor read of any.

One general principle in sewing is that you match the thread to the fabric (as to the fiber type - nylon, polyester, etc). If you're sewing nylon (we are), then "it is recommended" to use nylon thread. Also, threads come in (generally) two 'finishes': 'Bonded' or 'Soft' - with 'Bonded' having a sizing applied to it for a harder finish. That's generally what we want, although I do have some nice, 'Soft' TEX 45 mil surplus thread that sews real nice. Can you violate these recommendations (as to matching thread to fabric) and use polyester? Sure. Will the canopy disintegrate in the air? Probably not. Should you? Well, when I can find 8oz spools (several thousand yards) of bonded nylon for $3/spool (not your normal price), then why would I want the hassle of wondering if that polyester thread *really is* all that much better? (As one additional point, you will note in the thread properties chart (link above), the Anefil Nylon is about 10%-15% stronger than polyester for the same size - so why wouldn't you want the stronger thread (especially since it also matches the fiber type of the material I'm sewing)?)

Go read "A Thread of Truth" (above) - there's a whole lot more to learn there.

Where to get it? Keep your eye out for surplus deals - because you can find some truly great bargains if you just look - but the best (recent) place I've given some business to is Southern Threads in Chattanooga:

https://stores.ebay.com/SEWINGTHREADSRUS

Here's some TEX 35 threads:
https://www.ebay.com/itm/v33-Lightweight-NYLON-POLY-Alterations-Leather-Thread-/111209630290

... and some TEX 46:
www.ebay.com/itm/v46-Lightweight-Nylon-Poly-Alterations-Leather-Thread-/200844999287

... and some TEX 69:
https://stores.ebay.com/SEWINGTHREADSRUS/69-Tex-70-/_i.html?_fsub=3160354011

(I will admit that the costs of some of their TEX 46 and 69 threads are a little more than what I remember earlier in the year when I bought some -- but the TEX 35 are still some good deals - and useful in many places in a small scale parachute. UPDATE: I did some checking and one change these folks have made recently is 'Free Shipping' but even with that, the costs are about double what my orders back in January 2014 were. Oh, well - back to searching again.)

How to use such large (bargain) spools of thread? Well, one thing about this is that the thread needs to 'pay out' off the top of the spool - not spin round and round (like the little spools normally used). You can buy a thread stand something like this:

www.ebay.com/itm/1-PC-SINGLE-SPOOL-LARGE-IRON-CAST-BASE-THREAD-STAND-SEWING-MACHINES-/270971250629

... or, just make one out of an old broom handle, a small piece of 1/4" p/w and a small piece of aluminum welding rod (with a little bit of dacron fishing line and some Elmer's):



This last photo shows the 'context' for the homemade thread stand - looking at the rear of the machine from above. Note the little 'pin' where the more conventional (i.e. smaller) spools of thread would rest. The hole in the bottom of the broom handle slides down over this.




I'll leave off the actual materials in the canopy (actual fabric and reinforcing/binding tapes) until we get to that.

Until then...Can you Sew?? Just remember, the only difference between an expert in this and you (assuming you're not) is that the expert simply knows a few things about this that you don't. Sooo... go fix that.

-- john.

p.s. If, however, you are sewing a new uniform for Captain Kirk or some seat covers for a DeLorean-based time machine, this one may be more appropriate:



edits:
9-6-2014: added hints 6,7,8
9-10-2014: added hint 9, minor grammar edits and updates to the 'threads' section

Outline for this post:
Terminology
Relevant Canopy Shapes
The 'Tri-Spherical' (tm) shape

Before we can talk about parachute canopies, we need to be on a common ground with the terminology so that we can talk intelligently. The basic parts of a parachute are shown here:




Most of these terms are self-explanatory - but many folks use the term 'shroud lines' to describe 'suspension lines' (as shown). While both terms are encountered in the literature, the (recent day) tendancy in the industry is to use 'suspension', which I've adopted and will be what I use in these posts.

Note, also, that the 'confluence point' is where all the drag force produced by the canopy resolves to and not necessarily where the suspension lines end. If the suspension lines end upstream of the confluence point, the part between those two points are called 'risers'.

Some additional (analytical / design) terms not shown in the above diagram are:

S(o): The total surface area of the canopy. One should note that slots or gaps or other openings in the canopy design (or even the apex vent) are NOT subtracted from S(o) - that area is simply considered equivalent to solid fabric. Any lost drag through these openings are taken into account when assigning the Coefficient of Drag (CD) for the particular canopy design. IOW, if a canopy has a lot of 'ventilation' thru such gaps or openings, then it's 'CD' will (naturally) be lower (all other things being equal).

D(o) or Do: "Nominal Diameter" - which is the diameter of a circle with area S(o) - or, stated another way, the diameter of a circle that contains exactly the same square footage of fabric as the canopy in question. This metric of designating parachute diameters may (or may not) have any relation to any 'measureable' diameter of a particular parachute - but it places all canopies on an equal footing - so that relative performance characteristics can be discussed and compared intelligently.

C(D(o)) or CDo: Coefficient of Drag *referenced to NOMINAL Diameter - D(o)* - which, as noted above, places all canopies on that 'common ground'. It is, truthfully, a reference point to compare any canopy with any other - as it rates the actual EFFICIENCY of the canopy in creating drag. (For example,) a canopy design with a CDo of 0.80 can simply get more drag out of a square foot of canopy fabric than an alternate design with a CDo of 0.70. This makes those comparisons quite simple and rational and is the fundamental underlying reason that the professional industry uses this metric. Referenced to Nominal Diameter, CDo values will be lower than what is sometimes seen (especially here in the hobby) - the highest reported CD value for (solid or slotted) canopies is a CDo of 1.0 (for the Annular canopy (from which the Fruity Chutes Iris Ultra derives) - see Knacke_1978, Table 2.1 and 2.2, pg 75-76). Knacke_1991 shows a max CDo of the Annular even lower - 0.95. Some of the rotating canopies (the Vortex Ring, in particular) can get up to about 1.8 (Knacke_1978, Table 2.3), but this is a canopy seldom (if ever) seen in the hobby (I'm not aware of the existence of one in the hobby arena). The reader should note that there is a 'scaling issue' with parachute drag creation - and the CD will generally be LESS in smaller canopies than these figures reported (which are assumed to be full size canopies). I haven't seen any reports (that I can presently remember) that quantifies the drag reduction with scale (other than discussion confirming it is real), but it would be reasonable to conclude something on the order of 10% reduction with small, 'hobby size' canopies. Realistically, taking these factors into account, most hobby size canopies would show CDo's generally in the range of 0.60 to maybe 0.75. A concerted effort to document descent rates with all these instrumented (i.e. flight computer) vehicles now being flown is clearly indicated *provided* the data is referenced to the vehicle's *nominal* canopy diameter (so that accurate CDo values can be determined).

D(c) or Dc: Constructed Diameter - the diameter of the canopy *as constructed*. This will be the diameter of a canopy "as best as we can" lay out on the floor. A Flat Circular will be easy to lay out flat (naturally), but a Conical (or Hemispherical or Quarter-Spherical) cannot be laid out flat on the floor, as it is a 'shaped' canopy (see below) - there's too much 'extra' fabric within the perimeter (due to it's 3 dimensional shape). On 'shaped' canopies, D(c) will be smaller than D(o). With a Flat Circular, D(c) and D(o) are exactly the same.

D(p) or Dp: Projected Diameter - what many will call the 'inflated' diameter - the actual diameter of an inflated canopy. Since the skirt will contract under load, this number will be smaller than D(c) (a little) or D(o) (somewhat more and could be a fair amount). It is also virtually impossible to (accurately) measure or report (one would have to crawl up on an inflated canopy, which would, of course, distort the shape - not to mention being a rather foolish act). The only real way to determine this number is through accurate photo interpretation (and even then, the accuracy is somewhat variable). Due to it's questionable utility, the industry generally doesn't bother with it - since ranking (and designating) canopies with D(o) and CD(o) is far more accurate and workable.

L(s) or Ls: Length of the Suspension Lines - from the skirt to the risers (if any - or the confluence point if not). (Note that these 'length' values should use the lower case 'L', but it could be confusing, so I use the upper case 'L'.)

L(R) or LR: Length of the Risers - self explanatory.

L(r) or Lr: Length of the Reefing Line - self explanatory.

L(e) or Le: The 'Effective' suspension line length -- from the skirt edge to the confluence point (inclusive) - which is the sum of L(s) + L(R). If there are no risers, then L(e) = L(s).

L(e)/D(o) or Le/Do: The ratio of Effective suspension line length to Nominal Diameter - normally this value is unity (1.0) - but there has been study of other (typically, longer lines i.e. Le/Do values greater than 1.0). The Apollo Ringsails used Le/Do values of around 1.50 (I've seen slight variations on that number). This ratio is also the value used with the last several canopies I built (including the ringsail).

h(g) or hg: Height of Gore - This is simply the height of an (individual) gore segment - from skirt to apex - measured along the gore centerline.

e(s) or es: Width of Gore at the skirt (that's what the 's' subscript stands for) - This is the width of the gore segment AT THE SKIRT - which may or may not be the widest part of the gore pattern (extended skirt gores are somewhat narrower at the skirt than a little farther up the pattern (see Knacke)).

v(e) or ve: equilibrium (descent) velocity - what many will call the 'descent rate'. Sometimes referred to as V(v) (vertical velocity).

'Shaped' canopy: A canopy where the final, CONSTRUCTED shape is something other than a flat shape - something other than one that could be laid out flat on the floor. Conicals (single, bi- or tri-), Hemi- and Quarter-sphericals are examples of this type (see Knacke). The quarter-spherical is the newest shape - if a hemi-spherical is "half" of a sphere, then a quarter-spherical is a "quarter" of a sphere (or the top half of a 'hemi') Even Knacke_1991 doesn't talk very much on this canopy, although it is considered one of the most efficient canopy designs currently available today (and I have no explanation as to why - other than the fact that the 1991 text is now 23 years old and that may pre-date sufficient research data on the 'quarter's' performance). It is, basically, a 'curved' version of the tri-conical canopy design. One thing about too-strongly shaped canopies (like the hemi-spherical) is that it is considered a 'rigid skirt' - since there isn't a lot of room for the skirt to expand under high-dynamic inflation scenarios (i.e. high speed deployments) - which can result in canopy failure when the skirt band fails due to extremely high inflation loads. Rigid skirt canopies should generally be avoided for just that reason. Shaping a canopy is good (i.e. efficient), just don't shape it *too* much.

Bias .vs. Block construction: The general shape of a (inflated) parachute isn't always the shape it was constructed in. With the conical being one of the more efficient canopy designs (and, therefore, used quite regularly (in the professional/military regimes)), one must ask, "Why doesn't it look like a cone - that's what 'conical' means, right?" The reason it doesn't is because it is built with 'bias' construction. Fabric can be cut 'straight' (aligned in either the warp or the fill direction) or 'on the bias', where the major direction of a piece of fabric is at some angle (usually something approaching 45deg) to the warp/fill. Fabric articles assembled this way will stretch along that bias with very little distorting force. This is how the conical family of parachute designs don't rip themselves apart when they inflate (as the mid-gore stresses on a conical would far exceed the strength of the fabric were it not for that 'bias stretch'). A conical canopy canNOT be built (and be expected to survive) except 'on the bias'. The alternative to bias construction is called 'block' construction - where the main (or warp) fiber direction is aligned with the centerline of the gore - from skirt to apex (and the 'fill' direction is across and perpendicular to that centerline). Canopies constructed with 'block' construction do not have near as much give under stress, so either reinforcing or shaping of the canopy is needed to assure the stresses don't get out of hand. Constructing canopies (especially small scale ones like in the hobby) 'on the bias' is extremely difficult, as the slightest stretching force will distort the gore and make lining up adjacent points in the seams almost impossible (and it is equally difficult to get the gore back in shape after being distorted). This makes sewing the gore segments together a nightmare. One can mark 'points' at various places along the joining edges and use those to help keep things aligned, but it is still a difficult undertaking to construct a small-scale canopy with bias construction (and this is why I generally avoid it - maybe a little bias construction on my first canopy or two, but I quickly learned the difficulties and migrated to 'block' construction and have not moved back. That, however, has led to a canopy design that is more forgiving of the stresses that 'block' construction is subject to - I'll talk about that in a minute).

This issue of canopy shape is where the (more classic, but obsolete) Flat Circular design lends itself to the construction techniques generally practiced in the hobby - as (upon deployment and inflation) it simply 'folds itself around' the generally rounded shape we all recognize as normal for an inflated parachute (and would be your classic NON-rigid skirt). Better still would be the quarter-spherical - partially shaped (efficient), but still flexible enough at the skirt to allow for anomolous deployments without risk of skirt failure.

NOTE: Some of the newer fabrics are so 'infused' with various polymeric substances (silicone, polyurethane, etc - to reduce the ventilation of the fabric, i.e. make them less permeable), that 'bias stretch' is also greatly lessened (not necessarily preferred). Many of these fabrics feel (and respond) almost like plastic sheeting. If you can find some old military surplus canopy fabric (that is rather permeable), it's very obvious to demonstrate how much easier it is to stretch on the bias (and, conversely, with some newer fabrics, how much more resistant it is to deforming on the bias). That raises the question as to how are canopies that need that bias stretch constructed today? One must remember that circular canopies are pretty much obsolete for personnel descent (civilian or military) - except as reserve canopies. Most of the fabric generally available to the public today is assumed to be used for the cellular (primary descent) canopies - and this is the treated, 'lo-po' (low porosity) types. One can only assume that a (commercial) canopy manufacturer has his sources when he needs to make a 'run' of reserve, bias-constructed canopies (and I have no figures as to how many (manufactured per year) may be of this type.) Having said that, it is possible that these newer fabrics exhibit sufficient stretch that bias construction is, indeed, possible - when on the scale of full-sized canopies. We'd need a commercial manufacturer or rigger to answer that point with authority. Further still, this very trait in the newer fabrics *may indeed* make small scale canopies 'on the bias' possible - since this resistance to distortion is exactly what would be needed to keep the gore shape correct (during construction). However, again, if the fabric resists distortion (either through treatment or by being cut in 'block' fashion), then there is no reason to construct on the bias (as you would gain nothing).

The primary reason to spend time discussing this issue of 'distortion' of the fabric is due to one very important issue with small scale hobby canopies -- SCALE. In order to correctly and accurately create the designed shape of a canopy of the size used in the hobby will demand *accuracy* - and the smaller the canopy, the more precision is necessary. A 1/2" error on a full-size canopy is negliable. Not so on a small canopy (especially if additive on every gore). Strive for precision and accuracy - as that assures success in the final result - and the smaller the canopy, the less 'optional' that trait becomes.


==========

One additional shape that would naturally suggest itself (as to efficiency and to lending itself to something other than 'bias' construction (with its difficulties)) would be an elliptical shape - as it mimics the (ideal) inflated shape rather precisely (and, thus, equalizes the stresses along the radials). It was this thinking that informed my early canopy design efforts (that ultimately resulted in the geometry of the solid canopy photos in the initial post in this thread). I did not mathematically generate a 3d ellipsoid shape but, rather, generated a 'near-ellipsoid' from 3 circular arcs that resulted in the minor axis being approximately 70% of the major axis. I developed a computer program that would generate the coordinates and plot a gore pattern (on a dot matrix printer (does anyone remember these?)). I eventually started referring to this as a 'Tri-Spherical' (tm) design (borrowing from the 'Tri-Conical' canopy design in the literature). It was based on the constructed diameter - D(c) - something I might not do today, with my focus on nominal diameter - D(o) - but the difference in this design is only about 4-5% (Do = 1.05 Dc). It has shown to be a robust design - handling a few (thankfully, only a few) 'harsh' deployments without any damage to the canopy (particularly in the mid-gore region where the conical (constructed with 'block' construction) would have problems). Even so, I credit any 'robustness' more to the actual construction techniques (reinforced skirts, continuous line construction) than to any particular benefits of the actual canopy geometry. The logic behind trying to mimic the ellipsoid shape is to come as close as possible to the actual inflated geometry as the *constructed* shape - and, by definition, that would mean any excessive stresses (or concentration of stresses) would (or should) be minimized (for the structural engineers out there - think 'strain energy').

The basic geometry (in cross section or 'profile' view) involves a large radius central (apex or crown) region of 1.25x the major axis radius (call it R(c) - in keeping with D(c)), transitioning to a 1.0 Rc radius in the mid-gore region and finishing up with a 0.5 Rc radius at the skirt. So, for a 24" canopy (Dc), that would mean a 15" crown radius to a 12" mid-gore radius to a 6" skirt radius. As can be seen from the photos (1st post), there's very little slack fabric in the inflated shape and it mimics the classic ellipsoid very well. I had an email questioning the length of the gore (h(g)) and realized even before posting last night that I needed to get some kind of sketch out here to clarify the profile. Here's a sketch I did back in 1996 showing the profile and a comparison to a 'true ellipsoid' - showing it matches in excess of 99.4% of the ellipsoid:



The number shown is the multiple of R(c) for a gore segment height (i.e. h(g) -- or radial seam length) -so, for the 24" canopy, h(g) would equal 1.35 x R(c) or 1.35 x 12 = 16.2" (the computer output lists 16.232").

This geometry is also the foundation for the ringsail canopy I (eventually) built (and what we'll use down the road in this thread when we derive our own ringsail design).

NOTE: I added the (tm) notation on the 'Tri-Spherical' name - mainly to protect any future use that may would open up - altho I don't, at this hour, see that happening. I did a quick search at the Patent and Trademark Office and it seems the term is original -- and the work here *was* original on my part, so this just protects that future possibility.

That's about it for now.

-- john.

edits:
9-20-14 - 3am: minor edits to D(c) and the 'tri-spherical' text
9-20-14 - 5pm: added Tri-Spherical (tm) sketch and (tm) notice
9-21-14 - 1pm: added terminology on Length values (Le, Le/Do, etc)
9-22-14 - 2pm: additional commentary on treated fabrics and the robustness of the Tri-Spherical shape

Folks,

I realize this thread has been rather quiet for... (?? over 2 months now???!? wow) - but I wanted to get a 'status update' out here for folks not to give up hope. Work, Thanksgiving travels (and now) my 90 year old mother falling in the kitchen last week - breaking 3 ribs - has kind of interfered with my plans on this article. Altho household duties and 'patient recovery' has now been elevated to top priority, I feel as we get close to Christmas, there should be some time to devote to continuing the article series. Just bear with me.

My guess is we're about half way through the series on this -- I've been doing a little experimenting with alternative (i.e. simpler) construction techniques and have some new bits of knowledge to pass on, so it should be rewarding.

Again, just bear with me and we'll get going here again before the New Year.

Merry Christmas!

-- john.

Glad to see the thread isn't forgotten, but take care of Mom, she's the only one you have!

Looking forward to some updates, whenever....

Merry Christmas and Happy New Year!

this is great stuff! I just recently made a small parachute using the vatsaas plans and now I want to make more! thanks for the ebay links on nylon thread!

jcato said:

This is my 'Bulldog' canopy (in honor of my home state) - I didn't have to grit my teeth too hard in building this - mainly because it was THREE feet in diameter (compared to 4 feet for the 'War Eagle') I'm no fool. <g> Again, 24 gores - same general color patterning -- the silver represents the 'silver britches' of the Bulldogs. This, also, is no longer in my possession - but there was some 'trade' deal with Jim Balliro, a vascular surgeon from Tallahassee who used to fly with us here (I can't remember anymore what I got in trade - if it's less than a fine piece of real estate, Balliro got the better end of the deal <g>):

View attachment 182581


-- john.

Click to expand...

wow i'm in awe john, ever since i started to get into rocketry I've believed that the majority of people don't place enough emphasis on their 'chutes and ever since my first LPR flights I've wanted to do this better. I'm getting ready to go for my L1 cert in early March and the kit that i'm building, a madcow torrent, came with a 36", 8 line flat 'chute and I want to make something better. I would really like to make a ringsail, however I think that a ringsail might be a bit complicated for my first 'chute and I doubt that i could have it done in time for my cert filght, so I think that something like this might be more realistic. so if i you could help with some construction information that would be much appreciated. what type of fabric did you use to make these chutes? was the bulldog constructed on the bias or in block? was this a spherical, ogival, conical, bi-conical or tri-conical construction? I was trying to work out what the width of the bottom of the gore should be of a 15* conical and did it by Cd x 0.519 = hg and then using the formula to calculate a Chord, es = 2 x hg x sin ((360/24)/2) given that hg would be the radius and 24 gores in 360 would provide the angle. Is this sound calculation? I tried to use the formulas on page 24 of the Ringsail construction guide however I was finding that some of the formulas break down in small dia. chutes and the resulting circumferences would be smaller then the circumferences of Cd. when calculated using those formulas. have you ever found this to be the case. also how did you construct the seam joints and how do you attach the suspension lines. thanks for a great thread so far, i'm subscribed

Ted Cochran said:

View attachment 183564

This is the single slot. Might be a bit of a misnomer; all I really did was slice the gores and then re-hem to make up the gap. This chute is extremely stable in descent and opens very softly. I did a few other solid chutes and one parasail kite, then turned to other areas of rocketry. But I've always wanted to get back into chutes; maybe John will be the spark!

Click to expand...

love this disk gap band 'chute, would really like to make one either 24 or 32 gore. did you cut the whole way though the gore to create the gap and then use tape up the gore seams to join it back together or use the suspension lines to hold it together?

thanks for all the help.

Marty

I'm getting ready to go for my L1 cert in early March and the kit that i'm building, a madcow torrent, came with a 36", 8 line flat 'chute and I want to make something better. I would really like to make a ringsail, however I think that a ringsail might be a bit complicated for my first 'chute and I doubt that i could have it done in time for my cert filght, so I think that something like this might be more realistic. so if i you could help with some construction information that would be much appreciated. what type of fabric did you use to make these chutes?

Click to expand...

I made a custom chute for my L1, the first chute I've built. It was an elliptical shape with 8 gores, I think 30" diameter as constructed. As I recall, I made the ellipse about half as high as it was wide, then projected gores on to it in Rhino, added seam allowances, and cut out from template plotted from CAD (it's nice to work in an engineering office!). I left a small hole in the top to keep from having to join all those pieces together. The fabric was run-of-the-mill ripstop from our local outdoor fabrics store. I didn't have much choice; I think I got the one that was a bit lighter and softer feel. I like colors not found in nature for finding my chute, so it's a nice bright orange.

Anyone with basic sewing skills could make something similar with a little patience. If you want, I think I have the template still hanging around. I could post up a PDF for you to print at Kinko's.

Martymonsta said:

I would really like to make a ringsail, however I think that a ringsail might be a bit complicated for my first 'chute

Click to expand...

While I never want to be one to discourage an idealistic endeavor (how else would I have built my ringsail?), I might agree with you. You might could do it as a first canopy, but adding a 'deadline' (i.e. March) complicates your task enormously - which increases the chance it won't come out in your favor. Consider this: you will learn "precision" in your sewing skills in those first canopies - and it would seem vastly more logical to wait until those skills are developed. Building a ringsail is a serious commitment and I (personally) wouldn't want to risk all that effort on a canopy that may come out a little 'wopsided' (or, otherwise, less than a close approximation to that 'ideal' shape - and, thus, causing the descent characteristics to be a mess). Here you have this 'nice' (?) canopy that took an inordinate amount of time to build and it isn't worth flying because it 'flies' so horribly. Work to build precision in your canopy construction skills and THEN build a ringsail. You'll appreciate it a whole lot more that way.

Martymonsta said:

and I doubt that i could have it done in time for my cert filght,

Click to expand...

I wouldn't feel so bad - I doubt *I* could get it done by March and I've built one. As I noted above, building a ringsail to meet a 'deadline' is a prescription for disappointment. Learn some basic skills, and then set out to build that ringsail *with no particular flight (nor *date* for that flight) in mind*. Once you get it built, THEN worry about where (and when) you're going to use it. It will take longer than you think, I am rather confident on that.

Martymonsta said:

so I think that something like this might be more realistic.

Click to expand...

I'm assuming by "this", you mean something like the 'Bulldog' canopy - and, on that, I would agree.

Martymonsta said:

what type of fabric did you use to make these chutes?

Click to expand...

All my canopies are constructed with the F-111 -- 1.1oz/yd^2 ripstop nylon -- pretty standard. Several hobby canopies use the heavier 1.7 oz fabric -- and many kite shops use the lighter 3/4 oz fabric. All would work, but I prefer the lighter fabrics over the heavier ones and 1.1 oz is about as heavy as I would go.

Martymonsta said:

was the bulldog constructed on the bias or in block?

Click to expand...

With the exception of a few gores on one of my first canopies, all my canopies use 'block' construction. You will find (if you try it) that 'bias' construction is a nightmare on hobby size (i.e. under 10 ft) canopies -- it's just too hard to keep that gore 'in shape' from cutting through final seaming - it just warps out of shape too easily. Not worth the hassle.

Martymonsta said:

was this a spherical, ogival, conical, bi-conical or tri-conical construction?

Click to expand...

None of the above - these canopies all use the 'tri-spherical' geometry (see the latter part of my post #34 above for a description of that shape - but it's basically a 'rounded tri-conical').

Martymonsta said:

I was trying to work out what the width of the bottom of the gore should be of a 15* conical and did it by Cd x 0.519 = hg and then using the formula to calculate a Chord, es = 2 x hg x sin ((360/24)/2) given that hg would be the radius and 24 gores in 360 would provide the angle. Is this sound calculation?

Click to expand...

I'm not quite following this -- not sure where 'Cd' (Coefficient of Drag) enters into the dimensional aspect of the gore (unless you meant 'Dc' - "Constructed Diameter"). The width of the gore at the skirt ('es') is simply the constructed circumference divided by the number of gores. Admittedly, finding that circumference can sometimes get involved.

'Hg' (Height of Gore) is simply the arc length of the 'design shape'. If this were to be a hemispherical canopy, that would be 1/4 of the circumference of a full circle. A 'quarter-spherical' (i.e. the top half of a hemi) would need a little math, but it still is rather simple - and, in this case you are dealing with a smooth, continuous curve, so the math is simpler.

I can see where the '0.519' term comes from (Ewing, pg 24) - but, again, that particular canopy geometry is a rather complicated beast and I would work first with a simpler geometry for awhile.

The general principle of establishing your gore coordinates is to start at the apex, come down (along the canopy profile) 1" (or some simple number), then calculate the radius (i.e. 'X' coordinate) of the profile at that point - and use that 'X' value as the radius in calculating the circumference of this small segment - divide by the number of gores and you then have the gore width 1" down from the apex (measured *along* the canopy profile). Repeat for 2", then 3" etc down the gore until you get to the skirt. A tedious (but simple) operation - which is why I wrote a little computer routine to do it (but I imagine an Excel sheet on it could be set up rather easily). See here for what I mean about the 'X' term:

https://www.mathopenref.com/coordparamcircle.html

(my 'X' term is their 'x' term)

... and some explanation of arc length is here:

https://www.coolmath.com/reference/...ml#The_formula_for_the_arc_length_of_a_circle

Here, you set 'arc length' to 1 (inch) and solve for theta (in degrees). Transform that (i.e. set 't' to '90 - theta' to fit in the circle equations (first link above)) and then solve for 'x'. Calculate circumference of a circle with radius 'x' and divide by the number of gores. Repeat for an arc length of 2 (inches), etc. all the way down the gore [edit: 'canopy profile] - each time resulting in the gore width at those points *down the canopy profile*. I have, by convention, always started with a vertical line, representing the gore centerline and then plotted the gore 'half-width' on either side of that centerline.

Martymonsta said:

I tried to use the formulas on page 24 of the Ringsail construction guide however I was finding that some of the formulas break down in small dia. chutes and the resulting circumferences would be smaller then the circumferences of Cd. when calculated using those formulas. have you ever found this to be the case.

Click to expand...

I'm assuming you're talking about the Ewing text. You will find that dealing with the profile on pg 24 to be somewhat complicated (well, I did when I first set out on this project and elected to go back to my 'tri-spherical' profile and I haven't re-visited that profile since then (i.e. 20 years ago)). The Apollo ringsails were initially designed as 72 gore canopies, but some infolding upon deployment was solved with a rather simple removal of 4 gores - resulting in the 68 gore shape finally built and flown on the missions -- and this removal of those 4 gores is part of the reason the profile is a 'truncated ogive'. I approximated the same 'profile modification' by starting with my standard 32 gore profile, but only building the canopy with 30 gores - which is why it has 30 and giving it a slight 'truncated ogive' shape as well.

Martymonsta said:

also how did you construct the seam joints and how do you attach the suspension lines.

Click to expand...

I'll get into some of this in future posts here - but the seams are simply French fell seams and I connected all the 'rings' (of the ringsail) together with a 1/2" radial tape. Understand that I didn't/don't "attach" the suspension lines (in the conventional sense), as all these canopies are built with 'continuous line construction' - one, unbroken line from link, up and over the canopy, down to opposite link (for a pair of lines). There is no 'attaching' to the canopy per se - the lines are simply run inside the fell seams and zig-zag stitched (or 'tacked') to the canopy at the skirt and then at the apex on each radial seam to keep the canopy from slipping one way or the other.

Martymonsta said:

love this disk gap band 'chute, would really like to make one either 24 or 32 gore. did you cut the whole way though the gore to create the gap and then use tape up the gore seams to join it back together or use the suspension lines to hold it together?

Click to expand...

Understand that Ted Cochran is the builder of that canopy and he [edit: would] need to elaborate on how he did it. It could be done both ways (I used some of that 1/2" radial seam tape on that gap at Ring 4 (which was attempting to mimic the gap in Ring 5 on the Apollo canopies) - well the radial seam tape is one continuous piece from skirt to apex - I just introduced a little 'space' at Ring 4).


-- john.

Martymonsta said:

love this disk gap band 'chute, would really like to make one either 24 or 32 gore. did you cut the whole way though the gore to create the gap and then use tape up the gore seams to join it back together or use the suspension lines to hold it together?

Marty

Click to expand...

Thank you for the kind words. I did this very simply: I built a standard 12-gore canopy using a pattern of John's, cut it in half, hemmed the bottom of the top and top of the bottom, and then ran suspension lines through the tunnels created by the seams between the gores, bar tacking them at the top and bottom of the band and the disk. The only trick is to account for the fabric rolled into the hems so you can space the disk exactly that far from the band.

The neatest flying characteristic of this chute is its stability-- it never rocks.

BTW, I agree with John on the ring sail. When I got into it about 20 years ago, I made I think four parachutes. The first one took 40 hours or more (it was a 36", I think). I eventually got it down to 10-15. A ring sail would be weeks and weeks worth of effort--there are a lot of little pieces to tie together, and it's harder the smaller the chute. I've decided that well-made parachutes are worth every cent, and have bought mine since then. It's a fun thing to learn to do, and I enjoy it, but I, for now, enjoy building rockets more.

jcato said:

Understand that Ted Cochran is the builder of that canopy and he needs to elaborate on how he did it.

Click to expand...

That needs to be stated better -- it almost sounds like I'm placing a responsibility on Ted to elaborate - which was not my intention (although he graciously did -- thanks, Ted). I've edited the original, but felt I (myself) "need to elaborate" <g> - my (initial) thinking was it wasn't my place to speak for him and I'd defer to whatever he would add. Sorry, Ted - hope I got it right this time.

-- john.

Folks,

I suppose we could entertain a little 'diversion' on the main subject of this thread - but it's only a little (as it could be used in some senses for a ringsail design).

I've put together a little Excel spreadsheet that will calculate the gore coordinates for a (single)RADIUSED canopy -- i.e. a hemi-spherical or quarter-spherical design. The hemispherical design is obsolete (too much fabric for the drag created), but the quarter-spherical is a rather decent design that is 'current'.

Understand, this is for a SINGLE radius canopy (i.e. the canopy profile is generated via a SINGLE, non-varying radius). This is considerably simpler than an elliptical or my 'tri-spherical' design - which is why I could put this together this evening --- but it will provide a decent and robust design if you wanted to do a quarter-spherical - which isn't all that bad.

There's only two inputs: the RADIUS used to develop the profile and the NUMBER OF GORES in the canopy - shown up top in the lavender boxes. Units are what are called 'consistent' units - meaning if you enter 'inches', the outputs will be in 'inches' (or square inches for area) - if you enter feet (or meters), the outputs will be in feet (or meters), etc.

Other terms:

L - the distance down the gore (measured from the apex of the canopy) - this is in whole units, which makes pattern layout simpler.

e(l) - the width of the gore profile at 'L'

e(l)/2 - the HALF-width of the gore profile at 'L'
(This is a better way of trying to lay out the pattern -- draw a vertical line down your pattern paper - mark off the apex point and then measure down 'L' units - and then offset 'e(l)/2' units on either side of the centerline.) If you want to make sure your pattern is absolutely symmetrical (not altogether unimportant), fold your pattern paper in half vertically and use that 'fold' as your centerline - mark the half-width on the outside of the fold and cut the pattern out folded - getting both sides in one cut - thus assuring symmetry.

Go down the rows until you get to the row that corresponds to the 'H(g)' value (Height of Gore - up at the top) - or nearest 'L' value to it (go one beyond). If you need more values for 'L' than is shown (30), just highlight the last row (L=30 in this case) from columns B thru L and click and drag the little black box in the lower right of the highlight and you can expand the rows as many as you need - with the formulas updated as you go.

The default radius for the canopy in this sheet is 24 - as is the number of gores, but, again, these can be changed at will.

The other terms are defined at the top, but aren't that critically important (with the possible exception of 'S', the Surface Area of the canopy (segment, as appropriate)). If you want an explanation, chime in and we'll discuss as needed.

-- john.

View attachment radiused_gore_calc.xlsx

John and Ted thank you, your answers really clarified some things for me. I've ordered some 45gsm (i think 1.3oz/y^2) ripstop and i'm sure with the help of my wife, who is an avid quilter, i'll be able to make some 'chutes. I really look forward to the rest of this thread on making a ringsail and hopefully when I don't have a deadline i'll make one myself.