What are they for really? I think I can tell you the general philosophy but the truth is that perfectly good canopies have been built both with and with out them. And although most people have felt that they are a good idea they have at times caused problems.
So it goes like this. The classic thought is that you need them to help equalize the pressure between cells. It helps to fill the cells on opening inflating the whole canopy smoother helping to avoid things like end cell closure and off heading openings. Let's say on a good opening that the center cell opens first and starts to surge forwards and basically runs over the end cells leaving them behind un inflated. The front of the end cell is basically tucked under and not catching air. This is not uncommon. In theory the cross flow during opening would help to avoid this and would help to pop open the end cells after opening. It can be harder to get into deep enough breaks to get it to pop out and open with out them.
That was the conventional thinking. The truth is that there have been canopies built with out cross ports. For instance, Charter wound up buying this odd nine cell zp canopy in his first rig. It was an odd thing, I think it came out of south Africa. Damn, I'm brain locking, can't remember the name. It was the only one I'd ever seen. He got it cheep because no one had ever heard of it and Dallas is kind of snobbish about it's PD and Icarus canopies. It was kind of saber/stilettoish. It turned out to be a really nice canopy for him. He jumped the shit out of it. No cross ports. I seem to recall that the Strong Set 400's did not have cross ports. Maybe some one can confirm that? My memory is going. There are probable others but those are two that come to mind. It seems to be some thing that was tried when people were switching from F-111 to zp. It might have been an attempt to control the openings by slowing the inflation of the canopies by reducing the filling through the cross ports. I don't know that's just a guess. But the canopies seem to open and fly fine with out them. Or at least well enough. For example the I've seen the set roll tips under and it's not really prone to popping out on it's own with out help.
Problems. I told you about what was believed to be a cross flow issue on the early FX canopies. The angle of attack and pressure is not the same across the nose of the canopy especially in a turn. So there can be cross flow through the canopy. The flow on one side is no longer stagnant. You can create a dimple and start to cave in one side of the nose on a large lip or baffle that closes off the majority of the nose. Or At least that was the thinking when he was having problems at the time. So in a since the cross ports were actually making the canopy less stable. I always wondered if you could put a flapper valve over the cross ports on a couple of the ribs and get the best of both worlds. Say if you separated the canopy into groups of three cells with air lock valves so that the air could only flow outwards.
Personally I like cross ports. I've even enlarged them on some of my canopies for base. I think they would be a good thing in your design. You're a long way from a design where you might see any kind of problem. The vast majority of canopies have at least two cross ports of some type. For example some use mutable narrow vertical slits to better support the rib. Personally I think they were just having fun with their laser cutter. Look what we can do!
And if you want to put a nose lip on there I think you could come down 25% of the nose cut with no problem. It would probable help capture the airflow escaping the top of the nose at high angles of attack like on flare.
The real question you should be looking at is how you want to sew things. For example how are you going to sew the lip at the leading edge of the rib? Have you done a test yet. It might be a bit awkward on the non load bearing ribs. Don't buy your self problems you don't have to have. It's time to focus on construction. A rounder edge, basically curving the top skin down rather then a sharp turn might be easier to sew then a sharp turn. Just a thought.
Have you talked to the sewing machine shop yet? You're at a point where construction may start to dictate design. And there's nothing wrong with that. One is not more important then the other.
And there were the Atair Cobalts which either did not have crossports in the 3rd and 6th ribs in from the ends, dividing the 9 cell in thirds. (Although that is what they advertised, some apparently had very small crossports in those ribs). They were trying to promote having the center fill first, without opening up the outer wings, thus staging the opening. Opinions were rather mixed whether they achieved that or not!
Yet cross ports were enthusiastically adopted by early squares, after beginning without them. It must have helped to fill those end cells, particularly on opening.
Some canopies (which? how many?) don't bother with crossports in the center rib.
I suppose if a canopy (a) opens well enough anyway for whatever reason, (b) doesn't have a tendency for the end cells to tuck (because of good design regarding angle of attack and twist in the wing), then crossports won't be really needed unless one gets into the rare situation of actually having a tip fold in. So my gut reaction about that odd South African ZP thing that RiggerLee mentioned, is that it would be perfectly fine normally ... but I'd still be more wary of it in turbulence.
mutable narrow vertical slits
"Multiple" is what Lee meant. Yes, that does look like playing with the laser cutter input data.
Here's where I am with the project right now. I'm saving up a bit of money so that I can buy proper materials, but one thing I don't have is anywhere to build my new canopy. Sure, I can do the sewing on my home machine in my bedroom, but I don't have anywhere to cut out the pieces. I could really do with a rigging loft of something similar.
You can use sheets of masonite as a cutting surface. It's not ideal, and will wear out pretty quickly, but sheets are so cheap it doesn't really matter. Masonite is also nice for making patterns.
Actually I think the lack of cross ports in the center rib is probable more of a structural issue. There is a lot of airflow captured by the center cell of the nose on opening. I can see it stressing the rib and stating a tear there and then between the b and c you probable have the bridle attachment with all the tapes transferring the load down the rib to the bottom seam and out to the loaded ribs.
Even just 1/8 or 1/4 inch hard board works fine. Move all the furniture out of the way and lay it out on the floor. It's hard on your back but that's what I do now that I don't have room for a proper cutting table. But make your self a little rack for your roll of fabric. Just two t shaped end peaces made from 2*6 with a notch to hold a peace of conduit for the roll to turn on. It's worth it. And even when I had a full size cutting table I often climbed all over it as I cut so I might as well have been on the floor any ways.
Here's a thought. If you could get your patterns into a real drafting program you could ask some one to cut it for you. It would also bypass the issue of finding good fabric. Call Red at flight concepts. He might do it for you. Wouldn't it be nice to just get a box in the mail with all your peace's laser cut for you?
I agree a canopy does not need to have crossports, but they make the opening easier.
I have a BT Pro and as long as the end cell closure is even, it's not a big issue. But if I only get end cell closure on one side the canopy dives very steeply.
But isn't the crossport patented or something? I have a slight memory of I heard old canopies was made without crossports but with markings where to cut because of 'some reason'. Maybe I remember wrong or heard false stories.
Give it a try. It's the section between the B and C that you'll have to sew all the tape onto to carry the load of the bridle. The front cross port is the only other question. The center cell will be wide open catching a lot of high speed air. You'll just have to see how much load you get on that fabric. Worst case you'll get a tear starting at the cross port.
I only put attachment reinforcement load tapes on the center cell, and center top and bottom skins in an I shape. I just leave out the center cross port for that section. You shouldn't need to do this for the center 3 ribs, only the very center non load bearing rib.
And for what its worth, I build all of my harnesses on my base rigs with tightly zig zagged E thread out of type 8 webbing.
Riggerlee had even mentioned something about the flexon originally being TSO'd with bar tacks instead of 5 chord. IMO zig zagged E thread is fine for harness work in lighter duty webbings like type 8, or type 7 in the case of my speedflying harnesses. If you start working with type 7, or more than 4 layers, it stops being viable. It also takes a little longer to sew than a walking foot with 5 chord likely would.
Flexon TSO'd with zigzag? News to me. I tend to think that it could be done if it was designed properly so that ALL loads were in shear but with most typical harness designs at least some of the loads like those on the reserve risers tend to peal the upper junction. Climbing harnesses are mostly done with zigzag but they are also prone to abrasion of that stitching and are semi disposable with limited life spans.
@ineed2fly I thought there might be some force at the end of the reinforcing tape for the bridle on the topskin. I changed the v tapes on the loaded ribs to meet there so the force can go down the lines. Is this completely unnecessary?
@RiggerLee What do we think of this non loaded bottom seam?
I'd try to center the tape a little more to get a better bight with that bartac. I'm not sure you need to fold it back under like that. It's esthetically attractive but I don't think it's structurally necessary. Some manufactures have stopped doing it. If you think about it the only way to sew a seam like that will be with a down turn folder actually on the foot it self. That can probable be done but it would have to be custom. So why buy your self that pain in the ass.
It's not uncommon to have the v tapes meet that top skin tape some times they even reinforce the top skin panels on ether side. They do that on some base canopies where use large pilot chutes with out bags. But the top skin is actually a bit looser then the bottom most of the load should go down the rib to the bottom skin then out to the load bearing seam.
You're sewing's looking pretty good. I can't see exactly how you're doing your bottom seam. I see you're running you're stabilizer all the way to the tail. I'm honestly not sure how much you gain by that. I'm not sure how much you gain in flight and when you flare the tail will bend down quite a bit loosening it. Some people bring the stabilizer up a little sharper so the edge meets the bottom seam a little farther forwards on the cord. That way when the tail pulls down the back edge tape on the stabilizer is still relatively taunt. On the other hand the larger stabilizer kind of blooms out wards giving you a bit more surface. I guess the only way to look at it would be in a wind tunnel or maybe by kiting. I wonder if it would be noticeable in how the lift and drag change at those higher angles of attack across the control stroke. I wonder if it would even be noticeable. But this is just me rambling. It's time for you to ignore all of us and just go build a canopy.
Sewing machine. It's time for a sewing machine. Start hunting for a 112-w-116 singer or some equivalent. Worst case a 212-w-140 Singer It's actually a fine machine lighter then the 112 but it doesn't have a puller. neither of these have a reverse. If you really want something nice get like a Juki with a reverse. It's an oiler but you're going to run the hell out of it so that's a good thing.
Yeah! Whatever Riggerlee said, plus you might want to extend the stabilizers all the way to the trailing edge. Longer stabilizers will help (structurally) stabilize the rear corners (ala. Triathlon and Diablo).
I see you're running you're stabilizer all the way to the tail. I'm honestly not sure how much you gain by that. I'm not sure how much you gain in flight and when you flare the tail will bend down quite a bit loosening it. Some people bring the stabilizer up a little sharper so the edge meets the bottom seam a little farther forwards on the cord. That way when the tail pulls down the back edge tape on the stabilizer is still relatively taunt. On the other hand the larger stabilizer kind of blooms out wards giving you a bit more surface. I guess the only way to look at it would be in a wind tunnel or maybe by kiting. I wonder if it would be noticeable in how the lift and drag change at those higher angles of attack across the control stroke. I wonder if it would even be noticeable.
Hmm. And yes I am looking for a new machine. Well actually, I looking to use someone's loft, that way I have all their equipment and expertise on hand.
Circa 1990, Sandy Reid certified the Flexon harness sewn mostly with bar-tacks. Most of those bar-tacks were 3 inches long! Only the Flexon shoulder joints (directly below the 3-Rings) were sewn with 5-cord. Flexon was the first American harness with hip and chest rings and it survived all the heavy-weight and high-speed drop-tests required by FAA TSO C-23C.
The only problem was that bar-tack machines were not durable enough for sewing harnesses year-in-and-year-out. .. which is why Rigging Innovations had switched to sewing most of their harnesses with 5-cord by the time I started working there in 1994.
Also, as far as folding the bottom of the seam like that, I do the same thing on my canopies, but only for the bottom non load bearing seam. Only reasoning being, that I have a 1cm seam allowance on the top of the ribs regardless of it being a load bearing or non load bearing seam. But the bottom load bearing seam requires a 2cm seam allowance, the bottom non load bearing seam requires a 1cm seam allowance. So I have found it easier to just fold the non load bearing bottoms over to make up for the extra length, as opposed to making a completely separate pattern for the non load bearing rib that is just 1cm shorter on the bottom.
This guys at the point where he needs to make those kinds of seam decisions. He's getting ready to buy his first sewing machines soon and he needs to have his construction sorted out so that he can start looking at folders for the various seams. I've given him my two cents, for what little it's worth. How do you build you're seams? What kind of folders do you use? Where did you get them? Where any of them off the shelf or are they all custom jobs. Could he perhaps reference your orders to help get them set up right. My experience is that custom folders are expensive and some times take more then one try to get them right. He's kind of hitting a wall. He's not a sewing machine guy and it's intimidating for him. He could really use a bit of help here to get him over this next hump. A double needle is a pretty big commitment for him. He needs to get it set up right so that it will really work for him.
I'd give the guy a hand but he's on the wrong side of the ocean. I don't suppose there is any one local that could hold his hand through this? Come on you remember what it was like trying to buy your first industrial machine.