96 replies to this topic

[kairusan]

I'm curious as to what parts of a 1.5's frame — the verticals or the leading edge — receive the greatest amounts of stress from high winds. I've done some searches on the board and have found some conflicting opinions; most people seem to say that the leading edge takes most of the stress and the verticals absorb very little, but there are a few contrary opinions (including one from the Great Barresi himself, if I am not mistaken). Perhaps a bit of informal internet democracy (with commentary) is in order, so that this eternally vexing question might at last be resolved once and for all? Perhaps not? You decide.

[Kitelife]

Vertical spars, without question. These are the most unsupported spars in the kite.

Look at where the bridle attaches to the kite, and how the frame is held accordingly.

On the leading edge, there are multiple bridle points between the ends of the three spars.

On the vertical spars, only the ends are at bridle attachment points.

==

Impacts (crashes) are most likely to break a LE spar, whereas high wind pressure and gusts will blow out one of the verticals.

[melnsct]

directly from The Book Of Quad

[antman]

cant say i have broken a vertical but LErods.. i have done plenty

[DB Cooper]

For an initial pass I would have to say the middle rod on the leading edge would have the greatest stress. The vertical spars may have the least amount of support points, but the central leading edge spar has the most moment acting upon it. This would lead me to think it would have the greatest stress overall all; however, the outer rods on the leading edge ,where they meet the ferrules, would tend have the highest localized stress points. This is all barring impacts and just thinking about wind and such.

(Realize I have not flown much and have never broken a spar in flight, if JB says he breaks vertical spars in flight more regularly than leading edges and they are all the same wraps, etc then I would take his word. The old saying comes to mind "In theory, practice and theory are the same. In practice they are not")

[steveb]

Steve De Rooy tried out some old Sky Shark spars for verticals a few years ago and had one snap in a medium strength gust.
Each leading edge spar is supported at both ends and its centre; the verticals are only supported at the ends.

[quaa714]

While I've never broken a down spar, I have broken a couple of LE spars so my first reaction would've been LE being the more prevalent spar to sustain damage.
As it turns out, some of us briefly had this conversation with JB in TI this past weekend. He said then as he asserts above that the down spars are less supported then the LEs via multiple bridle points and are more prone to breaking/blowing out of their end caps and snapping and/or splitting the ends.

I suppose though, if you are using Quad sticks, the center attachment point to the sticks bridle would add a touch more support and possibly prevent the spar from potential damage.

[RevWizard]

When I look at the above photo of my second vented REV I flown by me in 1995 it really puts me to wondering.
The rods were 4-wrap(Revolution Equipped), length 91cm.

[FLY TILL YOU DIE]

Have only broken 3 rods ; 1 pro rod in my early days , my fault; and 2 race rods in winds I thought were safe ; all were uprights. They sound like a gunshot when they go!

JOHN

[FLY TILL YOU DIE]

When I look at the above photo of my second vented REV I flown by me in 1995 it really puts me to wondering.
The rods were 4-wrap(Revolution Equipped), length 91cm.

John, looks like you're about to do a bowl all by yourself!

JOHN

[Kitelife]

I've flown in 60+ mph multiple times, my findings have been proven time and time again.

While the center spar may bend like crazy, it's not the first to break under pure wind pressure.

Again, impacts change the equation altogether.

[Sailor99]

Interesting question, not least because I think it has two totally contradictory but equally correct answers! This would be why the two Johns appear to be equally vehement in their support of both the LE and the Verticals as being the most stressed. The problem lies in the question - what stress are we talking about.

Start off with a highly simplified situation which I shall make more complex: On the one hand the sail, which generated all the forces on the kite in the first place, is attached to the complete leading edge by the sleeve. . Similarly the bridle, which creates an equal and opposite force on the frame (it must do or the kite would fly away!) is attached at 5 different points on the leading edge. Thus the stresses placed on the leading edge are spread pretty evenly along its whole length by the sleeve and the 5 attachment points. However the sail is only attached at the ends of the verticals, as is the bridle. So the force is concentrated at the two ends, rather than being spread out along its length is focused at the ends thus setting up a beam (I mean beam very specifically here as a 'seesaw on a fulcrum' or two fulcrums in this case). However this is false - MOST OF THE TIME. As we said the bridle and the sail tensions totally cancel each other out otherwise the kite would fly away. As such there is no force what so ever on the frame as a result of the pull of the wind - MOST OF THE TIME.

So how come we need a frame if there are no forces on it what so ever!!!!

The clue is in that phrase MOST OF THE TIME. The bridle attachment points are not in line with the ends of the vertical spars, they are slightly inside. So they actually put the frame into compression. Similarly the sail is only attached to the ends of the verticals and the sail's center of effort is actually somewhere about a 1/3 of the way down the sail from the LE (as somewhat towards the center of the kite from the verticals as it happens). The both the sail and the bridle put the verticals into compression. On the other hand the bridle is attached at 5 different points on the LE and the sail attached right along the length of the LE. So the sail adds no compression to the LE and because of the multiple attachments the bridle adds very little. (To get the idea of this thing of a soft foil. With multiple attachment points it still has a tendency to fold itself up into a ball when flying, but not nearly as much as if it only had 2 attachments, and were it to only have 1 attachment then it would simply fold up into a streamer behind that one point!). So the verticals are under a lot more compressive tension than the LE. This is fine so long as the rods remain in column (ie straight). Compressive tension held in column is like Mohammad meeting the mountain - neither will give way. However if the rods ever got out of column then compressive force is deadly. The rods would snap with explosive force. Try this at home with a wooden dowel. Press with as much force as you like on the two ends and it will not break. You could even stand a car up on it (assuming you could balance the car!). But push just a little on the middle of the dowel when it is under the compressive load and it will snap like a twig. Yacht masts rely on the same thing. They are just a tube of aluminium, or more commonly carbon these days. But so long as all the wires (stays and shrouds) keep it in column it is immensely strong. As soon as it gets out of column, bang!

So the conclusion has to be John B is correct because the verticals are under much more compression than the LE and therefore will break explosively more easily.........or is he?

The other thing about that phrase MOST OF THE TIME is that the sail is not stationery. It moves backwards, forwards and sideways. The interesting one is when it moves forwards. This increases the airflow over the sail and therefore increases the forces working on it. As the section of sail between the verticals has much larger area than the outer sections (the verticals effectively break the 2 sails up into 4 each of which has air flows running in different directions and patterns) it loads up more and the center of effort of the sails moves much more towards the center of the kite. OK I really need to do a diagramme here, which doesn't work on forums. But if you take the two outer sails, the center of effort (CE) is about a 1/3 of the way back from the LE and half way between the verticals and the top corner. On the center two sails it is in a similar place. The average of each of these pairs of CEs is roughly placed on the 2 verticals (which are still in column remember) about 1/3 down from the LE. Load the sail up by flying forwards (or by a gust) and the CEs from the center sections of the sail increase in force and the average CEs move in towards the center. In this case you are going to set up two new forces on the kite:
1) The verticals now have a side load on them - the displaced center of effort of the sail is now towards the center of the kite and is applying a force to the verticals towards the center of the kite. This is when you are going to have JB's earth moving bang!
2) The displaced CE of the sail also puts a strong force on the bottom of the verticals pulling them towards the center of the kite. This is transferred to the outside trailing edges (leaches) which in turn takes the force to the tips of the LE. And the LE Bows (see John M's picture). If the LE were under significant compression it too would break at this point. But luckily carbon is resilient stuff and if there were no faults in it what so ever it would take a huge load of this sort to break it. Unfortunately there are always faults in a composite (that is a composite of carbon and resin in this case) lay up - it is not a fault as in a manufacturing error, more that in nature things can never be perfect). So if the forces become large enough then it will break, and again it will be explosive. Obviously if there is a significant fault in the lay up of the rod then the level of force needed to break it will come sooner - which has happened to me once. The other very significant fault you can introduce is an incorrectly inserted ferrule.

The final MOST OF THE TIME Moment is if you smack the kite into a lamp post in which case all the above goes out the window. - but I don't think that was what the OP was asking about.

So, the contradiction. JB is quite right in the conclusion he draws from his empirical experience. The verticals are under much more compressive stress. And because compressive load is totally destructive if the rods get out of column, and because something as simple as flying the kite forwards can get the rods out of column, they are the rods most likely to break IMHO. But John M is also right in that the LE is under the most stress from the moment of inertia (That is the bending force talked about above - strictly the cylindrical area moment of inertia, but that is splitting hairs!). By design the rods are very able to cope with this stress so long as there are no faults such as in improperly assembled LE to contend with.

And the answer to the OPs question is both the LE and the verticals are under more stress than the other - it is just different stresses in each case. So I voted for both!

(If you are interested in the math and theory behind these sorts of forces and stresses have a look for anything to do with rigging design. This wiki artical is also quite good but somewhat impenetrable:

http://en.wikipedia...._moment_of_area

And now you know why I like building boats and rigs!)

[quaa714]

Wow!!!!!!!!!
Jeremy, that was some response. Perhaps a bit technical but well said.
On the other hand, after looking at the picture posted by John, it appears clear from looking at it, that the more bend the LE takes on, the more the down spars would seem to bow out (compress) out from the sail, end cap to end cap, potentially snapping from too much force.
May not be the best description, however in my mind I can see clearly the negative effect to much bend on the LE would have on the ends of the down spars thus creating the explosive sound of the rod snapping.

[Choccy]

"Which parts hurt the most ?"

In high winds: Me
(all my spars down the left hand side (Marvin) anyway (Ref: HHGTTG))

Being 'funsized' I don't have the option to observe/try this even in the most modest winds!

I'm not even gonna recall the rusting knowledge from my Physics Degree days.
So sorry I can't answer your question.

Although I have never broken any rods yet, I did lose a vertical spar on a fast ascent once!

PS you're a bit young to be cantankerous ?

[andelscott]

Interesting question, not least because I think it has two totally contradictory but equally correct answers! This would be why the two Johns appear to be equally vehement in their support of both the LE and the Verticals as being the most stressed. The problem lies in the question - what stress are we talking about.

Great reply Jeremy - thank you that has helped me understand a few of the issues here.
But I may need to read it a couple more times to clarify the CE and sail design elements (and any impact on how I fly).

So the key word appears to be compression. I know that carbon fibre is great for bending - any number of kites use rods in a bent form - presumably with very mild compressive forces (just holding in position) compared to the larger lateral forces creating the curvature.

So are we saying that the compressive forces on the LE initiated by the endcaps and bungee, amplified as the sail deforms, are lower *per unit of length* than the equivalent (and not quite centred) forces on the vertical (per unit of length)? Or is length irrelevant, given that the rods are same construction (with the exception of the ferrules)?

[david ellison]

Yes thanks Jeremy - a great explanation from someone who clearly knows what they're talking about. You must have fun "playing" at work every day.

In my experience l.e.'s break when whacked by immovable objects or you've hung on too long with too few wraps as the wind increased. Vertical spars break when you're caught offguard by sudden increases - saw 2 go in quick succession recently when the wind gusted. I guess it's at that point that the forces shift abruptly.

[Sailor99]

Only some days David - other days (like today) I have to be an accountant . Of course when I was a boy I wanted to be a lion tamer.

[MrDenny]

I have noticed that Rev sells most of their kites with SLEs or "fatties" as I like to call them. I haven't seen a Rev with "fatties" used for verticals yet.

[Jeff]

I have noticed that Rev sells most of their kites with SLEs or "fatties" as I like to call them. I haven't seen a Rev with "fatties" used for verticals yet.

Probably because 3 spars end to end will bend more under less force than the single vertical spars. I doubt it's for strength against breaking as much as rigidity against deforming.

I've only seen spars blow out twice. Once on my kite, and once on someone else's. In both cases, they were verticals.

[RevWizard]

I am not saying one way or the other with the photos I presented. I was just presenting some of the factors for the game.

Now think of the point where vertical rods cross over the leading edge. This creates another factor.
Then you have to think what axis in reference to the wind effect the rods. The LE is effected heavily in the X-Y axis and less in the Z axis. The verticals are effected primarily in the Z axis and very little in the x-y axis.

It would seem to me that verticals might be effected more however it does not explain why Revolution came out with the SLE rod and some use two sets of rods in the LE where the verticals remain unchanged. Nor does it explain why I never broke a vertical rod.

Out of my experience since 1990. I have broken a couple LE's exploding and once believed previously damaged, where the verticals being the same type rod have survived.
In detail, I never broke a 4 wrap rod. I had a 6-wrap split. For the 3-wrap I lost a REV I rod apparently due to damage as it snapped in normal wind. For 2-wrap I have popped a REV I and REV 1.5 due to excess stress. I had a Rev I that was stepped on and crushed and two REV 1.5 with end splits due to inner ferrule slipping.

I think you would need a aerodynamic engineer, a stress engineer and a physicist working together in a wind tunnel to get the real answer.