|Skyshark P-Series vs PT-series|
Why are lower spreaders almost always tapered (PT) rods? Every plot of weight to stiffness I've ever seen shows that the PT rods have only a very slight advantage over the comparable P-rod. I've heard people say that because the tapered rods flex more at the ends, the shape of the wing is different, but couldn't you just alter the stand-off lengths and achieve the same thing?
I'll go ahead and apologize up front. I'm a mechanical engineer, and I prefer numbers to nuances. ;)
While there are always lots of numbers when it comes to sport kite design, there is a lot more nuance. Framing can be sort of a dark art. Tons of trial and error. This is why when something generally works, it gets used a lot. In this case the tapered lower spreader works better for a lot of reasons and it's what folks tend to go with as a default when setting up a new kite.
One of the reasons for using tapered spars in any part of the frame is to move strength/stiffness/weight around to where they are most needed. In the case of the lower spreader, the center T is a very high stress area and often needs a robust rod to survive normal use. By contrast, the LE end of the spreader has much less stress and doesn't need to be so beefy. In the days of kites with three widely spaced standoffs there was often more stress on the spreader out towards the LE, but most modern designs put the standoff forces closer to the center and on a beefier part of the spreader.
The overall stiffness of the spreader effects flight a lot. We use stiffness as a catchall term, but in the case of spreaders the critical concept is often more like "response" or "springiness" or something. Wrapped, tapered spars are often more responsive than straight spars like the Skyshark Pxx. The straight 'sharks are more of a compromise for durability and sometimes feel a bit less lively. While I think the P200 is the best all around spar ever made, there are times it's just not right for certain parts of a given frame design.
That said, I've had kites which prefer a straight tube for a lower spreader. Sometimes the extra stiffness out at the LE can make a huge difference in the overall frame stiffness. It locks in the LE more, and while the center t area may not be as stiff, it's a good tradeoff.
Ah, springiness is something I hadn't considered!
Don't think engineers can't quantify that, though. :D We deal with stiffness (spring rate - k) and "springiness" (effectively, damping ratio (c) in concert with spring rate) all the time.
I bet I could create a fixture that measured the damping factor of rods. Just need a decent DAC and pressure sensor..... Hmmm, lots to think about!
Part of the reason IMHO is that some of the most influential kites 10+ years ago evolved from previous designs rather than being designed from scratch. A kite designer doesn't disregard his previous work every time he produces a new design; hold a Nirvana next to an Opium to see just how close they are and how a few subtle changes really made the kite excel at what were then cutting edge tricks. With that in mind, using a tapered spreader compared to a straight spreader in the same kite gives more flex out near the leading edge which lets you keep a steep keel angle, maintain a cleanly defined channel between the two standoffs yet still get a very shallow angle out toward the tip which is crucial to getting a good backspin.
Yes you can get a kite with a straight spreader to backspin but you'll probably have a steeper outer sail angle and you'll probably use more tail weight.
10+ years ago talking to other kite designers, many were unclear what produced a good backspin, using tapered spreaders helped and now its become the accepted norm.
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