Re: Bridles - and I have read the archives

Subject Re: Bridles - and I have read the archives
From abw@peritas.com (Andy Wardley)
Date 9 Jun 1997 11:31:39 +0100
Newsgroups rec.kites

If you want to design a dynamic bridle from scratch, I would strongly recommend starting from a good static bridle. As a guide on how to create a dynamic bridle from a static one, use the simple knot- conversion process described on the Dynamic Bridle page.

This description talks about Benson Kites, but the principle works on many other kites. Even if it doesn't work, it should put you not-too-far-away from a suitable compromise. 

David Lindgren asks:
> How do I go about re-designing the bridle on my old Highflyers Viper
> (2-line delta)?

The simple answer is "starting from a few basic principle and with a lot of trial and error". 

> 1) How does one decide how long the bridle leg going from the spine
> should be?

A long bridle puts less stress on the kite's frame but is generally less responsive. If the kite doesn't turn well no matter how you adjust the bridle, you might try a smaller bridle. If the kite is very twitchy, or you see the frame bending, try a longer bridle. Normal length would be where the bridle legs are roughly the same length as the distance between the upper and lower bridle connection points. I say "roughly" because they are generally different sizes and may vary wildly, but it gives you an idea.

Make the outer bridle line (the outhauls) as a single length of bridle line running from upper to lower leading edge connector. You can then lark's head this line onto the end of the inhaul making for easy adjustment. You might want to make your bridle out of lots of knotty pieces of bridle line. Having simple overhand knots evenly spaced down the ends of the line mean you can adjust easily and fairly accurately (i.e. shorten each side by a couple of knots). A full test-bridle is useful (have short knotted lengths at each attachment point and tie the bridle onto these) and allows quick adjustment but can also get messy and very knotty! 

> 2) How does one then decide on the length of the bridling line that
> goes between the ends of the spreaders on the leading edge forming the
> two other bridle legs?
> 
> 3) Once these two lengths have been decided, how do you then decide
> the position of the knot that connects the two bits of line and
> determines the length of the two leading edge bridle legs.

Pick a random bridle length. Assuming it's not too way out, adjust the tow point up or down the outhaul until the kite flies reasonably well. If it doesn't take off until the wind gets ballistic, the kite is generally bridled too low. If the kite takes off and flies fast (perhaps over-flying at the top of the window) but lacks pull and responsiveness, the kite is bridled too high.

For any reasonable bridle dimension, you should be able to get the kite flying forwards at a good speed, with reasonable pull. As a rough guide, pull the bridle out tight and look at the bridle face on to the kite. The tow point will usually be about half-way along the lower spread and a little bit above it.

At this point, you can then look at how it turns, tricks, stalls, or whatever you are interested in. If the kite steers wildly, oversteers coming out of turns, or is otherwise erratic, the chances are that the tow point (where the lines attach) is too far out. That is, too far away from the spine. Shorten the inhauls and then adjust the tow point up or down, as before, to get the kite flying with this new bridle. If the kite refuses to turn, trying lengthening the inhaul.

Is the kite better or worse after this adjustment? Does it turn better or worse. Does it still fly forward as well?

If yes, try making a further adjustment until it becomes "too much". It is always a good idea to start by going too far with such adjustments so that you can quickly see the over-exaggerated effects of your bridle adjustment. Soon you will learn to recognise which adjustments cause which characteristics and you can tune your bridle accordingly.

If the kite flies worse, try adjusting the other way.

From here on in, it's about a lot of trial and error. When you get a bridle setting you like, mark it or measure it, but don't be afraid to carry on playing. There are often a whole range of different bridles you can create for a kite which vary immensely. Try them all (or at least some of them) and then stick with one you like.

You can probably get the kite flying in 10 minutes. In an hour, you could really understand the bridle and appreciate how different adjustments will change it. In an afternoon, with a few different kites, you can really understand most of what there is to know about static bridles.

The the fun really begins when you go dynamic.... but that's for another day...

As I hope I have conveyed, some of it is rule-based but a lot is trial and error to get a feeling for bridles and especially for whichever one you're working on. I am at the stage now where I can look at a static or a dynamic bridle and tell you fairly accurately how the kite will fly just by looking at the shape. When I create a new dynamic bridle from scratch, I make something that just "looks right" and 9 times out of 10 it will fly first time. Perhaps not perfectly, but good enough for starters.

[ As an aside, I recently flew a prototype kite from another kite designer. Before even flying the kite, I looked at the dynamic bridle, arrogantly said "Oh no, that's all wrong", changed the bridle and flew it. Luckily, the kite flew better than before... ]

Until you gain the experience to do this, here are a few basic pointers. None are set in stone, though - a lot depends on the kite, the flier, the wind and a hundred other things. 

> 4) If the kite is hung inside by its lines, how should it then hang? -
> should the nose be pointing up or down or should the plane of the
> frame be exactly parallel to the ground.

Generally speaking, fairly flat, perhaps with the nose up slightly. BUT, it is the aerodynamic centre of pressure that determines how the kite should be bridled, not the centre of gravity which the "hanging test" identifies.

One last thing: a well-designed bridle should be fairly tolerant of small adjustments. I never adjust my bridles by less than 5mm on a full-size kite. Normally, an inch (25mm) qualifies as a biggish adjustment, half-an-inch (12mm) a small one. If an adjustment of less than that makes a big difference then the bridle is way too sensitive. Try using a longer bridle. In all reality, the tolerance for error between the two sides of a bridle is probably a few millimetres so an overly sensitive bridle is just asking for trouble.

A