quiz for armchair kite designers

[tegirinenashi]

Question #1:

How about the "ultimate pulley" above the depower strap

The advantage is that center lines are always balanced. Can you think of a single disadvantage that would invalidate this design? Bonus point: design a single line safety system for it (therefore, the answer does NOT concern the safety system).

Question #2.

Delta kites feature briddle connected like this

which we'll call "oak tree branching", whereas hybrid/briddled C kites are usually designed like this:

which we call "cypress tree branching". What is advantage of oak compared to cypress, and why do you think it is more favorable for delta kites?

[tegirinenashi]

Answers:

1. Kite would be unstable to the point that it would have to be actively steered with significant amount of bar pressure. For example, when kite is in neutral at 12 o'clock with bar released would be not staying that way, starting slowly to roll, and move to the side, and will accelerate doing so. (However, this is just my intuition, as I didn't test it Please don't hesitate to disagree.)

2. The load on the bridle to LE connections is more evenly distributed in the oak tree branching. In the first approximation, we ignore the angles, so that the force on all 4 LE connections is roughly equal. Compare it to the cypress design, where roughly 1/2 of the tension from each center line would be on a wing tip, and only 1/8 would be at the center. As a result, the cypress tree designed bridle is closer to old-fashioned 4-line C kites where the center line tension is fully applied to the wing tip. Therefore, the cypress bridle design results in kite being more stable, but less depowerable.

[tautologies]

I do not think the pulley on the front lines will do much. I've used pulley bars (no depower ones) in the past and they had a pulley on the front lines. At the end of the day I could not feel much difference from just using a regular bar with a stopper....and that was with fixed back lines. I mean maybe for really good kiters. I cannot remember exactly how much the travel was. Maybe I remember it wrong.
Some people argued I was wrong, and it certainly might just be me, but I think the minor changes in power distribution just didn't impact the kite enough.

On the bridles, I am not clear what your point it. t is all about where the load is, like you mention, and certainly different models have different advantages and disadvantages. I doubt many here will intuitively understand the difference. At the end of the day it is just moving point of effort back or forth. It will certainly make quite significant changes to handling and turning, but without actually testing the bridles I think it makes it hard to guestimate. There are some initial factors tho...for instance looking at load distribution on the LE. When you have a bridle, each split point will on a balanced bridle split the load in two. So on your cypress the top connection points will only see 12.5% of the load each whereas in the oak model it sees 25% each. This is what moves the leverage point back and forth.

[tegirinenashi]

On the bridles, I am not clear what your point it. t is all about where the load is, like you mention, and certainly different models have different advantages and disadvantages. I doubt many here will intuitively understand the difference. At the end of the day it is just moving point of effort back or forth. It will certainly make quite significant changes to handling and turning, but without actually testing the bridles I think it makes it hard to guestimate. There are some initial factors tho...for instance looking at load distribution on the LE. When you have a bridle, each split point will on a balanced bridle split the load in two. So on your cypress the top connection points will only see 12.5% of the load each whereas in the oak model it sees 25% each. This is what moves the leverage point back and forth.

Yes, your numbers (12.5% vs 25%) are correct, I fixed the ones in my message. So, those connections points located close to the center act like 5th line. If you have twice more leverage on it, certainly the kite would be more depowerable? (An obvious assumption here is that it is easier to pitch the kite by pulling at the center as opposed to at wing tips, because the torque AKA moment of force is highest where the kite is width is maximal).

[jakemoore]

1) Regarding the ultimate front line pulley:

Didn't somebody actually call it that? A thread somewhere long ago inspired me to try with Flysurfer Speed 1 from 2005. I liked the idea that instability would cause the front line to lengthen when powering up one side of the kite, increasing the steering response. In practice it did not do much. Even the 7 meter speed on handles the entire span of my arms could not turn that kite by front lines.

Also, it seems to me that even a few inches of difference between front line lengths don't affect LEI flight characteristics much.

[fluidity]

I pmed tegirinenashi as he originally asked in the original thread instructions but he said he only got one pm (from me) and suggested I post it back to his thread. I've added and edited some bits too:

Hi tegirinenashi,
your questions are ones I've also considered in the past. It's a good exercise to work through...
I sometimes fly RC Delta gliders which use elevons which are combined ailerons and elevator(but no rudder action)
So for your Question #1, I would say that using a continuous front line running through an above bar pulley the effect would be to allow the kite angle to roll to follow the bar tilt angle but it would prevent the elevon action (one wingtip twisted relative to the other wingtip) and it would also prevent the kite steering like an aircraft does from rudder, but from the normal trailing edge brakeing which normally occurs from bar steer but couldn't if the kite simply rolls (tilts on the front to back axis with no wing tip deflection) to follow bar angle. Also keep in mind that with the pulley you suggested, the front line attachment points are no longer equidistant from the bar centre when you tilt the bar so you will be warping the front of the kite shape, i.e. with the letter "n" representing the leading edge, you are pulling one side of the "n" down and lifting the other which would create an unusual distortion.

So how does a kite really steer? I think it's primarily a yaw movement from trailing edge brakeing. If you look instead at the kite as functioning like a delta plane with steering from it's trailing edge like elevons, for starters it's not a small control surface rear elevon, it's the whole side of a kite that changes. As the angle of attack increases the kite slows on that side but it also lifts more. On a plane, that results in the *free wings* moving to create a roll and with the combined elevator action of the elevons the plane can pitch around the new pitch axis that has been created by the roll. On a kite, the roll action is completely constricted by the front lines which connect via either a V or a Y to the centre of the bar. (So on a kite, this elevon effect is useless)
What could work for the centre pulley is that the although the kite LE bridle distances to the bar centre change relative to each other (the uneven "n"), the kite can be redirected with bar pull in a different direction. Is it worth it? I don't think so. The beauty of using the current differential angle of attack system (bridle pulleys amplify this) system, is that the resultant rudder like yaw control give massively more redirection of the kite angle resulting in much faster loops. I don't think the front pulley would give nearly this amount of steering without a massively long bar!
And finally for your question #1, what happens when one LE bridle lifts higher than the other LE bridle? Dynamic instability: It will increase it's angle of attack, pull more line from the pulley, the other LE side will drop and eventually twist under the TE as the first LE lifts ridiculously high and stalls and/or folds over backwards. Unpleasant!

Question #2 (bridle mandelebrot designs)
"(1+1)+(1+1)" (oak tree)

"1+(1+(1+1))" (cypress tree) Note that the chain of links from the kite centre to the final bridle aggregation at the bottom is curved compared to the (1+1)+(1+1) bridle.
As a consequence under the additional lift of a gust, the kite's inflated LE shape can be distorted by the centre rising higher to straighten out the centremost bridle links on each side. This will narrow the kite to present a reduced aspect ratio profile and slightly reduce the effects of the gust. I'd look at using methods to release the TE instead though, much more rapid response.

Interesting the Kite you used to draw you bridle arrangement onto

Also I'll post this to the thread from wiki so we are all talking the same language:
An aircraft in flight is free to rotate in three dimensions: pitch, nose up or down about an axis running from wing to wing; yaw, nose left or right about an axis running up and down; and roll, rotation about an axis running from nose to tail.

The future of kites?
At the moment ILE power kites are intensively evolved but only over around a 15 year period?
The fastening of the lines to the sides constrains roll motions currently. For some really massive changes, we might see a move to high multiple Y splits and a centering movement of the side rear line controls. Using rear lines to control aerodynamics in a similar manner to a plane (probably a delta with ailerons) would mean bar pressure could be reduced to extremely light as the control would be for small control surfaces that only function with airflow. To go completely down this track would loose us the advantage of direct control but I think that the smarter designers will try to find a happy medium between firm feedback and efficient control with the result that lines move further in and control becomes a blend of user effort and a reaction of smaller control surfaces to reposition a kite in it's apparent wind.
Another focus should be the thickness of the leading edge along it's length. A fat leading edge helps the Coander effect and reduces stalling but it also resists torquing one end of the kite relative to the other. Get someone to hold one end on a no wind day and you take the other. Take turns twisting your own end and feel the torque transfer to the other end. Pump it more to enhance the effect and try again. How the C kite's ILE stays stable for in the air, steering resistance and for relaunch are all factors that need to be optimised and you can be sure that some of the designers are looking at things like where the inflateable leading edge most needs to flex or stay rigid along it's length...

[alexeyga]

Also, it seems to me that even a few inches of difference between front line lengths don't affect LEI flight characteristics much.

I used to have kites which had a few adjustment knots on both rear and front pigtails and on a few occasions I've actually managed to use different knots on front lines - which is roughly about an 1" difference. Trust me - that kite was really flying out of wack!!! Not only it was pulling to once side, but it felt like it was almost cork-screwing...

[grigorib]

Nope guys. The load splits 50/50 only if angle between line a-b1 is equal to angle a-b2 in a knot connection
Or if a line “b” is the same line running through line “a” pulley

[grigorib]

...

[tegirinenashi]

Yes, the 50/50 is first approximation only. However, it can be argued that if you designed a bridle with say 90/10 tension split ratio at some junction, then it would be poor design, and the weaker loaded line can be eliminated altogether without affecting performance too much.

Also, I would be very surprised if you can adjust the bridle geometry in such a way that cypress tree design becomes more depowerable than oak.

P.S. To clarify the title: "Armchair designer" is like "armchair philosopher".