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Author Topic: Arrrrgh - Help, taming the P60 parafoil  (Read 4124 times)
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Stuntman
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« on: November 15, 2009, 03:26 PM »

Help guys.

I just had a frustrating Sunday trying to fly my premier P60 parafoil. Its normally quite well behaved and steady as a rock. But yesterday it was an obnoxious beast.

I don't know whether it was just too windy (14-16knots on the ground) or what it was.  Undecided

It kept leaning off to the left. Then drifting right off and recovering with a powerful fast kill everything in its path zoom to the apex followed by a repeat performance. Shocked

In the end it caught another kite - cutting my nice spectra cored 400kg line in several places, damaging some of my line laundry and causing a catch the kite and line laundry race across the field.

I took the laundry off the line, played with tuning the bridles a bit, added some extra drag (may not enough)before giving up in disgust and packing it away (so I could play with other kites).

So I will be checking the bridles out over the next day or two - have read the tuning parafoils FAQ on the Gomberg page, but am looking for some wisdom from others more experianced than myself.

In short - HELP.

thanks, Shawn
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rncembal
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« Reply #1 on: November 17, 2009, 08:31 AM »

Often positioning the drag further away from the kite can help. 15ft lead to drouge. Spectra core can also hurt in high gusty wind ,it offers no buffer to to gusts.
Rob
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KaoS
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« Reply #2 on: November 17, 2009, 03:22 PM »


... added some extra drag (may not enough)...


Hi Shawn

You probably needed to do just the opposite, reduce the drag.  Sounds counter intuitive doesn't it?   But I've seen this time and time again, especially with parafoils in stronger, gusty winds. 

Parafoils tend to behave better when given the opportunity to react quickly to subtle changes in wind direction or strength.  Their natural behaviour is to turn and fly towards a new location, then settle.  So a small drogue or tail is ideal, as it keeps the parafoil facing into the wind without nailing it to the one spot.  Flowforms on the other hand are almost completely the opposite, because they tend to glide across the sky, not turn and head towards a different location.

I have some diagrams that explain this better, I'll dig them up and post them a bit later


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Kevin Sanders

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« Reply #3 on: November 18, 2009, 01:03 AM »

Have you ever had someone "help" you launch a kite with a long tail?  It can go one of two ways...

First way, they hold the kite up until you launch, then they let everything go



The kite (usually) launches and flies successfully.  If the kite is not in the centre of the wind window, it heads towards that point and the tail quickly follows

Second way, they hold the kite up until you launch, then continue to hold onto the tail... they think this will help attain a clean launch.  Unfortunately, it doesn't.  The kite inevitably veers to one side, doesn't correct and eventually comes down...



Adding too much drag to a kite (especially a parafoil) is very similar to holding onto the end of the tail of a kite.

Similar behaviour is seen if the drag for a particular kite is too far away from (behind) the kite.  The kite will try to turn and fly to a new point in the sky, but the point it is forced to rotate around is so far back, the kite can't turn.  So don't put a drogue a long way behind your kite and expect it to improve the kite's stability...it probably won't.
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Kevin Sanders

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« Reply #4 on: November 18, 2009, 03:20 AM »

Good stuff,  Smiley

I can make a link in my mind to aerodynamic stability with respect to aircraft - a longer tail moment - larger fin area can in fact make it difficult to recover from a bank - or worse a spin. Contrary to way you think it would work.

Adding more drag to the kite further away is similar, it increases the moment arm of the kite, and the kite cannot recover as once it deviates its flight path is in fact stabilised by the tail/drogue/drag inducing device. hmmm

The P60 doesn't have a huge drogue. Its connected to the foil as per the solid the lines below - would connecting the lines to the tips assist. I am thinking it would probably would make no difference but maybe it would. Something to try maybe

Also the rear two sets of lines I would consider loose in flight - ie the kite seemed to flying on the front set of lines. It has a "self adjusting cascade bridle". Would this be considered normal?

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spence602
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« Reply #5 on: November 18, 2009, 06:02 AM »

Shawn - over on the KB forum we've been working on a spreadsheet to help calculate the proper bridle line lengths for multi-lined SLKs.

If you have access, you can grab a copy of this spreadsheet, plug in the top-center line length, a few widths, and see if the rest of the numbers work out close to the length of your bridle lines.

The line lengths are calculated without consideration of the keel lengths, so measure from the bottom skin.  You may need to play with the angle to get the proper lengths of the lower lines.

Caveat Emptor:  We're still waiting on actual bridle measurements to validate the formulas in the spreadsheet.
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Spence
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« Reply #6 on: November 18, 2009, 08:13 AM »

You say it's "normally quite well behaved and steady as a rock" but then you went ahead and "played with tuning the bridles a bit". Well, unless you can accurately return it to the prior settings...

I've experienced similar symptoms with various kites in the past. It can be a very frustrating thing but my rule, learned the hard way, is that if nothing is evidently wrong or adjustments to drag don't work I'll write it off to fickle winds aloft, pack it away and fly something else for the day. Now if it does it again, I'll really look into tuning it.

As Spence said, you really have to make sure the bridle legs are even all the way to the kite face. Differences in keels, sewn loops, etc. can make otherwise even bridle legs uneven.

Your analogy to the moment of a solid airframe may not be quite applicable, as the kite and tail/drogue have at least a pivot point in the middle if not an almost entirely flexible length. The tail does, however, provide a pull that may not be in the intended direction of flight. Think of the kite having veered off to one side in a gust with the large drag-inducing tail remaining  more or less where it was. Now the tail is no longer directly behind the kite, and only partially pulling the kite rearward and partially pulling it to the side, possibly steering it further away from center depending on which force is stronger. Until the drag is centering again, this seems like it might exacerbate the situation.

And your comment about the middle and rear bridle legs being loose is curious. I know of framed SLKs that will fly fine off the front bridle legs in some winds, but it seems that without some light tension to keep things even, a soft kite might be susceptible to "warping" or twisting allowing an effect similar to airframe ailerons. DG's tuning suggestions talk about pulling in or letting out corner lines to counteract this effect. And the self-adjusting cascade bridles that I'm familiar with don't completely de-tension the middle or rear bridle legs.

Tuning is always an interesting and sometimes frustrating exercise and the proper tune can change in various conditions. If the kite was previously stable, I'd suggest that it be done in small increments and, if ineffective, return it to the original position.

Hope these comments provide a bit of food for thought.
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Bob
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« Reply #7 on: November 18, 2009, 08:30 AM »

depending on how long you have had your parafoil the skin might have stretched and/or the bridles have stretched....especially after a good heavy wind day.....happens.....take some time and put easily seen marks on the bridle location now and then tweak it....see if it is all the same length etc....
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KaoS
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« Reply #8 on: November 18, 2009, 02:37 PM »

It has a "self adjusting cascade bridle". Would this be considered normal?

I presume you mean that if you move the towpoint, all other bridle lines are free to move to compensate for the towpoint change?

If so, my personal opinion (based on a lot of bridle testing on the Gomberg Road Sign inflatables) is that this is NOT an appropriate bridle for a soft kite.  It is easy for the shape of the kite to change due to varying internal pressure, and for the bridle to allow that to happen.  For example, the tail end of the parafoil might bend backwards from the normally flat front surface.  A completely free self adjusting bridle allows this to happen - BUT IT SHOULDN'T.  The bridle should hold the front surface of a parafoil flat.

A much better alternative is to have prusik knots between the bridle sets, so you can easily adjust the various lengths and angles but control them as well.

..another 2 cents...
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Kevin Sanders

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« Reply #9 on: November 24, 2009, 02:55 AM »

You say it's "normally quite well behaved and steady as a rock" but then you went ahead and "played with tuning the bridles a bit". Well, unless you can accurately return it to the prior settings...

Only with looped knots. Returned to normal at the end of the session.

I've experienced similar symptoms with various kites in the past. It can be a very frustrating thing but my rule, learned the hard way, is that if nothing is evidently wrong or adjustments to drag don't work I'll write it off to fickle winds aloft, pack it away and fly something else for the day. Now if it does it again, I'll really look into tuning it.

As Spence said, you really have to make sure the bridle legs are even all the way to the kite face. Differences in keels, sewn loops, etc. can make otherwise even bridle legs uneven.

Your analogy to the moment of a solid airframe may not be quite applicable, as the kite and tail/drogue have at least a pivot point in the middle if not an almost entirely flexible length. The tail does, however, provide a pull that may not be in the intended direction of flight. Think of the kite having veered off to one side in a gust with the large drag-inducing tail remaining  more or less where it was. Now the tail is no longer directly behind the kite, and only partially pulling the kite rearward and partially pulling it to the side, possibly steering it further away from center depending on which force is stronger. Until the drag is centering again, this seems like it might exacerbate the situation.

And your comment about the middle and rear bridle legs being loose is curious. I know of framed SLKs that will fly fine off the front bridle legs in some winds, but it seems that without some light tension to keep things even, a soft kite might be susceptible to "warping" or twisting allowing an effect similar to airframe ailerons. DG's tuning suggestions talk about pulling in or letting out corner lines to counteract this effect. And the self-adjusting cascade bridles that I'm familiar with don't completely de-tension the middle or rear bridle legs.

Tuning is always an interesting and sometimes frustrating exercise and the proper tune can change in various conditions. If the kite was previously stable, I'd suggest that it be done in small increments and, if ineffective, return it to the original position.

Hope these comments provide a bit of food for thought.

Plenty of food for thought guys. Thanks. I've still got to pull it out and have a look. With 40+C and 95% humidity over the last weekend I didn't fancy spending a couple hours in the shed sorting it out (and of course Monday and today back to 20 and raining.  Roll Eyes When I do, I'll grab some photos of the bridle setup and post. Kaos its a similar setup to the Shannon parafoil (have a look over at Gomberg it details it there)

I really want to get it out again in some less strong winds before doing any other changes. But will check the lines back to the bottom skin and have a look at the spreadsheet to see if there's a correlation in the meantime.
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KaoS
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« Reply #10 on: November 24, 2009, 05:14 PM »


... its a similar setup to the Shannon parafoil (have a look over at Gomberg it details it there)...





As I suspected...

This is a great bridle for framed kites as the spine(s) provides enough rigidity to ensure the face of the kite keeps its shape from head to tail (usually straight line).  However, a soft kite can deform along it's vertical plane, and this type of bridle will self adjust to accommodate that deformation.  This doesn't often happen, but it is much more prevalent in strong blustery winds.  It wouldn't be quite so critical if the deformation was uniform and remained constant for a given wind, but it doesn't happen that way.  It is a constant battle between internal pressure trying to inflate the kite to it's intended shape and external pressure trying to move any part of the kite it possibly can.

A better solution is to use a prusik knot instead of a free running loop between the banks of bridles.  This allows you to make quick bridle adjustments, but doesn't allow intermittent kite deformation to override the intended configuration of the bridle.
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Kevin Sanders

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« Reply #11 on: November 24, 2009, 07:17 PM »

Nicely explained, Kevin! Drawing's ok, too.  Wink
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Bob
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