First off - how to test it :
(Line lengths has to be equal before you test - especially the rear ones are important - but this is clear to anyone, so the starting point and not worth using time on this one)
Set the kite up at 12 precisely, and depower so the rear lines are totally slack.
Do the kite fly straight now or not ?
If, it is not that bad no matter what.
If it flies a lot to one side, then :
1. Bladder twisted in the front tube.
I've seen this in some occasions, especially earlier when only C kites, and maybe the production process was a bit less "refined"
I dont think it can happen in a one-pump kite, as here it will always be straightened out.
But if the bladder is twisted a lot inside the front tube, it can really get a kite to fly a lot to one side.
One sign of this can sometimes be if the valve wants to "slip inside" the tube when pumping up - as then the bladder is definitely "stressed".
It happened often with older single-pump C kites I remember.
If you pump the kite on a day without any wind, and get someone to hold the kite up in the air with the bladder up like a "U" - and you stand back and look at the bladder from the front, you will be able to see if it is twisted, and sometimes you can see the struts lifted more in one side than the other also.
This is how you can visually tell if the fronttube is twisted.
If it is, then you can take the bladder out, and reinsert it again, as this will usually relieve all stress if inserted correct again, and pumped up very gently so it dont rotate.
A good idea could be to pump it up only sligtly, and then deflate again, and "shake" the fronttube - pump up a bit more now, deflate, and shake it again.
If done a couple of times, the bladder will fall into place and not be twisted, and it will be stressed like it should
If you just pump the kite up, maybe also with the front tube folded somewhere, you MIGHT get a stressed/twisted bladder.
If the kite still flies to one side, then there is #2 reason maybe -
2. Kite is sewn a bit "off" so it pulls to one side.
If it is a lot, it is a disclaimer of course.
If only a little, it is fine
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If okay, you do the second test - Pull the bar slowly, so rear lines are tight, kite still at 12.
If it turns to one side, correct the rear line length (or pigtails or knots or whatever you like best) till it flies straight at 12 o'clock.
Now the thing is, that ALL kites, and all brands and all types (C, Flat, anything) - will most often pull a bit to one side or the other, and nothing wrong with this, happens more often than not in fact..
If only a few centimers or an inch to one side (meaning - if you make the left or right rear line a bit shorter/longer it flies straight), you just adjust your lines or pigtails at the kite and no big deal.
But if it is more than 5cm/2inch, then the kite is sewn badly, or repaired badly, or has got too many tough and hard slashes in the water or beach
Although, some kites will still be able to fly fine and turn okay to both sides, EVEN with a lot of rear line difference in length - so if you have an old kite and no refund/disclaimer option, you might be able to fly fine with different rear line lengths, instead of ditching the kite.
Other kites will NOT be able to turn equally well to both sides, if you correct by means of rear line adjustment to compensate for some misalignment in the kite.
This is how I always test and correct ANY kite I get in my hands (also other newbies kites, or if someone wants help, so thats why I got the same experience with other brands).
It only takes a few minutes in fact, to determine how a kite should be tuned, so very easy when used to this.
Personally, I just HATE flying with a kite, where one rear line is just a cm too short or long
So I always correct this.
But I know that is just me - most dont care or even notice if it is only a little bit...
Dont know if this helps anyone or not - but these are my personal thumb rules and way of setting any kite up and testing if something is wrong.
Kindly, Peter Frank