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The Flexifoil Article

> I first posted this a couple of months ago.  Since quite a few people have
> requested it, I've updated it (marked ">" in the border) and posted it
> again.

> I still don't claim to be a flexifoil expert, but I've probably had more
> experience than most in flying large stacks of flexifoils, so I'm sharing
> some of my knowledge to help others avoid my mistakes.

To start at the bottom:

Handles:

> I'm still on a learning curve on the matter of handles.  The story so far:
> 
> 1) Peter Powell heavy duty handles.  Line must be fixed to the two holes, not
> the hook, which can snap off.  Anyway, these things are far too hard to take
> seriously.
> 2) Padded wrist straps.  These appear to give a reasonable degree of comfort,
> but are in fact a *very*bad* idea.  The human wrist is *fragile*.  Don't
> use them.
> 3) Combination of Peter Powells and Padded wrist straps.  A bit better, but
> the handles are still too hard, the straps are still bad for your wrists
> and the combination takes a little too long to let go of.
> 4) "Good grips"  (padded wrist strap with grip bar).  I must get round to
> padding the bar.  Still the wrist problem, they also take more than 0.5
> seconds to let go of.
> 4) Harness.  (with one of the above)  OK, so long as the wind is light enough
> (say, <10mph) that you don't mind being trapped in it.  A "quick release" more
> complex than letting go of the handle takes too long if you are being dragged
> along the ground at 20mph at the time.  (Will you be able to let go if you
> bang your head on the ground and get knocked out?)
> 5) Control bar.  Still havn't tried this.
> 6) Sky Claw (Padded 9" length of broom handle).  I think I'll try this next.

Line termination:

The flying line is, of course sleeved.  (If ever you doubted that Spectra
would cut on it's self, try tying a knot in the line on a flexi!)
When not flying, *always* stake your line in a decent dog stake or
a pair of heavy duty pegs.
Also, mark your pegs clearly, so that you can avoid sliding into them.
> If I am concerned that the stack may self-launch, I only stake one line.
> The thing won't fly on one line.
> (Note that if a big stack the thing does a clean self-launch, you can
> bet that it will either rip the stake out of the ground or break the line)

Flying line:

I have three lines:

300lb x 100'.  This was my first line.  It was originaly 150' long, but
I tried untangling a knot on an unstaked line.  The wind caught the sail
and I tried to hold it.  I was very lucky, the line cut through it's self
before it cut through my finger.  (It cut very deep in about 0.5 seconds
and I didn't even feel the injury).  The line is now about 100' and
I have learnt to treat it with a great deal of respect.  100' is really
to short because everything happens too quickly.  300lb is also too weak
to do anything serious with.

400lb x 300'.  Unfortunately, I can't fly this line straight up 
> in the UK
without ATC clearance, but it is a nice line for low wind because it is light
and the length allows you to spend more time flying in a straight line
(generating lift) and less time stalling in corners.  It makes for
impressive long low passes if you can find enough real estate.

800lb x 200'.  It takes a lot of power to fly this line, but I'd rather
use it whenever possible in order to play both safe and legal.  (forget
the 'plane - if anything powered flies into my lines, *I'm* going to get hurt)
The line slows down the foils because of it's diameter, cutting the lift
(you loose about one foil-worth of power due to the drag).

Bridle:

I attach the bridle lines to the flying lines with a D ring
> bought from a climbing shop.
I started with 1000lb Spectra as bridle, but it deteriorated rapidly.
Spectra is strong but had very poor abrasion resistance.  I Buy bridle
line from a boat shop.  I use about 4mm diameter braided line with a
breaking strain of around 350kg.  It resists wear (from the bridle rings)
much better.
You bridle the whole lot on just two lengths of rope.  You pull a loop
through the ring and make a larks head in the usual manner.
In order to space the sails equally, mark the rope so that you
can tie it accurately.
Flexifoil reccommend that you space 10' sails at 6' but I mark my bridle
lines every foot, so that I can easily change the spacing
as I wish.  I find that 5' spacing helps keep the stack flying more tightly
together (the foils are inclined to pull unevenly, where one stalls, the
sails above and below pull, making the stalling one fly, but causing these
two to stall, starting of an alarmin and violent fight).  4' spacing tends
to be too close without a good breeze, as the sails start to steal each
other's wind.  At 4' there is also a greater risk of the sails falling
between each other in a stall.
Marking every foot also lets you move the whole assembly
up or down to even out the wear.

> Flexifoil themselves use a different method.  They use a continuous
> length of spectra and attach the sails as follows:  They take a small
> length of seperate line, tie it into a loop and attach it to the line with
> double larks head.  They then take another short length of line, feed it
> into the centre of the flying line (like splicing), run it past the loop
> and back into the line, below the loop.  I think that the spliced line may
> be dacron, and be designed to take the wear of the ring moving against the
> flying line.

Spars:

Take care to insert the rods carefuly,  I know of someone who destroyed
a centre carbon (at 35 quid!) because it wasn't pushed all the way home)
My rubber stops havn't started to move yet, but the things won't fly right
if they do.
I havn't tried the lightweight spars.

> Ferules:
>
> Glue them on to the fibreglass rods with superglue if they loosen.  Also,
> buy a couple of spares, as they are cheap and you can never find a kite shop
> when you break or loose them.

Sail:

Note that it does have a way up!  It won't fly off the ground if it is
upside down.  Make sure that you puff it up properly before trying to launch.

Tails:

I've never tried flying a tail on a line behind the foil as suggested by
flexifoil, but I have run them in pairs (one at each end).  On one foil,
it's quite fun, but the speed does quite a lot of damage to the end of
a plastic one, quite quickly.  I did try flying 6 tails off 3 flexies,
but it slowed it down too much and made it too sluggish to fly.  More wind
might have helped, but it took *ages* to wind in all the tails and I
havn't tried again.

Launching:

The Flexifoil brochure states "Easy take-off and an astonishing rate of
climb puts the POWER KITE above your head in seconds".  This is a lie.
These things can be a pain to launch.  First check that it's the right
way up :-) (this is easy on mine, the top is a different colour to the
bottom).  Check that the bridle isn't twisted, the ring tape should lie
flat infront of the spar, the line should go through, round in a larks head
and out, without twists, over the top to the next sail.  Check that the
sail is inflated full of air.  (Although you can hold it open by hand,
I find that holding one end of the foil after assembly and spinning it
once round me helps to inflate it properly before attaching to the bridle.)

Launching it off the ground is difficult if the grass is long, because the
grass tends to pull the leading edge down, spoiling the shape and  blocking
the vent.  Launching off a cricket square is easy.

For an aided launch, I ask the helper to hold the front sail (ensuring
that they don't hold the vent closed!) and run a few paces towards me,
at the same speed as me, until the stack is off the ground.  It is
important that they let go soon as soon as it is flying, as some people
hang on until the thing is pulled out of their hand, upsetting the stack.

Many people use canes to run the foil up to launch it, but I have not found
this successful with a stack, I have been concerned about ripping the sail
on the end and stopped altogether when my wife pointed out that leaving a set
of thin stakes dug at an angle into the ground on a public park, with their
ends at about kiddie-eye-level was probably not a good idea.

I find that the easiest way to self launch is to peg out a pair of ropes,
raise them about 6 inches off the ground and rest the sails on top,
this allows easy movement and proper airflow and a good pull will produce
a stable launch.  In a good wind, you need someone to ensure that
they don't self-launch.

If the stack doesn't launch cleanly, you may have a violent fight on your
hands, the best move is to turn into a dive and move forward so that the
sails stall and get a chance to form their shape together.

Power:

A single flexi can rip across the sky at astonishing speed, producing loads
of pull.  If you fly two, then you can get about double the pull.  Above
two, things start to change.  Wearing trainers on grass, you can't pull
hard enough to get maximum speed, it'll drag you across the ground.  The
guys that go jumping do so by bracing themselves against something solid.
So far, I havn't tried this, I much prefer my body to stay in the number
of parts in which it was supplied to me.  I'm quite content with the
distance I can cover without this (an estimated 8' up and 30 across
with 3 foils on the 300' line).  Provided that you don't brace or stake
yourself, increasing the number of foils doesn't make a lot of difference
above about 3 or 4.  If one foil is like being dragged across a field at
5 mph by a bicycle, (you can stop it if you want), then 2 or 3 is like being
dragged at 5mph by a motorbike (you can try and stop it and you can
certainly effect it, but it'll probably win in the end). 6 or 9 is like being
dragged at 5mph by a car (you havn't got a hope of stopping it, but it is
still trying to take you at the *same* speed).  This means that although a stack
of 9 (which I flew at the Greenham festival recently, because I borrowed 3),
has potentialy got the power to rip your arms out of your sockets if you try
to stop it, I am able to sweep it in a controlled low pass, just a few feet
off the ground (normally one of the most powerful moves) by walking forward
to control the speed.
As for it lifting you off the ground, the lowest pull is at maximum altitude,
so although you may be off the ground some of the time, the kite will drop
you (sort of gently) as it rises higher and pulls less.
While a single or double flexi tends to be a vicious beast, a stack is much
more of a gentle giant.  You can't pull it hard enough to make it do anything
fast enough to take you by surprise.

> What about stronger wind?  I've tried twice in a 20-25mph wind.  I started
> with 6.  This launched with one sail inverted, pushing down rather than up.
> by the time I had got the thing above my head, I had slid over 300' of grass
> (this is with 200' line).  I was travelling, predictably, at about 5mph less
> than wind speed.  With the stack up, I was able to fly it ok, but I couldn't
> regain the ground that I had lost.  I didn't have enough field left to be
> able to recover if things went wrong during landing (I was also tired and
> a little scared), so I asked two men to hold me as I landed it.
> Considering that 6 10's may have been a little too much, I tried again with
> 4.  This launched perfectly and formed shape and started pulling almost
> as soon as it was off the ground.  Although the pull was mostly horizontal,
> it was so violent that I didn't touch the ground for (say) 30'.  I was
> travelling too fast to run when I landed, so I slid on my back, side, bum
> and belly until I reached the people standing at the launch point.  The kite
> had only achieved an elevation of 40 degrees or so and I only had 300' of
> field left, so I let go.  The sail landed 300' away.  My main regret was
> to have tried it while wearing shorts. (grin)

> Clothes:
> 
> Buy Army surplus trousers.  They are hardwearing and cheap to replace
> when you wear out the bum.

Landing:

The classy way to land a flexi is to sail it round to the side at low
altitude, and let it touch the ground with a spar-tip with zero forward
speed and negligible downward speed, then relax the line, so that it falls
back onto the ground, the right way up, ready for launch.  This is difficult
because normally you touch the ground with some forward speed and the sail
falls forward and upside down.  With a single flexi, this is recoverable
by pulling on the lines in the way described in the user notes to turn the
sail round, but this doesn't work with a stack.  Anything else involves
steering the thing into the ground.  The flexi *is* breakable, you can break
the spar by hitting the ground too hard, and someone at our club burst a cell
(in the middle of the nylon, not on a seam) on the shockwave as his single
flexi hit the ground head on from a power dive.  If I have to land my stack
in a restricted area, I put it into a dive and run forward as fast enough
to take all pressure off the lines.  With the pressure off, the sail produces
a considerable about of drag and the foils should hit the ground square-on
at a reasonable speed.

> Finally:
> 
> I'm happy to let others fly my stack.  I'm easy to spot:
> 
> Most of my kites are yellow and blue:
> Flexifoils: Currently (only) 6 x 10' yellow/blue with pink tips
> Rev I:        Yellow/blue
> Lightflight: Yellow/blue (This kite is possibly unique)
> 
> I fly near the Basingstoke junction on the M3
> 
> My black golf convertible has a *large* yellow/blue/pink graphic down
> the side, styled after a stack of flexies.  You'll recognise it if you
> see it, because you'll think "How could anyone put anything as gross as
> *that* on such a nice car?"
> 
> So, if you see me, come and say hello (even if you arn't brave enough to fly)

Andrew

> PS: I don't mind this article being copied to CIX, kite club news
> letters and the like, but please
> 1) tell me about it
> 2) retain my email address: gaffer@ibmpcug.co.uk
> 3) send me any followups
> 4) send me a copy of the publication, if appropriate
> Ta.
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