Flexifoil - Power Questions.

Flexifoil - Power Questions.

Post by Silas Murr » Mon, 21 Nov 1994 01:06:17


How do you work out how much pull a certain size of Flexifoil will
produce in a given wind? - I recently got a catalogue from Flexifoil
international, and I`ve got a few questions:

Why is it that 10`s pull 4x harder than the 6`s (as stated on page 9) -
why not 40 percent more as the sizes would suggest? - Would 2 6`s pull
a little harder than a single 10, because you would then have 12 feet
of kite? or do you need 4 6`s to get the same pull as a 10?

What is the Aspect ratio that is given for each ot the Flixi`s in the
catalogue (but not the other kites I notice), is this related to the
pull the kite has? - I can`t seem to find much of a mathmatical
relationship between the aspect ratios and lenghts of the flexis
- is kite weight involved. I`m assuming all flexis are the same width
(I`ve only ever seen my own 6 footer!) is that right?

It`s all a bit confusing! - I`m obviously missing something, but I
can`t seem to find any info on this in the FAQ`s.

Reason I`m asking is because I`m wondering what to but next. In some
ways I regret having bought the 6 (only 10 days ago! - I`m *HOOKED*
and need more pull! ;-) ) and wish I`d gone straight for 10`s.

Do I buy more 6`s, ditch my 6 and start buying 10`s to stack, or
stack different size kites? (not the best solution from what you good
folk of rec.kites have told me.)

Any help much apprectiated.

         _/_/_/_/  _/_/_/_/
        _/          _/_/                To find your kite
       _/_/_/_/    _/_/               You have to find wind
            _/    _/_/              in which to lose it first.       S;)as
     _/_/_/_/  _/_/_/_/

 
 
 

Flexifoil - Power Questions.

Post by Paul Bloomfie » Mon, 21 Nov 1994 09:13:07

Silas,

These are my opinions on flexi's (I fly 6x6' or 1x10')


Quote:
(Silas Murray) writes:

> How do you work out how much pull a certain size of Flexifoil will
> produce in a given wind?

You try it - there's no substitute for experience!

Quote:
> Why is it that 10`s pull 4x harder than the 6`s (as stated on page 9) -
> why not 40 percent more as the sizes would suggest?

The longer an aerofoil is compared to its width (front to back), the
more efficient it is.  This is why gliders have long thin wings.

Quote:
> - Would 2 6`s pull
> a little harder than a single 10, because you would then have 12 feet
> of kite or do you need 4 6`s to get the same pull as a 10?

Flying 4x6' is a totally different experience from 1x10'.  It's not just
the pull that matters, it's the way the pull comes in.  In a big wind,
a single 10' can snatch you off your feet as it accelerates across the
wind.  The pull on the stack will come in more gradually...

This is an inadequate description: you have to try it to find out.

Quote:

> What is the Aspect ratio that is given for each ot the Flixi`s in the
> catalogue (but not the other kites I notice), is this related to the
> pull the kite has?

This is the ratio of the wing's length to its width, as described
above.  It is simple to calcualte for a flexi, for a delta (with
non-constant width) it is harder and less relevent anyway.

Quote:
> - is kite weight involved.

No

Quote:
> I`m assuming all flexis are the same width

All I've ever seen are.  I'm not sure about the 16', which used to be
in the catalog, but I've never seen for real.

Quote:
> Reason I`m asking is because I`m wondering what to but next. In some
> ways I regret having bought the 6 (only 10 days ago! - I`m *HOOKED*
> and need more pull! ;-) ) and wish I`d gone straight for 10`s.

What to do next is find some other flexi flyers, and try their rigs.  
Where are you based?  There is more to flexis than pull alone -
controlling a 6 stack in a light wind is one of my favourites.

Quote:

> Do I buy more 6`s,

Yes, you will then have an impressive looking stack.

Quote:
> ditch my 6

No, any flexi is better than no kite atall!

Quote:
> and start buying 10`s to stack, or

Buy one, a solo 10' is my favourite flexi.  Find a freind or two, and
you'll have a stack.  I'd like to meet the man that can take more than
10 minutes of a 4x10' stack, so sharing isn't a problem!

Quote:
> stack different size kites? (not the best solution from what you good
> folk of rec.kites have told me.)

I agree, don't bother.

Quote:
> Any help much apprectiated.

No problem!


London, England | CompuServe 100420,142

 
 
 

Flexifoil - Power Questions.

Post by Dick Be » Mon, 21 Nov 1994 09:32:56

: Do I buy more 6`s, ditch my 6 and start buying 10`s to stack, or
: stack different size kites? (not the best solution from what you good
: folk of rec.kites have told me.)

Silas,  Probably your best solution now is to borrow two more 6 ftr's and
stack the 3 together.  In winds of 15 mph + you will find that the kites
are a hand full.  I normally stack 2 6 ftr's during the strong spring
winds.  After a few hours flying the beast you sure can tell how winter
has put you into bad shape.  Your body will hurt for days. :-)  
Again, just borrow a couple more kites and see how you like it.  It will
be cheaper that way.
--
--------------------------------------------------------------------------------*** Bell

 Dallas, Texas

 
 
 

Flexifoil - Power Questions.

Post by I want to Go Fly a Kit » Mon, 21 Nov 1994 09:10:05

Quote:
>Why is it that 10`s pull 4x harder than the 6`s (as stated on page 9) -
>why not 40 percent more as the sizes would suggest? - Would 2 6`s pull

The pull on any aero-foil shaped kite is more-or-less 90 degrees to the
kite top surface, and is almost wholly over the front half of the kite.
On a flexi, the rear half pulls -towards- you, to set the kite at an
angle to the wind without which there would be no lift (pull) at all.
The ends of a loaded flexi are curved down towards you, the pull from
these end sections is outwards, not directly away from you.
This `outwards' pull tightens the kite and makes it more effective, but
means that not every part of the length of the kite contributes equally
to the tension in the lines that you see as PULL.
On a ten footer, most of the kite is nearly straight with only the ends
bent down. In contrast, much less of a six footer is nearly straight.
Hence much less pull.

Quote:
>What is the Aspect ratio that is given for each ot the Flixi`s in the

Aspect ratio is Span (10ft or 6ft) divided by the chord (width) of the
foil. A six footer (6ft x 2ft) has an aspect ratio of 3. clearly the
value for a ten foot kite in larger. A high aspect ratio wing with the
same section is more efficient than a low acpect ratio.

Quote:
>                   - I can`t seem to find much of a mathmatical
>relationship between the aspect ratios and lengths of the flexis

Dont try too much maths, it makes the brain hurt.. :-)

Buy a ten footer as well. There will be winds in which even the six footer
is too big (though not very often). Also the six turns MUCH tighter than
the ten. I can easily ground launch a six, even when it is inverted, but
find it diffult to do at all with a ten.

Quote:
>stack different size kites?

While I have stacked a six with a ten, they are much better with an eight
between them, and better still all the same size.

        John Travell.
Solent Kite Flyers is a NEW Kite club based around Southampton, UK,

 
 
 

Flexifoil - Power Questions.

Post by Coreyki » Tue, 22 Nov 1994 14:20:04


Quote:
(Silas Murray) writes:
>What do I do with my flexi's? (Pardon the paraphrase.)

I humbly respond:  I've been flying flexi's since 1977.  Love the 6er in
stacks.  Moves good and you can add or subtract to control the power.
The 10 in light winds handled by a good flyer is beautiful to watch and oh
so graceful.  Flying it is an even better feeling.  Like being one with
the power of the wind.
I've read the other 4 postings and those folks explained the science
pretty well.  Shut off your computer and get out there and fly that flexi
some more.  None of us knows as much about flying as we think we do.

 
 
 

Flexifoil - Power Questions.

Post by David Sta » Thu, 24 Nov 1994 12:21:52


Quote:

>Silas,

>> I`m assuming all flexis are the same width

>All I've ever seen are.  I'm not sure about the 16', which used to be
>in the catalog, but I've never seen for real.

My old catalog shows the 12 footer to be 27 inches deep, whereas the smaller
kites are 24 inches deep.

Quote:
>> stack different size kites? (not the best solution from what you good
>> folk of rec.kites have told me.)

>I agree, don't bother.

Actually, stacking different size flexis can be both fun and instructional.
The usual way to stack the kites is with the larger kite to the outside of
the stack. This way, maneuvering seems rather natural. There is less stress
on the spar joints also. You'll find that you aren't able to make quite as
large control movements as you are used to with the big kite alone.

For something different, stack them in *reverse* order. This is best done in
a light wind - I find that an 8 footer with a 6 footer behind it, both with
light wind spars makes a good stack. Be sure to tape the spar of the smaller
kite *securely*.  

What will this do? It will increase the pull dramatically. The pull of the
smaller, outside kite will tend to spread the larger kite apart. (Use 5 to 6
foot training lines for the stack) While edge precision suffers, the airfoil
efficiency increases and you're in for some exercise in fairly light wind.




____|)avid

 
 
 

Flexifoil - Power Questions.

Post by Andrew Beatt » Wed, 30 Nov 1994 07:28:08

[Quick followup of bits that others have missed...]

Quote:

>Why is it that 10`s pull 4x harder than the 6`s (as stated on page 9) -
>why not 40 percent more as the sizes would suggest?

The 6' has 2 fibreglass tapered spars, they bend under strain.  The 10'
as 2 similar (identical?) tapered spars, but a solid carbon centre
section that hardly bends at all.  It is the much increased length of the
straight centre bit that delivers the extra power.

Quote:
>What is the Aspect ratio that is given for each ot the Flixi`s in the
>catalogue

It is The square of the span of the kite divided by the area.  For a
rectangular kite like a flexi, this is the same as span/depth.

High aspect ratios are generally more efficient.  Flexifoils have some of
the highest aspect ratios around (soft kites can't cope with much above 5)

Andrew
--
Kite FAQ's: ftp.hawaii.edu:/pub/rec/kites/faq)
For sale: 10' Flexis with std & UF Spars.
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