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Introduction
I should perhaps start by saying that whilst I am not an expert I have
made a few NPW's and a lot of mistakes. Mistakes are fine as long as you
learn from them. My remarks concerning aerodynamics are simplifications
but give, I believe, a good idea of what is going on. The NPW can be unpredictable
at times though so nothing is written in stone. If you are having problems
getting your Nasa wing to take off, then read on. An understanding of what
is going on with the wing should enable you to figure out what is going
wrong. I've also found an easy way of getting the bridling right. More
of that later.
The shape and bridling of the NPW FIVE is all important to the flying characteristics. A few cm here and there can make the difference between flying or not. It's easy to introduce slight errors when sewing it together and the stretch in the material can also be different in different directions. The bridle lines can also stretch or be too stretchy which will alter the shape of the skin in different airspeeds. Use at least dacron which has a stretch of ca. 2%.
Not to worry, it's not too hard to correct any errors in shape with
the correct bridling.
If you haven't yet built a NPW FIVE then I suggest you make one to
the plans by Peter De Jong , the ENPW. The "E" is for enhanced. http://www.xs4all.nl/~pdj/nasaplan.htm
If you use good quality materials and are careful with the measurements
then all should be well. If you have made one already (whatever design
you used) and it won't fly then read on.
Also have a look at the modifications at the end of the article before
you actually make your wing in case you decide to use any of them. I wouldn't
say that these will produce a perfect kite but if it's a first attempt
and maybe your materials aren't the best then cutting a few corners won't
hurt too much. The kite is fairly straightforward to make and you'll pick
up some sewing skills too if you don't already have them. (If you don't
then I suggest you start with cheaper material and a keel length of 1m
or so, it'll still fly even if it's a bit porous and won't matter too much
if the result is less than perfect.)
Construction
Follow the plans and make sure to include string reinforcements around
the perimeter, for the spinal attachment points (two vertical lines the
length of the spines and a line behind the nose for it's ties.
Then add short lines to all the attachment points (both wings and spines).
Say 20 cm for a keel length of 1m. These lines should be knotted every
cm for later adjustments. These short lines are not absolutely essential
if you follow the description later for adjusting the bridle, however,
you may at some time want to change the foil shape, angle of attack or
correct for stretch and then you'll need them.
Tie the nose as per the plan (here you can also add short adjustment
lines knotted every half cm a couple of cm around the correct length).
Mark all the middle knots with a coloured felt tip pen, say red. (Save
black for marking final positions as it shows up better.)
Shape
The shape of the skin determines how the wing flies, how it fills,
how it pulls etc. It can be shaped by the bridle for different results.
Some general considerations here may help you to decide on the shape you
want.
As with all airfoils the lift to drag ratio increases with increasing thickness (up to certain limitations). In other words a fat foil will pull more. Generally speaking it will also be slower than a thin foil and it will take more air to fill it. Even a single skin soft foil has to fill. The NPW is somewhat critical though and when you exceed the maximum thickness it doesn't loose power gradually but more or less totally. It's best, therefore, to err on the side of too thin to start with anyway.
Pressure in the NPW builds up from the trailing edge to the nose. In other words it fills from the trailing edge to the nose. The curve of the trailing edge, the so-called brakes, "trap" the air so the pressure increases. This increase is reflected back up to the nose. Problems with the nose folding under are a result of there not being enough pressure in the skin to keep it inflated. (This can occur especially when turning the kite. In this case you are spilling air from one side to make it turn. With four line flying you can compensate for this by pulling on the brake lines to assist turning and keep the nose inflated.)
The two ways of controlling the degree of filling are the amount of
wing pull-in and the amount of deflection of the brakes. Air is sort of
funneled from the tail up to the nose. As the area decreases from the trailing
edge to the nose, the pressure builds up more and more and this keeps the
nose inflated to at least the pressure on the outside of the nose. If it
doesn't then the nose won't inflate properly or will be prone to collapse.
If the flow isn't sufficient you can pull in the wings a little or
increase the brakes. Generally you will need to adjust the funneling primarily
with the wings. The flaps are used more to adjust for differing wind speeds.
During four line flight the brake lines help steer the wing and control
it's altitude. (Get it back to earth, for example, or take it to the top
of the window.) You will always need a certain amount of brakes though
or the air just spills off the trailing edge and no amount of pulling in
the wings will help.
The shape also has to allow the wing to fill properly. Here I mean
that it must be a smooth progression from nose to tail with no sudden bumps
or waves.
Bridling
Let's assume there are some errors in the shape, materials or whatever.
This means that the bridle measurements from the plan don't apply
to your wing. You can start there but if it won't fly you'll end up making
many small adjustments. It can take ages before you get it right and may
even result in you giving up.
In this case it's better to forget the bridle measurements. Start from
scratch. It's much easier and you'll probably get it close enough first
time for it to fly.
Tie on bunches of lines with lark's head knots to the wing at the middle
point of the short adjustment lines. You'll need the following bunches
to make adjustment easy in groups:-
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Go from the plan for the approximate bridle lengths and add a good bit for each line. If in doubt make the lines too long, it's better than having knots which can and will tangle. Code the ends of the lines furthest from the wing with coloured marker pens for A and B. For example a number of dots corresponding to the number of the line and red for A, blue for B. This makes it easy to find the right line to adjust. Take the groups together and here make sure to get the lines untwisted. Put a small rubber band on each bunch to hold them together but still allow adjustment by pulling individual lines through it. Take all the left bunches together and hold these together with a bigger rubber band just below the small rubber bands, ie closer to the kite. Do the same for the right side.
Now hang up the wing from the big rubber band in a suitable place, a barn, large room or a couple of trees outdoors on a still day. The left side and right side must be well apart. You need a space width of around twice the keel length of the kite. Hang it up to get the wing off the floor and at a convenient height to work on. Aim to get an angle between lines and floor around 60 deg. (it's not to critical) and make sure it's hanging freely from the floor. Hanging up
Two line flying is fine for fun and smaller Nasas but, in my opinion,
for a traction kite you can't beat four line flying. For four lines hang
up the handles level,
with the A1-9 and B1-4 at one end of the handle and A10-12 and B5-6 at
the other.
The NPW will then bunch up and make folds horizontally across the kite.
You may need to help it a bit if the material is stiff. You can put small
weights in the folds if necessary (gravel, sand, nuts and bolts or whatever).
With the kite hung like this you can get an excellent idea of it's symmetry
and smoothness from nose to tail.
The shape obviously changes once it's inflated and here you can get
an idea of the shape by pulling the nose or tail away from the main bunch.
A perfectly flat bunch will become a chord of a circle when you pull it
out, due to the lines radiating from the centre of a circle. Conversely
a flat kite in the air becomes a bunch with the nose and tail lower than
the middle.
The following guide is for a keel length of about 2.5m. Scale up or down for the size of your wing.
1) Start by levelling the whole wing parallel to the floor. Pull
in the wings so that the leading edge of the wings make lines roughly parallel
to the floor. This will entail adjusting all the B lines on each side.
Adjust the spines with the A lines left and right so that the spines are
also parallel to the floor.
2) Now pull up the nose ca.15cm on A1 and then make a smooth
curve through A2, A3, A4 down to A5. A5 stays unchanged. References to
A1 refer to the front of the nose, it works well to take the A1 line to
this point "c". See below
3) Pull up the tail by ca.18 cm on A12 and make a smooth curve
through A11 to A10 which remains unchanged.
4) Adjust the wings again so that they are going up a little
from the nose to B6. How much is a little? see the pictures below. Hanging
close up
5)Now check the distances from the hanging points to the middle
of the spine to make sure the overall bridle length left versus right is
the same. Check that the shape of the left and right halves are the same.
You can check the lengths of the lines left versus right if you wish but
differences in material and seams can mean the lines left and right are
not exactly the same. The important thing is the symmetry which you can
judge by eye.
To recap, the wings are now going up to the tips, the nose and tail
curl up from the floor. Go for a nice looking shape and you can't go far
wrong.
You can get a good idea of the shape in the air now by pulling the
nose or tail away from the main bunch.
The (almost) finished shape from the wing shown hung up can be seen
here.
That's about it. You're ready to try it on just the bridle. It's best to try it in a not too light wind, preferably a steady one. For four lines, attach the bridle bundles with a larks head to the handles. Pull it good and tight so no lines can slip and swing it up into the wind. Try to get it up high enough so that your body doesn't interfere with the wind flow.
If you had sufficient wind one of three things should happen, hopefully
#1.
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| 1) It will fly. Yippee. | Spend some time examining the wing closely to iron out any wrinkles then get out the long strings when you can see that no more fine adjustment is needed (yet). See below. |
| 2) It'll sit on it's flaps and refuse to go up. | Too much brake. Hang it up again and then let out the brake lines a few cm. Check the shape and try again. |
| 3) It'll shoot up and over your head and try to land behind you. | Not enough brake. Hang it up again and pull in the brake lines a few cm. Try again. |
| 4) It'll sort of fly but the nose doesn't inflate properly. | Not enough wing pull in. Hang it up again and pull in the wing points a few cm. Adjust the B2-B5 for a straight line to B1. Try again. |
| 5) It'll fly but you see lumps and irregularities. Take note of which points are affected. | Hang it up again. Can you see the irregularities now? Adjust the specific lines involved and no others. Try again. |
Did I say three? You may get a combination of a couple of points, just take your time and adjust one thing at a time, you can then back track if necessary. Once you've got it flying though, take care. It doesn't take much adjusting to ruin it. Stick to adjusting one thing at a time, you could make notes too so you can back-track.
Angle of Attack
This can be adjusted too but depends on the shape you have as to what
you need. The basic setup above should get it flying. If you increase the
angle of attack you'll get more pull and the kite won't fly so high, and
the converse. If the angle of attack is too great it might not take off
at all. You can adjust the angle by hanging it up again and changing the
angle of the whole wing to the floor (from nose to tail). As the hung kite
is upside down raising the nose with respect to the tail will decrease
the angle of attack and lowering it will increase it.
Tips
Once you have a wing flying just on the bridle it's time to really
test it.
It can help enormously if you get someone else to fly it so you can
examine the wing from different angles. When I got some pictures of "the
big blue boggart" (shown below) I saw straight away a couple of points
which could be improved on. I hadn't seen the wing from these angles before!
You will need a way of adjusting the brake lines for differing wind speeds. If you are flying four line then you can simply add some really strong line by the handles for the brake lines. Say 30 cm knotted every couple of cm. You will be attaching the main brake lines to this piece with a lark's head. For two line flying use the method given in the plans.
Long Lines
Tie the bridle lines together. It can be an advantage to leave them
as bunches at first as it makes adjustment of wing pull-in and amount of
brake easier.
I'm assuming you have equal lengths of all four lines already wound
up on your handles. Make sure the winding is in the same direction on both
ends of the handles so they will unwind together.
Take the wing outside and launch it by the bridle. Take the bridle down
to the ground and get someone to help you collapse it. Loop the bridle
over a stake or large nail driven into the ground, one for each side. Lay
something on the trailing edge of the NPW to hold it down if it's very
windy.
Lark's head your two or four lines to the bridles and walk backwards
paying out the lines. Let out say 8m of line at a time then get them even
and take the lines through the slots in the handle. This'll give you roughly
the right length for all four lines. Lauch now with a couple of good tugs
and maybe a few steps backwards. If it just sits there on the ground the
brake lines must go out some. The more you let them out the higher it'll
fly and it'll pull a little bit less. If you let out too much you'll run
into trouble with the nose not inflating so take it slowly testing a turn
at a time and make sure it's even left to right. You can then adjust the
lines for left and right lengths and degree of brakes to get it flying
evenly with no tendency to turn when the handles are even. At this point
you can mark the lines at the handle so you've got a marker for 8m line
flying.
Go back to the bridle fixed to the ground and let out another 8m of
line. Repeat this until you've got all the line out. With all the line
out you can have an adjustable brake by attaching to the knotted thick
line fixed to the handle. This is necessary for varying winds. With less
than the whole line out you just unwind one turn from the handle and take
it back through the slot.
Modifications
Nasa did a lot of testing of these wings back in the seventies I believe
and came to the conclusion that the NPW FIVE was the best shape. That said,
I have successfully made an 8 sq.m wing with radial spines from one quarter
of an old parachute. The material was very porous so it didn't turn out
as a real manlifter.
I've also made the 5 sq.m shown below with the wing tips cut off to
maximise on the use of the material I had. No problems. Just needed to
place B5 and B6 a little differently.
| The latest wings I've made with only one nose
tuck on each side. The one shown above is also like this.
The line bc is folded over to the line ef. The length of material we
need to "lose" is abcd minus aefd, ie the triangle "g".
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Take A1 in this case to the point "C" shown on the left. |
| If you want to get really radical then you can try skipping the A lines completely apart from the brake lines. I discovered this by accident when one set of A lines fell off while I was adjusting. The kite continued to fly and I didn't notice immediately what had happened. | Further experiment showed that it would indeed fly without A1-10. The adjustment of the wings and angle of attack becomes very critical but in a steady wind it works. I've only used this on smaller 2 line models. |
 On
the left you can see B4 needs shortening a little as does B6 (a very little).
Back. |
Here you can see the profiling nearly
finished. The nose has yet to be pulled up a bit on A1 and A2. You can
see they look a bit slack. Also there's a slight jump up from A8
to A9. A9 and A10 will be lengthened to lower these to the same line
as A5-A8.
Back.. |
The
small kite in the background is tethered to a tee line which runs from
to a tree to a flagpole. I've used a small 3 way swivel on the tee to prevent
most of the tangles. The kite will fly all day, or all week, all by itself.
This picture showed me I needed to pull in the nose a little more (a couple
of cm). You can see a slight fold behind the nose. The pull increased when
I did this too. Better airfoil shape, but necessitated a little more wing
pull in to increase the pressure behind the nose.
I
like this picture (my wife took it). It really shows the final shape well.
How many further adjustments can you see here? No prizes.
Handles on a shoestring.
A piece of broom handle around 25cm long will suffice. Drill holes
a couple of cm from the ends and countersink a bit, then saw a slot up
to the holes. Sand the holes and slots well so they don't wear the lines.
Take short lengths of real thick string (shoestrings will probably be ok)
or sleeving (ideal) and knot them so they can't slip through the holes.
The brake line ends need to be around 30cm and knotted every couple of
cm for brake adjustment for four lines.
Back
Diagram of the Lark's head you can find on Simo Salanne's site. It can
help removing the larks head if you tie a thumb knot at the last half cm
of the loop. It gives something to get hold of.
http://www.kfs.org/kites/simo/knots.htm
"The earth is flat, pigs can fly and nuclear power is safe."
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