Stunt Kite Design Question
Lloyd Paul Verhage
When designing model rockets, or airplanes, stability depends on the
relative positions of the center of gravity (CG) and the center of
pressure (CP). The CP is the "idealized" center of all areodynamic
forces and CG is the "idealized" center of mass (weight) and is the
point the rocket or plane will rotate about.
The most stable model rockets have the CP behind the CG. That way an
areo forces cause the rocket to pivot back to a nose up attitude.
The further back the CP from the CG, the more stable the rocket (there
are limits though). If the CP is above the CG, the rocket will flip
over (very unstable).
In modern high performance jets, they place the CG and CP very close
together. This allows the jet to spin in a snap, but makes it
difficult to control, hence computer controls.
Now on to kites (did you ever think I'd get there?). I suppose in the
radical designs, the CG and CP are very close together. In beginners
designed stunt kites, I imagine the CP and CG are further apart.
Further, if the CP is in front of the CG, then the kite will be unstable.
Is this in fact used to design stunt kites?
Does anyone take this into consideration when building stunt kites?
Thanks
Michael Graves
In article <3jkd1a$s...@fox.ksu.ksu.edu>, Lloyd Paul Verhage (lv...@ksu.ksu.edu) writes:
[snip]
>
>Now on to kites (did you ever think I'd get there?). I suppose in the
>radical designs, the CG and CP are very close together. In beginners
>designed stunt kites, I imagine the CP and CG are further apart.
>Further, if the CP is in front of the CG, then the kite will be unstable.
>
>Is this in fact used to design stunt kites?
>Does anyone take this into consideration when building stunt kites?
I use center of pressure on each half of the kite to initially
determine bridle dimensions on a new design. The optimal tow point
usually falls within 2" (in any direction) of CP.
Just quickly considering some of the kites I've made, they most
often have CP located close to CG. Mark Cottrell's book "Swept
wing delta stunt kites" has a brief section considering this issue.
Michael Graves
Marty Sasaki
In article <3jkd1a$s...@fox.ksu.ksu.edu>, lv...@ksu.ksu.edu (Lloyd Paul Verhage) writes:
|>When designing model rockets, or airplanes, stability depends on the
|>relative positions of the center of gravity (CG) and the center of
|>pressure (CP). The CP is the "idealized" center of all areodynamic
|>forces and CG is the "idealized" center of mass (weight) and is the
|>point the rocket or plane will rotate about.
With kites it is a bit different. Because they are attached to a
flying line, or lines, the CG alone isn't sufficient to figure out
where a kite will rotate and how it will fly.
The center of pressure is also very difficult to calculate since it is
a function of the lift and drag, which are functions of the real wind,
and the velocity of the kite. As a kite, especially a stunt kite moves
through the window, the angle of attack is constantly changing. The
changing angle of attack directly affects the lift. I suspect that the
CP is constantly moving while a kite is in motion.
|>Now on to kites (did you ever think I'd get there?). I suppose in the
|>radical designs, the CG and CP are very close together. In beginners
|>designed stunt kites, I imagine the CP and CG are further apart.
|>Further, if the CP is in front of the CG, then the kite will be unstable.
Moving the center of gravity upward does make a kite more stable,
especially in really light wind where gravity is a large portion of
the force on a kite.
|>Is this in fact used to design stunt kites?
|>Does anyone take this into consideration when building stunt kites?
When I frame a kite with a tapered spine, I put the larger diameter
(heavier) part of the spine closer to the nose of the kite. My kites
generally fly better (more control) when they have some nose weight.
In fact, the Katana flies better in really light air with a top
spreader than without one, mostly due to the weight, and the placement
of the center of gravity.
I think that most kite design is done by "seat of the pants" or SWAG
(Scientific Wild-Ass Guessing) engineering. Even those who use
computer modeling and such can be seen "tweaking" their designs to get
them to perform properly.
--
Marty Sasaki Harvard University Sasaki Kite Fabrications
sas...@noc.harvard.edu Network Services Division 26 Green Street
617-496-4320 10 Ware Street Jamaica Plain, MA 02130
Cambridge, MA 02138-4002 phone/fax: 617-522-8546
Brian W. Gordon
: I have a question about kite design.
: Thanks
This may or may not be in line with what you are talking about, but...
I recently built a prototype kite based on the Air Force's X-29
experimental aircraft. Like the jet, the kite had FORWARD swept wings
with a smaller delta shape configuration towards the rear.
Picture a large"v" with a smaller upsidedown "v" placed under the large
"v". Anyhoo, as you can imagine the kite's CP was well in front of its
CG and hence was EXTREMELY unstable. After about an hour of messing with
bridle I gave up. I think the longest flight before a crash was 15
seconds. The bridle was configured for two-line flight, but I think
4-line would only be slightly better. Needless to say I tossed the whole
heap of Tyvek and Rammin dowels into the dumpster on the way back to the
car. I have not given up on this concept entirely. I just don't have
the time, money and effort to put into this sortof thing just to see if
it will work.
The key to making most kites stable or unstable lies in the bridling
(some kites are just inherently unstable, see above). I once heard you
can fly a Mac Truck if you bridle it right.
Hmmm. I wonder of the same exact laws you spoke of (CP, CG) apply to a
tethered object like a kite as they do a free flying object like a rocket
or plane.
Keith Kidder
Brian Johnsen
Michael Graves <mgr...@leadingedg.win.net> wrote:
>In article <3jkd1a$s...@fox.ksu.ksu.edu>,
>Lloyd Paul Verhage (lv...@ksu.ksu.edu) writes:
>[snip]
>>Now on to kites (did you ever think I'd get there?). I suppose in the
>>radical designs, the CG and CP are very close together. In beginners
>>designed stunt kites, I imagine the CP and CG are further apart.
>>Further, if the CP is in front of the CG, then the kite will be unstable.
>>
>>Is this in fact used to design stunt kites?
>>Does anyone take this into consideration when building stunt kites?
Flying my single line stunt kites (fighters), when I give a tug on the
line the trailing edge folds up and unloads moving the CP forward in a
predictable and controllable fashion resulting in a nice straight
trajectory.
Giving the line slack moves the CP to the back resulting in the kite
going all wobbly again. It's a very important part of the design.
--
"Mike Lowery is the Nephew Of My Child", Warns Annoyed Troll
--
Brian Johnsen joh...@eskimo.com Seattle, Washington USA
Philippe Lepez
>I use center of pressure on each half of the kite to initially
>determine bridle dimensions on a new design. The optimal tow point
>usually falls within 2" (in any direction) of CP.
Michael,
Can you explain how you measure the CP location ?
Good winds.
Philippe.
--
Philippe Lepez (CP 125), | | Good kite
Universite Libre de Bruxelles, | Email: ple...@ulb.ac.be |+ Good wind
50 av. Roosevelt, | Phone: 32.2.6503553 |----------------
1050 Bruxelles, Belgique. | Fax : 32.2.6503323 |= A lot of fun !
Philippe Lepez
>Moving the center of gravity upward does make a kite more stable,
>especially in really light wind where gravity is a large portion of
>the force on a kite.
Marty,
I'm not sure in case of kite of what stability means.
Once I build a kite that was kite similar to an MEFM with a lower aspect
ratio. In low wind it has a tendency to flop on its back, tail first.
The obvious solution was to move the CG forward. I had nothing at hand but
coins and tape. So I taped coins to the top spreader. The kite didn't show
this problem anymore : you probably would have say it has a better stability.
But, the general behavior has completely changed. It has become much more
touchy. I was unable to slide it anymore : much less control and the kite
would rotate and drop a wing or another. The oversteer was terrible. I
would say that stability was worst !
Finally, I removed the coins. Maybe should I have re-tuned the bridle.
Now, if someone can give some insight on this experiment and how to obtain
the best of both situation, I would appreciate it.
Lloyd Paul Verhage
>In article <17...@leadingedg.win.net>, mgr...@leadingedg.win.net (Michael Graves) says:
>>I use center of pressure on each half of the kite to initially
>>determine bridle dimensions on a new design. The optimal tow point
>>usually falls within 2" (in any direction) of CP.
>Michael,
>Can you explain how you measure the CP location ?
Correct me if I'm wrong, but......
I would make a paper cutout of the kite and find the cut-out's
center of gravity.
Marty Sasaki
In article <3k46c4$b...@cbs.ksu.ksu.edu>, lv...@ksu.ksu.edu (Lloyd Paul Verhage) writes:
|>ple...@ulb.ac.be (Philippe Lepez) writes:
|>>Can you explain how you measure the CP location ?
|>
|>Correct me if I'm wrong, but......
|>I would make a paper cutout of the kite and find the cut-out's
|>center of gravity.
That's not a bad approximation when the kite is at rest. The exact
calculation can get pretty complicated. The center of pressure is a
function of the lift and the drag over a surface. Both lift and drag
are vector quantities (they have a direction as well as a magnitude)
and both vary over the surface.
For a wing in motion, more lift is generated (for a given area)
towards the leading edge of a wing than towards the trailing edge. The
center of pressure is therefore further forward on a wing than the
center of gravity of a cutout.
Michael Graves
In article <3k4i71$l...@netope.harvard.edu>, Marty Sasaki (sas...@netopd.harvard.edu) writes:
>
>In article <3k46c4$b...@cbs.ksu.ksu.edu>, lv...@ksu.ksu.edu (Lloyd Paul Verhage) writes:
>|>ple...@ulb.ac.be (Philippe Lepez) writes:
>|>>Can you explain how you measure the CP location ?
>|>
>|>Correct me if I'm wrong, but......
>|>I would make a paper cutout of the kite and find the cut-out's
>|>center of gravity.
>
>That's not a bad approximation when the kite is at rest. The exact
>calculation can get pretty complicated. The center of pressure is a
>function of the lift and the drag over a surface. Both lift and drag
>are vector quantities (they have a direction as well as a magnitude)
>and both vary over the surface.
I've never been asked to describe my method before, so I had to
think about it for a while. It's not the most scientific.
I find the distribution of sail area by bisecting each of the
three angles that forms each half of the kite. This yields a decent
static center of pressure, and a reasonable spot to center the tow
point for sake of experimentation.
Recently I have been trying to compensate for the fact that more
lift is gained along the leading edge. Just as an experiment I
shifted that static CP by an amount defined loosly by the
relationship between the linear amount of trailing edge/leading
edge. Like I said, not founded in the best science. It has,
however, helped me to do this experiment. I am yet sure of the
resulting bridle.
Michael Graves
Matthew Ward
location of the cg relative to the center of pressure.
I am certainly no expert, but I am an aerospace engineering student, and
I've been flying kites for a few years. I thought I'd mention
that the bridles are what always makes the difference between a stable
and radical kite. So I think that it matters more where the tow points on
the bridles are relative to the center of pressure. My thought is that
the only difference the cg position and the distribution of mass make is
in its turning ability.
Pleasant breezes,
Matthew Ward
Marty Sasaki
In article <Pine.SUN.3.91.950315191037.3451A-100000@sun14>, Matthew Ward <wa...@db.erau.edu> writes:
|>I am certainly no expert, but I am an aerospace engineering student, and
|>I've been flying kites for a few years.
This makes you as much an expert about these things as anyone...
|> I thought I'd mention
|>that the bridles are what always makes the difference between a stable
|>and radical kite. So I think that it matters more where the tow points on
|>the bridles are relative to the center of pressure. My thought is that
|>the only difference the cg position and the distribution of mass make is
|>in its turning ability.
In heavier winds I believe that center of gravity isn't that
important. In light winds it can be very important. This is because
the other forces are no longer dominate over gravity.
I agree that bridle position is one of the most important
considerations in stunt kites. Not only does the bridle change the
angle of incidence of a kite, but it also changes the shape of a
kite...
Andrew Hawken
: I agree that bridle position is one of the most important
: considerations in stunt kites. Not only does the bridle change the
: angle of incidence of a kite, but it also changes the shape of a
: kite...
I have been playing the the bridle on my Katana recently, trying to
drop the tow point towards the tail, and I find that once the kite is
in a stall, that it is very nose heavy, and it is difficult to keep the
stall stable. I guess that this is one place where the gravity does
overcome the other forces.
Andy
--
____________________________Andrew Hawken______________________________
"However many ways there may be of being alive,
it is certain that there are vastly more ways of being dead"
R Dawkins.
Home : 0895 420110 QMW : 071 975 5542
AIIT : 0494 677045 Email : A.Ha...@QMW.AC.UK
DStoutFAD
change in the angle of attack. The angle of attack depends on the sail
washout angle, and the center of gravity. The theoretical location for
the CP is always assumed to be at the 1/4 chord location of the entire
projected view of the flying platform.
With stunt kites, the center of gravity (CG) is below this location,
which is why kites stay out at the end of the lines. To determine where
your CG should be located you must do the following:
1. Calculate the panel aerodynamic center (PAC) for each unique vertical
geometric shape, called panels, of the stunt kite sail. The PAC will be
"Y" distance below the nose and "X" distance from the root chord of the
panel.
2. Use the panel aerodynamic centers to calculate the overall aerodynamic
center (AC) for the stunt kite sail. The AC will be "Y' distance below
the nose of the sail and "X" distance from the center spine for each half
of the sail.
3. Measure the CG from below the Nose of the sail. Fully assemble the
stunt kite and balance it on the center spine, horizontal to the ground.
4. Calculate the Static Margin (SM) for the stunt kite. Good team and
percision stunt kites have SMs ranging from 42 to 48 percent. Good ballet
stunt kites have SMs ranging from 48 to 49.9 percent. If the SM is
greater than 50 percent, the stunt kite will remain in a stall. The
various models of our Talon, for different wind conditions, were designed
for team, precision, and ballet and all have the same SM of 48.5 percent.
You can also adjust our CG by rotating the center spine and placing the
joiner low for ballet or high for percision, if your design allows. We do
this on our Talon-MF. This can change the CG about 1/4 inches or change
the SM 1.0 percent, which you will notice when flying.
As you fly your stunt kite from straight flight into a stall, the CP moves
foward from the 1/4 chord, until it stalls and the aerodynamic lift on the
back of the sail is broken, which causes the CP to move quickly back to
the 50 percent of the chord. The lower the SM percentage, or the closer
the CG is to the nose, the more quickly the stunt kite will recover from
the stall. Stunt kites with SM less than 40 percent also will have much
smaller wind windows. Stunt kites, such as our Talon with 4 standoffs
which provide washout angle for the tip panels, insure that the tips never
stall so that control is present when the center of the sail has lost
aerodynamic lift.
I hope this answers your questions. We have been using aerodynamics as
the basis for our stunt kite designs since 1990. If you need any
additional help, call be at (201) 347-4173 in the USA or sent a message to
the address provided below.
Sincerely yours,
Douglas K. Stout
Falcon Aero Designs
"Designing for the Future"
DStoutFAD@AOL
12-A Mayne Avenue
Stanhope, New Jersey 07874
DStoutFAD
Your aerodynamic center (AC) for each half of your stunt kite sail is the
where you first locate your bridle location. For each half of your sail,
the AC will be "X" distance from the center spine and "Y" distance below
the nose. Your bridle point directly over the AC "X" distance and about 1
to 3 degrees above the "Y" direction. The "Y" location of your bridle
point (BP) depends directly on the center of gravity, static margin and
the sail washout angle. This location is used as a reference point. For
percision, we move the BP 1 to 2 degrees out for our Talons. This is done
by shortening the outhaul. For ballet, we move the BP 3 to 4 degress out
for our Talons.
Sincerely yours,
Douglas K. Stout
Falcon Aero Designs
"Designing for the Future"
12-A Mayne Avenue
Stanhope New Jersey 07874
(201) 347-4173
DStoutFAD@AOL
DStoutFAD
gravity and the amount of washout angle at the tips dictate stability.
The horizontal center of gravity is were the stunt kite would balance, if
cut in half and measured from the center spine to the tip.
In almost all stunt kites, the hoizontal center of gravity is outside the
aerodynamic center (AC), further from the center spine than the AC. This
means the mass of the stunt kite will cause instability in a stall if the
tip panels are also in a stall.
In our Falcon, the standoffs were located at the horizontal center of
gravity, outside the AC, but the tip panels had zero washout angle, thus
oversteer and no stability in a stall. The zero washout angle is because
our designs use a cambered leading edge and leach line to maintain
silence. This does not allow the panels outside the standoffs to billow
and washout while flying.
In our Talon, we located the inner standoffs at the AC and the outer
standoffs provide 10 degress of washout angle. The tip panels almost
never stall under most flying conditions, thus providing great stability
in a stall, and speed control under all wind conditions.
Sincerely yours,
Douglas K. Stout
Falcon Aero Designs
"Designing for the Future"
12-A Mayne Avenue
Stanhope, New Jersey 07874
(201) 347-4173
DStoutFAD@AOL
Andrew Beattie
>4. Calculate the Static Margin (SM) for the stunt kite. Good team and
>percision stunt kites have SMs ranging from 42 to 48 percent.
Hmmm... I'm trying hard to follow this, but you didn't say what the SM is
or how to calculate it...
Andrew
--
New to rec.kites? START HERE! > To: w...@kfs.org
send an email message like this: > Subject: service
> http://www.kfs.org/kites/welcome/index.html
The proposal to split rec.kites is just a bad joke
Marty Sasaki
In article <3kbq68$p...@epsilon.qmw.ac.uk>, andrewh@hal (Andrew Hawken) writes:
|>I have been playing the the bridle on my Katana recently, trying to
|>drop the tow point towards the tail, and I find that once the kite is
|>in a stall, that it is very nose heavy, and it is difficult to keep the
|>stall stable. I guess that this is one place where the gravity does
|>overcome the other forces.
I'm not sure what you mean by nose heavy. Does this mean that it falls
away or towards the flyer? Does this mean that the nose drops to one
side or the other?
When a kite, any kite, not just the Katana, is stalled, the angle of
attack is very high and the lift is correspondingly low. Small forces
can easily upset the equilibrium of a kite that is stalled. Also,
remember that the Katana is meant to be somewhat radical. It is the
ability to respond to small changes that makes kites like the Katana
fun to fly...
Marty Sasaki
In article <3khs94$s...@newsbf02.news.aol.com>, dsto...@aol.com (DStoutFAD) writes:
|>As with all flying platforms, the Center of Pressure (CP) moves with a
|>change in the angle of attack. The angle of attack depends on the sail
|>washout angle, and the center of gravity.
The angle of attack also depends on the velocity of the kite (this
contributes to the apparent wind velocity). The shape of the kite also
changes the angle of attack.
The CP is also dependent on the shape of the sail, especially the
profile. The shape changes with wind pressure, further complicating
matters.
...lots of interesting calculations deleted...
After all of the calculations are done, you still have to take your
kite out and test fly it. Static calculations put you in the right
ball park, but there is no substitute for experimentation and fine
tuning.
|>As you fly your stunt kite from straight flight into a stall, the CP moves
|>foward from the 1/4 chord, until it stalls and the aerodynamic lift on the
|>back of the sail is broken, which causes the CP to move quickly back to
|>the 50 percent of the chord. The lower the SM percentage, or the closer
|>the CG is to the nose, the more quickly the stunt kite will recover from
|>the stall. Stunt kites with SM less than 40 percent also will have much
|>smaller wind windows.
Doug, what exactly do you mean by stall here? Most aerodynamic texts
talk about the stall angle as the point where lift drops dramatically.
Stunt kites operate at angles of attack both above and below the stall
angle.
In an airplane, as the angle of attack increases, the CP moves forward
which causes the nose of the plane to rise, which increases the angle
of attack, etc. Just after the stall angle, the lift drops
dramatically, and the nose of the plane drops. There is usually a
decrease in the forward speed as the nose rises, which also decreases
lift.
With a kite, the bridles keep the kite's nose from rising. If the kite
is rising from the ground, it's path will actually cause a decrease in
the angle of attack.
During a snap stall or a similar move, the forward speed of the kite
is suddenly decreased, which causes a sudden increase in the angle of
attack. The resulting angle of attack is very high, well behold the
stall angle, and the lift drops dramatically.
In both cases there is really no recovery phase...
!> Stunt kites, such as our Talon with 4 standoffs
|>which provide washout angle for the tip panels, insure that the tips never
|>stall so that control is present when the center of the sail has lost
|>aerodynamic lift.
Well, the tips stall later than center panels, which is a good thing,
especially in tight turns...
Kites with winglets, like the Katana-2 and the Freestyle Reactor (to
name two) have slightly different characteristics. Increased wind
pressure on the winglets cause them to bend backward which increases
the washout, delaying tip stalling. You can also snap these kites in a
way to cause the winglets to snap forward, which increases the angle
of attack and actually encourages stalling conditions. Snap stalls
with these kites can be really sharp and stability in the stall is
good.
Keep up the good work Doug...
DStoutFAD
distance between the Aerodynamic Center for the Sail and the Center of
Gravity, divided by the mean chord for the sail. You wish to obtain a
Static Margin within the ranges I have previously stated. When we design
a stunt kite using our Geokite(c) program, we input all the specifications
of the design, along with the mass of each component. We adjust the
design, which moves the calculated Center of Gravity until we obtain the
desired Static Margin. With a completed design, you can add weight to the
nose or tail of the center spine to adjust the Center of Gravity and
achieve the desired Static Margin.
Sincerely yours,
Douglas K. Stout
"Designing for the Future"
12A Mayne Avenue
Stanhope, New Jersey 07874
(201) 347-4173
DStoutFAD
rapidly back to the 50 percent of the mean aerodynamic chord. Airplanes
recover because the Center of Gravity is usually located between 25 and 35
percent of the mean aerodynamic chord, with an airplane containing a
horizontal tail surface. The distance between the Center of Gravity and
the 50 percent location allows for rapid recovery from the stall. Static
Margins for this type of airplane are ususally 10 to 20 percent. This is
what we used in my Control Line Percision Aerobatic Stunt Ships called
"Apparition" and "Apparition II". A flying wing must have the Center of
Gravity infront of the 1/4 Chord location for flight. Canards are even
more fun to establish the correct Center of Gravity. We designed a
Control Line Canard called "ISIS", which flew great, with the Center of
Gravity About 20 percent in front of the main lifting surface.
Most stunt kites have the Center of Gravity below the 1/4 Chord location,
which allows dramatic changes in the angle of attack to the direction of
the prevailing winds, without the design droping its nose and trying to
recover like an airplane. I have some old delta designs which had a high
Center of Gravity, and when you stalled the design overhead, it would drop
the nose and try to recovery like a conventional airplane. We extended
the center spine below the sail to lower the Center of Gravity, and the
undesired recovery problem went completely away. The wind window also
increased. That is why good stunt kite designs have a low Center of
Gravity, just above the 50 percent location of the mean chord.
As previously stated, when you leach a sail to gain more aerodynamic
efficiency, the outer panels can not billow and provide natural washout.
Washout is defined as a twist in the chord of the airfoil which allows the
panel which has been washed out to fly at a lower angle of attach that the
rest of the sail. Most aircraft washout the tip panels 2 to 3 degrees to
insure that the tips of the aircraft will remain flying and provide
control during landings, when the aircraft is flying very close to it
stall speed. When a stall occurrs in an aircraft with washout, the stall
starts at the center of the wing and then moves gradually out toward the
tips, thus maintaining lift over the surfaces that contain the ailerons,
allowing the pilot to maintain control, as long the tips don't stall. On
stunt kites, we impose a much greater change in the angle of attach, which
means the natural washout angle of the tip for an unleached sail or the
mechanically place washout angle in a leached sail must be much greater.
We have been experimenting with washout on our designs for over 3 years
and have found that on leached designs, an angle of 10 to 15 degrees is
about the right range. In fact, we install longer (1/4 inch) outer
standoffs to fly in winds greater than 15 miles per hour. This provides
more stability, but also induces more profile drag, which provides
additional speed control.
Dave Lord and I have experimented with wider tips and have found that
under turbulent conditions, tracking suffers. We found that additional
standoffs, used to generate the washout of the tips, provide the same
effect as the wider tips, but work in clean and turbulent air.
In closing, the detailed aerodynamic calculations or modeling that we
conducted to design our Talon allowed this design, including bridle
lengths, to be constructed and flown in competition during the 1994
contest season, right from the output of our computer modeling program.
The prototype was not changed in any way and was used for production. We
attribute this success and the same success of our recent design, the
Talon-2M, to the extensive modeling capabilites contained in our
Geokite(c) Program. This program was developed and has been extensively
improved for over 10 years. Calibrating the program during the
development of the Falcon in 1990 and 1991 was a lot of work, but the work
is now done of this generation of the computer model. We take the output
of Geokite(c) and draft up full size engineering plans in AutoCAD. This
ensures that the modeled design is provided within specifications for
sewing.
If you wish to also evaluated true airfoils, let me know. We also have a
PC based airfoil section wind tunnel computer model developed by Dr.
Eppler and refined through the use of a wind tunnel by Mike Selig at
Princeton University. It handes Reynold Numbers from laminar flow (
50,000) to turbulent flow (500,000). The program was design around the
transitional zone, which causes a lot of fun in model sailplanes. When we
used to design Radio Controlled Sailplanes, we would first develop an
airfoil which was tested using the wind tunnel program. Then we would
design the aircraft using the pre-program to Geokite(c) called Geosail(c).
Finially, we would run a performance program to generate Lift to Drag
ratios for the airframe, over a range of angles of attack. Since we do
not use true airfoils, we don't run the first program, and since it is
much quicker to construct a stunt kite than a 10 foot sailplane, we don't
see the need to run the performance program. The flying field is a much
better wind tunnel.
I hope you enjoy the above. It is rare when I get a change to relate this
type of information to others. I hope my spelling and grammar wasn't too
bad.
David Lord
>From: dsto...@aol.com (DStoutFAD)
>Subject: Re: Stunt Kite Design Question
>Date: 22 Mar 1995 23:47:31 -0500
>Dave Lord and I have experimented with wider tips and have found that
>under turbulent conditions, tracking suffers. We found that additional
>standoffs, used to generate the washout of the tips, provide the same
>effect as the wider tips, but work in clean and turbulent air.
Doug
I agree with what you say except I think that if you design the correct
size winglets to go with the proper spring constant (leach line tension)
then you can achieve the same results you get with fixed twist. And you
would get the added benefit of a wider wind window. It might be pretty tough
to model this in the computer program.
C.G. is most important in the lightest of winds and kites designed for these
winds have been a design struggle to keep them as light as possible so adding
ballast to trim C.G. is usually avoided like the plague. So one must resort to
changes in spreader positioning and sail area and aspect ratio to move the
C.G. The Banshee kite designed by Skip Parks was the only design I ever saw
that had a forward C.G. Its structure consisted of a bowed leading edge and a
short spine and nothing else, no lower or upper spreader. There were some wing
battens but they had little effect on C.G. I thought the Banshee handled
pretty good in light wind.
Dave Lord
lo...@eskimo.com
http://www.eskimo.com/~lord check it out
Michael Graves
In article <navarrolD...@netcom.com>, Larry Navarro (nava...@netcom.com) writes:
>
>have on the spin characteristics. The kite also has the
>upper spreader attached to the spine through a vinyl T
>connector, similar in design to many of the Prism kites.
>I'm also wondering what advantage this gives to the kite.
While Doug will likely respond in detail, my impression of the T on
the upper spreader is that it increases overall frame stiffness.
Just for sake of curiousity, what rod do the Talon use for the
upper spreader. The only kite I've flown with an upper T was the
Lite Flite Radial, which broke its upper spreader very frequently.
Michael Graves
Larry Navarro
I had a chance to fly a Talon this weekend for a short while
and do agree that the kite does have a very steady stall. The
pushed back wing tips seem to do their job. It was similar
in stall performance to the Reactor, which we were flying that
day. The Reactor has winglets to steady it in a stall. The
advantage to the Talon was that it would perform the radical
stunts like axles and coin tosses way easier than the Reactors
do. One thing I did notice about the Talon is that pulling
a hard turn to initate a spin will cause the kite to actually
slow down and plow slowly through the turn, as if it was floating
through it. The adjustable outhauls were set to the second
to longest position, and I don't know what effect this would
have on the spin characteristics. The kite also has the
upper spreader attached to the spine through a vinyl T
connector, similar in design to many of the Prism kites.
I'm also wondering what advantage this gives to the kite.
David Lord
>From: nava...@netcom.com (Larry Navarro)
>Subject: Re: Stunt Kite Design Question
Larry
Back in the November 91 issue of the AKA magazine Kiting I wrote an
article for the "Art and Science section called "Optomizing Delta Stunt KIte
Frames. Contained in the article is an analysis of the loads on the frame and
ways to strengthen the frame against these loads. One of the techniques was to
tie the upper spreader to the spine. The Prism people told me that after they
read the article and tried connecting the upper spreader to the spine it
increased the upper wind range on the Prism Radian by 10 mph.
Dave Lord
check out my web site http://www.eskimo.com/~lord
Larry Navarro
>upper spreader. The only kite I've flown with an upper T was the
>Lite Flite Radial, which broke its upper spreader very frequently.
>
>Michael Graves
>
top spreader. The top spreader/spine connector was a piece of
vinyl tubing, which I think might cause lower stress on the rod
than a stiffer connector like those used in my Eclipse.
Larry Navarro
Marty Sasaki
In article <lord.21....@eskimo.com>, lo...@eskimo.com (David Lord) writes:
|>>Dave Lord and I have experimented with wider tips and have found that
|>>under turbulent conditions, tracking suffers. We found that additional
|>>standoffs, used to generate the washout of the tips, provide the same
|>>effect as the wider tips, but work in clean and turbulent air.
|>
|>Doug
|> I agree with what you say except I think that if you design the correct
|>size winglets to go with the proper spring constant (leach line tension)
|>then you can achieve the same results you get with fixed twist. And you
|>would get the added benefit of a wider wind window.
I've done some expermenting with both winglets and standoffs to have a
fixed lower limit on the washout. Things have been promising, and I'll
post details after the next prototype is finished.
I also think that winglets provide a better tip shape than what we
usually have with a very pointed tip. Theoretically, the point that is
at the end of a stunt kite should generate the greatest amount of tip
drag...
David Lord
>From: sas...@netopd.harvard.edu (Marty Sasaki)
>Subject: Re: Stunt Kite Design Question
>In article <lord.21....@eskimo.com>, lo...@eskimo.com (David Lord) writes:
>|>>Dave Lord and I have experimented with wider tips and have found that
>|>>under turbulent conditions, tracking suffers. We found that additional
>|>>standoffs, used to generate the washout of the tips, provide the same
>|>>effect as the wider tips, but work in clean and turbulent air.
>|>
>|>Doug
>|> I agree with what you say except I think that if you design the correct
>|>size winglets to go with the proper spring constant (leach line tension)
>|>then you can achieve the same results you get with fixed twist. And you
>|>would get the added benefit of a wider wind window.
>I've done some expermenting with both winglets and standoffs to have a
>fixed lower limit on the washout. Things have been promising, and I'll
>post details after the next prototype is finished.
>I also think that winglets provide a better tip shape than what we
>usually have with a very pointed tip. Theoretically, the point that is
>at the end of a stunt kite should generate the greatest amount of tip
>drag...
Marty
Winglets do provide a better wing shape in terms requiring less washout to
eliminate tip stall in turns. If they are large enough they can be designed to
have no washout. I don't think pointed tips have any special characteristic to
create drag its just that they do not create an appreciable lift, either plate
(reaction) or Bernoulli. They are just there for the ride and create drag with
out creating any aerodynamic benefit. Larger tips move the tip vortex further
outboard and thus reduce their effect as a percentage of the total wing thus
reducing drag. They also contribute to plate lift. Most kites are so draggy
though I doubt this is much of a factor. It is probably in the handling area
where you will notice the improvement. Doug is also correct in that if your
primary goal is tracking then you want the wing tips to be relatively
inactive. He added just enough washout to get rid of the worst of tip
stalling. If you check my home page you will see my kite design of the month
posted. It is quite similar to your Katana II (I think that is the one, we
don't see them here in the northwest). When I designed it I had no knowledge
of your design and as obscure as I am I know you knew nothing of mine. But
they were both probably reactions to the same aerodynamic problem in delta
stunt kites, tip stall in turns. I started out with pointed tips and outboard
standoffs but quickly realized dynamic washout was the way to go. I have
designed large tips on some kites,but it is difficult to react the aeroloads
on the tips to limit washout. Alas, first too little then too much and a whole
new set of structure problems. Oh well, buggying is more fun anyway so I am
not very driven to work on the problem.
Dave Lord
check out http://www.eskimo.com/~lord
DStoutFAD
Test dive the Talon and make a 90 degree turn before you reach the ground,
using push/pull input. If the line tension starts to get lighter, move
the clip down, away from the nose, about 1/8 inches. If the line tension
starts to get stronger, move the clip up, toward the nose, about 1/8
inches. The manual is very detailed regrading this trimming.
Douglas K. Stout
Falcon Aero Designs
"Designing For The Future"
12-A Mayne Avenue
DStoutFAD
eliminate the movement of the upper wing spars during turns, when we use
any of the lighter frames. The upper joiner is standard equipment on our
Talon-F and Talon-MF, with Skyshark IIIP and IIP frames, respectively. We
use Skyshark VP frames for the center spine of these two models. This was
done to bring the center of mass of the design closer to the center and
provide strength and stiffness for our tight sail desigsn. Since our
kites are designed with stiff sails, this allowed the same performance
traits, found in the mid wind speed range, to be achieved at the low and
high ends. After a year of less than 2 mph wind flying, we found that
this help out performance during low wind routines. As Dave Lord stated,
it also maintains performance traits on the high end of the design frame.
Sincerely yours,
Douglas K. Stout
Falcon Aero Designs
"Designing for the Future"
Andrew Hawken
: In article <3kbq68$p...@epsilon.qmw.ac.uk>, andrewh@hal (Andrew Hawken) writes:
: |>I have been playing the the bridle on my Katana recently, trying to
: |>drop the tow point towards the tail, and I find that once the kite is
: |>in a stall, that it is very nose heavy, and it is difficult to keep the
: |>stall stable. I guess that this is one place where the gravity does
: |>overcome the other forces.
: I'm not sure what you mean by nose heavy. Does this mean that it falls
: away or towards the flyer? Does this mean that the nose drops to one
: side or the other?
Sorry, not very clear, what I meant was that it falls off to the
sides. The lower I set the bridle, the easier it is to stall, but
the harder it is to keep the stall stable. I am sure that this is
the same for any kite, I was just curious as to the cause, and wondering
how we can improve the situation. No criticism of the Katana intended,
I love it :-)
Marty Sasaki
In article <3loj4g$m...@epsilon.qmw.ac.uk>, andrewh@holly (Andrew Hawken) writes:
|>Sorry, not very clear, what I meant was that it falls off to the
|>sides. The lower I set the bridle, the easier it is to stall, but
|>the harder it is to keep the stall stable. I am sure that this is
|>the same for any kite, I was just curious as to the cause, and wondering
|>how we can improve the situation. No criticism of the Katana intended,
|>I love it :-)
I didn't take it as criticism. I was just trying to figure out what
you were saying. If the nose were actually falling towards or away
from the flyer, then that would be something completely new...
Some kites are more stable in stalls than others. Some require a
different touch to keep stable.
One thing to try if you are having troubles getting a kite to stall is
to shorten the outhaul bridles. This will often make the kite easier
to stall. You might have to move the center bridle points up to get
back forward speed if you do shorten the outhauls.
Ken Nealey
In a previous article, lo...@eskimo.com (David Lord) says:
Stuff deleted ...
>In article <3ltft9$k...@pobox.csc.fi> sal...@convex.csc.FI (Simo Salanne) writes:
Stuff deleted ...
>>In <lord.22....@eskimo.com> lo...@eskimo.com (David Lord) writes:
Stuff deleted ...
A couple of years ago, I did quite a bit of experimenting with this
concept (tie the upper spreader to the spine). From a structural design
perspective, this is an excellent concept. Further, it does
significantly increase the upper wind range of a given kite.
However, I observed that it can also induce various handling
anomalies. It is ***extremely*** critical that the upper spreader be
exactly centered at the spine connection point. The method Dave describes
(using a short piece of vinyl tubing) is the best overall method to
accomplish this. It is very important that the upper spreader can move
within the vinyl connector. If you choose to utilize a rigid connector
and a 2 piece upper spreader, you should also utilize a 4 point bridle
system. With one leg connected to the upper spreader center fitting.
If for any reason (varying sail loads or control input moments, etc), the
exact center relationship is altered, the turning performance will not be
the same for both left and/or right turns. This condition is due to the
billow of the sail (in the nose area - very critical) being different in the
left and/or right chamber. In general, it is my opinion that this is
more trouble than it is worth.
Leading edge bending (due to control input moments and/or wind load) can
be significantly reduced or eliminated
by the use of proper leading edge connector fittings (no vinyl tubing here) and matching the frame
stiffness to the desired wind range. For a wide wind range kite, my
preference is a tapered wrapped graphite frame with either the Easton
style Skyshark fitting or Power-Tite connector fittings.
Ken Nealey
David Lord
>Subject: Upper spreader/spine connector (Was: Re: Stunt Kite Design Question
>Date: 5 Apr 1995 07:12:41 GMT
>In <lord.22....@eskimo.com> lo...@eskimo.com (David Lord) writes:
>>ways to strengthen the frame against these loads. One of the techniques was to
>>tie the upper spreader to the spine. The Prism people told me that after they
>>read the article and tried connecting the upper spreader to the spine it
>>increased the upper wind range on the Prism Radian by 10 mph.
>>
>What kind of connectors are used to attach upper spreader
>to spine. Piece of vinyl tube with holes at right angle,
>is not very handy, to attach the sticks together the hole
>for spreader should be very tight, which makes assembly/
>disassembly difficult. Using x-connector and 2-piece
>upper spreader puts such a load at the end of spreader
>that it must be much stronger than 1-piece stick.
>Solutions?
Simo
I have been using a short piece of vinyl with holes at right angles. The
connection does not have to be unusually tight. If you look at the force the
connection is trying to react you will see it is partly rotational and partly
lateral displacement. The rotational component tightens the connection before
any significant lateral displacement takes place. Try it on one of your kites
and note the elimination of the bending of the leading edge.
Dave Lord
David Lord
>From: ac...@detroit.freenet.org (Ken Nealey)
>Subject: Re: Upper spreader/spine connector (Was: Re: Stunt Kite Design Question
>Date: 5 Apr 1995 20:55:34 GMT
>In a previous article, lo...@eskimo.com (David Lord) says:
>Stuff deleted ...
>>In article <3ltft9$k...@pobox.csc.fi> sal...@convex.csc.FI (Simo Salanne)
>writes:
>Stuff deleted ...
>>>In <lord.22....@eskimo.com> lo...@eskimo.com (David Lord) writes:
>Stuff deleted ...
Ken
I disagree with some of what you said. Decent vinyl tubing deflects very
little. I have used it on some high wind kites with satisfactory results. In
any case they would not deflect to a degree you could see at 90 feet while the
leading edge bend is visible from much farther. It takes a massive frame to
react the bending load. This load is caused by the triangle formed by the
bridle between the lower T connector and the upper spreader/leading edge
connection. If you look at the three triangles formed by the bridle on each
side of the kite, you will see that each has a piece of spar between the end
points except for the T to upperspreader/leading edge connection. To react
this load the leading edge is pulled toward the T. This causes both a
rotational and a displacement force on the upper spreader when the kite is in
a hard turn and one of the fly lines has low tension and the other has high
tension. The upper spreader does rotate slightly and displaces pushing the
opposite leading edge outward. It is the bending loads that break spars and
the vinyl connection of the upper spreader to the spine stops most of this
bending.
I also disagree with the use of tapered spars for high wind conditions. The
best use of tapered spars is for lower wind conditions where you can trade
some frame distortion for wing tip robustness. In high winds tip distortion on
tapered spars is way excessive.
Dave Lord
Ken Nealey
In a previous article, lo...@eskimo.com (David Lord) says:
Stuff deleted...
>Ken
> I disagree with some of what you said. Decent vinyl tubing deflects very
>little. I have used it on some high wind kites with satisfactory results. In
>any case they would not deflect to a degree you could see at 90 feet while
>the leading edge bend is visible from much farther.
In part, I agree; "decent" vinyl tubing has very little deflection. However,
vinyl tubing leading edge connectors do have their share of problems. It is
possible to install same one of two ways. The leading edge spar is installed
through both walls of the decent vinyl tube. Or, the leading edge spar is
installed through the center hole of the decent vinyl tube and through a hole
in one wall of same (the no - snag method). From a flight performance
perspective, the first method is better. However, since the decent vinyl tube
is perpendicular to the leading edge, it must be bent to an angle which will
permit installation of the spreaders. In either case (method 1 or method 2),
one wall (top or bottom) of the decent vinyl tube is under compression while
the other is under tension. In turn, this compromises the ability of the
decent vinyl tube to hold or grip the leading edge spar. Further, even with
decent vinyl tube, after a little air-time, the leading edge connection
loosens up even more. Try this simple test, take your favorite kite that is
equipped with decent vinyl tube and hold the leading edge spar while trying
to turn the vinyl tube - note the effort required to do so. Now, bend the
decent vinyl tube up at an angle (like if a spreader was installed) and
rotate the vinyl tube. Note that less effort was required to rotate the
decent vinyl tube. If you want to exacerbate the foregoing, use something
less than decent vinyl tube that does not properly fit the spars (holes too
large).
It is also equally important that the vinyl tube properly fit the spreaders.
That is if the spreader can be completely inserted and if the decent vinyl
tube can snugly grip the spreader and if the decent vinyl tube has adequate
resilience to withstand the stand-off and flight loads while maintaining the
integrity of the joint connection.
Simply stated, it is my point that proper, purpose made leading edge
connector fittings (yes - some purpose made connectors are worse than decent
vinyl tube), will improve the response of, and increase the wind range of a
given kite. This is relative to the same kite that is equipped with decent
vinyl tube connector fittings.
The connector that yields the most dramatic improvement (Power-Tite) does so,
in part, because of its unsurpassed ability to hold the leading edge and
spreader. All the while maintaining the most consistent, stable joint
relationship. I have seen and flown a Pro-Dancer with Beman Light-14 spars
and Power-Tite connectors in 15+ mph winds with virtually no frame distortion
or visible flexing of the leading edge due to input control moments (some tip
deflection - yes).
>It takes a massive frame to react the bending load. This load is caused by
>the triangle formed by the bridle between the lower T connector and the
>upper spreader/leading edge connection. If you look at the three triangles
>formed by the bridle on each side of the kite, you will see that each has a
>piece of spar between the end points except for the T to upper
>spreader/leading edge connection. To react this load the leading edge is
>pulled toward the T. This causes both a rotational and a displacement force
>on the upper spreader when the kite is in a hard turn and one of the fly
>lines has low tension and the other has high tension.
Again, in part, I agree; in straight flight, in the center of the window, all
legs of the bridle loaded, the bridle loads tend to pull the top spreader
connection point away from the plane of the spine and towards the T. However,
in straight flight, both sides of the upper spreader can be equally loaded.
This will result in the rotation of the leading edge toward the pilot. The
bridle loads tend to compress the leading edges towards each other. In part,
this is why it is so important to maintain the best possible joint integrity
(leading edge to spreader). Proper purpose made connectors are better at this
than decent vinyl tube. Poorly fit vinyls actually can permit the leading
edge to pull around the upper spreader. This can occur in straight forward
flight as well as during hard turns. The resulting symptom will be visible,
possibly variable, deflection of the leading edge spars.
>The upper spreader does rotate slightly and displaces pushing the opposite
>leading edge outward. It is the bending loads that break spars and the vinyl
>connection of the upper spreader to the spine stops most of this bending.
I agree, although displacement (due to compression between the upper spreader
and T) is momentary (depending on position in window and type of turn) as the
bottom leg (tow point to T) of the bridle loads and unloads (stand down-field
and watch the various legs of the bridle load and unload during a series of
maneuvers - while someone else is flying the kite). Then, rotation takes
over.
Perhaps I am leading a sheltered life, I have never broken a top spreader
(except when I was experimenting with connecting same to the spine) or a top
section of the leading edge. Lower spreaders and lower sections of the
leading edge, yea - I have broken a few. Well, since I like aggressive ground
work, maybe more than a few. Most of the time, I will even admit that I
deserved it (some times - not, it was the spars fault).
>I also disagree with the use of tapered spars for high wind conditions. The
>best use of tapered spars is for lower wind conditions where you can trade
>some frame distortion for wing tip robustness. In high winds tip distortion
>on tapered spars is way excessive.
> Dave Lord
I believe that I made reference to a "wide wind range" kite, not a "high
wind" kite in my earlier post.
I have been flying tapered wrapped spars (almost exclusively) since 1993. I
have flown them in all wind conditions ranging from less than 1 mph to in
excess of 35 mph and indoors. I have observed several other "high wind"
kites to exhibit more deflection than kites framed with the tapered spars.
The real killer in high wind is - speed. One of the most fundamental factors
associated with speed control in said conditions is spine stiffness. If the
spine is not stiff enough, the sail loads will push the spine into a concave
form. This can cause the chambers to merge into one, resulting in a very
pronounced increase in speed. By connecting the upper spreader to the spine,
in essence, you are reducing the unsupported sectional length of the spine,
which in turn will increase the effective stiffness of same. That is why the
aforementioned technique increases the wind range of a given kite. You can
accomplish the same thing (effective speed control - as described above) by
installing a spine of adequate stiffness.
Ken Nealey
Ken Nealey
In a previous article, djsk...@aol.com (Djskites) says:
>>>ways to strengthen the frame against these loads. One of the techniques
>was to
>>>tie the upper spreader to the spine.
>
>The Tremontana from Germany uses frontal spine and just a strong rubber
>ring that connects the top sreader to the spin, works well and does the
>job. It holds the whole kite nice and sturdy, and through hours of
>testing, it has never broke. I do not have access to those rings right now
>for my assembly of these kites. The first 6 I used a piece of reinforced
>tubing that works good. Sure would like to find those rubber rings.
>any other ideas?
>
>Dodd
Hi Dodd,
Is the ring that you describe an O-ring? If it is, measure the
inside and outside diameter as well as the thickness of the O-ring.
I can probably locate a source for you. Good Luck!
Ken Nealey
David Lord
I have used leading edge connectors that did not allow any significant
movement. On the Peregrine kite made by Jerry Sinotte were connectors that
locked onto the leading edge spar and were inserted into the upper spreader.
Some of these were made with 3 piece leading edges with the connector fitting
tightly on the internal ferrule between the spar sections. There are probably
no connectors used today that have less play in them than the ones Jerry used.
They had very little effect on leading edge deflection nor did a loads
analysis suggest they would. The rotational force I talked about is due to the
upper spreader being fixed to the leading edge, and as the leading edge bends
inward it bends at an angle to the spine. Mainly because it is already at an
angle to the spine and must bend in that direction. You can resolve this force
into two forces at right angles to the spine. One pushing along the upper
spreader and one pushing down at 90 degrees to the spreader. The opposite
leading edge is pushed out and up slightly. As a kite is flying straight the
loads are balanced but in a turn they are unbalanced and this bending is
exhibited. Even if breakage does not occur, and most of the time it doesn't,
there is distortion of the kite frame. All things being equal this will
degrade performance. It is difficult enough to design specific performance
into a kite when the frame is stable. counting on specific distortion is
insane.
As for tapered spars my contention is that their deflection characteristics
do not very much match the loads a kite frame will see. I believe one can get
a better match using straight sections of various sizes. Most leading edge
breaks are a result of contact between the tip and the ground and a smaller
diameter spar with a thicker wall (taper tip) will bend farther without
breaking. It will also bend easier thus more tip distortion during flight. You
cannot have it all. For a given weight you can trade robustness (more
deflection without breaking) for stiffness or vice versa. One must make a
judgement for each particular design which way to go. For ultralight kites I
believe one can make an adequate frame lighter with non tapered spars And for
high wind kites it is hard to find a tapered spar strong enough to maintain
tip stability. For ground work I yield to the tapered spar as the tip is more
robust.
In terms of speed a kite should continue to accelerate with increasing
wind speed until frame distortion ruins its characteristics. Once significant
distortion occurs then the kite is usually degraded especially if it was well
designed, but I grant you that some specific kite may turn more effecient
temporarily but at higher wind speeds still, it will hit the wall where
distortion causes drag to increase faster than the wind increase will speed it
up. For soft kites this is where the external forces on the kite are greater
than the internal forces and the foil loses shape. This happens to some extent
on framed kites since the sail is not rigid. The reason I mentioned the Prism
Radian in the earlier post is that it has the most rigid sail I have seen on a
commercial stunt kite and is capable of very fast flight in very high winds.
Dave Lord