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Author Topic: Doug Stout's stunt kite design tool  (Read 4923 times)
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Cydonia
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« Reply #15 on: May 10, 2012, 10:36 AM »

Hi Doug,

I read your article which is very interesting but I have some questions.

when you calculate subpanel aerodynamic center, you the project sail area or the total sail area?
In other words, what is the stand off influence in the subpanel aerodynamic center.

the second question is about the trailing edge.

in your calculation the trailing edge of each subset is linear. Is there any changes on equations if I use a curve trailing edge?

last question.

How did you "cut" the sail in subpanel. I suppose that it's depend on the position in the trailing edge

thanks a lot for your work

Cydonia
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Doug S
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« Reply #16 on: May 10, 2012, 02:11 PM »

Cydonia,

Thank you for the questions.  The center of pressure (CP) for each panel is calculated for the projected area.  In my Geokite Program, I also calculate the CPs for when you view the assembled kite from the top and side, since it has been my experience that these values help us determine the effect of the three dimensional aspects of the sail.  My old college physics class teaches us to look at the forces apply to an object in 3 planes for flying platforms, which Geokite does for us.  If you download the Geokite Program and the Evaluation File, you will see what I mean.  If you vary the length of the standoffs in Geokite, you will see the side and top CPs move in the charts for the Front View of the kite shape.

The current version of Geokite discussed in the article allows up to 10 panels per side to represent the shape of the kite.  After modeling a kite, I load the modeled kite shape into AutoCAD and draft the kite in 3D, and then flatten the design to make the patterns for the sail material.  While in AutoCAD, I use polylines and a fit curve function to provide the curve to the leading and trailing edges.  I have found that 10 panels per side allowed me to break the design into enough smaller sections to provide the desired sail shape.

With my provided model for the Bird of Prey, you can see the values for the rate of curve for the trailing edge, between the center spine and the 1st standoff and between the 1st standoff and the tip.  The rate of curve for the trailing edge outside of the 1st standoff is the minimal amount of curve that allows my leach line to keep the sail silent.  Please note with careful work, the rates of curve for panel of the indicated locations are the same, to allow for a pleasant looking curved edge.  A rate of curve can be observed for the leading edge, from the upper spreader to the tip, which allows the wing spars to provide just enough tension to assist the leach line in keeping the sail silent, if one so desires.

To input an existing kite into Geokite, just divide the kite up into easy to measure locations, such as distances from the center spine to each standoff, upper spreader, lower spreader, etc.  In the modeled of my Bird of Prey, I add extra panels between these easy to measure points to allow the sail to reflect a curved leading and trailing edge.  Please remember that Geokite is in a spreadsheet for a reason, which allows me to develop equations for the shape of the kite, I call the model.

Play with Geokite and have some fun with my Bird of Prey model before you input your own kites.  Draw a real simple kite with two standoffs and play with the length of the standoffs to see how the top and side CPs move.  I hope the above answers your questions.

Doug
« Last Edit: May 31, 2012, 08:32 AM by Doug S » Logged

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Cydonia
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« Reply #17 on: May 22, 2012, 01:48 PM »

Hi Doug,

I have another question:

when you calculate aerodynamic center on top position why don't you use vertical subpanel aerodynamic center equation?

Thanks in advance for your response

cydonia
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Doug S
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« Reply #18 on: May 23, 2012, 06:15 AM »

Cydonia

Thank you for your question.  The Top and Side Panel Aerodynamic Center (PAC) calculations are my own derivation to calculate numeric values to represent the depth of the sail and washout.  In simple terms, we are calculating the area and the PAC of each panel that we can see when observed from the 3-axis plane view of the kite:  Front (X,Y), Top (X,Z) and Side (Y,Z).  The calculation of the Front PAC uses the X and Y values for each panel.  For the Top PAC, the Y values are replaced with the Z values representing the depth of the sail for that panel, with the X values being the same as the Front View.  In the Side View, the X values are replaced with the Z values representing the depth of the sail for that panel, with the Y values being the same from the Front View.  In Geokite 8, the Output Sheet provides the results of these calculations for each of these three views (Front, Top, and Side), which results are used to plot the locations in the Front and Top Charts.

Please note that the PACs for the 3-axis views provide numerical values that one can relate to the characteristics/personality of a specific stunt kite, which will allow one to objectively compare the values from stunt kites with different characteristics/personalities.  Charting the PACs allows one to observe how stunt kites with similar or different characteristics/personalities relate to one another, providing one with a performance window of PACs that represent what you like as the desired characteristics/personality in a stunt kite.  This evaluation technique allowed me in the 1990s to explore refinements in my numerous stunt kite prototypes and develop the window of PACs for the characteristics/personality I was looking for, which was a very easy to fly and predictable precision stunt kite that could perform all of the precision and trick maneuvers know at that time.  Since the tricks have evolved, one must develop the window of PACs that will provide a stunt kite with the desired characteristics/personality to perform these tricks in an easy and predictable manner.

I hope this answers your question.

Doug
« Last Edit: May 23, 2012, 02:37 PM by Doug S » Logged

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n893
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« Reply #19 on: June 15, 2012, 07:12 PM »

Thanks for sharing all of this article... it's really open up my way of thinking of how complicated a stunt kite design process... Smiley Smiley Smiley
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Doug S
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« Reply #20 on: August 13, 2013, 05:32 AM »

Based on some research, I have revised this post.  I was humbled by the speeds that are currently being achieved.  I have flown control line combat model airplanes at those speeds, but it was many years ago.  It appears that a finely tuned speed kite is going to be challenging to fly.  Thus, it is a young manís sport.  Based on this, I donít think I could offer a design that would be faster than what is being currently flown.  I provided ae with some suggests on how to make what he is flying go faster, which he may have already taken into consideration.

I am in the process of updating Geokite because I have the itch to design something new in the dual line family.

Later,

Doug
« Last Edit: August 15, 2013, 06:14 AM by Doug S » Logged

"We never really grow up, we only learn how to act in public."
ae
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« Reply #21 on: August 13, 2013, 08:08 AM »

Ohh Speedkite,

I'm definitely interested in what you come up with.
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Doug S
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« Reply #22 on: August 30, 2013, 01:28 PM »

I haven't posted for a while because I have been busy updating Geokite and retrofitting one of my old prototype kites.  The output section of Geokite is reformatted and less pages to print.  I will add the simple equation to calculate the moment force for tail weight in trick related kites.

I have the itch to build a new stunt kite, which may be a pure precision machine or a pure speed machine.  Maybe one of each (ae will be happy about the speed kite).  I havenít decided on the size of the precision kite, but would like to keep the effective sail area around 900 square inches, if it is to be flown in normal moderate wind conditions.

Updated the fittings on one of my Falcon prototypes (January 1991), which has a 9 to 1 aspect ratio.  It was my first design that used a leach line.  Also added new standoff pockets and a second set of standoffs at the wing spar/lower spreader connection points.  Now it should axel without catching the lines.  Will be using this kite to evaluate the benefits of higher aspect ratio kites for the two potential builds.  I have other prototypes with different aspect ratios to be used in the comparison, which also have additional outer standoffs that were added after the initial construction.  All of the original 9 prototypes were large with 100 inch wing spans.  The January 1991 prototype was very efficient, but had a tip stall problem due to the leach line, with a sail with much less draft than what is being used today.  The new outer standoffs should remove the tip stall problem.

I modeled some of the speed kites and was amazed that the aspect ratios are not that much different than the run of the mill stunt kites.  The difference is the speed kites have a more uniformly tapered sail shape, less leading edge sweep, and very little draft.  If you want speed, I have been told that you donít washout the tips for stability or have a leach line.  They must have some entertaining tip stall issues.  No wonder they are hard to take off until they reach flying speed.  This will be one of the engineering challenges I will try to overcome.

Later,

Doug
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