Stunt Kite Design

I intend this to be an ongoing project. As I learn more and ideas occur to me I will add them. I apologize for this not being more organized but if I knew more it would be better.

Basic kite aerodynamics

The following is my personal view of kite aerodynamics. Most framed kites are pretty poor low speed airfoils to begin with. At launch, during a snap stall and for most ground work manouvers the angle of attack is so high that it is not in the classical sense behaving like an airfoil (unless one considers a deeply stalled one). It is only when the kite attains a forward velocity sufficient to cause the apparent wind vector to be within 20 degrees, or there abouts, of the kites forward direction that it starts to behave as an airfoil. The other conditions are when the kite is at the window edges, but this is not really a unique condition since the apparent wind and the real wind are nearly the same and meet the angle requirements stated in the previous sentence. For the rest of the time the kite is in some transition region either starting to behave like a classical airfoil or droping out of such a region. What conclusions can we draw from this? For one thing we can surmise that during the times the kite is not behaving like an airfoil the area of the sail is the dominating factor in making the kite fly. When the kite is in the region that it is behaving like an airfoil then wing efficiency dominates. Try and keep this in mind as the discussion continues.

Some Terms

Aspect Ratio. In the simplest terms this is the wing span divided by the chord. This applies only for rectangular wings. Aspect ratio is, for more complex shapes the wing span squared divided by the wing area. For delta shaped stunt kites you get a larger wing span by increasing the nose angle, this also has the effect of decreasing the wing area. Both contribute to a higher aspect ratio. What does this mean in terms of your kite? Well, the decreased wing area will make the kite harder to launch (or require more wind) and will give the kite less drive when it is in the below airfoil region. On the other hand the higher aspect ratio will reduce tip vortex drag and make the wing more efficient when it is in the airfoil region of flight.

Wing Loading. Wing loading is the weight of the kite divided by the area. The lower the wing loading the less energy it takes for the kite to launch and remain airborne. To get really low wing loading your kite must be constructed from the lightest materiels available. It is possible to build a delta stunt kite with wing loading numbers around 0.4 ounces per square foot (120 g/m2). As an aside, the optimum size kite for minimum wing loading is somewhere around the standard eight foot stunters up to maybe ten feet wing span. The reasons for this are that as the scale of the kite goes up the area increases as the square of the scale, but the spar stiffness requirements increase as the fourth power of the scale. This limits the upper size as the spar weight increases rapidly.

On the lower side given the reality of spar technology the spar efficiency drops dramatically below an internal diameter of about 0.200 inches (5 mm). If the wrap thickness was less (the fibers of a finer denier) then a smaller spar could be made with decent efficiency. The ratio of internal diameter to wall thickness is critical if maximum spar effeciency is to be maintained. Thus with the present wrap thickness of around .0075 in. per ply a 0.200 in. I.D. maintains the proper ratio. In addition to building the kite with as light of components as you can, you should design the sail to have as much area as possible. Battened large wing tips can dramatically increase sail area. As with aspect ratio though there are trade offs. Larger area sails increase drag once the kite is in the airfoil region of flight and load the spars more. The key is to find the right combination.

CLICK HERE for page two of kite design.

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