|
|
Close Help | ||||||||||||||
Washout.
Washout or Tip twist is characterized by the degree the wing tip is twisted. The purpose is to cause the wing tip to stall later than it would without the twist. With the proper amount of twist a kite can be made to turn at the window edge without falling out of the sky. Washout will also allow a kite to continue to turn well even as the bridle is adjusted lighter (nose pulled forward).
Washout can be achieved in a couple of basic ways. The first is fixed washout set by a pair of out board standoffs. The second is dynamic washout created by employing a larger wingtip with a batten to hold it. The aerodynamic load on the wingtip is reacted by the tension in the kites trailing edge. Getting the right size winglet with the correct tension in the trailing edge is the key. The combination of a leach line and an inboard standoff create a spring system that can be adjusted to maintain the amount of tension desired. The fixed washout system is easier to design and adjust but you pay a drag penalty at the window edges. The dynamic washout system provides drag at the middle of the window where aerodynamic forces are greatest but at the edges the tips pull in and give the kite a larger window. However as you turn the kite the tips are forced back into a twist to prevent stall.
Many of the latest kite designs are now starting to incorporate winglets to get dynamic twist. A pointed wingtip kite that has a fair amount of billow will get a small amount of washout at the tips. It is my opinion that the popularity of tapered spars for the leading edge of stunt kites is because of not only their ability to bend easily but also the added washout you get at the tips when the tips bend. A fatal condition is a kite with pointed wing tips, a shallow sail and a leach line to tighten the trailing edge. This results in zero washout and some times in washin. A kite with this configuration will stall during most any turn and flutter down to earth unless the bridle is set very heavy. This means much miore wind is required to fly the kite.
The first kite in my "Gallery" is a very large kite and I was never inclined to fly it at the higher wind speeds where the bridle could be adjusted heavy. With the heavy bridle adjustment the kite would turn ok, but with it set light so it could be flown in lighter winds the kite had lots of trouble finishing turns. With the addition of outboard standoffs to set some washout the kite would turn ok with the light bridle setting. There may be one or two of you out there who own one of Steve Shapsons huge eliminator kites. A set of outboard standoffs to set some tip twist will fix the kites reluctance to finish a turn when the bridle is set to fly at wind speeds around 5 or 6 mph. A more difficult design to get right is to use a single standoff on each side but move it to a compromise position farther out than is normal but not as far out as outboard standoffs. This can result in a very fine performing kite. An example of such a kite is the Jordan Air Millennium.
Leading Edge Camber.
Camber of the leading edge
generally occurs because the designer is trying to tighten
the sail. A small curve in the section below the lower spreader
is the result of taught trailing edge. The spar acts as the spring
reacting the tension in the trailing edge caused by the standoffs.
Extreme camber along the whole leading edge aids doing some
ground tricks like cartwheels, but the price is some reduced
wing efficiency due to a shorter wing span, and a pre-stress
on the spars that increases under flight loads. If it has
some positive kite aerodynamic feature I am not aware of it.
If one cuts the sails leading edge in a curve and the spars
are then reasonably straight then some leading edge billow
takes place creating some enhancement of lift when the kite
is in the airfoil mode of flight.
Click here for page 3.
RETURN to the LORD Main Page
