Into the Wind, suppliers of the Ultrafoil describe the kite thus:
“ITW UltraFoil 15 Kite
Ray and Jeanne Merry update the Flowform
With its uniquely vented design, the UltraFoil 15 has a very wide wind range. In light winds, its tall airfoils get more lift. The lower vents aid in initial inflation and the upper vents relieve pressure. In strong winds, the dynamic trailing edge vent opens to release a stabilizing jet of air. A high angle flier with good, consistent pull, it’s perfect for Kite Arial Photography and lifting Line Laundry. Made of ripstop nylon, it needs no assembly and has nothing to break. Wind range: 5 to 30 mph. 14-ft ripstop nylon tail. 8″ x 13″ case. 3′-0″ x 4′-8″. Use 150lb. Dacron line.”
I’m staggered by this description, this is the flowform principle as patented by Steve Sutton, I can’t see what’s been updated, sure the vents are a new shape but that seems to be the extent of the ‘update’ ..and it’s a kite that needs a lot of work before its even close to ‘perfect for kite aerial photography’.
My experience with this kite is in Bft 4 and 5 some 50 miles inland on a small offshore N W European island. We get wind of all kinds here, mainly South Westerly off the Atlantic and on the coast laminar flows are common. I don’t know what kind of wind Ray and Jeanne Merry fly their Ultrafoil in but my example eats huge amounts of sky in Bft 4 and gets downright scary in Bft 5.
The problem. As supplied the kite is prone to excessive oscillation in wind speeds above 10mph. The kite sways in increasing arcs as the wind picks up and, although inversion is unlikely, this is inadequate as a platform for photography. What seems to be happening is, once inflated, the wing behaves like a closed parafoil, it just can’t cope with higher flows in the intended flowform manner- it ends up acting as a near rigid sail rather than a channel for the flow.
If you think of the kite as a pipe it’s unable to cope with the flood of air as a smooth stream, much of the flow is disrupted by the mouth of the pipe and this causes instability.
Solution. To improve the stability of the kite and mimic the Sutton pattern of vents, a series of holes are cut to increase the airflow through it to reduce the ram air ‘bounce’ effect in wind-speeds over 10mph or where airflow fluctuates. Placing the holes is determined by where the fabric will take them. Holes too near the cell mouths could cause the edge to lose stiffness and curl up so the holes are on the trailing side of the Merry vents.
On the underside 4 new holes are required. The multicoloured panels get in the way of the centreline of the outer cells:
placing the hole offset to the middle of the triangular panel should minimise weakness.
Holes are 8.5cm dia. which is the diameter of the big end of a pint glass.
The 8 holes add 1426 cmsq to the available venting of 352 cmsq bringing a total of 1751 cmsq. This doubles the flow rate.
Bridle. The bridle is extended to 3.80m long (from the supplied length of 2.3m)
Becot detail. Venting the side keels has 2 functions:
- reduces the risk of side draught collapsing the outer cells (this is unlikely given the high pressure in the cell but as the pressure is reduced by adding holes it becomes a bigger risk)
- dampens side slip (where the keel can act as a vane) by spilling excess flow to balance side to side movement.
The vents add to the porosity of the drag surface as well as dampen weaving.
At around 20mph the weaving begins, the increased throughput has smoothed the movement now so I think I’m on the right path. The modifications have raised the wind range to 20mph so far. I still have a drogue to test as well as further ‘Becot’ venting to the outer keels.
Aspect ratio,chord & cloth. The Ultrafoil 15 (and 9, but not 30) shows some of the characteristics of its predecessor the Sutton Flowform. Christian Becot showed stability of the design can be improved by modification of the airflow. The movement of the kite is, in part a result of its aspect ratio- it’s longer than it’s wide. Kites in general are more stable with a longer axis along the wind, parafoils, being closer to an aircraft wing are different, they are much more rigid than a sparred sail when inflated and are consequently more dynamic- they are prone to producing great power with a tendency to movement, getting the balance between flexibility (to absorb flow variation) and lift is difficult.
It looks like Ivo van Olmen took Don Mock’s MiniMock 6 cell design as the starting point for his 4 cell Explorer and this has a wider than it’s tall aspect; the result is a kite that is steady, pulls like a horse but has a rocking motion about the tow point. Ivo chose a heavier, more porous ripstop fabric with looser seam stitching and I believe this, along with a deeper (slower) chord and a bigger tail vent produces greater stability.
The Explorer 1.6 flies on its ear at 20mph but keeps its line, you can fly it in gust knowing it won’t suddenly oscillate wildly across the window, release the line tension (this can be tricky- sometimes I have to run down wind if its seriously overblown) and it recovers predictably.
The only modification to the Explorer has been to reduce the weight of the bridle and add corner tow points for a Y line towed drogue.