The impact of a tail on the stability of the DL Trooper is something of a revelation to me- a simple thing can turn an unstable low AoA kite into a steady lifter albeit with decreased lift.
Kite stability is a constant worry for the Kapper and when a paper on the traditional use of kites by the pacific peoples came to light on the KAP forum ( Ho’olele Lupe -An Analysis of the Ancient Practice of Hawaiian Kite-flying) I was intrigued to see Damian Sailors used a ‘Beaufort stability curve index’ to evaluate kite performance using an index range between 1= (stable) and 0= (not stable) based on kite behaviors:
- 1.0 Kite remains stable in an upright position and can be left “unattended”.
- 0.9 Kite remains stable in an upright position but must be “attended”.
- 0.8 Kite remains upright but moves about a little.
- 0.7 Kite remains upright but moves about fairly regular.
- 0.6 Kite moves about at regular intervals,dips and twists a little
- 0.5 Kite moves about at regular intervals,dips and twists a lot.
- 0.4 Kite does complete revolution Approximately every 30-45 seconds.
- 0.3 Kite does complete revolution approximately every 15-30 seconds.
- 0.2 Kite is continuously rotating.
- 0.1 Kite is continually rotating and takes maximum effort to fly.
- 0.0 Kite is not flyable.
This is a very useful, practical description of kite stability. There may well be more scientific methods: it is possible to break down stability as components of the lift equation but lift alone is not a good indicator of stability, a kite with great lift can can have poor stability across a range of wind-speeds (a good example of this is the J pattern parafoil).
Theoretical kite stability criteria
The description of the forces acting on a kite and the resultant effects of aspect, balance and lift have been explored in depth: this was the work that gave birth to the aeroplane at the turn of the last century. Many descriptions rely on theoretical flow conditions which are in constant flux in the boundary layer- they are a great aid in kite design but lack the ‘real world’ effects on kite behavior in chaotic flow.
2 examples of the theoretical approach:
1. Formula for determining the torque at the tow point of a kite. The turning force at the tow point is proposed as a stability indicator at the Glen centre for aeronautical research hosted by NASA research this is a good indicator of raw performance and should reveal the correct geometric relationship between AoA CoG and the tow point for a given aspect of kite:
T = – L * cos(a) * (yb – cp) – L * sin(a) * xb- D * sin(a) * (yb – cp) + D * cos(a) * xb + W * cos(a) * (yb – cg) + W * sin(a) * xb
T= the net torque with a positive torque being in the clockwise direction.
L= the lift,
D= the drag,
W= the weight.
xb and yb = the co-ordinates of the tow point,
cg = the location of the centre of gravity
cp =the location of the centre of pressure.
2. Formula for determining sideways motion of a kite, Nicolas Wadsworth has derived the equation:
0 < d/L < ALρ/M
d = distance sideforce is ahead of CG
A = effective side area of kite
L = spread of side area
ρ = density of air
M = mass of kite.
To determine the sideways deflection (and hence the oscillation) of a single line (box) kite which goes some way to show the aspect of the kite surface has a big effect on stability here
The edges of the envelope
The theoretical methods show how a design would behave in response to a given set of forces at a given time. Kites fly in a constantly changing set of forces and exploring this dynamic for each of my kites is a necessary part of the process of building the confidence needed to use them to lift a camera …for example I was accosted by an anxious onlooker as I launched my kite downwind of an overhead powerline. Having a firm knowledge of how the kite would behave across its entire performance range I knew the path of the kite would be inside the safe zone in front of me. The apparently random movements of many kites justifies the concerns of bystanders: any kite in proximity to a lethal overhead wire looks like a short cut to A&E.
Finding the limits of kite performance is a trying business involving the least rewarding aspects of kite flying: at one end to willfully set out in unpleasant weather and torture a kite with a screaming line and at the other to watch it fail to lift in a feeble breeze after much line tugging and the inevitable birds-nest of spilled line to untangle.
Searching for the sweet spot: Ho’okauahe’ahe
Most kites have a wind-speed they work best in, the flying angle, lift and line tension will be optimal and movements across the face of the wind will be slow and controllable, Damian Sailors uses the Hawaiian term Ho’okauahe’ahe to mean ‘fly steady as a kite’ something I see birds (particularly raptors and seagulls) do a lot – hang in the air absorbing flow variation as if pinned to the sky. That absorption of flow variation is the secret of the sweet spot at the middle of the wind range for a given kite.
The wider the wind range the kite is stable in the better. The more flexible a kite is the more variation in flow it can absorb, the work on dynamic bridles and spars has got 5-25mph as a good sweet spot to aim at. So far 7mph is the effective minimum wind-speed I can work in with a 800g rig.
The 4 Modes of failure
Kite stability is determined by 4 failure modes:
- Overblown state.
- Launch failure.
- Structural defects.
- Design defects.
A fifth ‘dirty wind’ aspect (such as rotor reaction and lull collapse) can happen inside the upper and lower limit of the wind range but are usually evidence of an overblown or under-blown kite.
The end of stable flight at the wind-speed limit varies from kite to kite: cell collapse in foils, dip and dive at the low end for deltas, heel and spin for flowforms, ‘float and drop’ for boxes. There are many kites which simply spin and dive when overblown and others which heel and drop to the horizon as if pushed out of the wind window into the ground.
The reaction of a kite to a failure mode can be
- A rapid or gradual transition from stable to unstable.
- An ‘edge of performance’ state allowing recovery to stable flight by intervention
- Irrecoverable (crash)
Structural failure modes result from the distortion of the kite geometry, asymmetric fabric stretch has caused one of my deltas to develop an uncorrectable heel, unmatched spars, shroud line tension, seam splits, sail puncture and bow/ dihedral failure are common structural causes of instability.
A few kites have a desirable characteristic that allows them to be pushed to the horizon and maintain a ‘heeled’ condition (usually with some loss of lift) until wind pressure drops and they recover to the apex of the wind window. The more gradual the failure the more reaction time there is to adapt to the airflow.
Some KAP stability descriptors. KAP demands kite stability so I suggest the ‘Sailors scale’ for KAP kites might look like this :
- 1.0 Stable flight in wind range.
- 0.9 Stable flight with some movement: yaw/rocking ameliorated by tail/drogue.
- 0.8 Kite occasionally tracks to left or right but consistently returns to apex.
- 0.7 Heels to one side in gust without toppling.
- 0.6 Big movements across centre of wind in gust with predictable recovery.
- 0.5 Erratic movements across face of wind.
- 0.4 Kite needs constant attention to remain airborne
- 0.3 Tendency to spin/top or tip
- 0.2 Kite unable to rise to apex, does not centre on apex of wind window.
- 0.1 Kite is continually rotating and takes maximum effort to fly.
- 0.0 Kite does not fly. Fails to rise, falls to ground, flies at a very low angle.
Kite stability is, of course, not only dependent on the characteristics of the kite. I have known truly awful kites fly majestically in coastal flows and really good kites fail if they are put before too bigger wind. Buffeting or rapid cycles of gust and lull is a very uncomfortable place for any kite.
General cases for kite stability
- The bigger the kite the more stable it will be
- A ‘long’ aspect ratio (a shape that is longer downwind than across) is more stable than a short one.
- Short line oscillations are common- line length and weight effect stability.
- A tail or drogue is essential for some kites in some (higher speed) flows
- Coastal or laminar flows will forgive most kite defects.
- Any left/right asymmetry will generate instability.
A tale of 2 deltas. Of late I have been flying a 2008 Dan Leigh R8 (a 9.5′ delta) and been astonished at its stability in light wind, the delicate balance of spar flexibility and CoG placement make this kite a beautiful thing to fly, slow movements even at the point of lift failure all of which come with a drop in power compared to steep AoA of the Levitation (also a 9′ delta). The delicate lift/power characteristics of the R8 make an ideal autoKAP platform where pin point position and steep flying angle are less crucial than managing line on a walk over awkward terrain. It’s early days with it but I’d estimate the lifting capacity of the R8 at about 2/3rds that of the Levi. Dan Leigh and Chrisoph Fokken have arrived very different outputs from the 9′ delta pattern ; they are both stable and both highly valued lifters in my flight.
A walk along the upcast of this creek with the R8 in tow captured the wonderful meander of the ancient drainage channel. Walking the bank is hard work in the summer as it’s chest high with thistle and nettle- too much to deal with and a radio, I let Dan Leigh do the lifting and James Gentles’ clickPan Pro drive the rig. These marshes have been subject to drainage since the Roman occupation.