Copyright © 2005-2019 by Robert Moore · All Rights reserved · E-Mail: email@example.com
Flying techniques and other issues
Flying a kite is easy, is it not? Well, one would think so, however, it is surprising how many people find it difficult to launch a kite let alone have it fly for any longer that 5 or 10 seconds. Yet there is are lot of people who have no problems what-so-ever. The problem may be confined to those people who have a poor understanding of physics, a lower Visio-spatial IQ or lack hand-eye coordination. Often people ask me:
"How do you get your kites so high"? "I can barely get my kite off the ground". Whether it's some intrinsic characteristic of the person's brain wiring, some innate inherited talent for the way things work and an understanding of how our world is put together, I am not sure. Perhaps it's just learning. Perhaps it's the same reason some people are mechanics or rocket scientists and others are musicians. Anyhow even a few of those people who do build and fly kites for a hobby don't necessarily know how to fly big kites high but most understand immediately when I relate big kite, high flying techniques to smaller kites at normal altitudes. There are a few things that aren't normally issues flying recreational kites from hand on 300 ft of line. They are, line weight, line sag from line drag, accumulated line tension from line drag, delayed kite response, kite out of sight and need for a winch to control the line.
I could teach most kite enthusiasts the mechanics of flying kites to record altitudes but would they have the will? Could they be bothered? Do they want to? Does it "float their boat"? I don't think it's hard from a technical point of view. It just takes, dogged determination, a moderate amount of mechanical aptitude, a big kite, some wind and a bit of luck. There are enthusiasts on the kite builder's forum that show the same dogged determination as me such as Doug La Rock who built a massive Delta with small geometric applique patterns. This determination is is just applied in a different direction. His kite must have over 1km of sewing and hundreds of man hours invested. There are other people who are passionate about their hobbies. They show a similar unwavering determination. Perhaps I have an obsessive streak, I don't think I'm obsessive. I can think of lots of people who show that characteristic and far more determined and chose much more physically and mentally challenging targets. Marathon runners, mountain climbers, vintage car enthusiasts, artists. We all get the same sense of achievement, the same buzz from reaching a target, the same buzz from getting close. I don't know how I will feel after breaking the record. Is the journey more important? Is it the personal challenge? I can think of lots of people at the opposite end of the spectrum such as those who seek instant gratification and reward for little effort. There are some people like that who look at me with puzzlement and say "why on earth would you want to do that"? "What’s the point"? "What money do you get at the end"? "Is there a prize"? "Is it a competition?"
I don't bother trying to explain in much detail or rationalise and justify my activities. I just say because it pleases me and because I want to. I guess sometimes I may bore people stupid with my stories of the kite record attempts. I think if I were a hermit I would still do it and get the same pleasure from reaching the target. In many respects it is an internal battle, a battle to stay "alive", not to have my spirit crushed by the steam roller of modern society, to stand out from the sheep, literally, at Cable Downs, a metaphor for my life.
A kite is a tethered flying device, object, thing, work of art, toy, aircraft; it’s whatever you think it is but it must fly on a string to qualify. It relies on moving air for the energy to fly aloft. Without wind it doesn't usually fly, discounting those clever flyers that can jerk a line to keep a kite aloft and even do tricks. Aside from those skilful folk, generally no wind = no fly. Too much wind = beak and/or crash.
Apart from those unfortunate folk who try to fly their kites backwards, upside down or into the wind, most people who fail to fly kites do so because they don't understand their local wind and the effect of topography and obstructions on wind. They are unable to judge the wind or fly from the wrong side of the field. We setup at the furthest point of the field from the direction the wind is blowing from. We take into account the position of trees and the likely direction of the line as the kite rises. it is no good assuming the line is going to remain heading in the same direction throughout the flight, it rarely does. If we don't plan the positioning of the trailer before launch we may have to move it mid-flight. That includes all the ancillary equipment, shelter, table and generator. We do have to rotate the trailer from time to time so the line exits directly between the line guides. Mostly the trailer wheels have enough stiction on the airstrip to not require wheel chocking but occasionally the trailer starts to move with high line tension. With more massive kites or kite trains the trailer would need to be anchored with guy ropes and large pegs.
A general rule is you should have the wind at your back when holding the string. The only differences between flying my kite and your kite is the size of the kite and my string is attached to a winch. If that seems to be stating the bleeding obvious, you would be surprised that there are some people who don't get it and try to do odd things like flying cross wind, into the wind, kite upside down and the line tied to the tail.
If the wind is steady and well over the minimum speed required then 100 meters of line is walked out with the winch running in reverse at slow walking pace. Before that is done the line end is attached to the bridle attachment point. The line has no knots as they weaken the line significantly so the line end is sleeved and loop spliced to make an enclosed loop. The bridle is a three legged heavy set of lines that distribute the kite's pull evenly over the length of the front fibreglass tube spar. The length of the bridle legs are set with special locking slip knots at the attachment point. This determines the kite’s angle of attack or how much the kite's top tilts into the wind before it leaves the ground. Too little angle of attack and the kite will not rise and if it does it tends to overfly beyond vertical. Too much angle of attack and the kite becomes skittish then does not develop enough lift at higher flying angles. Its ultimate flying angle is also lower. The angle of attack is established by trial and error. The knot is marked with permanent marker and alternate positions for different winds may also be marked.
So far the spreader for the 120 sq. ft. DT Delta has remained a fixed size with 15 mm FG tube and 1.3 mm wall thickness. There are heavy Dacron loops attached to the 2 rear spars at the spreader cross over point. The spreader is inserted inside these loops when assembling the kite. These loops restrict the bowing of the spreader and so increase the stiffness and power the wings develop. We seem to have discovered a very good combination of spar weight, stiffness and spreader position. The bridle dimensions also contribute to the kite's flying qualities. After the kite is positioned on the ground, upright and facing into the wind, it is ready for launch. It requires good judgement and clear signalling from the kite handlers to the winch operator. When the handlers feel sufficient wind they signal to the winch operator then thrust the kite into the air. If the wind is above the lift-off threshold then it will rise, slowly at first then faster and faster as it climbs. In marginal wind we put out longer line lengths which can be over 500 meters. We use walkie-talkies to coordinate the launch in these circumstances. It is more likely the kite will fail to stay aloft if long line launches are required, however, at least the line is being reeled in so it's not as tiring as it could be. If we encounter those conditions then we now will not try for 5 or 6 hours to "flog a dead horse" like we did in 2005. The optimum flying angle is about 60 degrees but maybe more or less depending on the air temperature (density), thermals and wind speed. I try to keep the kites as light as possible consistent with strength, stiffness and instrument load. The kite weighs 3kg bare and 3.5 kg with instruments, batteries and light. The GPS telemetry unit, batteries and Garmin GPS is in a high density foam box inserted into a pouch on the lower rear of the kite. The radio and GPS aerial are oriented so the kite flying at 60 degrees will orient the aerials for the strongest GPS reception and radio telemetry signal transmission. The strobe light is a 3 LED high intensity tubular cycle light with a relatively narrow beam attached to and pointing down the flying line. The strobe is a 3 LED, super bright white light with a 10 Hz clustered strobe pattern of about 1 Hz. It is visible from 5,000 ft. in 1/2 light near dusk. It is not visible in daylight from more than a few hundred meters.
Rather than functioning like an aircraft strobe, that is, to make an aircraft more visible to other airspace users 24 hour a day, the kite strobe is to help the winch operator know the position of the kite when it drops within 2000 ft. of the winch and then it is critical to drive the winch when the kite is nearing the ground.
For the first few thousand feet of the flight the kite is usually clearly visible flying at a angle of between 50 and 60 degrees from horizontal. that's providing the wind is 10 knots and above and so the line can be released at 2 or 3 knots. Any lower than that then the line release needs to be slowed to between 1 & 2 knots. The alternative is to release almost as fast as the wind then stop, pause for a few minutes, regain some altitude then reverse the winch for maximum speed for 10 seconds or so. That way the kite gains more altitude than it loses and the kite climbs in zig-zag steps. In a wind of 20 knots, the line can be released at the full speed of 5 knots without loss of flying angle. However 20 knots at ground level on an inland location usually is accompanied by gusty conditions and much greater wind speeds up high. When the kite is beyond visible range, we rely on the telemetry to provide position, altitude, climb rate and horizontal speed. We have learned to use this information like our eyes. Without it we would be pretty much lost. We don't just use the telemetry but we observe the line angle and feel the line tension. In 2012 we will have a line Tensiometer which will add greatly to our record attempts.
A side view diagram of a theoretical kite track. It is NOT the trace of the line. The path would be much jagged than the illustration but the approximate path is shown and illustrates how the kite’s path usually describes a saw-tooth pattern in marginal winds. If the line were just released at a steady speed than the kite would go to ground. The alternate release and retrieval gives energy to the system. Some kite enthusiasts ask me "why don't you just release line until the kite nears the ground then winch up?" The answer is that while this may work on a beach or other area without obstacles such as trees, at Cable Downs, getting line out of trees is not easy and the line can touch the ground well before the kite. The line is released as far as possible without snagging the in trees. It can be a delicate operation requiring balancing risk of kite loss, line damage and maximising altitude gain. The whole point of the exercise is get as much line out as possible while keeping the kite flying and the line out of trees. Kite altitude = line out and kite angle and is all dependant on lift, exceeding kite weight, line weight, line drag and kite drag. The energy for this is provided by wind. There is no magic or mysterious forces involved, it is all clear cut and measurable, although complex, science. On a particular day with a particular air density, pressure, temperature and humidity and wind profile with exactly the same wind speeds, a particular kite will fly at x altitude with x line out providing the kites potential is maximised by proper winch operation. A rather simplistic statement and the devil is in the alignment of all these factors. It is taking advantage of these conditions. The first step is understanding the conditions. Knowledge is everything apart from the personal attributes of physical and mental tenacity and the ability to generate confidence in other experts to work as a team.
Here is a small section of an excel routine I developed. Kite flying is not a well developed science however theoretically it should be solvable using established aerodynamic knowledge. That has triggered me to try to solve the puzzle by generating computer simulations of high altitude flying. These attempts are relatively crude but I can't know everthing. Here is an excel spreadsheet that generates fairly good results which match actual flights. More data is needed to make the calculations more accurate. NASA has built a kite modeller in Java script However, it is flawed for high altitude flight as it doesn't include line weight or drag in the simulation.
These diagrams represent simulations of the 2 classes of altitude records, the single line, single kite record and the multiple kite or absolute record. Both require varying line strengths to reduce line drag and weight. For the train record, the line lengths and strengths are only educated guesses.
The 1919 train record was accomplished with a train of eight
Flying a kite really high is easy providing wind is there. Getting it back is sometimes a problem! In this shot, 15 minutes after launch I am struggling to keep the kite aloft and the kite is about 500 metres away and only 100 metres high. The day was warm at 11 am with the Herald photographer/reporter snapping a shot after flying from Sydney, staying 2 hours, then submitting a very negative story for the Sydney Morning Herald Saturday edition.
It is often more frustrating to attempt to fly a kite in very marginal conditions rather than sitting in the shade drinking coffee. We were here to break the world altitude record and it is tempting to "flog a dead horse" (sorry horse lovers) trying to keep a kite aloft in marginal conditions. Conventional wisdom would tell us to relax and wait for better conditions but we are only on the air strip for a week. To break the record on any one day needs at least 8 hours continuous flying. Even in ideal conditions it was 7 hours 46 minutes and we need to fly only in daylight hours. It was disappointing that the Herald should publish such a distorted negative article about failure and defeat when this is a marathon effort of 10 years of repeated attempts requiring perseverance and dogged determination. I am glad I was never deterred by this negativity or the few others online, particularly on my YouTube videos which seems to attract a fair number of keyboard warriors and judgemental pseudo experts. Apart from the Herald story, all the other papers have been great with very positive and accurate reporting. Radio interviews have been very positive apart from a single interview on a Sydney commercial station that was a little flippant and slightly trivialising in its style.
Top is the 4 kites used for record attempts over a 10 year period. The black DT delta was used April 2004, April 2005 and was lost in October 2005. In April 2007, September 2009, September 2010 and September 2012 the white and black kite was used and had a high of over 12,200 ft. In September 2011 the blue, yellow and purple kite reached over 14,000 ft. In September 2014 the turquoise and black DT flew to over 12,300 ft. then the next day broke the record with a flight to 16,009 ft.
The bottom image is a Grund kite of the German weather station at Lindenberg in early 20 century Germany. This was one of a series of similar box kites inspired by Hargrave's box kites of the late 19th century. A train of eight of these kites of various sizes reached 31,955 ft but were not retrieved due to the steel flying line breaking.
Foil kites have a curved leading edge where the air clings to the surface over the leading edge and it behaves like an aerofoil wing of a fixed wing aircraft. Framed kites such as deltas and diamonds don't have smooth flow at high angles of attack, that is, when on the ground and climbing. They simply use the air push against the face of the kite and some negative pressure at the rear inside the turbulent zone to exceed the weight and drag. Unless at the kite's angle of equilibrium for that wind speed and kite design, drag and lift ratios are higher producing higher line tension but not high flying angles. At high angles of attack the air pressure on the front of the kite forms curved cloth surfaces and pseudo aerofoil shapes. They then may develop smooth-flow aerofoil lift but at any angle other than 70 or 80 degrees the kite is in a state of perpetual stall, that is the air stream separates over the leading edge and cascades in a turbulent stream and the kites are mostly in a perpetual state of stall. The best framed kites do produce aerofoil lift but only at low angles of attack at high flying angles. You could imagine a kite facing into the wind on the ground at launch at a very high angle of attack, for instance at a bridle set angle of 10 degrees. It is almost vertical apart from a slight slope into the wind. There is no way an air stream is going to stick to the rear surface in a lifting flow. The air stream separates at the leading edge of the wings and the air cascades in turbulent flow. As the kite rises the angle of attack reduces and the kite’s vertical speed increases then slows towards the kite zenith. At the kite's maximum altitude, all the forces of lift drag, weight and line tension are in balance and kites, like deltas develop smooth flow aerofoil lift. Other kite designs are not as efficient and never develop smooth flow aerofoil lift, for instance, flying at a maximum 45 degree angle of elevation.
This excel formula demonstrates the relationship between line diameter and strength is not linear but exponential. The means that the relationship between the line cross sectional area and line strength is linear. Doubling the line diameter increase the line strength by Pi x R x R. For instance a Dyneema SK78 braided line with a diameter of 0.8 mm and a cross sectional area of 0.504 sq. mm, has a breaking strength of 134 kg (300 lb). A Dyneema SK78 braided line with a diameter of 1.6 mm has a cross section area of 2.01 sq. mm and a breaking strength of 534 kg (1,176 lbs), nearly four times the breaking strength but only double the diameter of the 0.8 mm line. It is this property that favours flying bigger kites to high altitude because although the ratio of kite area to line weight remains the same, the ratio of line drag decreases. A bigger kite has less work to do lifting long line lengths because the wind drag is less per square metre of kite area. It is helpful to know these forces when selecting a strategy including kite sizes and line strengths
Images courtesy of the American Kitefliers Association
Diagram: Bob Moore
Image: Sydney Morning Herald
excel charts by Bob Moore
Image - Bob Moore
Kite Altitude World Record