Hi Chris (& Harry),
Excuse the long post, but the issues are fascinating & complex.
(You must mean 5000 sq meters or 50,000 sq ft, not 50,000 sq meters, for a large tall ship sail area. JoeF might edit the typo out of the list archive)
A key point is that a displacement vessel is hull-speed limited & not able to accelerate like a kite & sweep a vast area in apparent wind many times true wind. So your correct estimate of about 5 megawatts for the ship is much less than the potential of a kite of similar area. The 5000 sq ft sweeping wing under a 17,000 sq ft lifter mentioned could in principle be sweeping a quarter mile loop at around 300 kts & roughly match or exceed the power of the tall ship's greater sail area in the same true wind. Existing parafoils like the 80's Flexifoil can do 100+ kts.
Yes, actuation force required to stabilize a kite increases with wind speed/load. The simplest way to input this force is to pull strings from the ground, not fly servos. A beauty of passive-control is that it is self-powered. In rising wind the kite gets increased actuation force for key stability mechanisms. Holland identified "snowplow stability" where a side gust kicks a leading edge back & the kite yaws (kiter's "roll") to meet the new wind direction. Weathercocking assisted by a tail does the same correction but with useful dampening & added drag. These mechanisms scale reasonably.
Unfortunately the pendulum stability of a CG aft of the bridle-point/ tether-axis does not increase in higher winds & the kite becomes more prone to flat spin (kiter's "roll" or "loop"). Adding tail adds aft mass & water ballast might be taken on, but only to the point the added mass becomes a greater liability. Active onboard actuation does not scale well, weight penalizing the kite critically in normal wind, but a "quiver" of actuation packages to match conditions mightserve.
You are so right that flying clouds of "sail" in the sky will be a challenge, but what a glorious job it will be, a New Golden Age. The problem will be broken down into managable pieces (~100 sq m units?), just as a tall ship rig is a thicket of stability & control features allowing the "ants" to cope. A useful analogy is how crystaline solar cells cannot be made large & must be ganged. Another is to remember how discrete transistors gave way to integrated circuits. Kite elements can be interlinked in arrays for enhanced stability & even achieve general synchrony, effects already seen in festival kite trains & arches.
Harry, you make a good point about kite trains & how multiple elements can support each other. This will make a good topic to discuss further.
NOTES- 1) A kite with sufficient ground clearance can loop periodically without anything bad happening. There is a small loss of altitude to make up, which a lifter module handles well for a looper module. 2) A deeply keeled kite can lock into a dive by random initial conditions. 3) Weak active actuation can interfere with the harmonic process of repeated undesirable looping (chaotic control). Similarly, a small trim input can poise a lifter kite against the handedness of prevailing wind helicity. 4) Wind chaos near a kite's limits creates momentary crisises to deal with. "Punch turning", as used by stunt fliers, is the sudden release of "stored" tension on one side of the kite (punching motion) to shift CP. This does not require high real-time force, but the tension has to be replenished, perhaps by the same tiny servo-winch that provides trim input.
Re: Edeiken Tragedy
Your interest in failure cases is ultimately very valuable. In computer science such analysis is processed as case-based reasoning (CBR) as inspired by best legal & medical practice. It is a worthy compliment to Alister's connectionist genetic algorithm (GA) approach to solve poorly defined problems. but is opaque compared to CBR's transparency. CBR over GA would be so cool.
Poor Edeiken was swept up by the bridle lines as The Monster launched unexpectedly with what was described as the "roar of a 747". Several lines had parted due to poor attachment & one of these coiled like a snake around an ankle. When it released him he was about 100 feet high. Kay Buesing, director of the World Kite Museum witnessed the fatality & described it to me just last week.
The 25th anniversary of the tragedy just passed & the AKA magazine devoted a section to it which i clipped. Its yours for you file when we next meet.
The PTSD continues to traumatize fun kiting & the AKA's highest prize is the Edeiken Award. Peter Lynn describes being tripped by sweeping lines on launch in his excellent big=kite safety piece on Gomberg's site. Another common big kite accident is like ballooning where tag line handlers don't let go & are lofted.
The Monster only came down when it parted its 3 inch Kevlar cable. Aft bridles should be elastic & passively dump excess tether force by reducing AoA, allowing lighter lines. Traditional kiting changes tethers with wind changes. at the drop of a hat I was astounded watching Jim Patton promptly reverse the completed hour-long launch process of his 1000 ft classic train simply to substitute lighter line in anticipation of changed conditions. As with sailing one must fly the hell out kites to fully maximize performance.
Some Lessons- 1) Stay out of the lateral scope of the launch/flight & never stand or walk inside the bridle/control lines of a big kite, 2) Kite Killers required for high consequence operation, 3) No loose lines or seams allowed, 4) Controlled launch mechanism required. 5) Hook knife required for all handlers & maybe a clip on carabiner/body tether if one is somehow dragged aloft.
The Controlled Launch requirement can be met by various means. A kite can be stoppered like a spinnaker or packed like a parachute, lofted, & popped at altitude. Towing or winching a kite aloft is a powerful technique. Stands with releases might be practical, etc.. Foxholes &/or trenches may be useful on mega-kite field. Large kite handling robots might be a solution.
Current giant kites are still belly launched & tend to flip over as one side gets more lift, a severe scaling limitation restricting handler ants to the current 10,000 sq ft. A giant parafoil or sled might best be launched flaked & laid on its side so it can be constrained from the key nose point with launch crew ideally always outside the scope. Such a prepared kite inflates progressively by unfanfolding, stands up, & lift off; a pretty sight. This is how sleds self relaunch after landing on a side. This launch method avoids launching directly into the power zone. Note also that an overly short-lined kite tends to oscillate violently as the line's natural frequency matches/couples with the kite's natural frequency, a hazard of short-line launch.
--- On Sun, 7/5/09, christopher carlin <christopher. m.carlin@ btinternet. com> wrote:
From: christopher carlin <christopher. m.carlin@ btinternet. com>
Subject: Re: [AirborneWindEnergy ] Re: Tripod Tether COTS AWE Demo
To: AirborneWindEnergy@ yahoogroups. com
Date: Sunday, July 5, 2009, 12:47 PM
I find the numbers that get bandied about here very interesting and may be about to draw an incorrect analogy but a big full rigged ship has about 50,000 sq meters of sail which at about 25 knots of breeze must put out on the order of 4000 horsepower. To hold that thrust must require a 2 or 3 inch diameter cable. Now if the wind goes to 40 knots the load will be up by a factor of 4 I believe. While the weight to be supported won't change. I think what I'm getting at is the lifter kite will need active control of some sort to keep the cable size reasonable. I'm not suggesting that it can't be done just that it will be required. I do think managing these large spreads of "canvas" will be a challenge to put it mildly.
By the way who was Edeiken and what is the accident story. I collect engineering accident stories. They are very useful in doing safety analysis.
On Jul 5, 2009, at 9:00 AM, dave santos wrote:
The Lifter Kite provides "passive control", the autonomous flight function, without avionics. Without the lifter the unstable sport kite would quickly crash. That's why these demos are so easy & cheap compared to "active control" (or helium aerostat) based AWE. You'll find a lot of related discussion in earlier posts & linked off JoeF's site.
The largest kite in history, the biggest wing ever, was Osborne's parafoil "lifter" flown near Ilwaco here (Long Beach, Washington), just over 17,000 sq. ft., so the scaling potential of this approach is vast. (Such a lifter could easily loft a hot 5000 sq. ft. looping parafoil as its multi-megawatt power element.)
Sadly, Osborne's Monster killed Edeiken on its maiden flight, but many lessons were learned. The fact that a community college sewing teacher could create such a wing is significant. Osborne also set the "official" kite endurance record (180 hours) on this same beach. Stormy Weathers topped 12,000 ft. altitude here with his sparred sled, a lifter (lifting line). Many of KiteLab's AWE experiments take place on this hallowed spot. The World Kite Museum is also located here. The winds are great year around.
Kite show lifters carry large exotic "line laundry" aloft & lifting AWE elements is a natural application. The greatest professional practitioners of giant show lifters are Peter Lynn & David Gomberg, with much online info available. KiteShip's (Culp & Jordan) huge OL traction kites can be stabilized as super lifters (KiteLab has stabilized an OL). Jordan's large parafoil lifters are some of the best ever.
Its been an honor to meet & learn from these pioneers, who have considerably advanced & perfected the art of giant kites. Its funny how certain "kite gods" forget the festival ancestors & fall for UAV aerospace to develop AWE.
--- On Sat, 7/4/09, Lou Groner <lgroner@gmail. com> wrote:
From: Lou Groner <lgroner@gmail. com>
Subject: [AirborneWindEnergy ] Re: Tripod Tether COTS AWE Demo
To: AirborneWindEnergy@ yahoogroups. com
Date: Saturday, July 4, 2009, 9:21 PM
What is the point of the lifter kite? Can the looping kite not also provide lift?
--- In AirborneWindEnergy@ yahoogroups. com
, dave santos <santos137@. ..> wrote:
> Hi folks,
> Attached is a video clip of the looping sport-kite under aï¿½lifter-sledï¿½driving a tripod-tether linked to aï¿½hand-cranked generator. The video might have been better but the camera batteries died.
> The attachedï¿½ jpeg shows the entire kit. Starting clockwise from the top-ï¿½(1) cheap X-kite sport-kite,ï¿½(2) cheap Premier 14 sled,ï¿½(3) tripod-tether lines & pulleys,ï¿½(4) tripod mount forï¿½the (5) hand-cranked generator,ï¿½(6) tuning sheets,ï¿½(7) sand anchors,ï¿½(8) plastic sand trowel.
> A much more powerful version of this concept is underway....