How does tether drag vary with diameter and/or material? I have looked, but haven't found a source for this. Can anyone help?

-- Wayne

WL, before you get very far into this whole project you should really look into getting a proper aerodynamic engineer, or at least an educated visionary, on side. Stuff like you're asking has SO much impact that such a guy could answer in a more integrated manner.

Anyway, back to the fun.... I've seen tests done where the drag of a simple round wire or rod had the same drag as a proper symetrical airfoil that was 10 times as thick as the diameter of the wire or rod. That's why I was suggesting an extruded slit foam jacket that would fit over the tether wire as it came off the reel. The foam being formed onto the wire as it goes out and split off of it and stored on a special low tension reel as it came off. Perhaps the foam fairing sleeve could contain the data communication and power leads to the kite/wing.

And for what it's worth I think you're on to something good here. Like you say it's a well know fact that if there's wind at all that it's much stonger even just a few hundred feet from the surface. I've come around to believing that your wing/kites can work well but I think there's a LOT of work and study that you/we have only just touched on in order to make it work as efficiently as I think it could be.

The other night I jumped over to http://www.grc.nasa.gov/WWW/K-12/airplane/foil2.html and played for a bit. It was interesting to note that even using the stock sized wing of 5x20 feet I was generating almost 800 lbs of lift just before stalling at only 40 mph and over 1200 at 50 mph with a 14% thick 7% camber wing. Sized up to something DC3 like at 70x15 and the numbers go to 8400 lbs at 40 mph and 13,100 at 50 mph. Even at 20 mph that large wing still has 2100 lbs of lift to work with. Not bad for a kite I must say.

In the end drag is always the enemy and just because kites fly despite the drag does not mean they are efficient. I've seen cometition models of various types "kite" up their towlines and can honestly say that I have never seen a classic kite that is capable of getting that much speed out of a simple pull. If your ARTAGS ever see the light of day then I think you'll find that most of the time the tether line will never angle back more than about 20 to 30 degrees from the vertical and will often be closer to vertical than that. But in that mode I suspect there will be plenty of lift to do what you are trying to accomplish.

Perhaps the guys with the number tallent can correct me if I'm wrong but at even a high angle of attack close to the stall the L/D is still around 15 to 1 ( I'm basing this on a Clark Y at Cl=1 and Cd=~.3) Converting that to vectors shows a very straight angle. Of course the tether will add it's own drag but I still can't see the angle to the wing being too far downwind although the tether may be leaving the reel at a lower angle what with the drag and weight of the tether assembly.

It's mandatory first to determine how much tether there will be!
The weight of the material will affect the catenary shape and size.
It will -not- be a straight shot to the kite, except when the kite is relatively close.
The area for computing drag therefore will vary along the length of the tether depending on the "droop".
Way back when I was nimble, I could tow my Nordic A-2 gliders in circles when the lift wasn't good. There can be a lot of latitude
in vehicle position relative to the ground.
Our towlines at the time always sagged, even when the plane was overhead.

Thanks gentlemen,

Bruce,

Could foam really be practical going onto and off of a tether as it goes onto and off of a spool? Thanks for your confirmation of these basic possibilities.

I would appreciate it if both of you would read or re-read postings 1 and 7 in the thread: ARTAGs -- Autonomous Reciprocating Tethered Airfoil Generators and vote for the ARTAG design of your choice. I would appreciate your reviews. And while you're there please tell me why my postings are as wide as a mile, when other's postings look so reasonable.

But you're right Bruce. My professional experience is in embedded software and electronics. I could automate the sensors, servomechanisms, flight patterns, etc. I am rapidly coming up to speed regarding the aerodynamics. I would greatly prefer that someone knowledgeable and committed do that, but as yet no one has stepped up to the plate, so I learn as best I can. I sincerely appreciate your help. I am thinking though of approaching some schools of aeronautics to find someone who might wish to apply for a grant parallel to the one I am applying for. I could get a grant to develop the automation. Another could get a grant to develop the aeronautics. Could either of you recommend someone who would be interested in working with me on this. The grant application deadline is approaching the end of January.

I firmly believe this project could really take off if we can get enough people and/or resources on board to reach critical mass.

Regarding the earlier paper I wrote, you may remember that I read somewhere that doubling the tether diameter multiplies the drag by 2.5 The effect of this is that if all of the kite dimensions are doubled, the lift quadruples and the tether drag multiplies by 2.5 meaning that the over-all lift to drag ratio improves by a factor of 1.6 The point of all of this Bruce, is that if we are successful in small scale, we should have a smashing success if full scale.



-- Wayne

As far as the foam or other shaped fairing goes I don't see it being a basic hardware store generic stuff. I'm thinking of either some form of extruded high desity polyethylene foam or perhaps an extruded plastic form that has a split or zipper joint in it that is payed out onto the tether cable as it passes off the reel. The idea being that the fairing would not have to be subjected to the horrendous crushing forces of the cable on the drum with 10,000 lbs of tension.

Yeah, scaling up is always a good thing.

I've quickly read over your other post and there's so many items to consider that I really want to take another look. I may just post separate answers for each item or I'll be at the 'puter all night.

One thing I'd like to know. Is the ARTAG going to be lighter than air or is this just an inflatable wing using regular air?

Any single one of these will work generating power.
But economically, it can't compete.
The "other" concerns; volume of space just one of these requires relative to the large number of ground-based generators in the same area; the environmental hazard of a malfunction; wind dropping below flight-sustainable values with the tether fully extended; a broken tether...
As the generator is now on the ground, raise it on a pylon, screw on a prop....

Bruce,

Knowing as little as I know, I thought that [link]http://12.45.56.6/%7Ebirdworks/bwaven.htm[/link] might make a good basis from which to develop a heavier-than-air small scale prototype. Making inflatable wings, lighter-than-air or otherwise, is probably an excercise all in itself. In order to get the dollars flowing in I need to be able to demonstrate the generation of electricity in sufficient quantities to be interesting. After that, we can focus on the inflatabale wings that should bring improvements.

Tall Paul,

Starting out of the gate, the ARTAGs would be demonstrated on ground, but their real place is probably off shore where there are vast reaches of ocean just ready to suck in some unsuspecting kite. But that is why we self-launch only in a good breeze and reel the kite back in when the wind is insufficient (self-landing). It hasn't been said in so many words, but very large scale commercial versions of these kites would probably have something similar to the Local Area Augmentation System (LAAS) that will allow aircraft to launch and land using GPS in just a couple of years. Until then, we will probably have to rely on small inexpensive GPS modules that are equipped with Wide Area Augmentation Systems (WAAS). This allows accuracy to within 3 meters. This should be just fine when we are flying. But when we are automatically landing it is going to be a real question whether we will be able to extrapolate the GPS readings to accurately guide the kite back to a safe landing, in the proper configuration for another automatic launch when the wind picks back up.

No point in pooh poohing these free flying kites because they have to land in low wind. Granted they would be downed and inactive, but during such periods wind turbines would just be standing idly by also. The fact of the matter is that each of these points will require development and refinement. We can only develop and refine a few features at a time.

-- Wayne