Hi Pierre,

          Yes, Flettner rotors (L/D = 3 to 4) used as pull-kites should be able to fly at a high elevation angle. So the tethers should be shorter for reaching a given altitude, as compared to a cross-wind moving kite.

          An SRVK will have a L/D that may only be 1 to 1, and a tether angle of 45 degrees. That it because it produces torque, and torque creates high drag. So an SRVK or an FRVK will not have a high elevation angle – as you noted.

PeterS

Hi Pierre,

          Yes, as far as I know, which isn’t much, the longer the tether, the greater the land use. But using a tether angle greater than 45 degrees doesn’t substantially shorten the tether, so I’m not clear on why you want to achieve a higher tether angle than 45 degrees.

          There would be no advantage, as far as I can tell, to fly an SRVK at a tether angle greater than 45 degrees. To do so, it would be necessary to sacrifice a lot of torque and power. Yes, the wind speed will be a little stronger higher up, but that won’t be enough to compensate for the reduction in power required to lower the L/D ratio of the SRVK. A higher tether angle than 45 degrees would mean a loss of power because, as far as I can tell, some torque would need to be sacrificed in order to create more lift. Torque increases VAWT kite drag, and drag lowers the L/D ratio

          If I were trying to design a pull kite using a Flettner rotor or a Sharp Rotor, I would start by assuming a conservative L/D ratio of 2 to 3 rather than an optimistic 3 to 4.

          Many published papers contain typos. So it’s hard to know if an author has actually made a mistake. If I disagree, I may be disagreeing with a typo. Or, I may be wrong. So I try to consider the preponderance of evidence.

PeterS

Hi Pierre,

          That all sounds right to me.

          A HAWT type rotor that is tipped to the wind reduces its swept area. And if the blades are fixed, then the angle of attack cannot be optimum during a full revolution. But variable pitch might be able to partially compensate. If the blades were Sharp Rotors, that would keep the blades at their optimum “angle of attack” during a full revolution.  That should be the case for the Sharp Rotor HAWT Long-Pull Kite.

PeterS

Hi Pierre,

          The attachment didn’t open.

PeterS

Hi Pierre,

          The attachment is quite helpful. I can see that a tether angle greater than 45 degrees does in fact significantly reduce the land area.

          The website wouldn’t open for me. Don’t know why.

          In any case, now I do appreciate why you want to use a higher tether angle. Thanks for explaining.

PeterS

From: Pierre BENHAIEM [mailto:pierre-benhaiem@orange.fr]
Sent: Saturday, August 18, 2018 12:00 AM
To: AirborneWindEnergy@yahoogroups.com
Cc: Peter A. Sharp <sharpencil@sbcglobal.net

Hi DaveS,

          My reaction to the ACK project is different from yours. I read a lot of research papers and many of them are done as senior thesis papers. There are usually a number of authors. The quality tends to be low, most often because the students have not adequately searched the available literature, so they tend to duplicate what has already been done, often without realizing it. Such groups typically have a faculty advisor. What bothers me is that, although doing most any project will be an “educational experience”, the students too often learn poor research methodology. And research methodology is the most important thing for them to learn at that stage of their education.

          With respect to their results, the ACK project was a high school level project, or maybe a first year college engineering project. As part of a senior project, if that is what it was, it was an inadequate demonstration of competence for professional engineers about to enter the work force. They should have been held to a higher standard. They demonstrated sloppy work, which implies that their faculty advisor condoned sloppy work. So I fault their faculty advisor for not doing his or her job.

PeterS

Hi Pierre,

          Thanks for posting this information.

My reaction to the LTA blimp kite is that it is too heavy and too expensive and could be improved by using a different design. For this  design, saving weight is critical for minimizing costs.

I would use the huge blimp body as a type of wind speed enhancer (shroud). Mount a single, small HAWT inside of the nose of the blimp. The opening in the nose would be considerably larger than the HAWT diameter. Then use slanted, radial channels behind the rotor that extend out to where the blimp diameter is the largest. That creates a Bernoulli effect speed up of the air passing through the rotor.

The air flowing around the blimp will be speeded up considerably at that maximum diameter, and so will provide a strong suction effect due to its lowered pressure. That will suck additional air through the rotor, thus further multiplying its RPM and its power. So the generator could be small and light.

It should be possible for one small, high RPM HAWT in the nose to produce as much power as the two, large, low RPM HAWT mounted on the wing tips. The total weight would be greatly reduced. And the total drag should be greatly reduced as well by eliminating the large, external HAWT. So the L/D ratio of the blimp as a whole should be much higher.

Mounting large rotors at the tip of the short wings has the effect of reducing airflow over part of the wings which are in front of the HAWT rotors. So eliminating those rotors will increase the lift of the wings, thus further increasing the L/D ratio of the blimp kite as a whole.

By feeding air to the slots around the largest diameter of the blimp, the boundary layer on the rear portion of the blimp should stay attached longer and be less turbulent. That should lower the drag of the blimp and further enhance the L/D ratio of the blimp as a whole.

PeterS