Can you share your experience with your lifters?

I am thinking about the same.

@Rodread Looking at one of your latest videos, I am thinking when you have so much drag on the kite that if falls pretty far back into the window. It could be stalled, and fly better with a lower AoA on the bridling.

Btw I really enjoy your videos

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Just an update. IMHO high flying kites are generally more useful than downwind kites. So I am interested in L/D and such. I measured this the other day.

image

I added a hollow tail to calm down left-right motion.

67 degrees should mean a L/D ratio of 2.4. I believe this is quite good for a Pilot type kite, in particular one that flies in very little wind and pulls like nothing else.

High wind needs to be solved for this kite though

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That is really interesting talllak. Such measurement interest me and all the kitewinder team. If you could measure flight angle, pull and wind speed (at height) that would be wonderful.
With a variety of kite… Even better

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I have some observation-question to Rod and to Olivier about the size of the lifter in regard to the rotor swept area, and also about the elevation angle. Please correct me as I am not sure for the numbers.
Daisy: the tilted rotor swept area is about 6 to 18 m² (after cosine loss) with respectively one and three layers; the lifter is only 2 m², and the elevation angle is about 30-35°.
Kiwee-one: the non (or little) tilted rotor swept area is a bit more than 1 m², the lifter area is 2 to 6 m², the elevation angle is about 55° (a high) value).

So a tilted rotor would allow save some lifter area, even by considering the difference of both elevation angles (which would require more accurate calculation), no?


2.32m outside radius
1.32m inner radius

sections length circumference pi dia radius
6 1.7 10.2 3.141592654 3.246760839 1.62338042
ring radius outer tip radius inner tip radius total disk swept area inner hole area wing swept area flying angle 1 flying angle 2 flying angle 3 projected swept area 1 projected swept area 2 projected swept area 3
1.62338042 2.32338042 1.32338042 16.95812039 5.501821211 11.45629918 30 35 40 9.92144612 9.38445089 8.776034322
Radians 0.523598776 0.610865238 0.698131701
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For kiwee one flight angle is depending on wind speed. Flight angle at 10m/s windspeed is greater Than 60deg. At that speed, our theoretical extracted power is around 240 watts (by memory). Our measured extracted power is around 210 watts. Wind turbine Drag is about 70 newton.
Thing is, the farthest you are from Betz, the less power you produce and also the less drag. Right now we are really cloth from Betz. Maybe rod is not that cloth. Thus he has less drag… Rod ?
Another difference, kiwee propeller is cloth to the kite, daisy is far from the kite, cloth to the anchor.

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That suggests the swept area is a bit more than 1 m². Please have you the lifter area for these measures, as 60 degrees is a very high value? The 210 W value shows also a high efficiency of the belt transmission.
It would be interesting to know the required lifter area, using an elevation angle of about 35 degrees like Daisy, then comparing the respective materials that are used.

Yes à little more than 1 m2.
Regarding lifter surface area, as soon as you have consistent wind, kiwee can operate with a 2m2 pilot kite. If the wind is weaker, we need a 4 m2. That is why we choose 4 m2 pilot.
60 degree is not a really high value in consistent winds, more of a common one.
210W is the propeller value. You have to had right angle efficiency and belt transmission efficiency in the calculus if you want to look at power production. For flight angle concern, only propeller characteristics matters

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Please have you the drag and lift values at 10 m/s wind speed (or both and the elevation angle) for the 4 m² pilot alone (without the wind turbine)?

I don’t, talllak has began some measurements, maybe he has that

Assuming wind speed is 10 m/s, the turbine drag is 70 N, the elevation angle is 60 degrees, the kite area is 4 m², the wind turbine would be 1.2 m² and its drag would be equivalent to a 1 m² area perpendicular to the wind and with a drag coefficient of 1.

60 degrees should mean an L/D ratio of 2, that with the wind turbine.

Now what is the L/D ratio of the kite alone? Let us take the elevation angle value of 67 degrees from @Tallakt’s assessment, so a L/D ratio of 2.5 (the cosine is 0.3907), then see how it can work.

Let us try with a reasonable lift coefficient of 1, so a drag coefficient of 0.4:
Lift = 1/2 X 4 [kite area] x 1.2 [air density] X 1 [lift coefficient] X 100 [squared wind speed] = 240 N.
Drag = 96 N.
The L/D ratio with the turbine (adding 70 N) is 240/166. That doesn’t work as this value is far below 2. Or that suggests a cosine of 1: (240/166) = 0.69, so an elevation angle of 46 degrees, or a higher angle value than 67 degrees for the kite alone or another variable to be changed.

Now let us try with a yet reasonable lift coefficient of 1.6, so a drag coefficient of 0.64:
Lift = 1/2 X 4 [kite area] x 1.2 [air density] X 1.6 [lift coefficient] X 100 [squared wind speed] = 384 N.
Drag = 153.6 N.
The L/D ratio with the turbine (adding 70 N) is 384/223.6. That suggests a cosine of 1: (384/223.6) = 0.58, so an elevation angle of 54 degrees. It is better but not still 60 degrees.

In fact that works from a very high (although not impossible) lift coefficient of 2.9, implying a high drag coefficient of 1.16:
Lift = 1/2 X 4 [kite area] x 1.2 [air density] X 2.9 [lift coefficient] X 100 [squared wind speed] = 696 N.
Drag = 278.4 N.
The L/D ratio with the turbine (adding 70 N) is 696/348 = 2. It looks correct now.

One question was also a comparison with a tilted rotor like Daisy.
For it let us evaluate Kiwee-one with the same angle as Daisy, so about 35 degrees, that on the basis of the last numbers, using 2.9 for the lift coefficient as this coefficient would allow to reach 60 degrees with the turbine.

35 degrees should mean 1: (0.8191) = 1.22 for the L/D ratio, so 696/570.
570 – 278 = 292 N, so a little more than 4 turbines (70 N X 4) in all.

Now we extrapolate from the second calculation with a lift coefficient of 1.6.
35 degrees should mean 1: (0.8191) = 1.22 for the L/D ratio, so 384/315.
315 – 153 = 162 N, so more than 2 turbines (70 N X 2) in all.

This can be wrong, so please correct me. Probably some variables should be a little different and known in order to obtain a more precise calculation.

Even only two turbines for a kite of 4 m² at 35 degrees would mean a good efficiency (power to weight ratio) of this system. And it could be four turbines. The weight (0.5 kg per turbine) is not yet taken account but looks to be low in regard to the lift of the kite at 10 m/s wind speed.

For what I see, an installation with tilted rotors would have an equivalent efficiency (power to weight ratio) in small scale but this topic should be deeply studied.

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Pierre, all this calculus seems to be right.
We do not measure flight angle right now, just a guess from what I see.
I think you forgot one important thing: the system you design shall work on the larger wind frame possible. 10m/s is a really favorable condition. Kiwee start at 5 m/s. At this windspeed, flight angle is much lower and weight of the flying parts has a huge impact.

I didn’t forget this. It was the reason why I already mentioned this on my previous long post:

Assuming 5 m/s wind speed, this model of calculation would begin with the corresponding angle of elevation and would work in the same way. But it would be different for the weight as I mentioned.

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To complete my previous message:

“Much lower”: even using a 4 m² instead of a 2 m² lifter as you mentioned? This confirms the higher relative value of the weight at lower wind speed and was one of the three reasons I used 10 m/s wind speed, the second being you provided some data with this value, the third being 10 m/s wind speed is often mentioned as a reference value.

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Hi. Today we had good conditions, 5 m/s wind average gusting a lot and sunny.

I brought out my Peter Lynn SSSL 3 m2 pilot with a 10 m hollow tail. I was ready to test:

  • wind speed at kite with anemometer
  • movement of kite with gyro/accel
  • tether force over time
  • remote controlled release

As you can see I have been quite busy in my workshop.

I started out testing the kite with only the remote release. I will release details of this design once I have tested it a bit more. Anyways, at the time I was flying, the wind had picked up gusting over 9 m/s.

The kite flew for 5 minutes with quite a large pull (estimated 20-40 kg force). At one point it started pulling to the right and I was able to save it by pulling the tether, but the second time it was impossible to save the kite.

I can conclude that the SSSL flies as well underwater as in air, it was quite a job to pull it out of the river currents. I did not measure underwater L/D, sorry.

I could also conclude that the radio receiver I used actually does work after submersion in salt water, as does my tiny LiPo battery.

Anyways, I can only conclude that a mass hanging of the trailing edge seems more efficient in terms of pointing the kite to zenith rather than a tail. The tail will stabilize the left right movements but not help much in terms of pointing to zenith at higher winds. (I would like to test this in the future in a more scientific manner)


See how well the kite flies underwater!

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Great to see someone actually building something!
What would you like to lift with that kite?
We’ve got a new category Homebrew suggested by @Rodread. Every system by a forum member gets a thread and they can share their progress there.

I was planning to lift other kites that I am testing. So sort of a huge Yak shave there. The Homwbres thread came just a little late for my post…

6 posts were split to a new topic: Building a RC pilot kite

I think my analysis was wrong as were the results. An explain is below:

It is an affair of tangent, not cosine.

So the L/D ratio is 2.4 which is Tan(elevation angle of 67°).

Here the difference is tiny, but some other differences are larger as shown below.

As lift to drag ratio is 2.4, drag = 100 N.

240/166 = about 1.445, so Tan(55°30’). The elevation angle is 55 degrees 30 minutes, so far higher. Now, by taking account of the lightly corrected L/D ratio of 4, we have 240/170 = about 1.41, so Tan(54°40’). The elevation angle is now 54 degrees 40 minutes.

384/223.6 = about 1.717, so about Tan(59°40’).

1.22 = Tan(50°40’). Here the difference is very large.

For that L/D ratio should be 0.7, so Tan(35°), not 1.22.

If we keep the initial (lightly corrected) numbers for the kite (240 N lift, 100 N drag, 4 m², lift coefficient of 1, so a drag coefficient of 0.416), and the initial numbers for the turbine drag (70 N, 1 m², drag coefficient of 1), we obtain a L/D ratio of 0.7 by the following way:

0.7 = about 240/340; we withdraw 100 N of kite drag; 240 N of drag turbine remain, so the drag of more than 3 turbines (3.428).

Of course with lift coefficient of 1.6, and yet more with that of 2.9, and drag coefficient x 1.6 then x 2.9 respectively, still more turbines could be lifted with the kite of same area (4 m²), at an elevation angle of 35 degrees:
with lift coefficient of 1.6: 384/548, so more than 5 turbines (5.54);
with lift coefficient of 2.9: 696/994, so 10 turbines.

These results are far from the previous.

Subtract the weight of the flying parts from the lift in all cases. Indeed when the wind speed is low this weight takes a larger impact. Example with the weight of flying parts being 25 N: the 4 m² kite is assumed to have a lift of 240 N at 10 m/s wind speed. At 5 m/s wind speed it will have a lift of only 60 N (240/4), then 35 N by taking account of the weight of the flying parts (25 N), the drag being that of the kite, so 25 N (100/4), + the drag of the turbine, so 70/4 = 17.5 N. Lift to drag ratio = 35/42.5 = 0.823, so Tan(39°40’).