# High drag coefficient

Geez I think my rescue parachute is like 30 years old! My glider is like 40. Well I have a few gliders here now, just because I have the room for storage, so people leave me with all kinds of stuff. I even have an old “standard Rogallo” - same model as I had in the 1970’s, different color…

So anyway, yes a string of parachutes is about the lowest level of design thinking for AWE, something a child could (and did) think of long, long ago, - in fact, the idea brings to mind the character “Captain Obvious”. One more way to bring the worst-performing type of drag-based wind energy device (Savonius) into the sky. Will it prove superior, or even iuseful? Hard to say. At least it “seems” re;atively simple. But of course, AWE as a whole “seemed” relatively simple 15 years ago, when so many were pursuing it. Today such people are realizing that wind energy turns out not to be as easy to improve upon as they originally thought. Who knew?

Updated preprint:
Airborne Wind Energy System based on steerable Rogallo rescue parachute
DOI: 10.13140/RG.2.2.32994.13762

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I had mentioned the document that I identify below, but without realizing the consequences on the significant variations of the Cd as a function of the Reynolds number, as shown in Figure 3.
https://asmedigitalcollection.asme.org/FEDSM/proceedings-abstract/FEDSM2009/43727/2285/346666

I quote again this passage on the abstract:

The Reynolds number is varied from 78000 to 3900000 (1 m/s to 50 m/s). It is found that, for a parachute without a vent at the top, as the Reynolds number is increased from 78000 to 800000, the drag coefficient is decreased from about 2.5 to 1.4, and then as the Reynolds number is increased to 1500000, the drag coefficient increased to about 1.62 and it stayed constant for higher Reynolds number up to 3900000.

Note that Figure 3 shows a particularly rapid decrease in Cd as the Reynolds number increases from 78,000 to approximately 390,000, i.e. an air speed from 1 m/s to 5 m/s.
We can deduct from this that the drag coefficients Cd but also the lift coefficients Cl will be lower at a wind speed around 10 m/s compared to those for a sink rate lesser than 4 m/s.

So are updated the publications related to high drag coefficient (which could be not so high in the use conditions with high winds (and lower Cd?):

To complete:

Excerpts:

2. Literature review 2.1 Drag Analysis
Drag analysis is a weak function of the speed of descent and decreases exponentially at higher velocities, influenced by a combination of factors, including the Reynolds number at high speeds and changes to the canopy during high speeds.

Conclusion : […] as canopy size increased, both drag coefficient and stability increased substantially.

Another publication:

Indeed this publication could be more relevant for a preliminary assessment, because the high diameters of the studied parachute can match the dimensions of the investigated AWES. Table 1 mentions diameters from 29.6 ft. to 156 ft. for a drag coefficient Cd from 0.67 to 1.1 according to an increase almost consistent with that of the diameter, then 183.8 ft. and 189.6 ft. for respectively Cd of 0.84 and 0.92. And Fig. 17 represents a decrease in Cd ranging overall from 1.1 to 0.7, while the rate of descent increases from 20 FT/SEC to 35 FT/SEC to stabilize after, which corresponds to average wind speeds. We therefore see that the drag coefficient Cd decreases significantly when the air speed increases from 20 FT/SEC to 35 FT/SEC.