Public Lab Research note

This is part of a series on diy-wind-sensors.

LEAFFEST Aeropod flight

by cfastie | October 03, 2017 02:59 03 Oct 02:59 | #14993 | #14993

A LEAFFEST goal was to fly both an Aeropod and a SkyPod to compare their sensor results. Both can measure wind speed, temperature, humidity, and barometric pressure. The wind was not strong enough to lift both on the same kite, but we did fly the Aeropod under a seven foot Rokkaku kite. A really nice thing about the Aeropod is that it weighs a lot less than my camera rigs, so it doesn't take much wind to lift it.
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Above: The Rokkaku had no trouble lifting the Aeropod. Photo courtesy Bryce Taylor.

Above: I tried to fly a camera rig, and it took this photo when it was about as high as it ever got. Bryce was documenting the flights with video so I will soon be famous.
To facilitate the original plan to lift both devices with one kite, I changed the suspension of the Aeropod. I was worried that the Aeropod hanging by a 15 foot line from the kite line might swing into the kite line or into the other rig. So I made a three foot long stiff pendulum from a carbon fiber tube. It has the same flexible attachment to the kite line as KAPtery pendulums, so there is some resistance to swinging back toward the kite line. .

Above: A three foot carbon fiber pendulum replaced the 15 foot string.

Above: A stiff wire at the top of the pendulum allows the kite line to be wrapped around it (it must form a continuous spiral for the entire length of the wire). The reinforced tubing is flexible so it damps swinging, but I insert a short piece of carbon fiber tube in the top to stiffen it and help keep the pendulum from swinging back toward the kite line.
Above: A hole through the bottom end of the pendulum shaft allows a locking cotter pin to hold a swivel. A short piece of string attaches to the top of the Aeropod post.

My pendulum weighs four times more (48 grams) than the 15 foot long string suspension that arrived with the Aeropod (12 grams). I don't yet know whether there are any other disadvantages to the pendulum. One problem that is solved by the long string is that it offers no resistance to spinning. The Aeropod measures wind direction, so the horizontal shaft must be able to point freely into the wind. A nice ball bearing swivel ships with the Aeropod, but it is probably irrelevant because the string will twist more easily than the swivel will swivel. With a 15 foot long string, those twists will not make much difference. The much shorter string at the lower end of my pendulum could conceivably get so twisted that it could resist the free rotation in response to changes in wind direction. But when flying on a kite line, the wind is almost always coming from the same direction, and I didn't observe any spinning of the Aeropod. So other than the weight difference, the pendulum could be a good solution.
Above: Temperature, wind speed, and elevation at the Aeropod during a 20 minute flight. The wind speed was not as variable as these data suggest (see text).

Above: Wind speed and direction during the 20 minute flight. These results suggest that the wind was almost directly from the north, and that was confirmed by casual observations on the ground. Note that the outlier wind direction data points during the second half of the flight are all associated with periods of low apparent wind speed (when the wind was less effective keeping the Aeropod pointed into it).

In the graphs above there appear to be three periods when the wind speed dropped. Those are times when I was walking down wind with the kite line. The first one was an attempt to coax the kite a little higher which is what happened when I walked back up wind. The apparent wind speed tripled when I did that. The second and third dips in wind speed are when I was pulling in kite line to end the flight. To make my job easier, I walked down wind while winding in line. Then I turned around and walked up wind which increased the measured wind speed and temporarily slowed the descent of the kite (and Aeropod).

Above: The same temperature and elevation data from the graph above is presented here as an atmospheric profile. The Aeropod temperature sensor responds quickly enough that the data show the relationship caused by the adiabatic lapse rate.

Above: Relative humidity shows a linear relationship with elevation during the ascent, but that relationship changed during the descent. I'm not sure why that was. Maybe some more humid air blew in at the end of the flight.
Above: The relationship between wind speed and elevation is weak. That probably has a lot to do with my walking up wind then down wind with the kite line.

Atmospheric profiles of environmental variables seem to be the goal of the Aeropod program. Although I had 1000 feet of kite line out, about 350 feet were above the Aeropod, which climbed about 430 feet (140 m) above the ground. The temperature and humidity profiles are more or less as expected (graphs above). The wind speed profile does not show much of a relationship with elevation. I guess that is primarily because, unlike temperature and humidity, wind speed measurements include information about both the movement of the wind and movement of the Aeropod. When I was walking around with the lower end of the kite line, I was exerting as much influence on measured wind speed as elevation was. Getting good data for a wind speed profile will require careful kite line handling.

This was my first flight with an Aeropod. Maybe I am now ready to attempt to attach both an Aeropod and a Saturn V Rig to the same kite line. The Saturn V Rig will carry two SkyPods -- one recording GPS data, one recording wind speed, and both recording temperature, humidity, and barometric pressure. The Saturn V Rig can also be taking photos, but I have not decided whether to orient everything so I get photos of the Aeropod or photos to stitch into a spherical panorama. It might be possible to do both (a spherical panorama with the Aeropod in it), but then I would be doing things not because they are easy, but because they are hard.

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