Testing and outfitting the UW ultralight – October 26, 2017

This morning, I flew the ultralight for what is probably the last time in 2017 unless we have exceptional weather in December (I will be gone most of November). I took it to 1,100 ft. AGL – about four times as high as my previous flight, loitered for a short time, and landed again after only 8 minutes total time in the air. 

I had planned to stay up much longer and to try to burn through most of the rest of the fuel (about 3.5 gallons) before putting the plane away for the winter.  But the winds aloft were quite strong, and for a time, my ground speed (as measured with GPS) when heading directly into the southerly wind dropped to only 2 knots!  Given the 30–35 kt airspeed of the Zigolo,  the winds between ~600 and 1,100 feet must have been pushing 30 kts or so. 

In fact, I spent much of my time at altitude hovering almost motionless over a particular farm house about a mile southeast of the air park!   I realized that if I had headed downwind from the airport after takeoff instead of upwind, I might have had difficulty returning!  Something to think about when planning future science flights.

In addition, there was pretty obvious wind shear accompanied by gusty turbulence between 600 and 1,000 ft. AGL.  One more reason not to push my luck on this particular day, especially given the likelihood that that gustiness would migrate downward toward the surface over the coming hour as the rising sun heated the surface and eliminated the nocturnal temperature inversion that was, up until now, decoupling the stagnant surface air from the stronger winds aloft.

Jonathan had fastened a Kestrel 5500 weather logger to the front of the plane, and the figures here and below (click to enlarge) show the measured temperature and dewpoint profiles.

The temperature profiles tracked surprisingly well between ascent and descent, suggesting temperature sensor response time and repeatability aren’t problems for the Kestrel. We clearly see what was still left of the noctural inversion, which had probably extended to the surface a couple hours earlier when the surface air temperature was closer to 2-3 degrees. If I had been able to get off the ground a hour or so earlier and do multiple profiles, it would have been possible to track the evolution (this would be a great reason for a sunrise takeoff after a calm, clear, dry night!). 

The dewpoint was a full degree lower on an ascent than on descent, only minutes apart. At first I thought of sensor lag but then realized that the difference would be flipped (moister on ascent than on descent).  Then it occurred to me that my descent took me over a 500′-diameter pond that began about 800′ northeast of the approach end of the runway, and that could well have added enough moisture to the overlying air to increase the dewpoint by a degree (at that temperature, only about 0.1 g/kg of added moisture would be required).  [Edit: It’s also possible the humidity sensor simply drifted by that amount over 8 minutes.]

Flight track as viewed toward the southeast. The departure/ascent track begins on the right.

For future flights, it could be interesting to fly a low circle around a similar-sized pond under similar conditions (cool and dry) and see whether we can detected a plume of humidity on the downwind side.  

Total materials cost for this boundary layer sounding was about half a gallon of fuel plus oil, or maybe $2.  For comparison, a conventional radiosonde launch typically costs at least $200 in expendables.

PS: I captured GoPro video from the above flight and will post it at my next opportunity.

Comments are closed.