Thermal Detection on the ground?

March 19, 2014
Will Gadd

I receive a fair number of questions about ice climbing and flying, and often write back to people. I think I’ll start putting more of the answers up on here for others to comment on as well. Here’s one I found interesting:

 

Hi Will,
I saw your part 3 about thermals from the internet. My hobby is free flight model airplane contest completion and try to pick good thermals. I  know the sun heats the earth and creates thermals. I’m trying to invent a thermal detector to pick when flying my planes. I’m using a strain gage sensitive load device that measures lift on a 32 inch disk about 10 or 20 feet in the air on a pole.
    My question is how much lift would there be on the disk with a normal thermal? I would like to know what to expect in PSI  or grams per square foot etc. I’m trying to set the sensitivity of a analog readout
     Please help me if you can
             Jack
——
Hi Jack, interesting.

That low to the ground I think thermals would have a distinct horizontal component, maybe more than a vertical component. Thermals “pull” air in from the sides as they release upward. If you watch the grass as a thermal releases it “swirls” most of the time.
So maybe measuring changes in wind direction and strength on a hot , relatively calm day would give you more of a clue than trying to measure “lift” on your disc?
When searching for thermals low on a paraglider one of the best indications of a nearby thermal is rapid changes in ground speed.  If your speed accelerates and your sink rate decreases while your wing pitches slightly forward you’re often flying toward a thermal.
If your speed decreases and your sink rate decreases but your wing still pitches forward a bit then you’re flying away from a thermal. If your speed decreases but your real sink rate (not a pendulum swing that your vario measures) remains constant or increases then you’ve just hit a “gust” front on or are in some mechanical turbulence off a ridge or other feature. Novices often try to turn in increased windwind without a thermal component, confusing the decrease in speed and their wing falling back behind them (instead of pressuring and pitching slightly forward or at least not falling back dramatically) with a thermal…
If I were you I would put up a bunch of poles with wind streamers as far you can reasonably can. You can watch the thermals coming as the wind streamers start flapping in sequence. If you watch two flags on the opposite sides of a big field they often will point toward each other when a thermal is releasing out of the field. Airports have this happen a lot on light wind days–the wind socks at either end of the runway will be pointing toward the middle of the runway as that is where the thermal is releasing. That can be frustrating for towing hang gliders and paragliders into the air too.
The simplest “analog” readout would be one pole with some streamers on it. Watch them change direction, get your model into the air.
Curious what you find, please let me know!
Best,
WG