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Where is the next thermal?
On Sep 25, 6:51*pm, Frank Whiteley wrote:
On Sep 25, 5:08*pm, wrote: On Sep 24, 5:02*pm, Martin Gregorie wrote: On Wed, 23 Sep 2009 17:53:24 -0700, snead1 wrote: Shat is a thermal detector? Typically an DC amplifier watching a tiny, naked fast response thermistor on a 5m (15ft) pole. The thermistor should be sensitive to air temp, so it is fitted with a sunshade to keep direct or reflected sunlight off it. They are also typically high resistance units (20K is a sensible minimum) so the sensing voltage doesn't warm them. If this happens the detector is sensitive to wind speed - something we don't want. Output is normally an analogue dial or a chart recorder built from RC servos though I have seen one with an audio output tone that rose and fell with temperature. Thermal detectors can be quite sensitive. Full scale deflection with a 0.8 C temperature change is not uncommon, so the better units are designed to let the zero setting track average day temperature. All have a gain control, needed because the temperature swing as a thermal blows through rises during the day, peaking in mid-afternoon. Some people use digital thermometers, but there are problems - the sampling rate is often far too slow (usually every 3 or 10 seconds) and the sensitivity to small temperature variations is limited by the display. I've seen none that can show changes of less than 0.1 degree.. That's probably more than you wanted to know, but there you go. -- martin@ * | Martin Gregorie gregorie. | Essex, UK org * * * | Does anyone have experience with using thermal detectors to increase the "get away rate" when auto or winch towing full size gliders? Bill When driving the winch, I often advise whether to turn left or right at the top of the launch. *Most don't listen. Frank Radio exchange immediately after winch launch:-) Frank: "Why are you circling right? I said turn left. Me: "I'm climbing at 8 knots." Frank: "Oh." Actually, a system which could reliably locate thermals in the vicinity of a winch would be extremely useful even though most pilots find a thermal and soar away on better than 50% of their launches in thermic conditions. AFAIK, there are three methods that show promise. The cheapest is anemometer/wind vanes on the four corners of the airfield which can show surface inflow to a thermal. Connecting these with radio modems and plotting the wind vectors on a laptop screen should indicate the surface location of a thermal. Wireless weather stations are off-the-shelf items. Both lasers (LIDAR) and microwave (RADAR) can directly detect vertical air motion by reflecting the beam off entrained dust, pollen and/or insects. None of these are exactly cheap but that has to be weighed against the utility of reliably finding thermals from a cheap winch launch. |
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