Info needed on Wind Direction and Strength Computer???

A while back in Soaring magazine I read in an article of a computer/instrument in a sailplane which calculates the wind strength and directions. According to what I remember, the pilot had to start by flying a few circles in order to let the computer make its calculations, and there after it would automatically (and continuously?) update the wind strength and direction.

I have two questions:

1) How is this possible? How can you calculate wind strength and direction if you do not have another source to derive the information from? (E.g., if I know what the sialplane HEADING (not ground track) is, and I can calculate ground track and ground speed from GPS, together with true airspeed, I can see how one can derive that information. Is there another way? Is there something simple I am missing? (But it is difficult to determine the sailplane heading...!)

2) What are the leading brand names (and model numbers) of computers/instruments to calculate wind speed and direction?

Thank you in advance for your reply.

Cameron cpn66 at hotmail dot com

Cameron a écrit :

GPS computer can do it. If you turn is constant, computing drifting on all available points of your circle (which is oval) is enough to give wind velocity. You need at least two turns, but more is better.

With ground speed and track data from a GPS, if it is assumed that flight in consecutive circles (i.e. thermalling), one can use the movement of the centers of the circles to calculate wind drift (which may or may not be an accurate indication of wind speed). If it is assumed that the circles are flown at constant airspeed, vector addition on the segments between GPS fixes can be used to calculate a wind vector (speed and direction).

If TAS is available in addition to GPS, it is possible to calculate the wind vector in a more direct fashion, in circling flight or straight flight on a few different headings. The quality of the result is very dependent on the accuracy of the reported TAS and on the accuracy and latency of the ground tracks reported by the GPS. Finally, adding a digital compass to the system makes it possible to calculate the wind vector in straight flight on a single heading, but the compass must be calibrated and accurate to within a degree or two to provide meaningful results.

All GPS-based flight computers, that I am aware of, do some form of wind vector calculation, along with headwind/tailwind components (using TAS) for final glide. Wind calculations are also performed by most GPS-based glide software on handheld computers, as well as many GPS-based flight recorders. You'll have to be more specific about what you are looking for...

I understand that you can compute wind directing and velocity using drift. However, to compute accurately, would that not need the pilot to hold both a constant bank angle, airspeed, and altitude to compute? Holding all those variables constant for several complete turns while circling (thermaling) would seem very difficult to do...

Thanks.

Cameron cpn66 at hotmail dot com

lots of stuff trimmed.

On top of all of this you have the fact that the drift in the thermal is not actually at the same speed at you will feel when you roll out and head off on track. Thermals don't drift downwind quite as fast as the wind.

I think the thermal drifts as fast as the wind, but the circles flown by the the glider don't. As we all have experienced it, and as Reichman explains it very well in his book, when we stay in a thermal, we often need to re-center it by moving our circle upwind, this is due to the oblique shape of the thermal combined with the fact that the glider sinks in the thermal, i.e. its climbing speed is lower than the upward speed of the rising air.

What differences in speed and direction did you see?

How do they stay behind the thermal without running into it? Or are they orienting themselves directly downwind of a gaggle before flying towards it?

Hmmm.... didn't think of that.... Maybe I should read Reichman after all.... By the way: the original poster talked about 'first flying a couple of circles'. I think he did NOT mean thermalling. That would re-validate the system of measuring the drift, as the effect that one tries to stay in the thermal will not occur.