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May 15, 2011, 01:54 PM
Team Hello Kitty
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Wind speed gradient discussion

After yesterday's windy contest it got me thinking just how strong the wind was gusting at launch and flying altitudes. Also, with knowledge of the wind gradient in the landing pattern, it can be a big help in understanding the need for extra airspeed on descent through a steep wind gradient during final approach. i.e. airspeed rapidly decreases while sink rate increases (ref)

Below is a chart showing the wind speed factor to apply to a handheld wind meter reading taken at 6 feet above ground. This will produce a fairly accurate estimate of wind speed at different elevations. The calculation is based on excellent references from these sources (Excel spreadsheet attached if you want to play with it): ref1 ref2

Wind shear exponent to use in calculation:
Smooth Surface: 0.10
Low grass/fallow ground: 0.15
High grass/low row crops: 0.18
Tall row crops/low woods: 0.20
High woods w/ many trees, suburbs, small towns: 0.30
General rule of thumb (independent of ground cover): 0.14

Assumptions for chart below:
1. Flat grassy ground with medium-length grass
2. Wind speed meter held at 6' above ground

Practical Summary: For a given wind reading taken at 6 ft at the SVSS grassy field, the wind speed at 75' is 1.5x; at 500' and above it is ~2x.

Example uses:
1. Say you took an average wind speed reading of 15mph at 6 ft above ground. To estimate wind speed at 250 feet, multiply 15 * 1.8 = 27mph.
2. Say the wind gusts at 6 ft above ground were registering 25mph. To estimate the gusts at 750 feet, multiply 25 * 2.2 = 55mph.

Question: Given a 15mph average wind speed reading at 6', and given a flight altitude of 750', how fast will a glider travel downwind in mph and feet/min when it is thermal circling with one circle every 6 seconds? (i.e. a relatively tight circle)

Any other thoughts on best practices in high wind conditions?

Chris B.
Last edited by SoaringDude; May 15, 2011 at 02:02 PM.
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