THE GEEK
GETTING THE MOST OUT OF CROSS COUNTRY FLYING WITH A SIMPLE GPS Knowing wind direction and strength while soaring is critical in order to find and work lift, avoid getting rotored onto hillsides and to land safely. The calculations in this article involve simple addition and multiplication only. If you understand the problem you are trying to solve, the units and so forth, you can easily get the answer! All soaring pilots should know these equations for their own safety. Never fiddle or look at your GPS when close to other gliders or hills, or if you have critical flying tasks to complete. This is a well known cause of accidents. GPS Satellites provide your GPS unit with its position in terms of geographical coordinates (Northing and Easting), and your time at those coordinates. From continuous measurements of position (coordinates) and time, your GPS unit calculates groundspeed as follows:
Wind calculation by Cruising Method Calculating your headwind or tailwind component while cruising in a straight line. This method is useful to help calculate your best speed to fly.
Speed = Distance / Time e.g. 100 kts = 100 Nautical miles / 1 hour I prefer setting speed in knots on my GPS, not kph, because measuring in knots assists me in making wind calculations while flying. There are only two wind calculations you need to know as explained below: Wind calculation by Circling Method Calculating true wind speed and direction while circling. Firstly, you need to convert your indicated airspeed to true airspeed by adding 2% per 1000 ft to Indicated Air Speed to get True Air Speed (this is approximate only). E.g. at 10,000 ft, 10x0.02 = 20% increase from IAS to TAS; to simplify this calculation we use 10 x 2 = 20% increase for 10,000 ft. For example, at 100 kts and 10,000 ft, in your head just say 100 kts indicated x1.2=120 knots true airspeed. For 5000 ft 100 kts indicated airspeed, in your head just say100 kts x 1.1 = 110 knots true airspeed. This table gives more examples at different altitudes for 70 knots indicated airspeed. Altitude (feet)
Firstly, do constant air-speed 360’s to calculate wind. It doesn’t matter what your constant indicated airspeed is, so long as it is constant. And no correction is required for Indicated Air Speed to True Airspeed because they cancel out due to taking readings on reciprocal headings, that is, your fastest and slowest leg. Secondly, True wind speed = (heading on your 360 with highest groundspeed – heading with lowest groundspeed) / 2 e.g. (70 ktGS – 50 ktGS) / 2 = 10 kt True wind. True wind direction is your heading with highest groundspeed. Simple as that! 38
April 2009
From indicated to E.g. at 70 kts indicated True Airspeed
20,000
+40%
98 kt true airspeed
10,000
+20%
84 kt true airspeed
5000
+10%
77 kt true airspeed
2500
+5%
73 kt true airspeed
Secondly, take the ground speed from your GPS and add or subtract it from your true airspeed calculated in step one above for headwind or tailwind component. For example at 100 kt IAS at 10,000 ft, +20% = 120 kt TAS and if GPS reads say 70 kt you have 50 kt headwind component, which is quite different to the wind direction and strength we just calculated in the circling method above; however, if you’re heading straight into the wind or directly with the wind, it is true wind strength not just head or tailwind component that you are measuring while straight line cruising.
