INTERNATIONAL
FROM THE COCKPIT
Bill Lavender bill@agairupdate.com
Distance to Make a Turn
With Google Earth® and other software, this tracking can equate into an accurate calculation that can fit a variety of scenarios.
A 6 | agairupdate.com
Recently, the National Agricultural Aviation Association composed excellent data to refute proposed setback distances from wind turbines and other tower-like structures that may be encountered by ag-pilots. The NAAA is commended for addressing this and has done a very good job of it. The Association used two methods to calculate a more appropriate setback, far more than the 500 feet proposed by a wind farm sponsor. One method was a theoretical mathematical calculation using an AT-802. The other method, which I believe provides the best possible scenario, is analyzing the GPS turn tracks of different aircraft at different phases of the application. With Google Earth® and other software, this tracking can equate into an accurate calculation that can fit a variety of scenarios. Just for kicks, in this editorial I decided to play around a little with calculating the diameter in feet of a turn at different airspeeds and bank angles. See if you can follow the math and apply it to your situation. The following is how to estimate your diameter of a turn at various airspeeds and turn rates (bank angle). Let’s use some basic rules-of-thumb that addresses this exercise. For the bank angle of a turn, there is a simple way to calculate that is readily remembered. If we deal in knots, for a standard rate two-minute turn you simply add 12 to the first number of your airspeed to find a bank angle which will result in a 3° per second standard rate turn. For example: 60 knots = 6 + 12 = 18° of bank, 90 knots = 9 + 12 = 21°, 120 knots = 12 + 12 = 24°, and so on.
For an easy, mental arithmetic rule-of-thumb to find the diameter of a standard rate two-minute turn with a no-wind condition, you simply multiply the first, or first two numbers of your airspeed by 6.4 and add a couple of zeros to obtain your turn diameter in feet. Some examples are: 90 knots = 9 x 6.4 = 5,760 feet, 100 knots = 10 x 6.4 = 64 = 6,400 feet, or 120 knots = 12 x 6.4 = 7,698 feet. To determine the diameter of a standard rate oneminute turn, use a 3.2 factor instead of 6.4. Suppose you want to make 180° turn by turning downwind initially (racetrack) with a direct 10knot crosswind and little, or no crab angle before the turn. A 3° per second turn rate equals 60 seconds (180°/3) and a 6° per second rate of turn equals 30 seconds. An air mass moving at 10 knots is about 17 feet per second. So, 60 seconds x 17 = 1,020 feet for the 3° per second rate turn to shift downwind (the higher 6° rate of turn is calculated as 30 seconds x 17 = 510 feet). Add 1,020 feet to the diameter of the 3° per second turn rate with a true airspeed of 100 knots and 22° bank angle to 6,400 feet and the aircraft will be about 7,420 feet downwind, over one nautical mile (6,070 feet). This could be an example of the distance between GPS lines with a racetrack configuration. However, on the return racetrack pass, you would be turning into the wind. Let’s see how that works out using the same angle of bank, constant airspeed and no crab angle. The diameter of the turn is 6,400 feet turning into an air mass that is moving 1,020 feet per minute equals a tighter turn diameter of 5,380 feet; a difference of 2,040 feet. That is quite a significant difference that ag pilots compensate for in their racetracking mostly with
