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Originally Posted by scifi
(Post 10533983)
Just wondering if the OPs original formula should have had a divide by 2 sign, instead of a minus 2 sign...?
Still gives about the same answer... . |
Originally Posted by scifi
(Post 10533983)
Just wondering if the OPs original formula should have had a divide by 2 sign, instead of a minus 2 sign...?
Still gives about the same answer... . |
Circumference of a circle = 2Πr.
r = (Half Circumference) / Π. Rate 1 turn... 360° turn takes 2 minutes... 180° takes 1 minute. so... Ground speed in NM/Min. 180 kts --> 3NM/min. So in 1 minute you cover 3 NM and in this example that's half the circumference. So if you assume Π ≈ 3, your radius become ≈ 1 NM. Ground speed in NM/Min. 120 kts --> 2NM/min. So in 1 minute you cover 2 NM and in this example that's half the circumference. So if you assume Π ≈ 3, your radius become ≈ .66 NM. Ground speed in NM/Min. 210 kts --> 3.5NM/min. So in 1 minute you cover 3.5 NM and in this example that's half the circumference. So if you assume Π ≈ 3, your radius become ≈ 1.15 NM. |
From the since rescinded US Air Force AFMAN 11-217, Volume 3, page 65:
Turn Radius Calculation. The following two relationships provide the distance required to turn an aircraft 90° using 30° of bank. This distance is the aircraft’s approximate turn radius. These formulas are particularly useful when determining lead turn points when planning to perform a radial-to-arc or arc-to-radial portion of an instrument procedure Formula 1: Turn radius [in nm] = (True Airspeed [in knots] / 60) - 2 or (Mach × 10) – 2 Formula 2: Turn radius [in nm] R = (True airspeed [in knots] ÷ 60)^2 / 10 or Mach^2 × 10 If you are flying in against a ground-fixed reference (e.g. DME arc, VOR radial, FMS Track-to-fix or course-to-fix leg), use ground speed in lieu of true airspeed |
Originally Posted by Le Flaneur
(Post 10535111)
Formula 1: Turn radius [in nm] = (True Airspeed [in knots] ÷ 60) - 2 or (True airspeed in nm/minute × 10) – 2
Formula 2: Turn radius [in nm] R = (True airspeed [in knots] ÷ 60)^2 ÷ 10 or (True airspeed in nm/minute)^2 × 10 In each case, the bit before the "or" is OK, but the part after should use True Mach, not TAS nm/min. |
Originally Posted by DaveReidUK
(Post 10535125)
Those formulae don't make sense.
In each case, the bit before the "or" is OK, but the part after should use True Mach, not TAS nm/min. |
Originally Posted by DaveReidUK
(Post 10530342)
One of the simplest rules-of-thumb that I've seen (assumes limiting bank angle of 25°) is to square the groundspeed (in nm/min) and then divide the result by 9, to give radius in nm.
Turn Radius with a full proof. Also suggests dividing by 10 as it's clearly easier and "in most cases is close enough". To get a better approximation of dividing by 9 you could divide by 10 and add 10%. So - taking the example of 25 25 / 9 = 2.78 25 / 10 = 2.5 2.5 + 10% = 2.75 To see the difference between 1/9 and the new approximation 1/9 = 0.111111 Dividing by 10 and adding 10% can be expressed as 0.1 * 1.1 = 0.11 which is very close to 0.11111. It's about 1% different. |
Originally Posted by jimjim1
(Post 10535963)
Dividing by 10 and adding 10% can be expressed as
0.1 * 1.1 = 0.11 which is very close to 0.11111. It's about 1% different. |
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