Radius of turn
 

Anyone have a cunning rule of thumb for radius of turn with respect to groundspeed? ie turning onto a dme arc

Two USAF rules of thumb (we're not any better than a thumb):

Mach No. squared, i.e. M.80= turn radius of 6.4 nm

Groundspeed (in nm per minute)- 2 i.e. 360 Knot GS = 6 nm/min = 4 nm turn radius.

There are others for the mathematically inclined (not me), if you need more accuracy than that you are not flying, you are computing.

:D :D

Alteburger,

GalaxyFlyer's fomula is predicated on (I believe) a 30 degree bank angle. If your using 20 or 25 or Std Rate, you'll need more of a lead point. But, you knew that already.

PT:8

Yes, for a bank angle of 25 degrees, I use G/S divided by 100 and find it works out consistently for me. Thus...

360 / 100 = 3.6 NM
300 / 100 = 3.0 NM
250 / 100 = 2.5 NM and so on. :ok:

In a Rate One turn situation:

Half a circle is performed in one minute.
Therefore: PixR= TAS/60 ; R being the radius of turn and TAS/60 the distance flown in the air during one minute if you expressed TAS in mph, kph, or kt.

R= TAS/(Pix60)

Pi being 3,1415... the product (Pix60) is fairly close to 200.

Therefore: R= TAS/200

If TAS is expressed in Kt the latter formula gives you the radius of turn in the air (for a rate one turn) in NM.

You can replace TAS by GS which gives an approximation of the raduis of turn on the ground (though in a turn for a given wind speed conditon, the Gs is not constant).

Application:

You are tracking inbound to a VOR/DME to take up a 5 Dme arc.

Your ground speed indicator reads 120 kt.

You can assume that the radius of turn is R=120/200=.6 NM

The latest distance reading at which you turn to position on the arc is 5.6 NM.
However I would suggest to give yourelf a bit more leeway and turn at 6 rather than 5.6 and then adjust the rate of turn accordingly.

Thanks very much for your time guys. Shall try them out and show that LNAV how to do it!

Maybe the easiest is :

one third of your NM/minute


example 180 kts => 3NM/min => 1 NM is your radius