Radius of turn

[Alteburger]

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

[galaxy flyer]

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

[PlaneTruth]

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

[OzExpat]

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:

[Leo45]

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.

[Alteburger]

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

[LEM]

Maybe the easiest is :

one third of your NM/minute


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