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onebob

F50 - VOR Intercept

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Having a problem when flying towards a VOR, arriving at the VOR the airplane starts to oscillate from side to side, starts about 1.5 miles from the VOR then carrys on until about 10 miles past. the go to/ from arrow works as normal.

No flight plan, take off from Runway 26 KCLM and fly towards VOR TOU 112.20. at 10000'  …   AP on with nav and Alt hold

window 10 (1809) Prepar3d v4.3 …. Falco f50 ver 3.4

edit ..re-edit the heading

Edited by onebob

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13 minutes ago, onebob said:

Having a problem when flying towards a VOR, arriving at the VOR the airplane starts to oscillate from side to side, starts about 1.5 miles from the VOR then carrys on until about 10 miles past. the go to/ from arrow works as normal.

No flight plan, take off from Runway 26 KCLM and fly towards VOR TOU 112.20. at 10000'  …   AP on with nav and Alt hold

window 10 (1809) Prepar3d v4.3 …. Falco f50 ver 3.4

 

bob

If the autopilot in the Falcon is using default FSX/P3D autopilot coding, then this is a common problem that seems to affect most default P3D and FSX aircraft. As you get close to the VOR, the CDI needle starts to become very sensitive, and will usually start moving off of center, eventually going to full deflection left or right as you pass the station - if the autopilot remains in NAV mode, it will start to respond to the fluctuations by rolling one way or the other to track the moving CDI.

The solution is to go to heading mode when approaching the VOR, and not re-engage NAV mode until past the VOR, when the CDI settles down.

Even less sophisticated real-world autopilots will require this technique when tracking VOR radials for the same reason - the instability of the CDI when very close to the station. (Especially if there is any crosswind).

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4 minutes ago, JRBarrett said:

The solution is to go to heading mode when approaching the VOR, and not re-engage NAV mode until past the VOR, when the CDI settles down.

Thanks Jim,

I see we are up to ver 3.5 , just downloaded.

 

bob

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There is a so-called 'cone of confusion' over a VOR. For a side view of the cone of confusion, think of a large V with the point of the V sitting on the VOR and the legs of the V extending upward. The cone of confusion exists between the tops of the V legs. The angle between the sides of the V is about 100 degrees (each leg of the V makes a 50 degree angle with the vertical). So if you are, for example, 4 NM above the VOR (a little over 25000 ft) then the radius of the cone is 4 x tan(50deg) = 4.76 NMs, and the width, or diameter of the cone of confusion that you have to fly through is twice that, almost 10 miles in this example (distance between the legs of the V) . So not a trivial distance. Perhaps a more convenient way to think about it is that for every 1000 ft you are above the VOR station, the width of the cone of confusion is about 0.4NM. So in Bob's flight example above, if the VOR station was about at sea level and Bob was at 10,000ft, the cone of confusion was about 4NM wide (10 x 0.4).

As Jim points out above, the technique is to 'fly the cone' in Heading mode and NOT chase the CDI needle. When inbound, switch to heading mode as the CDI starts to move off to the side, and when the TO/FROM flag flips, turn to your desired outbound heading. You should then be close to the desired VOR radial when you exit the cone (the CDI starts to center again) and you can switch back to NAV mode to pick up and track the outbound radial.

How accurately the cone of confusion is modeled in FSX/P3D I can't say.

Al   

Edited by ark
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18 hours ago, ark said:

There is a so-called 'cone of confusion' over a VOR. For a side view of the cone of confusion, think of a large V with the point of the V sitting on the VOR and the legs of the V extending upward. The cone of confusion exists between the tops of the V legs. The angle between the sides of the V is about 100 degrees (each leg of the V makes a 50 degree angle with the vertical). So if you are, for example, 4 NM above the VOR (a little over 25000 ft) then the radius of the cone is 4 x tan(50deg) = 4.76 NMs, and the width, or diameter of the cone of confusion that you have to fly through is twice that, almost 10 miles in this example (distance between the legs of the V) . So not a trivial distance. Perhaps a more convenient way to think about it is that for every 1000 ft you are above the VOR station, the width of the cone of confusion is about 0.4NM. So in Bob's flight example above, if the VOR station was about at sea level and Bob was at 10,000ft, the cone of confusion was about 4NM wide (10 x 0.4).

As Jim points out above, the technique is to 'fly the cone' in Heading mode and NOT chase the CDI needle. When inbound, switch to heading mode as the CDI starts to move off to the side, and when the TO/FROM flag flips, turn to your desired outbound heading. You should then be close to the desired VOR radial when you exit the cone (the CDI starts to center again) and you can switch back to NAV mode to pick up and track the outbound radial.

How accurately the cone of confusion is modeled in FSX/P3D I can't say.

Al   

Thanks for your detailed explanation.

bob

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