Should FSX show the same heading for 550 miles?

[idahosurge]

I have the VRS FA18E, if I start at San Diego and put the plane on a heading of 85 degrees and set the autopilot to that 85 degree heading when I get over El Paso, which is 550 miles away, the aircraft hud says that I am on a heading of 88 degrees even though a heading of 85 degrees is still selected in the autopilot.  The farther I fly the more off it gets.

However, with the Accel FA18A, the Aerosoft F16 and the Cool Sky MD80 if I do the exact same thing, when I get to El Paso the aircraft instruments for these three planes still say that I am on a heading of 85.

I think that the VRS FA18E is right, that you actually can not stay on the same heading that you will drift if you fly long enough, but I would like some confirmation from somebody who actually knows, like a real pilot.

Given that the VRS FA18E is right, would anyone know why the other three are wrong?

Rod

[Rocket_Bird]

Different systems modeling and so on.  You can set a heading on your autopilot, and a little wind can put you off track, resulting in different indicated heading than what you have planned.  Its possible that the discrepancy between your different aircraft may be the result of either varying winds or perhaps that a particular model was modeled to aggressively maintain a heading as opposed to a flight track. 

In real life, you almost never see a plane simply set a heading to fly to a destination 550 miles away though.  You typically would use airways and way points. 

[Brett_Henderson]

Wind won't effect your heading (especially with an autopilot holding it).. it will change your track, though.

What's most likely happening, is that one of the models has an HSI that is not slaved to the compass.. so as the HSI suffers gyro-drift, the autopilot follows the HSI which now differs from the HUD (which is in sync with the compass).

You can test this theory with the keyboard shortcut for synching the gyro to the compass..

[idahosurge]

Thanks Brett, I will give it a try!

Rod

[idahosurge]

Well Brett I tried pressing "D" in the VRS FA18E and there is no change it still drifts and drift may not be the correct workd to use.

You stated in your post that the HSI does not agree with the HUD.  Just to clarify, that is not what is happening.

In the VRS FA18E I leave San Diego on a heading of 85 and I have this heading in the autopilot.  When I get to El Paso the HSI and the HUD say that I am on a heading of 88, but the HSEL bug in the HSI is still set for 85.

Of the four four planes I have checked only the VRS FA18E does this.  I am sure that it is the way that VRS modeled their FA18E.

What I am really wondering is if your are in a real airplane would you see this?  If you set your heading to 85 and then fly 500 miles would you be at a compass heading of 88 or something along those lines even though your HSEL is still set to 85?  Or does it depend on the plane and how advanced the autopilot if?

Maybe you would see this in an FA18E, but not an MD11 or B757 because the FA18E's autopilot is not that advanced, but the MD11 and B757 do have advanced autopilots.  The autopilot function in an FA18E versus the one in an MD11 or B767 is really not meant for the same application.

Rod

[1olehippy]

Hey guys,
Your answer lies in the shape of the earth & the location of "magnetic north".  If you keep your compass needle pointed directly at "MN" you will actually be flying an arc. To fly a straight line your magnetic compass is going to show a variation. The longer the straight line, the more variation. If your compass is off 3 degrees & you are over El Paso, bingo you did good. Early auto pilots were based on INS as opposed to todays GPS.  But either way you will show a variation on your compass that with a little math can be calculated over the distance you plan to fly.

[idahosurge]

Thanks Ole Hippy!

[Brett_Henderson]

[quote]Hey guys,
Your answer lies in the shape of the earth & the location of "magnetic north".

[Brett_Henderson]

...... or maybe the HUD displays your track ?

Heading = where the nose is pointed.

Track = the line you're 'drawing' on the ground as you fly.

Course = the line you WANT to be following.

[Brett_Henderson]

Well.. it's not the magnetic variation (difference twixt true and magnetic)..

Here's a snippet from the sectional at El Paso.. I circled the variation (I think it's callled a line of declination)... Anywhere along that line, the difference twixt true and magnetic is, 9 degrees / 30 minutes..

[idahosurge]

[quote]If you set an autopilot to follow a heading, and the HSI (or DG, or HUD), is slaved to the compass.. your

[Brett_Henderson]

The Accel F18 is functioning properly...

Another variable...  Are you downloading real weather ?  Or flying the same, predefined weather (allowing for the same wind every flight) ?

The only thing I can come up with (aside from a flaw in the VRS HUD), is that the HUD displays your track, which differs from the heading because of wind..

'Ctrl + Z'  will also tell you the winds..

[idahosurge]

[quote]The Accel F18 is functioning properly...

Another variable...