Question For Real Pilots

[Gianluca]

Please bear with me as this is a very basic question and many of you will undoubtedly roll your eyes in disbelief. However, I'm a big believer in asking when I don't know something, so here we go. I noticed that generally Heading Indicators have two knobs. I understand what they do mechanically, but I'm unclear as to how to use one of them. I often use the bug ring to set the desired heading when using the autopilot. However I don't understand how the other knob (the one that turns the compass card) is used in the real world and under what circumstances. Could someone give me a real life scenario under which they would use the knob that turns the compass card?

Thanks a bunch.

Gianluca

[Jeff S KDTW]

Gianluca -

That is certainly not a basic question that someone would roll their eyes at. :-)

The purpose of the "other" knob, as you mentioned, is to turn heading indicator/directional gyro. It is turned so as to set the DG relative to the heading indication displayed on the magnetic compass (typically mounted on or above the glareshield (dashboard)).

The heading indicator/directional gyro is a gyroscopic device that, like other unslaved gyroscopic instruments, suffers from "gyroscopic precession." Precession is the phenomenon occurring in rotating bodies in which an applied force is manifested 90 degrees later in the direction of rotation from where the force was applied. What....:-eek Basically, due to forces applied to a gyroscopic instrument (friction and such), the reading of the instrument progressive becomes in error. Therefore, pilots are reminded to periodically reset their heading indicator/directional gyro when in straight and level, unaccelerated flight relative to the magnetic compass.

This explanation could go on at length. But I will spare you. ;-) With one last note, a "slaved" indicator does this "resetting" process automatically.

Hope this helps,

[link:www.dreamfleet2000.com|http://www.dreamfleet2000.com/gfx/im...UM_SNYDER.jpg]

[Gianluca]

Wow, Jeff. I'm sure glad I asked. Thanks a lot. I wonder if Flight Simulator simulates gyroscopic precession. I personally have never used "that" knob in any of my many virtual flights to reset the heading.

Many thanks again.

Gianluca

[someone]

It does "simulate" it, but it's way overexagerated. Real DG's are not allowed to process more than 3° in 15 minutes or they can not legally be used for IFR. In FS it will precess 3° in 15 seconds.

The option for this is under realism settings.

[hr]
http://home.cablelynx.com/~jchaney/C47_sig.jpg
[font size="1"]Learn from the mistakes of others, you won't live long enough to make all of them yourself.[/font][table cellpadding="2" cellspacing="10" border="0"][tr][td][font size="1"]System Specs:[/font][/td][/tr][tr][td][font size="1"]Intel Pentium III 933MHz
Asus CUSL2-C Motherboard
384MB PC133
Geforce2 GTS 32MB
SoundBlaster Live! 5.1
30GB ATA100
Windows XP Pro[/font][/td][td][font size="1"]Intel Pentium IV 1.5GHz
ASUS P4T-M Motherboard
256MB PC800
GeForce 4 MX 440
Onboard AC '97 Audio
60GB ATA100
Windows XP Home[/font][/td][td][font size="1"]COMING SOON:
Intel Pentium IV 2.66GHz
Asus P4PE Motherboard
1GB PC2700 RAM
ATI Radeon 9700 Pro
SoundBlaster Audigy2
80GB WD SE HD
Windows XP Home[/font][/td][/tr][/table]

[ToddNicholson]

I turn "gyro drift" off in FS because it is so overly exagerrated. You certainly could turn it on, but you'd be adjusting the DG all the time - simulated, but not very realistic.

Regards,
[link:www.dreamfleet2000.com|http://www.dreamfleet2000.com/gfx/im...NICHOLSON.jpg]

[Gianluca]

Thanks. Great info.

Gianluca

PS: By the way, being of a curious nature I looked up some further info on the internet regarding gyroscopic precession and found this neat site. It may prove to be helpful to others who, like me, might struggle a bit with these concepts.

http://physics.nad.ru/Physics/English/gyro_txt.htm

[ladamson]

Other good reasons to use the "bug" heading when not using A/P.

ATC tells you to fly runway heading ---- set the bug

ATC tells to fly new heading

Just set it to runway heading before takeoff, so you can make a straight climbout with use of rudder.

Ladamson

[flyinggriffin]

If anyone asks why? Its because occasionaly you suffer brain fade and totally forget what heading you were told to fly. The bug helps you prevent those moments when ATC lets the whole world know you have suffered a Blonde Moment.

Its also helpful to set the final approach course with the heading bug. Every little bit helps when your in the clouds.

When FS2002 first came out I let the gyros precess untill I had to reset the DG three times on an NDB approach. Great way to end up a smoking hole in the ground in real life.

[ladamson]

For the above reasons, I'm very happy that Aircraft Spruce (parts supplier) was out of "bugless" directional gyro's when I bought mine, although the "bug" was about $150 more. When you're over 50, brain fade is something to think about! :)

Ladamson

[Jeff S KDTW]

Larry -

Brain fade is not something that only those over 50 experience, us thirty-somethings....uhmmm.....errrr.....whoops, I forgot what I was going to say.

Nevermind, ;-)

[link:www.dreamfleet2000.com|http://www.dreamfleet2000.com/gfx/im...UM_SNYDER.jpg]