please explain this one
Thread Starter
Joined: Aug 2005
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From: Glens o' Angus by way of LA
please explain this one
Is there something wrong or am I just being a dummy.
Line up on 27, wet compass shows 27 set HI to 27 and set off in very light variable wind. Fly about an hour north then turn around to head home, set the Garmin 496 for "go direct" back to my home field and follow the pointer that extends from the wee plane on the screen. The Garmin shows my Course, Bearing and Track all 180 BUT the wet compass and HI show I am flying 210 !
What am I missing here?
Line up on 27, wet compass shows 27 set HI to 27 and set off in very light variable wind. Fly about an hour north then turn around to head home, set the Garmin 496 for "go direct" back to my home field and follow the pointer that extends from the wee plane on the screen. The Garmin shows my Course, Bearing and Track all 180 BUT the wet compass and HI show I am flying 210 !
What am I missing here?
Thread Starter
Joined: Aug 2005
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From: Glens o' Angus by way of LA
I understand that bearing doesn't but course and track must use either magnetic or true as a reference, or am I completely off the reservation here?
If that was the case then wouldn't my course and track be different to reflect that?
Maybe a strong westerly wind, tracking 180 but heading 210
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From: An ATC centre this side of the moon.
Joined: Oct 2011
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From: South-East, United Kingdom
Do you have anything metal placed close to the Compass. I put my Zaon MRX up on top of the instrument panel and it was at the time a little too close to the compass and was knocking it off by about 20 degrees. Other than I can only guess one of the two screws that calibrate it have moved and your compass needs recalibrating. I would agree its a bit stange but I often find the wet compass is never as precise as a GPS reading.
Joined: Nov 2001
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From: UK
Hmmmm magnetic heading is where the aeroplane is pointing. Track is the actual direction of travel over planet earth, the difference is the correction for drift due to wind. 30 degrees is a lot of drift so it's likely that the compass is subject to some sort of interference too.
You need to revise your basic navigation and make sure that there's nothing within a foot of the compass that might have magnetic properties.
You need to revise your basic navigation and make sure that there's nothing within a foot of the compass that might have magnetic properties.
Joined: Aug 2012
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From: Wales
Hi, if you assume that magnetic = true for the purpose of an easy calculation, then you can use a velocity triangle to determine the wind velocity. The Hypotenuse side is your airspeed, and the Opposite side is the wind speed. Remember OHMS? Opp/Hyp means Sin.
So for 30 degrees, sin 30 = 0.5
Which means your cross wind is 0.5 of your airspeed... Say 45 knots if you are in a 90 knot cessna.
(Even allowing 3 deg magnetic variation, its still 41 knots.)
So for 30 degrees, sin 30 = 0.5
Which means your cross wind is 0.5 of your airspeed... Say 45 knots if you are in a 90 knot cessna.
(Even allowing 3 deg magnetic variation, its still 41 knots.)
Joined: Aug 2007
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From: 18nm NE grice 28ft up
Thinking out the box here. Did you check the wet compass in level flight northbound?
Does your wet compass read differently in the level attitude compared with the more extreme 3 point attitude you have with the bigger wheels?
D.O.
Does your wet compass read differently in the level attitude compared with the more extreme 3 point attitude you have with the bigger wheels?
D.O.
Joined: Oct 1999
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From: UK
This reminds me of a chap (non pilot) I used to take flying in the Chippy (him in rear cockpit) occasionally. One day we had a good ground speed and explained that was because of a healthy tail wind.
"Oh, I could tell that", he replied.... "by the wind blowing in under the back of the canopy!".
"Oh, I could tell that", he replied.... "by the wind blowing in under the back of the canopy!".
Joined: Jun 2002
Aviation Qualifications: PPL
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From: east ESSEX
UV, the latitude nut caters for `transport drift` ,not Earth rotation.The Di must be reset to the best `compass` every 15 minutes,as it will still drift,due Earth rotation..
Joined: Jan 2011
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From: England
Sycamore
The latitude nut is simply a metal nut mounted on a threaded stud which protrudes from the inner gimbal. Altering the position of the nut creates an imbalance of the gimbal. This in turn creates a wander rate which compensates for (part of) the wander of the gyro.
The latitude nut is not accessible to the pilot, so it can be adjusted only in the instrument workshop........................1
Earth rate wander (in degrees per hour) = 15 x Sin latitude. So for a given latitude it is constant.
Transport wander = West - East Ground speed x Tan Latitude / 60.
The west-east ground speed can be adjusted (within limits) at will by the pilot................2
If you look at statements 1 and 2 you should see that the (constant) position of the latitude nut cannot compensate for the (non constant) transport wander, but it can compensate for the (constant) earth rate wander.
the latitude nut caters for `transport drift` ,not Earth rotation.
The latitude nut is not accessible to the pilot, so it can be adjusted only in the instrument workshop........................1
Earth rate wander (in degrees per hour) = 15 x Sin latitude. So for a given latitude it is constant.
Transport wander = West - East Ground speed x Tan Latitude / 60.
The west-east ground speed can be adjusted (within limits) at will by the pilot................2
If you look at statements 1 and 2 you should see that the (constant) position of the latitude nut cannot compensate for the (non constant) transport wander, but it can compensate for the (constant) earth rate wander.
Last edited by keith williams; 21st February 2013 at 23:03.
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From: Scotland
Piperboy, If you flew north for an hour was that on the wet compass or the GPS?
Frinstance in a westerly of 15knots you will be 15nm east of where you thought in a hour if yr on the wet compass. If you then turn round & fly south on the wet compass fr another hour yr gonna be 30nm East of home, or thereabouts, aren't you? So the GPS go to is telling you where home really is innit? Never mind all this wind triangle deviation gobbledegook.
Frinstance in a westerly of 15knots you will be 15nm east of where you thought in a hour if yr on the wet compass. If you then turn round & fly south on the wet compass fr another hour yr gonna be 30nm East of home, or thereabouts, aren't you? So the GPS go to is telling you where home really is innit? Never mind all this wind triangle deviation gobbledegook.
Last edited by Crash one; 21st February 2013 at 23:13.
