Compass Dip Error and Acceleration Compensation in Aircraft Navigation

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Discussion Overview

The discussion revolves around the compass dip error and acceleration compensation in aircraft navigation. Participants explore how acceleration and deceleration affect compass readings, particularly when an aircraft is heading east or west. The conversation includes technical explanations of compass mechanics, the influence of magnetic dip, and the implications for navigation accuracy.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant describes how an aircraft's compass shows a false northerly direction during acceleration and a southerly direction during deceleration, attributing this to the arrangement of the compass and its pivot point.
  • Another participant argues that if the compass is pointing north while heading east, it should rotate counterclockwise due to torque, but questions why a clockwise adjustment is needed.
  • Some participants suggest that the tilt of the compass during acceleration is due to the fluid dynamics within the compass, comparing it to the behavior of water in a glass when accelerating.
  • There is a discussion about the torque experienced by the compass needle, with some participants questioning whether it should experience any net torque if it is symmetrical.
  • A later reply explains that the center of mass of the needle is not at the pivot, leading to rotational acceleration and deviation from magnetic north during acceleration.
  • One participant notes that using a GPS receiver could mitigate the issues caused by compass errors, but emphasizes the importance of understanding the magnetic compass as a backup.
  • Another participant expresses confusion about the concept of dip and its effects on the compass, seeking clarification on the mechanics involved.
  • A participant reflects on the asymmetry of the compass design, questioning the necessity of maintaining level despite dip error when acceleration and turning errors are more significant.

Areas of Agreement / Disagreement

Participants express differing views on the mechanics of compass behavior during acceleration and the necessity of adjustments. There is no consensus on certain technical aspects, particularly regarding torque and the implications of the center of mass of the needle.

Contextual Notes

Some participants highlight limitations in understanding the compass mechanics, particularly regarding the definitions of torque and the effects of magnetic dip. The discussion also reflects varying levels of familiarity with aircraft navigation systems.

rajeshmarndi
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When an aircraft heading east/west accelerates, it shows false northerly direction, similary on deacceleration shows southerly.

I know the magnet in the aircraft, dip (not at the equator) bcoz it tries to align with the magnetic lines. To avoid this, the magnet needle is pivoted to a float around which is mounted a compass card in a fluid and an dip-compensating weight(this is where the cg is) just below the pivot. I'm not sure if i have explained the arrangement correctly.

Since the pivot and cg do not coincide, it shows tha above error on acceleration and deacceleration.

Lets take on east heading the aircraft accelerate, since it shows falsely northerly direction, the compass card rotate CW, so why does it rotate CW and not CCW?

also why does the compass card tilt forward, when it is floating in the fluid.
the picture of the compass tilting forward can be viewed at http://www.pilotsweb.com/navigate/art/accel.jpg
 
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If you are heading east and the compass is pointing north then it is pointing to your left. The torque on the needle will cause it to rotate counter clockwise. Imagine you are in a considerably fast car at a stop sign and you hold your left arm out straight to your side (out the window). Now launch from the stop and your arm will want to rotate counter clockwise. So the adjustment would need to be clockwise.

The reason it tilts forward is because of the acceleration. Hold a glass of water while launching from a stop sign.
 
To take a random guess, I'd guess it is because the water is denser than the compass. (otherwise it wouldn't float) When you accelerate the water is forced backwards and pushes the edge of the compass towards the frong of the aircraft. Since it cannot move that way it tilts instead. And the reverse for deceleration. A similar effect can be seen with a helium filled balloon in a car. When you accelerate the balloon travels FORWARD, and when you decelerate the balloon travels BACKWARDS in the car, which is reverse from what you and me would normally expect from our everyday experience.
 
LostConjugate said:
If you are heading east and the compass is pointing north then it is pointing to your left. The torque on the needle will cause it to rotate counter clockwise. Imagine you are in a considerably fast car at a stop sign and you hold your left arm out straight to your side (out the window). Now launch from the stop and your arm will want to rotate counter clockwise. So the adjustment would need to be clockwise.

The reason it tilts forward is because of the acceleration. Hold a glass of water while launching from a stop sign.

Why is the needle experiencing any torque? If it is symmetrical then it shouldn't experience any torque correct?
 
Drakkith said:
Why is the needle experiencing any torque? If it is symmetrical then it shouldn't experience any torque correct?

It is still some mass some distance away from the pivot. Even if the entire arm is the same thickness.
 
LostConjugate said:
It is still some mass some distance away from the pivot. Even if the entire arm is the same thickness.

Yes but both sides of the needle are equally far away from the pivot, so why would it experience any net torque?
 
Drakkith said:
Yes but both sides of the needle are equally far away from the pivot, so why would it experience any net torque?

Oh I didn't know that! I need to fly more airplanes.

Well then why does it need a clockwise adjustment?
 
LostConjugate said:
Oh I didn't know that! I need to fly more airplanes.

Well then why does it need a clockwise adjustment?

No idea.
 
Drakkith said:
Yes but both sides of the needle are equally far away from the pivot, so why would it experience any net torque?

The center of mass of the needle is not at the pivot. The moment of the weight of the needle about the pivot is balanced by the moment caused by the angle of dip of the Earth's magnetic field, to make the needle balance horizontally.

When the plane accelerates in an East-West direction, the offset of the CM causes a rotational acceleration in the horizontal plane around the pivot, and that causes the needle to deviate from magetic north.

The deviation can be in either direction, depending which hemisphere the plane is flying in.

Of course the easy way to fix this is to use a GPS reciever instead of a compass, but you still need to know how to use the backup magnetic compass in case the plane's electrical systems fail.

As the OP said this, only affects the compass reading when the plane is accelerating, in particular when you are flying a turn (i.e. the plane is traveling round a horizontal circle). If you don't allow for this effect, you will leave the turn flynig on the wrong heading, and have to make another turn to correct the error.
 
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I don't understand Alephzero. I thought the needle was being pulled equally in two directions by the magnetic field. What causes this "dip"?
 
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This thread did a great job of explaining this to me. Thanks [itex]\aleph_{0}[/itex]! The part that I did not understand originally is that the center of mass of the needle is not centered over the pivot, but is in fact slightly along the north side of the needle to prevent the needle dipping downward in the northern hemisphere.

Several FAA documents describe the CG as being "beneath the pivot point" which is a horrible and inaccurate description of the situation. This image: http://williams.best.vwh.net/compass/img13.gif has been repeated in 100 variations, and is equally pointless for someone actually trying to understand what is happening! Once you think of the south side of the needle as "heavier" than the north end, (since it has more mass) everything clicks. Also remember that the north end of the needle points AWAY from the "N" on the compass. If north is straight ahead, the needle points forward but you are reading N on the back of the compass.

My further question: The reason for this correction/asymmetry is to keep the compass level in-spite of dip error. Who cares? The compass still reads the right thing despite the dip error, and then you wouldn't have acceleration or turning error, which is much more dramatic! Am I missing something?
 
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