Compass Dip Error and Acceleration Compensation in Aircraft Navigation

In summary, when an aircraft heading east/west accelerates, the magnet needle in the compass is subjected to a torque due to the offset of the center of mass from the pivot point. This causes the needle to deviate from magnetic north, leading to false readings. To compensate for this, the needle is tilted forward and a clockwise adjustment is needed. The center of mass of the needle is not centered over the pivot point, but is slightly along the north side to prevent it from dipping downward in the northern hemisphere. This can be confusing as some FAA documents describe the CG as being "beneath the pivot point," but understanding the weight distribution of the needle helps to clarify the situation.
  • #1
rajeshmarndi
319
0
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
 
Last edited:
Physics news on Phys.org
  • #2
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.
 
  • #3
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 baloon travels BACKWARDS in the car, which is reverse from what you and me would normally expect from our everyday experience.
 
  • #4
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?
 
  • #5
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.
 
  • #6
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?
 
  • #7
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?
 
  • #8
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.
 
  • #9
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 accleration 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.
 
  • #10
I don't understand Alephzero. I thought the needle was being pulled equally in two directions by the magnetic field. What causes this "dip"?
 
  • #11
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 [Broken] 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?
 
Last edited by a moderator:

What is compass acceleration error?

Compass acceleration error is a phenomenon that occurs when a compass is affected by external accelerations, causing it to deviate from its intended direction. This error is also known as deviation error.

What causes compass acceleration error?

Compass acceleration error is caused by external accelerations such as changes in speed or direction of a moving object, magnetic interference from nearby objects, or improper compass calibration.

How does compass acceleration error affect navigation?

Compass acceleration error can cause incorrect direction readings, leading to navigation errors. It can also make it difficult to maintain a steady course, especially when navigating at high speeds.

Can compass acceleration error be corrected?

Yes, compass acceleration error can be corrected by calibrating the compass properly and taking into account any external accelerations that may affect the compass readings. In some cases, special compensating equipment may be used to minimize the error.

What are some ways to reduce compass acceleration error?

To reduce compass acceleration error, avoid placing magnetic objects near the compass, calibrate the compass regularly, and use additional navigation techniques such as GPS to cross-check the readings. It is also important to maintain a steady speed and direction while navigating.

Back
Top