# Compass acceleration error

1. Aug 11, 2011

### rajeshmarndi

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: Aug 11, 2011
2. Aug 11, 2011

### LostConjugate

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. Aug 11, 2011

### Staff: Mentor

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. Aug 11, 2011

### Staff: Mentor

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

5. Aug 11, 2011

### LostConjugate

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

6. Aug 11, 2011

### Staff: Mentor

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

7. Aug 11, 2011

### LostConjugate

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

Well then why does it need a clockwise adjustment?

8. Aug 11, 2011

### Staff: Mentor

No idea.

9. Aug 11, 2011

### AlephZero

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 travelling 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. Aug 11, 2011

### Staff: Mentor

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. Nov 14, 2012

### Meekohi

This thread did a great job of explaining this to me. Thanks $\aleph_{0}$! 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: May 6, 2017