# Compass acceleration error

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.

Drakkith
Staff Emeritus
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.

Drakkith
Staff Emeritus
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?

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.

Drakkith
Staff Emeritus
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?

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?

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

Well then why does it need a clockwise adjustment?

No idea.

AlephZero
Homework Helper
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 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.

Drakkith
Staff Emeritus
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.