[TRACK REQUIRED] "Whiskey Compass" rolls during AB use (Pics)

ST0RM · October 25, 2019

When the AB is initiated, the compass on the right panel rolls vertically, rendering it unusable. My trk file didnt save, as I was in S/P, but I did take these screen shots in VR. Notice the fuel flow and engine RPM during each to show when the issue occurs.

randomTOTEN · October 25, 2019

This is correct behavior.

A wet compass can only be considered valid during straight and level, un-accelerated flight.

Bouli306 · October 25, 2019

This is correct behavior.

A wet compass can only be considered valid during straight and level, un-accelerated flight.

True, but during my flights in the Viper I never saw so much deflection during acceleration. IRL it stays readable.

randomTOTEN · October 25, 2019

It's movement appears correct for the forces on it. Perhaps the interior case dimensions have not been implemented or a collision model built for the card and case? The inertia on the card/magnet appears normal in my General Aviation experience.

ST0RM · October 25, 2019

General Av doesnt get the acceleration you'd require to get anything close.

And I too have experience in fast jets and heavies. Sorry, but this isnt correct.

randomTOTEN · October 25, 2019

Physics of a whiskey compass don't change based on the quantity of G's applied to it. It reacts according to the acceleration and moves freely until the card strikes the interior of the case. It's correct as is.. but I'm willing to concede that the deflection may be excessive.

Unless you have special correction mechanisms which make it different from the equipment installed in a basic Cessna 172?

Frederf · October 25, 2019

AQC-3 has a trapezoidal shaped card profile since the pilot is looking down on it from above. The DCS one is spherical. Details of the ACQ-3 design are needed to estimate how sensitive the card orientation is to linear acceleration. One would think this effect would be minimized deliberately. The goal is to have the orientation of the card determined mostly by the magnetic field and weakly by local acceleration (e.g. gravity plus any maneuvering). Since the magnetic field has no concept of upright getting the card upright needs local acceleration. To this end the CG of the card has to be below the pivot point.

Remember that the compass is also seeking the field line alignment in the vertical plane. So during acceleration (e.g. North) there are two torques acting on the pitch of the card: magnetic field and local acceleration. If the local "down" acceleration effect is small compared to the magnetic torque then the card will remain fixed to the magnetic field. If the local acceleration effect is dominant over magnetic alignment then the pitch of the card approaches that of the local vertical (e.g. 20kt/s acceleration is 1g so 45° pitch tilt).

In practice the tilt will be somewhere equilibrium between the mag field and the local horizontal. The balance is dependent on design and the local field strength. The design of the compass ideally has the minimal local vertical effect required to keep the compass card suitably upright.

There is also the concept "dip-correction weight" which attempts to level the card due to the dip in magnetic field. This requires some CG-pivot displacement to function as well. Without the weight the inclination of the magnetic field would align the compass card to it in the vertical. This balance is set to the dip of the operating latitude. So redeploying to the southern hemisphere requires getting the compass changed.