Accelerometer showing different values than F2-view

[TOViper]

The accelerometer in the cockpit shows 7g.

The indication in the F2 view shows 7.7g.

 

See pics, they where made during pause (not active pause).

See attached track for more detail in case necessary.

 

This seems an obvious bug, but which indication is correct?

@RagnarDa, can you please take a look?

 

2021-02-12_Viggen_accelerometer_wrong_indication.trk

[TLTeo]

Can you check Tacview as well? If I recall correctly, that's also slightly different and it's related to whether you average out over longer or shorter periods of time to calculate the acceleration.

[Flappie]

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Edited February 12, 2021 by Flappie

[TOViper]

On 2/12/2021 at 9:11 PM, Flappie said:

100% reproducible.

Here: 8 G according to the cockpit, 9.7 G according to the F2 view.

 

 

The accelerometer can only show 8g max.

@TLTeo: If you fly a steady turn with 7g, you would notice the same error (confirmed of today, did a very long constant 5g turn, value is 0.5g off at 5g)

Edited February 17, 2021 by TOViper

[Flappie]

Oh, I see, removing my post then.

[Machalot]

Could possibly be caused by the two quantities being measured at a different point in the aircraft. If the F2 view reports acceleration at the mass center, while the IMU is mounted in a typical location just fore or aft of the cockpit, then there's a lever arm between the two measurement points that results in a measurement difference proportional to the pitch rate acceleration (typically called "R times q-dot" or "omega-dot cross R").

[TOViper]

Hm, .. not in a constant turn.

 

If we'd consider your assumption correct, wouldn't then the g-meter (fed by a measuring unit placed forward of the c.g.) in the aircraft show a higher value?

As you can see in the pics, the opposite is noticeable: the acceleration in F2-view shows a higher value, not the accelerometer in the cockpit.

 

[Machalot]

4 hours ago, TOViper said:

Hm, .. not in a constant turn.

 

If we'd consider your assumption correct, wouldn't then the g-meter (fed by a measuring unit placed forward of the c.g.) in the aircraft show a higher value?

As you can see in the pics, the opposite is noticeable: the acceleration in F2-view shows a higher value, not the accelerometer in the cockpit.

 

It all depends on what points are used, of course.  It's possible the F2 view refers to some other point instead of the mass center. It's common to model aircraft motion using a coordinate system with an origin at the nose or even in front of the nose, called "Aircraft Station Zero" (AS0).  If F2 shows the acceleration of AS0, it would be consistent with what you're seeing.  This effect disappears in steady flight of any kind. 

 

It's also possible (though I think unlikely) the F2 measurement is true acceleration instead of "specific force" (aka "sensed acceleration" or "proper acceleration") which excludes gravity and is shown in the cockpit.  This would show up as 0 g in level flight in the F2 view, while the cockpit shows 1 g of specific force.

 

Another possibility is that the g-meter includes only aircraft Z-axis measurements while the F2 view is a combination of two or three axes, reflecting thrust/drag and/or sideforce.  To test this you could try accelerating or decelerating in level flight and watching for a deviation from 1.0 g, ignoring Mach tuck.

Edited February 15, 2021 by Machalot

[TOViper]

On 2/15/2021 at 5:16 PM, Machalot said:

Another possibility is that the g-meter includes only aircraft Z-axis measurements while the F2 view is a combination of two or three axes, reflecting thrust/drag and/or sideforce.  To test this you could try accelerating or decelerating in level flight and watching for a deviation from 1.0 g, ignoring Mach tuck.

 

So, why don't you prove your idea?

 

[Machalot]

1 hour ago, TOViper said:

So, why don't you prove your idea?

 

Honestly because it doesn't bother me, and I would rather fly for fun than run science experiments to find pesky software bugs (enough of that in my day job).  Hopefully there is somebody else on this forum who likes to test this kind of stuff!

[TOViper]

LOL, ok boy, then I will do a quick check next time I fly the beast