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autotrim question


fulcrum_64th

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Hello,

 

I just installed 2.5 OB and played around with the M2K...the M2K auto trim is a bit weird: the plane keeps on increasing the pitch (at a small rate like a fraction of a degree per second) instead of trimming for the attitude.

 

I don't recall that behaviour last time I flew the M2K (a few months ago)

 

Best Regards

 

Fulcrum

:pilotfly:

 

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The previous behavior was incorrect. Now you have to use altitude hold AP. The aircraft trims for 1g which will typically result in a sight pitch up.

[sIGPIC][/sIGPIC]

Reminder: SAM = Speed Bump :D

I used to play flight sims like you, but then I took a slammer to the knee - Yoda

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Hello,

 

I just installed 2.5 OB and played around with the M2K...the M2K auto trim is a bit weird: the plane keeps on increasing the pitch (at a small rate like a fraction of a degree per second) instead of trimming for the attitude.

 

I don't recall that behaviour last time I flew the M2K (a few months ago)

 

Best Regards

 

Fulcrum

 

There is also a new 300 pages manual that will explain you what you need to know. :thumbup:

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That is incorrect. 1G is a specific value of acceleration. This acceleration changes in some proportion with the distance to the gravitational center. The higher you go, the less acceleration (as in less than 1g) you experience. Even though the difference is tiny, it will result in a slight up-trim.

 

1G should be level flight as stationary on the ground is the force of gravity (1G) so I don't know how it's programmed

[sIGPIC][/sIGPIC]

Reminder: SAM = Speed Bump :D

I used to play flight sims like you, but then I took a slammer to the knee - Yoda

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The vertical load factor to hold a particular dive or climb angle is the cosine of that angle and so will be always less than one with the exception of zero dive or climb.

 

What you may not take into account is that only the vertical component of acceleration is being measured. There are left-right and forward-rearward components too but these are not considered by the FCS. So with the airplane bolted to a metal frame with its nose straight into the sky a human in the seat will feel 1g but it is 1g into the back of the seat. The FCS will consider that 0g because none of that acceleration is in the direction of the bottom of the airplane.

 

So if the Mirage FCS is a g-command system and trim is a g-bias then there is no stable direction at e.g. 1.1g command. In fact as climb progresses the difference between straight line g and commanded g will increase as will the rate of turn.

 

This may make you ask how to design an FCS for stability in pitch. If at 1.0g commanded and level everything is fine forever but consider a small upset in pitch and how plane responds. If upset is downward (say -0.01 degree) then g to maintain this is 0.999g, but commanded is more at 1.000g. If commanded > straight line then nose pitches up. This is positive stability: an upset downward causes a correction upward.

 

But look at a positive upset to +0.001 pitch, required g is 0.999 but commanded is 1.000. Commanded > straight line so nose pitches up! This is negative stability. An upset to +pitch produces a reaction which makes more +pitch.

 

So if this half-instability is not desirable then what can be done? I don't know details but this was addressed recently by RAZBAM to change some FCS laws. This divergent behavior at small pitch up shouldn't happen I think now. I would have to look.

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