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A320 Mach trim

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A320 Mach trim

Old 18th Jun 2020, 18:53
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A320 Mach trim

Good evening,

I'm looking for information about the Mach trim on A320. Any hint welcome.

Thanks

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Old 18th Jun 2020, 19:43
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Do you mean high speed protection, I'm not sure it goes fast enough to need mach trim.
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Old 18th Jun 2020, 19:57
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Iím pretty sure, that if it needs one, it is embedded deep in the FBW logic.
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Old 18th Jun 2020, 20:11
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As automatic trimming is active in the normal flight regime, any trim change due to mach effects would be countered in the same way as any other trim change.
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Old 18th Jun 2020, 21:46
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The term 'Mach Trim' is normally associated with conventional flight control systems which have to provide a feel force feedback to the pilot.

With increasing Mach No the aircraft might suffer reducing stickforce-speed relationship which defines stability; Mach Trim applies a corrective trim offset to mask the effects of this change.

The A320 experiences this change, but because the FBW design does not need any feel-force feedback to provide 'trim feel' awareness, any change or aerodynamic non-linearity is 'hidden', managed, within the FBW system.

Other aspects of trim are within the FBW computation, which enable 'equivalent manual' trim functions.
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Old 19th Jun 2020, 09:53
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On the subject of Mach Trimmers does anyone know why the B747 classic had no need for a Mach Trimmer? Always intrigued me.
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Old 19th Jun 2020, 14:25
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Originally Posted by safetypee View Post
The term 'Mach Trim' is normally associated with conventional flight control systems which have to provide a feel force feedback to the pilot.

With increasing Mach No the aircraft might suffer reducing stickforce-speed relationship which defines stability; Mach Trim applies a corrective trim offset to mask the effects of this change.

The A320 experiences this change, but because the FBW design does not need any feel-force feedback to provide 'trim feel' awareness, any change or aerodynamic non-linearity is 'hidden', managed, within the FBW system.

Other aspects of trim are within the FBW computation, which enable 'equivalent manual' trim functions.
This makes a lot of sense- especially when you consider that degraded flight control laws have a lower Vmax.

Correlation or causation, do you think? Do the 777 and 787 have Mach trim?
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Old 19th Jun 2020, 16:23
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747 lack of Mach Trim

Meikleour, 747 lack of Mach Trim.
D Davis provides good reasoning in 'Handling the Big Jets' (3rd edition).

The design of the 747 trim system provisioned Mach Trim, but it was not fitted for certification. It is not clear if the deletion was a pre first-flight design decision or as the result of early flight testing.

Davis describes the 747 as having good longitudinal stability and control; large control surfaces and good trim characteristics.
The stick force gradient at high speed was low, but did not reverse. Although these characteristics did not meet 'the letter' of BCARs ( assume that they did meet the differing FAR ), it was accepted for UK certification - judgement call. Noting that the VC10 and Trident had similar characteristics, but did have Mach Trim fitted.

From the narrative I suspect that the lack of 747 'Mach Trim' was a compromise in relation to the more emotive CAA/FAA discussions on stick nudgers / stick push in that era.

Last edited by safetypee; 19th Jun 2020 at 17:50. Reason: Typo
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Old 19th Jun 2020, 18:05
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Thaks for all,

Mach trim is mentioned in FCOM without details.
I expected to see some trim movement at high Mach numbers, close to maximum.
Couldn't observe any, both in auto and manual flights,
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Old 19th Jun 2020, 20:59
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safetypee: Thanks for that - it got me to dig out my old copy of HTBJ (and let the moths out!) Intersting that the c. of p. did not travel far enough aft at high mach to produce instability.
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Old 19th Jun 2020, 21:22
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Check Airman, 'Do the 777 and 787 have Mach trim?'

I don't know.
Assuming that most modern high-speed aircraft will have some stability change with increasing Mach, then this has to be managed somewhere in the control system. Exactly where and how, involves choice of design / operational concept, control law, mechanical engineering, and 'normal' trim in manual flight.

My previous post simplified this under the term FBW; more specifically, it involves the design objective of the control law, depiction of stability to the crew - control inceptors, and followup trim. All of which enable considerable variations on a theme, not least how a particular function is defined, or named, within software.

With an 'old school' view, modern FBW aircraft do not have 'Mach Trim'; the term is best reserved for conventional 'hands-on' aircraft.

And then #9, it is named, but not seen.
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Old 27th Jun 2020, 10:45
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When I'm stressed out and tired I pick up my copy of HTBJ and read a chapter or two... sometimes I also read Fireworks:The Art Science and Technique by Dr.Takeo Shimizu.
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