A320 Mach trim
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.
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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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.
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.
Correlation or causation, do you think? Do the 777 and 787 have Mach trim?
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.
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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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,
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,
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.
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.
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.
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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Directly from Airbus:
For a given Mach number and a given AoA, the EFCS takes into account the natural pitch up effect of the wing for this Mach number and this AoA, and applies on the elevators the appro-priate longitudinal pre-command to counter its effect.
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I think you are trying to know about what's known as Mach tuck. This was pronounced on B707 where the beyond certain Mach aircraft pitches down and Mach trim prevents that. Modern aerofoils have much less this phenomenon. In A320 since it is flight path stable aircraft flight control system maintains the flight path by doing whatever it takes even in manual flight. There's no manual trimming unless in direct law.
