Some comments re the previous two posts -
As for % MAC the diagram does not contain MAC dimensions... You would need to know the width of the MAC.
Not the case, I'm afraid, except to the extent required to draw the MAC fan grid.
Stab trim is related to the MAC in a one to one relationship as defined in the OEM AFM/WBM
The sheet designer has avoided going through the MAC grid scale and gone straight to the stab trim setting. However, the fan grid is, nonetheless, an MAC scale grid, regardless of the fact that it has been cast as stab trim. I presume that this model has (or the operator has decided to use) only one flap setting ? Normally, an aircraft will have multiple flap settings with different MAC-stab scale relationships. In this latter case, the MAC is plotted as the fan and then one will (usually) put the stab trim settings as separate scales for each flap setting. Otherwise it gets very messy trying to have multiple fan scales for each stab setting relationship.
The vertical axis is well marked in Tons.
Correct and absolutely stock standard.
The horizontal axis I presume is in Inches (or centimetres.?).aft of datum position.
Absolutely
NEVER the case. On a trimsheet, the vertical scale is
ALWAYS moment (or IU, more commonly - two ways of looking at moment). Please keep in mind that the envelope usually is presented in one of two layouts - weight x CG (which is the certification data) and weight x moment/IU (which is of use in calculations as we sum moments, not CGs).
There is no simple %MAC that suites all (such as 20 to 35%.)
Not the case. If the MAC grid is plotted as an overlay, you get the typical van-shaped grid. In this case, that is scaled as stab trim. However, it is just the MAC grid, rescaled to suit.
or that would give an envelope with straight vertical edges..
An envelope on a trimsheet with vertical fore and aft limits suggests that it has been (incorrectly) plotted as weight by CG and is
ALWAYS a suspect design. Unless the curtailment analysis just happens to work out that way, it is presumed to be
WRONG until proven to be correct. On the other hand, the usual weight by CG envelope in the AFM/POH will have (some) vertical limits. However, this is not the case for a moment envelope such as seen on
ALL trimsheets (except the occasional example which has been drawn by a "designer" of little technical competence.
the important bit was the weight and stab trim setting.
Correct. While the FT fraternity is always concerned about MAC, the line pilot generally has no interest in the value beyond noting that the loading puts the CG inside the approved envelope at all stages during flight.
As these points are critical and indicate some incorrect training, I'll endeavour to find some model data and put an MAC overlay on the sheet to show how it works. Alternatively, if any of the posters have flown this sheet/aircraft model, perhaps you can email me some details for the particular aircraft.
If you want a good story (ie technically competent) regarding trimsheets, generally, there is a detailed thread on Bob Tait's pilot training website at
https://www.bobtait.com.au/forum/rpl-ppl/5236-aircraft-trim-sheet-loading?limitstart=0
(The site moderator leading the discussion in that thread is a long standing Australian aircraft weights engineer with an extensive trimsheet design background.)