I have one on file if all else fails ...
Not difficult to run one up if you would like to have a go at it. For a simple GA aircraft try the following ....
(a) pick a datum for the trim sheet (suggest the aft envelope limit)
(b) general form of the entry IU line and envelope is
IU = C1 + (FS - datum) * weight / C2
where
IU is entry IU for running calculations
C1 is a fudge number constant to move the IU line to the left to get rid of negative IU .. ie pilots prefer doing it with positive numbers .. can be anything you like to make the trim sheet pretty ..
FS any loading point under consideration
datum is the chosen trim sheet datum
(FS-datum) reworks the manufacturer's datum arms to your chosen trim sheet datum
weight is the weight for FS as you go around the envelope (or weight appropriate to the particular calculation being done at the FS)
C2 is a convenient constant to get "nice" numbers
(c) general form for the trim lines is as above without the C1 constant. FS is the particular loading arm (as per the manufacturer's datum. Again (FS-datum) does the translation for you. Use a sensible "standard" weight interval for each line to give you a nice looking trim sheet chosen to be a balance between ease of use and accuracy.
(d) to draw the thing, start with a vertical scale of IU.
(e) draw in the envelope at the bottom of the chart
(f) draw in each loading arm line. Having worked the delta IU for the chosen standard weight for a particular line, draw in the ticks to suit. If you prefer the slopey line style to reduce error, draw a second line and plot convenient diagonal lines so that there is a bit of overlap
(g) draw in the entry IU line at the top
(h) if you want to have MAC lines, do the MAC to FS conversions, calculate high and low weight IU values and plot these
Be prepared to have several goes at designing the trim sheet, playing with constants and scaling until you get something which looks nice and works easily. After you have designed your first 200-300 sheets you'll find that you can go straight to the final interation with very little effort.
Run some error checks and constrict the envelope limits as calculated to allow for the errors so that you don't go outside the "real" envelope. Plot these "revised" envelope values in lieu of the AFM figures. The importance of this part of the exercise should neither be under-rated nor hurried ... those of us who design these things for a dollar might be called upon to defend our design in court.
Above all, check the regs in your own country to see if the sheet needs formal approval by an approved weight control person before it can be used for calculations in anger .....
Last edited by john_tullamarine; 17th Jan 2004 at 20:47.