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3 Answers

W&B reduction factors

Asked by: 3620 views ,
General Aviation

Do anyone know why some reduction factors are Moment/1000 and other reduction factors are Moment/100?

Also, if you divide by the reduction factor in the beginning in order to get a smaller moment to work with (makes math easier)...you have to remember to multiply your total moments BY the reduction factor in the end before obtaining the CG.  I actually prefer NOT to use the reduction factor...less chance for mistakes. 

I also prefer the "airplane CG location" chart as opposed to working with "loaded airplane moment" chart.  Which do you guys prefer?

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3 Answers

  1. Steve Pomroy on Jan 28, 2011

    Hi John Doe.
    By “reduction factor”, are you refereing to the scaling factor that is sometimes used on the W&B evelope diagrams?  The main reason these are used is to reduce the number of zeros, and therefore the amount of clutter, on the diagrams themselves.  They aren’t there to make the math easier, but to make the graphic more readable.  In fact, with regard to the math, they effectively make it more difficult since they requier an added step (simple though it may be, it’s still an added step).
    My suggestion would be to not use the scaling factor at all during your W&B calculations.  Applying the factor, and then un-applying it later just increases the chances of making an error.  When you apply your results to the envelope, apply the scaling factor that your manufacturer has chosen to use (if any).
    As for which type of envelope I prefer, I do have a preference for the “Weight v. CG” envelope instead of the “Weight v. Moment” envelope.  But this is really nothing more than personal preference.  Both diagrams provide the smae information.

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  2. John A Lindholm on Jan 28, 2011

    If you get frustrated with reduction factors doing cg/moment calculations, you are really in for a treat when you have to deal with % of MAC.

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  3. Kent Shook on Jan 28, 2011

    John Doe,
    I’m with you – I don’t use the reduction factors (more chances to make a mistake, and removes some of the accuracy too) and I much prefer the CG envelope chart to the Moment envelope chart (which seems to be a Cessna thing). The moment chart’s shape makes the envelope much narrower and more difficult to read accurately.

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