Weight & Balance calculation

[Greg Arehart]

I've been doing W&B calculations on my 9A and note in Van's instructions that he defines the CG envelope based on between "15 and 28% of chord" which is then converted to inches. Just a couple questions for the aeronautical engineers out there: Are these two numbers (15, 28) true for all wing designs, or specific to each airplane? and How does one do such a calculation (ore determine the numbers) for a wing that is swept?

Just curious....

thanks,
greg

[Ironflight]

Greg,

As you suspect, CG envelopes are unique to specific aircraft designs. They are a result both of analysis and flight testing, and affect both stability and handling characteristics. In the simplest terms, the aft limit is determined by the point at which the pitch stability goes negative, while the forward limit can be defined as the point where you can't get (or keep) the nose up for landing. It gets considerably more complicated than that in the details, but in broad brush strokes, that is a good beginning. Qualitative flight testing can show that while an airplane might be considered stable at a calculated aft limit, it might be objectionable enough that the limit has to be set farther forward.

There are many good books on the subject, and not all of them say the same thing....

Paul

[plehrke]

For swept wings you can use this method to determine the MAC and then the % stability is based on that chord position and length.

http://www.lferc.com/Tips/mean_aerodynamic_chord.php

or

http://en.wikipedia.org/wiki/Chord_(aircraft)