G'day everyone,
I have a question regarding the FWD limit.
I understand that when you need to add or subtract weight and the aircraft is forward of the forward CG limit, you can use the chart to plot the current CG and then add the weight and moment to determine where it intersects the FWD limit.
However, I find this method can be difficult to get an accurate answer because it depends on how precisely you can plot and read the graph.
Is there a way to mathematically double-check the result, rather than relying solely on the graph? Or is the graph generally intended to provide a rough answer and get you close enough to the correct value?
Thanks in advance for any help.
Simply take the new weight from the graph where it intersects the forward limit and work out both C of G at that weight and the Forward limit. Compare the two, add or subtract weight and then recalculate the C of G and Forward limit. Time consuming and you may not have spare time in the exam for multiple calculations, but you may achieve a more accurate answer.
However, I find this method can be difficult to get an accurate answer because it depends on how precisely you can plot and read the graph.
Perfectly OK in the field for pragmatic solutions, providing that you exercise a little conservatism with the process and make sure that your solution puts you a little inside the envelope.
Is there a way to mathematically double-check the result, rather than relying solely on the graph?
Indeed, run a simultaneous equation solution. This goes directly to the correct answer, bypassing the graphical bit. Not warranted and you have better things to do with your time. More importantly, to get the answer "accurately" requires the simultaneous solution of one linear and one quadratic equation. Fine brain teasing but definitely not warranted. The next comment is the double-checking approach.
Compare the two, add or subtract weight and then recalculate the C of G and Forward limit.
This is a sensible, iterative alternative to the above and can be used, quite easily, to get to whatever precision answer you might seek. Only useful for Echo exam questions. For other aircraft, you would have to figure out the equation for the forward upper limit line which is a waste of time, generally. The previous discussion is how you would approach the problem in your flying.
With the iterations, you use the graph to get close to the answer, check the CG by reference to the CG line equation and then iterate once or twice to get to a closer answer. Again, the main aim is to ensure that you end up with any residual error putting you on the conservative side of the limit line.
Engineering specialist in aircraft performance and weight control.