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Alternative FWD calculation

• Posts: 12

lcanning created the topic: Alternative FWD calculation

Hey there guys.

So I'm coming to the end of the Bob Tait text book and I've had alot of FWD limit calculations.

So I found this other method instead of using the chart to draw a line and estimate how much it takes to be on the boarder of the FWD Limit. why cant I try and calculate it by finding the total moment, ARM & weight and then calculating the FWD Limit (2360 x 0.27 2400) and seeing if Im in or out.

Doing this method has got me right down to decimal to see if Im in or Im out. But alot of the questions towards the end book use the line method but in doing this calculation, they seem to be out by 1 or 2 mm.

Please let me know if im doing the wrong thing right now but to me it seems quite logical to do it this way.

• John.Heddles
• Offline
• ATPL/consulting aero engineer
• Posts: 833

John.Heddles replied the topic: Alternative FWD calculation

why cant I try and calculate it by finding ...

Of course you can.

The problems are

(a) in the real world, as a pilot, you would never calculate the cg from an equation. That would require you to figure out the equation for each and every Type/Model you flew. Give that a miss. The situation in the exam is just an artefact of the examinations and, really, is quite sad to see.

(b) running the figuring by plotting and reading off the intersection is fraught with error due to the similar slopes of the lines. No way around that, unfortunately, other than being very careful and using points well apart for the line.

(c) if you want to get "better" accuracy/precision (which is rather pointless, but there you are) you have a choice of running a simultaneous solution of a linear and a quadratic equation (which is sillier than words) or running a time-wasting iterative process to find a solution which matches your ideas of what precision you are after.

Plotting the line and reading off the cg is fine, just so long as you do so conservatively. From an engineering viewpoint, the accuracy can be acceptable. However, should you find yourself in court after a serious incident, just make sure that the final answer you end up with is inside the envelope so that you can be held to be squeaky clean. Otherwise you risk being hung, drawn and quartered by a rigid legal system.

Engineering specialist in aircraft performance and weight control.
The following user(s) said Thank You: lcanning