Just some quick thoughts - it takes a long time to analyze in detail why small aeroplanes fly!
Let us assume, for convenience, that the CG is on the trailing edge of the wing, the wing lift is acting upwards somewhere in front of this and the tailplane is also lifting upwards at the end of its boom. As the model is flying in balance the wing lift times its distance to the CG must equal the tail lift times its distance to the CG. Now if the boom length is increased the moments will no longer be in balance, the tail lift working on a longer boom will make the model nose down. Presumably Bernie's program calculates a new, lower, angle of attack for the tailplane to reduce its lift so the model's total lift coefficient will be reduced and the flying time will go down.
However wily aeromodellers know that you don't always take calculations at face value; they know that the longer boom will make the aeroplane more stable so they move the CG back to make the stability margin what it was before. With the CG moved back the tailplane has to do more work so the tail angle of attack is increased, more or less, to where it was before so the longer boom will not give a reduction in performance if the aeroplane is properly trimmed. Indeed with the better dynamic stability and the slight improvement in downwash conditions you will probably put on a few seconds (as long as you keep the weight down).
John
Received on Fri Feb 24 2012 - 08:00:50 CET
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