Adam,

I see what you mean, but I'm not surprised that the Geistware NP estimate is off.

I made a mistake interpreting "your" equations. I use the formula

CGTarget(As %of MAC) ~ 10+40*TailVol

for a CG estimate with 15% Margin. The 25+40*Vbar version adds the 15% margin back in, so it is an estimate of the NP.


The Neutral Point calculation in the Geistware Site assumes the lift coefficient for each slice of the tail and wing are same. So total lift is proportional to area and is acting at 25% of the MAC for both the tail and wing. The NP can then be calculated by solving for the point where there is no pitching moment. It seems to do the calculation correctly. How accurately this describes reality is debatable. If you add the tail efficiency kluge that is used in the Simons formula, you will get numbers more to your liking. Try an example and reduce the tail area to 0.6*TailArea.

You can then go a step further and reduce the tail volume again by the downwash term (1 - (de/da ))~(1-.33)=.66 then the two estimates are very close indeed. The Geistware version has a (WingArea+TailAera*.6*.66) in the denominator vs. WingArea for the Simons formula. All this assuming as/aw=1.

Carl