Hi guys,

I'm building a 3.5 meter 747 400. See pictures posted on www.scalesoaringaustralia.com and go to the forum. (anyone can look it just a small club web site. The Fuz is nearly done. I need to calculate the wing lift verses cord including the washout etc. I will be using a different root and tip section. I need to know the stall speed for each part of the wing. A formula would be helpful and I can put it into a spread sheet. I was going to use RG15 root and Clark Y tip as it has worked before on a 4 meter U2. I want to know the true stall speed.

Well, when using 2 different airfoils at the root and the tip this calculations gets a lot trickier, becasue you will have to calculate the spanwise lift distribution and find the wing's lift coefficient from there. That I know of, this is very complicated to do (i'm trying to code a small program in VB to do this, but it's very complex). From my point of view, you could get XFOIL which is free (get it here http://raphael.mit.edu/xfoil) and use it to analyse the airfoils you will be using.

Or, if you can get the profiles for each part of the wing (say, 20 sections spanwise) analyse each one of them with XFOIL and take the overall arithmetic mean of the Max Cl values. Using this method, expect some error (up to 10%) in the stall speed you get from the following formula.

Then get the mean max lift coeficcient with the results you got and calculate the stall speed using this formula:

Ss = sqr((weight(oz) * 3519) / (sigma * Cl * speed(mph))

where sigma = air density factor (1.0 for sea level, 0.87 for 5000ft 0.74 for 10000ft)
Cl = Lift coeficient

If you need any help with XFOIL, please feel free to e-mail me at [email protected].