

Jan 21, 2013, 06:03 AM  

Quote:
MAC calculation document I then used the methods outlined in the document to find the centre of area by again intergrating the area moments numerically (chord.x.dx over the span and span.y.dy over the chord and dividing by the total area). The prototype model flew with the CofG at 43% of the root chord, the rot chord being taken as the centreline. So directly taking the CofG position at 90° to the centreline means it intercepts at 22% of the MAC. See the updated sketch from Excel, which is created using a graph and the calculated points. To make sure I have this right, I have taken the planform and considered it as a simple delta wing with a tip chord of zero at the outermost point of the wing. This sort of balances out the areas outside of the delta shape and the cutouts, and the delta I have taken has 98.4% of the area of my model, so pretty close for a cross check. I have then calculated the location of the MAC and the MAC using the cross strings method. The result is in the second sketch, the delat shape is in orange with the rough MAC chord lines. These are very close to the integrated values I came up with. The simple MAC is 6mm (1/4") shorter and 11mm (1/2") inboard. The key is it confirms the numerical integration was OK. Is your point about the 10% MAC an observation that a flying wing such as the B2 ought to balance at 10% MAC or from my graph that the CofG looked more like 10% rather than 22%. 

Jan 21, 2013, 11:47 AM  

The 43% of centreline chord is what is on the plan so that is what I stated first, I am new to all of this theory, empircally it flew at 43% of the centreline chord. If my calcs are correct and the MAC is in the right location then I have a CofG 3% in front of the NP if this is at 25% MAC but as you say, I may not have the maths perfect.
If again I assume my MAC is correct, the setting the Cof G to 48% of the centreline chord brings the CofG to 30% of MAC and behind the 25% NP. No wonder it reared up when it got to flying speed. What this is teaching me is that as I put it together forward CofG is critical and I must not think that it is only part of an inch backwards it'll be OK because it wont and for early flights moving it furtehr forward than the plan may be a good idea. ImagesView all Images in thread 
Jan 22, 2013, 06:29 AM  

Found on the net some calcs for the B" from Aero Eng students at Virginia Tech Aerodynamic calcs for B2 and that has a page where the shape of the wing has been plotted from an on line 3 view drawing along with the resultant calcs for MAC and MAC location.
If I scale the quoted 86ft wingspan down to my models size then my calc for MAC length and the MAC spanwise location agree with my calcs within 5mm (1/4"). Additionally, within a few mm the model agrees with the planform and measurements in this analysis, the same 35° (to one decimal place) LE angle and centreline chord. It seems that the Chris Golds plan is a good outline vs the full scale. You can download the program he wrote to generate MAC from Aerodynamic tools and look under Planform Analysis. The program works but I do not have the numbers for my model with me at the moment to cross check. 
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