Marwen, first off 48 + 6 = 54 and not 52. And when you get the average for the wing chord by adding the root and tip and dividing by 2 you have already allowed for the shape and the average chord is the average chord. No need to divide by two for the shape. So it's 27 x 60 which is 1620 sq inchs. You've got yourself a Vulcan like "balsa overcast" there....