Why is the centre of pressure of a finite wing ahead of the quarter chord point predicted by 2D flow over the airfoil? Also what is a "LOW" aspect ratio in terms of numbers. The literature has terms like "very low A.R" "low A.R" "Moderate A.R" and "High A.R", but the actual ratios in terms of numbers are not given.
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$\begingroup$ very low aspect ratio denotes a slender body where its length l is far bigger than its width b: l >> b. Think fuselages, external tanks, projectiles and the like. For wings of varying chord over span, it is preferred to use AR = b²/S (S = area). Low AR is roughly b²/S < 2, moderate AR means b²/S around 5 and high AR is b²/S > 10. But there is no firm definition - every author is free to pick his or her own boundaries. $\endgroup$– Peter KämpfCommented Aug 13, 2016 at 10:11
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The center of pressure is defined as the point on the wing/airfoil where there is a zero net moment. This definition means that it is a function of the lift and subsequently of the lift coefficient. Airfoil theory overestimates the slope of the lift curve due to the absence of wing tip effects. With regards to the second question - I think the answer is rather subjective. Mathematically the limit of a very low aspect ratio is a 2D airfoil moving through unconstrained medium. How high an AR can go is mainly dictated by structural soundness of the wing. For real aircraft, have a look at the classic example of a low aspect ratio wing - F-104. Typical high aspect ratio aircraft are most gliders. I would point you to the exceptionally interesting read that is Theory of wing Sections by Abbott and Doenhoff and also any book on aerodynamics, e.g. Anderson
