By placing a control volume around the airfoil section and its wake, we can use the momentum equations in integral form to find the time rate of change of momentum into and out of the control volume. This is sometimes called the “wake rake” technique. One common way of estimating the steady drag on a 2-D airfoil section is to estimate the momentum loss in the wake of the airfoil by measuring the velocity profile downstream of its trailing edge, as shown in Fig. However, the measurements must be properly corrected for any 3-D effects associated with the airfoil configuration in the test facility. This has the advantage of measuring both the pressure and viscous shear drag. Alternatively the drag may be measured with a force balance. The problem is complicated, however, with the formation of supercritical flow and a shock wave. However, for this to be accurate a good concentration of pressure points is required in the region of high suction pressure (usually the leading edge region). Pressure drag may be estimated by the integration of pressure with respect to airfoil thickness as previously described. In addition, at higher Mach numbers there is a source of drag known as wave drag, which is important whenever a shock wave forms of the airfoil. There are two sources of drag:įigure 7.29 Measurement of drag by wake momentum deficit approach.ġ. operating in attached flow are typically nearly two orders of magnitude less than the lift forces at the same AoA. Many modem helicopter rotor designs take advantage of the low pitching moment benefits produced by reflex cambered airfoils.ĭrag forces on airfoils. It is apparent that only a small amount of reflex at the trailing edge is required to negate the pitching moment produced by the positive camber near the leading edge. The results for A and A 2 are given byĪnd by satisfying A - A2 = 0 we find that a = 8.28 and b = 0.875 with p - 0.31. e., Aj - A 2 = 0 using the thin-airfoil theory). Which can be solved for a and b under the assumption that Cmi/4 = 0 (i. This leads to the two simultaneous equationsĪp3 + a(b - l)p2 - abp = 1 and Зар2 + 2a(b - l)p - ab = 0, (7.83) We may define a specific cubic camberline such that dyjdx - 0 at x = p and ydm - 1 at jc - p. While this equation describes a family of camberlines, of specific interest in this case are the values of a and b that will produce a camberline with zero pitching moment. Where m is the maximum camber as a fraction of chord, and a and b are general coefficients. Many authors have investigated low pitching moment reflexed airfoils based on cubic caiuberliues - see Glauert (1947) and Houghton & Carpenter Generally, for a given airfoil, the addition of reflex camber to an airfoil with camber in the nose region gives a significant reduction in Cm with only a minor reduction in maximum lift capability and a small drag penalty.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |