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#18
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Mark James Boyd wrote:
... I did a few calculations of an imaginary glider with a stall speed of 32 knots, a min sink speed of and a wingspan of 87 feet. In a 50 degree bank at 54 knots (good thermalling speed if you believe) www.stolaf.edu/people/hansonr/soaring/spd2fly/ the fuse and ASI says you are at radius 180 ft circling every 7 seconds. The inner wingtip is 3/4 of that distance, and 3/4 of that airspeed, and should be stalled. The outer wingtip is 5/4 of that distance from center, and 5/4 of that airspeed, and producing excellent lift. ... I don't completely agree with your computations. I agree with the 54 knots, i.e. 43 knots multiplied by the square root of the load factor at 50 degree bank. However the radius I find for this speed and bank is 66 m (sorry, I prefer to do my calculations in metric, because I know the formulas for metric data), i.e 216.5 ft. A wingspan of 87 feet translate into 26.5 m, the inner wingtip is inside the circle by an amount which is the half wingspan multiplied by the cosine of 50 degree, this is 8.5 m or 27.8 ft. The ratio of the two radii is .87 rather than ..75 and the speed at the inner wing tip is 37.4 kt. Anyway even with your values tis doesn't implies the inner wing tip is stalled, because stall depends on AOA rather than speed. Of course you need an increase of AOA in order to compensate for the lower speed in order to keep an equal lift on both wings. Some difference in AOA between both wings is already provided by the simple fact that the glider is sinking, i.e. both wings have the same vertical component of velocity but different horizontal ones. The complement is provided by aileron deflection, which change not only the AOA but the whole airfoil shape, so that the action is an increased Cl due to both changes in AOA and shape. The stall case would be if the needed Cl would be higher than the maximum achievable Cl, but this can't be decided just from the value of the speed at wing tip. |
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