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Convincing story "F1y1n" and I agree with most of it.
However when banking, the loadfactor of the glider increases. E.g. at a bank angle of 45° the loadfactor is 1.41. The result of this is that the polar diagram of the glider moves with a factor sqrt(1.41) = 1.19 to the right and down. So when one likes to stay clear of stalling speeds for what ever part of the glider, flying speed should go up by 19% when banking 45° compared to the "normal" flying speed without banking. "Normal" meaning something like flying with an IAS where sink rate is minimum or may be a little faster to ease control of the glider. Karel, NL Ventus-2cxT "F1y1n" schreef in bericht om... The point I'm replying to is: I'm convinced that in very long wing gliders at high angles of bank and slow speeds (and ergo light weights too), the inner wing is significantly slower than the outer wing, and tacking on some knots is most efficient (to keep the length of the inner wing nicely above stall)... I grant you that the AOA is slightly higher for the inner wing due to the contribution from the sink, but this is negligible. Consider a 45deg bank, 45 knots. The turn radius (at the fuselage) is about 50 meters, so for a 15-m glider the speed of the outer wingtip is about 50knots, and the speed of the inner wingtip is about 40knots. If the sink rate in this configuration is 1.5 knots, the difference in AOA for the two wingtips is about 0.4 degrees. You will notice that (for a good reason!) this is much less than the typical twist of a wing. You cannot stall the inner wingtip in a steep turn without stalling both wing roots first! For the same reason, the inner wingtip is NEVER on the back side of the polar when thermaling. If it was the wing roots would already be stalled. To answer the original question - should one speed up when thermalling with a steep bank - the answer is no. There are too many factors that come into play - the twist of the wing as a function of position, the wing profile as a function of position, the drag produced by the aileron deflection needed to correct for the overbanking tendency as a function of speed, and so on. In the end, these effects will tend to cancel each other: if you speed up a little to bring the wing roots to the front side of the polar you will a) create more drag on the wing tips and b) need more aileron input to correct for the overbanking torque and hence create more drag. I suspect that amount by which one should speed up or slow down to optimize the sink rate in theory will be much smaller than the speed of the turbulent currents in the thermal, and thus utterly irrelevant in practice. Your time will be better spent flying cleanly and in the core of the thermal rather than trying to nail the speed to within 0.2 knots. (Chris OCallaghan) wrote in message . com... You are confusing AOA with sink rate. The sink rate is the same across the airfoil, but AOA is dependent on sink rate and forward speed, so: If an airfoil has a forward motion of 10 and sink rate of one, then its angle of attack can be measured -- about 5.7 degrees. If we then slowed its forward speed to 9 while maintaining a sink rate of 1, the angle of attack would be higher: 6.3 degrees. We agree that the angular speed is the same across the span. We agree "that the inner wing is flying slower." The sink rate is the same across the span. As you've stated, this is a given: the wings are fixed to one another. Since AOA is dependent on both sink rate and forward speed, then the inside wingtip must have a higher AOA. Inner wing slower, higher AOA. Outer wing faster, lower AOA. Lift is dependent on both AOA and speed. So even though the outer wing is at a lower angle of attack, it is moving through the air more rapidly, and producing slightly more lift than the inner wing. With resulting overbanking tendency. Balance this knowledge against the sailplane's response to a turning stall. Inner wingtip typically drops first. Why? Because it has a higher AOA. No aggrevation from the aileron required. |
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