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#51
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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. |
#52
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I was taking issue with an oversimplification, one you made for the
sake of convenience, I'm sure. Pet peeve of mine. In an earlier thread on slips, I asked whether anyone had thought through the notion of using a slip (dihedral rolling moment)to counteract the overbanking tendency . I didn't note any responses, though it might prove interesting to develop the idea in the context of this thread. Dodging the math, I'll also add that a few extra knots while circling gives a great deal more aileron and rudder authority. Since smooth, elevator cores are the exception rather than the rule, the more effective your controls, the quicker you can correct for or take advantage of turbulence, then get the controls streamlined (or as close a practical) to minimize drag. The lower the speed, the greater the drag for a given control input, and the longer you'll have to leave it in to achieve the desired change in direction. Cheers, Chris |
#53
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F1y1n wrote:
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 snip 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. I thank you for taking the time to do some math, but unfortunately I don't have quite the time to verify it, and I've two references (Carl Herold and Dick Johnson) who seem to encourage what I've suggested. And my math must be poor. If a glider has 2 knots of sink near stall, and stalls at about 40 knots, then straight it has an AOA of 3 degrees (sin 3 deg = .05...)? Ahhh...that can't be right... I'll need to look closer at this, but I certainly hope at least you gained something for yourself from this discussion too... |
#54
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#56
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You'll want to add the angle of incidence of the wing. Assume it is 15
degrees to the fuselage center line and you're getting close to typical critical AOA. The math, in fact is a little more tenuous, but not much. Just assume your airspeed is the path through the air. You also have vertical speed, so you have the hypotenuse and one side. Some trig yields horizontal speed, a little more yields AOA at the fuselage centerline, then add AOI for AOA of the airfoil. A bit psuedo scientific, but perfectly acceptable for the sake of the discussion. At least it gets us numbers that look more like the stuff we see in tables. |
#57
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Robert Ehrlich wrote:
Mark James Boyd wrote: ... And my math must be poor. If a glider has 2 knots of sink near stall, and stalls at about 40 knots, then straight it has an AOA of 3 degrees (sin 3 deg = .05...)? Ahhh...that can't be right... call this last one incidence while in France we call this "calage" because we use "incidence" for what you call AOA :-) In the US, "calage" is where I would go to learn math and franch, and how to spell patato. By the way, I love the bread, fries, and kissing. You can keep the dressing, horn, doors, and braids. :P But thanks for the Statue of Liberty! |
#58
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Mark James Boyd wrote:
... And my math must be poor. If a glider has 2 knots of sink near stall, and stalls at about 40 knots, then straight it has an AOA of 3 degrees (sin 3 deg = .05...)? Ahhh...that can't be right... ... No, 3 degrees is the angle between the airflow and an horizontal line. In order to obtain the AOA you have to add the angle of the fuselage axis with this horizontal line (nose up attitude) and the angle of the wing chord with the fuselage axis, you english speaking people call this last one incidence while in France we call this "calage" because we use "incidence" for what you call AOA :-) |
#59
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nafod40 wrote:
F1y1n wrote: This I do not agree with. The angle of attack of both wings is the same. It always helps my understanding to look at limiting cases. If you take the wingspan to an extreme, the inner wing would reach all of the way to the center of the circle, and its airspeed would be zero. It would be descending, though, so its AOA would be 90 degrees. Certainly different from the outer wing. Yes, but we are far from this limiting case. As someone else pointed, in the case of a typical glider making a typical turn the difference is a few tenth of degrees. This explains why it can't counteract the effect of the speed difference and we all have experienced this overbanking tendancy that we must counter with outside stick. So the speed difference make a little difference in AOA but a much more noticeable difference in lift. We cancel this difference by using outside stick, i.e. introducing a difference in lift coefficient by changing airfoil and AOA. But when we are sufficently close to stall AOA, i.e. maximum lift coefficient, this change is of no help and may even have the opposite effect if this brings the inner wing tip to a lower lift coefficient. Then the inner wing will drop, and then, as now both wing have a significative difference in their sink speed, this introduces a significative difference in their AOA. |
#60
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I'm with Bob on this one.
From an aerodynamics manual: "However, many other parameters influence the lift that a wing produces. The most basic is the configuration of the wing, specifically the position of the trailing-edge flaps, leading-edge flaps or slats, and spoilers. As the trailing-edge flaps are extended, the curvature (or camber) and area of the wing are increased, and the wing will produce more lift at the same AOA (fig. 2). Note that although the maximum lift is increased, the AOA at which stall occurs is actually less because the wing cannot sustain the higher lift levels up to the same AOA. The airflow separates earlier. Wing-mounted speed brakes or spoilers have the opposite effect. They reduce the lift at a given AOA; they also reduce the maximum lift achievable but, surprisingly, increase the AOA at which stall occurs." Note that, on a long winged glider with ailerons near the tips, a deflected aileron on the down wing changes the camber of the wing in that area. With the overbanking tendency of the down wing, there is inevitably some down aileron on that wing (to stop the bank) and the above aerodynamics apply. Allan "Bob Kuykendall" wrote in message om... Earlier, (F1y1n) wrote: ...The angle of attack of both wings is the same. The air is impacting both wings from exactly the same angle - the wings are connected by the fuselage.... I think I disagree on what might appear to be a technicality: The wings are connected not only by the fuselage, but also by the control system that the pilot uses to balance the lift distribution between the two wings. Since the inner wing is going substantially slower, and as you point out later has only a slightly greater angle of attack, something has to increase the Cl on the inner wing in order to keep from rolling into the turn (the overbanking tendency). That something is the pilot applying slight opposite aileron, increasing the inner wing's effective angle of attack. That may only be a couple of degrees of deflection down on the inner wing, but its also the same or more up on the outer wing, decreasing its effective angle of attack. The deflected surface has a substantial effect on the pressure distribution of the airfoil in front of it, and can make a big difference on its stall characteristics. Thanks, and best regards to all Bob K. http://www.hpaircraft.com/hp-24 |
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