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#31
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That's pretty impressive Chip.
I would think that using rate of change of curvature (or absolute curvature) to estimate the thickness of the overall airfoil would be really challenging. It's kind of like using an accelerometer (or variometer) to estimate your altitude - it's possible if you're really good at integrating the values, but you need very accurate measurements to keep the integrated sum from drifting pretty significantly. Since these dial guages are generally only good to 001' or so, it seems like they're best at estimating surface waviness. I'm not disputing what you did - I'm just amazed that you were able to do it. Can anyone tell me if it really matters if the overall thickness of the wing is off by a few thousandths. This is the difference between sanding down to the spar wave and building up gelcoat in the spar wave. I think most people sand down if they can and build up if they have to. Even if I make my wing .005' thinner overall from sanding, that's around 1/10 of 1% of the total thickness - I'm not sure if the design/manufacturing tolerances are that good to start with and I don't know if the factory allows for some shrinkage in the initial construction. Of course if my spar wave is .05' instead of .005' than maybe the answer changes. At what point does it start to matter? I thought the main objective of reprofiling these days was to remove surface waviness - but this adds a new level of complexity - should I even worry about it? 9B At 17:54 18 September 2004, Chip Bearden wrote: Be aware that the gauge alone will *not* detect errors in the shape of the profile or thickness of the wing. For that you need accurate templates. Actually, it is possible to do *some* profiling of the wing using a dial gauge. You need the airfoil coordinates and the dimensions of your dial gauge (the distance between the contact points of the 'feet', and where between the feet is the dial gauge). This assumes that the thickness is correct and that the very nose of the leading edge is also correct, two big assumptions. I believe it was Rudy Alleman who published a paper in Technical Soaring (early 80s?) on comparing airfoils from one glider to the next this way. I derived a slightly more elegant (but no more accurate) solution and wrote a BASIC program to do the number crunching about 20 years ago. Basically for each chord length (spanwise station) in question, you use three points on the segment of the airfoil spanned by the dial gauge to calculate radius of curvature (any three points lie on a circle). Then you can calculate what the dial guage should read at that position for the correct curvature. That's a potentially useful number but I found that the easier way is to calculate how much the dial gauge should change moving from, say 3' aft of the leading edge to 4' aft on a 28' chord. If it's supposed to 'unwind' (i.e., the curvature is getting flatter) .010' but it actually drops more than that, then the curvature is getting too flat too soon, and vice versa. It helps to print out a strip of paper with the actual readings every inch or so and tape it to the chord line so you can do the deltas from one point to the next as you slide the dial gauge along the wing. I had a little trouble at first visualizing what was wrong when the actual numbers didn't agree with the calculations. It's especially difficult when you come to a bad spot that spans more than the dial gauge itself. Let's see, the needle went clockwise .005' too much which means that the curvature is too sharp. So the back feet are sitting in a depression. When the dial gauge slides into the depression then...what? I started putting small pieces of tape on the wing to build up the low points, so the dial gauge feet could rest on the 'reprofiled' wing. Only then did it become obvious there was a large 'flat' spot over the spar cap on my old LS-3 caused by shrinkage. Moving from front to back there'd be no tape, then .002' of tape, then .004' of tape, then ..006', then .008', then .006', then .004' and so forth back to zero. Then the two sharp dial gauge needle reversals we had seen made sense (one was the sharper point at the forward end of the flatter spot and the other was at the after end of it). It was too extensive to sand out so we sprayed gel goat over the flat spot and built up that area, then used the dial gauge to get the correct contour. The results were dramatic in improved performance. Templates are a far more accurate way of profiling a wing, but you can learn a lot, and even make some small adjustments, with a good dial gauge. It helps tremendously if you have the airfoil coordinates, of course. That was easier then than now. Interestingly, when I first ran the program against the Wortmann series on the LS-3, I found some strange discontinuities (i.e., the deltas weren't a smooth curve in several spots) even though I had used the corrections that Dr. Wortmann had published. It wasn't until I cranked in Dan Somers' subsequent corrections that all the deltas smoothed out. Chip Bearden |
#32
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Andy, see comments below:
I would think that using rate of change of curvature (or absolute curvature) to estimate the thickness of the overall airfoil would be really challenging. It's kind of like using an accelerometer (or variometer) to estimate your altitude - it's possible if you're really good at integrating the values, but you need very accurate measurements to keep the integrated sum from drifting pretty significantly. You're right. It's probably poor for profiling the whole wing. But it worked reasonably well in picking up flaws that were bigger than the wave gauge but substantially smaller than the chord. If I'd known I had a significant problem when I started, I'd probably have tried to make templates. But by the time I finally figured out what the problem was, I felt like I could go ahead with the dial gauge. Frankly, an experienced tuner like Hank Nixon would pick up that kind of problem simply by running his hand over the wing! Can anyone tell me if it really matters if the overall thickness of the wing is off by a few thousandths. This is the difference between sanding down to the spar wave and building up gelcoat in the spar wave. I think most people sand down if they can and build up if they have to. The main reason we sprayed to build up was that this was the second time the glider was in for sanding and there simply wasn't enough gel coat over the leading and trailing edges of the spar cap; I was already seeing dark spots. You're right about overall thickness. The later LS-3a wings (I had an earlier "3") allegedly were as much as 2% thicker than spec in some places. That's a half inch on, say, a 24" chord!!! From memory, Dick Johnson found similar things on his PIK-20 and just extrapolated the airfoil for thicker sections since he had the coordinates for both 15% and 17% sections. I thought the main objective of reprofiling these days was to remove surface waviness - but this adds a new level of complexity - should I even worry about it? 9B I sanded/contoured my LS-3 three times in the 12 years I owned it in addition to extensive sealing and other tuning. The only thing I've done to my ASW-24 over the past 12 years is add winglets. One reason is that the '24 wing is amazingly stable over time (maybe one of the best composite gliders ever). The other reason is that I was single when I owned the '3 and with a 9 to 5 job. Now I'm married with two children, and average 60 hour weeks! If you're really worried about it, I recommend selling your '27 and buying a PW-5. Chip Bearden |
#33
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Earlier, Andy Blackburn wrote:
Can anyone tell me if it really matters if the overall thickness of the wing is off by a few thousandths... Based on template checks of several top-ranked contest ships, I will opine that it does not. For a few of the ships, you'd guess on the basis of the template mismatch that the right and left wings weren't even for the same aircraft. And yet these were all straight-flying, go-like-stink gliders. So you'll find me in the camp that says that waviness below .004" is first priority, canopy and wing root sealing is second, and fidelity to profile is a distant third. Thanks, and best regards to all Bob K. http://www.hpaircraft.com/hp-24 |
#34
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