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#1
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How does spar protrusion affect performance
On the top surface of the wings of a 17m Kestrel I
am looking at, the spar is clearly visible and you can feel it (it stands out slightly proud) below the surface of the wing itself. This is uniform down the whole spar and on both wings. Is this 'normal'? Will it affect performance?? Thanks Chris |
#2
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yes it's 'normal' and yes it will affect performance
it needs fairing, filling and refinishing. Ian "Chris Davison" wrote in message ... On the top surface of the wings of a 17m Kestrel I am looking at, the spar is clearly visible and you can feel it (it stands out slightly proud) below the surface of the wing itself. This is uniform down the whole spar and on both wings. Is this 'normal'? Will it affect performance?? Thanks Chris |
#3
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tango4 wrote:
yes it's 'normal' and yes it will affect performance it needs fairing, filling and refinishing. Ian Yes. According to Richard Johnson a laminar flow airfoil not only needs to be accurate in shape , but it needs to have no "waves" larger than +/- .002" in 2", in order to extract best performance. If you can see and feel the bump of the spar you are violating both of these criteria. Fairing and refinishing a wing, and making sure the airfoil is correct, is not a small task, but doable. I am (finally) finishing up my RS-15 wings. I know the shapw is accurate and I know the +/- .002" waviness limit is not exceeded (this is best checked with a dial indicator). Regards, -Doug "Chris Davison" wrote in message ... On the top surface of the wings of a 17m Kestrel I am looking at, the spar is clearly visible and you can feel it (it stands out slightly proud) below the surface of the wing itself. This is uniform down the whole spar and on both wings. Is this 'normal'? Will it affect performance?? Thanks Chris |
#4
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Chris Davison wrote:
On the top surface of the wings of a 17m Kestrel I am looking at, the spar is clearly visible and you can feel it (it stands out slightly proud) below the surface of the wing itself. This is uniform down the whole spar and on both wings. Is this 'normal'? Will it affect performance?? Thanks Chris It happens to some extent to all GRP gliders as they age (some say PIK's are immune - hmmm...), and it's due to shrinkage of the GRP. It will affect performance because it will trip the laminar flow into turbulent flow, this may or may not be beneficial depending on what part of the polar you are flying on - I think you can safely assume it will adversely affect it. You will need to get the wing re-profiled to fix it. Rgds, Derrick Steed |
#5
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At 16:30 10 July 2004, Chris Davison wrote:
Is this 'normal'? Will it affect performance?? Yes and yes. As others have said, you'll need to have the wings reprofiled to fix it. However, you should also ask yourself whether it is worth fixing. You are looking at a few hundred hours of work by someone who knows what they're doing (or several hundred by someone who doesn't). How much time and/or money are you willing to expend, given that there is no guarantee that you'll end up with a noticeable increase in performance? A related anecdote: my partner and I have a LAK-17A which has quite obvious spar bumps on the top and bottom wing surfaces. We're perfectly happy with the performance, so we haven't done anything about it. A top US contest pilot had flown with us a number of times in his new ASW-27B (when we were using 15M tips), and noticed that he couldn't keep up in higher speed glides. So, he spent several thousand dollars getting the bottom of his wings reprofiled. He is now much happier with his ship. Of course, he hasn't asked for a comparison glide since the reprofile job 8^) Marc |
#6
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Marc Ramsey wrote:
A related anecdote: my partner and I have a LAK-17A which has quite obvious spar bumps on the top and bottom wing surfaces. We're perfectly happy with the performance, so we haven't done anything about it. A top US contest pilot had flown with us a number of times in his new ASW-27B (when we were using 15M tips), and noticed that he couldn't keep up in higher speed glides. So you are going to make impressions of your "unique" airfoil in case LAK fixes their process? That way you could get them to build you another glider (should you need one) with assurances you will still have the magic bumps. ;-) Regards, -Doug |
#7
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Doug Hoffman wrote:
So you are going to make impressions of your "unique" airfoil in case LAK fixes their process? That way you could get them to build you another glider (should you need one) with assurances you will still have the magic bumps. ;-) Not to worry, I don't think LAK is any more capable of fixing their processes than Schleicher 8^) Marc |
#8
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Marc Ramsey wrote:
Doug Hoffman wrote: So you are going to make impressions of your "unique" airfoil in case LAK fixes their process? That way you could get them to build you another glider (should you need one) with assurances you will still have the magic bumps. ;-) Not to worry, I don't think LAK is any more capable of fixing their processes than Schleicher 8^) Of course. How naive of me. :-) Btw, here at Ionia we have a LAK-17A driver whose ship also exhibited some problems at the surface over the spar. Although I can't recall if it was protrusion or depression. Regardless, the local repair guru, David Nelson, did a repair job on it and it now looks flawless. The LAK-17A driver seems to really like his ship as well. Regards, -Doug |
#9
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Doug Hoffman wrote:
tango4 wrote: yes it's 'normal' and yes it will affect performance it needs fairing, filling and refinishing. Ian Yes. According to Richard Johnson a laminar flow airfoil not only needs to be accurate in shape , but it needs to have no "waves" larger than +/- .002" in 2", in order to extract best performance. If you can see and feel the bump of the spar you are violating both of these criteria. Fairing and refinishing a wing, and making sure the airfoil is correct, is not a small task, but doable. I am (finally) finishing up my RS-15 wings. I know the shapw is accurate and I know the +/- .002" waviness limit is not exceeded (this is best checked with a dial indicator). I thought Johnson's criteria was +/- .004". -- Change "netto" to "net" to email me directly Eric Greenwell Washington State USA |
#10
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"Eric Greenwell" wrote in message ... Doug Hoffman wrote: tango4 wrote: yes it's 'normal' and yes it will affect performance it needs fairing, filling and refinishing. Ian Yes. According to Richard Johnson a laminar flow airfoil not only needs to be accurate in shape , but it needs to have no "waves" larger than +/- ..002" in 2", in order to extract best performance. If you can see and feel the bump of the spar you are violating both of these criteria. Fairing and refinishing a wing, and making sure the airfoil is correct, is not a small task, but doable. I am (finally) finishing up my RS-15 wings. I know the shapw is accurate and I know the +/- .002" waviness limit is not exceeded (this is best checked with a dial indicator). I thought Johnson's criteria was +/- .004". -- Change "netto" to "net" to email me directly Eric Greenwell Washington State USA R.Johnson states .003" to .004" peak to peak in Soaring May 1998 The contact of the gage is held in the centre by a 2" cart. The Johnson measuring cart picks up only limited deviations info. Longer wave can not be picked up readily and longer waves can be just as detrimental if they exceed certain limits (That is where attention to details and craftsmanship is so important.) I would like to refer Doug to an article by Vic Saudek "Airfoil Smooth-Curve Measuring" of 1987 in Soaring for further reading. Regards Udo |
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