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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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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 |
#5
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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 |
#6
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Eric Greenwell wrote:
I thought Johnson's criteria was +/- .004". Thanks Eric and Udo. I got the +/+.002" from a Soaring article *not* written by Mr. Johnson. My mistake! I tried looking at some of the flight tests from the SSA website and +/-.003" to .004" seems to be what Mr. Johnson uses as his benchmark, as Udo says. Now I'm feeling a lot better about my wing reprofiling work! ;-) Regards, -Doug |
#7
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Doug,
it is "not + or -" the total is max .004" Udo |
#8
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Eric Greenwell wrote:
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". Eric, After reviewing the articles I could find with more care, and with Udo's input, I think .008" peak-to-peak (or +/- .004") is too large a wave for laminar flow. Regards, -Doug |
#9
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Doug Hoffman wrote:
I thought Johnson's criteria was +/- .004". Eric, After reviewing the articles I could find with more care, and with Udo's input, I think .008" peak-to-peak (or +/- .004") is too large a wave for laminar flow. I'm sure you are right - I meant peak to peak. -- Change "netto" to "net" to email me directly Eric Greenwell Washington State USA |
#10
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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 |
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