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#1
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How well polished does a glider need to be?
I'm crewing this season, and no doubt my pilot will be expecting me
out at 7am with Mer in hand. My question is (and I'm sure it's been debated before, I did search, but want fresh input) how worthwhile is polishing? I can understand keeping the fuselage as slippery as possible to reduce skin drag, but I've heard that glider wings would still work as well as possible if their surface was 40 grit sandpaper! Also take a look at a golfball -- it's dimpled specifically in order to reduce drag, again implying that a smooth surface may not be perfect. On the other hand I realise that bugs, chips in the gelcoat etc. are bad news as they locally seperate the boundary layer, so you don't want those around. Dan |
#2
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How well polished does a glider need to be?
As a 'glider sander' and surface contourer I usually stop at 400 or 600 grit
and find the performance at least equal to a highly polished surface. There is some wind tunnel data that suggests a slightly rough surface actually keeps the boundary layer energized for greater laminar flow whereas a glossy surface trips the flow to turbulent sooner. One school of thought says that the big advantage of gelcoat is that it can be resanded and polished each year to keep the airfoil contour perfect. When it's sanded off, just recoat and start over. The downside to a non-glossy surface is that it will pick up handprints very easily. The only way to remove these is more sanding. Bill D "Dan G" wrote in message ... I'm crewing this season, and no doubt my pilot will be expecting me out at 7am with Mer in hand. My question is (and I'm sure it's been debated before, I did search, but want fresh input) how worthwhile is polishing? I can understand keeping the fuselage as slippery as possible to reduce skin drag, but I've heard that glider wings would still work as well as possible if their surface was 40 grit sandpaper! Also take a look at a golfball -- it's dimpled specifically in order to reduce drag, again implying that a smooth surface may not be perfect. On the other hand I realise that bugs, chips in the gelcoat etc. are bad news as they locally seperate the boundary layer, so you don't want those around. Dan |
#3
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How well polished does a glider need to be?
On Wed, 9 Jul 2008 04:26:33 -0700 (PDT), Dan G
wrote: I'm crewing this season, and no doubt my pilot will be expecting me out at 7am with Mer in hand. My question is (and I'm sure it's been debated before, I did search, but want fresh input) how worthwhile is polishing? I can understand keeping the fuselage as slippery as possible to reduce skin drag, but I've heard that glider wings would still work as well as possible if their surface was 40 grit sandpaper! Also take a look at a golfball -- it's dimpled specifically in order to reduce drag, again implying that a smooth surface may not be perfect. There's a major difference between blunt bodies like a golf ball and slender ones like an airfoil. Drag on the former is mostly pressure drag, while drag on the latter is dominated by skin friction. If the boundary layer on a golf ball never separated, with the flow from all sides rejoining smoothly on the back side, there would be no pressure drag at all...but in real life, on a smooth ball, it will separate by the time it gets halfway around. Consequently, the back side of the ball is in a low pressure area and there is lots of pressure drag. The dimples create small-scale turbulence on the surface, adding energy to the airflow, which delays the separation of the boundary layer to about the 3/4 point. That keeps high-pressure air on part of the backside, reducing the pressure drag. The dimples also increase the skin friction, but since that is small on a sphere, it's a positive tradeoff. Skin friction drag depends heavily on surface roughness: every time an air molecule impacts the forward face of a bump, it extracts a little bit of energy from the aircraft. rj |
#4
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How well polished does a glider need to be?
On Wed, 09 Jul 2008 08:13:04 -0600, Ralph Jones
wrote: On Wed, 9 Jul 2008 04:26:33 -0700 (PDT), Dan G wrote: I'm crewing this season, and no doubt my pilot will be expecting me out at 7am with Mer in hand. My question is (and I'm sure it's been debated before, I did search, but want fresh input) how worthwhile is polishing? I can understand keeping the fuselage as slippery as possible to reduce skin drag, but I've heard that glider wings would still work as well as possible if their surface was 40 grit sandpaper! Also take a look at a golfball -- it's dimpled specifically in order to reduce drag, again implying that a smooth surface may not be perfect. There's a major difference between blunt bodies like a golf ball and slender ones like an airfoil. Drag on the former is mostly pressure drag, while drag on the latter is dominated by skin friction. [snip] I forgot one other effect of the dimples: they make lift. A golf ball is normally hit with backspin, and the curveball effect creates a little bit of lift, making the ball a super low-performance glider...;-) The dimples magnify the effect somewhat, by generating a little extra flow circulation around the ball in the proper direction. If you've read up on the spinning mines that were used against the Ruhr Dams, it's precisely the same effect: the backspin on the mine kept it pressed against the surface of the dam as it sank. rj |
#5
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How well polished does a glider need to be?
As a 'glider sander' and surface contourer I usually stop at 400 or 600 grit and find the performance at least equal to a highly polished surface. There is some wind tunnel data that suggests a slightly rough surface actually keeps the boundary layer energized for greater laminar flow whereas a glossy surface trips the flow to turbulent sooner. Bill D Hi Bill, I have not seen any research data as it relates to sanding. When sanding, it appears, mixing of air takes place sooner and laminar flow is reduced. Sanding with 360 to 400 grit works in some in some cases with the FX 67 and similar airfoils. It is just an other form of a turbulator in my opinion. In the case of sanding, the run of laminar flow is reduced, because of that a more energetic transition can takes place and attached turbulent flow can be obtained in an otherwise unfavourable pressure recovery as with the FX 67. The same can be achieved by placing turbulator bumps at about 40% and 50% chord with a 1" inch spacing, the bump is 3/16"diameter and .025" high, this allows one to keep the polished surface, plus a positive flow trip. If there is a vestige of laminar flow left in the region, it can move in-between the bumps. At climb speed the wake spreads at 35 degree, give or take. When cruising, the wake pattern is more parallel at first and then spreads. I have used this technique to solve some short comings on my original HP18. In either case it is not optimum I rather see the airfoil fixed or at least corrected so it meets it intended design criteria. Similarly, HP18 owners complained about wing drops on there take off rolls. I recommended to place a turbulator 50% chord on the top surface, within the span of the aileron. which stop the wing drop problem. Udo |
#6
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How well polished does a glider need to be?
Do whatever you pilot wants you to do. A pilot wo is confident that his
glider is in the absolute best possible shape ist far more important than half a percent of theoretical gain. |
#7
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How well polished does a glider need to be?
At 11:26 09 July 2008, Dan G wrote:
I'm crewing this season, and no doubt my pilot will be expecting me out at 7am with Mer in hand. My question is (and I'm sure it's been debated before, I did search, but want fresh input) how worthwhile is polishing? I can understand keeping the fuselage as slippery as possible to reduce skin drag, but I've heard that glider wings would still work as well as possible if their surface was 40 grit sandpaper! Also take a look at a golfball -- it's dimpled specifically in order to reduce drag, again implying that a smooth surface may not be perfect. On the other hand I realise that bugs, chips in the gelcoat etc. are bad news as they locally seperate the boundary layer, so you don't want those around. Dan I believe the difference between the wing and the golf ball is a result of low reynolds numbers ,you get some strange affects when you increase the size of aerofoils . With regard to crewing in my competition flying I like to rig,water ballast,grid and polish while grid squatting.It gives you something to do and moves any finger marks . Jon |
#8
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How well polished does a glider need to be?
On Jul 9, 4:26*am, Dan G wrote:
I'm crewing this season, and no doubt my pilot will be expecting me out at 7am with Mer in hand. My advice is wipe off any water spots from dumping as soon after landing as possible if flying in a hard water area, then knock off any bugs on the leading edges, then have a few beers and sleep in late in the morning. Final wing cleaning, at least in the dusty SW USU, should be done on the grid shortly before takeoff. We all know if make no difference to contest scores, it's just occupational therapy, and best done by the pilot anyway. Andy |
#9
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How well polished does a glider need to be?
Well, my ASW 19 had the same gelcoat for 33 years now, so talk about
sandpaper! But hey, it still fly!...:-) Richard |
#10
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How well polished does a glider need to be?
Dan,
How much you polish depends on several important things. 1) How old is the pilot? If the pilot is old then you can just say you polished the wings and he will not be able to tell the difference! 2) Is rain a possibility? If you wax or polish a wing so that water beads on a laminar flow wing the pilot will kill you....don't polish but wipe or sand it! 3) What nationality is the Pilot? If the pilot is German you better start a couple days ahead of time because perfection is the only acceptible job. If the pilot is South West American, he will not recognise his own glider if you polish it so don't. If the pilot is Australian then you are probably mistaking your pilot for someone else as Aussie pilots dont need crew. 4) Can you see the fiberglass weave on the wings? If so then just mist the wing and dry if with a cloth and chase bugs away from the glider as best you can. 5) What is for dinner? If its a McDonalds burger then just clean the canopy. If its a nice steak dinner with desert then polish for as long as it would take to eat your dinner. You will figure out what else is important after the first time crewing ;-) Ray On Jul 9, 4:26 am, Dan G wrote: I'm crewing this season, and no doubt my pilot will be expecting me out at 7am with Mer in hand. My question is (and I'm sure it's been debated before, I did search, but want fresh input) how worthwhile is polishing? I can understand keeping the fuselage as slippery as possible to reduce skin drag, but I've heard that glider wings would still work as well as possible if their surface was 40 grit sandpaper! Also take a look at a golfball -- it's dimpled specifically in order to reduce drag, again implying that a smooth surface may not be perfect. On the other hand I realise that bugs, chips in the gelcoat etc. are bad news as they locally seperate the boundary layer, so you don't want those around. Dan |
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