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Wing Loading
Is there a practical limit to wing loading?
I'm looking for aero dynamic information, not statements about max gross weight. Can you load your glider up to the point that, even with strong and reliable lift, you're at a disadvantage to lighter ships? I was just thinking of the old days when I read that some contest pilots tried, or considered, using salt water for ballast because it's heavier. |
#2
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Wing Loading
On Apr 25, 7:38*pm, "Dan Marotta" wrote:
Is there a practical limit to wing loading? I'm looking for aero dynamic information, not statements about max gross weight. *Can you load your glider up to the point that, even with strong and reliable lift, you're at a disadvantage to lighter ships? I was just thinking of the old days when I read that some contest pilots tried, or considered, using salt water for ballast because it's heavier. For every condition there is an optimum wing loading. More is very often not better. There are weight optimums just like speed optimums for given thermal strength, thermal width, cloud streeting, and ridge or wave conditions. Too often I have seen pilots put on too much water because they falsely believe the old statements that more is better. You can create models for the correct amount of water if you can account for all the above factors. Simple models can look at just thermal strength but the larger circling diameter can make a big difference on achieved climb rates. The more you can fly straight the more water is useful. If you are flying classic thermals without streets then often less is better. If you need maneuverability to work near ridges less is better also. |
#3
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Wing Loading
I have found using enough ballast to minimize how often I bang my head on the canopy works quite well.
Mike |
#4
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Wing Loading
Thanks, Tim,
I know all these things. This is sort of a mind exercise so let me try a different approach... I was curious if there's a point analagous to the drag bucket where the L/D for a given speed would take a sudden dip given higher wing loading. I'm visualizing the polar curve taking a sudden trip downward past a certain weight. Yeah, I know... When the wings break off! I'm really having trouble putting my thoughts into words... "Tim Taylor" wrote in message ... On Apr 25, 7:38 pm, "Dan Marotta" wrote: Is there a practical limit to wing loading? I'm looking for aero dynamic information, not statements about max gross weight. Can you load your glider up to the point that, even with strong and reliable lift, you're at a disadvantage to lighter ships? I was just thinking of the old days when I read that some contest pilots tried, or considered, using salt water for ballast because it's heavier. For every condition there is an optimum wing loading. More is very often not better. There are weight optimums just like speed optimums for given thermal strength, thermal width, cloud streeting, and ridge or wave conditions. Too often I have seen pilots put on too much water because they falsely believe the old statements that more is better. You can create models for the correct amount of water if you can account for all the above factors. Simple models can look at just thermal strength but the larger circling diameter can make a big difference on achieved climb rates. The more you can fly straight the more water is useful. If you are flying classic thermals without streets then often less is better. If you need maneuverability to work near ridges less is better also. |
#5
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Wing Loading
On Apr 26, 9:39*am, "Dan Marotta" wrote:
Thanks, Tim, I know all these things. *This is sort of a mind exercise so let me try a different approach... *I was curious if there's a point analagous to the drag bucket where the L/D for a given speed would take a sudden dip given higher wing loading. *I'm visualizing the polar curve taking a sudden trip downward past a certain weight. Yeah, I know... *When the wings break off! I'm really having trouble putting my thoughts into words... "Tim Taylor" wrote in message ... On Apr 25, 7:38 pm, "Dan Marotta" wrote: Is there a practical limit to wing loading? I'm looking for aero dynamic information, not statements about max gross weight. Can you load your glider up to the point that, even with strong and reliable lift, you're at a disadvantage to lighter ships? I was just thinking of the old days when I read that some contest pilots tried, or considered, using salt water for ballast because it's heavier.. For every condition there is an optimum wing loading. More is very often not better. There are weight optimums just like speed optimums for given thermal strength, thermal width, cloud streeting, and ridge or wave conditions. Too often I have seen pilots put on too much water because they falsely believe the old statements that more is better. You can create models for the correct amount of water if you can account for all the above factors. Simple models can look at just thermal strength but the larger circling diameter can make a big difference on achieved climb rates. The more you can fly straight the more water is useful. If you are flying classic thermals without streets then often less is better. If you need maneuverability to work near ridges less is better also. My view: For given conditions, performance is a smooth function of wing loading. No sharp curves, drag buckets, or discontinuities. If you add 1 lb/ft^2 you glide a few knots faster, but give up a few hundred feet in the next thermal. The biggest question is how much you can fly straight. There is a discontinuity in the advantage of ballast when you can fly straight without thermaling. There is a maximum desirable wing loading. If the wings let you fly at 50 lbs / ft^2 you wound not want to fill up. Designers make modern gliders so that they carry enough water to win strong days at world contests in strong conditions. Most of the time that is far more water than you need. Most pilots fly with too much water. They are hoping conditions get better ahead. But they pay a price until it does, and it often doesn't. I've been in several contest flights with me empty, a gaggle full and 3 knot thermals. The water made no difference at all. Filling up with water and flying too slowly removes much of the advantage of water. If the gaggle is flying at 65 knots because nobody wants to be first, then trying to climb at 3 knots with full water, you will outfly them empty. Where water really hurts is if you get low and have to core tight thermals. Even if they are strong, you can waste a huge amount of time here, then finally dump and magically core the lift. One more reason that water is not as beneficial for isolated thermals as it is with streets. John Cochrane |
#6
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Wing Loading
On Apr 26, 8:58*am, John Cochrane
wrote: On Apr 26, 9:39*am, "Dan Marotta" wrote: Thanks, Tim, I know all these things. *This is sort of a mind exercise so let me try a different approach... *I was curious if there's a point analagous to the drag bucket where the L/D for a given speed would take a sudden dip given higher wing loading. *I'm visualizing the polar curve taking a sudden trip downward past a certain weight. Yeah, I know... *When the wings break off! I'm really having trouble putting my thoughts into words... "Tim Taylor" wrote in message .... On Apr 25, 7:38 pm, "Dan Marotta" wrote: Is there a practical limit to wing loading? I'm looking for aero dynamic information, not statements about max gross weight. Can you load your glider up to the point that, even with strong and reliable lift, you're at a disadvantage to lighter ships? I was just thinking of the old days when I read that some contest pilots tried, or considered, using salt water for ballast because it's heavier. For every condition there is an optimum wing loading. More is very often not better. There are weight optimums just like speed optimums for given thermal strength, thermal width, cloud streeting, and ridge or wave conditions. Too often I have seen pilots put on too much water because they falsely believe the old statements that more is better. You can create models for the correct amount of water if you can account for all the above factors. Simple models can look at just thermal strength but the larger circling diameter can make a big difference on achieved climb rates. The more you can fly straight the more water is useful. If you are flying classic thermals without streets then often less is better. If you need maneuverability to work near ridges less is better also. My view: For given conditions, performance is a smooth function of wing loading. No sharp curves, drag buckets, or discontinuities. If you add 1 lb/ft^2 you glide a few knots faster, but give up a few hundred feet in the next thermal. The biggest question is how much you can fly straight. There is a discontinuity in the advantage of ballast when you can fly straight without thermaling. There is a maximum desirable wing loading. If the wings let you fly at 50 lbs / ft^2 you wound not want to fill up. *Designers make modern gliders so that they carry enough water to win strong days at world contests in strong conditions. Most of the time that is far more water than you need. Most pilots fly with too much water. They are hoping conditions get better ahead. But they pay a price until it does, and it often doesn't. I've been in several contest flights with me empty, a gaggle full and 3 knot thermals. The water made no difference at all. Filling up with water and flying too slowly removes much of the advantage of water. If the gaggle is flying at 65 knots because nobody wants to be first, then trying to climb at 3 knots with full water, you will outfly them empty. Where water really hurts is if you get low and have to core tight thermals. Even if they are strong, you can waste a huge amount of time here, then finally dump and magically core the lift. One more reason that water is not as beneficial for isolated thermals as it is with streets. John Cochrane As XX once said....... "" its hard to find a garden hose at 3,000 agl and put more water in!!!"" We do have dump valves! Its better to have and dump, than wish you had more. Now when to dump and what to dump to, will bring alot of hoop laaaa's around the fireside chat that goes on and on and on. # 711. |
#7
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Wing Loading
I am sure there is a point where the polar falls off a cliff, however I
doubt you would ever get enough water in!! Flying older generation gliders like a standard cirrus with winglets retro fitted, they climb exceptionally well and carry water very well also. The day has to be pretty crap (2 knot climbs) for me not to carry water! In newer generation gliders which already have a higher wing loading, I think it is much harder to call! JC Standard Cirrus 566 (UK) |
#8
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Wing Loading
On 4/26/2012 8:39 AM, Dan Marotta wrote:
Thanks, Tim, I know all these things. This is sort of a mind exercise so let me try a different approach... I was curious if there's a point analagous to the drag bucket where the L/D for a given speed would take a sudden dip given higher wing loading. I'm visualizing the polar curve taking a sudden trip downward past a certain weight. Yeah, I know... When the wings break off! I'm really having trouble putting my thoughts into words... "Tim Taylor" wrote in message ... On Apr 25, 7:38 pm, "Dan Marotta" wrote: Is there a practical limit to wing loading? I'm looking for aero dynamic information, not statements about max gross weight. Can you load your glider up to the point that, even with strong and reliable lift, you're at a disadvantage to lighter ships? I was just thinking of the old days when I read that some contest pilots tried, or considered, using salt water for ballast because it's heavier. For every condition there is an optimum wing loading. More is very often not better. There are weight optimums just like speed optimums for given thermal strength, thermal width, cloud streeting, and ridge or wave conditions. Too often I have seen pilots put on too much water because they falsely believe the old statements that more is better. You can create models for the correct amount of water if you can account for all the above factors. Simple models can look at just thermal strength but the larger circling diameter can make a big difference on achieved climb rates. The more you can fly straight the more water is useful. If you are flying classic thermals without streets then often less is better. If you need maneuverability to work near ridges less is better also. If I'm accurately understanding what you're pondering, I think you're asking if there is (are) any physical reason(s) to expect that increasing wing loading for a given glider will ultimately 'uncover' any presently 'generally unconsidered' gotchas that will result in 'something like a laminar airfoil's drag bucket effect' on the glider's polar. My short answer: "Yes." Two aerodynamic possibilities: 1) Reynolds number effects (which, arguably, make laminar flow airfoils possible in the first place...and would eventually - because of increasing glide speed necessary to support the increasing weight - result in breakdown of presently-existing laminar flow runs), and 2) mach effects. The former might appear as a glider-based (as distinct from a wing-profile-based) drag increase, while the latter would take us back to polar-detectable Chuck Yeager days. The time for swept wing sailplanes may be at hand! Bob W. P.S. The operational Me-163B (arguably a non-laminar glider optimised for assisted climb/speed over thermalling had operational wing loadings varying from ~20-psf empty to 45-psf full-up; its operational max speeds ranged from 515 mph at sea level to ~600 mph between 10,000 and 40,000 feet. No thermalling pireps in any of my sources... |
#9
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Wing Loading
At 16:13 26 April 2012, Justin Craig wrote:
I am sure there is a point where the polar falls off a cliff, however I doubt you would ever get enough water in!! Must be best to carry as much as possible... even if its only to dump on any following pilots. Releasing 10 gallons of (preferably ice cold.) water in the middle of a 5m/s thermal is the easiest way to seed some rain and make it just a 1m/s thermal ! |
#10
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Wing Loading
On Apr 26, 9:13*am, Justin Craig wrote:
I am sure there is a point where the polar falls off a cliff, however I doubt you would ever get enough water in!! When I ran the numbers for what later became the Concrete Glider episode on Mythbusters, that point was where you get transonic flow and start hemorrhaging energy into shock waves. I worked the numbers back through L=1/2*rho*v^2*Cl*A and figured that the upper limit was on the order of 60000 lbs. I thought it was really cool that you could cast a 15m sailplane in concrete and rebar, ballast it with tungsten to get the CG right, and it would smash right along at about 40:1 at a few hundred knots. Of course, launching it would be a real bear, but that would be somebody else's problem. But, no, they wanted a concrete glider that could do a roll-off launch from a hilltop with Jamie or Adam at the controls. That was a non- starter, because a Part 103 aircraft was not in the cards. They might have done so if they'd allowed themselves the kind of fiberglass reinforcing mesh that Rob Wheen used in the University of Sydney's concrete hang glider, but the Beyond Productions researcher I was working with said they wanted it all or at least mostly concrete, with perhaps some pieces of rebar. Of course, they ended up scaling the episode way back, and made hand- launch model gliders instead. I think even that could have been a cool demonstration of how the rho*v^2 works; they could have taken a styrofoam toy glider, copied it in cement, and then launched it from an airplane or fast moving car to show that it would achieve the same glide ratio as its polystyrene cousin, just at a greater speed. Thanks, Bob K. |
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