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#11
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Wing Loading
Great responses Bob K and Bob W. That's what I was looking for!
"BobW" wrote in message ... 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... |
#12
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Wing Loading
On Apr 26, 7:32*pm, Bob Kuykendall wrote:
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. Did you know the Germans made a "glide bomb" with reinforced concrete wings during WW II? http://www.flickr.com/photos/67307569@N00/2658890478/ |
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