Ustik, it's not fair to compare the cubic wing loading from the full size to the model. That's too big a change and doesn't take into account the whole reynolds number shift for that big a change. And I guess it depends on your tolerance for flying bricks but when you're talking 34 oz/sq foot on anything that is sub 100 inch span I would suggest that this is well and solidly in the flying brick category. A much nicer weight to achieve a scale'ish look to the flying would be more in the 8 to 10 lb range. But to make it look like it's really hanging in the sky like the real ones do I suspect the weight will need to be more in the 5 to 7 lb range.

Let's see..... The 1/12 model would have 6.8 sq feet of area. At a sailplane like 10 oz/sq ft that would imply a 68 oz, or just a little over 5.5 lbs all up weight. Sailplanes with that wing loading typically can fly at around the 20 mph mark with ease. And I'd say that the illusion of scale speed could be well, if not accurately, maintained up to likely 16 oz/sq ft which would give a weight target of 6.8 lbs. I think that the 6.8 is quite do-able as long as extreme measures are taken to making the airframe weight come in low enough. Even 5 to 6 lbs is do-able if this design does not require solid skin covering but instead uses open structure that is covered with material of some form. I'm not familiar with the plans mentioned so I can't comment further on them.

In the 20 to 26 oz/sq ft range you'd have a model that many would consider a complete success but the flying speed would definetly not look scale like. But as the loading hits 30 and up in this wing area range bad things will become the norm.


Another test I like to do in these discussions is hit up NASA's Foilsim II to get an idea of how slow something can fly or how fast it needs to fly to avoid any harsh issues.

http://www.grc.nasa.gov/WWW/K-12/airplane/foil2.html

I set up the simple span and chord to provide 6.8'ish sq feet and then setting the camber to 3% and the angle of attack to achieve a lift coefficient of 0.75. At 0.75 you're up in the high lift region but should be comfortably away from the stall of most higher lift style airfoils. With those set I altered the flying speed until the lift amount hit 5.5 lbs. The flying speed to achieve this was 20.25 mph. A very nice sailplane like speed even if it is well above the 12.5 mph scale cruise speed.

Foilsim says it would need to be down in the 2.0 lb all up weight range to fly at a true to scale speed of 12.5 mph. If you're willing to cheat on the airfoil and use a higher camber with 6% camber and run it up near a Cl of .9 and where it's getting a little close to the stall speed for most folks comfort then you could get away with a 2.5 lb model. You can see why I was saying that you'll need to compromise for the sake of reality. Not to say that a 2.5 lb model is not do-able but it would need to be constructed like a rubber scale model with lots of minimalist strips and covered in only the very lightest of films or jap tissue and dope. Handling and flying of such a model would require the calmest of conditions and care. Oh, and the wing loading on a 2.5 lb model would be only 6 oz/sq ft. Right smack dab on the money for wing loadings enjoyed by the lightest "gasbag" sailplanes from the early days of gliding like the Olympic 99 and others of that era.

Incidentally at 14 lbs and a Cl = .7 for a safe'ish landing speed or minimal cruise Foilsim says that the flying speed would be 34 mph. It doesn't sound like much but it would make a 6 foot DC3 model look more like a WW2 fighter doing a quick pass.