Scales for a flying glider fleet

[ReynoldsSlumber]

Per a question from Siwi, I posted the method I've been using to locate the center of gravity on these planes over on the OV-10/Cessna 337 thread.

[ReynoldsSlumber]

Here's a first look at what kind of fleet of outdoor gliders I might build. The bands of color indicate the range of scales I'm considering for each size class of plane, and the bold text indicates what scale I've tentatively decided to use among those. Some of the choices of prototypes are different than for indoor gliders, as I'm thinking about what might look the most impressive soaring overhead.

Also I'm trying to think practically in terms of what configurations would better at withstanding the acceleration of a rubber band catapult launch. For these I think I'd finally have to skip the rounded leading edge airfoil and use a cambered flat airfoil, so that the wings flex when it crashes ("when," not "if"), rather than buckling. Think I'd get these models laser printed, so that moisture doesn't instantly wreck the markings.

[ReynoldsSlumber]

See my separate 337 and OV-10 thread—again I've posted the pictures and construction details of this modification to a Scissors & Planes plan there. More observations about this series of scale indoor gliders:

• I'm surprised and relieved to find that, for adequate roll stability, I don't need to increase the wing dihedral compared to the prototype. Outdoor gliders like Whitewings have pretty pronounced dihedral, and Ojimak's build instructions recommend substantially increased dihedral compared to the real thing. I think it's because outside you need an excess of roll stability in order to be tolerant of various launch attitudes as well as wind gusts. You want to be able to huck or slingshot the glider in an upward spiral and then have it roll wings nearly level, and you don't want a puff of wind to send it into a downward spiral.
  Funny enough, when I had the crash net up for the OV-10D and tried out the A-4E again, on one throw when I gave it way too much nose-up attitude, it stalled and went into a spin. Very realistic-looking in that sense too.
• I'm finding the straight wing planes more controllable than the swept wing ones, so perhaps there's a difference in roll or yaw attitude sensitivity.

Here's the flying glider fleet so far, three in 1/72 and one in 1/200:

[ReynoldsSlumber]

Lockheed USAF U-2R Dragon Lady in 1/105 scale, downscaled from Paper Trade 1/50 plans: See the build thread. I meant for this model to be 1/120, to fit with my prior scale scheme, but miscalculated when readying it for printing. That was for the best, though, since even at 1/105 much of it was frustrating to build. To be honest, it caused a bit of an existential crisis for this fleet project, because however much I might desire the end result, the build process has to be fun too, or else it's not a good hobby project for me. I'm addressing the problem by changing the scale scheme, as detailed below. Also I'll try to be more discerning about eliminating features that aren't visually significant at the scales I'm using.

Meanwhile, after one too many flights ending in furniture collisions, one of the wings broke off at the fuselage join. It was readily fixed, though with a bit of telltale glossy dried glue evident along the seam amidst all the flat black. Similarly, I had to repair one of the tails on the Cessna 337.

Bell USAAF P-39Q Airacobra in 1/72 scale, downscaled from Fiddler's Green 1/30 plans: See the build thread. I thoroughly enjoyed building from this simpler plan! That fact reinforced that I need more practice before tackling some of the more convoluted aircraft shapes on my build list. The method of fashioning the nose cone from sheet craft foam again worked handily.

[ReynoldsSlumber]

Scale scheme: In the spirit of being realistic about making sure builds are fun—it helps if each plane's fuselage is at least as big across as a pinky finger is—I'm bumping up the common scales to be used and adding a fifth scale category. Though adding a category means fewer planes at a common scale to compare to each other, I'm trying to avoid tiny frustrating models as well as paper behemoths vulnerable to crash damage, so I had to bite the bullet and do it. I chose scales that have been used for plastic models and the like before.

• Indoor gliders: 1/72, 1/96, 1/110, 1/175, 1/288
• Outdoor gliders: 1/64, 1/96, 1/144, 1/250

Paper selection: Having compared almost 20 types of paper to date, my new favorite for small- to medium-sized models is Domtar Lynx Digital 32 lb bond/118 gsm. It's thicker and therefore stiffer than the same weight of Domtar Cougar paper because Lynx is not pressed as hard during manufacture. As such it is "lower quality" for its intended use but better for paper gliders. Its whiteness and smoothness for inkjet printing are much better than art papers, and it takes to curvature and gluing fine, though it's a little vulnerable to buckling if one is mashing it into place for a fuselage join or the like. Meanwhile I still haven't found a favorite paper in the 130-160 gsm range.

Flying characteristics: Here's a plot of aircraft wing loading versus mass for the glider models I've built so far. For floaty fliers, looks like they should be under 10 Pa. Pascals = Newtons per square meter, and 10 Pa = 0.0001 atmosphere, which doesn't seem like much, but it's right between the wing loadings that insects and small birds fly at, so in that way it makes sense. I picked mass for the x-axis in order to compare different model sizes and how effective glue discipline was when making them. I was surprised that the outdoor F4U model was so light and lightly loaded, given the thicker paper and glue quantity involved in making it. I found out the kit for it uses lignin-free paper; gotta get me some of that. Also surprised that such a light wing loading fares well outdoors. Wing aspect ratio, in terms of span divided by chord, is also a major factor for glide performance; later I'll try plotting wing loading against that.


Gram scale: To weigh the models, I bought a centigram scale, very handy. It's also useful for trimming brass wire nose weights to size, takes a lot of the guesswork out of it.

Gliding speed: Speaking of measuring things, when I was making guesses about flight speeds before, I was overly optimistic about how slow they were going. With the P-39, I timed it out to about 18 ft/s, which indicates my earlier guesses were about a third low.

Undercambered wing with thin aft portion: On several models, when I glued the aft portion of the wing together, I found that it made the wing bend into a gradual gull wing shape, I imagine due to glue shrinkage. It's not a good look; it kind of ruined the look of the OV-10 from some angles. On the U-2 it's the opposite of the way the real aircraft's wing flexes. On the P-39 I instead tried just gluing the trailing edge, and it seemed to work better. We'll see how it goes on planes with skinnier wings.

Finger grip: I upgraded the plastic to 0.010" thick PVC, which is plenty stiff, yet it doesn't crack when bending tabs for attaching it to the belly of a plane.

Vellum canopy: The tracing paper translucency effect is especially noticeable in person when light shines through it. I'm using this method now on all but the smallest windows. For the P-39 I kept the blue gradient printing in the window areas; usually I blank those areas out so that more light can get through. I'm using 24 lb bond/90 gsm vellum in most cases. For smaller canopies I'm looking for something lighter, though not as light as the pad of 25 lb tag/41 gsm vellum that I also bought to check out.

Fleet overview, with less accurate models made for practice in italics:

• 1/72: A-4E Skyhawk, Cessna 337, OV-10D Bronco, P-39Q Airacobra, F4U Corsair (outdoor glider), F-117 Nighthawk; 1/80: F7F Tigercat
• 1/96: LTV Scout B (not part of the glider fleet of course!); 1/105: U-2R Dragon Lady
• 1/200: 737-200; 1/230: Concorde

With 6 complete fleet models done in the past half year out of about 80 on my wish list, that's pretty good progress! And lots of fun to go. As a few of the desired prototype aircraft aren't available as appropriate downloadable plans, I'm looking forward to at some point trying out some design as well.

[paperairforce]

Great planes and interesting thread! Small gliders are what got me started in paper models - namely the "Flying Paper Scale" line once available at the Smithsonian museums gift shops. But these are much better scale and really good looking.

[ReynoldsSlumber]

Thanks paperairforce!

Here's a model on which I'm putting the finishing touches:


And the prototype for a companion model that I'm making next:

[gomidefilho]

Although my models are static, I have always seen in paper models since I discovered Ojimak and Trotsky's models great potential for creating gliders, your thread demonstrates this exactly. Thanks.