1/96 Beechcraft Starship glider with spinning props

[ReynoldsSlumber]

The latest installment in my scale indoor glider series is a half-size downscale of the Murph's Models Beechcraft Model 2000 Starship business plane in stock markings. For the main parts I used my new favorite paper, Canson XL Pen and Ink art paper (80 lb text/118 gsm), and for detail parts I used 24 lb bond/89 gsm paper.

Because the pusher propellers are well-shielded from hitting things during landings (and during crashes), this is the first time I've included them. Rather than build them static with feathered blades, I went ahead and made them to spin freely in the airstream. This was super fun! It was also as quite challenging at this small of a scale, and it came with a significant weight penalty. Even though each propeller assembly only weighs 0.15 g, once the nose weight to balance them out is accounted for, the addition of this feature amounts to about 0.5 g out of the plane's 4.2 g total mass.

Modifications for aesthetics:

• Wing leading edge cuffs: Added small pieces where the highly-swept inner wing meets the less-swept outer wing, to make it look more like the gracefully-curved shape of the prototype
• Vellum windows: Additionally made the nose structure under the pilots' windscreen out of vellum, so that it wouldn't show through the windscreen
• Propeller direction: Corrected the starboard propeller to rotate the same direction as the port one (unlike the P.180 Avanti, which has counter-rotating props)

Modifications for flight:

• Undercambered airfoils: I did this in the outboard wing sections but didn't try to modify the inboard sections. It didn't pan out as I had hoped in any case, as will be described in the upcoming flight report post on this thread.
  
• Plastic finger grip: As seen above
• Spinning propellers: The main event! For strength, I added through-the-wall tabs to the roots of the propeller blades and mounted them to the spinner paper before shaping it over inner cone layers.
  
  I used small quilting sewing needles for the axles, with tiny plastic discs in front of and behind the prop as "thrust bearings." To make the props spin freely, I had to open up the axle holes using a wire a few hundreds of a millimeter larger in diameter than the needles. Minimizing axle friction is critical, since even a tiny amount can overcome the feeble windmilling torque of the air. The blade pitch is fairly coarse, just enough to make the props spin given the slow airspeed, so that they don't cause too much drag. For mounting each prop at the back of the nacelles, the pointy end of the needle axle is inserted into a bulkhead-mounted paper roll.
  
• Foam nose tip: This got a serious workout during the arduous flight trimming process
  
• Wire nose weight: The center of gravity range on the real thing is notoriously narrow, and so it is here. Going by the aircraft flight manual as well as by an R/C modeler's build report that I found, it's supposed to be between the second-to-last and last cabin windows. At first I had it at the front edge of the last window, but the plane dived uncontrollably.
  
  I coiled more wire in the shape of a keychain ring and spiraled it in through a small hole under the nose, followed by some glue. The CG ended up at the back edge of the second-to-last window, which may have made it a touch too nose-heavy for easy gliding.
  

[ReynoldsSlumber]

Freshly built, this model was very satisfying. Then came the struggle to make it fly.

Because the canards straddle the nose joint, I had to guess the CG and finish the assembly before starting test flights. As described before, the CG ended up too far aft, so I had to add more nose weight after the fact. I then had to give the plane an absolute truckload of nose-up trim, both to the canards and to the wingtips. The initial careful airfoiling of the wing outer portions was for naught, since I had to heavily reflex the trailing edge upward, to the point where I think most of the lift is coming from the highly swept inner portion of the wing. I also had to deform the wings upward overall to get dihedral for roll stability, since when I first attached the inner wing sections, they inadvertently introduced unwanted anhedral. All of this mashing of the wings into shape is unfortunately evident in the outer wing surfaces, and the wingtip vertical stabilizers are now tilted forward compared to how they should be. The costs of the first version of a small-scale flying model, I suppose!

With all that, the model ended up heavier than I would've liked, and it doesn't get an efficiency advantage out of its high aspect ratio wings, given that the aft-swept tips are mostly lifting downward for trim. So in the air, this model is kind of a brick. It does glide though! When thrown with a touch of bank, it seems to have a Dutch roll response, kind of a little waddle.

You can't really see the props spinning while the Starship is flying, though you can hear a faint whirr from them, which is cool. Even just blowing on the props to spin them is a hoot.


When I make a P.180 Avanti, I'll put spinning props on that too. Though they'd be far too fragile to put on a model's nose, I wonder whether I could get away with putting forward-facing props on twin-engine planes like the P-38? Perhaps if I fly it into a suspended sheet rather than into a badminton net. Flying into a net, the engine nacelles on the VC-25A (747) model have taken quite a bit of punishment.

Here you can get some idea of what the light coming through the vellum windows looks like.