Flying Models Centerfold Plan - May 2013
Back in the July 2012 issue of Flying Models I had speculated on the possibility of an radio control (R/C) conversion of the Baby Biwinger model aircraft free flight (F/F) design. I figured using newer technologies available along with carefully selected materials, a judicious redesign of certain structural elements and a very aggressive weight reduction program would yield a radio controlled conversion of the original 13-inch span model. With changes to reduce the weight by at least 50% for the airframe and appropriate mods to allow for a proper c.g. with the lighter weight propulsion unit (8.7 grams for the electric motor and prop vs 18 grams for the Cox Tee Dee .010) was indeed possible.
On December 8, 2012 I was able to successfully fly for the first time one possible solution to this design problem with very satisfactory results. The new model flies very much like the Ember 2 which is where the receiver came from (only three channels were needed) and used the gearbox/motor from a ParkZone P-51 (PKZ3624) and a GWS 5034 prop and adapter spinner.
The target weight I had figured should be somewhere between that of the ParkZone J-3 Cub and their P-51, so I would be looking at 28 to 34 grams. That would be about right for realistic purposes. The weight of the raw material would be critical and I searched through all the 1⁄32 sheet balsa stock in town and finally found two sheets of 1⁄32 × 3 × 36- inch balsa that turned up at 8.9 and 9.2 grams respectively.
The model component weight breakdown is shown in the box on page 37 and I hope will serve as a reinforcement for the above message.
The key components to a strategy to reduce the weight on this project so that I would be assured a flyable model at the end of the program were as follows:
Since “one picture is worth a thousand words” I will let the pictures do most of the talking for the remainder of this article.
The original inspiration for the subject of this article (above).
Upper and lower 1⁄32-inch sheet balsa wing blanks weigh 2.7 grams and 2.4 grams respectively.
Fuselage 1⁄32-inch sheet balsa side skins (two required) are 2.0 grams each, with extra length so that c.g. could be adjusted to compensate for difference in motor package weight.
Horizontal tail components of 1⁄32-inch sheet balsa, 0.65 gram total.
Vertical tail components of 1⁄32-inch sheet balsa (above left). 0.20 gram total. Built up bulkheads F1, F2, F3 and F4 (1⁄16 sheet and strip) (above center). 0.5 gram total weight for all four. Laminated front bulkhead (above right) (four layers of 1⁄32 × 1⁄8-inch balsa strip). 0.2 gram total.
Top and bottom 1⁄32-inch sheet balsa skins (above left), 0.4 gram and 1.0 gram respectively. Completed empennage assembly (above center), 1.0 gram. Front view of fuselage sides (above right) and bulkheads assembled.
Side view (above left) of fuselage sides and bulkheads assembled. Detail of 1⁄16-inch sheet balsa motor mounts (above center) for new front end of Baby Biwinger. Length was adjusted with motor assembly attached to provide suitable rough c.g. location with empennage taped in place on fuselage. Mounts were then trimmed to fit. Wire landing gear installation attachment to forward edge of laminated front bulkhead. As can be seen in the photo the piano wire has been formed to follow the inside contour of the 1⁄32 sheet balsa fuselage skin. 30-minute epoxy used here for durability. Initial forming of nose section sheets (above right) for electric motor installation.
Initial mounting of empennage to fuselage (above left) and fitting of rudder and elevator pushrods. ParkZone Ember 2 receiver (above center) mounted on the pair of 1⁄8 × 1⁄4-inch receiver mounts. Receiver and pushrod assemblies (above right) ready for installation into the fuselage.
Bottom view of the pushrods and receiver installed (above left) form the lower front of fuselage and are located for final positioning. Close up of forward cabin (above center) close out mounted to fuselage. Wings mounted to fuselage (above right) along with interplane struts.
Installation of upper fuselage skin with cutouts (above left) for pushrods. 1⁄32-inch sheet balsa base (above center) and 1⁄16-inch sheet balsa intercostals for battery mount. Battery mount (above right) installed between laminated front bulkhead and F3. Cut relief in F2 and forward part of wing as required to nest.
Bottom skin (above left) installed with cutout for battery mount. Battery provisions (above center) and close up of area before the additional lower forward skin is added. Ironic that the final flying weight of the R/C conversion Baby Biwinger (above right) is within a very few grams of the weight of the engine and period propeller that the original freeflight Baby Biwinger was designed for. It bears dramatic witness to how far the technologies in this hobby have come, particularly in the past 10 years, since the last one Daniel had built still had to be freeflight. There was no alternative commercially available back in that time period for a model this size. Up until now, Peanut Scale was still strictly a freeflight event.
For those wishing to try something like this on some other model design that they may be thinking about, here are some general ideas and principles for the conversion to R/C of the many existing F/F rubber, CO2, and small glow engine powered models that are currently out there.
Go for aircraft that offer the lightest possible wing loading (lots of wing area for lightest possible wing loading for your first try).
If possible, substitute a thin cambered airfoil section in place of a “scale” airfoil. The ParkZone Sukhoi 26M has a “regular” scale-like airfoil while their P-51 has a thin cambered airfoil. The P-51 is much easier to fly and has better low airspeed characteristics even though both it and the Sukhoi 26M share many other features like radio, motor unit and basic weight.
WW I and Pre WW I Pioneer aircraft have a lot to offer in this category even if they may be lacking in some other areas. Remember that these models operate in an entirely different Reynolds number range than their full size counterpart. This is why bird and insect wings look the way they do. Nature knows that the air molecules look bigger to these smaller flyers with different viscous effects and follows her rules accordingly.
Some free flight sheet balsa models featured a 1/16-inch square or 1/16 × 1/8 balsa LE sanded round as part of the airfoil design. After thinking about this particular design characteristic for a while I realized that this was probably to prevent damage to the leading edge of the wing as much as anything else. Free flight models tend to collide hard with the first thing they encounter when at the flying field—things like radio antennas on cars, field boxes, barbed wire fence, hedge apple trees, etc. So since you should be able to steer an R/C model to avoid such damage, this feature could well be eliminated in the interest of reduced drag and weight and increased flight performance. The choice is yours.
Come up with a “serious” target weight and try to meet it.
As nice as it would be to have the ability to move the radio equipment from one plane to another, I have found that the lightest and “permanent” one with these systems. Welcome to the age of disposable electronics.
Make sure you can see the LED from outside the aircraft to be sure you have solid radio contact.
Keep the trim and decoration simple, clean and to a minimum. Besides reducing weight it also makes the model simpler to build. As has been voiced in the past by many Peanut Scale advocates in the hobby, a few well executed details always outperform a large host of half hearted details and are always lighter. I could have painted the tires on the wheels black on this model, but like Dorothy’s ruby slippers, those red wheels can transport those who remember them across time and space to a place found only in their youth. The model also does not have any clear protective coat on it either in the interest of reduced weight.
Use readily available sub assemblies if possible. I find the pushrod replacement part kits from some of the micro R/C aircraft on the market make perfect “kits” for pushrods for your project with only slight modification and the price is not all that unreasonable for what you get in the package.
So there you have it. One solution for transposing this very desirable F/F model into a very flyable and good looking R/C version with as little effort as possible. With the above general guidelines in mind you should have very little trouble converting any of the following suggested examples on this abbreviated list to a micro R/C model of your own design.
These are just some of the considerations for a micro R/C subject and I’m sure there are many others that would work and ideas that are applicable. The real trick is to keep the project as simple as possible and therefore, as light as possible. Too much complication usually means a less than satisfactory model. The original plan for the F/F Baby Biwinger is included in this article for your convenience.
This article originally appeared in the May 2013 Flying Models.