Solving RC Airplane Design Challenges
Experience gained from the Blackburn Type D Monoplane
One of the fun aspects of drawing up a radio control model airplane plan is solving the many RC airplane design challenges you will face. Following is a discussion of some of the issues that needed to be solved as I came up with the final layout and CAD plan for the Blackburn Type D monoplane.
Blackburn firewall and metal wing tubes mounted on plywood platform
Copy success!
One design approach I always like to follow is to copy the success of others. Modelers are great at sharing their ideas and design techniques. Keep a catalogue of unique and innovative methods that other modelers have used to meet a particular model aircraft design need. These techniques can be easily cataloged and adapted to your future projects.
The Sig Demoiselle provided design insights for the Chickadee followed by the Blackburn
I was particularly impressed with the layout of the Sig Demoiselle, in particular the use of plywood reinforced balsa dowels to mount the wings into metal tubes. I adapted this approach to my Chickadee airplane design using similar moments, areas and incidences taken from the Demoiselle.
Chickadee to Blackburn
The Chickadee flew so well that I met the RC airplane design challenge of the Blackburn by essentially overlaying the Chickadee outline to the dimensions of the Blackburn. I made a few modifications such as lowering the wing from a parasol attachment to the fuselage top installation for the Blackburn. But the wing area, airfoil and positive incidence were taken directly from the Chickadee. This worked out well, with no need for a Blackburn prototype build for test flights.
Note how the landing gear is fixed to ply platform
The Blackburn has a narrow fuselage structure, characteristic of the designs of that early era of flight. I had to widen the upper fuselage slightly to allow enough room for the 350 milliamp NiCad battery that I employed. This task was easy to do using TurboCAD, and led to the design of the friction fit removable balsa battery hatch cover.
The Blackburn adapted the Chickadee wing tube mounting system
There was sufficient fuselage volume for the receiver and electronic speed control. However the servos for the elevator and rudder would not quite fit in. As I did not have enough space in the forward section of the fuselage due to the battery, I decided to install the two servos aft of the cockpit and allow them to poke slightly out of the fuselage side. This was a good design compromise, made for easy servo control arm installation and is hardly noticeable on the finished model.
Blackburn fuselage strength
Perhaps the major RC airplane design challenge for the Blackburn was ensuring that the fuselage had sufficient structural strength, yet was light enough for successful electric powered flight. The Blackburn fuselage has a triangular cross section that did not look like it would provide a sturdy enough structure. I could see no good way to reinforce the lower fuselage section to meet strength goals.
Battery mounts in plywood tray - note cutout for wire access
I finally decided on an innovative approach for the fuselage that worked perfectly on the Blackburn and can be applied to any number of future designs. I made a 1/16 inch plywood platform on which I installed the wing panel metal tube mounts, electric motor, two servos, landing gear, receiver and ESC. In short, any critical aircraft component that needed a stable foundation was attached to this plywood base plate.
The Blackburn tail section is kept light for weight and balance purposes
This plywood plate was strong enough for all these critical components. The plate was glued onto the top of the fuselage structure, adding strength to the balsa components of the fuselage assembly underneath and aft. With this arrangement, the stress induced on the model by the wings, motor or a hard landing is completely absorbed by the plywood platform and protects the fuselage from any sort of damage.
Blackburn cowl
The final design challenge with the Blackburn was fabricating the rather unique cowl shape. I studied a series of Blackburn pictures obtained from the internet, as well as several YouTube videos of a full scale reproduction of the Blackburn in flight. The cowl was unusual in that it expanded rapidly from the narrow fuselage to enclose the large rotary motor.
Underside view of the Blackburn
To ensure the cowl did not interfere with the electric motor, I decided to mount the motor onto the Blackburn model while still under construction. I then built the cowl around the motor. Using 1/8 inch square balsa, I roughed in the outer dimensions of the cowl, and filled in with 1/16 inch sheet balsa planking. Some additional balsa on the front allowed for rounding the finished cowl. I covered the cowl with aluminum iron on Monokote to simulate a metallic finish. The final result is easy to construct and provides a pleasing look to the finished model.
Any RC model airplane project that you undertake will have its own set of design challenges. Creating a set of model airplane plans is truly a case of where the rich get richer. The more plans you prepare and design challenges met and solved, the greater your depth of knowledge and experience that can be used to successfully complete future projects.