Rough Prototypes

The Merriam Webster definition of a prototype: “An original or first model of something from which other forms are developed or copied; an initial example that is used as a model for what may come afterwards.”

ParkZone electronics installed in the Spin prototype

ParkZone electronics installed in the Spin prototype

The use of prototypes has been a standard procedure for full scale aviation development since the Wright brothers first flew a powered aircraft in 1903. Model aircraft follow along the same path. As we advance ideas or, for example, learn of new power or construction methods it is wise to build an initial vehicle to determine the feasibility of this approach before constructing a finished variant.

View of the top half of the Spin's stand-alone rudder

View of the top half of the Spin’s stand-alone rudder

Pilots are familiar with the common term “X plane” regarding full scale aviation, “X” being shorthand for experimental. From the first X plane, the Bell X-1 rocket aircraft that Captain Chuck Yeager flew to break the sound barrier in 1947 to today’s concept fighter aircraft, the use of prototypes is a foundation approach for aircraft development.

We can use the idea of a prototype for our model aircraft designs. In particular, I use the term “rough prototype” to describe a basic initial design that captures the characteristics of what I am trying to model – wingspan, tail areas, general weight, moments, etc. – without spending a lot of time and effort creating a finished product.

Using the theory of a rough prototype I can ensure that the overall plane design will fly. Once that is known I can make a follow on complete and finished example.

When I started to design indoor electric radio control models, in the era before widespread use of lightweight lipo batteries, the selection of motors and nicad batteries was an essential skill. The early brushed motors had limited power and flight weight was a primary concern. The use of a geared propeller was a common technique to provide sufficient thrust to fly.

Fokker Spin nose

Spin prototype nose section

The introduction of brushless outrunners greatly simplified motor selection due to their exceptional power output. However, attention still needed to be spent on the right setup of the relatively heavy nicad battery pack.

The Chickadee was the first time I embraced a prototype design approach. In fact, the entire aircraft is more or less a prototype. The fuselage employs minimal structure. The wing is mounted on simple posts that allow a ready modification of incidence. There is no fuselage enclosure, allowing full access to the servos and electronics. Every component is available for adjustment to get the model ready for flight.

This approach paid dividends as the wing incidence was initially too low. The Chickadee had plenty of power as it raced along the ground trying to take off. But I could not get the model to actually rise from the ground, even with full up elevator.

The solution was simple, raise the leading edge of the wing to increase the incidence setting. The model flew fine after that.

The wing and tail surface areas, moments and incidence settings of the final Chickadee were applied directly to my first build of the Blackburn. As the wing for the Chickadee and the Blackburn were essentially the same, as well as the fuselage length, I did not feel the need for a dedicated Blackburn prototype.

Fokker Spin fuselage

Spin frame fuselage and motor

Instead, using the built in tools of TurboCAD I took the wing from the Chickadee plan, copied it to my Blackburn sheet and literally lowered the wing (with the correct incidence derived from test flights described above) onto the top of the Blackburn fuselage.

To keep the correct incidence derived from the Chickadee, the Blackburn’s wing trailing edge is slightly below the Blackburn fuselage top and the wing’s leading edge slightly above the fuselage top line. These measurements are critical.

The precision of TurboCAD next allowed me to carefully dimension the wing fore and aft hardwood mounts, onto which are glued the metal wing tubes. The demonstrated flight performance of this arrangement in the Chickadee translated perfectly to the Blackburn.

My next prototype design was for the Fokker Spin. I truly needed to prepare an “X-plane” for the Spin. I had no idea how the aft-swept wings would behave in flight, if the ParkZone micro-electronics and motor had enough power nor how the all-moving upper and lower rudder and lack of vertical fin would work.

Detail of the Chickadee nose section and control system layout

Detail of the Chickadee nose section and control system layout

From experience with the ParkZone motor on other models, I decided 30 inches was about right for the maximum wingspan. I then sized the fuselage and tail surfaces areas around the wingspan dimension.

I put the ParkZone electronics on top of the Spin’s fuselage for ease of installation. I used a conventional (i.e. non-scale) landing gear setup and a bamboo stick for a tail skid. The motor was mounted in a simple balsa clamp. I did not spend any time installing the distinctive rigging as that would have no effect on prototype flight characteristics and could be readily added to the final version.

Wing attachment scheme for the Chickadee

Wing attachment scheme for the Chickadee

This quick build approach paid off. The Spin had adequate power and flew well. With this knowledge in place I made a few design tweaks and proceeded to construct the final variant with all scaled details in place. I even had sufficient weight margin to install some attractive, yet fairly heavy spoke wheels that I had in my parts bin.

In summary, consider taking the time to build an initial prototype of your model airplane design to work out various design issues and then go all out making a final version. This approach will take off a lot of pressure on that first flight, always an exciting combination of apprehension and anticipation, followed by satisfaction when your original design takes to the skies for the first time.

Author: Gordon McKay