Fokker Spin – Prototype

 

Fokker Spin top view

Fokker Spin Prototype

Following is a discussion of my thoughts and actions as I drew up a set of original plans for the 1911 Fokker Spin prototype RC model aircraft.

This information will be of use to anyone wanting to design their own radio control model airplane. The process of viewing the full scale aircraft, anticipated likely design tasks and coming up with solutions is all part of the fun and skill development required to create your own model airplane layout.

Spin plans

Fully detailed CAD plans for the 28 inch wingspan Spin are for sale here.  You will need to enlarge them to full size, ratios are shown on the plan.  Also, you can easily enlarge the plans via the tiling option as you print the Adobe file.

Fokker Spin history

Anthony Fokker was the Dutch aviation pioneer who designed such well known World War I fighter aircraft as the E.III Eindecker and the Dr.I Triplane. Fokker, with the assistance of two colleagues, designed the Spin in 1911 to teach himself how to fly. This remains a remarkable feat for someone just 21 years old.


“Spin” is the Dutch word for Spider, which is a perfect description of an airplane that appears to be surrounded by flying and rigging wires. The Spin is a great candidate for a radio control model plane. The Spin has character combined with sufficient wing size and tail moments. The Spin looks to be straight forward to capture as an RC model. Follow along as I describe the design process I followed to build and fly my prototype of the Spin.

Fokker Spin

Fokker Spin prototype model top view

The Spin has ideal proportions for a successful model. The wing area looks to be about right for a slow flying aircraft. The stabilizer and elevator surface areas look to suitable. The small size of the rudder, combined with no vertical fin, will be a design challenge. I was also mindful of the full scale Spin’s very short nose moment as this could offer a challenge with the proper location of the model’s center of gravity. Tail heavy RC models must always be avoided.

The Spin also offers a simple layout. The fuselage is a basic wood frame with the motor mounted in front. The wings attach to the fuselage. The main concern for the tail section is the all-moving top and bottom rudder. Fokker used extensive rigging to hold the contraption together. These flight wires are easy to include on a model and add a lot to the visual appeal.

Determine model size

Based on my experience with earlier model designs and ready to fly aircraft, the thrust of the ParkZone geared electric motor was about right for a 28 inch wingspan and a final weight of under 2.3 ounces.

With knowledge gained from my earlier designs of the Chickadee and Blackburn, I had a good idea of the wing and stabilizer incidence settings required for smooth and stable flight.

It appeared at first glance that the Spin prototype’s wing had a bit more positive incidence than required. But my history with the Chickadee, where the initial low wing incidence actually precluded takeoff, led me to use this setting for the Spin. Flight test showed that this set up works well in the air and is characteristic of the low powered, high drag aircraft of that early era of aviation.

Spin in flight

Prototype Spin in flight

The Spin has a distinctive aft sweep to its wing. This sweep was an attempt by Fokker to design in some inherent stability to the aircraft. I had never designed a swept wing aircraft before. I thought it would be interesting to see how this aspect of the model would work out.

The wing’s sweep presented a new design undertaking with correctly figuring out the center of gravity location. A swept wing’s CG location is much different as compared to a conventional straight wing.

An internet search came up with the correct method of figuring out the mean aerodynamic chord (mid-span for a constant chord swept wing), then placing the CG at a standard 30% aft of the wing leading edge at this mean chord location.

Airplane modifications

I knew that the short nose moment, or distance the Spin’s nose extends in front of the wing leading edge, could present a problem with adding nose weight to correct a tail heavy condition.


I decided as part of the Spin design to make the nose slightly longer, about half an inch in this case. While the electric motor, battery and flight electronics are lightweight, the further forward they can be located is the easiest way to preclude a tail heavy situation. This minor modification is hardly noticeable on the final version of the Spin, and is all a part of the model design process.

Spin tail section

Spin Prototype tail section

I knew the model would have a fair amount of structure in the tail section, and structure equals weight. As it turned out I was correct in my suspicion of a tail heavy model. I needed to add some weight to the nose section to get the CG within range.

A handy method to draw the three view outline is resident in TurboCAD. TurboCAD allows you to import a scanned image into a layer on the drawing, in this case a Spin three view. You can easily resize and rotate the imported image as required. I did this by resizing the three view to get to a 28 inch wingspan.

I then drew an outline of the Spin over the three view image with the TurboCAD line tools. I needed the outer dimensions of the wing, fuselage and tail section to build the prototype. I extended the nose a bit as discussed earlier to help with the center of gravity. I drew the fuselage slightly wider to accommodate the ParkZone electronics. I also carefully set in the desired wing incidence on to the side view, as this needed to be precisely built into the model.

Fokker Spin CAD plans

Spin CAD plans

Note that with TurboCAD you need to make these sketches for just one half of the airplane. Using the Mirror command it is a simple matter to make an identical opposite half of the model. Once this rough outline, or CAD sketch of the Spin was complete I printed out a full size plan and proceeded to build a flying prototype.

 

 

Building the prototype

I used 0.050 inch carbon rod to reinforce the two balsa wing spars. This provided plenty of strength. The fuselage mounted wing dihedral blocks provided the correct dihedral and incidence. A simple balsa wood jig kept the aft wing spar at the proper height as the wing was constructed over the building board.

The fuselage and tail surfaces are built over the plan outline. For this quick-build prototype, a simplified music wire landing gear was epoxied to a plywood plate in the fuselage, with bamboo skewers glued in place for a tail skid. This conventional landing gear scheme is a “placeholder” for the scale landing gear set up installed in the final version of the Spin.

Fokker Spin nose

Spin prototype nose section

The rudder contained some unique design elements. The Spin does not have a vertical fin, so some method was needed to keep the rudder post firmly in place. I employed a 1/16 inch dowel for the main structural element of the rudder assembly. I glued in place 1/32 inch plywood washers to the top and bottom of the stabilizer and inserted the rudder dowel. I then added two more 1/32 inch ply washers top and bottom and glued these washers to the freely rotating dowel. The two 3/32 square inch balsa rudder frames are glued to the dowel.

I used three degrees right and down thrust while gluing the motor in place. The control electronics and battery were located as far forward as possible for CG considerations. The control rods are 0.025 inch music wire with direct runs to the rudder and elevator. I included a brace guide for each control rod run so that there is no flexing of the music wire.

First flight

Fokker Spin RC plane

Final version of the 1911 Fokker Spin

The prototype Spin took off without incident on its maiden flight. The weight came out at 2.0 ounces and the motor had sufficient thrust. Handling was smooth and responsive. The elevator and rudder provide plenty of positive control, and the motor provides ample power. The lack of a fixed vertical fin did not seem to matter. In short, the prototype Spin is a pleasure to fly.

The final version of the Spin offers lots of opportunity for adding more scale detail, from the rigging and flying wires to including a distinctive dummy motor. These visual features are easy to add and will be part of my final version of the Spin.

As I mentioned at the beginning of the page, Spin plans are for sale at my old website, IndoorFlyingModel.com. My goal is to get a PayPal button up by mid-August 2013 so you can purchase a set of Spin CAD plans here. Thanks for visiting!

Author: Gordon McKay