About the Unit:

A little history. After many trials with several RK049 fan units, I got impatient with the poor starting characteristics of the Cox TD-051 engine. I purchased a Kress-709 unit and a TD-09. This is a better starting engine. But the quality of the Kress polycarbonate duct was not up to my standards. Out of the box, the front and rear ends of the duct were not trimmed perpendicular to the axis of the duct. This required some effort to fix. After a few years the duct had many small cracks coming from the mount holes. It did not look flight worthy.

Since my RK-049 duct still looked great, I thought it would be straight forward to make my own duct, modeled after the RK-049. The size of the TD-09 cylinder head was not compatible with a simple round 3" ID duct. So I did some work, and what resulted is this design. It has a 3" ID inlet, and a egg shaped exit. The cylinder glow-head has to be removed to get the central core unit out, just like the RK-049/TD-051 design, but the cylinder slides right under the rear two rings.

I bolt my fan units to the fuselage as part of the load transfer path. I use 2-56 bolt going through holes in the front and rear rings. It works great, and the units always fit tight.

Flying the Unit:

The fan unit has been installed in the X-32, in preparation for flight in 2006. It fits great.

The fan unit has run on the test bench. It works! The TD-09 engine is much easier to start than the TD-051. The bare fan unit makes a lot of sound. This should be less noticeable when installed in an airplane.

The only real problem is balancing the rotor. This took so time to figure out. It seems the Du-bro magnetic balancer does not fit the hole tightly. The shaft of the balancer had to be wrapped in tape so that the rotor was snug on the bare balancing shaft.

Construction Highlights:

The prototype used a balsa plywood for the rings. I thought this material was a little too soft so I added .007" carbon fiber strips to the OD. The balsa play is very light, but you may want to substitute lite-ply instead. Don't use regular plywood, the rings will be too heavy.

The shell of the unit is made from pre-made fiberglass sheet stock. This is easy. Get a piece of window glass, and coat it with mold release. Lay 4 layers of 0.5 oz fiberglass cloth on the glass, making sure there are no wrinkles. Iron the fiberglass if it has any creases in it. Using polyester finishing resin, fill the weave so that the fiberglass to soaked. Make sure the fiberglass cloth is laying flat on the glass. Once the resin has cured, the sheet can be removed from the glass. I recommend you let the fiberglass resin cure for a week. The sheet will be a little soft after only one day of curing, and it is possible to cause delamination if the sheet is bent too much when soft.

The duct is cut out using the flat pattern. Reinforcement strips are cut from the excess sheet, and glued to one end of the duct as shown. Once that is dry, the duct is rolled into a tube and placed inside of the rings. The mating edges of the front portion of the duct are now glued together, forming a round tube with an inside diameter a little over 3". Don't glue the rear mating edges yet. The discarded "inside" portions of the rings are pushed into the duct to force the sheet to expand and match the ID of the rings. This is done for all four rings. Now the rear mating edges of the duct can be glued. This will be a little tough; the transition region between the middle two rings will deform a little, and this is OK.

Once the duct is glued, rings "A" and "B" are glued to the duct. The ring "D" is also glued at this time. Just make sure ring "C" is already on the duct or you will have a real problem! Since the duct is semi-transparent, the location of the screw holes in ring "B" can be found simply by installing the inner unit, and marking ring "B" as required. Drill holes through ring "B" and screw the duct to the inner unit.