project pattern
W
Redux
ith the servos installed, the next construction item is installation of the firewall(s). The most com- mon mounting method for geared inrunner motors uses a pair of mounting structures. The Hyde soft mount attaches to the front of the Neu F3A-1 gear- box, and is mounted on the primary firewall, which is located just aft of the nose ring. The secondary mounting point is a very light- weight circumferential friction fit “tail ring” at the aft end of the motor mounted on tabs or a partial former. The primary task of the tail ring is to limit the shear motion of the motor such that the thrustline is main- tained and the motor movement is limited to rotational.
Depending on the particular motor, direct drive inrunner/outrunners can be mounted singularly on the nose ring, or aft firewall, or primarily on the nose ring with a tail ring support. Caution needs to be taken when mounting solely on the nose ring as not all fuselages have sufficient structure to carry 100% of the mounting load on the nose ring. Direct drive motors are typically not soft- mounted; quite simply, the very minor re- ductions in transmitted noise and vibration do not outweigh the additional weight/com- plexity of the softmount. Key to a good hard mounted setup is a very solid firewall(s) such that the mounting structure(s) does not flex under loads, which can be quite high re- sulting from the large amount of rotating mass of a large outrunner spinning a large diameter prop.
Given the large number of motor options (including glow), many modern kits do not in- clude the firewall(s), but some do, and some have templates for the manufacturer’s pre- ferred motor option. Absent of the correct firewall(s)/template(s), various online forums can be a good source to connect with other builders that may have used the same mo- tor/plane combination. Lastly, if all else fails, the “toothpick” method I used for the instal- lation of the mounting structures for Yuri is easily adapted to virtually any power plant installation.
by dave lockhart with algirdas ungulaitis You can reach Dave Lockhart via e-mail at
davel322@comcast.net
Prior to embarking on our toothpick mis- sion, the motor itself needs a little prep work. The cooling inlets at the aft end of the motor and the cooling exhaust outlets for the fan are taped to keep any debris from en- tering the motor. The prop adapter assem- bly for the Neu F3A-1 uses a shaft adapter with a tapered shoulder that mates to the thrust washer. Once tightened, it can be rather difficult to remove the thrust washer, which is required to service the gearbox. Should you have one or more thrust washer pullers compatible with the Neu F3A-1, you can skip this step.
As I’ve yet to see a thrust washer puller
that can be used in all situations, I prefer a more customized, but very simple approach. I drill and tap the thrust washer for a pair of 4–40 bolts, and drill a matching pattern in an oversized prop washer. It is then very easy to leverage the thrust washer off by tightening the 4–40 bolts against the prop nut. As the Neu F3A-1 will not utilize the nose ring as part of the mounting structure, the first step is to cut out the middle 80% (give or take) of the nose ring. The amount re- moved minimally needs to exceed the diam- eter of the thrust washer; any additional material removed is primarily a weight sav- ings measure. I would not advise taking out any more than about 90%, as doing so would cut into the corner/fillet area which adds substantial strength to the nose of the plane.
The second step is to prepare an under-
sized firewall that can be bolted to the Hyde mount. Essentially the undersized firewall is a donut/washer shape with the inner ra- dius slightly larger than the gearbox of the Neu F3A-1, and the outer radius approxi- mately 80–90% the diameter of the fuselage at the cross-section where the firewall will be mounted. 1⁄8-inch balsa or lite ply scrap works well for the undersized firewall, but the material/thickness is not very critical, so long as it will hold its shape and accept CyA glue. The third step is to prepare a “spinner
shim”. This shim is again washer shaped with the outer radius matching the spinner backplate, and the inner radius being some amount greater than the thrustwasher. I typically use 1⁄16-inch scrap balsa for this shim. The thickness of this shim is critical, as it sets the distance between the spinner backplate and the nose ring. The spinner shim is lightly tack glued to the back face of the spinner backplate.
The fourth step is a bit messy; having more than two hands can be helpful. With the Neu F3A-1 mounted on the Hyde mount, the undersized firewall attached to the Hyde mount, and the prop adapter with thrust washer attached to the gearbox output shaft, the whole assembly is slid into the fuselage through the canopy opening. The spinner backplate is then placed on the prop adapter, followed by a propeller (or a pro- peller hub), with the prop nut, spinner, and spinner bolt. With the full propeller/spinner assembly gently tightened, the fuselage is rotated to a vertical position such that the motor/mount/undersized firewall are hang- ing in the fuselage. Masking tape is then used to firmly affix the spinner to the fuse- lage sides, top, and bottom. I’ve not found it necessary to tack glue the spinner shim to the fuselage nose ring, but this can be done. The next step can literally involve tooth-
picks, however, I prefer to make my own “toothpicks” from scraps of 1⁄16-inch balsa wood. For this technique, the ideal toothpick is approximately 3⁄16 inch wide, tapered to a point at one end, and about ½ inch longer than the distance between the undersized firewall and the fuselage side. Balsa tooth- picks of this variety are much easier to glue than traditional toothpicks which have much less gluing area. Armed with a pile of toothpicks, the process is quite simple. Glue a toothpick onto the undersized firewall with the pointy end of the toothpick just touching the inside of the fuselage. Once a sufficient number of toothpicks are in place to define the exact shape of fuselage, this step is done. Describing this process is more difficult
PHOTOGRAPHY: DAVE LOCKHART
Simple but effective thrust washer removal (above left) can be done with nothing more than an oversized prop washer (drilled in two places), prop nut, two 4–40 bolts, and two drilled/tapped holes in the thrust washer. A precise template for the front firewall is prepared in place with the assembled motor, mount, and spinner taped in place. With an undersized front firewall (at right) attached to the motor (aligned to the nose ring), toothpicks will be used to define the exact outline of the firewall.
26 DECEMBER 2012
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