project pattern
W
Redux
ith the equipment installation completed,
retention of the
canopy and belly pan hatches are in order. While many meth-
ods are used today, all have pros/cons. The ideal method would be lightweight, vibra- tion proof, wear resistant, able to handle substantial airloads, and allow fast removal and attachment. Airloads can be quite high on designs that have very large hatches (i.e., canopies extending from the nosering to the wing trailing edge), or have large aero de- vices (T-Canalizers or strakes) attached to them.
It is very rare to find a single method/in- stallation that achieves all the desired goals, especially with glow, where vibration is a substantial issue. Mounting demands for the hatches for Yuri are relatively low, as the filleted flanges present on the mating surfaces add substantial strength, the parts fit is very accurate, and the hatches are rel- atively small in size.
I want to stress the importance of secure,
rattle free retention systems. Any part that can move or rattle will do exactly that, and the amount of movement will increase expo- nentially as parts wear. Even electrics pro- duce vibration; whether from air impulses from the prop, cogging of the motor, or cog- ging of the gearbox. A little extra time spent getting the parts fits of the hatches accurate and rattle free will reduce maintenance time in the future.
Additionally, the noise generated by a rat- tling hatch can be quite annoying in several respects. Firstly, it is entirely possible for a large rattling hatch to contribute a full deci- bel or more to the noise meter during a sound test. Secondarily, the quiet “whoosh” of an electric or the purposeful growl of an IC can be tainted by a loudly rattling hatch. Attachment options are many including tongue and groove structures, pins, tabs, “keyhole” slots, a variety of spring latches and “one touch” locks, and of course screws. I will briefly describe several options, which along with the pictures herein should con- vey the concepts. Most of the hardware bits are available from F3A Unlimited, which currently has the most comprehensive stock I am aware of for retention bits. The SebArt Wind S Pro has preinstalled a
by dave lockhart with algirdas ungulaitis You can reach Dave Lockhart via e-mail at
davel322@comcast.net
combination of pins, tabs, and a “one touch” locking mechanism. The pin at the front of the canopy has a large surface area and is nylon to reduce wear. The tabs on the sides and rear maintain alignment of the canopy under side loads. The canopy is held in place by the “one touch” mechanisms (one on each side), each of which is essentially a dowel pin with a circumferential relief slot that is engaged by a spring loaded “key” in the re- ceptacles mounted in the fuselage. This system fits the bill for being fast and simple, but is only suitable for fairly robust structures (actuation pressure is notice- able). Additionally, as the retention force is primarily lateral, the vertical movement of the mating surfaces is not substantially re- strained. I would not use this type of reten- tion system on an IC powered plane without a thin layer of silicone or vibration pads (such as those made by Secraft) on the mat- ing surfaces.
The Wistmodels Bravo has a system for retaining the canopy that is the best design and execution I have ever seen. The fuselage flange is slightly recessed (from the fuse-
Abbra belly pan mount
through the circular part of the keyhole slot, then slide aft to the narrow portion of the slot. Once slid aft, the bolts are captured lat- erally and vertically. A small retention screw at the nose of the belly pan is used to keep the belly pan from sliding forward (un- likely, but possible). This system requires a fair degree of precision to have all bolts en- gage equally, but is quite positive and ro- bust when done correctly. Additionally, as the mating surfaces wear, the bolts can be tightened slightly to restore the system to zero slop.
The belly pan mount on Bryan Hebert’s Alferma is another example of detailed, pre- cise craftsmanship. The rear of the belly pan is held in place with locating pins on each
Bravo canopy retention
lage) around the full perimeter. The outer perimeter of the canopy precisely fits the fuselage recess, and the base of the canopy has a raised ridge that precisely fits the in- ner flange of the fuselage side. This tongue and groove arrangement prevents lateral movement of the canopy and captures the front edge of the canopy.
PHOTOGRAPHY: DAVE LOCKHART Wind S Prolocking mechanism 54
The security is further enhanced by six sets of small magnets embedded in the fuse- lage flange and canopy base. The ultimate retention of the canopy is completed by a spring loaded latch at the back of the canopy. This system is excellent for any ap- plication, with the only downside being the extremely high degree of skill needed to pro- duce the accurate fit tolerances. The Mach Composites Abbra has a very long belly pan (to allow electric and IC con- figurations) and uses a system of “keyhole” slots to retain the belly pan. The flanges of the bellypan are reinforced to accept 8–32 flush head nylon bolts (installed vertically to the flange). The fuselage flanges are rein- forced with backing strips of lite ply, and then “keyhole” slots are cut to accept the 8–32 bolts from the belly pan. The bolts fit
PHOTO: BRIAN CLEMMONS Alferma belly pan retention
side and sides are fully supported by a tongue/groove structure. The belly pan is re- tained using a “one touch” type of spring sys- tem. Quick and easy to use, but very de- manding of precise construction to avoid rattles and rapid wear. Shifting back to Yuri: I put a pretty high priority on fast removal/attachment of the canopy as the motor Li-Pos will be accessed every flight, as well as the Li-Pos for the ra- dio. As the canopy is of reasonable size, does not support any aerodynamic devices (T- Canalizer or strakes), and is quite sturdy, I chose to use a quick release latch at the rear with a slotted/bolt capture in the front. I did not add any pins or tongue/groove surfaces to the sides of the canopy as flanges are very rigid and maintain alignment without any additional support.
Starting with the front slotted/bolt cap- ture, a shaped piece of 1⁄8-inch light ply is used to reinforce the fuselage flange, and a small fillet of epoxy/microballoons is used to
APRIL 2013
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