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Nov 03, 2016, 04:15 PM
Folding Spacetime
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Frame widening a Trex 250

As many will attest, the Align Trex 250 doesn't have a lot of room for mounting a flybarless receiver. It seems that the frame sides are just slightly too narrow for many of the most popular FBL receivers on the market to fit within the frame.

Many 250 owners have resorted to cutting out portions of the frame sides to allow the receiver to fit onto the bottom plate, between the frame sides. Some prefer to de-case the receiver and then mount the electronics directly into the frame. Others, myself included, have accepted mounting the receiver onto the outside of one of the frame sides. Side-mounting the receiver does work, but it leaves the helicopter imbalanced and exposes the receiver to relatively greater potential crash damage.

So, when it came time to do some work on one of my older 250s, a four-year-old CopterX 250, I took the opportunity to attempt installing a Spektrum AR7200BX receiver inside the frame. This called for widening the lower frame ever so slightly. Widening the frame would require creating spacers between the frame sides and the bottom plate, the motor mount, and the belt drive-gear box. The spacers would have to be thick enough to provide clearance between the AR7200BX and the frame sides, but not so thick as to require longer frame screws.

I had thought about using matchbook paper for the spacers, but then I worried the paper might not maintain a uniform thickness when tightened into the frame. Next, I thought about creating the spacers out of old credit card plastic. I ruled out credit cards upon considering that cutting small, detailed shapes into the credit cards would be unduly challenging, and the credit cards might be thick enough to require longer frame screws. Eventually, I decided to use some plastic healthcare cards for my spacers. The healthcare cards are thinner than typical credit cards, and super easy to cut, yet they are thick enough to widen the frame just enough to accommodate the AR7200BX.

During cutting the healthcare cards, I tightly clamped each card to the frame side and then cut the cards with a model scalpel. I took my time, using the frame side as a guide. I carefully removed any portion of the card that covered a screw hole in the frame or would be visible from outside the helicopter. Crafting the spacers became a lot easier once I was able to remove the clamp and install the unfinished spacer between the frame side and the motor mount.

After trimming the cards and assembling the lower frame, I was pleased to find there's just enough width between the frame sides to accommodate the AR7200BX within the lower frame. I used an Align 3M gray pad to attach the AR7200BX to the bottom plate. During attaching the AR7200BX to the gray pad, I placed two sheets of copy paper between the AR7200BX and each frame side to ensure that the AR7200BX is centered within the lower frame. As a final touch, I used a black indelible marker to darken any portion of the healthcare cards that remained visible on the outside the helicopter.

Although the antennae of the AR7200BX now extend neatly through the frame side of the 250, the antennae do provide a constraint on where the AR7200BX may be located within the lower frame. It's possible to face the servo sockets of the AR7200BX toward the tail, but I didn't want all the servo wires extending out of the rear of the lower frame. Also, I didn't want the AR7200BX extending rearward out of the lower frame just to accommodate the antennae. Instead, I opted to orient the servo sockets forward, toward the motor. Although this places the servo connectors awfully close to the motor, and it can be challenging to plug in the servo connectors, the wires don't actually touch the motor. Itís also somewhat difficult to access the setup button and see the lights on the AR7200BX. But Iím willing to live with these difficulties because I think keeping the wiring inside the frame gives the 250 a clean appearance.

I have flown this helicopter a few times, now, and I find it flies every bit as well as my Align Trex 250 that has a side-mounted AR7200BX. The chief difference, however, is that the AR7200BX is more well protected within the lower frame, and the helicopter feels more balanced with the AR7200BX centered within the helicopter.
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