Friday, July 18, 2008

LiPo Balance Connector Compatibility

Perhaps the main problem with Lithium-Polymer batteries (other than their tendency to explode when damaged) is the lack of a standard for their balance connectors. This limits which chargers and balancers you can use with a given pack. To address this, Evan at RC Accessory has put together a great list of which brands comply with the four different variations out there in the wild. Check out his Balance Tap Configuration Guide for the details, and remember that some brands using identical connectors have the polarity reversed. Good grief!

Thursday, July 17, 2008

Align T-Rex 450S Build Notes

The Align T-Rex 450 series is perhaps the best known of the mini-size electric helicopters. Many reviews are available online, including this thorough one at T-Rex Tuning . In this article I'm going to sum up the positive and negative experiences I had building the 450S aluminum kit, and include a list of tips to help you out.

Pluses: The kit includes three hex wrenches, two screwdrivers, two kinds of thread locker, a bag of tie wraps to secure all the wiring, and an antenna tube for FM receivers.
The ESC has a ferrite ring already installed on the receiver lead, no need to buy one separately.
The brushless motor comes with two pinion gears, a 12T and a 13T. If you're new to collective pitch helis like I am, go with the 12T for now.
The most complex parts of the head and tail assemblies are already built, even in the S kit (not just in the 450SA ARF).
The manual shows all the parts and steps for the factory-assembled components, which is handy if you need to repair or replace them.
Align includes a bag of spare parts consisting of extra screws, bolts, nuts, ball links, etc., although they label the bag "screw parts".
Minuses: The screws for the ball links are too short. On my Hitec HS-65MG head servos, the screws just barely emerge far enough through the servo horns to get the nut on. In the case of my Futaba 9650 tail servo, the screw was completely unusable, and I had to replace it with hardware I had on hand.
The screw holes in the frame are too close together for the HS-65's. I used a rotary tool to elongate these holes into ovals, which allowed the screws to go in straight without stressing the servo case.
You have to solder all six bullet connectors and the power input connector on the ESC. In addition, the bullet connectors are incredibly stiff to put together. Use a pair of pliers to compress the male bullet and force it into the other side.
Some parts of the manual only have diagrams instead of numbered, written instructions, so it's easy to miss adding a small part. Even worse, some steps like bolting the main gear to the main shaft are not shown at all.
The tie wrap cutouts in the frame are positioned too close to the head servos, and could cause interference with the ball links.
The plastic tail servo mounts are fairly short and only have one screw each to hold the servo. I used them, but you might want to consider an aftermarket replacement.

Tips: Download and watch the Finless Bob instructional videos from HeliFreak, they are incredibly useful and free. You will have to register with the site, but it's worth it.
Although the kit includes hex wrenches, a 1.5mm hex driver with a real grip (like a screwdriver) will make life much easier.
Buy a good pair of ball link pliers. You could do without them, but it'll be much more frustrating.
Take the time to do things right. If something isn't moving smoothly it could affect flight control or burn out a servo, so take it apart and adjust it.
If you're thinking of moving to 2.4 GHz, go for it. Dealing with the long FM antenna is a pain and it doesn't look as good.
Make sure to check the front-to-back CG. Especially with the large 9650 tail servo, it's very easy to end up tail heavy. Put the battery and ESC as far forward in the nose as possible to offset it.
Install tie wraps in the frame before putting any servos in, and keep them open until all of your gear is installed, including the ESC.
If using an FM receiver, cut two holes off a servo horn and run the antenna through them, sliding this piece almost up to the receiver. Then run the antenna through a hole in the frame which is smaller than the servo horn piece. This will prevent any strain on the antenna from yanking on the receiver.
Make sure to keep one wire between the motor and ESC disconnected while setting up the radio to prevent the blades from spinning up accidentally.