1 Compass Atom 5.5 Stretch Kit Proving bigger is better! Author: Jimmy Anderson The Compass Atom 500 is a hardcore, top-of-the-line helicopter right out of the box. It s cyclic response is instant, crisp, and fast. But like most helis that swing 425 blades, it relies on high headspeed just to get decent collective response. You can slap on a larger pinion or run a fancy governor, but as stock, you will never get the feeling of float that makes larger helis feel awesome. But Compass now has a solution that brings together two sweet worlds. With the Atom 5.5 stretch kit, you get the insane power and inexpensive LiPo s of a 500, with the amazing collective authority and low disk-loading of a larger heli. What s Changed: Blades: The most obvious change is in the blades. In the kit, Compass includes a pair of top notch 510mm carbon fiber blades that are identical to Rotor Tech s 515 s, other than being slightly shorter. Four spacers are included for the blade grips, and two brass bushings are also included to reduce the bolt holes from 4mm to 3mm. Boom and Belt: To accommodate larger blades, Compass includes a longer boom and belt. The belt, other than being longer than stock, is identical to the original. However, the boom is a little different. Instead of only being longer, the new boom is also beefier. The walls of the boom are noticeably thicker than stock, which is very reassuring when you consider that the Atom utilizes a no boom support design. Motor Pinion: An 8 tooth pinion is included to lower the headspeed and increase torque to the rotorhead. Tail Pushrod: A longer tail pushrod is needed to connect the tail servo to the tail rotor. The longer one is stainless steel and includes link ends. Installation: Putting all the new parts on is fairly simple. You need to drop the motor out to install the pinion. For the tail, you need to remove the boom, separate it from the tail case, and pull the tail gear assembly out of the tail box. This includes removing the roll pin and sliding out the tail shaft. You need to remove the old tail pushrod and install it onto the new boom with pushrod guides. You re fine using two pushrod guides, but I opted to add a third to reduce any possible in-flight flexing. Before putting on the blades, install the reducer bushings. Use blade grip spacers if needed I found that one on the bottom fits the blades perfect.
2 Tips for a Hassle-free install: When removing the original pinion, be careful not to strip the grub screw. Test feel it, and use ONLY an allen driver that does not have a worn head. If the grub screw seems excessively tight, use the barrel of a soldering iron or a lighter to melt the thread lock on the screw. BE CAREFUL removing a pinion with a stripped grub screw is a royal pain, and older Atom kits use a very small grub screw that may easily strip. Don t forget to reset the gear mesh after reinstalling the motor. It helps to slide a blade grip bolt into a bushing before pressing the bushing into the blade hole you want to make sure those bushings go in straight and true. Removing the roll pin from the tail gear can be tricky. Use the shank of a drill bit or an allen key to push out the pin. Some other items that help are a table vice, vice-grip pliers, drill bits, and a hammer. When you reinstall the roll pin, be sure that you put it in the right way, and that you put it in on the proper side of the tail gear. NOTE: Be sure that you watch for the tiny bushings that go between the sides of the tail gear and the tail box bearings. You do not want any side-to-side play of the tail shaft/gear assembly when everything is put back together. For the best 3D performance, it s recommended to use KBDD 84.5mm tail blades. You will need to sand down the roots to fit them into the tail blade grips, but the longer blades will help keep tail performance as good as possible. Flight: The flight characteristics of the stretched Atom are nothing but awesome. The stock motor and ESC are capable of delivering plenty of power. Even with the significantly longer blades, cyclic speed is nearly as fast as stock. But the collective power and stability increase hugely. Large maneuvers such as power loops and large rainbows are much easier. The heli tends to maintain its speed much better and quick stops are much snappier. Every type of flight is smoother. I found it necessary to reduce expo in my radio to make up for the added stability of the bigger blades. There is much more float in flight, making the heli feel like one in the simulator. I continue to use KBDD 500-size blades (70mm; 4mm root) and have not yet found an issue with them in most kinds of flight, including backwards circuits. However, the tail will break slightly in hard transitions and tends to blow out occasionally in wind or long, fast tail slides. I will likely change them out for KBDD 84.5mm blades. The longer tail shifts the center of gravity rearward, but makes the heli only slightly tail heavy, even with a full-size tail servo. (Running Turnigy 2650, 30C batteries that weigh 466g.) Surprisingly, you only lose 20-30 seconds of flight time after stretching the helicopter. And the shift in CoG is virtually unnoticeable in flight. Using 3000- to 3300-sized batteries (500-550g) would likely be optimum, for both flight time and balance. The only issue I ve found in flight
3 with the stretch kit has to do with flying in moderate to heavy winds. In 15-20mph+ winds, the larger rotor disks acts like a sail when you put it up vertical. This is because the rotor disk is very large and the heli is quite light. Doing rainbows, tic tocs, stall turns and walls accurately is more difficult I found that the batteries were significantly hotter after flights in wind also. Highend batteries (Hyperion, Thunder Power, etc ) should have no problem, but older, cheapo packs will not take this beating as easy. However, flying in mild or no wind seems to stress the power system very little more than with the stock setup. Maiden Flight Tips: When you stretch the Atom, you increase the rotational inertia of the rotor system quite a bit. The Atom is designed to be short and low-profile, with a high center of gravity. These traits all revolve around performance. But you need to take caution when you first fly the stretched Atom, because those traits also make the helicopter sensitive to proper setup. Any vibrations or imbalance will be exaggerated by the larger rotor disk. I recommend taking off from grass for your first setup flights, until tracking is dead on and any vibes are tracked down and resolved. If you have any imbalance, the heli can resonate on a hard surface such as asphalt or concrete, resulting in violent shaking and possible damage. Once the heli is setup properly, however, it handles just as well as any other helicopter and there are no issues flying off of hard surfaces. This need for proper setup is a very small price for the awesome flight characteristics that are achieved. Final Thoughts: I would recommend this kit to anyone who wants to get their Atom flying better. The only drawback I see is the expense and limited selection of 510-520mm blades. I have to say, though, that the blades included in the kit are very nice. They are stiff, as wide as Mavrikk G5 425 s, and run really well on the heli. Their quality is way up there, also. Overall, this kit is typical of Compass: well thought-out, tough as nails, and with performance high on the list of priorities. Heli Setup Used for the Review: Motor: stock Compass 1080KV ESC: stock Hobby Wing 60A Pentium Batteries: Turnigy 2650mAh 30C ( 466g ) Cyclic Servos: JR Z3650 MG Tail Servo: JR DS8900G Gyro: Spartan DS760 Tx / Rx: Futaba 8FG / RS8000HS
4 Photos don t do the Atom justice. With the stretch kit, the heli looks better proportioned and even more aggressive.
5 Top: Stretch boom. Bottom: Stock boom The walls of the new boom [right] are noticeably thicker.
6 Mavrikk G5 Pro 425 s share the same chord width as the included 510mm blades.