Photos by Susan Harum
Mirror of article to appear in the online section of Model Airplane News (TBA)
The E-Flite Blade CX2 is a very stable helicopter that can be flown by a total beginner, which explains its commercial success, especially when combined with the low cost of its spare parts. With more than a foot-wide rotor, it is a little too big to fly indoors, so it does not take long before pilots bring it outdoors, and this is where the problem starts; it is too stable. Unless the wind is calm, the CX2 simply does not have sufficient forward speed to fight against it. The CX2 Manual actually says that the slightest amount of wind can cause the rotor blades to spectacularly strike each other. The aluminum rotor head of the later releases sets the top rotor higher to prevent that.
The original E-Flite Blade CX2, a successful little machine (www.e-fliterc.com).
This lack of cruise speed is not linked to an inherent limitation of coaxial rotors, but because the CX2 was designed to be a beginner's helicopter yet with full 4-channel control. For example, the real Kamov Ka-50 can cruise at 170 mph, loop, roll, sideslip, and "funnel fly" better than helicopters designed with the standard large main and small anti-torque rotors.
The Kamov Ka-50; immune to retreating blade stalls (photo by Sergey Ryabtsev)
When the wind is about 5 mph, the CX2 take-off and landings must be heading exactly into the wind, with cyclic full forward. Any deviation and it dramatically drifts sideways and loses ground to the wind; this can quickly get unmanageable.
There are several options to increase speed. The first option is to move the battery forward by breaking the tab preventing it from sliding forward in its compartment. The battery can be set forward until just being able to hover with cyclic all the way aft. This adds a lot of wear and tear on swashplate and linkages, and then it is impossible to fly backwards anymore.
Another option is to increase the throw of the swashplate servos. To prevent beginners from overcontrolling the rotor, the swashplate pushrods are factory-set to the servo holes closest to the shaft. It is simple to connect them to outer holes. The corresponding increase of pitch range of the lower set of blades allows more cyclic, with the benefit of more squirrelness. However, this increases the aerodynamic loads, and the lower blades are more likely to bend upwards enough to strike the upper rotor.
The real Sikorsky X2 (photo by Ashish Bagai).
The option retained here is adding horizontal thrust with an extra motor and propeller in the tail, just like the Sikorsky X2 and its predecessors. This is made possible by the CX2 having a 5th channel available, a rotary knob on top of the transmitter. Pilots have used this free channel for retractable gear, tiltable LED searchlight, camera control, steerable chin gun, and dropping various loads; why not a boost propulsor engine?
Among the few available various motor & propeller units, the Direct Drive Tail Motor Conversion Kit EFLH1320, designed for the Blade CP series, looked like a good choice. The motor casing is 7/16" diameter, the 2-blade high-pitch propeller 3 1/4", and the unit (rated for up to 11.1 Volts) consumes about 2 Amps under 7.4 Volts. At that voltage, it provides a static thrust of about 40% of the weight of the CX2.
Component view of the modified tail.
Originally, the plan was to chop the tail boom and completely hide a motor into it, like a semi-scale model of the Sikorsky X2, but this is not possible using the CX2 airframe. Apart from the limited throw of the controls mentioned above, another reason the CX2 is a beginner's helicopter is that it does not have true cyclic control, but instead a gravity pendulum design. When "losing it", this allows releasing all the controls, and the helicopter will stabilize itself the right way up. If the axis of supplemental thrust is too low, it will pitch the fuselage up and tend to make the helicopter swing backwards, somewhat negating the extra forward thrust it provides. As a result, the boost motor and propeller has to be mounted as high as possible (also good for ground clearance). In the "CX2X2", it is practically aligned with the top rotor.
Assembled view of the modified tail.
The original fin was chopped halfway with kitchen scissors, and a 1/4" thick blue foam pylon was cut to a shape fitting snugly into the tail, between the remaining flanges of the fin. If you want to leave your original CX2 intact, this operation can be done on a spare tail (Part EFLH1256); it is the only structural modification. The pylon has a 3/4" chord to provide some yawing stability. Motor, pylon, and tail are simply attached to one another with electrical tape, which absorbs vibration and is crash-resistant. Red electrical tape and CX2 also have the same color.
For varying the speed of the boost motor, a GWS ICS50 speed controller was used, being rated for 2 Amp continuous (4 Amp peak). This controller is plugged into the 5th channel (marked Gear) of the receiver, and located in the tail boom, as far away as possible from the receiver.
To compensate for the significant moment of a 19 g motor added practically at the tip of the tail boom, the battery was relocated from its dedicated compartment under the rotor shafts to the floor of the cockpit. It happens to fit snugly inside the cockpit, while resting against the forward tab of the battery compartment. Even so, when balanced from its flybar, the CX2X2 is a little tail heavy, and needs full cyclic trim forward for hovering in place. To reduce the tail heaviness, the pylon could be mounted closer to the main rotors without any problem (in the Sikorsky X2 rotors and prop nearly touch each other). Ready for take-off, the CX2X2 weighed in at 254 g, an increase of 11 % on its nominal weight of 227 g, so it is able to handle the extra weight.
With the boost motor off, the CX2X2 behaves the same as the CX2, without adverse effect of the shorter tail boom. Because of the increase of longitudinal inertia, it is more sluggish when yawing fast when hovering. With the boost motor half-power, it shows an increase of speed without fishtailing. For easiest flying, set the rate of climb and yaw with the left hand, and control the forward speed with the thumb and index of the right hand, while keeping pitch and roll relatively constant. In theory, when going fast, the power (and the Amps that go with it) could be shifted by lowering the RPM of the rotors (the "collective") while increasing the RPM of the boost motor, but you would need a computer radio to do that.
At full power of the boost motor (and this probably shortens battery life), the CX2X2 goes fast enough in a straight line to be concerned with radio-control range. The intent is not zooming around though, but being able to beat the wind. You will never again have to land flying backwards, where the rear of the skids have a nasty tendency to catch obstacles and risk tripping the helicopter over. For about $30 of materials, you now have the option to turn on that boost prop and fly an "all wind" helicopter that is still stable.
PS. The first prototype had the tail motor mounted much too low (YouTube)