RE: Eflite Blade CX
A degree of left or right "spinning" or yaw is unavoidable on these helis.
The heli does not seem to have a heading hold gyro, so as the voltage from the batteries/bec varies the heli will tend to yaw either left or right, and/or may change in flight.
If the yaw is uncontrollable without minor (2-4 ticks of the joystick) rudder inputs, your heli needs tuning.
Note that it is somewhat NORMAL to start out with the trim set say, 2-5 ticks one way, and by the end of the flight have to set it completely over the other way to keep the yaw under control.
That is the price you pay for the low cost electronics...
Usually a difference in "lift" between the two sets of blades can result in one direction being favored over another... any form of binding which causes one blade not to spin as freely as the other also complicates matters.
The aluminum bearing holder and shaft upgrade helped this out a lot on my heli AFTER I discovered the balance issue with the numeric markings on the blades.
Ideally all four blades should have the same number, but matched pairs do work well.
If the metal gears are too tight against the large plastic gears, then imperfections or out of roundness in the large gear will result in vibration.
The 2 pieces of paper rule applies here.
As noted, this heli uses a quartz piezo gyro. These types of devices can and will vary according to temperature, which is why the holes in the cowl can be effective to cure a degree of yaw variation during flight.