Rotor blade tracking
Rotor blade tracking is an often overlooked task on simple 4 channel FP (Fixed Pitch) electric rc helicopters, such as the Walkera Dragonfly 4, yet is as equally important as balancing the main blades.
The term tracking refers to the line that the blades take through the air, in their horizontal plane as they spin.
To check the tracking of the rotor blades you need to have the spinning blades at eye level (Yikes!) -- you can either get someone you don't like much to hold the helicopter firmly while you fire it up, or you can do it yourself.
Either way, a lot of care must be taken. Spinning rotor blades don't pay much attention to what they hit, and a nearby face is an easy target, so do this very responsibly!
Above: checking the rotor blade tracking - with care!
But before you get to this stage, you need to mark one blade on its leading edge with a highly visible colour. Coloured tape such as electrical insulating tape is good, as is a good size blob of coloured ink (pref. white or yellow) from something like a paint pen or marker pen or even correcting fluid. The important thing is that you can easily identify one blade from the other when they're spinning.
Holding the helicopter at arms length, or getting someone to hold it for you, fire up the main rotors so that they are at 'flight-speed' - anywhere between half and full power is good.
Look at the blades straight-on at eye level, and you'll see one of two things... either you will see just one blade profile, or you will see two at different heights. If the latter is the case, then the blades are tracking badly and need to be adjusted.
Above: the top illustration shows two different blade profiles, one above the other; the marked blade is flying higher than the unmarked one. The lower illustration shows correctly tracking blades, both following the same line in the horizontal plane.
Why do the blades track differently?
If the Angle of Attack (the angle of the blade in relation to the airflow) of one blade is slightly different to the other, then that blade will generate a different amount of lift to the other one. If this is the case, the two blades will follow a slightly different path through the air, the blade with the greater angle of attack flying higher.
Badly tracking blades can cause vibration in the same way that unbalanced blades do, and they can also cause instability in flight.
FP (Fixed Pitch) rc helicopters don't have the same level of adjustment that more complex CP (Collective Pitch) rc helicopters do, and adjusting the blade's angle of attack on an FP helicopter can be done in two ways.
The first way, and this is the way usually stated in the manufacturer's instruction manual, is to gently twist one of the blades. Either increase the angle of attack of the lower flying blade by twisting the leading edge upwards/trailing edge downwards, or decrease the angle of attack of the higher flying blade by twisting the leading edge downwards/trailing edge upwards.
Personally I'm not a fan of this method because there's no guarantee that the blade will hold its new shape.
The better method is to adjust the angle of attack by placing tape between the top surface of whichever blade you choose, and the rotor holder. This involves removing the blade in question from the holder.
To increase the angle of attack, if you've removed the lower flying blade, put a small strip of tape on the trailing edge side of the screw hole, as shown in the photo to the right.
To decrease the angle of attack of the higher flying blade, place tape on the leading edge side of the screw hole.
Depending on the thickness of the tape, add one or two strips and then replace the rotor blade into its holder and check the blade tracking again. Remove the blade and add tape as necessary, until both blades have the same angle of attack, and so track correctly.