RC Heli Main Rotor Blade Tracking & Adjustment
After my very first blade strike in the grass, I realized that my rotor head seemed to be a little out of alignment (noticeable through the apparent vibration and sound of the blades in the air). It was time to check and adjust the main rotor blade tracking...
There are two popular methods to perform main blade tracking, visual inspection and the paper contact test.
Technique #1: Visual Inspection
|Notice how the black marked blade is lower|
This method is fairly dangerous, so it is strongly recommended that the paper contact test be used instead. Nonetheless, the Heli manual and other references describe this test, so I'm still noting it here.
- Mark one of the blade edges so that you can distinguish it when viewed from the side (at the same level as the blade). For example, I marked the leading edge of one blade with black stripes using a permanent marker on the white painted surface of the blade. You'll want to avoid putting anything such as tape on the blade as this could impact the balance or flight characteristics.
- Fix the heli on the ground with a suitably heavy object (e.g. 2x4) across the skids. This should help restrain the heli and keep it grounded even under moderate throttle. I don't place too much trust in this restraint as I am concerned that the skids could pop out (only held in by friction of clear rubber rings). As insane as it sounds, I have seen a recent video posted on the forums showing a guy who held onto the heli with his hand (ie. only 2-3' from his face) to do this test -- clearly this is an idiotic thing to do!
- Power up the heli with enough throttle to match your current flight style (e.g. good hover speed if you're a beginner).
- While at a safe distance (recommendations have suggested 10-15'), crouch down to bring your eyes down to the same level as the blade. This can obviously be very dangerous (and hence the distance). To add to your personal safety, I would strongly recommend wearing eyewear (e.g. sunglasses) which may help minimize serious injuries should a blade come loose at speed.
- The danger here is that the rotor head may experience a catastrophic failure, causing a blade to become free. Even on a small heli (10" long blades), at a headspeed of 2000 RPM, the outer tip is traveling at a velocity of over 200 km/h! At this speed, the 15g blades have an apparent weight upwards of 20kg!
- Try to sight which blade is lower/higher, and adjust the pitch control link responsible for this blade. You can only rotate by half rotations, so there is a finite limit to the amount of precision in this adjustment. Make sure you note down which linkage/blade you are adjusting, and amount (number of half-turns) and direction (pitch increase or decrease).
- You can either increase the pitch of the low blade or decrease the pitch of the high blade. In the Blade CP Pro manual, it recommends lowering the high blade if the head speed in hover is low. Shortening the Pitch Control Link will decrease the pitch.
Pitch Control Link Adjustment
At first it wasn't very obvious how one adjusts the Pitch Control Link on a heli such as the Blade CP Pro. It is actually very easy. Simply locate the small plastic ball socket holding one end of the link and pop it off. You can do this by wedging a pair of needle-nose pliers or other solid rod into the gap between the socket and the Paddle Control Frame.
|Top View of the Control Link||Side view of the Link|
Technique #2: Paper Contact Test
A far safer alternative to the visual inspection test, this one simply measures the contact points of the blade tips on a piece of paper. There is little danger in exposing your head to the spinning rotor blades! Basic steps:
- Fold a piece of paper in half length-wise.
- Mark the tips of each rotor blade in a different color. Felt markers and crayons work reasonably well, although you will want to select colors that have a lot of contrast. I chose red and black.
- Somehow force the heli to remain locked to the ground and stable. I used a heavy-duty tripod leg across the skids, but other choices would probably prevent yaw better.
- Power up the heli until sufficient headspeed has been reached (appropriate for your style of flying / hovering).
- With the folded edge of the paper held vertically, carefully advance this edge towards the outer ring made by the main rotor blade tips.
- When you get close enough, the blades will start to make contact with the paper, and should leave a mark. Note that you will have to hold onto the paper firmly with two hands, one of which will be under the blades, the other above. Don't move the paper any more into the path of the blades as is necessary (should only require a few millimeters).
- After you have observed that both colored marks have been transferred to the paper, determine which blade must have been tracking higher than the other.
- As described in the Visual Inspection test above, adjust the pitch control links accordingly.
While performing this test, I ran into two difficulties: First, it was hard to find colored markers that remained wet long enough for it to transfer effectively to the paper. Second, it became apparent that one blade had slightly longer reach (diameter) than the other, so when I could feel the initial contact with the paper, only one blade was actually making contact. I had to press the paper in a little further (extra couple millimeters) before the second blade could leave its mark.
Main Rotor Head Balance
With the heli strapped down properly, you can carefully examine the Cap Head Screw (the screw at the very top of your rotor head assembly) while it is powered up and get an idea of the rotor head balance. If the balance is set correctly, the screw should appear to be almost perfectly stable. The center depression of the screw should make a nice, sharp hole. If you see some vibration or wobble, then it is time to adjust the main blade tracking or flybar / paddles.