While the topic of blade tracking may seem on the surface very basic and unworthy of a complete technical article, it is not. As one of my technical instructors has often said, “us old guys tend to take these routine matters for granted”, but remember that new people enter the aviation business all the time. The same can be said of our hobby. I have noticed a lot of incorrect recommendations on the Internet based upon something read out of context or incompletely reasoned to full accuracy. This sometimes is based on lack of experience or is simply due to plain old ignorance. These machines are potentially dangerous, and quite frankly they have the power to kill and disable. My intention here is to make people safe, competent and supply the reasons for the how and why of rotor tracking. Some situations may seem extreme by present standards but it is these cases that have us scratching out heads in puzzlement. This is a major reason for the column. Correct blade tracking is necessary to get the most from your model helicopter both in flight performance and mechanical longevity.
The newcomer often wonders what exactly is blade tracking. Look at the normal out of track diagram and it should give you an idea as what to expect in the way of a snag. Well the best way to describe it is to understand that all rotor blades attached to the main rotor should fly in exactly the same plane under all flight conditions. Looking at the rotor edge wise should indicate an image no thicker than the thickness of the airfoil. Any thicker than this and the blade tracking needs to be corrected. In technical terms the blades must have the same tip path plane for best tracking. We are simply making a blade fly higher or lower by changing its pitch relative to the other blade. The common method of correcting an out of track condition is to adjust the pitch link of one or both blades. In the case of both they would be adjusted oppositely and half as much as compared to the single pitch link adjustment technique.By making adjustments we can cause the low blade to fly higher and/or the high blade to fly lower. This decision should be based on the desired hovering rpm. If the hovering rpm is spot on adjust both links equally but oppositely and in the correct direction. This maintains the relationship of total rotor thrust to the collective stick position. If the rpm is low, bring the high blade down by decreasing its pitch with the respective link. For a high rotor rpm situation raise the low flying blade to decrease the rpm. These considerations will allow you to leave your existing throttle curves and linkages close to the nominal settings or to those you so carefully perfected, thus giving you the advantage. The advantage of proper track is a reduction in rotor vibration. This in turn means lower stress fed into the servos, electronic equipment and airframe components. Parts will stay tight and last longer. Your machine will be less prone to RF glitches from loose metal parts. Metal fatigue will be reduced in side frames, plastic fins and canopies will not crack as easily. Fuel will be less prone to foaming encouraging better engine performance. Visibly your machine can look like a well honed mechanism with all its parts operating in harmony. This will be a personal reflection of your modeling skills at the flying field.The fore mentioned adjustment method is fine when everything is normal or in your favor. With such a complicated device as a helicopter, established methods can sometimes yield incorrect results. Indicative of this might be the case one side of the rotor in track while the other is not. It may also manifest itself as tracking errors associated with the application of cyclic or collective control inputs. On rare occasions tracking errors will occur with rotor rpm changes. Sometimes blades will go in and out of track during different times in a single flight. There are many root causes for such problems and a little bit of thinking and investigation should lead to a source of the matter. If you are interested in these and their cures read on!
Many times a machine will not track after a crash even though the obvious parts were carefully replaced. Notchy grip bearings with flattened balls can cause the feathering action to hang up on one blade while the other does not. Sometimes in the case of thrust bearings this is not readily noticed since the grips need to be loaded or pulled apart while rotating the grips to feel this roughness on the bench.The head type will have to be considered since a bent single through spindle can rotate altering the grips position and thus sometimes shifting the blade tracking in and out during flight. What sometimes happens is the spindle takes a set or seats under certain rotor G loads then shifts rotationally to seat under other rotor G loads. The dual spindled rotor head design can have the spindle fixed to the blade grip so it rotates with feathering action. Tracking errors and aggravated vibration levels will be evident with a bent spindle. If the spindles of the dual spindled design do not rotate as is the case where the bearings are in the blade grips, expect more tolerance to very minor spindle imperfections. Also consider the fact that a bent pitch horn shifted closer to the spindle will change blade pitch ata higher rate for the same control movement as compared to one located further away at the normal specification. In this bent pitch horn case the track condition will change with cyclic and collective control inputs. Everything needs to be symmetrical! Think about what a bent rotating spindle (fixed to the grip) will do to the spacing between the spindle and pitch horn during feathering action! This may sound stupid but what about the person who accidentally installs the mixer or collective scissors geometry differently on one blade grip side? How often have we been distracted during a building/rebuilding session by the phone or worse? Considering this distraction factor you may have more than poor tracking to contend with if things are not double checked. In any event the control linkage perspective diagram regarding tracking should point you in the correct direction.
The flybar paddles need to be at identical incidence because cyclic blade pitch control is slaved to the flybar. The indication here is very obvious sighting the paddles but a secondary symptom might be a poor track condition if taken to extremes. Personally I have not had this happen to me and out of curiosity I radically put one of my Raptor paddles off by many degrees before the track would change. Just something to keep in mind since there are various configurations of rotor systems and setups out there. The cyclic centering feel did change substantially when taken to extremes. Eyeballing the paddles to a flat incidence at neutral cyclic will give you ample accuracy. A typical pitch link adjustable out of track state will have a single blade flying high or low but not both. The type of blade and construction can cause problems. Usually this problem will lay with wooden blades or blades with an incorrect airfoil design for a specific helicopter size. Most manufactures have this nailed down for the most part but there are glass blades on the market that suffer from this.
First lets take the case of blade covering and how it physically relates to the airfoil. Two types are currently on the market, the non-adhesive shrink tubing type and the self-adhering sheet type. For the smaller sized helicopters the shrink rap covering maintains enough surface tension and stiffness with the blade that it does not lift off the top low pressure area of the blade. As soon as the airfoil gets wider the concentrated low pressure area of the blade becomes larger allowing the covering to lift thus altering the airfoil. Since things like this are never identical one blade will pop out of track before the other as rotor rpm or lift is increased. In my opinion 46 sized blades are at the threshold and really should have not have shrink tubing. The 30 size and below seem fine. The worst offender is the wide chord 60 blades which are becoming more or less fazed out, as might be found on some older Schluter machines.The relationship of the blade chord and chord thickness to the span, seems for the model helicopter tracking to have an ideal ratio or limitation. The wider and thicker the blade chord the more lift that will developed at similar angles of attack. In effect this will make tracking adjustments more sensitive on the wide chord blade. Further more any minor manufacturing or building defects will be more prone to show up. With more lift comes more drag and this can cause one blade to lag at lower rpm in the blade grip before the other during cyclic control inputs. Besides the loading on the pitch links being very different we also have one airfoil swept more than the other thus altering its lift to drag ratio. By running the rotor up to rpm flight for dynamic alignment, and then locking the blades to the grips through blade bolt tension, will help this lag problem, as will a higher rotor rpm. With smooth glass blades it is often hard to get enough friction between the blade root and the blade grip in this situation. Lucky for us that most glass blades are built in the proper proportions. The reverse can be true in the case of a blade that is too thin and/or has a narrow chord. In the extreme if the blade is about to be over pitched at high collective settings, a cyclic input can violently pull a blade out of track. The blade can also lag under this condition and aggravate matters. All I can say is reasonably fit the blade size to what the model was designed for. We often hear talk of mismatched span/chordwise c of gs causing tracking difficulties. This seems a thing of the past fortunately with the better quality kit or ARF wooden blades. Still it should be something to be aware of. Basically it takes more lift or tracking adjustment to alter the track on a blade with the weight more outboard as compared to one with its mass closer to the root. Both can statically and dynamically balance (under certain specific conditions) but if a blade can change its track differently from the other for the same pitch change amount, it cannot be expected always to stay in track.
Thinking about how our blades are controlled by a small control rod attached to the blade grip which moves up and down during every cyclic rotor rotation should draw us to the importance of accurate rotating linkages. If one blade has excessively sloppy rod ends compared to the other we are asking for problems, especially if the blade design can load the pitch link in different directions at distinct blade angles of attack.
Now that we understand something about blade track we should have a close look as to the actual procedure and the safety issues. Since we have to view the blade track we will be in a potentially hazardous location, in line with the rotor. Watching the track and operating the machine can be done safely but if you have a mate who can view the rotor, matters will be best. I might suggest that the new flyer have a buddy help set up the helicopter. In any event keep 15 to 20 feet away from the helicopter. To view and isolate the track one blade needs to be isolated with a contrasting colored blade tip. The blade grip should also be marked with the same color to avoid confusion. Think about which way the link or rod needs to be adjusted to raise or lower a blade in flight, since rotor heads are available both with leading and trailing pitch horns on the grips. When you make an adjustment remember what you did where. You can even write the adjustment down so if at the next run matters get worse, you can go back to make proper adjustments in the other direction. Remember that if you over adjust, the tracking will go through center and out of track once again. The first run should be a ground run followed by light on the skids to encourage disk coning or rotor loading. This will get you well into the ball park. Next adjust in the hover to finalize the situation. One half turn on the rod end is the limit of tracking resolution which is also dependant on the thread pitch of the control rods. Often the best place to adjust your track is with the link closest to the blade. By doing this the rest of the rotating control geometry can retain its symmetry. Adjusting the long rod feeding the mixer from the swashplate can introduce mechanical errors. I realize some machines come with short fixed links at the blade grips but these can often be replaced with adjustable types.
There will be cases on certain rotor heads where the mixers are mounted on the blade grip pitch horns, where two links on the same blade will have to be adjusted together to keep the mixer level. For some reason this is often neglected in their respective manuals. Just consider the rule that mixers should be level at half stick with a level swashplate as this is all I am saying.
While very remote, it is possible to have a blade set with an individual incidence difference as it relates to the blade root. They don’t have to be cheap wooden blades either since a mixup can happen at packaging when a product is modified. Excessive compensation for this can introduce differential errors as the pitch horns will be non-symmetrical in location or position. As the collective or cyclic is moved there will be tracking errors that cannot be mechanically corrected since the positional pitch horn is fixed to the blade grip. These types of rarities are easily corrected by notifying the manufacturer for replacement or refund.
I have heard and viewed some pretty unsafe tracking practices. One fellow wanted to tie his machine down to the ground because he could not fly and do the tracking under load. When he tried this, the machine violently shook from ground resonance almost destroying itself and him in the process. Not all machines will suffer to this degree but many can! This next one takes the cake. After reading some historical information concerning full scale helicopters and how the old guys did it, one gentleman tried the tracking stick method. The theory is that you poke a stick into the rotor with a flexible tip containing a colored pigment to be deposited on the blade tracking the lowest, then adjust as required. To top this off the aircraft was to be secured to the ground or a run-up stand. As far as I am concerned the stand is nothing more than a good support for accurately aiming shrapnel. I once viewed a picture of a person actually holding a running helicopter in his hands, by the skids, as he performed a power check! Trust me you don’t want these people flying behind your back so lets keep it safe guys.
There will be the very rare case when tracking will be fine in the hover, vertical climb and decent but faulty in forward flight. It is possible that the blades are not matched close enough to one another. The fix usually is a new set of blades. While I am on the topic I’d like to answer a related query directed at me about trim tabs on full size rotor blades. Basically our models don’t require these due to the stiffness of the rotor disk including torsional blade rigidity. The reason they are in place is usually to correct tracking problems in forward flight at various airspeeds. The tab simply twists the more flexible blade at the tip being most effective in the forward flight mode. It has little or no effect in the hover on many full size machines. You would be surprised at what flexing and oscillation a large flexible blade will do. A camera is sometimes mounted to the rotor hub to film such blade actions for further rotor blade/hub design and analysis.
Finally as you move up in the hobby your helicopters will become smoother and smoother as will your desire for better blades. As a beginner wooden blades tracked properly will be more than adequate. Remember as we move up in helicopter quality the theory of diminishing returns takes over as flight improvements become smaller. You don’t have to have the best but you should always make the best of what you have. Happy tracking.