Building Blades and Statically Balancing Rotors
In this hobby new comers are usually confused by what they often hear and read regarding the construction and balancing of main rotor blades. I hope to put rumours to rest and demonstrate simple effective construction procedures. Some of the information is historical by nature since many of todays blades usually come pre-built. The basic balancing rules rules still relevent so hopefully a bit of knowledge might be applied elsewhere.
A rotor like any rotating mass needs to be in balance to reduce vibrations down to a tolerable level. By carefully building the blades minimal weight adjustments will be required when mounted to the rotor head. We have what is known as the spanwise center of gravity. By this I mean that there is a point along the blade length where it will balance like a levelled teeter totter. Now if this point is the same on both blades and both weigh the same then a modeler can expect suitable balancing results. There is also a balance axis known as the chordwise center of gravity. This point lies 90 degrees to the span. The distance this c of g can move is small due to the short chord length. By the mere fact that the point where weight may be added is on such a short lever, very large amounts of weight are required to move the chordwise balance point very small amounts. Generally speaking, with most quality laminated blades this point will fall into acceptable tolerances automatically with proper spanwise balancing. Should you desire to check the chordwise balance before covering they can be hung by the blade bolt holes to verify that they fall in line to each other.
Wooden blades come with slots machined or routed in the tip end. This is where the lead weight will be bonded. The lead weight serves three purposes. It enables the blades to be matched to each other and also offers a higher inertia rotor. Higher inertia means the machine will be more stable with the added benefit of better autorotation qualities. Thirdly weight added here helps to move the chordwise c of g (forward) inline with or closer to the blade’s center of pressure. This makes life easier for the servos and flybar since a aft chordwise c of g can create an unstable rotor. That I shall leave for another time to explain. Blades usually require that the plastic root end fittings be glued on for structural integrity. This is best done prior to installing the tip weight.
Before performing the actual construction you should have on hand some 2 inch wide masking tape, 30 minute epoxy, an X-acto knife, and a piece of angle aluminum (or similar device). A precision scale is very handy to have and almost an absolute requirement. Note: As a somewhat less accurate alternative method, which is still loosely acceptable, the blades can be mounted in the rotor head and the unit balanced from the flybar which will be supported at both ends. The dissimilarity here is that while the unit may balance fine statically, the individual blade spanwise c of g and weight are different to that of the other blade. While small differences here are no big deal, an excessively large c of g variance can induce slight vibrations during blade coning and/or cyclic rotor disk deflections.
Here is a better way to balance rotor blades. Initially check where each individual blade balances on the knife edge and mark lightly with a pen. This point should be within 2-3mm of each other before balancing. Although the blades will be covered with shrink tubing or sticky back covering, it is a good idea to fuel proof the root end and possibly the tips of the blades with paint for the covering to overlap. Tip: I have used extra coats of paint here to help match up c of gs (after weighing) when conditions permitted. The blade tip end can be sealed with a coat of C/A as an alternative. Next weigh the blades and record the results making sure one blade is marked for reference. One blade will be lighter and require weight, but the addition of such may move the spanwise c of g in the incorrect direction. This is where the old noodle comes in. By allowing some edge distance in the weight grooves we have the option of shifting the longitudinal position of the lead. For example an inboard balance point can be moved outboard to the desired position by locating the weight more outboard in the slot. The reverse is true for opposite conditions. Through this reasoning along with matching the overall blade weights to one another, the c of g will also be identical for both rotor blades. I like to have the balance points set matched within a pencil mark width. In a nut shell, make the blades weigh the same then slide the weight in the slot to acquire the same spanwise c of g. Normally .250″ edge distance is enough space between the weight and the end of the slot. Remember to mark the weight with the respective slot noting balance location or slot weight position.
After reading the above paragraph one can reasonably assume that if the blades weigh the same and a teeter-totter blade balance or similar device is level, the individual blade spanwise c of g’s will match. It is all in how you use a tool so to speak. The next step is to bond the lead to the blade. There is positively no need for a mess here.Apply tape over the slot making sure the marks for weight location are visible. Cut the tape out which covers the slot. Mix up some epoxy and fill the slot about 1/3 full. Clean the lead with lacquer thinners and insert it into the proper blade and location. Trowel and add epoxy as required to acquire a level bubble free surface. Do the same with the other blade. Remove the tape carefully and wait until the glue cures. Any slight ridge may be sanded or filed away. Later you should recheck weight and c of g while admiring your fine glue job. Mass specifications should still be very close but can be fine tuned with a tiny drill using lead shot epoxied in place, if need be near the weight groove.
Finally the dreaded covering job! Heat shrink is easiest but does not adhere to the wood as does sticky back covering. The covering overlap should be 5mm and started at the lower trailing edge. The finishing edge will be trimmed in line with the trailing edge, overlapping the 5mm initial starting point. Make sure the covering starts at the same root position since you don’t want to throw out the span wise balance. Carefully work the covering throughout this stage avoiding wrinkles and bubbles. Even though you have carefully covered the blades the matching will not be perfect even though it will be very close. Don’t worry because this may be used to an advantage. When you install the blades to the rotor head during “assembly balancing” try both positions noting which mounting situation requires the least amount of corrective action.
There are several tools available for final main rotor hub and blade balancing. The key here is to make sure the blades are lined up properly. Moving a blade chordwise c of g forward of the feathering axis is called sweeping the blade forward, moving the blade behind the feathering axis is commonly referred to as sweeping the blade aft. Through this sweeping action one will find a position where the blades will remain at a horizontal flat pitch with the pitch links removed (zero sweep). The blade bolts should be tightened to maintain this chordwise balanced location. It is now time to finish the overall assembly (spanwise) balance either by using a Dubro balancer or adjusting for a level teeter-totter condition using the flybar as the fulcrum. Blade tape or extra covering may be used at the tip for final compensation. Even though it may slightly alter the individual blade c of g it will not be enough to matter, and you are after all completing a final assembly balance. You may alternately add weight in the form of washers under the blade bolt and/or use a longer bolt since again corrective mass location isn’t going to matter for practical reasons.
Last but not least is the flybar and head assembly balance. This can be balanced without the blades installed. Make sure both paddles have the same distance from center using a calipar. Small collars may be installed on the bar and slid/secured to a position for balancing. The Dubro balancer works very well for this task by mounting the head assembly onto a dummy shaft.
Sometimes one may inherit a set of improperly balanced blades. Well, the weight is already bonded and the blades are covered so some alternative method might be sought. A thin strip of lead can be glued and pinned at the inboard blade root, space premitting and/or extra raps of covering applied at the tip. These two methods should allow a quick salvage of the c of g situation and get you flying. Even though there are always options, you will have to be the judge of any particular situation. If in doubt ask others for sound advice.
People can argue the finer points of blade construction and balancing, but the end appearance through smoothness of operation is what matters. Hopefully this has given you a realistic insight into basic tools and methods for balancing rotor blades. Happy balancing!