Mounting and Balancing Rotor Blades

Mounting and Balancing Rotor Blades

ABOVE : This is a picture of the author, taken some years ago, with two of his scale helicopters. It took a lot of work to get these reflex wooden rotor bades sanded, covered, and balanced, but just watching them in flight made it all worthwhile.

UPDATED :  

Rotor blades have come a long way since I entered this hobby more years ago than I care to admit. Way back then, we had the advantage of buying rotor blades not only of varying lengths but also of varying chords and airfoils. We also had rotor blades with square tips, angled tips, round tips, a tapered blade from root to tip, and some blades even had washout that allowed the rotor blade to delay the stall at the blade tip. Different airfoils were also available. Some blades were flat bottomed, which were particularly useful for scale applications, while other blades had a reflex airfoil for improved autorotations; and of course, there was the full symmetrical airfoil for aerobatics. That was the good news.

Weighing-rc-helicopter-rotor-bladesA digital gram scale, accurate to 1/10th of a gram, makes It easy to find the weight of each rotor blade. These scales are Inexpensive and readily available at office-supply stores.

The not-so-good news was that all of these rotor blades were made out of wood, and none came ready to fly. They were fairly rough-cut from wood, so each had to be finely sanded to produce a smooth surface. Each blade was covered with a vinyl-type material that had an adhesive on one side, much like shelving paper.

This material had to be trimmed in such a way that the edges would not catch the air or the covering would be blown off in flight. The next step was to cut the covering around the root of the blade so that a mounting reinforcement could be glued in place. As a final step, you had to put a little epoxy on the exposed wood tips to keep them from attracting dirt and moisture.

As you can imagine, this was a tedious process and one that required a certain amount of knowledge, skill, and patience.

In this day and age, I don't think you can even find one of these wooden rotor-blade kits—that is, except in my attic, where I might have one or two left. The good news is that we are now in the buy-and-fly mode. Although we don't have the variety of different types and styles of rotor blades as before, it's much easier to purchase a set of composite rotor blades, install them on your helicopter in a couple of minutes, then go fly. In most cases, this works out very well. The companies do an excellent job of matching rotor blades so that they are ready to fly right out of the box.

Because I am old school, however, I still check the blades' balance to discover if they need a little tweaking here and there. I certainly don't think we can eliminate all vibration in our helicopters, but this extra effort at least will reduce the potential for unwanted vibration. Let's take a closer look at how I do it.

BALANCING ROTOR BLADES

Checking the chordwise balance of your rotor blades is a very simple, quick, and inexpensive process. All you will need is a round pencil and a suitable gram scale, which can be purchased at an office-supply store for less than $20.

The first step is to weigh each rotor blade on a gram scale, which should be accurate to 1/10 of a gram. It's great if both blades weigh the same, but that is rarely the case. If their weight is slightly off, place a small piece of covering material on the light blade until the weight matches that of the heavier blade.

It's better to apply a slightly larger piece of covering than needed, and then cut off a tiny amount as you continue to weigh the blade.

Keep the backing on the covering material to cover the sticky side, however, and do not apply the covering material until the next step.

Wooden rotor blades RC helicopter

ABOVE : Wooden rotor blades came with many different airfoils to optimize this type of flying. They were also available with standard square tips, round tips, orthe angled tips shown here.

On a flat, smooth table (without any fans or air-conditioning going), place the heavy rotor blade on a round pencil and rotate the pencil to find the center of gravity (CG).

Mark the balance point, which is centered on top of the pencil, with a pencil and transfer this mark exactly to the light blade. Place the light rotor blade on the pencil and move the covering material as needed to bring the blade in balance over the mark you just transferred.

Be careful when applying the covering to the blade: Wrap it evenly around the leading edge of the blade so that the oncoming wind will press it in place. Each rotor blade now will have the same weight and the same balance point, and they will be ready to mount onto the helicopter.

MOUNTING ROTOR BLADES

Mounting rotor blades requires a certain understanding of how each rotor blade reacts in flight. As the helicopter maneuvers, the lift throughout the rotor disc changes to vary the lift. This is accomplished by changing the angle of attack of each rotor blade as it completes one revolution.

As the angle of attack is increased on one portion of the rotor disc and decreased on another, the helicopter will rotate in the direction of least lift. As the angle of attack of the blade is increased, however, the induced drag of that rotor blade is also increased. This increased drag will have a tendency to move the rotor blade back from its normal extended position.

Radio control helicopter rotor blade design

ABOVE : It was common for wooden rotor blades to come with slots that would accept lead strips to add weight to the blades. Light rotor blades were used for aerobatics, while heavier ones were used for hovering and scale applications.

As you can imagine, even the slightest movement of the rotor blade will cause the entire disc to vibrate.

A rather obvious solution would be to tighten the rotor blade to its blade grip to eliminate this movement. That is virtually impossible, however, because we have no idea when the rotor blade is in its correct and fully extended position.

Therefore, when mounting the rotor blade to the blade grip, it should be mounted loose enough to allow the centrifugal force to bring it into its correct position but not loose enough to where it will swing freely back and forth as the angle of attack changes. The technique I use is to tighten the rotor blade to the grip just enough so that it will maintain its position when the rotor disc is rotated 90 degrees but still loose enough that it can be moved with your hand.

REPAIRING ROTOR BLADES

As a general rule, it's almost impossible to repair a damaged rotor blade. Years ago, with wooden rotor blades, it was possible to remove the covering and make minor repairs to the blade. Because everyone now uses some type of composite rotor blade, however, these repairs are very limited.

If you go through the above balancing technique with a new set of rotor blades, they should be as vibration-free as you can get them. I have seen, however, where a good set of rotor blades seem to lose their balance over time, as witnessed by a slight amount of vibration in the helicopter. Because rotor blades can't just lose their balance, there has to be a reason—and here it might take a little detective work to find the cause.

RC helicopter blade balancing

ABOVE : By balancing the rotor blade at two different angles, the chordwise CG can be found at the Intersection of the two balance lines. A more forward CG Is used for stability, while an aft CG Is better for aerobatics.

One of the first things I check in such a situation is the mounting hole on the rotor blade. Large, heavy rotor blades, turning at a high rpm, produce a tremendous amount of centrifugal force, and it is possible for the mounting hole to elongate slightly. Even the smallest amount of elongation will take them out of balance. If this is the case, however, I have not found a suitable repair method. The blades have given you all they have to offer, and it's best to replace them and install a new set.

A more common problem is what we used to call "hangar rash." This is just the normal bumps and bruises everything gets from normal use. It could be that you hit the blade on something or scraped the tips slightly in the grass or dirt. If this is the case, it should be fairly easy to repair with some CA or filler, as long as the problem is not near the structural part of the blade. And once the repair is made, be sure to go through the balancing technique I mentioned above. A smooth helicopter with a well-balanced set of rotor blades will allow your helicopter to perform at its best and give you many  hours of fun flying.