Chapter 4 : Helicopter Weight and Balance

In this chapter, we will tackle the helicopters' weight and balance just as we have tackled in the fixed-wing aircraft. There are differences between the fixed wing and the rotary wing on the weight and balance consideration because obviously, the helicopter can fly vertically and hover. There is designed weight on every helicopter whether it's a full scale or an RC model depending on the engine size, main rotor blade diameter and pitch. Just don't go beyond what is stated in the manual and you won't go wrong. A light aircraft flies better than a heavy one.

The distribution of weight is as important as the weight consideration in a helicopter to fly it properly. A poorly balanced helicopter is difficult to fly and also dangerous because it might cause damage to people and property if it went out of control. Usually, the center of gravity is located along the main rotor shaft ( see Fig. 7 ). The reason is the main rotor head is the pivot point of the three major axis ( see Fig. 12 ). So whenever you are in a hovering flight, it will not be as hard to control the helicopter. This is very important on the first stage of learning. All the controls should be stabilized.

Looking at the illustration ( see Fig. 19 ), some RC pilots prefer to adjust the center of gravity in front of thw canopy to simulate the full scale helicopter. In this situation, the aircraft is out of trim, meaning you have to create a counter acting force to stabilize it. Since the the center gravity is ahead of the main shaft, the main rotor will tilt by itself ahead and have a tendency to fly forward instead of hovering first before transition to forward flight. So in order to offset the tendency to tilt down, we have to apply a force on the opposite side like pulling the control stick backwards which in turn will tilt the swashplate that controls the main rotor mass so it will fly in the vertical direction like hovering or vertical take-off and landing ( see Fig. 20 ). So if you are just learning to fly an rc heli, the best choice is balancing it with the center of gravity in lined on the main shaft.

Fig. 19 Center of Gravity Located Ahead of the Main Rotor Shaft

Fig. 20 Application of Opposite Force to Counteract C.G.

So then how will you balance it? unlike a full scale heli that weights tons, the helicopter model can only be balanced using your two hands with your fingers. In a real helicopter, balancing it requires calculation of weight distributed againts the distance from the fulcrum or the place where we want the canter of gravity to be located. In this case main shaft is our desirable place. ( see Fig. 21 ) In this illustration, we should equalize the moments on both sides to balance the aircraft. Distance multiplied by the weight is the "moments". Using some simple algebra we can get the result and can determine it both sides are equal.

Fig. 21 Illustration of Weights and Moments in a Helicopter

In a model RC helicopter, all we have to do is to use both fingers on both sides of the flybar. Lift it slowly approximately 2 inches from the surface and see if the skid is parallel to the surface or ground. By moving or relocating the internal components like the receiver, servos, batteries, gyro we will be able to balance it. But, in some cases where relocation of radio components is inevitable, putting some lead weights on the front or inside the canopy is effective.

Now we go on the balancing of the main rotor blades. This is one of the most important aspect of set-up. Unbalanced rotor blades results in excessive vibration, loss of power which in turn affects the flight characteristics of the heli. It also might cause the damage of the chassis, radio equipments inside the helicopter. So before attempting to start the engines and fly your heli, make sure the main rotor blades are balanced.

Using a propeller balancer, the main rotor assembly which includes the rotor head, blades,flybar and shaft should be placed on the balancer in order for it to rotate freely. If the blades are not balanced, drilling some holes or slots on the lighter blades and putting some lead is an effective method.

Since a helicopter generates in own lift, in an engine failure, it cannot glide like an airplane. It will rely solely on the momentum of the rotating blades with out power. So in order to accomplish this, there is a feature called "auto rotation". The main rotor blades will continue to rotate until it losses it's momentum. To improve this, the blade tip is inserted with lead tip weights. ( see Fig. 23 )

Fig. 23 Lead Tip Weight for Auto Rotation