Chapter 2 : Why and How Helicopters Fly

Now we are familiar with the parts of a RC helicopter from our previous chapter, now lets look deeper on why and how helicopters fly. Obviously the rotating blades of a helicopter are responsible why it can defy gravity. Looking at the cross section of the airfoil shaped rotor blade, it has an angle of attack or what we call the pitch ( see Fig. 6 ). As the rotors turns and achieved the velocity needed for the aircraft to lift the ground, it generates a downward force ( see Fig. 7 ). As we have tacked in our previous chapter, the fixed pitch type is only dependend upon the speed of the engine but the collective pitch can vary the pitch angle so even at maximum RPM it can fly or hover whatever altitude you desire ( see Fig. 6 ).

Fig. 6: Illustration of Heli Blade Airfoil

Fig. 7: RC Helicopter in Hovering Flight

The way helicopters fly is very similar to a fixed wing aircraft on the aerodynamics side. The only difference is helicopters don't need to move forward to gain airspeed for it's wings to be effective. In fact it can generate it's own lift. That's right. Because of the rotation of it's rotary wing, unlike a fixed wing aircraft, it can produce it's own lift ( see Fig. 8 ) The airfoil section of the rotor blade below shows how it goes against the relative wind, the air that passes through an airfoil.

Fig. 8: Airfoil Section Along the Relative Wind

Among the heavier-than-air machine, a helicopter has a unique ability of hovering flight. It can fly suspended in the air (see Fig. 7). All the forces that acts in a helicopter during hovering are balanced: Lift = Weight & Thrust = Drag. Since there was no forward motion, Thrust & Drag is equal to Zero. So how will it move forward? In order to do that we should create an unbalanced situation for it to move forward. Provided that the C.G. ( center of gravity ) is located within the main shaft of the main rotor. Later I will discuss the reason regarding the importance of C.G. location. Again going back to the imbalance topic, if we want to move the heli forward, we should create an uneven distribution of lift. Making the lift behind cyclic rotors' lift less than that of the front so that the main rotor mass will tilt forward (see Fig. 8). The control mechanism responsible for controling the tilting of the main rotor mass is the swash plate.

Fig. 8: RC Helicopter in Forward Flight

Theoretically the forces that acts on it will look like Fig. 9. Since the helicopter is moving thorough the air, it behaves differently from hovering since forward flight need more power. With a little analysis using using right triangles, you can see that there will be a additional lift requirement on the lift component, the resultant force: lift-thrust vector depending on the angle of tilt. That will be the resultant of the thrust and lift vectors see Fig. 9. ( Sorry if this is quite confusing).

This explains why the helicopter needs less power while hovering that in forward flight. You will notice this when you are actually hovering a helicopter and transition to a forward flight, a slight loss of altitude will occur so you will increase power to maintain altitude until it gains momentum and climb its way up. Like wise, when you want to transition to a hovering flight again, it will suddenly gain altitude and you will decrease the power.

Fig. 9: RC Helicopter Vectors

Now we will tacke the phenomenon called gyroscopic precession. If you apply a force on a rotating disk it will not tilt on the location where you applied the force, instead it will react 90 degrees away from it on the direction of rotation. So in flight, when you are controlling the rotor blade mass, for example you want it to fly forward, you tilt the rotor forward but actually the control input is 90 degrees before the direction of rotation ( see Fig. 10 ). Again as an example, if you are looking at the top view of the helicopter, and the main rotor blades are rotating in a clockwise motion when you want to tilt the main rotor blades in the forward direction, you are actually making the control input on the left side and then because of the law of gyroscopic procession you will notice that the main rotor is tilted forward. You can observe this when watching an RC helicopter on the ground about half throttle. And later you will know how it is controlled using the flybar mechanism.

Fig. 10: Gyroscopic Precession Phenomenon

Since we have explored the rotary wings' some important facts, let me add some more regading the distribution of lift. I guess this not very important just an additional knowledge. From the tip to the root blade, the thrust is not equal. The tip has the greatest thrust and decrease as you go to the root. This is because the angular velocity at the tip is much greater the root ( See Fig. 11).

Fig. 11: Angular Velocity of The Main Rotor Blades

Do you know that a real helicopter is very limited on the rotational speed of it main rotors? That's a fact. Model or RC helicopters can be designed to rev up a very high speed rotors. The reason behind it is the length of the main rotor blades. Longer rotor blades can approach to an angular velocity to the speed of sound ( or mach 1 in other terms ). When a body, particularly an airfoil approach the speed of sound, it will behave differently than below mach 1. That is why full-size helicopters need to keep their angular velocity to a minimum ( See Fig. 11).