Ya, I know, very basic stuff, but now we get back to your high school physics class - specifically Newton’s third law of motion. You know the one that states that "for every action there is an equal and opposite reaction."
It is pretty easy to understand how this law applies to all helicopters. That big rotating rotor is creating a lot of reactive torque in the opposite direction of rotation. Therefore, if our rotor is spinning clockwise, our helicopter wants to spin counter clock wise. This reactive torque is not constant either. Any change in the engine power to the rotor or change in collective pitch will produce a corresponding change in torque.
There are basically three ways helicopters deal with this reactive torque:
This is called a coaxial rotor and because the rotors are each rotating in opposite the directions, the reactive torques from each are canceled out.
With electric toy and micro coaxial RC helicopters , this changing of torque between the two blades is very simple. Two electric motors are used to spin each blade. If one motor is slowed down slightly and the other sped up, there is a corresponding reduction of torque produced by one of the rotors and and increase in torque from the other - heli will turn. This makes control simple and inexpensive.
I didn't think there was a MD 520 RC NOTAR helicopter out there yet, but to my surprise, the brilliant minds at the Vario RC helicopter company have one - here she is in action.
Most helicopters use tail rotors to control yaw – RC and real alike. The main reasons for this... tail rotors are simple and they work. In our RC helicopter world there are two ways to power the tail rotor and adjust the amount of thrust they produce.
You have to be able to adjust the amount of thrust coming off the tail rotor because as we learned earlier, the amount of reactive torque from the main rotor is always changing. The other reason is so we can turn the helicopter in both yaw directions (left and right).
Controlling Tail Rotor Thrust With Pitch
The most common method is by using a tail rotor that gets its power from the main engine or motor of the helicopter. By means of gears, some of the power going to the main rotor is taken down the tail boom by using a drive shaft or belt. This power is then used to drive the tail rotor through a simple right angle gear box. The rpm of the tail rotor in this set up is dependent on the engine/main rotor rpm which is more or less constant, especially in a hover.
Our Helicopter controls the amount of tail rotor thrust by changing the pitch or angle of attack of the tail rotor blades. The tail rotor is basically a variable pitch propeller – we increase the pitch of the tail rotor to provide enough thrust to counter act the torque of the main rotor.
If we apply even more pitch, we create more thrust force than main rotor torque force and the helicopter turns against the main rotor reactive torque.
The picture to the right shows a large amount of pitch producing thrust to the right - this turns the heli to the right.
If we decrease the pitch of the tail rotor to the point where it is not producing enough thrust to counter act the main rotor torque, the helicopter turns in the direction of the main rotor reactive torque.
This picture illustrates a left helicopter control tail rotor command. There is very little pitch and next to no side thrust. The heli will turn to the left due to the reactive torque of the main rotor not being canceled out by the tail rotor.
Controlling Tail Rotor Thrust With Speed
The other method that a RC helicopter controls tail rotor thrust is by using a dedicated small electric motor to power the tail rotor. This method is very popular on most small micro and mini RC electric helicopters, but never used on larger heli models.
Instead of getting power from the main engine, there is a small electric motor at the end of the tail boom that powers the tail rotor. Because the speed of the motor is controllable, there is no need to use a variable pitch tail rotor. The thrust force can be controlled by simply speeding up the motor to create more thrust or slowing it down to create less thrust.
Several advantages of this system are: less weight(no tail rotor servo and push rods), no complex gearing and drive linkages, and it is inexpensive.
The disadvantage is the tail rotor control is vague and you are always 1 step behind what is going on because the motor won't react fast enough to give good holding power. To tell you the truth, electric tail motors are rotten in most cases and you should avoid them at all costs. If you have read the
heading hold gyro
section, you already know why we want and need fast acting tail rotor control.
Tail Rotor Example
Let’s examine exactly what is going on here. If we have a main rotor that is spinning clockwise the reactive torque wants to spin the body of the helicopter counter clockwise. To counter act that force, we need our tail rotor to be pushing thrust out to the right – in effect trying to rotate the helicopter clockwise. When the counter clockwise force from the main rotor is exactly balanced out by the clockwise force from the tail rotor, our helicopter will have no yaw movement.
As you can see, tail rotors work very well at counter acting the reactive torque from the main rotor. There is just one problem. Remember that the torque from the main rotor is always changing. What do you think that makes controlling the tail rotor like? If you said “almost impossible,” you understand this process very well.
Unlike full size helicopters, our smaller RC helicopters produce violent changes in reactive torque with just the smallest engine speed or pitch adjustment. This is yet another reason why RC helicopter control is so challenging.
Thankfully we have the gyro to help us out. They take care of all these sudden torque changes so we can focus mainly on cyclic and lift helicopter controls. There are even Heading Hold gyros now that not only correct for the sudden torque changes; they actually keep the helicopter pointing in the same direction. If you are hovering with the nose pointing north, that is where it continues to point unless you give it a command to turn – amazing!
I have just one more interesting item to note regarding tail rotors. It has nothing to do with the actual yaw helicopter control, but it is something you might notice when hovering. Because the tail rotor is pushing sideways to counteract the main rotor torque, we have to apply a little bit of right or left cyclic to balance out the sideways thrust of the tail rotor.
For example – if our tail rotor is producing thrust to the right to counteract the torque from a clockwise rotating rotor blade, it will be trying to push the helicopter to the left. To balance this out, the main rotor will have to be trimmed to provide a little bit of right cyclic so the helicopter doesn’t continue drifting left.
I only mention this because a lot of people will notice their helicopter is tilted slightly to the right or left when in a stationary hover. Many will think they have set something up wrong or the heli is out of balance. This is not the case at all; your helicopter is simply doing what it has to in order to balance out all the various forces acting upon it.
Out of all the RC helicopter controls that have been discussed in the Theory and Control section; yaw is usually the easiest to understand. It also requires the least amount of concentration from the heli pilot. In other words H.B.G.E. (Heli Breezy Gyro Easy!)
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