Tail Rotor Used To Both Counter Torque & Provide Yaw Control

Torque and helicopters go hand in hand - but what RC helicopter controls are used to correct for torque, and for that matter what exactly is torque?

A nice basic description of torque is "rotating force". I think you can already see how that applies to RC helicopters. Helicopters have a very large rotating mechanism and coincidently it is called a rotor – obviously the name refers to rotation.

Yes, 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."

Reactive Torque

It's 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 clockwise. This reactive torque is not constant either. Any change in the engine power to the rotor or change in rotor pitch (collective or cyclic) will produce corresponding changes in torque.

There are basically three ways helicopters deal with this reactive torque:

  • The most common way is by using a tail rotor. The tail rotor is simply a propeller that provides thrust in the opposite direction to the reactive torque produced by the main rotor. The idea is for the thrust coming off the tail rotor to equal the amount of reactive torque to keep the helicopter from spinning around - we call this movement YAW.

  • Another way a helicopter controls yaw that is very popular with micro electric RC helicopters and toy helicopters, is by using two counter rotating rotor blades stacked on top of one another.
Counter Spinning Rotors Cancel Out Reactive Torque From Each Rotor

This is called a coaxial rotor and because the rotors are each rotating in opposite the directions, the reactive torque from each is canceled out.

Yaw control of the coaxial set up on real or hobby grade helicopters with collective pitch is very complex. It requires changing the pitch angles of each rotor to induce a corresponding change in reactive torque to the left or right while the rotational speed of each rotor remains constant.

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 yaw control simple & inexpensive, yet very effective.

  • The third and least known way of yaw control on helicopters is called NOTAR (no tail rotor). This system uses air pressure created by an internal ducted fan inside the helicopter. This moderately pressurized air is piped down an over-sized tail boom and released out an adjustable thrust nozzle at the back of the tail boom. In addition, the tail boom is also shaped so the air flowing past it from the main rotor downwash gives some side thrust. It's basically a big fat airfoil that provides some side ward lift in the opposite direction of the main rotor torque.

I didn't think there was a MD 520 RC NOTAR helicopter out there yet, but to my surprise, Vario RC helicopter company has one.

Here it is in action

Tail Rotors

Most helicopters use tail rotors to control yaw – RC and full size 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 (yaw) the helicopter in both directions (left and right).

Controlling Tail Rotor Thrust With Pitch

Variable Pitch Tail Rotor Driven By Main Rotor Motor Through Gears

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 (torque tube) 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, greatly minimizing torque spikes.

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 that can vary the amount of right & left thrust to yaw the helicopter in either direction. With just the right amount of pitch, it provides the correct counter thrust to maintain a state of equilibrim between the main rotor torque and the tail rotor thrust so the heli will not spin/turn at all.

Lots of Tail Rotor Pitch  Blowing Air Right Turning The Heli Right

If lots of pitch is applied, more thrust force from the tail rotor is produced 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 tail rotor pitch blowing air to the right, thrusting the tail to the left causing the heli to turn/yaw to the right.

Very Litte Tail Rotor Pitch Providing No Thrust Resulting In Main Rotor Torque Spinning Heli Left

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

Electric Motor Driven Fixed Pitch Tail Rotor Varies Speed To Adjust Tail Thrust

The other method that a RC helicopter controls tail rotor thrust is by using a dedicated small electric motor to power a fixed pitch 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/motor, 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/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.

Coreless Direct Drive Tail Motors Work Pretty Good On Micro Sized RC Helicopters

There are exceptions to this rule however. Coreless direct drive tail motors work well on smaller size micro helicopters such as the Blade mSR & 120SR. This is because the tiny motor and fixed pitch tail rotor can both accelerate and decelerate fast enough to provide a decent tail hold. Taking it one step further, direct drive brushless tail motors on the Blade mCPx and Trex 150DFC work almost as well on these tiny RC helicopters as variable pitch tail rotors work on the larger RC and full size ones. 

RC Helicopter Controls - 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 blowing air out to the right, thrusting the tail to the left – 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 (in a state of tail thrust to main rotor torque equilibrium), our helicopter will have no yaw movement at all.

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 and very fast acting 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.

Enter The Tail Gyro

Tail Gyro Provides Automated Yaw Detection & Correction

Thankfully we have the humble little 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!

Simple Vector Analysis

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 which I also cover in my flight school section. Because the tail rotor is pushing sideways to counteract the main rotor torque, the heli has to trimmed out to have a little bit of right or left cyclic to balance out the sideways thrust of the tail rotor. This can either be done by dialing in a little right or left cyclic trim or is done automatically by the flybarless system gyros if you are flying FBL.

For example – if our tail rotor is blowing air out to the right, thrusting the tail to the left to counteract the torque from a clockwise rotating rotor blade, this thrust will also be pushing the entire helicopter to the left. The full size helicopter term for this is called left tail rotor translating (LTRTT). To balance this out back into a state of equilibrium, the main rotor will have to be trimmed to provide a little bit of right cyclic vector 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 and remain in a state of hover equilibrium.

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 thanks to the gyro.

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