by John Salt - Updated June 2022
The RC helicopter rudder (generally referred to as the tail rotor), makes up the last of our RC helicopter controls - specially Yaw & Anti-Torque control!
Let's determine how we counter torque and control yaw on our RC helicopters, along with pros and cons of various yaw control methods.
Basically what works and what doesn't, dependent on type & size of RC helicopter.
Let's start with Torque!
Torque and helicopters go hand in hand. What is torque?
A nice basic description of torque is rotating force.
I think you can already see how that applies to all RC helicopters.
Helicopters have a very large rotating mechanism and coincidentally it is called a rotor – obviously the name implies "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 "For every action there is an equal and opposite reaction."
It's pretty easy to understand how this law applies to all helicopters. That big rotating rotor is creating a lot of torque reaction", which I often call "reactive torque" that occurs in the opposite direction of main rotor 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 main rotor speed (RPM), or change in rotor blade pitch angle (collective) will produce corresponding changes in torque reaction.
Basically, the more drag (the more the main rotor blades "bite" into the air), the more reactive torque is produced. This drag is not affected only by rotor speed and pitch angle, but also dependent on factors such as size/shape of the rotor blade and the number of blades on the rotor head.
Tri or multi blade rotor heads for example, will generally produce more reactive torque when compared to a two bladed rotor head because they produce more drag; given the same rotor speed, pitch angle, and size of blades.
Regardless of size, speed, shape, or number of main rotor blades, there are basically three ways helicopters deal with this reactive torque they all produce:
1. Tail Rotors
The most common way to counter the main rotor torque reaction is by using a tail rotor. In our hobby, 2 bladed tails are by far the most common, but you will see 3 and even sometimes 4 on some scale RC helicopters.
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 type of tail rotor is known as the Fenestron tail rotor (derived from the Latin word fenestra for window).
This is essentially a multi bladed, variable pitch, ducted fan tail rotor housed in an oversized vertical tail fin assembly.
It is rarely seen on RC helicopters due to the added cost, weight, complexity, poor tail authority (on smaller RC helicopters), and cross wind challenges, over that of a traditional tail rotor.
In full size helicopters, the main reason fenestron tail rotors are used is for safety.
Statistically, one of the most common injuries around full size helicopters is from people accidently walking into the spinning tail rotor, often resulting in a fatality .
If you've ever been around a full size helicopter while its running, you know the spinning tail rotor can become totally invisible in certain light conditions. A fensestron does away with that danger.
The other safety benefit is to the helicopter tail rotor itself. It's less likely a tail/boom strike will cause tail rotor damage on a fenestron. Tail rotors getting caught in power lines, guy-wires, etc. are also much less likely with a fenestron.
For our RC hobby however, the main reason to have a fenestron is because they look cool and they are more challenging. It has always been a long time desire of mine to eventually build a scale heli with a fenestron tail for those two reasons and I was fortunate enough to finally get my first fenestron heli kit as shown below.
2. Coaxial & Tandem Main Rotors
The second method an RC helicopter rudder control is by using two counter rotating rotor blades stacked on top of one another.
You will find this type of torque & yaw control on most micro electric RC helicopters and toy helicopters.
This is called a coaxial rotor and because the rotors
are each rotating in opposite directions, the reactive torque from
each is canceled out.
Yaw control of full size coaxial helicopters is mechanically 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, and the lift component also remains more or less constant.
On small fixed pitch coaxial RC helicopters, yaw control is much simpler to execute by simply spinning the top and lower rotor at different speeds to each other.
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 reaction produced by one of the rotors and and increase in torque reaction from the other - heli will turn. This makes yaw control simple & inexpensive, yet very effective.
Using two completely separate rotor systems spinning in opposite directions to cancel out the torque of each are seen on Tandem Rotor RC helicopters. These are not at all common in our hobby due to the added cost and complexity, but they do exist. Generally being custom built by true RC helicopter enthusiasts.
There are by the way, a number of micro coaxial tandem rotor RC helis on the market, but they are basically using a pair of coaxial rotors. The torque cancellation on them is coming from the stacked coaxial rotors, not the fact there are tandem rotors spinning in opposite directions.
The third and least known way of yaw & torque control on helicopters (full size and RC alike) is called NOTAR (no tail rotor).
This system uses air pressure created by an internal ducted fan inside the main body of the helicopter.
This moderately pressurized air is piped down an over-sized tail boom and released out slots that run down the length of one side of the boom, along with an adjustable thrust nozzle at the back end of the tail boom.
The air flowing out the side slots in combination with the downwash of air from the main rotor produces an area of low pressure on one side of the boom (coanda effect) to counter the reactive torque. Basically the boom acts like a big fat airfoil that provides some sideward lift in the opposite direction of the main rotor torque.
I didn't think there was an RC MD 900 NOTAR helicopter out there thinking not enough air pressure could be generated by a small, internal ducted fan, but to my surprise, Vario RC helicopter company has one.
Here it is in action
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 well. 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).
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 diverted down the tail boom by using a drive shaft (usually called a torque tube) or a long rubber toothed belt (belt drive tail).
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 speed change torque spikes.
In general, the tail rotor on most RC helicopters with variable pitch tail rotors will turn about 4 and a half times for each single revolution of the main rotor (a 4.5:1 ratio).
We control 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 equilibrium between the main rotor torque and the tail rotor thrust so the heli will not spin/turn at all.
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.
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.
The video I made below goes over RC helicopter tail rotor operation in more detail covering:
The other method that RC helicopter rudder 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 (successfully anyway) 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.
Note, motor driven fixed pitch tail rotors never reverse direction to change thrust, they rely on main rotor reactive torque to yaw the heli one way, and thrust direction from the tail rotor to yaw the heli in the opposite direction.
I only mention this because I get so many question from people new to the hobby thinking there is something wrong with their helicopter or tail rotor motor because it only spins in one direction. Nothing wrong, that is exactly how they all work.
Several advantages of this system are: less weight (no tail rotor servo and push rods), no complex gearing/drive linkages so parts count & complexity is reduced, it's usually more robust and it's cost effective.
The main disadvantage is the tail rotor control and holding ability can be poor.
The larger the RC helicopter is, the worse it gets due to the increase in inertia; ie. the tail rotor can't respond/react fast enough to change the thrust required to yaw the helicopter or keep the tail hold locked.
Yaw control is vague, and tail blowout is common on such helicopters in other words.
To tell you
the truth, conventional brushed electric tail motors paired with anything but smallest of micro sized RC helicopters are rotten in most cases, and you should
avoid them at all costs. They are also under enormous stress and therefore burn out quickly in most applications.
If you have read the heading hold gyro section, you already know why we want and need fast acting tail rotor control.
There are exceptions to this FP motorized tail rotor rule.
Coreless direct drive tail motors can work quite well on smaller size micro helicopters such as XK's line of micro collective pitch helicopters such as the K100, K110 & K120.
This is because the tiny direct drive coreless motor (which has very little spinning mass) in combination with a small fixed pitch tail rotor can both accelerate and decelerate fast enough to provide a decent tail hold when the heli is small & light weight (less inertia to overcome).
As they are not being "over worked" on most micro size helicopters, these little coreless motors also tend to have fairly long lives.
Tail motor failures however, even with this small size are still one of the biggest complaints. Most people who have helis such as these generally keep a spare coreless tail motor on hand for replacement.
If you fly often, it's not a matter of if a coreless tail motor will fail, but when - they are a consumable item.
Taking it one step further, brushless direct drive tail motors such as on the Align 150X, OMP Hobby's M2 & M1, Eachine's E160/180 and most of Blade's smaller helicopters work almost as well on these tiny RC helicopters as variable pitch tail rotors work on the larger ones.
The tail hold with a direct drive brushless motor on these micro size helis is outstanding! It's all but impossible to get a blowout to occur, even when trying hard to induce one.
Powerful direct drive brushless motors speed up and slow down very quickly giving impressive thrust control rivalling variable pitch.
Brushless motors also last a very long time, even when under such demanding workloads and I've personally never had to replace one (yet).
I often get asked what is the largest size heli that a fixed pitch, motor driven tail can work well with.
There is no one specific size generalization because as we have already discussed, there are several variables involved including tail rotor motor type, weight of the heli (amount of inertia the tail has to overcome). Along with main rotor size, shape, rotational speed, and number of blades, which all affect the amount of reactive torque produced.
With that said, it has been my experience that brushless direct drive tail motors can work well on micro helis up to absolutely no larger than 250 size (spinning 250mm main rotor blades).
Coreless direct drive motors work well up to about 120 size (spinning 120mm main rotor blades).
Conventional brushed (not coreless) direct or gear driven tail motors don't work well regardless of size - stay away from them!
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 thrust 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 large full size helicopters, our smaller RC helicopters produce violent and lightning fast 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 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.
With the introduction of Flybarless systems, the tail gyro is now built in the flybarless unit along with the two cyclic gyros.
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 on day 4 in my flight school section.
Because the tail rotor is pushing/thrusting sideways to counteract the main rotor torque, the heli has to be 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 tendency (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. The smaller and lighter the heli in relation to the amount of tail thrust produced, the more pronounced the lean will be.
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 to remain in a state of hover equilibrium.
Out of all the RC helicopter controls that have been discussed in the Theory and Control section; RC helicopter rudder control is usually the easiest to understand. It also requires the least amount of concentration from the heli pilot thanks to the gyro.
I'll conclude this page with a couple videos. First is one I made of basic maintenance to a variable pitch tail rotor. I'm only including it because it may help a few people understand the operation of a variable pitch tail rotor if they can see the various components of one close up.
The next video is a good "full size" helicopter controls torque video. The exact same principles apply to our smaller model variety. If you look closely at the full size helicopter (R22) in the video, you will even see the slight lean caused by tail rotor translating tendency cyclic compensation.
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