RC Transmitter Controls – Rudder
This is probably the most underused control on most radio controlled planes. Many beginners probably wonder why its there because they get to use it so infrequently. Actually, its one of the most important rc transmitter controls for your model. Its use, to keep a plane going in a particular direction, is a skill that is the hallmark of an accomplished pilot. It is infrequently used on its own but mainly in combination with other controls.
When you are learning, unless you have a three channel plane, the rudder tends to be overlooked so that you can concentrate on the other controls. The problem is that once you have learned to fly without using the rudder, it is tempting to carry on that way and ignore its correct application.
You can fly without using the rudder but you can’t fly well without using it!
What It Does
The rudder is usually a part of the airplane fin or vertical stabilizer assembly right at the back of the plane. This is because the fin needs to be behind the CofG (You remember what this is of course). To understand why this is necessary, imagine trying to fire an arrow from a bow backwards!
Below is a picture of the transmitter showing the main control stick functions. The Rudder and Throttle stick is on the left.
Our vertical stabilizer performs the task of keeping the model flying straight. On a full size airplane the stabilizer usually has a symetrical aerofoil section so that when everything is lined up and the plane is flying straight into wind, the fin has a zero AoA which produces no net sideways force. If anything happens to cause the plane to try to turn off course then an AoA is developed and, as with a wing, lifting force is produced to push the tail of the plane sideways, rotating it about the CofG and re-align the plane into the airstream.
In this instance we have ignored the rudder as it performs this task along with the fin as one assembly. Maybe we don’t want our plane to be totally in alignment with the oncoming airstream. Possibly other forces are acting on the model, pushing it out of line with such force that the fin assembly alone is incapable of rectifying the situation. Now you need a rudder.
Last time we discussed the effect of changing the AoA on a wing using ailerons to create lift differential. Changing the AoA changes the wing camber to create more or less induced drag and the consequential increase or decrease in lift. The same principles apply to our rudder.
You need to consider your fin and rudder as a vertical wing. You can demonstrate this to yourself with your model. stand behind it and apply some right rudder with your transmitter stick. You will see the aerofoil shape of your fin changing. it becomes a cambered wing shape with the underside of the new shape on the right side. This will produce a net lift force to the left which pushes the whole tail to the left, rotating the model around its CofG and causing the nose of the plane to turn right. Of course left control input will have the opposite effect and push the tail assembly right and the nose to the left.
When is it Used?
Many trainers feature tricycle (trike) undercarriages with a steerable nosewheel. This nosewheel is often controlled by the same servo as the rudder. When the rudder moves to the right, so does the nose wheel and similarly, left rudder will provide left turn of the nose wheel.
During taxi-ing to the take-off point the rudder control will be used to steer the plane on the ground. Providing the steering has been correctly set up and the model is pointing directly into wind, there should be no further need for rudder control during the take-off phase.
Tail draggers are different! The tailwheel is usually connected to the rudder again to provide a steering function as before, so left rudder will push the tail of the plane to right and the plane will turn left. Conversely, right rudder will push the tail of the plane to the left and the plane will turn right.
Now, taildraggers have a distinct tendency to swing to the left as they accelerate forward under take-off power. This is due to a torque reaction to the rotation of the propeller.
The solution is to hold a little up elevator to keep the tailwheel in firm contact with the ground and add small amounts of right rudder to keep the model running straight into wind. Once the plane is running with some speed, release the up elevator to allow the tail to rise so the model is moving forward on its front wheels. Retaining the up elevator could cause the plane to take-off before you have sufficient airspeed, resulting in a dangerous stall condition. You will find this technique a bit tricky at first but with practice you will develop the feel for the models responses and master it.
Rudder In Flight
So we’ve dealt with the use of rudder during take-off so now let’s consider what it does once the plane is in the air. You need to understand that the rudder doesn’t behave as a steering control in the air. All it really does is to swing the nose, temporarily, to the side. A quick application of rudder will not turn the model and as soon as the rudder input is released the plane will swing back to its original flight path. Rudder input alone does not make any long term changes to the models direction of flight.
This pure Yaw effect is used in a variety of situations.
1) In combiation with a “dihedral” wing to initiate a turn.
2) Where a model has a tendency to suffer “Adverse Yaw”, it is used in conjunction with the ailerons to counteract that tendency.
3) To control orientation and correct heading of the plane during a landing approach.
4) To initiate a “Stall Turn”.
5) To hold up the nose of the plane during Knife-edge flight.
6) To help maintain altitude during a “Rolling Circle”.
As a rookie, unless you are using a three control model, you can ignore number 1. Also you can forget about numbers 4, 5 & 6. You are not going to be learning these maneouvers in the early days.
We’ll briefly discuss No. 1 just in case you have gone for the three channel model. When a Model with dihedral is made to Yaw across the on-coming airflow the outside wing attains a greater AoA than the inner wing and so produces greater lift (We’ve discussed the reasons for this in the previous posts). This causes the plane to bank in the direction of the Yaw and the use of Elevator causes the plane to turn in sympathy with the Yaw. I don’t intend to go any more deeply into this explanation as I am assuming most readers will have decided to learn on a four channel plane.
Although most trainer types are design to eliminate “Adverse Yaw” it is possible, under some circumstances, to find your plane being affected by this problem. Instead of going through the explanation of this again, I take this opportunity to refer you to its dicussion in our previous post on rc transmitter controls – Ailerons.