August 7

RC Transmitter Controls – Ailerons

Time to get yourself a little light refreshment so you can concentrate and absorb information on another of the rc transmitter controls. Today we are going to look at the Ailerons.

What Do Ailerons Do?

 

aileron roll

As you can see from the diagram above, when we move one aileron up and the other down the plane responds by rolling in the direction of the “UP” going aileron. So what is happening here? Quite simply, the “UP” going aileron is reducing the effective lift of the wing to which it is attached and the “DOWN” going aileron is increasing the lift of the wing to which it is attached. Because of this imbalance of lift, the plane will naturally roll toward the “UP” going aileron.

Lets look a bit more deeply into why this happens. A wing creates lift by bending the air flowing over its surface. The amount of this bend depends on three things. 1) the Aerofoil section of the wing, 2) the Angle of Attack (AoA) and 3) the speed at which we bring about this bend. If we increase the speed at which this bend happens by increasing our forward airspeed, increased lift results. If we bend the air more we also get more lift.

Assuming we dont speed the aircraft up, then it is the second point we are interested in.

Creating Bend

Thicker more cambered wings create more bend than thin wings at a given airspeed. Irrespective of the aerofoil shape of a wing, we can increase the amount of bend in the air by changing the AoA (Do you remember what this stands for?).

As the AoA goes up, so does the obstacle presented to the on-coming airflow hence it has to bend even more to get round this profile obstacle resulting in more lift.

What we can learn from this explanation is that changing the AoA is in effect changing the “Camber” of the wing so deploying an aileron does exactly this. A “DOWN” going aileron increases the camber of that part of the wing and creates more lift whilst an “UP” going aileron descreases the camber, reducing the lift. Now we know how ailerons work.

Using Ailerons In Flight

 

Aileron Effects

Let us assume we have a model plane flying straight and level with all forces in balance. In this situation we know that the amount of lift created by the wings is exactly equal to the weight of the model. Lets assume our plane weighs 10lbs (approx 4kg) then lift is also 10lbs (4kg). We apply some left aileron control from our transmitter without moving any other controls. The plane willl roll or bank to the left.

I’m going to have to get a little technical now because lift will always remain perpendicular to the plane of the wings. So we have to consider this lift as having two components.

1) Vertical lift

2) Sideways Turning Force

Turn lift-weight distribution

In this diagram we can see that the lift is inclined to the left. It can now be represented by two forces, once vertical and another at 90 degrees to the vertical force pointing toward the centre of our turning circle. We give this force a special name – Centripetal. The sum of these two forces is equal to the lift which has remained unchanged. The lift is still 10lbs (4kg) but the vertical component is now only about 8lbs (3.64kg). This means that we no longer have sufficient vertical lift to maintain the same altitude.

We have a problem now! We no longer have enough lift to keep our plane in the air and it will start to descend. The only thing we can do is to increase the overall lift so that the vertical component is equal to the weight of the plane. This is essential if we want our plane to turn without loosing altitude. If you recall, when up elevator is applied more lift is generated by increasing the AoA. This puts a bigger bend in the airflow resulting in more lift.

I want to get back to our ailerons for a moment. The right one is down, bending the air more and increasing lift. The left one is up, bending the air less and producing less lift. You are now adding elevator, increasing the AoA and putting a bigger bend in the airflow. The righthand wing is experiencing two lots of bend increase, aileron induced and elevator induced.

This double effect is the weakest link in our lift chain. It is easy, if we are not careful, to push the overall AoA to a point where the air is no longer able to follow the wing camber and we hit the dreaded STALL condition. The result is a sudden flick to the right out of the turn.

Lift and Drag

I have told you that lift happens as a result of our wing aerofoil causing a bend in the airflow. Now let me also tell you that air does not really want to bend in this way, it prefers to flow on in a straight line! Force is required to cause the air to bend, the bigger this bend the more force is required. Where does this force come from? It comes from the “Induced Drag” that results from the bend forced into the air.

There are two types of drag, the first is “Form Drag” created by the shape of the wing as it pushes air out of the way to move forward. This drag does not change, irrespective of whether the aileron goes up or down. The second is the “Induced Drag” caused by the change in the bend forced into the air. The down going aileron increases the lift induced drag  whilst the up going aileron decreases it.

The important thing to understand here is that there can be no lift without drag. The more lift we create, the more drag is involved.

Adverse Yaw

Adverse Yaw

The next problem to overcome is caused when you employ the ailerons to bank the airplane, the drag on it becomes asymmetric (look this up in the dictionary). The drag on the wing with the down going aileron increases and the drag on the wing with the up-going aileron decreases. Asymmetrical drag causes the plane to Yaw in the direction of the increased drag (to the right in our example) in the direction of the up-going wing, not what we desire at all. This effect is known as “ADVERSE YAW” and can cause a plane to point its nose out of the turn.

Turns done in this way look untidy  but more seriously,  if the model is yawing out of the turn then its nose is not pointing in the direction it is travelling. We call this “Side-Slipping”, an attitude that causes added drag and a loss of airspeed.

Let’s Review the Aileron rc transmitter controls effect

Here is a picture of the transmitter showing the positions of the four main control functions. The Aileron and Elevator stick is on the right.

Transmitter Controls

Imagine your model is in a turn with ailerons deployed. The wing on the outside of the turn is causing a bigger bend in the air than the inner wing so that it can create more lift and hold a bank angle. This bank angle is partly redistributing the overall lift inwards to cause the plane to turn but at the same time loosing some vertical lift. You compensate for this loss by applying a little up elevator to increase the AoA and obtain some more lift to maintain altitude. Add to this the fact that due to adverse yaw, the plane is side-slipping and loosing airspeed to the detriment of the lift.

The reaction may be to apply more up-elevator to increase the AoA and generate more lift. Now the outside wing tip with down going aileron is in even further danger of overstretching its ability to sustain smooth air bend. This is a particularly difficult situation to find yourself in especially when coming on to final approach with possibly lower airspeed, a definite recipe for a stalled wing. Experienced pilots will usually make this final turn with a nose down attitude and gentle descent to avoid to much airspeed loss. A low approach can always be extended by applying a little more throttle once the plane is straight and level again.

Attitude Correction

What can we do to combat Adverse Yaw? This is not a trait common to all models but where it occurs there are two remedies.

1) Using Rudder

2) Employ Aileron Differential

Method 1) involves applying some rudder in the turn to push the nose into the turn. Many people do this manually which means that turns are a combination of controlling ailerons, elevator, rudder and throttle, all four principle controls. This is good practice and worthwhile mastering when you come to fly more advanced models.

Method 2) involves reducing the amount of downward throw compared to upward throw of the ailerons. This is called “Differential Throw” where upward travel is greater than downward travel. The effect of this setting is to reduce lift on one wing instead of enhancing lift on the other wing.

Aileron Differential

If you are using only one centre servo to drive both ailerons this setting will normally be created by a mechanical offset of the pushrod/ servo output arm/ control surface horn. If you have two servos, one in each wing, this differential can be set up again mechanically or on your transmitter through endpoint adjustment or by using a specific aileron differential menu option available on some transmitters. I suggest you discuss this with your tutor although most purpose designed trainers rarely suffer from this problem.

I think that’s covered just about everything there is to be said about how the ailerons  via your rc transmitter controls work so we’ll take a break and come back again next time with something different (and perhaps not quite so complicated!) -Enjoy.

 

Colin

 



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Posted August 7, 2015 by Colin in category "General Flight Training

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