Differential on Ailerons for Radio Control Aeroplanes
Several posts ago I gave an in-depth explanation of Setting Up Servos For RC Planes. As a result of this I was asked by a couple of readers for an explanation of the need for differential on ailerons for radio control aeroplanes.
This is an interesting subject and quite an advanced subject that doesn’t normally concern those learning to fly rc planes. Most trainer type models do not suffer from this phenomenon because of the way they are designed. However, as the question has been asked I will bite the bullet and try to explain things in as simple a way as possible.
You may ask why aileron differential is necessary and what it is. Let me handle the first question then we can go on to discussing how it can be achieved.
In the diagram on the right the view shows an aeroplane that has control inputs trying to cause a right hand turn. The left aileron has moved downward and the right aileron upward.
The theoretical result of this control input is to create greater lift under the left wing and a reduced lift under the right wing. As a consequence the plane should bank over to the right and there will be a tendency for it to start a turn to the right. This is then helped by the input of a small amount of up elevator to increase the overall lift generated by the wings.
Unfortunately, there is a complication associated with the change of lift under each wing. The down going aileron increases induced drag of the left wing to a greater extent than that of the right wing. Lift and drag are inexplicably linked – the more the lift, the greater the drag. You can see in the diagram the drag arrows at the wing tips are out of balance, that at the left wing being greater than at the right wing.
The initial effect of this imbalance in the increased drag is to try to rotate the plane to the left around its horizontal axis. This is what is meant by Adverse Yaw and is a problem that arises in many types of aircraft.
The usual way of counteracting it is by applying a rudder input in sympathy with the aileron control. In other words, initiating the turn by applying right rudder marginally before applying right aileron. The effect of this is to commence a rotation around the horizontal axis which reduces the lift of the right hand wing.
Once this occurs the application of the right aileron turn input becomes more effective and the desired right hand bank is achieved.
Why Aileron Differential?
I have just explained that Adverse Yaw can be counteracted by using rudder so why the need for Aileron differential?
Well, many people who fly rc model planes find co-ordinating rudder input and aileron input together somewhat difficult. So to make life easier it is possible to use aileron differential to accomplish the same result.
First of all what is aileron differential? Quite simply it means that the ailerons are arranged so that they move more in one direction than the other. Usually this is more UP than DOWN.
I have already mentioned the fact that a DOWN going aileron generates more induced drag than an UP going aileron. It is the extra drag that causes the adverse yaw that we want to avoid.
If we arrange for the aileron that goes up to move more than the one that goes down, we reduce this additional induced drag whilst still achieving the bank angle we need to make a turn.
Arranging Aileron Differential
We can arrange this differential in at least three different ways.
- Raked Torque Rods
- Offset Servo Output Arm Position
- Via Transmitter Programme
We’ll take each of these in turn.
Raked Torque Rods
The torque rods we are talking about are those attached to the control surfaces and connected to the ‘servo arms’ by the ‘pushrod’. In the illustration here it is called the ‘control horn’.
You will find that on most planes the control horns are fixed to the underside of the control surfaces, unlike this illustration.
Here we see the normal correct arrangement where the clevis connecting holes are perpendicular to the hinge line. This guarantees equal movement either side of the neutral position. We need to change this so that, with the control horn below the surface, there is more movement upward than there is downward.
To achieve this the coupling of the clevis connection to the control horn needs to be forward of the hinge line. The further forward of the hinge line we set this point the greater the differential between up and down aileron movement.
Ok, I know this looks like a complicated drawing but let me try to explain it before you run away in despair. It’s not so difficult to understand.
First of all take the servo. As and Bs represent the linear movements either side of the neutral position when the transmitter aileron stick is moved to its maximum deflections both up and down. These are identical distances.
Now move right to the circle describing the arc of potential movement for the control arm or horn. The dark line at the bottom shows this attached to the underside of the control surface and extending down and forward to a position where it has a ‘forward offset’.
The two dotted lines linking the servo output arm to the control arm show the range of transferred movement to this horn at full deflections fore and aft.
Finally we see the range of movement of the trailing edge of the control surface as Ac and Bc. it is clear to see that the down movement (Ac) is considerably less than the up movement (Bc).
This has created a differential of movement that reduces DOWN aileron but increases UP aileron. As UP aileron does not induce so much drag as DOWN aileron we have balanced out the tendency of the drag to cause an adverse yaw about the horizontal axis to the left.
I hope that this has made some sense to you.
Offset Servo Output Arm Position
This is the alternative mechanical solution to our problem. Most servos currently available have a round toothed output shaft or spline to which the output arm is attached using a small self tapping screw positioned centrally.
This means that the output arm can be positioned on the toothed shaft in a wide array of positions. Normally this position is when the connection between the arm and the pushrod are at right angle to each other (90 degrees).
To achieve our differential movement at the aileron with more up than down, we need to position the servo output arm with a measure of forward bias.
In the diagram on the right we have positioned the servo output horn several degrees forward of the perpendicular line through the spline axis. The result is that the linear pushrod movement As will now be less than Bs.
As Bs is responsible for driving the control surface upwards, with the pushrod running left to right, you can see that the surface UP movement will be greater than the DOWN movement.
This has achieved for us exactly the same result as the Raked Torque Rod/Control Horn in the previous example.
Via Transmitter Programme
Most modern digital transmitters have programming facilities that enable the pilot to make adjustments to the magnitude of control movements in each direction independently.
This means that by going into the transmitter setup mode one can increase or decrease the upward or downward, left or right movement of each control surface. You will need to follow the radio gear manufacturers instructions to use this facility.
Summarising Differential on Ailerons
I hope that the above explanation has not proved too difficult for you to absorb. I know it is a fairly technical aspect of Radio Controlled model airplane flying but it is important to have an appreciation, first of all, why Adverse Yaw occurs and, secondly, how we can eradicate it.
If you are having difficulty understanding my explanation, I suggest you talk to your club tutor about differential for ailerons and how to set it up mechanically. In my opinion adjustments at the transmitter should be a last resort, not the first port of call for solving this kind of issue.
If you have enjoyed this post, be sure to visit my main site www.rookiercflyer.com to find loads more useful information for beginners.
Best of luck,