July 31

RC Transmitter Controls – Throttle


I hope that the website has or is giving you sufficient information for you to make a great start in your journey to mastering model plane flight. In the next sequence of posts I aim to give you a more detailed insight into the way your rc transmitter controls effect the plane in flight.

This post will deal with the THROTTLE and how it actually works. The information is valid for both Glow and Electric planes so lets ask the question:- “what does it do?” Here is a picture of the transmitter showing the two primary control sticks. The Throttle and Rudder stick is on the left.

Transmitter Controls

 RC Transmitter Controls – Throttle Adds Power

In essence, if you open the Throttle you feed more “juice” to the power unit, whether that be a Glow engine or an Electric motor. More fuel for a glow engine and more amps for an electric motor. The consequence of this is that more “Power” is developed. This extra power will show up as an increase in the propellers rotational speed. As the propeller speeds up it moves more air which results in greater “Thrust”.

The four basic forces acting on your model are:-

Thrust (created by the power unit),

Lift (generated by the lying surfaces),

Weight (the total mass of the materials in your plane)

Drag (the resistance of the air to the total frontal area of the plane).

When all four forces are in balance, the plane will be flying straight and level at a consistent speed because Lift is exactly matching Weight and Thrust is exactly matching Drag.

Lift,Thrust,Weight & Drag

So what happens if we increase our throttle setting so that the propeller is spinning faster? First of all we get more power which results in increased thrust. Now thrust is greater than drag and the end result is that the plane will speed up until once again thrust is equal to drag.

The actual value of the increase in speed will depend on the way drag is increased and this is mainly affected by the type of model. A sleek smooth profiled plane will create less additional drag and a greater increase in forward speed than, say, a WW1 biplane  with lots of rigging and blunt nose, two sets of wings and inter-plane struts causing the drag to increase rapidly resulting in a much smaller increase in speed for the same amount of added power. This basic principle holds good for any model and the final result will be an increase in airspeed.

 Other Effects

One might imagine that this was the end of the story. Unfortunately this is not the case! Aerodynamics are riddled with complications and anything we do to change a single dynamic will have other knock-on effects.

Increasing the power to your engine or motor not only increases forward airspeed, it also increases the amount of lift created by the flying surfaces. There is a classic lift equation, which I’m not going to bore you with, which says that for a given wing, a particular air temperature and air density, the amount of lift we get depends on both the Angle of attack (AoA) of the wing and the forward airspeed.

angle of attack

 You will notice on this diagram that a component (CP) called the “Centre of Pressure”. This component of lift moves forward on the wing as we increase the Angle of Attack. The centre of pressure of an aircraft wing is the point where all of the aerodynamic pressures may be represented by a single force vector.

I don’t want you to worry about this as you would need to investigate aerodynamics for greater understanding. All you need to know is that the greater the angle of attack, the further forward this vector moves and the nearer we get to the stall condition of the wing. Creating a stall is not desirable when learning to fly so we’ll leave this for another day.

For a simple life all we need to know is that when we are flying our model lift comes from two basic components, the Angle of Attack (AoA) and the Airspeed. So taking these two things, our lift results from the amount we bend the air flow over the wing influenced by AoA and how fast it bends due to the airspeed.

Varying these two influences we have two methods of controlling the flight of our plane. Firstly we can increase the AoA and fly more slowly so we are creating a larger bend in the airflow or we can reduce the AoA and fly faster so that most of our lift is coming from the speed at which we bend the airflow. The way we vary these two components gives us the ability to infinitely control the flight pattern.

You need to understand that it is entirely your decision  how you apportion this lift but you can only do this if your plane is correctly trimmed to allow proper use of the throttle.

Meet the Throttle Banger

This is the guy who knows only two throttle settings. At take-off he bangs the throttle hard forward to maximum and never touches it for the duration of his flight until he wishes to land. At this point he pulls his throttle stick back to the lowest setting and drops in on complete idle. This kind of flying really doesn’t need a proportional throttle facility, all that is needed is a High and Low switch setting.

We all enjoy a bit of speed now and again but flying in this manner means that our pilot is missing out on so much more enjoyment. This flying is basically two dimensional and is ignoring the third dimension. To get the most out of our flying experience we need to be able to control the balance between AoA and Airspeed.

In my next post I will be discussing the Elevator but I need to mention it at this point for you to understand how we trim our plane for straight and level flight. Our elevator is the main control by which we are able to vary the AoA. I will go into this relationship in greater detail next time but for now just accept that the elevator is used to “Trim” our plane to fly straight and level at different speeds.

If you were to suggest to our throttle banger that he fly a little slower he would probably tell you that if he did, his plane would start to fall out of the sky – and he’s probably right! Why? Because it is trimmed to fly flat out all the time. Basically he is relying on airspeed alone to create all his lift so to keep his plane airborne he needs maximum speed all the time.

Getting In Trim

Trim Adjustment

Mode 2 Transmitter Trims

So what is our best option if we’re to avoid becoming another Throttle Banger? Providing our plane has a well matched power plant the best setting for straight and level flight is somewhere between half and two thirds throttle setting. Achieving this setting is simplicity itself. All we do is to get our plane flying in a straight line at about half throttle and adjust the elevator trim on the transmitter so that the model retains its altitude in level flight.

If you struggle to get your model to fly straight and level at this throttle setting, it suggests that you model is underpowered. If you can obtain level flight at very low throttle settings then your model is overpowered. The later of these characteristics is more desirable than the former.

Having obtained this cruise setting with a good balance between airspeed lift ( from power thrust) and AoA lift (from elevator trim setting) you have the best of both worlds. With the throttle at your cruise setting you fly straight and level, increase the throttle a little and you start to climb, reduce the throttle below cruise setting and your plane will start to descend.

From this you will begin to appreciate that your throttle isn’t actually your speed control, its your UP and DOWN control.

Wherever you are in a circuit the ability to control this up and down motion is an ideal scenario but where it really comes into its own is during landings. When you are set up on your final approach you can use the throttle, not elevator, to control the descent onto the landing area.

If you find yourself landing short, a little more throttle will add lift and you’ll make progress toward your ideal landing point.

If you are a bit high on approach, reduce the throttle to increase the rate of sink.

Using the throttle to control the rate of descent, instead of using elevator, will give much better control over your landings. Excessive use of elevator in the final stages of a landing approach can cause difficult stall conditions, the last thing you need in this situation.

OK, that’s it for this post, the first of four explaining your rc transmitter controls. If you’ve enjoyed this post why not visit my website: www.rookiercflyer.com where you’ll find a whole host of useful information aimed at the newcomer. I’ll see you next time with an in depth explanation of Elevator Control.

Happy flying.

Colin