August 14

RC Transmitter Controls – Elevator

Hi again, go get yourself a cup of coffee (or tea) and lets look at another of your RC Transmitter Controls. This time we’re going to investigate the Elevator, that bit at the very back of your plane that moves up and down to alter the attitude of the model.

Pitching Up and Down

The complete assembly at the rear of the plane comprises a Horizontal Stabilizer (the fixed bit) and an Elevator (the moveable bit). Some aircraft have an all moving tailplane where the stabilizer is pivoted and moves about this pivot. Most trainer planes have a fixed stabilizer and moveable elevators so this is the arrangement we will concentrate on.

Elevator Control

What you need to understand is that the elevator is not used to make the plane go up and down! Before we deal with the elevator we need to look at what the complete tailplane (stabilizer and elevator) actually does. Many people are under the impression that the tailplane holds up the rear of the plane. in fact  on most planes the tailplane generates downthrust instead of lift.

How is this I hear you asking. Well, its all to do with plane’s Centre of Lift in relation to its Centre of Gravity. The centre of lift is the point through which all the models upward lift can be considered to act. The centre  of gravity is another point through which all of the models downward weight is acting. A stable model is one where the centre of gravity is in front of the centre of lift. This means that under normal conditions the lift is acting behind the centre of gravity, it tends to push the tail of the model up resulting in the plane wanting to dive into the ground. To prevent this happening we have to create “down push” on the tail to keep the nose up. This the job of the tailplane assembly.

This effect can be physically tested in your workshop. Usually the centre of pressure (centre of lift) is about half  way between the wing leading edge and trailing edge (50% of wing chord). Support your model at this point under both wings. It will immediately fall nose forward because the centre of gravity is in front of where it is supported. Push down on the tail end of the model treating it like a see-saw to make it sit level. The force you use to achieve this balanced position is the force required of the tailplane to keep the plane level.

What the Elevator rc transmitter control does

Quite simply the elevator moves up and down to change the effective aerofoil  cross section of the tailplane. This varies the amount of balancing down-force delivered by the tailplane. As a consequence this variation in the force pushing the tail down you can make the model adopt differing pitch attitudes from nose down to nose high depending on how much you vary the force. So what the elevator does is control the pitch-attitude of the model. Here is a picture of a transmitter showing the two main control sticks, the Elevator and Aileron stick is on the right.

Transmitter Controls

From a practical aspect, raising the elevator creates more tailplane down-force pushing the back of the model down and so the nose goes up. If the elevator goes down, tailplane down-force is reduced and the planes natural nose down tendancy dominates and the tail comes up.

Putting Theory into Practice

You’re flying along straight and level and you decide to squeeze a little up-elevator. What happens now? You have increased the down-force produced by the taiplane causing the model to “pitch-rotate”  and the whole fuselage pivots around the Centre of Gravity (CofG), sending the tail down and the nose up.

Now we must consider the implication of this pivoting and its effect on the wing. Providing the wing is firmly fixed to the fuselage (as it should be) then the wings’ Angle of Attack  will increase. This will increase the lift coefficient produced by the wings’ aerofoil section and so your wing will create more lift.

Angles of attack

Before this our model was flying straight and level because lift was equal to model weight. Now we have a surplus of lift over model weight so the effect is to bring about an upward acceleration that pilots call Climb. If you recall our previous post we decided that an increased throttle setting was what caused our plane to climb!

Let me finish and you’ll see more clearly. Because of our elevator change the model is sitting slightly nose high and and as a consequence the wing is presenting a larger area to the on-coming airflow. This has  created more drag and as we haven’t touched the throttle the forward thrust from the engine is still the same. This now means that the drag now exceeds the thrust and so airspeed decreases, slowing our model down.

Things are getting a little complicated now so stick with me and concentrate.

I’m going to say this twice:


Once again:


Got that? Good, hang on to it. What does this mean from a practical viewpoint? Well what it means is that quite small changes in airspeed can cause larger changes in lift.

Lets take an example. If we reduce our airspeed by only 10%, our lift goes down by almost 20%, nearly twice as much! If as a result of this reduction our lift falls below that required for level flight then the model weight will exceed the lift resulting in a downward force. Newton’s law of gravitational force tells us that our model will take on a downward acceleration and the model will start to descend.

The interesting observation here is that our use of elevator has given us both climb and descent! It can’t be both so which is it?

Up or Down or What?

To get a true answer to this question we need to know a great deal more about the aerofoil of our wing, an area I don’t propose to go into here. Different aerofoils have different lift to drag ratios at different angles of attack (AoA). these ratios will effect the way our plane behaves.

If for a given aerofoil the lift grows faster than drag then our model will climb but if drag grows faster than lift then our model will descend. Now true as these statements are, apart from some very specialised aerofoils,  the Lift:Drag ratio for many common ones is pretty constant and we can assume that the two effects often cancel each other out.

The actual outcome from changing the angle of attack slightly is that the model will experience an initial amount of climb which is not sustained. As the drag increases and the airspeed falls off the model will end up flying straight and level again but with a slight nose high attitude and at a slower airspeed.

The result is that we can determine the main function of the elevator is to change the angle of attack. It determines that, for a given amount of thrust, the proportion of lift due to airspeed and the proportion that is due to the wings aerofoil effect resulting from its lift coefficient.

  • Up Elevator = less speed-generated lift and more aerofoil lift
  • Down Elevator = more speed generated lift but less aerofoil lift

Having said all this, there is a limit to the amount of angle of attack change you can bring about using your elevator. You can’t keep getting more and more lift by increasing the angle of attack. The defining point of the limit is the Stall Point.

Wing airflow

In this diagram the upper wing aerofoil is moving through the air creating lift whereas the lower wing aerofoil has reached an angle of attack that has caused the air to delaminate from the upper surface of the wing. this results in a failure of lift generation and the plane will stop flying. This usually happens when the angle of attack reaches around 15 – 20 degrees.

As we increase the angle of attack the lift increases smoothly until the critical angle is reached at which point it decays almost instantly. The sudden nature of this transition  necessitates the avoidance of this critical angle of attack. We have now discovered another function of our elevator. Not only does it determine our angle of attack, it also causes our model to stall.

Understand this: “Aeroplanes do not stall because they are flying too slowly. They stall because the elevator stick is too far back and the resulting angle of attack is beyond the critical value. this can happen at any airspeed and in any attitude”.

In Conclusion

The elevator is probably one of the most important rc transmitter controls. It does not control height, it controls angle of attack and the proportion of lift you gain from different sources. It is possibly the most important control. You can fly without one aileron and you can fly without a rudder but you can’t fly without an elevator. Always be aware of the potential for excessive elevator use pushing you into the stall danger zone.

Next time I’ll discuss the ailerons in detail. Don’t forget to pop in on my website:, you’ll find lots of useful information to help you.

Keep practicing.




August 12

Find Centre Of Gravity

This post covers a very important aspect of setting up your model plane to fly correctly and be fully controllable once airborne. Any plane, whether it be full size or a model, needs to be balanced both in pitch (fore & aft) and laterally (side to side). Today we are going to look at the factors affecting the “Pitch” stability.

The Pitch Stability of your model plane depends almost entirely on you locating the correct Centre of Gravity (I apologise to my American readers for the English spelling of “centre”). So it is most important that you find Centre of Gravity (CofG) of your plane at the outset. The term “pitch stability” refers to the longitudinal balance of the plane whilst flying. A nose heavy pitch will cause the plane to be unresponsive to controls and drop its nose when power is reduced. A tail heavy pitch will make the plane climb excessively when power is increased but,even worse, it will be over responsive to the controls and, in the very worst case, impossible to fly.

The neutral point and centre of gravity of a model plane
Neutral Point & Centre of Gravity

If you purchase an ARF plane the correct CoG position should be provided by the manufacturer. In the first instance ensure that your plane balances at this position. This should ensure that your plane is flyable from the word go.

In our diagram above the CoG is shown both ahead of and behind the “Neutral Point” and the “Aerodynamic Centre”.

Neutral Point (NP)

Without going into detail, this is a mathematical analysis of the longitudinal static stability of a complete aircraft (including horizontal stabilizer or tailplane) that yields the position of centre of gravity at which stability is neutral. The significant word here is “static”, i.e. when the plane is stationary without lift being generated. This position is called the Neutral Point.

Aerodynamic Centre (AC)

 It has been found both experimentally and theoretically that, if the aerodynamic forces on a plane are applied at a location 1/4 of the way from the leading edge of a rectangular wing at subsonic speed, the magnitude of the aerodynamic moment remains nearly constant even when the angle of attack changes. This location is called the wing’s Aerodynamic Centre and because it is affected by dynamics during flight and has to take account of lift generated by the wing, it is normally positioned slightly ahead of the Neutral Point.

Top view of plane with constant chord wing
Constant Chord or Rectangular Wing

In order to obtain good Longitudinal Stability the Centre of Gravity (CofG) should be as close as possible to the main wings’ Aerodynamic Centre (AC). This occurs because the lift due to the horizontal stabilizer has only a very slight effect on conventional R/C models.

Most model aircraft with rectangular wings (typical of a trainer design) have an “Aspect Ratio” of between 4.5 and 8.5 . Now I hear you asking: “What is the Aspect Ratio (AR)?”  This is the ratio of wing span (the full length of the wing, tip to tip) to wing chord ( the width of the wing, leading edge to trailing edge). So if we have a 60″ wing span and a 10″ wing chord, our Aspect Ratio is 60/10 = 6. The maths to find a formula for locating the position of our CofG is fairly complicated and I don’t intend to strain your brain cells with such tedium. So here is such a formula that you can use to calculate the CofG position for any plane where the Aspect Ratio (AR) falls within the range 4.5 to 8.5.

CofG position = 0.1 + (0.4 x V-bar)

Here we have a new term to explain. “V-bar” is the “tail volume ratio”. In other words the size and area ratio of the stabilizer compared to that of the wing and its distance behind the aerodynamic centre. Practically it is found by the following method:

1)  Measure wing span – S

2)  Measure wing chord – C

3)  Mark a point 25% of the wing chord from leading edge of wing.

4)  Measure the “mean” stabilizer chord (mark a point halfway along one half of the stabilizer and measure the chord at that point) – TC

5)  Mark a point 25% from leading edge of the stabilizer at this “mean” chord position.

6)  Measure stabilizer span( tip to tip both sides of the stabilizer) – TS

7)   Measure distance from the 25% wing mark in (3) to the 25% stabilizer mark in (5) – TA(tail arm)

Stabilizer Area(SA) = TS x TC       Wing Area(WA) = S x C     

V-bar = (SA/WA) x (TA/C)

So now let’s repeat the final formula for your CofG:

CofG position = 0.1 + (0.4 x V-bar)

depending on the units of measurement you have used, this figure will be the number of those units measured from the wing leading edge to your Centre of Gravity location.

Worked Example

Let us do a calculation based on some typical dimensions for a trainer plane:

S = 60″      C = 10″    TC = 6″     TS = 20″     TA = 28″

Stabilizer Area (SA) = 20 x 6 = 120 sq.ins.

Wing Area (WA)= 60 x 10 = 600 sq.ins.

V-bar = (120/600) x (28/10)

= 0.2 x 2.8 = 5.6

CofG position = 0.1 + (0.4 x 5.6) = 2.34″ from wing leading edge

I know all this sounds complicated but if you take the time to do a practical exercise with your trainer you’ll find it is easier than you think. Try it and have some fun, it will be interesting to see how close you are to the kit manufacturers suggestion.

Check Your Centre of Gravity

Once you have calculated the theoretical CofG position it’s time to check it. There is a very simple way to do this using just your fingertips.

Finding The CofG Using Fingertips
Fingertip CofG Locating Method

Mark the position of the CofG on both wing tips lightly in soft pencil and using just one finger of each hand place them under the wing tips in alignment with the marks and gently lift. If your plane is correctly balanced it should remain horizontal. If it drops its nose it is nose heavy and if the tail drops, it is tail heavy.

Usually slight adjustment to the location of either the receiver battery in a nitro plane or the lipo in an electric plane should correct the situation. If such adjustments do not correct the balance then you will need to add some additional weight either to the nose or the tail. Because the nose moment is much shorter than the tail moment, the amount of weight for the tail will be proportionally less than for the nose.

Tool to find Centre of Gravity

You can actually buy a special balancer for more accurate results. Below is a photo of this device:

A frame for finding the CoG of a model plane
CoG Balance Frame

You can purchase this frame on-line in the USA by clicking on the image. UK visitors can buy using this link: CofG Balancer

I really hope that you will have a go at this practical exercise to find centre of gravity for your own plane. Take your time and I think you will feel a sense of achievement at the end of it. Good luck and come back and let me know how you get on.


August 8

Tuning Nitro RC Engine

This time I am dedicating a post to those of you who have decided to go down the Glow or Nitro Engine power route. Don’t worry if your preference is electric power, I will be posting plenty of material to keep you interested. In the meantime I want to look at how tuning nitro rc engine, glow engine or internal combustion (IC) engine, call it what you will, is achieved.

nitro-engine drawing

I have purposely made this drawing large so that you can see all the relevant part descriptions. Probably the most interesting feature of this two stroke glow engine is the rear remotely mounted High Speed Adjustment Needle Valve. Below is a picture of a carburettor where the High Speed Needle is part and parcel of the assembly.

nitro-engine carburetor

Why would manufacturers want to separate this needle from the carburettor? Mainly for safety reasons so that ones fingers are as far as possible from the spinning propeller whilst making adjustments. When this needle is integral with the carburetor, it is very important to ensure knuckles and fingers do not come into contact with the propeller. Below are photos of glow engines using the two options.

You will notice that the rear needle valve is connected to the carburetor by a short length of silicone fuel tubing.

Why Do You Need To Tune Nitro RC Engines?

Our Two Stroke Glow or Nitro Engine runs on a special fuel made from a mixture of Methanol (the fuel content), Oil ( for lubrication) and, sometimes, Nitromethane (to help idling and transition from low to high speed). This is also the component that gives our engine its “Nitro” name. The manufacturer’s instructions will give you the correct relative percentages of these ingredients appropriate to their engine. Your local model shop is bound to have available supplies of suitable fuels.

Tuning your engine revolves around getting the correct mixture of air and fuel into the carburetor. Air is the largest component by volume. Mixing the two in the carburetor produces a wet fuel gas that becomes the right mix when the volumes of each are correct. When we talk about fuel/air mixtures we relate everything to the fuel content. You will hear reference made to “Rich” and “Lean” mixes. A “Rich” mix indicates to much fuel whilst a “Lean” mix indicates to little fuel.

Just to give you some idea of the mix ratio of Fuel to Air,  for one kilo (2.2lbs) of fuel, we need approximately 4.5 kilos (9.9lbs) of air, a ratio of almost 5:1 air to fuel by weight. So we can start to appreciate that a small increase in fuel content will give a “rich” mixture whereas a small reduction in fuel content will give a “lean” mixture. The mixing of fuel and air for an engine aspiration system has the technical name “stoichiometric” which chemically means the relationship between the quantities of materials that are involved in a reaction. So stoichiometrically, more liquid fuel in the mix means a “Rich” mixture whereas  less liquid in the mix means a “Lean” mixture.

Air enters the engine, not by suction, although this does account for a very small amount, but mainly as a result of atmospheric pressure. This can vary depending on the prevailing atmospheric conditions e.g. hot, cold, wet, etc. These conditions will affect the amounts of air entering the engine at any time. Such changes are very small but may affect your engine tuning at times.

To help you understand how your nitro engine works, let’s take a look at some diagrams.


2 Stroke Engine Diagram
2 Stroke Engine Diagram

For the purpose of this explanation the important areas are:-

a) crankcase

b) intake port

c) combustion chamber

d) exhaust port

e) piston

The main reservoir of air is in the “crankcase“. The air/fuel mixture reaches this area through the carburettor. The “piston” travels down the cylinder and forces the air/fuel mix (fuel gas) up the “transfer or intake port” to the “combustion chamber“. As a result of this transfer a vacuum is created in the “crankcase”. Atmospheric pressure takes over here and draws another shot of air/fuel mix through the carburettor. The two illustrations below should help to clarify this procedure.

2 Stroke Cycle Diagram
2 Stroke Cycle Diagram

How much of each component is drawn in depends on the settings of the “carburettor” so we need to look closely at the two possible adjustments that can be made at the carburettor.

Adjusting A Carburettor

Model aircraft engines usually employ one of two types of carburettors.

1) Fuel Metering

2) Air Bleed

Fuel Metering carburettors have mixture needles at each end of the barrel assembly. These enable adjustment for both High and Low rpm. In the photograph below the main high rpm adjustment needle is on the left and the low speed fuel adjustment needle is in the centre of the control arm nut on the right. When we come to discuss low speed settings we’ll see a view of this needle in close-up.

two needle carb
Fuel Metering Carburettor

An Air Bleed carburettor has a main mixture needle and a small hole located somewhere on the main section of the body and a small bolt with a spring or locknut that can be adjusted to restrict the amount of air entering through this hole.

Air Bleed Carburettor
Air Bleed Carburettor

At this stage it is important to understand the distinct difference between the two types.

Air Bleed Carburettors adjust fuel flow for high rpm but adjust air flow for low rpm.

Fuel Metering Carburettors adjust fuel flow for both high rpm and low rpm.

Please note very carefully; DO NOT touch the slow running setting of a brand new engine! The manufacturer will have set this for you before despatching the engine and in 98 out of 100 cases this will be spot on. If you interfere with this setting you may have great trouble getting it right again so leave well alone.

Having said that, I need to give you the information you need should it be necessary to make an adjustment to this slow speed setting, so here goes. This should only be undertaken once your engine has been run in (see below). Carefully close the idle needle right down as far in as it will go (be careful not to over-tighten it) and open the main needle. Take a dressmakers pin and insert it into the “venturi” (air intake) of the main carburettor body and close the throttle barrel to hold the pin in its position. Fit a length of fuel tubing to the fuel intake nipple and start blowing. You should find that it is completely blocked at this point. Now very slowly unscrew the idle needle until the smallest amount of air from your blowing passes it. That’s it!

Run the engine, set the high speed needle, then come back to idle. Any further adjustment (if needed) will be very small – probably less than 1/8th of a turn. This illustration will show you the effects of idle needle adjustment.

Idle Needle Lean - Rich Adjustment
Idle Needle Lean – Rich Adjustment

To check whether your idle mixture is correct, with engine running at idle, push fully forward on the throttle transmitter control lever. If your engine stops dead your setting is too lean so  open the needle a very small amount (about 1/8th of a turn) and try again. If it now hesitates when you throttle up, it has gone rich and you need to take the needle back almost all of the 1/8th turn you opened it.

Fuel Metering Screw
Idle Needle Adjustment

Note that this method is for a fuel metering carburettor. An air bleed carburettor is adjusted for slow running the opposite way. turning the screw out ( anti-clockwise) increases the amount of air intake and as a result leans the mixture whereas screwing the bolt in (clockwise) reduces the amount of air bleed and richens the mixture.

Now, I will say it again, If at all possible avoid changing the setting of the Idle Needle especially on a new engine.

Running In a Nitro Engine

First of all, why is it necessary to “run-in” a new engine? Manufacturers produce the component parts of an engine to very fine tolerances and often minute bits of metal and other microscopic particles can get left behind. For this reason it is necessary to not only create a perfect mating fit between the various moving parts but to flush out any of these minute foreign bodies. To do this plenty of lubricant and flushing oil is desirable to keep rubbing parts cool and to flush the system.

The method of running in a nitro engine will depend on the type of engine you have. Basically there are two types of two stroke engines,  “non-ringed” or “ringed“.

The ideal way to know when your engine is running at its optimum is to use a”tachometer” to observe the actual rpm setting.


Tachometers (Tachos) are readily available from most model shops and internet suppliers and if you intend to use nitro or glow engines long-term  are a very worthwhile acquisition. The settings you make using this tool will be far more accurate than those made by relying on your hearing alone. If you would like to buy one from a UK supplier click the following link:- UK Tacho or if you prefer to buy from a US based supplier click the following link:- US Tacho.

Non-Ringed Engines

Engines without a piston ring (non-ringed) require a fairly brief and simple running in process and can be done almost at full throttle rpm throughout. Just ensure that the high speed needle is opened a little more than optimum so that the engine is running slightly rich.

The way to do this is to open the high speed needle several turns to ensure a plentiful supply of fuel. Start the engine and gradually turn the high speed needle in a few clicks at a time, waiting between each adjustment, allowing the engine speed to settle. Keep adjusting until the rpm is hardly changing with each click of the needle. Once there is no further increase in rpm for the next click, you have reached the maximum rpm. Immediately turn the needle back at least two clicks and wait for the rpm to settle. This will richen the mixture and provide extra lubrication for the moving parts. The ideal running in rpm will be between 500 to 1000 below maximum. You need to do 6 to 10 runs at this setting by which time your engine should be ready to open up for full rpm.

Ringed Engines

again, 6 to 10 runs are necessary at a rich setting close to top rpm which you will establish exactly the same way as for a non-ringed engine. The technique I use is to start the engine holding the cylinder head. Allow it to run until you can no longer stand the amount of heat building up in the cylinder head. At this point, stop the engine and allow it to cool down completely to absolute cold (no residual heat).

Start the engine again and repeat the finger test. When you have completed the chosen number of runs using this hot/cold technique, start to turn the high speed needle in a click at a time whilst watching the tachometer to see that the engine speed continues to increase. Wait at least 30 seconds between adjustments, especially with remotely mounted needle valve carburettors to ensure everything has stabilised.

You should detect small increases in rpm with each click of the needle. Once you reach a point where the next click fails to cause an increase, you know your engine is running at full rpm. If you continue to turn the needle in a few clicks the rpm will start to drop off or fall back. If you reach a point where the fall of of rpm continues you have leaned your engine mixture too much and it is beginning to overheat. Immediately open the needle one full turn to richen the mixture. This is most important if you don’t want to do permanent damage to your engine. the additional fuel will help to cool the engine and increase the essential lubrication.

Re-adjust the needle to attain maximum rpm again using your tachometer. When you reach the point where the next click does not increase the rpm, you have reached the maximum. Turn the needle back two clicks and watch the rpm to see it doesn’t continue to reduce but settles just below maximum. If it continues to fall it needs a couple more running in sessions.

When you get to the point where your engine will sustain full rpm for at least 5 minutes, you can consider it fully “run-in”. At this point restart your engine and turn the needle out two clicks or until it is about 1000 rpm below maximum. This is the ideal run setting and you should now be able to leave this needle alone indefinitely.

Now that you have learned the technique of tuning Nitro RC Engine and before we leave this post I would like you to consider this point. How often does a car driver use maximum rpm? Ask any full size aircraft pilot how often he opens the throttle to maximum? You will be surprised at the answers. So why should you expect to need maximum rpm from your plane engine continuously? If you need maximum power from your engine all the time, your engine is too small for the plane or the plane is too heavy for the engine. An upgrade is needed!

Food for thought. Happy flying.




August 8

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.

Transmitter Controls

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.


Rudder Function

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.

Nose wheel/rudder servo

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.

Steerable Tailwheel


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.





August 8

RC Transmitter Functions Reviewed

Today I want to discuss with you some of the more significant control functions on an RC Transmitter. As the range of rc transmitter functions become more sophisticated so the available range of adjustments and controls increases.

At one time the only variables on a transmitter were the two control stick gimbals and ratcheted trims at the side of and below these control sticks.

early futaba tx

On this early Futaba transmitter there are just four channels for the main control surfaces and throttle. It appears that the owner of this transmitter was using this with a very simple three control set up, throttle, rudder and an auxiliary control. In fact this could well have been for a boat but we will never know.

You will notice the four trim adjustment ratchet switches to the inside and below the sticks. The central switch is the ON/OFF switch. There does appear to be a fifth control knob on the top of the case the purpose of which is uncertain.

Gradually things like servo reversing switches and dual rate switches started to appear. Today we are faced with an array of buttons, knobs, switches and a liquid crystal display screen (LCD) that would do a space shuttle flight deck proud. Even a basic beginners six channel outfit will have quite a range of switches to contend with.

Transmitter Controls

Here is a modern six channel transmitter showing the principle control setup for Mode 2 operation. Let’s visit each of these controls in turn:

Left Gimbal stick – Throttle & Rudder Control

Right Gimbal Stick – Elevator & Aileron Control

Lefthand Upright Trim Switch – Throttle Trim

Lefthand Horizontal Trim Switch – Rudder Trim

Righthand Upright Trim Switch – Elevator Trim

Righthand Horizontal Trim Switch –  Aileron Trim

We have used this diagram in previous posts to discuss the effects of the main controls. If you look carefully you will see other switches, knobs and buttons arranged around the transmitter case. We will also come to these in due course.

I mentioned that this diagram showed a transmitter set up for Mode 2 operation. There are, in fact, four possible arrangements as illustrated here:-


Modes 3 & 4 are rarely used but there are a good number of more mature flyers, such as myself, who learned on Mode 1 especially in the UK.

Nowadays almost without exception Mode 2 is used to teach new pilots although it doesn’t really matter which mode you learn, all are valid and work. The choice will depend on your tutor’s preferred mode if you learn with a club. Should you be unhappy with your tutors mode, discuss the possibilty of changing to your choice of mode with the club training officer.

If you recall, the last series of posts covered our principle controls and we discussed in depth what each one does. If you refer back to the second image above, you will see the effects of applying these controls displayed alongside the transmitter. Throttle – Climb & Descent, Rudder – Left & Right Yaw, Elevator – Angle of Attack (AoA) and Aileron – Left & Right Roll.

Trim Levers

OK, now we need to understand the use of the Trim Levers. These are positioned alongside the appropriate main sticks and are used to adjust the centre point of each servo. As the name suggests, they are used to trim the plane in flight.

Let us imagine, for example, that just having taken off our plane wants to keep climbing when all controls except throttle are at a neutral position. A couple of clicks of down trim on the elevator should push the tail up and bring the plane into level flight.  Similarly, if the plane wants to roll to the left, a couple of clicks of right aileron trim should level it out for you.

In the early stages of your training program your tutor will make the necessary adjustments for you, eliminating the need for you to concern yourself with them.

On older transmitters these trim levers were of an analogue nature and the lever was left in the adjusted position. Unfortunately many people found that this position was easily changed accidentally after flying had finished. This meant that often adjustment was necessary at the start of each flying session. On modern transmitters these trims are switched digitally and cannot be changed inadvertently when the transmitter is switched off.


This is an adjustment that is made in the transmitters programming mode and allows fine adjustment of the servo centre point when setting up the model before flight. Sub-Trim adjustment is often used to ensure that the servo output arm is exactly at 90 degrees to the servo or that the control surface is exactly at neutral.

Ideally this should be done mechanically by adjusting a clevis at either end of the push-rod. Occasionally this is not possible so Sub-Trims can be used.

End Point Adjustment (EPA)

This is another programmable adjustment that can be used to reduce or extend the amount of servo travel either side of the neutral point. Most servos are set by the manufacturer to move 30 degrees either side of the neutral  or centre point, i.e. 60 degrees of total travel and is referred to as 100% movement.

Using the End Point Adjustment this movement range can usually be increased to 125% or decreased to 0%. This should not be used to correct for major errors in setting up the servo and control surface movement mechanically.

Whenever possible make as many adjustments to obtain the correct range of movement through physically setting the servo arms, pushrods and linkages.

Your End point Adjustments should always be as close to 100% as possible. Setting to much control surface deflection and then reducing it via the End Point adjustment results in lack of servo accuracy and a reduction of servo output torque (the power available to move the control surfaces).

One area where End Point Adjustment is particularly useful is in setting up “Aileron Differential” (more “up” than “down”).

Aileron Differential

Dual Rates (DR)

This facility is usually available via switches on the front of the transmitter, above and adjacent to the main control sticks. In the photo below you can see where this particular manufacturer has positioned a single switch to change rates on Ailerons, Elevator and Rudder for ease of use whilst continuing to control the model with the main sticks.

dual rates 1

The idea behind Dual Rate Switches is to enable small or large control surface movements to be set and available at the flick of a switch. This is most useful when setting up a model for first flights.

Large control surface movements will often make a model over sensitive and unduly responsive during a landing approach. On the other hand it is possible that for some models larger control surface movements are desireable when flying slowly.

On some transmitters the programming will allow this facility to be designated to just one switch for all three control surfaces (as in the above photo) or to single switches for each individual control.

Exponential (Expo)

Another programmable facility, Expo either increases or decreases the rate of response of the servo around the centre or neutral point. The overall range of movement is unaffected by this setting.


setting Expo

Manufacturers treat positive or negative exponential differently. Be sure to check the correct sense when using this facility on your chosen transmitter. Dual Rate and Exponential facilities are often grouped together on the same screen.

Model Memory

This is a facility common to most programmable transmitters and permits the storage of unique settings for a range of different models. Once a particular model’s settings have been entered into the programming they are stored in the model memory location selected.

Often there is also a facility to copy a set of parameters from one memory to another. This can be useful as the basis for setting up another model.

There are a couple of manufacturers that provide a desirable “Model Match” feature that prevents a receiver being activated unless the correct model has been selected at the transmitter.


Nearly all modern transmitters provide at least one timer. The prime purpose of this facility is to warn the pilot when a flight is nearing the end of its chosen duration.

This facility can normally be designated an “up” timer or a “down” timer. The “up” version counts from zero up to a pre-determined duration whilst the “down” setting counts down to zero from a pre-set chosen duration.  Each choice will usually provide audible warnings as the termination point is approached. There is often a choice of switch designation to initiate this facility.

Servo Reversing

This is simply a way to make the individual servo(s) operate in the opposite direction. It is useful where, having mounted a servo and connected it to the control surface, it operates in the wrong sense for your purpose.

Typically if your rudder turns to the right when you give left rudder at the transmitter then a quick reversal in the transmitter programme will rectify this problem.

Fail Safe Setting

Although this is set at the transmitter, it is actually controlled by the receiver. The idea is to set the servos and in the case of an electric model, the Electronic Speed Controller (ESC) so that they go to a pre-set state should signal connection between the transmitter and receiver fail.

Typically this could be to drop the throttle to idle or off in the case of an electric model, the rudder to a very gentle turn setting whilst aileron and elevator return to a neutral setting. This should reduce its impact speed and prevent it from flying away.

For the time being this should be sufficient knowledge for you and your tutor to ensure that your trainer plane is  eminently flyable.

There are more rc transmitter functions available within the programming facilities but these are more suited to advanced flying techniques. Over time you will be able to learn about your radio gear and familiarise yourself with these sophistications.

If you are considering buying your Radio Control Gear soon, can I suggest you visit my page on the website covering Selecting the Best Radio Control Equipment for you.

See you next time.



August 8

Choosing The Right Propeller

All those years ago when I learned to fly rc planes advice concerning choosing the right propeller was brief and unqualified: “For a 40, you’ll need a 10″ x 6″ and if it’s a 60 you’ll want an 11″ x 7”. No reasoning, just accept that this was correct. As my experience grew I discovered that this was a very vague and inaccurate way of looking at prop choice. Experience showed me that models should be propped for the airframe, noise limits, performance and not the engine.

Model Aircraft Propellers
Model Aircraft Propellers

The two numbers that designate propeller sizes, e.g. 10 x 6, 11 x 7 (read as ten by six and eleven by seven) are measurements in inches –  the first number indicates the exact diameter of the rotating prop arc or the length of the propeller, whilst the second number is the pitch of the propeller blades. The pitch is the theoretical distance the propeller would travel forward during one complete revolution in a fluid (air) with no drag, resistance or slip. A 10 x 6″ propeller measures 10″ in from tip-to-tip and would theoretically go forward 6″ during one complete revolution.

The theoretical pitch measurement is used as propellers are not 100% efficient, nor is the air it’s travelling through of consistent density and free of movement and air currents. It compresses as the propeller goes through it, and the propeller’s shape, material and finish all affect its performance and efficiency during flight. Some propellers may flex and twist with increased loads, reducing their efficiency. All of this means that the 6″ pitch of our airscrew is just a guide, and in reality doesn’t exactly translate into distance at all.

A propeller is actually nothing more than a wing with an aerodynamic section set at a specific AoA that we call the pitch. ( There’s that abbreviation we came across before, do you remember? If you’ve forgotten go and take another look at the post on Elevators). This generates lift as it is pushed through the air by the rotation of the engine shaft. Each part of this ‘rotating wing’ will have a different airspeed, low near the centre and high near the tip. To ensure that each part of the prop generates the same amount of lift from root to tip, the ‘wing root’ angle of attack is set higher than the ‘wing tip’ (hence the ‘twist’ in the prop).

Airfoil & Changing AoA of Propeller Blade
Airfoil & Changing AoA of Propeller Blade

The Right Choice of Propeller

Choosing the right propeller is just as important as the choice of engine. The propeller size will fundamentally affect the way your model flies. The wrong one may damage your engine and you should always be guided by the manufacturer’s instructions initially. There are normally a number of choices available as manufacturers recognise that a number of factors will affect propeller choice. These include model size, drag, weight, wing loading, engine type, fuel and even the altitude of your flying site. All are important considerations for correct propeller selection.

You also need to consider the material the propeller is made from. This must be suitable for the expected operating rpm of your engine. General sport models and trainers with motors turning props at up to 12,000 rpm are suited to glass reinforced plastic (GRP) types. It is important to understand that propellers for glow engines are totally different to those made for electric motors and the two are not interchangeable.

Having said all this, we will look at making a good selection next so don’t worry that choosing the right propeller is going to be too difficult.

Propeller Selection

Generally speaking a higher pitch prop will pull the model faster in level flight whilst a lower pitch prop will help it to take-off in a short distance and climb quicker and easier.

Basic trainer
You’re not going to go far wrong by using the suggested prop sizes given by the engine or motor manufacturer. For a low-powered .40 – .46 glow engine I’d usually look to fit a 10 x 6″, 10 x 7″, 11 x 5″ or 11 x 6″, using the shallower pitches if extra pull is needed for take-off. If you fly from long grass or the model is heavily built, this might be something to consider. For an equivalent electric outrunner it would normally be an 11 x 5 or 11 x 6, 12 x 4 or 12 x 5. For more powerful motors I’d favour the higher pitches.

Propeller Preparation

A GRP propeller will usually need some preparation before being fitted to your engine or motor. The first thing required is to remove the sharp edges. Don’t try to sand GRP propellers to remove the moulding ‘flash’, use a sharp tool (e.g. craft knife or Stanley knife) with the blade held perpendicular to the edge of the prop to remove it.

Once the edges are smooth, check the hole in the middle. In many cases this will need enlarging to fit your engine’s crankshaft. A special-purpose prop reamer or drill press is required to enlarge the hole, being careful to make it straight and perpendicular to the prop hub.

Propeller Reamer
Propeller Reamer

Under no circumstances should you use an oversize screwdriver or round file to enlarge the hole. Anything that causes your propeller to run out of true will produce damaging vibration and wear problems elsewhere. Click the image above or this “Reamer” link if you wish to order this item on-line.

Balancing Your Propeller

Propellers must always be balanced before fitting to your engine. This can be achieved by using a commercially available prop balancer. Cheaper examples are available whereas others can be quite expensive. Most are totally frictionless magnetic affairs as the examples below. Click on the images to compare and order your selection on-line.

Economy Prop Balancer
Economy Prop Balancer
Mid Range Prop Balancer
Mid Range Prop Balancer
Top Flite Prop Balancer
Top Flite Prop Balancer

Suspend your propeller on the balancer and carefully remove material from the heavy blade until it is in equilibrium. Don’t remove any more than necessary and be careful not to change the prop’s shape., Never remove material from the blade faces or reduce the length of one side. Alternatively, the light blade can be coated in fuel proofer to get the balance correct. When you’ve finished and it is perfectly balanced, write a ‘B’ on the hub in a permanent marker colour that contrasts with the propeller material to remind you that this one is balanced.

Keep The Noise Down

Depending on where you fly, noise may or may not be a major consideration. In the UK there are strict controls on noise emissions from model aircraft engines. You should check this out with the British Model Flying Association (BMFA) to ensure you don’t exceed the limits. There are also guidelines regarding the distance you are permitted to fly with relation to “noise sensitive buildings”. I believe there are similar regulations in the USA. The AMA will be able to advise you.

These regulations apply to all flying models but those powered by Glow, Nitro or IC engines are the most difficult to keep below the set levels. With many engines of this type, the noise generated by the spinning propeller can often be greater than that emitted from the silencer.

Propeller noise can be reduced to acceptable levels by selecting a propeller that allows the engine to turn at an rpm producing a rotational tip speed of less than 350mph. For the technically minded amongst you, the following formula enables you to calculate this figure.

Propeller tip speed in mph = [(3.142 x diameter in inches) x rpm] ÷ 1056

To achieve this figure isn’t always practical, but it’s a good figure to use as a basis. Noise reduction on many flying sites is now such a concern that some engine manufacturers’ .40-size engines are made to run on 10 x 8″ or 11 x 7″ size props instead of the more traditional 10 x 6″.  All of this is aimed at reducing the tip speed of the prop and enabling overall noise production.

So there you have it. With a little experimentation you really can get the best from your model just by choosing the right propeller. Finally a word of caution:- Mind Your Fingers and Never use a propeller that has any sign of cracking or other damage. Even if not for your own safety, at least consider those around you.



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.