February 5

Setting Up And Tuning RC Nitro Engines

For many RC flyers there is a certain magic to the sound andEngine Checks smell of Glow/Nitro Engines powering their models around the skies. If you are one of these then the ease of operation and reliability of your engine is an important aspect of your hobby.

In this post I aim to try and help you make life as easy as possible when it comes to operating your rc nitro engines.

Until fairly recently all of my planes were powered by glow/nitro engines and I managed to get the starting and running down to a fine art. There was rarely a time when I needed to spend ages trying to get my engines to perform correctly at the field. Sad to say , this was not so for many of my fellow modellers who often spent a great deal of time trying to get their engines to start and run reliably.

Preparation and Priming

The key to a safe and reliable start up procedure is just this; preparation and priming. These two aspects ensure  confidence in our installation and the knowledge that our engine will start reliably. Let’s take these two subjects in turn.

Preparation

a) Tank Installation and Plumbing:  Although the fuel tank arrangement is not too critical once the engine is running, there are some basic precautions that need to be considered.

Glow Engine and Tank Installation

The fuel tank needs to be installed as close as possible to the engine. Ideally its centre line on or very slightly below the level of the carburettor spray bar assembly as shown here. This arrangement helps avoid flooding of the carburettor when the tank us full. It also gives the muffler pressure a good chance of maintaining fuel flow to the spray bar throughout the flight from tank full to tank empty.

An additional consideration when installing the engine and tank is to ensure that the fuel line from tank to spray bar does not rise above the full level at the top of the tank. With the fuel line as shown in the diagram positive pressure, due to gravity, will ensure that fuel flows to the spray bar for initial priming and is retained throughout the starting procedure.

The diagram above only shows two lines, one for fuel to3-line-tank plumbing for easy de-fueling spray bar and the other for muffler pressure. I personally prefer a three line system as shown here. This avoids having to remove the line from the spray bar in order to refuel/defuel the tank. Note, however, that you need to cap off the extra fuel/defuel line before starting the engine.

Another aspect of this setup is the use of two clunk lines running to the bottom of the tank. Many installations have the fuel/defuel pipe alongside the vent/pressure pipe inside the tank. The setup shown here enables defueling without having to invert the model. It does ,however, require an extra clunk weight to keep the line at the bottom of the tank.

There is also another slight modification that can be made to this arrangement and that is to put a length of rigid fuel pipe from the engine feed line clunk, approximately half the length of the tank, to prevent the tube doubling back on itself ( a “hang up”) in the event of a heavy landing.

As a precautionary part of the engine and tank installation process be sure to remove any burrs from the ends of rigid tubes and around access holes in bulkheads, etc. Check that there are no pin holes in any of the flexible silicone tubes both inside and outside the tank.

Another good precaution is to fit a fuel filter in the fuel fill line. Some people also fit fuel filters in the tank to spray bar line but I feel that so long as the fuel is filtered on its way into the tank, it shouldn’t be necessary to filter it again.

Check that there are no kinks or tight corners in any of the feed, pressure and fill lines that might restrict fuel flow.

b) Engine Integrity

Before you actually fit the engine into the plane check it over for any potential air or gas leaks. To do this carefully inspect the fit and tightness of the front  shaft housing to the crankcase, the back plate to the crankcase and the cylinder head to the crankcase. All of the screws, nuts and bolts, etc. should be checked for tightness.

Ensure that the carburettor is correctly seated in its spigot hard against the rubber “O” seal ring. Make sure there is no dust or other debris in the spray bar assembly and jet.

Some fuel adjustment needles can be a fairly slack fit in theirAir Bleed carb 2 threads and can move in or out under engine vibration. To prevent this, a small section of silicone fuel tubing can be slid over the needle thread before refitting it. this will create a resistance to free movement of the needle.

The next thing to do is to check the glow plug fitted to your engine. Do this by removing it using a suitable glow plug spanner, connecting it to your glow plug driver and observing that it glows a bright red colour.

Make sure it is the correct type of glow plug. Consult the engine manufacturers data sheet for guidance here. If you are a newcomer to rc flying I suggest a glow plug with an “idle-bar” would be appropriate. This type helps retain heat in the plug element when running at idle and tick-over speeds.

c) Throttle Servo Linkage SetupThrottle Arm Positions

To ensure full range of control the throttle to servo linkage should be arranged so that there is full movement between “tick-over” and “full throttle” with final cut off being achieved via the throttle trim control or throttle cut switch at the transmitter.

If you look vertically down on the air intake of your engine’s throttle barrel then the three barrel air openings that correspond to the arm positions shown here will be those Throttle Barrel Settingson the right. It is important to ensure that the servo output arm positions the throttle control arm at the correct positions for you engine to run correctly and give you the full range of control required.

The opening at “idle” setting should be between 1 & 2mm and should give  tick-over of between 2,500 & 3,000 rpm. The final adjustment will be dependent on the size of your propeller and the fuel used.

Getting Ready To StartProp 2 o clock

Your propeller should be fitted so that the blades are at a 8 o’clock/2 o’clock position when pushed against compression as shown here on the right. This ensures a smart positive flick can be made.

Close the main needle valve so that flooding of the carburettor does not occur while you fill the fuel tank. Once the tank is full you can open the needle again. It needs to be set 1.5 to 2.5 turns from the fully closed position.

If your tank is installed as suggested above the fuel delivery tube from tank to spray bar should fill under gravitational pressure. It may not have been possible for you to achieve a straight line between the tank and spray bar and any unavoidable hump in the tube may prevent the desired flow of fuel.

This can be rectified by placing your finger over the air intake of the carburettor (finger choking) and turn the propeller over until the fuel is drawn through. If the fuel starts to flood into the carburettor by siphon action, raise the nose of the model a little.

Alternatively, if the fuel refuses to flow into the feed tube, raise the tail of the model and repeat the choking procedure or cover the exhaust outlet with your finger and smartly flick the propeller over a few times. This creates a pumping action via the exhaust pressure system.

Priming The Engine & Testing For Fuel

A properly primed engine will usually start immediately so follow this procedure carefully.

With a full fuel line up to the spray bar and the throttle fully open, choke the carburettor using your finger as explained above. Turn the propeller over four or five times to draw some fuel into the engine crankcase.

Take your finger off the carburettor air intake and flick the propeller over repeatedly until the engine feels loose and free. Do not skimp on the flicks, it may take ten or more to ensure the fuel reaches all parts of the inside of the engine.

You will tell this has happened when the propeller turns freely over compression with a wet ‘plopping’ sound. If this does not happen, choke the engine again another two turns and repeat the priming procedure.

The best way to tell if this priming has worked successfully is to connect the glow plug to a fully charged glow driver. Firmly grasp the propeller and slowly turn it through compression. You should detect a kick as it goes over top dead centre (the compression point).

If this kick does not occur the engine is not ready to start and will require further priming or the glow plug is not functioning correctly or it is flooded. This should not happen under the routine above but excessive priming could cause the problem.

Starting and Setting The Main Needle

You are now ready to start your engine for the first time. This is done with the throttle set to ‘fully open’. The reason for this is that if there is any problem with the slow running jet setting, it will not affect the start-up unless someone has screwed the slow running needle fully in. Note that future starts are made with the throttle set to about 1.3rd.

Assuming you have correctly primed the engine it should start on the first or second flick of the propeller or with a mere touch of a starter. Keep in mind that your throttle is set to fully open so ensure the model is securely tethered or restrained and be prepared for a burst of full power!

If your engine bursts into full song but cuts soon after, it is too lean and you will need to open the main needle another half turn. Re-prime your engine and try again.

The other extreme is that the engine starts but runs rough and eventually stops. This means that the needle is set too rich and requires screwing in half a turn at a time until the engine continues to run.

In the happy event that the engine continues to run from the start you are ready to make adjustments to the main needle to obtain maximum revolutions.

You will know when the correct setting is achieved if by raising the nose of the plane upward toward the vertical the engine neither gains or looses revs.

If a drop in revs is detected with nose up, place the model back on the ground and open the main needle slightly and check again.

An alternative approach is to keep the plane on the ground and squeeze the exhaust pressure tube flat. This will eliminate the pressurisation from the tank. If the engine looses revs then a slight opening adjustment of the main needle is required.

Slow running & Throttle Response

All new engines should be set up for reliable slow running before they leave the manufacturer and should rarely need adjustment to the slow running jet. In all of my years of experience with new engines I have only once needed to make an adjustment to a slow running jet.

My advice would be to check the throttle response of your new engine by steadily closing and opening the throttle and unless it does not behave as you would expect, LEAVE WELL ALONE!

If for any reason the pick up from idle is not consistent then a SMALL adjustment may be required. I DO emphasise here the word ‘SMALL’. The difference between reliable and unreliable ‘pick-up’ is usually no more the 1/8th of a turn of the adjustment screw.

Depending on the make of engine you have purchased there will be one of two types of carburettor.

The ‘air-bleed’ type. This has a small screw with a spring on the sideAir bleed Carb of the barrel assembly that partially closes off a small air bleed hole in the front of the body.

If, when the throttle is smoothly opened the motor speeds up but then slows and cuts, then the air bleed screw is set too lean (too open).

The solution here is to turn the set screw IN no more than 1/8th of a turn. Re-start the engine and check the response again. If it still fails to keep running, adjust the screw another 1/8th of a turn although this should rarely be necessary.

The other type of carburettor is the ‘dual or twin needle’ type. As you can Twin needle carbsee, this type has a secondary needle to control the slow running which is situated in the centre of the rotating throttle barrel.

In this instance, rather than screwing the needle in to create a richer mix of air and fuel, the screw is turned OUT. The difference being that you are increasing the amount of fuel in the mix rather than, in the case above, reducing the amount of air in the mix.

An over rich mixture at the idle position will cause the engine to be reluctant to speed up smoothly and you may detect fuel being sprayed out of the carburettor. Stop the engine and make the appropriate adjustment to rectify this.

In the case of the ‘air bleed’ type turn the screw OUT 1/8th of a turn or, in the case of the ‘twin needle’ type, turn the needle IN 1/8th of a turn.

Repeat these adjustments until the engine speeds up to full power without hesitating when the throttle is opened fully in about one second.

I repeat what I said at the beginning of this section: New engines should NOT require adjustment to the slow running jet.

General Problems

If your engine gives you trouble starting then it is likely that one or other of the following possibilities has arisen:

  1. The battery is not properly charged or the glow clip / leads have a bad connection or continuity.
  2. The glow plug has failed, is the wrong type, the filament has become distorted or the engine requires a longer ‘run-in’ period.
  3. The carburettor is worn or is poorly machined.
  4. There is a gas leak: at the plug washer; around the plug’s central electrode seal; on the carburettor or engine.
  5. The fuel filter is blocked or the main carburettor jet is dirty ( often bits of silicone rubber from the feed tubes can cause this).
  6. The tank clunk has become ‘hung-up’ (doubled back on itself) or the muffler pressure nipple is blocked.
  7. The throttle servo linkage is ‘wandering’ in operation due to excessive vibration and wear.

Glow plugs are disposable items and do need to be replaced from time to time. Repeated replacement due to ‘blowing’ them is often a result of running an engine too lean at high load. Excessively high compression ratio can also cause this problem as can the presence of metal swarf and bits of grit either in a new engine or due to slow disintegration of an old engine.

Final Thoughts

There is no valid reason why modern RC Nitro Engines should present their operators with problems providing they understand the workings of said engines and know how to get the best from them. This post is aimed at helping you do exactly that and, providing you follow the guidelines closely, you should gain great enjoyment from their operation.

Don’t forget that these miniature power plants can bite you! Treat them with the greatest of respect and don’t take chances. Whirling propellers and human flesh are not the greatest of friends. They also get very hot so beware!

Please don’t hesitate to contact me via the comments facility here at the end of this post if you need my help. If you have found the post helpful, be sure to visit my website www.rookiercflyer.com especially if you are new to the hobby. Everything you need to know about getting started is there.

Looking forward to hearing from you.

Colin

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

December 4

How RC Planes 2 Stroke Engine Works?

If you have followed my website www.rookiercflyer.com you will know that I have tried to cover both Nitro/Glow powered planes and Electric powered planes. In either case a basic understanding of the way the power is generated is helpful for the flyer. This post will be directed at those of you who have decided to use Nitro/Glow engines and is aimed at helping you understand how a 2 Stroke Engine Works.

2 stroke diagramA Simple Explanation of How a Model 2 Stroke Engine Works

First of all let us ask ‘why is it called a 2 stroke engine?’ Quite simply it is because every part of the energy producing function happens as the piston goes through one full cycle and the crankshaft goes through one full rotation. This means that the piston completes ‘two strokes’, one up and one down.

Each of these ‘two strokes’  achieves two objectives. On the ‘UP’ stroke we get both Induction and Compression. Let us analyse these two actions in detail.

Induction

This is the word that describes how the fuel/air mixture is drawn into the engine. As the piston rises up in the cylinder (as shown in the first part of the diagram above) air is drawn in through the carburettor and passes into the crankcase under the piston.

This takes place because the crankshaft is hollow and has an opening that is directly in line with the intake barrel of the carburettor and is opened once in each rotation. The piston rising in the cylinder creates a vacuum within the crankcase that has to be filled.

As the hole in the crankshaft lines up with the carburettor barrel, a volume of air rushes in to fill this vacuum. As it does so it draws with it a small quantity of fuel that is controlled by the needle valve in the carburettor. This mixture of air and fuel rushes into the crankcase

Compression

At the same time that this is happening, a previous intake of air and fuel is being  compressed into the cylinder head by the rising piston ready to be burned. We will look at how it gets into this space shortly.

OK, so now we reach the point where the piston has to return ‘DOWN’ the cylinder. This is where we get two more operations called Power plus Exhaust/Transfer

Power

All of the energy produced by the engine is created as a result of burning the air/fuel mixture in the head of the cylinder. This happens because the residual heat in the glow plug filament ignites this mixture producing gases that expand very rapidly. This rapid expansion forces the piston back down the cylinder.

Exhaust/Transfer

The descending piston compresses the fresh air/fuel mixture previously drawn into the crankcase. As it descends almost to the bottom of its downward stroke, it uncovers an outlet in the side of the cylinder called the ‘Exhaust Port’. The expanding gases resulting from the burning of the air/fuel mixture escape through this port.

Very slightly lower down the cylinder, on the opposite side another opening is uncovered called the ‘Transfer Port’. This is a passage in the side of the engine between the crankcase and the cylinder to carry charges of air/fuel mixture. This enables a new charge of air/fuel to enter the cylinder above the piston from the crankcase.

Once the burnt exhaust gases and waste lubricating oil have exited the Exhaust port and the new air/fuel charge has entered the cylinder the whole procedure begins all over again.

In just one complete revolution of the engine all of the above has taken place.

Exhaust/Transfer Conflict

You may already be wondering why the new air/fuel mixture doesn’t escape while the exhaust port is open at the same time as the transfer port is opened. This all comes down to a bit of clever design work by the manufacturers.

Special transfer or ‘boost’ ports are  incorporated that direct the air/fuel mixture away from the exhaust port and back into the cylinder. A German engineer developed this modification some years ago and it was called ‘Schnuerle’ Porting after him.

A 2 Stroke Engine’s Moving Components

In the above explanation we have talked about the ‘Piston’ and ”Crankshaft’. These are the two principle moving components but there are other items that are essential to the successful operation of these two.

Piston & Liner

These are central to the operation of our engine and the seal between them is all important. Most engines you will encounter use one of two methods to achieve this good fit:- a) Close Tolerance Fit or b) Ringed Fit 

Close Tolerance Fit    non-ringed piston and line

The first of these relies on a very accurately engineered fit between the piston and the liner. The picture to the right illustrates such a piston and its cylinder liner.

Engines manufactured in this way are often listed as ‘ABC’ types. This designation refers to the materials used in the manufacture of the piston and its cylinder liner.

‘A’ refers to the piston which is made from an alloy of Aluminium.

‘B’ refers to the metal from which the liner is made and that is Brass.

‘C’ refers to the fact that this Brass liner is plated with Chromium.

Most modern engines of this type are manufactured with a very slight taper inside the cylinder liner, wider at the bottom than the top. This means that the fit of the piston within its liner gets better as the piston rises. The principle reason for doing this is because most of the heat from the burned air/fuel mixture is absorbed at the top of the cylinder  causing this part of the liner to expand more than the lower part. The expansion causes the liner to reach a more parallel state so the piston fits the same along its full stroke.

Modern metallurgy has provided new finishes for cylinder liners so we are starting to see such acronyms as ABN where the liner has a Nickel plating instead of Chrome.

Ringed Fit

The construction of the engine is much the same as for that aboveRinged Piston apart from the fact that there is a groove running around the top of the piston just below the crown. Into this groove is fitted a spring steel ring that forms a tight fit against the walls of the cylinder liner.

In some engines this ring is manufactured so that when air/fuel is being compressed into the cylinder head or the exhaust gases from the burned charge add pressure it presses even more against the liner walls to form an even better seal.

Cylinders with ringed pistons do not normally have tapered liners as the ring is capable of flexing against the liner walls to accommodate extra expansion at the top of the cylinder.

Up and Down To Round And Round

In the photograph above right the piston is shown with a rod protrudingConrod from its inner cavity. This is the conrod (short for ‘connecting rod’) that connects the piston to the crankshaft (photo right). If you refer to the picture above you will see that a metal pin, called a gudgeon pin, passes through the piston and the smaller hole in the conrod. The gudgeon pin is retained in place by spring steel circlips, one at each end.

The bushed hole at the other end of the conrod fits over the stub on the rear face of the crankshaft balance plate (see photo below). The accurately positioned cut-out in the crankshaft is positioned so that the charge of air/fuel from the carburettor is allowed into the crankcase at exactly thecrankshaft right time.

This crankshaft is the part of the engine that converts the up and down motion of the piston into a rotary motion suitable for driving the propeller. It is mounted inside the crankshaft held by either bronze sleeves or ball bearings. One in front of the conrod bearing and balance plate and the other over the narrow part of the shaft.

Controlling Air/Fuel Mixture

In the discussion above the carburettor was mentioned. Now wetwo needle carb will look more closely at this important piece of equipment and how it works. To the right here is a photograph of a standard two needle type of carburettor found attached to model two stroke engines.

It is called a two needle carburettor because it has just that, two needles, one to control the high speed running mixture and another to control the low speed running or idle. The hand adjustable high speed jet is located on the left in the photograph whilst the idle control needle adjustment screw is hidden from view in the end of the pushrod link fastening on the right.

The fuel intake nipple that takes in fuel from the tank is located on the left hand side of the main body adjacent to the high speed mixture jet. The air intake barrel is shown at the top whilst the crankcase intake is at the bottom. This fits into a matching boss on top of the crankcase and relies on a neoprene Carb throttle openingring to form an air tight seal when the retaining screws are tightened.

The small screw to the right of the carburettor body is an adjustment for the range of movement of the rotating air/fuel intake mixture barrel (throttle barrel). In the photograph (right) the throttle barrel is shown about half open. In the fully open position the hole in the barrel will be totally in line with the air intake hole whereas when fully closed the solid part of the barrel will completely close the air intake. This movement is controlled by the throttle servo via a pushrod connected to the rotating arm attached to the barrel.

Running through the centre of the throttle barrel is a jet with a needle controlled by the ratcheted finger adjustment. This directly controls the flow of raw fuel from the tank into the carburettor. Screwing this needle in restricts the flow of fuel into the carburettor causing the air/fuel mixture to become leaner. Screwing it out increases the flow of fuel and makes the mixture richer.

Some manufacturers have this needle valve assembly (NVA) remotely mounted at the rear of the engine for safety reasons (keeps fingers further away from the propeller) where it is connected to the carburettor fuel intake nipple by a short length of fuel tubing.

At the other side of the carburettor at the centre of the fixing for the Idle jet Screwpushrod lever is the slow running jet. this only comes into action when the throttle has been closed to below halfway. It is used to fine tune the fuel flow at low throttle settings. You will notice that when you turn the pushrod lever back and forth, the barrel moves in and out of the carburettor body. This is because the movement adjustment screw runs in a ‘helical’ groove cut into the barrel.

As the throttle barrel moves inward the idle needle engages with the main fuel jet and starts to restrict the flow of fuel. This control is independent of the main needle. The more the barrel closes the more it restricts the flow of  fuel. The idle needle has a very fine thread because it has very critical effects on the low speed mixture and needs only tiny adjustments. Screwing it in makes the mixture leaner whilst screwing it out richens the mixture.

Concluding How 2 Stroke Engine Works

I hope that my explanation of this complex piece of kit has helped you understand how the power is produced. The 2 stroke engine works well as a propulsion system for model planes but it has some disadvantages in certain circumstances. Firstly it is noisy and secondly it is messy, it can also prove problematic at times. Providing you are happy to accommodate these facts you should find your 2 stroke engine works well for you.

Till the next time, happy learning.

Colin

 

 

 

 

 

 

 

 

September 19

RC Planes Beginners Choice – Arising Star Trainer Reviewed

Seagull Arising Star Trainer

Seagull arising star
Seagull Arising star

This review is of interest mainly to those of you who wish to go down the Nitro/Glow Power option. Having said that, with a little help, it would not be difficult to adapt this model for Electric Flight.

On opening the box there was a pleasant surprise. The contents were nicely packed and were extensively prefabricated. Good instructions are provided .

At first sight, this appears to be an ideal rc planes beginners choice.

Pre Assembled Components

Tubes for push rods were ready fitted for all control surfaces along with the same for throttle and front wheel steering. Everything was well assembled without glue gun excesses.

The aileron hinges were nicely pinned so that there is no chance of them pulling out in flight. A motor mount suitable for most 2 and 4 strokes in the recommended engine size range was ready fitted.

The nose gear wire leg had a flat nicely filed to take the steering horn in the correct position.

Final Assembly

The fuselage sides and wing centre section skinning are not made of balsa but some light weight thin harder wooden material. This is stiffer than balsa so be careful when cutting it as it is fairly brittle and could split.

Assembly was quite quick and simple following the maker’s instructions. Although the EZ connector type links for the control surfaces are different, they Arising star wing tubedo work well but be sure to read the instructions carefully.

The only modification I did was to join the two wing halves over the joiner to make a one piece wing. I did this by coating the centre ribs with epoxy as well as the joiner before bringing them together to set.

I was very careful to ensure that the trailing edge of each wing met correctly and were clamped together to ensure the correct alignment of the panels. I decided that the supplied tape for holding the wing halves together was no longer required so this was discarded. So far this has been proved totally satisfactory. An SC46 two stroke engine was fitted with an 11 x 7 Master prop. Arising Star engine installation

First Flight

Since we have a good runway with a smooth surface, I used the lightweight factory supplied wheels. If you need to fly off grass these could be exchanged for slightly larger wheels but be sure to go for light weight versions.

I had heard some reports that the nose leg wire would bend quite easily but it seems that an improvement has been made in this area. The undercarriage wire was very solid and held up well to a couple of bouncy landings.

The first flight was uneventful. After a good roll-out a smooth lift-off was achieved at about 3/4 power. A little down trim was required to fly level at half throttle training speed. Even at 1/3rd throttle she remained stable with no tendency to stall whilst holding a very small amount of up elevator that could be trimmed in for slower flight.

This model flies very smoothly, turns easily with only a small amount of up elevator necessary for the steeper turns. Loops, barrel rolls and some inverted flight were all carried out with ease.

Landing was relatively easy. The plane slows down to idle speed without stalling or dropping a wing and remains very controllable right down to the ground.

Conclusion

All in all, a very nice stable platform for any beginner to rc model flying who has chosen to go down the Nitro/Glow engine power route.

Becoming an R/C pilot won’t cost you an arm and a leg with the Arising Star Trainer. Built mainly from top-quality light balsa and ply and covered with genuine Hangar 9 Ultracote, the Arising Star is anything but cheaply made.

Assembly takes just a few evenings and all the hardware needed to finish putting it together is included.

The Arising Star’s gentle, self-righting, flight characteristics make it easy to master the basics of radio controlled flight. The above characteristics and affordable price make the Arising Star outstanding value and a great rc planes beginners choice of  for anyone looking to earn their wings.

Arising Star Basic Specifications:-

  • Wingspan – 63ins (160cm)
  • Wing area – 645 sq.ins (41.6 dm.sq)
  • Suits – 40-46 2-stroke. (52 4-Stroke)
  • 4 channel RC with 4 servos

Please note, to complete your model, you will also need:

Radio Gear

Propeller

Epoxy Glue

Engine

Fuel Lines

Fuel

Starting Equipment

You can purchase this kit on line by clicking the following link:- Seagull Arising Star 

Come back soon for another page of essential rc flight know how.

Colin

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.

Tachometer
Tachometer

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.

Colin