December 18

Adhesives for RC Planes

modelling glues

There is a wide variety of adhesives types available for repairing and building RC planes.  They all have a useful purpose but it is important to know what they should not be used for.

Inevitably you will have to use some kind of glue or adhesive in the future, either to build your own model or to effect a repair following damage to any of your RC planes. You will need to be prepared for this eventuality and it will be helpful to understand what types of glues are best for the particular requirement you have.

Glue Properties

There are certain things you need to understand about the properties of any glue that you decide to use.  This understanding will help you select the correct glue for a particular task

Glue Types

Most glues used to build model airplanes fall into one of two types:

  • Evaporation Types

These glues are solvent or water-based and dry and set by evaporation of the solvent.

  • Chemical Cure Types

These glues involve a chemical process that causes them to cure and set.

Chemical types can be further broken down into One or Two-Part glues.

Two-Part glues rely on the mixing of the two parts in a specific ratio before the glue can be used.  These glues do not shrink significantly and have good filling properties. Two-part glues include Epoxies.

Multi-Part adhesives should be mixed on a non-porous surface or in a similar container. Porous surfaces such as cardboard and untreated wood will prevent the glue from being mixed in the proper proportions as some of the components will soaking into the surface.  This could result in the glue not curing properly and compromise its final strength.

One-Part types may or may not shrink. Such glues that cure wnen exposed to the atmosphere include Cyanoacrylates (AKA Super-Glue or CA) and Silicone Sealant.

Excess glue of both types can be wiped up with an appropriate solvent while wet or scraped off with a blade after it is cured.

The expressions “Dry” and “Cure” will be used to describe the process of setting and hardening. For all practical purposes these mean the same thing.

Properties

Strength

Generally speaking, the stronger the glue, the heavier they are.  It is important to select a glue that is strong enough to do the job without adding excessive weight.  There is no good reason to use epoxy when gluing wing sheets together but good reasons not to.

Slow drying glues tend to create a stronger joint than fast drying glues because they have more time to soak into the wood.  Fast-drying glues tend to create a more brittle joint.

Matching Adhesives to Materials

Most glues are intended for use with certain materials.  The use of a wrong glue for a particular job can cause problems that include excess weight, difficult finishing and joint failure.

Fuel-Proofing

Fuel tanks can and do split open as a result poor assembly, defective moulding, bad design or as a result of a crash.  It is essential that the fuel tank compartment be coated with something fuel-proof such as epoxy resin or polyurethane (paint).  On a Glow/Nitro powered model all firewall joints should also be glued with a fuel-proof glue.

This precaution is not so necessary in electric powered models as the possibility of impregnating the structure with fuel is most unlikely.

Ease of Sanding

Often you will need to sand across a joint between two pieces of wood.  If the glue used is significantly harder than the surrounding materials, the glue will not sand away at the same rate as the materials it is bonding. This can result in an unsightly ridge that will be visible through the final finish or covering.

Pot-Life

This expression refers to the useable life of the glue after it has been dispensed or mixed in an open container.

Working Time

It is important to understand that glues that cure tend to heat up.  In the pot, they will cure faster than in a thin film.  This means that many of these glues can still be worked after being applied to a part even though the glue in the pot has become too thick to use.

The time given on epoxy packaging is the working time, not the curing time.  So, a 15 minute epoxy has a 15 minute working time.  Cure time is usually 30-60 minutes depending on the brand and climatic conditions.

Cure Time

This describes how long a glue takes to fully harden for practical purposes.  Most glues that cure tend to continue the chemical curing process for months.

Shelf-Life

This is the length of time a glue or adhesive can be stored on the shelf before it goes bad. Some glues last better than others so do not buy any more glue than you can reasonably use within about one year after you purchase it even though manufacturers make claims that their glues have shelf lives of years.  Shelf life is strongly affected by the climate (heat, humidity, UV light, etc.).

Surface Protection

Sometimes an adhesive is used to protect a surface. An epoxy resin coating on wood will provide a smooth, long lasting surface to mount a servo using foam tape.

Its strength is irrelevant in this instance. Exposed, cured epoxy withstands exposure to the environment better than most other adhesives while creating a non-porous surface that foam tape adheres to well.

 

Aliphatic Resin (Also called Carpenters Glue)

Aliphatic resins are most useful, inexpensive, light and strong. Super Aliphatic Resin It is the primary adhesive I use to build Balsa and Plywood based flying model aircraft.  Its longer working time lets me make adjustments to ensure everything is as it should be before the glue sets hard.

This glue is water-based so it allows me to create neat joints. The excess glue is easily removed using a damp sponge or paper towel before it starts to set.

Features

  • Not Fuel Proof – They are fuel resistant to a point but will not withstand continuous exposure to raw fuel.
  • Clean-up – Use water while wet, Acetone when dry.
  • Use for – General construction.
  • Do not use for –

Non-porous surfaces,

High-stress areas (firewalls, landing gears, etc.),

Edge joining sheets of balsa — it does not sand as easily as soft balsa and will leave a nasty ridge when you try to sand the sheet flat.

Laminating broad areas — it will cause severe warping.

 

Cellulose-Based Glue (Balsa Cement)

This glue comes in a tube and is used for balsa wood models. Balsa Cement It is fast-drying and lightweight.  The solvent in this glue has a strong odour and should only be used in a well ventilated area.

Balsa cement is ideal for joining sheet wood together.  It sands very easily and makes a strong edge to edge joint.

Features 

  • Fuel proof – They are fuel proof to low nitro fuels.
  • Clean-up – Dope thinner or acetone even after it has dried.
  • Use for –

Stick and tissue model.

Edge joining sheet wood because it is very easy to sand.

Gluing some types of plastic parts to wood models.

Sealing dissimilar grains to make sanding easier.

  • Do not use for –

Non-porous surfaces with the exception of some plastics that it are capable of melting in order to create a bond.

High-stress areas (firewalls, landing gear, etc.).

 

Contact Adhesive

Contact adhesive is heavy and has very limited uses in model-building. contact adhesive The one benefit is that it doesn’t warp sheeting badly enough to cause problems.

It is applied to both surfaces and allowed to dry to the touch. The manufacturer will advise the best time for this to occur either on the container usage instructions or leaflet. The two parts are brought into contact with each other and they are instantly and permanently stuck together.  You do not get second chances to align things when using contact cement.

Features 

  • Fuel proof – Unknown.
  • Clean-up – Lacquer thinner, mineral spirits or a dedicated thinner.
  • Use for –

Laminating broad contact areas (fuselage doublers, creating your own balsa ply sheets, etc.).  If you use contact cement to glue sheeting to a foam wing core, be sure the cement is foam-safe.  Most contact cements you see in hardware stores will melt right through the foam and ruin the core.

  • Do not use for –

Anything else.

Cyanoacrylate Adhesive(CA)

Also called Super Glue

CA glue will instantly bond almost anything to anything else and indeed it is very versatile. CA Adhesive The only substances that resist it seem to be some soft plastics.

It is very good for bonding hard plastics, resin and metals to each other. It is not easy to use and should perhaps be avoided by novices and children. The thin formulas cure in one or two seconds and often seem to stick fingers to the components to be glued much better than it sticks the parts together.

Although cyano will stick most things together, the quality of the surface is important.  Any dust or grease on the surface will substantially reduce the strength of the bond.  Roughed surfaces stick better than smooth ones, so a light sanding of the surfaces that are to be bonded is worthwhile.

Excess should be immediately wiped off with a paper towel.  Any residue should be left to harden and then scraped off with a sharp knife.  If fingers, or other human tissue is accidentally bonded, it should be carefully separated with superglue debonder If you decide to use Cyano always have a debonder available for such incidents.

Once open, CA deteriorates quickly and it very easily blocks the nozzle of the bottle.  To reduce this problem, always wipe the end of the nozzle, gently tap the bottle on a hard surface to encourage any glue on the inside the nozzle to go back down and put the top back on straight away.

If you are not going to use a bottle of superglue for some time, the shelf life can be extended by putting it in the fridge.

Features

  • Fuel proof – No. Nitro methane dissolves cyanoacrylates.
  • Clean-up – Acetone or nitro-methane.  Some companies make debonders that are a mix of these items.
  • Use for –

Hardening threads cut into wood.

Gluing difficult to clamp items when you do not want to hold the part for the two hours another glue would take to dry.

Use for some types of dissimilar joins such as carbon fibre to wood.

  • Do not use for –

High-stress areas.

Areas exposed to raw fuel.

Clear plastic parts.  CA can fog the plastic.

Foam.  Standard CA dissolves foam.

An accelerator is made for use with cyanoacrylates.  It sets CA glues instantly but tends to make the joint more brittle.

 

Epoxy Glue

Epoxy resins are strong but add considerable weight to the structure.  Epoxy glueThey are good for bonding high-stress areas as well as different materials that other adhesive will not bond together.  Additionally, epoxy can be used for laying up fibreglass cloth and making fibreglass parts.

There are basically two kinds of epoxy resin.  One is used as an adhesive whilst the other type is used for laminating and has a thinner consistency.

I advise using slow-drying epoxy (30-minute to 4-hour working time) whenever epoxy is called for.

5-minute epoxy should be used in special circumstances only.  It is heavier, does not cure well and gets brittle with age. Generally, epoxy should only be used on load-bearing components. It is probably important enough to get it repaired right correctly in your workshop and not at the field.

The only “special circumstance” I would use 5-minute epoxy for is to create a smooth, non-porous surface for servo tape. Allow the epoxy to cure fully (at least over night) and then give it a wipe with alcohol to remove any residue before applying the servo tape.

  • Fuel proof –Yes.
  • Clean-up – Lacquer thinner, acetone or alcohol before cured.  After cure, a heat gun will soften it so that it can be scraped off.  Soaking in a strong solvent will break it down eventually but could well ruin the component it is attached to..
  • Use for –

High-stress areas.

Applying fibreglass cloth.

Fuel proofing firewalls and fuel tank compartments.

Bonding fibreglass and carbon fibre.

Mix with micro balloons (microscopic glass beads) to make a putty that can be formed into fillets and sanded easily.

Bonding dissimilar items such as metal to wood.

  • Do not use for –

General construction.  It is heavy, stronger than necessary and difficult to sand.

 

Hot Glue

Hot glue is very heavy.  I would never use it on any part of a wood built HotGlue & Gunmodel other than to perhaps fix components within the structure where there is no load bearing implication. Once set it has good vibration absorption properties.

  • Fuel proof – Unknown.
  • Clean-up – Contact Manufacturer for advice.
  • Use for

Fixing RC components and other accessories inside the model

Building foam models used sparingly.

  • Do not use for

Model Building using traditional materials.

 

Silicone Adhesive

Flexible, relatively strong and fuel proof.  It can be bought in small silicone adhesivetubes large enough to complete smaller jobs. The types available from DIY stores in larger tubes will go off and set in the tube long before it gets used up.  There is a high-temperature variety that can be used to seal mufflers and other engine parts (with care).

  • Fuel proof – Yes.
  • Clean-up – A special silicone sealer remover can be purchased from hardware stores
  • Use for

Sealing holes that you may need to access later.  For example, if you do not want exhaust residue to climb up the landing gear leg and go into the wing, you can seal the landing gear area with silicone.

Attaching landing gear fairings so they flex instead of break when the landing gear flexes.

Sealing the seating between wing and fuselage sides.

Attaching servo trays inside the fuselage.  I have never done this, but have been told it works well.

  • Do not use for

General construction.

Load-bearing members.

 

White Glue

White glue is very economical and is strong enough for many modelling tasks. White Glues It looks very much like PVA glue, being white and water-based but it is more viscous.  It has limited use for most modelling applications, but is very good at bonding wood,

  • Fuel proof – No.
  • Clean-up – Water.
  • Use for –

Small wood or paper models.

  • Do not use for –

Plastic

Metal

High stress areas

 

Glues For Building RC Planes Using Foam Products

Electric RC planes and foam building material has revolutionized the way park-flyers are built. We are still exploring the hundreds of bonding materials to find those that do not damage our foam, have a strong hold and weigh as little as possible.

Here are some of the best glues to use on foam like Depron, EPP, EPS, EPO, paper-faced Foam Board, etc.

GORILLA GLUE

As far as strength and weight considerations go, Gorilla Glue is a great choice.Gorilla glue It dries white and sets in about 30 minutes.

It foams up a little to fill gaps, something you need to watch as it tends to squeeze out of joints and onto clean surfaces. Cover the surface with clear tape immediately after gluing. This not only helps keep the foamed-up glue in, but it also hold the two parts together until the glue dries.

To help speed-up the setting process, wet both surfaces to be glued right before applying Gorilla Glue.

FOAM-SAFE CYANOACRYLATE (CA)

For the ever-growing foam business within RC, new foam-safe CA glues Foam Safe CAhave appeared in the last few years.

They come in various viscosities like thin, medium and thick. Most commonly used is the thick, slow-curing CA. This is due to foam’s porous properties and the need to fill the gaps in it.

Since CA doesn’t hold the strongest of bonds, it is not the best choice for models that will endure high speeds or stress on the airframe.

HOT GLUE

Hot Glue comes in solid “sticks” of various lengths and diameters and is melted with anHotGlue & Gun electric gun that also squeezes it out of a nozzle tip.

Many modellers like hot-glue for building with foam. It holds a strong bond and is easy to work with but it stays a bit flexible, a good thing in a some applications.

Make sure the gun is set to low temperature and you won’t damage the foam. With Hot-melt Glue you’ll get a strong, inexpensive bond in seconds.

EPOXYEpoxy glue

Epoxy is a popular, strong and inexpensive adhesive but it adds a bit of weight to models.

There are a large selection of Epoxies to choose from. It is sold as two chemicals in separate tubes or bottles in gel form. You mix the two by way of a stick or paddle and apply it to the surfaces with this tool.

 

Summary

The variety and range of glues and adhesives available from model suppliers, DIY and hardware stores is vast. Building or repairing RC Planes requires just a few specialised glues so you don’t need to buy lots of different ones.

My advice would be to invest in the following initially:-

Two part Epoxy

Super Aliphatic Resin

Medium Viscosity Cyano

These three will enable you to carry out most of the gluing jobs you will encounter unless you are using a foam material. In this case I would suggest you buy:-

Gorilla glue

Foam safe cyano

Epoxy resin

As and when you come across a job requiring something different, just refer to the articles above to decide which is the best option for your needs.

There is one specific area I haven’t discussed because it is such a specific application and that is gluing clear plastic components such as canopies.

Sticking clear plastic parts presents problems. The fumes given off by polystyrene cements and even cyano glues can make the clear parts become foggy.  It is very difficult to use epoxy resin glues on clear parts without smearing the glue on the parts.

A number of modelling suppliers have produced their own special glues specificallyCanopy Glue designed for clear plastic parts and these are generally called ‘Canopy’ Glues.  They tend to be of low viscosity so it is easier to apply them in small amounts using a fine brush

Not only do they not affect the clear parts but they dry completely clear so any excess is almost invisible The bond is not as strong as some of the glues in the previous sections, but it is normally quite adequate.

If you have any questions please feel free to contact me through the comment facility at the bottom here. Also if you have enjoyed this post please feel free to share it on any of your social media sites. You may also enjoy my main site www.rookiercflyer.com especially if you are just getting started in RC planes.

Till the next time, all the best.

Colin

 

 

 

December 11

Understanding & Caring For NiMH Rechargeable Batteries

4.8V New Technology NiMH Receiver Pack
4.8V New Technology NiMH Receiver Pack

Those of you who have chosen to go down the Glow/Nitro power source for your trainer will have to use a battery to power their receiver. This is most likely to be a NiMH Rechargeable Battery.

On the other hand, if you’ve chosen an electric motor option then it is very likely that your receiver will take its power from a ‘Battery Elimination Circuit (BEC) integral with your Electronic Speed controller (ESC). Some ESCs are supplied without a BEC option in which case you may choose to power your receiver via a separate battery.

Many people new to rc flying have very limited knowledge of NiMH batteries and how to care for them. This post will provide all the information you need to get the best out of your NiMH rechargeable batteries.

A Bit Of History

The first miniaturised rechargeable batteries suitable for radio control aircraft applications were Nickel-Cadmium (NiCd) types. The technology for these batteries was perfected in the 1950s. The only alternative at the time was the throw-away alkaline batteries which, although having greater capacity than the NiCds, could not be recharged.

These NiCds would only run for 1/10th to 1/5th as long as alkaline types. The big advantage was that when the batteries did run down, they could be recharged several hundred times.

Unfortunately the Cadmium used in the cells is poisonous and harmful to the environment. Disposal of NiCd cells presents an environmental issue to such an extent that several countries in Europe have banned NiCd batteries for just this reason.

The introduction of Nickel-Metal-Hydride (NiMH) batteries during the 1990s provided a safe alternative. With very similar properties to NiCds, but with higher capacity, and more importantly, no super-toxic components NiMHs rapidly became popular with rc modellers.

Electrical Characteristics of NiMH Rechargeable Batteries

The characteristics we are concerned with are as follows:-

a) Voltage  b) Maximum Current  c) Capacity  d) Self-Discharge Rate

Let’s take these one at a time.

Voltage

Although a NiMH battery voltage starts out at about 1.4V it drops to 1.2V almost immediately. As it discharges in use, the voltage remains relatively constant, dropping only to about 1.1V just before it is fully depleted. 

Maximum Current

Although a battery produces a voltage, the equipment it is powering requires that voltage to provide current, measured in Amperes (A). Think of voltage as water pressure and the current as the rate of flow. The water pressure won’t be any good if the tap ( I believe that this is called a ‘fawcet’ in the USA) holding it back is closed. Batteries are limited as to how much current they can produce by their internal resistance. Your tap can’t provide as much water as a fire hydrant, even though the pressure may be the same.

NiMH batteries are able to deliver significantly more current than disposable alkaline batteries. This makes them well suited to high-current devices like our more powerful servos and even smaller electric motors such as those used in smaller planes.

Capacity

Using our analogy of the water pressure and rate of flow as Voltage and Current, Capacity is the total amount of water available from the container it is stored in.

A good AA NiMH battery has a maximum capacity of about 2.5Ah although they are available in smaller capacities. Theoretically a 2.5Ah NiMH can deliver 2.5A for one hour. Such a battery could, for example, deliver 0.5A for 5 hours (since 0.5 × 5 = 2.5) or maybe 1A for 2.5 hours, even 10A for 15 minutes. The common factor here is that Amps x Hours = 2.5

Self Discharge Rate

Traditional NiMH batteries have one serious disadvantage: self-discharge. This means that a fully charged NiMH battery will lose more of its charge the longer it is stored before use.

A good NiMH battery’s self discharge rate is about 1% per day. Thus, after about a week it would be at 93% charge and after a month, 73%. After three months it would only be holding 40% of its fully charged state.

This is not an issue so long as one gets into the habit of recharging these batteries the night before a flying session. The 1% overnight loss is of no great significance.

Low Self-Discharge NiMH Batteries

NiMH technology has progressed recently and we now have a new generation of low-self-discharge batteries which can retain up to 85% of their charge after sitting idle for a whole year. This is a major breakthrough for rc flyers as it now enables us to charge a transmitter and have sufficient power for several weeks use before having to re-charge.

Many modern transmitters have very low current consumption so that even modestly rated NiMHs will last well. The voltage level of transmitter batteries is usually shown on the front status display. Receiver batteries are required to drive several servos and a receiver so cannot be expected to last so long. Even so, several flying sessions should be possible with these NEW GENERATION NiMHs.Battery Capacity Tester

Because it is not necessary to recharge after every flying session, it is all the more important to check the remaining capacity in the battery at the beginning of each session.

I strongly recommend you invest in a Battery Capacity Checker as shown here. USA visitors can order this on-line by clicking the image right. UK visitors can purchase by clicking this link below

Battery Capacity Checker

How to Charge NiMH Batteries

There are two types of charger available suitable for charging NiMH batteries. They are commonly described as “DUMB” or “SMART” types.

“Dumb” Chargers or Overnight ChargersDumb Battrery Charger

This type charges the battery very slowly, typically taking 14 to 16 hours to fully charge a dead battery. It is very important to understand that when the battery is full, the charger will continue trying to charge. As the battery is already fully charged, the surplus charge is turned into a small amount of heat. This low heat won’t harm the battery as long as it isn’t allowed to continue for too long.

If you use a “dumb” charger it is important to remove the batteries from the charger when the charging is complete. Most dumb chargers are designed to charge at a rate of C/10 (battery capacity ÷ 10). This takes 14 to 16 hours for a full charge. However, this is only the case if the batteries were fully depleted before beginning the charge and the charger is actually rated to supply the batteries full capacity divided by 10 (e.g. a 200mA charger will charge a fully discharged 2000mAh battery in 14 to 16 hours. Partially depleted batteries will reach full charge sooner. The problem is knowing when this has happened.

You may be wondering why, if the charge rate is C/10, the charge time is 14 to 16 hours. one would expect this to be 10 hours. It is recommended , at this charge rate, that the time should be 1.5 x 10 – hence the anticipated time of 14 to 16 hours.

IMPORTANT: If you use the charger with batteries of a higher capacity than it was designed for, a full charge will take longer than 14 to 16 hours. Thus, in the case of a 1000mAh pack, using a “dumb” charger with an output of 50mA the charge time will be around 30 hours for a totally discharged battery (1000/50 x 1.5).

In short, properly charging with a dumb charger is a guessing game.

“Smart” ChargersSmart Battery Charger

“Dumb” chargers became popular because they are inexpensive to make. Although overuse of such a charger can damages the battery, this damage takes the form of a gradual reduction in capacity rather than a complete failure.

Fast “Smart” chargers have become very popular. In addition to not overcharging batteries, they charge much faster, typically in one to five hours. The reason fast chargers are not “dumb” types is that overcharging at these higher rates can result in critical damage to the battery.

A good fast charger normally uses one of two methods to determine that the charge is complete:

  1. a) −ΔV (negative delta-voltage) This method detects the voltageNiMH Delta Peak Detection Graph drop that a NiMH battery exhibits if you attempt to keep charging it when it won’t take any more. (The ∆ symbol represents “change”). The graph right shows the charge characteristics of a typical NiMH battery using a ‘constant current’ charger which has cut off having sensed the delta peak voltage drop.
  2. b) ΔT (delta-temperature ) This method detects the temperature increase once the excess charging current starts getting turned into heat.

Some chargers are designed to use both methods. −ΔV as the primary method and ΔT as a backup in the event −ΔV should fail.

Memory Effect – Myth or Fact

As NiMH batteries age the time between recharges gets shorter. A phenomenon called “memory effect” is blamed for this. It is considered to have been caused by the repeated partial use of the battery’s capacity before recharging it. It seems to “remember” that only part of its capacity was used before it was recharged, and thus refuses to deliver more than that.

The only circumstance under which true memory effect can occur is in cases where the charge and discharge cycles are identical every time. One of the few situations where this occurs is in satellites that are orbiting the Earth. These charge their batteries using solar power for some period of time, and then operate from them during the night. These cycles are exactly the same length every time.

NiMH Battery Life

NiMH batteries looked after properly will last for years. They do eventually deteriorate and there is a limit to the number of charge/discharge cycles that they can tolerate. Manufactures typically quote 300-500 cycles but these are FULL charge/discharge cycles and normally ‘Fast’ charges of 1xC or more.

The NiMH rechargeable batteries in our radio system should last far longer, probably for 1000’s of part discharge cycles.  The more carefully we use and recharge these batteries the longer we can expect them to last.

I hope this post has been helpful and informative for you. If you have enjoyed reading it you may get further help from my full website at www.rookiercflyer.com especially if you are just getting started in model plane flying. Please feel free to share it with groups or individuals you think could benefit from it.

Catch you again very soon.

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