Tuning the radio controlled airplane engine

One area of learning the basics of flying radio controlled model airplanes that we have barely skimmed over up to this point is the tuning of the nitro powered model plane engine.  This is a very important part of getting ready to fly.  But, before you fly with a brand new engine, it should be properly  “broken in”.

You will hear discussions on the “right” way to break in a new engine, but really there is no one way that is the best way.  What is important is that you run the engine either on a test stand or mounted in the airplane until it starts easily and runs smoothly without excessive heat.

It is important that you select the proper fuel for the engine.  You have choices of the type of lubrication that is in the fuel as well as the nitromethane (nitro) content.  Model engine glow fuel can either contain castor oil or synthetic oil, or sometimes a combination of both.  As far as the amount of nitro in the fuel is concerned, basically for your trainer 10% to 20% will give you adequate performance.   Check with your hobby dealer or your instructor to find out what fuel is commonly used in your area.

Although you can successfully break in your engine while it is mounted in the airplane, there are a couple of reasons why I like to use a test stand for at least the first few tanks of fuel.  When the rc model airplane engine is running, oil will be blown out of the exhaust and if you use a stand, you won’t have to clean the plane.  Also, if your test stand is mounted at a comfortable work height, it will be easier to work with the adjustments and make needed repairs.  Either way you need to run a few tankfuls of fuel through you engine at various operating speeds and fuel mixture settings.

The engine should be fitted with the proper size and pitch of propeller as recommended by the manufacturer, before starting the break in process.  If you are going to leave the engine mounted in the plane, make sure that is well secured in place.  Either use some type of hold down device or have a trusted individual hold the plane.  While tuning and breaking in the engine, you will have to run it at varying speeds, so you will want the radio system turned on.  Remember, if you are doing this at the flying field, you must be cleared to use the frequency that your transmitter is set up with.  Also, if there are other people flying their models, you should be away from the flight line to help with the noise problem.  These are some more reasons that I prefer to use a test stand at home for the break in.

With your test stand securely mounted to a table or a workbench, it is time to get the engine running and tuned.  You will need a source of energy for the glow plug and some means of connecting it to the glow plug.  We will go into setting up a flight or field box in a later post.  Starting and running the engine has its own set of safety hazards.  You will need sufficient room all around the test stand to allow you to keep away from the propeller and especially away from the ends of the prop.  Tip:  A black or other dark colored prop will be nearly invisible while it is spinning.  To make it more visible, dip the ends in some white model airplane dope. Never stand in line with the propeller.  If the prop is cracked or damaged in some way, at high speeds it could shatter and someone could be hurt.   When the engine is running it is safest to stand behind the engine.

Any internal combustion engine needs three things to start and run.  Air, fuel, and a source of ignition are all required in the right mixture to make the engine run properly.

Fill the fuel tank with fuel by disconnecting the fuel line from the carburetor and connecting a fuel bottle or pump to the fuel  line that is connected to the tank.   After the tank is full, reconnect the fuel line to the carburetor.

You need to make an initial adjustment to the needle valve setting before you start the engine.  Check with the manufacturers instructions on the recommended inital setting.  If you don’t have the instructions, it is common practice to gently screw the needle valve all the way clockwise until the needle contacts the seat.  Be careful not turn it too tight, because you could damage the needle valve or the seat.   Back off the needle valve two full turns.  There is a mark on the knurled knob th help you count the turns.

Finally, we are at the point of starting the engine.  It takes longer to explain how this done than it does to actually do it, so please bear with me.  The engine has no fuel in it at this point and it will not be able to draw fuel from the tank without a little coaxing.   Do not connect the battery to the glow plug just yet.  If you are going to start the engine by hand, place you finger tightly on the carburetor’s air inlet port and turn the prop counterclockwise ( looking from the front) and watch the fuel line to see when the fuel reaches the carburetor.  Don’t overdo this step because you can easily “flood” the engine and make it very difficult to start.

Now, it is time to connect the battery to the glow plug and start the engine.  Open the throttle a little (you will quickly learn how much) to allow some air into the carburetor. Give the glow plug a few seconds to heat up and then using a “chicken stick” to keep you fingers off the prop, flip the prop smartly counterclockwise until the engine fires and begins running.   At this point it will not be running smoothly.  Slowly open the throttle and listen to the sound of the engine.  It may take several attempts to get  the engine to run through the full range of speed, but keep slowly turning the needle valve in until the engine breaks in to a full two-cycle sound when at full throttle.   Then back it out about 1/8th to 1/4th turn and let run as rich as you can and still keep it running.  At this point unhook the battery from the glow plur.

You have to be careful not to lean out the engine until it has had a tank or two of fuel run through it.   The terms “rich” and “lean” refer to the ratio of fuel to air.  A rich mixture is a high fuel to air mixture and a lean mixture is a low fuel to air mixture.

When you have it running on the rich side vary, the speed by changing the throttle setting.   Don’t let it run at a constant speed too long at a time.  The engine can “load” up with fuel if it runs too long at a slow speed and it could possibly develop excessive heat if you let it run wide open for too long while it is still brand new.  Let the first tank of fuel run through the engine while varying the speed from slow (just above idle) to wide open at a very rich fuel mixture setting.

Watch the amount of fuel in the tank so that you don’t run it empty, because as the tank gets empty, the fuel mixture rapidly gets leaner and the speed will increase dramatically and that is not a good thing to do at this point.  You should stop the fuel flow by pinching the fuel line between the tank and the carburetor with you thumb and finger thereby stopping the engine.

Le t the engine cool down slowly and repeat the fueling and starting process.  This time and each successive time you should not change the initial needle valve setting.   Run three or four tanks of fuel through the engine and each time you can work your way closer to getting it up to full two cycle, but do it slowly.

When you have completed the “breakin” process, reinstall the engine in the plane and be prepared to go through fine tuning when you start working with the instructor at the flying field.  He will want to make sure that it is running properly so that you can benefit the most from your training.

I want to make this blog as informative and accurate as possible so as to provide the best possible instruction for beginning modelers.  So, I welcome comments from beginners and experts alike.   Thanks for your support and I’ll see you next time.

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One Response to Tuning the radio controlled airplane engine

  • Dwight,
    The blog looks very good. I can tell that you know your subject material very well. However, it’s over my head. No pun intended.
    Are you familiar with a ham radio connection to RC planes? I know of some hams who are into RC flight. Is this a possible topic for a future posting? If you want more info check out http://www.arrl.org, web site of the Amateur Radio Relay League.
    Best wishes.
    John