Model Aircraft Engines
Posted by Scale Model Aircraft on
July 9, 2009
This latest insallment on the scale model aircraft blog aims to cover the basics and purchasing of model aircraft engines.
So first up, the basics.
Model aircraft engines are similar to their full size equivalents. They are available in 2 stroke and 4 stroke glow (sometimes referred to as 2 cycle and 4 cycle), diesel, gasoline and jet form. You can also find Wankel rotary engines but these are incredibly rare.
Engine Size
All model aircraft engines are expressed as a given size, which is their volumetric capacity in one hundreths of a cubic inch. For example, an engine with a .60 cubic inch capacity would be called a 60.
Glow Plugs
Nitro engines differ from a standard gasoline/petrol engine in that they use a glow plug instead of a spark plug to ignite the fuel-air mix each revolution. The glow plug needs to be heated initially with an igniter, but continue to run after this. They are also one of the most common parts to fail, so having a few spares is a good idea.
Carburetors
This is where the fuel is added to the air being sucked into the engine. An engine typically has one carburetor, which can be adjusted with an idle screw and a mixture screw. The idle screw simply adjusts the idle speed of the engine while the mixture screw adjusts the mix of fuel to air which has major performance impacts on the output. The last important part is the rotating barrel. As you increase the throttle on the controls, the barrel allows more air into the engine, thus producing more power.
Model Aircraft Engine Types
Glow Engines
Two stroke glow engines are the most popular in the model airplane engines. They are capable of very high RPM and produce more power as they fire once per revolution opposed to once per two revolutions in a four stroke. They are also cheaper, easier to run and easier to maintain.
Four stoke glow engines produce more low rev torque and sound great.
Gasoline Engine
These are generally used in much larger RC airplanes. Fuel for these is less costly which can offset the extra initial cost for the engine itself. There are many variations of gasoline engine – 2 stroke, 4 stroke, magneto ignition, electronic ignition etc. If you are even considering a gasoline engine, chances are you’ve been in the RC hobby scene for a while and already know what you are talking about.
Diesel
Diesel model airplane engines are not very common among model aircraft builders, but they are still an option. These engines provide a high torque and thus are able to move a larger propellor. They also do not require a glow plug or spark plug, as diesel/air mixtures ignite under pressure with a little heat generated by friction. Diesel engines are often used in vintage aircraft to preserve authenticity and due to the large propellors.
Gas Turbine
Exactly like a real turbine engine, but small. Incredibly loud and powerful, these engines are only used by the experts in the field. Obviously, they are very expensive! But, they are this cool.
Purchasing a model airplane engine
Before you go looking for an engine, you need to know how powerful it needs to be. Sometimes model frames are given a specific number i.e. “40″ as they are ideally suited to a 40 engine. If this is not the case, you need to work out your model airplane’s dimensions, weight and know what performance you want it to have.
Chances are, especially if you are a beginner, your best choice of engine is a two stroke glow. Model engine specifications are given with any motor you buy, unless it is used or out of the box. Power and torque specs need to match the ones required for the size of your aircraft. If you are buying your first engine, your best bet is to buy from a store.
If you have a bit of experience and know what to look for, one of the cheapest ways to get your hands on an engine is Ebay. However buying motors from an untrusted source can be problematic.
As much as possible, you should try to buy engines “Brand new in box”, as most of the time these will be exactly the same as if you bought it from a store. Similar to ANY ebay purchase, checking seller feedback is incredibly important to ensure you do not get ripped off.
Buying an engine that is used or not in the box, there is no way to tell what you will get. Sometimes you can pick up a really good deal but it is also very easy to buy a piece of junk. Buying a modifed model aircraft engine is probably the most risky move, and you should stick away from these.
Ask the seller a lot of questions to figure out the history of the engine – how much it has been used and how old it is etc. If you get clear answers then you are most likely in luck. If the seller tries to avoid the questions, saying things like they don’t know, they are selling it for a friend, or give vague or incomplete answers… avoid them. Once again, check their feedback and make sure it is positive over a decent amount of time.
However, there are exceptions to all of this. You may pick up an absolute bargain because someone is selling an engine they have no idea about. Conversely, you may get ripped off by someone who appears genuine. Use common sense, and hope for the best.
There are also other sources to buy online. Check out model and RC aircraft forums. Often companies may sign up and do group buy discounts or just offer better prices to forum members.
