Which engine?

Which engine?
Like many other things, a good engine is worth it's weight in gold. A sub-standard engine that won't run will send you to the crazy house and maybe (God forbid) make you give up this hobby altogether.

Most helicopters will have a recommended engine. The most popular type of engine in R/C helicopters is the glow engine which takes a fuel mixture of nitromethane, synthetic (or castor) oil and methanol. There are other alternatives though, including electric powered helicopters and petrol engined helicopters.

You can get either two-stroke or four stroke engines.

There are a number of different engine manufacturers around, OS, YS, Thunder Tigre, Novarossi, Rossi, etc.

Again, engines come in all different sizes. In the two stroke line for helicopters there are the 30 size engines followed by the 46, 50, 60 and lately the 80 and 90 sizes for larger helicopters.

You want to match the engine capacity to the size recommended by your helicopter kit manufacturer.

When you're looking to buy an engine, you've got to take into account a number of things. Some of these are:

How much it costs.
Obviously important in any purchasing decision.
Fuel requirements.
Some engines take a different fuel mixture than others. For example, some manufacturers recommend a higher nitromethane requirement than others. Nitromethane isn't cheap, so you might want to take that into account.
Engine reputation.
Some engines are absolute dogs to get running correctly. Make sure you ask around and get some experienced opinion before purchasing.
Local knowledge.
If you're getting into your first engine, you'll probably need all the help you can get to get optimum performance from your power plant. Try and get one of the more popular engines at your field.

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Two Stroke
Two strokes are by far the most popular helicopter engine. Most aren't too hard to set up and produce good, reliable power.
One of the OS line of engines. Very popular modelling engines.

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Four Stroke
Four strokes aren't very popular just yet. They often require a lot of tuning and can be somewhat underpowered. It sounds like a tractor but it can really haul.
OS's 52 four stroke engine.

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Electric
Electric powered helicopters are slowly becoming more popular as battery technology advances the capacity of batteries allowing for extended flight times.
Electrics are popular for indoor flying and are often installed in small helicopters. However, there are larger electric helicopters which can easily do 3D aerobatics.

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Petrol
Petrol engines are based upon chainsaw or weedeater engines. They take a mixture of petrol and two stroke oil as used by chainsaws. Petrol's have tonnes of torque at low speeds which means they can often hover at around 30% power (depending on your setup).
They're very economical to run both because the cost of fuel is low and their fuel consumption is also very low. For example, most petrol helicopters will fly for about 30-40 minutes on a tank of gas.
Hanson modified Zenoah G23 petrol engine.