Nitro (or glow fuel) is a liquid fuel used to power small specialist reciprocating (piston) internal combustion engines. Nitro is made chiefly of methanol (wood alcohol), nitromethane and natural or synthetic oil; other additives can include dye (so you can see the fuel level in the tank), anti foaming agents, and corrosion inhibitors.
The addition of nitromethane has several advantages, including more power, more reliable engine idle, and cooler running, at the expense of more expensive fuel and faster fuel consumption. Helicopter engines typically give the best performance if run on 15% to 30% nitro.
Nitro engines achieve combustion from compression and a glow plug (similar to a diesel engine). The glow plug is heated electrically with a glow starter when the engine is first started, and afterwards is kept hot by the reaction of the methanol with platinum catalyst in the glow plug wire.
Nitro engines are sized by their displacement, measured in cubic inches (or, really, hundredths of a cubic inch). The engine in a typical model helicopter may have a displacement of 0.30 to 0.50 cubic inches, and so are called 30 or 50 sized helicopters.
Both methanol and nitromethane are poisonous, primarily if ingested, but to a lesser extent through inhalation and absorption through the skin. Reported LD50 (death of 50% of test populations of rats) of both substances for ingestion are:
- Methanol: 5628 mg per kg of body weight.
- Nitromethane: 940 mg per kg of body weight.
Nitromethane is degraded by UV light and should therefore be stored out of sunlight. Methanol is highly hygroscopic and will absorb water from the atmosphere, which will affect how well the fuel burns.
The byproducts of combustion of nitromethane are highly corrosive. For this reason, before storing a nitro helicopter, the tank should be refilled and drained with fresh nitro to flush out combustion gas used to pressurize the fuel system during flight. The engine, too, should be cleaned, both to clear out combustion gas, and remove and remaining methanol that will serve to absorb water and corrode the engine internally; this can be done by disconnecting the fuel line and attempting to start the engine, then when the engine will no longer start, running the starter again for a few seconds at full throttle.
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