Moore-VARIO
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Joined: 1/1/2002
From: CA
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While brousing Rotory Modeler Magezines' website, I copied this article to my clipboard a long time ago.
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HIGH PERFORMANCE HELICOPTER FUELS
Nitromethane: can truly be deemed "Liquid Horsepower" for model helicopters. For additional power and generally smoother running, our flying machines can't really afford to live without it!
If the engine performance of your R/C helicopter is proving to be a bit of a problem for a whole tank of fuel, and you suffer from overheating as the tank empties during hover, or the idle is difficult to get "just right", it may be that a dose of good old fashioned "nitro" is what the engine doctor would order.
Nitromethane is a liquid that has been around for a long time and is used to contribute towards smoother running and increasing power in varying amounts of many model fuels. In addition to altering the power output of an engine, it also contributes towards cooler, cleaner running, smoother idling, and easier starting. Of course, the positive affects of nitro must be weighed against the cost of the magic liquid, which can add from $2 to $4 per gallon per 5% of nitro.
Other than the plain fact that nitro is probably the easiest way to increase the power output, or smoothen the run of a model aircraft engine, the whys and wherefore's of nitro are not well understood by most modelers (or anyone for that matter, based on the trouble I had finding information on the subject) so what follows will attempt to shed light on the mystery and help heli flyers decide whether they need use it, and if so, how much.
Nitro is manufactured in production volumes by mixing nitric acid and natural gas (or other hydrocarbon base) under high temperature and pressure. It can be made in the laboratory by some complicated mixing and distilling of acetic acid, sodium carbonate, and sodium nitrate which is rather hazardous.
The element that is most important is the oxygen which "disassociates" from the liquid at high temperatures. While Methanol has almost the same amount of oxygen (50%) by weight, it is the overall "mix" that contributes to the unique nature of nitro, allowing a much higher fuel flow and the typically inert nitrogen which can serve to "soften" the shock of the combustion process and inhibit pre-ignition (this is not to say using nitro prevents pre-ignition).
All fuels, whether gasoline, methanol or nitro (which incidentally can be burnt at 100% mix like most fuels) have a "stoichiometric" (I brought this word in in a wheelbarrow!) or chemically correct air to fuel ratio, at which they theoretically (as calculated by chemists on paper) burn the most efficiently in air. With gasoline it is 14.9:1 (air to fuel) with best power at 12.7 and best fuel consumption at 15-16:1. Gas puts out 2.78 kilo Joules of energy per kg. Stoichiometric methanol burns best at 6.5:1 or twice the liquid (by volume) for the same amount of air as gas and produces 2.67 kJ per kg, slightly less than petrol, but typically produces 10% more power due to the temperature drop of the mixture as it vaporizes, which produces a more dense mixture (higher density = more power). Methanol burns twice as much liquid as gasoline because it carries its own oxygen supply along with it (50% by weight).
Methanol can also run 40% rich and still make good power because of this. This excess fuel contributes to cooler operating temperatures. Nitro burns at a big 1.7:1, or 37% liquid, 63% air, or nearly three times as much liquid as methanol. Energy at stoichiometric = 4.05 kJ per kilogram or 1.5 times that of methanol. This is where the effects of nitro become important.
Getting fuel into an engine is never a problem. The problem with producing power from a given engine is getting the air in! Hence, the use of superchargers, turbochargers, special manifolding, porting and valving arrangements on modern car engines.
With model engines in general, we don't have the luxury of supercharges, etc. (the 0S 120 Supercharged four stroke being the exception). So Nitromethane actually provides "chemical" supercharging, introducing up to 3.8 times more liquid overall or 5.5 times more oxygen per liter at 100% "stoichiometric" mix, meaning more fuel (methanol) can be burnt, because of all the extra oxygen (the oxidizing agent).
For example, a methanol only mix provides 400 grams / liter of oxygen (gasoline has zero oxygen). At 20% nitro, there is 3.14 kJ/kg of energy and 436 g/l of oxygen, and because at 20% nitro the correct mixture or air / fuel ratio is about 4.2:1, a 35% increase in fuel flow will occur, which means around 47% more oxygen ends up in the engine when tuned correctly.
I know this sounds complicated, but I did check my math repeatedly, and it all makes sense if you remember that we are talking about quantities here in two different situations: specific quantities per liter and quantities per liter at the "stoichiometric" mixture fuel flow!
This increase in oxygen availability and fuel flow amounts to richer running. For example, the main needle has to be opened further to flow the correct amount of liquid to match the incoming air (which |
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All Forums >> Glow Engines, Gas Engines, Fuel & Mfg Support Forums >> RC Fuels >> Do we really need Nitro??? 
Do we really need Nitro??? - 
Nitro - 
Do we really need Nitro??? - 
Thanks Guys nice Replies - 
but nowadays it's being used to mean any fuel with zero nitro. 
