Nitromethane is great stuff. It makes simple glow-ignition engines start easily, run dependably, and produce more power. There are engines designed to run with no nitromethane. These engines are difficult to start and tune. But flying with high nitromethane fuels is not advantageous for the causal sport flyer. After some book and field research, here is my conclusions. If technical ramblings bore you, skip to the conclusion!
What exactly does nitromethane do? As I understand it, here are the main two functions:
Octane is the measure of a fuel's ability to resist self-ignition due to compression. The timing of the ignition of fuel in a cylinder is critical for good performance and economy. The fuel is supposed to be ignited ignited a little before the piston reaches the top of its compression stroke. This allows time for maximum pressure to build as the piston continues its upward path. In most spark engines, this advanced timing is increased as engine RPM increases. When your car "pings" or "knocks" when pulling hard, the fuel is igniting on its own, before the spark plug gets a chance to ignite it, resulting in overly-advanced timing. The maximum pressure occurs well before the piston reaches the end of its upward travel. This is caused by too low an octane fuel being ignited under pressure, or by carbon deposits which act as a glow plug and ignite the fuel prematurely. This of course reduces power and can harm the engine. On the other hand, while late ignition reduces power and economy, it is not likely to harm your engine. Note that while using too high an octane fuel is a waste of money, the engine still runs fine, because ignition at the proper time is still effected, thanks to the spark plug.
But igniting the fuel in a glow engine depends in part on pressure to ignite it. The glow plug itself is not hot enough to ignite glow fuel that is not under pressure. Methanol has a high octane, and does not easily ignite due to pressure. Methanol is used as a fuel for high-performance competition engines in part because of its high octane, and in part because it runs cool. The nitromethane lowers the octane of the glow fuel so that it will more easily ignite under pressure. This is why nitromethane makes a glow engine easier to start and idle more reliably.
This means that the timing of the ignition of the glow engine is affected in part by the fuel we use. Using less nitromethane retards timing. Using more nitromethane advances it. So while too little nitromethane may cause excessively-retarded ignition, this isn't going to harm the engine. On the other hand, using too much nitromethane could cause preignition and engine damage.
Other factors affect timing too. These include the selection of glow plug (the hotter the plug, the earlier the ignition), air temperature (hotter air = earlier ignition), atmospheric pressure and altitude (higher air pressure or lower altitude = earlier ignition). Even prop selection is theoretically a factor. A larger prop decreases RPM and increases the load on the engine, effectively advancing ignition.
Engine design factors such as compression, RPM, and valve/port timing also effect glow ignition timing. Your engine is designed to use a certain range of nitromethane content. For example, Supretigre recommends 5% to 15% for their their GS40 engine.
The power produced by an engine is limited by the amount of fuel it can burn. The glow fuel that the engine burns is not only the fuel the engine burns. Alcohol and nitromethane won't burn in a vacuum. Your engine also burns air (specifically, the oxygen in the air). As you richen the carburetor to provide more fuel than there is air available to burn it, power decreases. We can increase power by providing more air. Methods of doing this (some more practical than others!) include using a blower to force more air into the engine, using a bigger carburetor throat, and flying on cool days at low altitudes. Better yet, use a bigger engine!
We could also increase power by using a glow fuel that does not require as much air. Nitromethane needs less than half the amount of air to burn as alcohol. So, all other factors being equal, we could more than double the power of our engine by burning straight nitromethane! In fact some engines are designed to do just this. So why don't R/C flyers do this? First of all, it costs about $30 per gallon. Not prohibitive for some, but there is another downside. There are fewer calories in a gram of nitromethane than a gram of alcohol. So we have to burn more of it to do the same amount of work. Even if you doubled your power so you can now fly as fast at half throttle as you could at full throttle burning alcohol, you still can't fly as long or as far.
The fact is that nitromethane isn't added to sport fuels to provide power. The power added by putting an additional 5% nitro in your fuel would not be detectable without a tach.
Nitromethane isn't added to sport fuels to provide power. Its function is to make starting easy and provide smooth idle and running. You won't notice the difference in power if you increase the nitro content 5% one way or the other. Sport engines are designed to run on alcohol, not nitromethane. If you need more power, get a bigger engine.
Unless you have already discovered 5% fuel like I have, you really should try a lower nitro fuel. I have switched from 10% to 5% (that's HALF the nitro!) and can tell no difference in performance, starting ease, or idle. The stuff runs great. Not only will you save a few of bucks a gallon, your engine will run cooler, you will get more flight time out of each tank of fuel.
IMPORTANT! The nitro content will affect your needle valve settings. If you go to a lower nitro content, you can lean the engine a bit. IF YOU INCREASE NITRO CONTENT, BE SURE TO RICHEN THE MIXTURE!
I have not tried 5% fuel in cold weather yet.
If you don't like the results of lower-nitro fuel, your investment is not wasted. Save it. After you have used a pint or a quart of your usual fuel, top the jug off with your 5%. You won't notice the difference.