Fuel Details

Model engine fuel is a mixture of methanol (60%-80%), nitromethane (5-15%, sometimes even 30%) and lubricants (at least 18% recommended for my engine, but up to 25% exists). Typically japanese engines are spec'd on high nitro percentages, while european engines run on lower percentages.
My heli can fly about 17 minutes on a tank of 300cc (tank volume from chief aircraft inc. ), so that makes slightly less than 1 liter per hour. The local shopman claims that his Mosquito (Robbe) with .45 engine also runs at 1 liter per hour, but I already knew that my engine is tuned very rich.

Methanol

Methanol (CH₃OH, sometimes called methylalcohol, wood alcohol or meths) is the fuel that drives your engine:
2 CH₃OH + 3 O₂ -> 2 CO₂ + 4H₂O + 22688J/g (Evaporation: 1100J/g. Heating up: 2.5J/gK)
Assuming the engine temp is 125°C and ambient is 25°C, the heating up and evaporation together requires 3600J/g and 22688 comes out, giving a net 19088J/g. Note: I'm not sure how far the methanol is heated before it burns, so this maybe wrong. I read somewhere that the temp inside is about 50°C higher than the head temp, and somewhere else I read about tests measuring between 180°C and 200°C.
Interesting quote (from here): "Methanol is the first oxidation product of methane. Further oxidation produces formaldehyde, then formic acid, and finally carbon dioxide and water (CO2 & H2O), which is ultimate - you cannot burn water! All of these godies occur in your engine and produce heat which does the work.".

Nitro

Nitromethane (CH₃NO₂) burns as follows:
4 CH₃NO₂ + 3 O₂ -> 4 CO₂ + 6 H₂O + 2 N₂ + 10531J/g (Evaporation: 560J/g. Heating up: unknown)
(It can also explode without air: CH₃NO₂ -> CO₂ + NH₃!)
Again assuming the engine temp is 125°C and ambient is 25°C, and assuming the same heating up numbers as for methanol, the heating up and evaporation together requires 3060/g and 10531 comes out, giving a net 7471J/g.
As you can see, the same amount of oxygen can burn twice as much nitromethane as methanol. Thus, the nitromethane kind of replaces the oxygen required to burn the fuel. So instead of oxygen you can cram more fuel into the engine. If you do so, you will of course run more quickly out of fuel now! This power increase is not really dramatic, about 45% more than pure methanol when using 50% blend (reference), but you dont use such high percentages of nitro in model engines. Typical values as 15% probably give some 13% power increase. But another good reason to add nitromethane is to get a more smooth running engine at idle speed.

Some people suggest that more nitro decreases the temperature of the engine, I did not yet figure this out exactly, but from trying 5, 10 and 15% I have the impression that this is indeed the case (although the effect is very small). Here follow the facts that I collected in trying to explain this. NOTE: I'm not a chemics expert so the following is probably not complete and maybe partially incorrect.

Because nitro requires only half the amount of oxygen as compared to methanol, you could theoretically tune the engine twice as rich with 100% nitro. (More recently I heard a mix ratio of 6.5:1 for air:methanol and 2.4:1 for air:nitro, which is even close to a factor three.) Assume you burn 1 gram methanol or 2 gram of nitro per second (note: a .5 cubic inch engine uses about 0.4g/s. Later note: we should probably use l/s instead of g/s?). Methanol would give a net heat production of 19088J, while nitro would give 2*7471=14942J. As this heat is produced per second, we have 19088 and 14942W. So methanol produces 28% more heat (although we assumed that twice as much nitro can be crammed into the engine!).
On the other hand, the burning nitro gives much more volume increase. To understand this, remember that plain air contains only 20% oxygen next to 80% nitrogen. Therefore the real equations are more like
4 methanol + 6 oxygen + 24 nitrogen -> 36 exhaust
while
4 nitro + 3 oxygen + 12 nitrogen -> 24 exhaust
So with methanol we put in 34 which is expanded to 36 (6% volume gain apart from temp rise), with nitro we put in 19 which is expanded to 24 (26% volume gain apart from temp rise).
Note that the efficiency of the engines must be pretty low: 736 W = 1 HP so the heat production of a 0.5 engine is in the order of 0.4*15kW=6kW=8HP while their claimed power delivery is only 1.8HP.
The tricky point of the above reasoning is that the ignition moment of nitro may be different from the ignition point of methanol. If for instance nitro would bearn quicker than methanol, you probably would have to tune the carburetor mixture richer to slow down the burning, in order to get the engine run smooth. Concluding, I'm still at a loss on what to expect from running more nitro.

What is also very important with respect to the temperature of your engine is proper lubrication: more friction gives a higher temperature.
A nasty aspect of nitro is that it when it comes in contact with water (which is produced by burning fuel!), nitric acid comes free. Nitric acid is difficult to neutralize for after-run oil, and may cause corrosion of the bearings.
Maybe important for the do-it-yourself fuel mixers: nitro is stable for about 2 weeks, protected from light, at temperatures up to 60° C. But I think that longer storage is no problem, as I have no problems with longer-stored nitro on my shelves. Also I heard that nitro is more stable once mixed with methanol. I have flown with a year old mixture without problems.
You can find more details on nitromethane on power master fuels.

Oil

The oil has to mix properly with the methanol (therefore car oil is not good), and should not burn in the high temperatures of the engine. The 'smoke' that comes from your engine is mainly fine drops of oil, only when your engine gets very hot you may see real smoke. As helicopter engines usually are hotter than plane engines, wonder oil (pressed out of the seed of ricinus tree, C₁₁H₁₀O₁₀, eg Castor oil) may burn. Therefore, synthetic oil is safer to use for helicopters. Some recommend adding a little (3%) wonder oil because of the anti-corrosive layer it covers your engine with. Lately I heard that carbulin contains a few percent castor already.

Toxic

Methanol is highly toxic. "Short term exposure may result in major temporary or permanent injury; may threaten life" (see chemrest.com). You can get blind and worse if swallowed. Be careful with this stuff! Nitromethane is not very toxic. "short term exposure may result in minor injury that is reversible.". Some oils are toxic, especially those that dissolve in water (eg, Klotz). Others are less toxic. I learned from reckless loony at RunRyder forum that Castor oil can be made non-toxic by removing the toxic recin by boiling the oil in water. The recin dissolves and the cleaned oil can be skimmed off the water. I have also seen 'hydrogenated castor oil' in sun creams etc.

Water in the fuel

Avoid getting water into the fuel. Even leaving the bottle open for some time will cause water to contaminate your fuel, because methanol rapidly 'catches' the water/moisture out of the air we breathe... So avoid superfluous contact with air and keep your tanks closed as much as possible!

downunder-RCU at the RC Universe forum did a few tests on a 80% methanol 20% castor mixture (no nitro) to see how much water is acceptable. He found that 2% or 3% water does not give any noticeable differences. However with 4% water the "engine won't start, runs awful, won't accelerate".
Also he found that with 4% water, after 10 minutes all the oil had gone to the bottom of the bottle. With 3% water, the oil also separates, it only takes a bit longer. Also he found that the amount of water that can be added (before the oil separates) depends linearly on the temperature: at -10°C 0.25% water is already the limit, while at 39°C 2.5% seems the limit on the long term.
I once read that you can put contaminated fuel in the freezer, that the water will freeze and that you can get out the ice. I have no idea if this really works.

Fuel prices

To give you an impression of the fuel prices, here are the prices of Quartel Pijnacker per gallon (3.8 liters), incl. BTW (VAT). (NOTE: I don't update this frequently, this is just to give an indication, and I'm not responsible for mistakes)

Coolpower 5% nitro	&euro 20,- (Vasco modelbouw, 2005) (fl. 39,- Quartel 2001)
Coolpower 15% nitro	€ 24.95 (VASCO modelbouw, 2005) (fl. 49,- Quartel 2001)
Coolpower 30% nitro	&euro 32.95 (Vasco Modelbouw 2005) (fl. 79,- Quartel 2001)
Q-Mix Heli C0, 5 liters, 16% Carbulin oil, 0% nitro (Quartel)	€ 12,50 (3'04)
Q-Mix Heli C5, 5 liters, 16% Carbulin oil, 5% nitro (Quartel)	€ 14,50 (april'04; was €21,- between 8'02-6'03,they lowered the price :-) )
Local brand fuel, 5 liters (2 brothers Bussum), 5% nitro, 16% titan oil	fl. 39,- (2001)
Local brand fuel, 5 liters (2 brothers Bussum), 15% nitro, 16%? titan oil	fl. 51,50 (2001)
Tornado, 5 liters (april'01), 5% nitro	fl. 50
Tornado, 5 liters 15% nitro (april'04)	€35,-

For comparison, methanol costs &euro 2.15 per liter (fl.23,50 for 5l), nitromethane &euro 19,20 per kilogram (€ 21.89 per liter) (Quartel, 2004 - has become cheaper in 2004! ) and oils per liter fl.16,15 ("risel" oil), fl.28,95 ("h300" synthetic oil) and even fl.47,10 ("tex" synthetic oil). (from the pro-models 2001? catalog). Carbulin oil (and its replacement P2000 oil, Quartel, 8'02-3'04) €11.95 per liter.

Mixing yourself

I like to have the recommended 18% oil in my fuel for the OS, so I have to add a little oil to the Q-Mix (€4.20/l) that I am using. Also my engine runs a bit hot with 5% nitro, so I increase that to at least 10%, and since I get increasingly into 3D I switched to 15%.
The following table shows how much to add to come from one blend to another

For example, to get from the standard 16%oil/5%nitro C5 mix to 18%/10%, you have to add 59.7ml nitro and 37.5ml oil to each liter. This costs €1.75 per liter, and you end with 1.097 liter, giving a total price of €4.25/l.
Also note since the price reductions it's cheaper to mix starting with C5. If aiming at 15% you save &euro 0.55 per liter starting with C5 instead of C0. Both ways, you will save a lot compared to brandname fuel, for instance Coolpower 15% costs &euro 25 per gallon(3.8 liters) = &euro 6.57 per liter, which is €1.22 =23% more expensive per liter!