2) Fuel System

Often misunderstood, the fuel system operates on very straight forward principles.
  • I find it helpful to think of the fuel being pushed into the engine rather than being sucked in.The air around us is at a pressure of 1000 millibars or 14.7 psi, give or take a bit, when the piston travels up the bore in one of our engines it lowers the pressure in the crankcase and the pressure outside pushes air in through the intake, the greater this pressure differential the faster the air will travel - it is not sucked in, it is pushed in. As air moves faster it's pressure is lower (this is the way a wing produces lift, the air passing over the top travels further and therefore faster than the air passing under the wing, the pressure on top is lower than on the bottom and the wing is pushed up by this pressure differential), the needle valve is positioned in the intake and often across it at a point where the intake air is travelling at high velocity, it is therefore at a point of low pressure, the fuel in the tank and line is at a higher pressure and is pushed through the needle valve by the pressure differential.
  • Whenever this pressure differential changes,the amount of fuel pushed in to the engine will change. A model in flight is subject to great changes in 'G' force and depending on the position of the tank there will be a change in fuel delivery. You can use this to your advantage if you understand it, though you will see many examples where that is not the case.
  • A control line model with the tank mounted outboardof the engine will lean out as the speed and line tension increases,if the tank is mounted inboard the it will richen up as the speed increases, successful speed and racing fliers all understand this and use it to manipulate the fuel feed and therefore the engine run. You will have seen and possibly may have experienced the leaning out example on one of the many profile design models with the tank stuck on the side of the fuselage behind the engine.

  • Another example of an outboard tank problem is a stunter which is supposed to switch between 2 stroke in manouvers and 4 stroke when flying level but tends to break into 2 stroke and not return to 4 stroke, such a model probably has the tank in an outboard position and is leaning out as its speed increases, it keeps going faster and leaning out more till the horsepower and drag balance, install a narrower tank and it is possible to have it so it only leans out when the nose is pointing up. Once you have the position close to right you can pick adjustments of 1mm and even less on fast models.

  • For control line I can't think of a situation apart from models suited to bladders such as F2D, where you shouldn't be using a uniflow tank, it is a good design which reduces the change in fuel pressure at the spray bar as the fuel is drained from the tank. A non uniflow tank will lean out more and more as the fuel is used up, you cannot prevent it. A properly made and installed uniflow will deliver fuel at a nearly constant pressure throughout the run - use them they are good.

  • Keep your fuel clean, cool and dry. I don't use filters in my models, in my experience anything which comes through in the fuel which will block a needle valve will also block an in line filter, so my advice is to filter the fuel before it goes in the filler bottle or as it goes through your pump, if you are using a syringe to fill then either make sure the fuel in the can is clean or use a filter as you fill the syringe and take it off or turn it around to fill the model, don't drop the end of the fuel line in the dirt, I have seen people fill their syringe using an in line filter and then push the fuel back through the filter into the model without turning it around, any crap on the filter will be in the tank.

  • Cool and dry, well you can only try, methanol will evaporate leaving you with a mixture different to the one you thought you were using so keep the lid on your can. Also methanol will absorb a lot of water if you give it a chance and while you won't notice a bit there is a point at which your engine won't run as well as it should, and the same goes for diesel fuel but the ether in your mixture will evaporate very quickly, particularly in hot weather, so try to find some shade to store it and don't mix big batches, as obviously the longer you take to use it then the more time you are giving your fuel to absorb water and evaporate.