Post by Modelman on May 4, 2005 11:58:16 GMT
Your Fuel Tank
Goodness knows, we have enough trouble keeping our "simple" little engines running properly - these seem to be a gazillion detail items that can cause them to run badly or not at all. Now we introduce another whole set of potential problem areas that affect the engines operation - the fuel tank and its set-up.
Obviously the engine must have fuel to run - but how EASY it is for the engine to draw fuel from the tank can have lots of effect on just how WELL the engine runs in various flight conditions. Essentially, the engine creates a vacuum in the carburettor throat while running; normal air pressure in the tank (or muffler pressure if you use it) then forces the fuel into the carb through the needle valves that you have diligently set.
The problems that can arise from your tank set-up can be grouped into several distinct areas:
Tank height:
The farther "up" we must draw the fuel to get into the carb against the force of gravity, the less likely it will arrive in sufficient quantity. And as we manoeuvre the plane applying "G" forces, the forces required become even greater. So we can't have the tank too low in the plane with respect to the engines carburettor. Most experts seem to agree that the centerline of the tank should be, optimally, about 3/8 - 1/2" below the centerline of the carb barrel. The tank should be mounted in the plane as near that goal as possible - but in some designs that can't be done! If you can't raise the tank, then lower the carb; since the carb is usually about an inch above the centerline of the engine, just side mounting the engine will lower the carb by an inch!
Head Pressure:
The actual pressure of the fuel going to the carb is determined by the air pressure in the tank PLUS the pressure exerted at the tank pickup point by the depth of the fuel in the tank. This extra pressure exerted by the fuel (the "head") makes it easier for the engine to draw fuel when the tank is full, harder when the tank is empty. The result - your engine will lean out some as the tank level gets lower. There isn't much we can do about this, although pressurizing the tank with muffler pressure eases the effect somewhat. Also a smaller tank (less difference in in height between full and empty) helps.
Distance of fuel tank from the engine carb:
The farther back in the plane that your tank is located from the carb center, the farther "up" the fuel must travel when the plane is going uphill! This is why we point the plane's nose up when setting the main needle; its the worst case situation. The farther the tank is back from the engine, the worse the situation. Remedies? A smaller (shorter!) tank is better. The tank should be located as near the firewall as possible. And the engine should be back as close to the firewall as practical.
Fuel Lines:
Should be as straight as possible, and as short as possible. The longer the line, the more "drag" the fuel has as it is pushed to the carb. And any bends in the line constrict the flow some.
Fuel Tank Mounting:
The tank should be isolated from the airframe as much as possible by mounting it in foam rubber. Too little foam or any direct contact with the airframe can transmit vibration to the tank, causing the fuel to foam up - then the carb is being supplied with air as well as fuel! Too much foam or too soft foam can cause the tank to shift during manoeuvres, causing the fuel supply line to bend sharply or even kink.
Taking some extra care and planning while installing the fuel tank can save you a lot of grief - as well as a lot of dead stick landings!
Goodness knows, we have enough trouble keeping our "simple" little engines running properly - these seem to be a gazillion detail items that can cause them to run badly or not at all. Now we introduce another whole set of potential problem areas that affect the engines operation - the fuel tank and its set-up.
Obviously the engine must have fuel to run - but how EASY it is for the engine to draw fuel from the tank can have lots of effect on just how WELL the engine runs in various flight conditions. Essentially, the engine creates a vacuum in the carburettor throat while running; normal air pressure in the tank (or muffler pressure if you use it) then forces the fuel into the carb through the needle valves that you have diligently set.
The problems that can arise from your tank set-up can be grouped into several distinct areas:
Tank height:
The farther "up" we must draw the fuel to get into the carb against the force of gravity, the less likely it will arrive in sufficient quantity. And as we manoeuvre the plane applying "G" forces, the forces required become even greater. So we can't have the tank too low in the plane with respect to the engines carburettor. Most experts seem to agree that the centerline of the tank should be, optimally, about 3/8 - 1/2" below the centerline of the carb barrel. The tank should be mounted in the plane as near that goal as possible - but in some designs that can't be done! If you can't raise the tank, then lower the carb; since the carb is usually about an inch above the centerline of the engine, just side mounting the engine will lower the carb by an inch!
Head Pressure:
The actual pressure of the fuel going to the carb is determined by the air pressure in the tank PLUS the pressure exerted at the tank pickup point by the depth of the fuel in the tank. This extra pressure exerted by the fuel (the "head") makes it easier for the engine to draw fuel when the tank is full, harder when the tank is empty. The result - your engine will lean out some as the tank level gets lower. There isn't much we can do about this, although pressurizing the tank with muffler pressure eases the effect somewhat. Also a smaller tank (less difference in in height between full and empty) helps.
Distance of fuel tank from the engine carb:
The farther back in the plane that your tank is located from the carb center, the farther "up" the fuel must travel when the plane is going uphill! This is why we point the plane's nose up when setting the main needle; its the worst case situation. The farther the tank is back from the engine, the worse the situation. Remedies? A smaller (shorter!) tank is better. The tank should be located as near the firewall as possible. And the engine should be back as close to the firewall as practical.
Fuel Lines:
Should be as straight as possible, and as short as possible. The longer the line, the more "drag" the fuel has as it is pushed to the carb. And any bends in the line constrict the flow some.
Fuel Tank Mounting:
The tank should be isolated from the airframe as much as possible by mounting it in foam rubber. Too little foam or any direct contact with the airframe can transmit vibration to the tank, causing the fuel to foam up - then the carb is being supplied with air as well as fuel! Too much foam or too soft foam can cause the tank to shift during manoeuvres, causing the fuel supply line to bend sharply or even kink.
Taking some extra care and planning while installing the fuel tank can save you a lot of grief - as well as a lot of dead stick landings!
