Help w/CG and fuel tank location, giant scale!!!
07-17-2004, 07:18 PM
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Help w/CG and fuel tank location, giant scale!!!
Ok, I know I'm new to this sport, and having recently learned to fly but never having built........
I have a giant scale Pober Pixie I have restored to flying (I think) condition. I am placing receiver, battery, and fuel tank while working on a guessed CG of 25% of the wing chord. Only thing is that the tank used to be placed in front of the former that houses the instrument panel............and this doesnt make sense to me because it seems well forward of the projected CG point to be placing an item that changes in weight as fuel is used. At 32 oz of gasoline am I making a mountain out of a mole hill?.....or should it be moved aft of that former to the general vicinity of the CG? The whole plane weighs in at 20 lbs approximately, and maybe that weight change is insignificant?
Also, there are two brass tubes to the fuel tank.....one obviously goes to the engine, and the other I am told goes to a vent.....
Do you then use the vent to fill the tank too? How is the vent routed and what does one do to keep the fuel from pouring out the vent, as both brass tubes enter the fuel tank at the same level....and even if they didnt I can't see the sense in having a vent that would leak when you fly inverted? I can't seem to find a diagram on this task....
Thanks in advance for your help guys.....
I have a giant scale Pober Pixie I have restored to flying (I think) condition. I am placing receiver, battery, and fuel tank while working on a guessed CG of 25% of the wing chord. Only thing is that the tank used to be placed in front of the former that houses the instrument panel............and this doesnt make sense to me because it seems well forward of the projected CG point to be placing an item that changes in weight as fuel is used. At 32 oz of gasoline am I making a mountain out of a mole hill?.....or should it be moved aft of that former to the general vicinity of the CG? The whole plane weighs in at 20 lbs approximately, and maybe that weight change is insignificant?
Also, there are two brass tubes to the fuel tank.....one obviously goes to the engine, and the other I am told goes to a vent.....
Do you then use the vent to fill the tank too? How is the vent routed and what does one do to keep the fuel from pouring out the vent, as both brass tubes enter the fuel tank at the same level....and even if they didnt I can't see the sense in having a vent that would leak when you fly inverted? I can't seem to find a diagram on this task....
Thanks in advance for your help guys.....
07-17-2004, 09:03 PM
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RE: Help w/CG and fuel tank location, giant scale!!!
umm, ok, what motor are you using, I assume it is a gasoline engine, not glow. 32 oz seems kind of big for gas, you might be able to use a smaller tank. In any case, you balance the airplane dry so if your tank is in front of the cg, you will be nose heavy on take off. usually this is not a problem. if you have the room and have a gas engine, you can move the tank onto the approximate cg. if you want, but you don't have to.
The tank. Actually, I usually use 3 lines. One goes from the fuel pick-up line (clunk) to the carburator.
Second is a vent, with a brass tube bent so that it opens at the top front of the tank, attach fuel line to this and let it hang down through the bottom of the fuse, open to the atmosphere (if gas).
Third line is a fill line, brass tube bent so that it opens at the bottom front of the tank, attach fuel line to this, let it hang down through the bottom of the fuse, and plug it.
There are many other ways of plumbing your airplane, depends on the installation and your preferences.
Fuel doesn't spill out of the vent since it is the only open line, I suppose it's the same reason soda won't spill out of your drinking straw unil you take your finger off the top of it.
good luck.
peter
The tank. Actually, I usually use 3 lines. One goes from the fuel pick-up line (clunk) to the carburator.
Second is a vent, with a brass tube bent so that it opens at the top front of the tank, attach fuel line to this and let it hang down through the bottom of the fuse, open to the atmosphere (if gas).
Third line is a fill line, brass tube bent so that it opens at the bottom front of the tank, attach fuel line to this, let it hang down through the bottom of the fuse, and plug it.
There are many other ways of plumbing your airplane, depends on the installation and your preferences.
Fuel doesn't spill out of the vent since it is the only open line, I suppose it's the same reason soda won't spill out of your drinking straw unil you take your finger off the top of it.
good luck.
peter
07-18-2004, 01:36 AM
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RE: Help w/CG and fuel tank location, giant scale!!!
Starting with the GAS tank. A full up 20 pound plane will likely not need a 32 oz. tank. What size engine are you using? That will determine the tank size. Think about using a 24 oz. tank to reduce weight. Your gas engine will burn between 1 and 1-1/2 ounces of gas a minute.
A two line tank uses one line with a "T" fitting to provide gas to the engine and to fill and drain the tank. The fill line is run from a fuel dot or left loose, but has a plug of one type or another in the open end when the plane is running or flying. The other line is a vent line that has a tygon fuel line running to someplace outside the plane. Usually out the bottom of the fuselage and dumping from there.
The gas tank requires a gas resistant stopper, not one for glow fuel use. Gas will quickly eat the glow fuel stopper and permit draining the tank into the plane. Dubro and Sullivan both sell gas stoppers, or gas conversion kits. By the way, medium sized Tygon fuel line will be fine. Use small electrical zip ties to secure all fuel line connections, including inside the tank on the brass lines and at the clunk. It's a cheap way to insure that the fuel lines stay on the connections during flight.
To the C/G issue. Try to assemble the plane without installing the tank and radio equipment. Check to see where the c/g is without all the working parts. Now set the tank and radio gear on the surface of the plane to see where the C/G will come out as you move the various parts around to find the correct C/G. Try to install the components in locations inside the plane that closely match the places they were sitting when you were checking the C/G. It may not come out perfect, but you can get close.
It's always better to attempt to place the gas tank at or very near the C/G to prevent a shifting of the center of gravity as the gas is used up. Having the tank too far forward or aft can cause a very large shift in the C/G if you're not careful. Too far aft and the plane could become uncontrollable as the gas is burned off. Placing the tank immediately forward of the C/G reference would likely end up the best location.
With a gas engine, you need not concern yourself with having to keep the tank close to the engine. Gas engines have a pump, permitting the tank to placed almost anywhere in the plane without effecting the engines' performance. Remember the C/G though.
Bets of luck in your efforts, and good flying.
A two line tank uses one line with a "T" fitting to provide gas to the engine and to fill and drain the tank. The fill line is run from a fuel dot or left loose, but has a plug of one type or another in the open end when the plane is running or flying. The other line is a vent line that has a tygon fuel line running to someplace outside the plane. Usually out the bottom of the fuselage and dumping from there.
The gas tank requires a gas resistant stopper, not one for glow fuel use. Gas will quickly eat the glow fuel stopper and permit draining the tank into the plane. Dubro and Sullivan both sell gas stoppers, or gas conversion kits. By the way, medium sized Tygon fuel line will be fine. Use small electrical zip ties to secure all fuel line connections, including inside the tank on the brass lines and at the clunk. It's a cheap way to insure that the fuel lines stay on the connections during flight.
To the C/G issue. Try to assemble the plane without installing the tank and radio equipment. Check to see where the c/g is without all the working parts. Now set the tank and radio gear on the surface of the plane to see where the C/G will come out as you move the various parts around to find the correct C/G. Try to install the components in locations inside the plane that closely match the places they were sitting when you were checking the C/G. It may not come out perfect, but you can get close.
It's always better to attempt to place the gas tank at or very near the C/G to prevent a shifting of the center of gravity as the gas is used up. Having the tank too far forward or aft can cause a very large shift in the C/G if you're not careful. Too far aft and the plane could become uncontrollable as the gas is burned off. Placing the tank immediately forward of the C/G reference would likely end up the best location.
With a gas engine, you need not concern yourself with having to keep the tank close to the engine. Gas engines have a pump, permitting the tank to placed almost anywhere in the plane without effecting the engines' performance. Remember the C/G though.
Bets of luck in your efforts, and good flying.
