Moving The CG ?
03-09-2004, 03:22 PM
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Moving The CG ?
I have a DP Edge. The planes manual shows installing the fuel tank behind the firewall. I have it installed it there now(23oz tank) The plane flys good with the CG 5 1/4" from the leading edge.
I want to relocate the fuel tank further back on thr CG line. Do I have to change the 5 1/4" CG that now balances the plane to a different CG measurement ? I am afraid it will be to tail heavy if I just move it back. I am thinking with the fuel tank full I have over a pound up front.
I want to relocate the fuel tank further back on thr CG line. Do I have to change the 5 1/4" CG that now balances the plane to a different CG measurement ? I am afraid it will be to tail heavy if I just move it back. I am thinking with the fuel tank full I have over a pound up front.
03-09-2004, 06:37 PM
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RE: Moving The CG ?
The short answer;
If you are moving the tank back from the firewall, but ahead of the 5-1/4" CG location, you will not need to change the 5-1/4" CG point. You will need to check the balance again, but you will be very very close as it is.
The long answer;
The plane was designed with the fuel tank up front, and with it balanced while empty (typically). If you move the tank back well behind the CG and balance it with the tank empty as per the plans, it would be fine while empty. However, it can't fly with an empty tank. When you fill the fuel tank you would be adding substantial tail weight shifting the CG rearward and possibly well past the limit the designer had in mind. So the tanks location can have a marked effect on the flight characteristic.
If you move the tank back but keep it forward of the CG line, and balanced it with the tank empty as you normally would, you will be fine. When you fill it for flight you will be adding nose weight, and slightly less nose weight than you would originally. However, since its balanced empty in both cases and the tank is forward of the CG, the most rearward CG position on an empty tank in both cases would be the same. In either case, the CG is well within the limits the designed had in mind.
If you move the tank right over the CG location, and balance it with the tank empty, the CG will be the same whether the tank is full or empty. Because of this, I would move the CG forward just a tad to 'simulate' the CG you would have with the tank more forward with a full tank. Then, you can adjust it during flight testing to get the optimum CG. This is probably the most preferred location for the tank, since once you get the CG adjusted where you want it, its always the same from take off to landing, regardless of the amount of fuel in the tank.
If you move the tank aft of the CG position, you need to balance the airplane with a full tank of fuel, as well as a empty tank. If you don't you may have the correct CG when its empty, but may easily add substantial weight aft of the CG location when you fill it thus causing a tail heavy condition well out of the design specs. Balancing it with the fuel full will give you a worst case condition for setting the initial CG, and it will become more nose heavy as the flight progresses. Balancing it empty will give you the CG location after you burn off all the fuel during a flight. You then need to look at the empty and full CG locations, and see if they fall within the ranges in the manual. If not, you need to move the tank forward and repeat the CG measurements to get both the empty and full CG locations within specs. This setup gives you the opposite effect of a normal fuel tank mounted forward of the CG, in that on take off you will be heavier and more tail heavy, whereas on landing you will be lighter and more nose heavy.
The ideal situation is a neutral position with the fuel tank right over the CG. The next best, is to have the tank just forward of the CG. The worst would have the tank rearward of the CG location.
I typed this in quickly on the fly, so I hope I explained it clearly enough.
If you are moving the tank back from the firewall, but ahead of the 5-1/4" CG location, you will not need to change the 5-1/4" CG point. You will need to check the balance again, but you will be very very close as it is.
The long answer;
The plane was designed with the fuel tank up front, and with it balanced while empty (typically). If you move the tank back well behind the CG and balance it with the tank empty as per the plans, it would be fine while empty. However, it can't fly with an empty tank. When you fill the fuel tank you would be adding substantial tail weight shifting the CG rearward and possibly well past the limit the designer had in mind. So the tanks location can have a marked effect on the flight characteristic.
If you move the tank back but keep it forward of the CG line, and balanced it with the tank empty as you normally would, you will be fine. When you fill it for flight you will be adding nose weight, and slightly less nose weight than you would originally. However, since its balanced empty in both cases and the tank is forward of the CG, the most rearward CG position on an empty tank in both cases would be the same. In either case, the CG is well within the limits the designed had in mind.
If you move the tank right over the CG location, and balance it with the tank empty, the CG will be the same whether the tank is full or empty. Because of this, I would move the CG forward just a tad to 'simulate' the CG you would have with the tank more forward with a full tank. Then, you can adjust it during flight testing to get the optimum CG. This is probably the most preferred location for the tank, since once you get the CG adjusted where you want it, its always the same from take off to landing, regardless of the amount of fuel in the tank.
If you move the tank aft of the CG position, you need to balance the airplane with a full tank of fuel, as well as a empty tank. If you don't you may have the correct CG when its empty, but may easily add substantial weight aft of the CG location when you fill it thus causing a tail heavy condition well out of the design specs. Balancing it with the fuel full will give you a worst case condition for setting the initial CG, and it will become more nose heavy as the flight progresses. Balancing it empty will give you the CG location after you burn off all the fuel during a flight. You then need to look at the empty and full CG locations, and see if they fall within the ranges in the manual. If not, you need to move the tank forward and repeat the CG measurements to get both the empty and full CG locations within specs. This setup gives you the opposite effect of a normal fuel tank mounted forward of the CG, in that on take off you will be heavier and more tail heavy, whereas on landing you will be lighter and more nose heavy.
The ideal situation is a neutral position with the fuel tank right over the CG. The next best, is to have the tank just forward of the CG. The worst would have the tank rearward of the CG location.
I typed this in quickly on the fly, so I hope I explained it clearly enough.
03-09-2004, 08:34 PM
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RE: Moving The CG ?
Stephen,
Thank You, for taking the time to fully explain your answer. I intend on installing the fuel tank on the CG.and from what you say I understand.
Happy Flying
Chris
Thank You, for taking the time to fully explain your answer. I intend on installing the fuel tank on the CG.and from what you say I understand.
Happy Flying
Chris
03-09-2004, 08:54 PM
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RE: Moving The CG ?
What engine are you using on this plane?
It may not run right with the tank further away. You might have to use a fuel pump.
If it is a gas engine, or a YS engine then you will be O.K.
Jim
It may not run right with the tank further away. You might have to use a fuel pump.
If it is a gas engine, or a YS engine then you will be O.K.
Jim
