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fuel tank at balance point/center of gravity

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Old 08-25-2010, 02:57 PM
  #1  
georgebonn
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Default fuel tank at balance point/center of gravity

The fuel tank is usually located just behind the firewall, i.e. much forward of the plane's balance point. The plane is balanced with an empty fuel tank. This results in the plane being significantly nose-heavy on takeoff and during most of the flight. In theory, it appears that ideally the fuel tank should be located at the balance point. This would result in a rather long fuel supply line to the engine, perhaps causing problems.

I would appreciate comments about this, especially from those who have experienced this or have theoretical knowledge about the concept.
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Old 08-25-2010, 03:07 PM
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MinnFlyer
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Default RE: fuel tank at balance point/center of gravity

It actually doesn't add THAT much weight to the nose, but you are correct.

Many people who use gasoline engines or YS brand engines DO put the tank on the CG because those engines have built in pumps.

Putting it back there without a pump can result in lean runs on a climb (unless you care to add a pump)
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Old 08-25-2010, 04:01 PM
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TedMo
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Default RE: fuel tank at balance point/center of gravity

This is typical for most all planes but should not give you any problem. You are right it should be balanced without fuel. I have never noticed any significant change in flight with the fuel burning off.
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Old 08-25-2010, 06:20 PM
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scooterinvegas
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Default RE: fuel tank at balance point/center of gravity

I do! I normally fly till I run out of fuel and bring it around dead stick for the landing, last weekend I only flew a few min and brought it in the check something and was SHOCKED and how fast my plane was going when tring to land. Over shot and to come around to try a 2nd landing. I about ran out of runway before getting it turned around. For a few min I was wondering what was different. Then it hit me, full tank of fuel! It makes a difference.
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Old 08-25-2010, 07:07 PM
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Default RE: fuel tank at balance point/center of gravity


ORIGINAL: georgebonn

The plane is balanced with an empty fuel tank.
For a regular layout, that is correct; however, the concept is to balance for whatever condition locates the CG at its most aft position.
In that way, the plane will never be tail heavy.

For models with the tank located by the tail or just behind the CG, the balance is to be done with a full fuel tank.

Many models designed for precision aerobatics, have the location of the tank closer to or at the CG, as well as a fuel pump, in order to avoid even sutile changes in trim.
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Old 08-25-2010, 08:42 PM
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Default RE: fuel tank at balance point/center of gravity

Some of my planes I have located the tank over the CG and it does effect these planes, all stunt or pattern type. Not enough to ever worry about but i do run YS and OS with pumps so i can do it. The difference isn't worth the price of a new engine though. If I'm out of pumpers then I set the planes up just like everyone else and use the engines I have on hand.
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Old 08-25-2010, 09:59 PM
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JohnBuckner
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Default RE: fuel tank at balance point/center of gravity

Many installations can if they use Tetra or Jett Bubbless tanks can position the tanks on the CG with no fuel delivery problems and no pump or pressure systems. I have done many bubbless installations with the tank as far as 7 inchs aft of the the firewall.

A bubbleless type has far superior fuel delevery characteristics than a conventional muffler pressure system.

John
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Old 08-25-2010, 10:13 PM
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summerwind
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Default RE: fuel tank at balance point/center of gravity


ORIGINAL: JohnBuckner

Many installations can if they use Tetra or Jett Bubbless tanks can position the tanks on the CG with no fuel delivery problems and no pump or pressure systems. I have done many bubbless installations with the tank as far as 7 inchs aft of the the firewall.

A bubbleless type has far superior fuel delevery characteristics than a conventional muffler pressure system.

John
Ditto on the BB tanks big John.
i have some "gee that looks like work" comments on my Jett powered models at the field, but they really aren't..........the payoff is the consistent run from end to end, and like you say i posistion the tanks on the CG without issue.
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Old 08-26-2010, 02:32 AM
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guille2006
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Default RE: fuel tank at balance point/center of gravity


ORIGINAL: georgebonn

The fuel tank is usually located just behind the firewall, i.e. much forward of the plane's balance point. The plane is balanced with an empty fuel tank. This results in the plane being significantly nose-heavy on takeoff and during most of the flight. In theory, it appears that ideally the fuel tank should be located at the balance point. This would result in a rather long fuel supply line to the engine, perhaps causing problems.

I would appreciate comments about this, especially from those who have experienced this or have theoretical knowledge about the concept.

George; your observation is rigth. You balance the plane with most safe-rear CoG location; when you fill the tank CoG goes forward with this result: GIVES YOU MORE LONGITUDINAL STABILITY TO THE PLANE. You will also receive another consequences but we modellers don't care about them: less maneuvrability (how much less? depends on the design), more drag and thus less endurance or more power needed...
As a airplane designer I'm used to deal with that problem, as a modeller I don't care since I use pumped engines and I install the fuel tanks near the CG. In any case my advice is to place the CG where the manufacturer indicates; If you want to move it backwards do it but: DO IT VERY CAREFULLY AND JUST A FEW MILIMETERS AT TIME. If, for any reason you don't want to have that CG movement during flight, just get a perry pump or pumped engine and place the tank as close to the CoG as you can...
Hope it helps
CHeers
G.
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