CG/Weight/Wing Loading

How is the weight of the fuel taken into account in the placement of the CG. In other words, is the airplane balanced with a full or empty or partially full tank of fuel? This would make a difference in wing loading and weight calculations also. Thanks.

>How is the weight of the fuel taken into account in the
>placement of the CG. In other words, is the airplane
>balanced with a full or empty or partially full tank of
>fuel? This would make a difference in wing loading and
>weight calculations also. Thanks.

It is normally caluclated with the tank empty. Everyone needs fuel to run, but the comparisons are made with the tank empty.

If you actualyl MEASURE the gram weight of say 100 cc of fuel you would find its VERY minimal. Alot of people pick up a fuel bottle and think " gee there's a lot in there" when in reality its marginal.

im not saying there is none, but i measured it a while back and off the top of my head , it barely registered my scales.

Not sure about fuel but water weighs 8 pounds per gal or 128 ounces per gal. Wow there are also 128 fl ounces in a gal so 4 ounces of water would weigh.... well heck.... 4 ounces. Our fuel may be slightly heaver, don't know if thicker makes it heaver.

I officially retract what I said.

I went and weighted an empty tank it weighted X..
I filled it up then weighted it..
To my surpise and I rechecked it 3 times, For 60 cc of fuel which is standard 20 %oil 5 % nitro. It weighted 120 grams

Seems like a LOT more than is possible, but my scales are accurate and I did it 3 times as I said.

Maybe it IS a consideration however, where' tanks sit is usually jsut in fromt of the wing and less chance of noseing forward?

Phil,

The 'specific gravity' of fuel is less than water, which weighs 1 oz per fluid oz (Ideal lab conditions -- Sp Grav is less at our usual Summer temperatures.) As I recall:

Oil (synth and castor) goes about 0.98 Sp Grav

Methanol is about 0.8 Sp Grav

Nitro is heavy, at about 1.13 Sp Grav

So, a 10% nitro, 22% oil fuel (the rest is methanol) has a Sp Grav around 0.87. Our tanks are sized by fluid ounces, so a fill in a 4 oz tank weighs about 3.5 oz. How'd I get that? Figure per "ounce"

Oil -- .22 of volume, weighs (.22 x .98) 0.216 oz
Nitro -- .10 of volume, weighs (.1 x 1.13) 0.113 oz
Meth -- .68 of volume, weighs (.68 x .8) 0.544 oz

Summed up = 0.873 oz per fluid oz volume. ...times 4 fl oz = ~3.5 oz.

And most models weigh around 2 lb, or (much) more. 3.5 oz of fuel in a 32 oz model IS ~11% of total (dry) weight, but where it is placed reduces the effect on CG. (Figure 'location' of fuel weight at half the tank's length.) Sure, some effect, but the model tends to get less noseheavy as fuel burns off and we get to the more demanding maneuvers.

For the record... 120 Grams is 4.2 oz

So i think we are on the right path to solving this mystery!

Phil,

I hope it was clear without being spelled out, that 'fluid ounce' is a volume measure (1.8 cu in), like cc or ml, and the other 'ounce' is a measure of weight or mass, like gm, Kg, pound, etc.

The difference between mass and weight units is a bit clearer in the Metric system, but I was trained in the USA/Imperial and still have to do some mind work to speak metric with its natives,