[CryhavocMike]

I'm not a chemistry "guy" (took one chem class in college many years ago) so I was hoping I could get some help to a problem I have here.

I've been trying to precisely figure out the weight and balance of a model airplane. One of the variables is the fuel used. It's woefully inconsistent from manufacturer to manufacturer. But, the ingredients are pretty much the same. Methanol, Nitromethane, and Castor oil. What changes is the amounts of each and whether the ingredients were mixed by volume or by weight. Also, model airplane engines don't fair well with lots of water in the fuel so the Methanol is dehydrated as much as is probably possible in a production environment.

Now I know you could just say "measure the weight on a scale" but I would have to do it for every fuel made so I was wondering if there's a way to figure out, or get at least a good estimate before purchasing. As an example, One manufacturer lists 1 gallon of fuel as containing 10%Nitromethane, 20%Castor oil, and the balance Methanol, mixed by volume. If I have a fuel tank that holds 10ounces of fuel by volume, will the fuel weigh 10oz? I'm thinking it won't but....

Thanks,
Mike

[chenbeier]

You are right it won't. Beside water no other liquid can be set equal weight to volume. For this you need a factor. How  is this one called?  Your fuel contains 0,1 gal Nitromethane, 0,2 gal Castor oil and 0,8 gal Methanole.

[Arkcon]

Every liquid, or mixture, has a density, which is a function of weight per volume.  You can weigh, as best as you can, a volume that you've measured as best you can, and expect the ratio to remain the same.

The 10 oz fuel by volume is some sort of industry jargon as a way of measuring what you have and what you've put into the plane.  You also need a factor to compare it to things you can measure.

The question is:  What do you need to know?  What can you measure, and how accurately?  What do you know about the composition really?  Working with this, we can see what you need to do, to get the result you want.

[Borek]

So far I am flying electric RC models only, but I feel your pain

The best thing you can do IMHO is to buy a https://en.wikipedia.org/wiki/Hydrometer for the range of densities you expect. This way you can always easily check the fuel density experimentally and be sure you know what it is, calculating the mass from density and volume is trivial. Trying to calculate the density it from the information you have will be next to impossible.