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Q-talk 91 - Fuel Flow

Dave Richardson Stow, OH

Measuring the fuel flow rate on our planes is not only a smart thing to do, it is required by regulations. 150% of the fuel flow rate anticipated at full throttle in a climb attitude is the minimum FAA requirement. Perhaps your personal minimum fuel flow rate is higher than 150% to help you sleep at night. Our header tank arrangement provides different flow rates based on the total quantity of fuel located there. The various engine types available for both Q1 and Q2xx designs affects the climb attitude that your plane will produce, too. Once you choose the attitude angles you want to measure and the header tank fuel quantities you want to test, you will need a simple way to calculate the gallons per hour using smaller volume and time increments.

I have been using a two-cup household measuring cup and the second hand on my watch for my measurements. Once I had all the raw data, I needed a way to convert the 2 cups per seconds into gallons per hour. After conferring with a former Math major at work, we developed a very simple formula to do the trick. You only need two raw data points to calculate the rate; the number of cups measured and the number of seconds it took to measure that amount.

Multiply the number of cups collected in one timing by 225. Divide the result by the number of seconds it took to fill the cups. This creates the number of gallons per hour flow rate. See the written formula below.

No. of Cups x 225

---------------- = Gallons per Hour

No. of Seconds

This is an example using the formula: Two cups of fuel collected in 38 seconds. 2 cups x 225 = 450 450 / 38 sec. = 11.842 Gal/hr

Obviously, the magic is in the number 225. All the conversions have been reduced down to that one multiplier.

If you measured the fuel in fluid ounces instead of cups, there is a simple way to do that math, too. Since there are 8 ounces in a cup, you can divide 225 by 8 to get the new magic number for ounces, 28.125. For example, if you had a measurement of 25 oz in 45 seconds, the formula would look like this:

25 oz x 28.125 = 703.125 703.125 / 45 sec. = 15.625 Gal/hr


You can order a printed copy of Q-talk #91 by using the Q-talk Back Issue Order Page.