# Using the Drag and Cost Calculators

The Calculator is really three separate calculators, configured to give you the option of a set of linked calculations.

You can calculate:

• Equivalent Flat Plate Area (E.F.P.A.) by entering an aircraft's top speed at Sea Level, and the Horsepower required at that speed.
• Drag by using the calculated EFPA or entering an EFPA you hove otherwise obtained
• Cost by using the calculated Drag or entering a Drag figure you have otherwise obtained, and then also entering a Speed, Fuel Cost, and estimated Prop Efficiency and Specific Fuel Consumption.

You can begin at any point in the the series.

### A word about Equivalent Flat Plate Area

There are two main approaches to performing these calculations.

1. Ignore the numerical value of the propellor efficiency, knowing that as long as the efficiency is roughly the same at both the flight regime where EFPA was calculated form and the regime where the Cost is being calcuated for, the efficency factors will cancel each other out.
This is the method used in the book, because it is simpler to calculate.

This method gives us what we call a Gross Equivalent Flat Pate Area (G.E.F.P.A) which represents more directly the work the engine is doing to turn the propellor shaft. Since this is also what the fuel is being consumed for, all our calculations correspond to this. You can think of this as the drag of the propellor blades adding to the drag of the aircraft, though slipstream losses are also effectively included as well.

If you prefer to work with GEFPA, then use the Gross Drag Calculator or set the Prop Efficiency figure in the Calculator to 100 percent, and the figures will become GEFPA instead of EFPA.

2. Take the propellor efficiency into account when calculating EFPA and then also into account when calculating the Cost for a given amount of Drag.
When following this method we get an EFPA figure that represents roughly the actual equivalent area being pushed through the air.