# 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.

- 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.

- 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.