From Cathay Pacific notes:
Specific Fuel Consumption
Specific Fuel Consumption is a measure of the fuel consumed by an engine. There are two types of specific fuel consumption:
1.Thrust Specific Fuel Consumption [TSFC]
2.Power Specific Fuel Consumption [SFC]
TSFC is defined as fuel-flow per pound of thrust produced [FF/Thrust]
SFC is defined as fuel-flow per horsepower produced [FF/HP]
Fuel-flow should be measured in units of pounds of fuel per hour, rather than gallons per hour. This is because the chemical energy in the fuel is a function of the mass of the fuel. A gallon of fuel expands or contracts with temperature. Therefore, a gallon of cold fuel contains more energy than a gallon of warm fuel.
The units of TSFC and SFC will be:
TSFC = lb per hr/thrust lb
SFC = lb per hr/HP
Converting the Drag vs. Velocity Curve
A perfectly accurate conversion of the drag curve into a Fuel-flow vs. Velocity graph must take variations in engine and propeller efficiency into account. However, we will find it easier to break the process into two steps. We will therefore conduct a simple aerodynamic analysis first, in which we will assume that:
§ TSFC is constant for a jet
§ SFC is constant for piston and turbo-prop engines.
FF vs. Velocity for a Jet
We will start by converting a Drag vs. Velocity curve into a Fuel-flow vs. Velocity curve for a jet aircraft. This will be very easy because the TSFC is a constant.
Remember the definition of TSFC:
TSFC = FF/Thrust
We will assume that:
TSFC = FF/Drag (i.e. we assume thrust = drag)
Therefore:
FF = TSFC x Drag
Hope it helps a bit.
FB