Hello,

Long time ago, the flight instructor taught me this old rule of thumb :

Fuel flow of a piston engine (expressed in litres per hour) at cruising
power (75% of rated power) = rated power (expressed
in HP) divided by 5

This is very convenient for flight preparation an quite accurate -
On the DR300-120 i used to fly, the 120 HP O-235 really
did burn 24 litres of avgas per hour at cruise speed - but that
does'nt mean much for fuel efficiency.

BSFC is directly related to fuel efficiency, but it is rather
difficult to get data about cruise conditions (throttle not
necessarily wide open)...

So, I propose the following chart computed with data of
standard gasoline and gas oil (for diesel engine) :




For the old DR300, the 120 HP engine at 75% produced
90 HP for a fuel flow of 24 litre/h...
That is  0.266666 l/HP/h

Using the chart, that reads directly that a little
more than 29% of the fuel was
transformed in useful shaft power - and
merely 24% in propulsive power thanks to the not
too badly chosen propeller.

Philippe (inveterate metric user)

.266666 Liters is 0.070 444  gallons
or .44 BSFC. That sounds about right.

Paul Lamar