A common element for each of these operating problems  is the specific range; that is, nautical miles (NM) of flying  distance versus the amount of fuel consumed. Range must  be clearly distinguished from the item of endurance. Range  involves consideration of flying distance, while endurance  involves consideration of flying time. Thus, it is appropriate  to define a separate term, specific endurance.  speed. Blade loading is expressed in pounds per square foot  and is obtained by dividing the total weight of a helicopter by  the area of the rotor blades. Blade loading is not to be confused  with disk loading, which is the total weight of a helicopter  divided by the area of the disk swept by the rotor blades.  Fuel flow can be defined in either pounds or gallons. If  maximum endurance is desired, the flight condition must  provide a minimum fuel flow. In  Figure 11-11  at point A,  the airspeed is low and fuel flow is high. This would occur  during ground operations or when taking off and climbing.  As airspeed is increased, power requirements decrease due  to aerodynamic factors, and fuel flow decreases to point B.  This is the point of maximum endurance. Beyond this point,  increases in airspeed come at a cost. Airspeed increases  require additional power and fuel flow increases with  additional power.  The ability of an aircraft to convert fuel energy into flying  distance is one of the most important items of aircraft  performance. In flying operations, the problem of efficient  range operation of an aircraft appears in two general forms:  Cruise flight operations for maximum range should be  conducted so that the aircraft obtains maximum specific range  throughout the flight. The specific range can be defined by  the following relationship.  1.  To extract the maximum flying distance from a given  fuel load  2. To fly a specified distance with a minimum  expenditure of fuel  Figure 11-11.  Airspeed for maximum endurance.  Range Performance  Figure 11-10.  Absolute and service ceiling.  Indicated airspeed (knots)  Standard altitude (feet) Service ceiling  Absolute ceiling  Best angle  of climb (V$_{x}$)  Best rate  of climb (V$_{y}$)  24,000  22,000  20,000  18,000  16,000  14,000  12,000  10,000  8,000  6,000  4,000  2,000  Sea level  70 80 90 100 110 120 Fuel flow/power required (HP)  Speed  Reference line  A t a l t i u d e  A  B  Maximum range at L/DMAX  Maximum endurance at  minimum power required  Applicable for a particular Weight Altitude Configuration 11-9  flight hours specific endurance = pounds of fuel  flight hours/hour specific endurance =  pounds of fuel/hour  1 specific endurance =  fuel flow  or  or 