Aircraft cruise performances

Aircraft cruise performances


Aircraft Cruise Definitions:


Fuel flow: It is the amount of fuel that goes throw the tubes.

Specific Fuel Consumption (SFC): It is the fuel flow required for getting the unit of power. SFC=FF/Power.

The lower the SFC is the least fuel we need to get power.

Specific Range (α): It is the distance we move per unit of power.

α=Distance in nautical miles / kilograms of fuel or α= True air speed/ Fuel flow


Cruise Flight


We consider all forces are equal, the system is in equilibrium. That means that Thrust is equal to Drag, and Weight is equal to Lift.



First I would like to highlight the difference between Range and Endurance.
Range is the distance and Endurance is time. So how can we fly more nautical miles?, How can we be more time in the air?.
When do we fly the maximum range?


When do we fly the maximum range?


The main purpose of this flight is to move the greatest distance with the least amount of fuel.


As the fuel we are burning during the flight it is proportional to the required power. To reach the maximum range. The relation between power required versus true air speed must be the lowest (Power required/True Air speed). As you can observe this relation is the lowest when power required is low (low fuel consumption) and the true air speed is high.


Power required (Pr) = TAS x D, so if we divide the Power required by TAS, Results that this relation is accomplished when we have the minimum drag.


When do we get the minimum drag?


The minimum drag is reached when the induced drag is equal to parasite drag. As a pilots we can fly at minimum drag speed when the angle of attack corresponds to the maximum fineze which is (Cl/Cd)max


Cl : lift coeficient

Cd : drag coeficient


Considerations about the range 


Every factor that affects our power required, will change the fuel consumption and the range we can fly.

Weight: The higher the weight is the higher engine power we need, so we required a higher fuel consumption.


Altitude: The altitude has two opposite effects. On one hand the power required increases (high fuel consumption) and on the other the TAS increases. We normally are flying high because the TAS affects more, and also we get many benefits to the lowest flight time,such as maintenance issues, we can do more flights ……


Wind: The wind does not affect the power required nor the true air speed. But if we have tail wind our ground speed increases leading on an increased ground distance (range).


Some books say that when we have tail wind is recommended to reduce a bit the speed. In the pictures bellow you have the explanation.


Maximum endurance flight


Endurance is the maximum time that an aircraft is able to be on the air. So we must burn the least fuel, which leads in the least Power required. As you can imagine all factors affecting the Power required is affecting the endurance


Considerations of maximum endurance flight


Weight: increase of weight leads in an increase of required power, which means and increase of fuel consumption, so de endurance decreases.


Altitude: An increase of altitude leads on an increase of density altitude, which means an increase of power required. The result is that endurance decreases.



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