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I had a question regarding thrust calculation for an aircraft during cruise. For an aircraft flying at a constant cruise altitude and a constant Mach number (level unaccelerated flight), what exactly is the weight we use in the equation T = W/(L/D) while calculating the thrust?

What I understand is that since the mass of the aircraft is continually reducing due to constant fuel consumption, therefore the lift requirement will change. Assuming a constant L/D ratio, that implies the drag will change leading to a constant change in the thrust. Therefore the cruise thrust is not a constant value. So how can we apply the above equation for a calculation of the thrust anyway? Is it a thrust at some specific point during the cruise?

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The equation refers to instantaneous thrust in unaccelerated flight. The weight in this equation is instantaneous weight.

Indeed, this weight changes over time. This is for example the reason we have to use the Breguet range equation instead of just divide thrust by thrust specific fuel consumption to calculate the range of an aircraft.

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  • $\begingroup$ I have seen in JD Anderson, Introduction to Flight that there is a calculation of a definite amount of thrust in the examples, where it takes a fixed weight for the calculation. Is it the weight at the beginning of the cruise phase? Maybe it is the maximum thrust required since the weight is the highest at the beginning of the cruise. And obviously, any other thrust requirement would be lower. So when we specify/if we want to specify a cruise thrust, it is always taken at the beginning of the cruise? $\endgroup$
    – Harjot Singh
    Apr 11, 2021 at 13:08
  • $\begingroup$ I think it should be clear from context. If estimating engine requirements, obviously you should be looking at maximum thrust (i.e. at the beginning of cruise). If it's about small GA planes, fuel represents a smaller fraction of total mass and doesn't change as much during a short cross country cruise. Is there a particular example you are stuck on? $\endgroup$
    – Sanchises
    Apr 11, 2021 at 17:26

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