We will start by looking at the magical asymmetry of the wing. What makes this piece of hardware fly? Remember the four forces exerted on the airfoil that are etched in every pilot’s mind?
Let’s take them in concert: The thrust compensates for the drag and the lift counters the weight. Given the relative constancy of the airfoil, it is the thrust that ultimately decides flight. Without thrust the airplane would stay put. So, knowing the variability of the thrust, various modes of flight can then be contemplated (high power for take-off, cruise power followed by specified lower power for landing). Now look at the relationship between, thrust, time, weight and drag. An airplane will progressively become lighter as it consumes fuel and with the change in the weight of the fuel, the angle of attack would change ever so slightly to maintain the same lift that will ultimately affect the airspeed. This change in the angle of attack will help increase the relative thrust to drag ratio. Atmospheric conditions will vary the thrust produced by the engine especially high density altitude which will reduce the power-plant efficiency but, on the flip side, it will also reduce the atmospheric friction and hence the drag. That is why turbo-charged aircraft can maintain their thrust at higher altitudes and fly faster. This preamble explains the varying conditions in-flight and now we will consider how they impact 1.3 Vso.