Refer to figure.
The maximum endurance speed, VMAX END, is found at the lowest point of the power required curve, which is also the minimum power required speed, VMP and Best RoC speed, VY.
As altitude increases, the TAS for a given IAS increases. This fact would cause the power required curve just to be stretched out to the right.
But, power required is Drag x TAS, so the increase in TAS also causes the power required curve to move up.
Thus, the combined effect is that the power required stretches up along its tangent from the origin.
As a result, the lowest point of the power required curve, which corresponds to VMAX END, moves to the right and increases.
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