here (Chapter 8). A soft sf writer merely imagines a winged humanoid form and assumes that such a being would be capable of flight whereas a hard sf writer carefully considers the limitations imposed by the laws of physics, then scientically rationalizes an organism that might be large and heavy enough for intelligence and yet also capable of winged flight.
Thus, in Poul Anderson's History of Technic Civilization, a ship of the Grand Survey finds the Ythrians and Nicholas van Rijn is shipwrecked among the Diomedeans, the latter referred to as "Wing Men" in the title of an early edition of the first van Rijn novel. In both cases, Anderson tells us much more about these species than merely that they are intelligent and can fly. How do they live? How do they use their power of flight? How does that affect their society and world view?
ERB wrote imaginative action-adventure fiction. Anderson wrote imaginative and scientifically rationalized action-adventure fiction and serious speculative fiction. Thus, Anderson is like a scientifically educated ERB.