Why are early wing planforms mostly rectangular? I am looking at the history of wing planforms, and I am having trouble finding reasons for why the wright flier and other early aircraft were built with rectangular wings. I thought it was due to being more structurally secure, but I can't find any sources that explicitly say that, or describe the thought process of early designers.

Is my reasoning correct, and are there any other reasons for the decision to use a rectangular wing (vs a tapered wing)?
The benefit of tapered wing lies in its proximity to the elliptical lift distribution while retaining much of the structural benefit of a rectangular wing. But we owe this knowledge to a few things:


The Kutta-Joukowski Theorem: published in 1906 by Nikolai Y. Joukowski and influenced a great deal by Martin W. Kutta.
The Lifting Line Theory: published in 1919 by Ludwig Prandtl and inspired by the work of Frederick Lanchester. The term induced drag was coined by Max Munk, a colleague of Prandtl in 1918. Although Lanchester had published some results on aspect ratio and finite-wing aerodynamics as early as 1907, they weren't taken seriously by his compatriots and had very little impact as a result.


The Wright Flyer flew in 1903. By all accounts, the Wright brothers designed it through trial and error. As a result, the insight of tapered wing couldn't have been known to them during the Flyer's design and through much of the First World War.

In any case, most of the early airplanes were bi/triplanes with external struts being their structural members. The drag from the struts would've overwhelmed any benefit gained from tapered wings.