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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)?

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    $\begingroup$ I would assume a square wing is much simpler to produce. The design and construction techniques required for a tapered or swept wing would be out of reach for such an early aircraft as the Wright Flyer. Additionally, the benefits of a swept or tapered wing are at higher speeds, and may not even be noticeable at the speeds which early aircraft operated. $\endgroup$ – zymhan Nov 14 '19 at 19:50
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    $\begingroup$ I always thought taper was specifically for the lift efficiency of the wing due to less lift at the tips of the wing. Is that more relevant at high velocities (regardless of what early engineers knew)? $\endgroup$ – Boto Nov 14 '19 at 19:55
  • $\begingroup$ Actually, it may be more relevant at lower speeds and higher angles of attack, as far as drag, and more relevant at higher speeds for wing loading and roll rate. Taper is seen in gliders. The story may be something like this, observed as a natural phenomena by the Wrights, understood and described by Kutta-Joukowski, masked by struts and wires in the biplane era, foreseen for use by Prandtl and Munk, applied in the 1930s as higher speeds and mono wings were developed. $\endgroup$ – Robert DiGiovanni Nov 15 '19 at 11:39
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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:

  1. The Kutta-Joukowski Theorem: published in 1906 by Nikolai Y. Joukowski and influenced a great deal by Martin W. Kutta.
  2. 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.

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  • $\begingroup$ The Wrights had a wind tunnel and extensive experience designing and flying their designs, even before considering adding an engine and going powered. They certainly did not need Kutta, Joukowski, Prandtl, or Munk to tell them a rounded trailing edge did not pull as hard in the wind. Their biggest mistake was a 2 year hiatus in building while they travelled and sold their "flying machine". But the ones who develop the theoretical understanding are certainly appreciated, as they are the true foundation builders of refining art into science. $\endgroup$ – Robert DiGiovanni Nov 15 '19 at 6:19
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The rectangular "Hershey Bar" is a safe reliable, and yes, easier to build design.

But if you look more carefully at the Wright Flyer wings, you can see they were already rounding off the trailing edges to decrease drag. Tapering to a point serves the same function, and also enables one to build a bit more lightly due to reduced torque stress from the end of the wing. Same reason a prop is usually tapered. It evens the load along the wing.

The common sea gull is a good example of tapering done nearly to perfection.

But aircraft mimicking this design lack the fine control gulls have of their wings and trade decreased drag for increased probability of often fatal "tip stalling".

"Washing out" (reducing) the wing tip angle of attack helps remedy tip stalling, as well as deploying slats at lower speeds.

The rectangular wing is fine for many recreational aircraft. Another drag saving device that can be used is the Hoerner wing tip. Basicly, it angles the bottom of the wing to more of a point when it meets the top, helping push the downflow of the vortex away from the top of the wing.

Tapering can be seen on gliders, and even on the Cessna 172, but it is not absolutely required in aircraft design if fuel savings or optimal glide range is not an issue.

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  • $\begingroup$ "[...] reduced torque stress [...]" I think you mean bending stress, torque stress is about the spanwise axis. Also "But if you look more carefully at the Wright Flyer wings, you can see they were already rounding off the trailing edges to decrease drag" did they specifically give this as a reason? I'm not disputing it, but if you have a source I would like to read it. $\endgroup$ – AEhere supports Monica Nov 15 '19 at 2:28
  • $\begingroup$ F×d works either way. The Wrights had a wind tunnel. Their handiwork can be seen on the elevator too. This is pretty basic stuff. I marvel at the sea gulls. $\endgroup$ – Robert DiGiovanni Nov 15 '19 at 6:10
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A rectangle gives the maximum area for a given span, and for something like the Flyer, running on 12 hp, they needed all the wing area they could get in the lightest possible package, so rectangular it is. Plus it's the easiest structure to build with minimum weight, since all the joints are simple 90 deg ones and the simple cross braced ladder structure is easy to make. There are some (Fokker DVIII springs to mind), but overall you don't see many tapered wings prior to the 1920s.

The Wrights weren't engineers, just bicycle mechanics, but they really were geniuses. They taught themselves all the math required. They weren't really building on the work of others; they started out using Lillienthal's theoretical framework during their glider experiments, discovered that it was deeply flawed so as to be unusable, and built their own wind tunnel to work out their own numbers, to develop their wing from scratch. On top of that, they designed and built their engine from scratch because nobody made one light enough. Pretty amazing.

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Wing ribs are usually made with the aid of a template or tool. For metal wings, it could be the male and female parts of a mould used in a press. For wooden wings it's a board with blocks of wood that hold all the pieces in place while the glue dries. Either way, a rectangular wing only needs one tool for each rib, while a tapered wing needs a different tool for every two ribs (assuming you can reuse it for port and starboard wings)

In these days of CAD and CNC machining, it's easy to forget how much effort it took to scale drawings by hand, and build tools and templates that accurately matched those drawings.

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