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The first airplane by the Wright brothers had a double wing. This concept continued through World War 1 and into the 1930s - why was this concept popular?

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    $\begingroup$ Not all older airplanes are biplanes. See the Bleriot XI that crossed the channel in 1909. $\endgroup$ – usernumber Aug 27 '15 at 9:30
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    $\begingroup$ @Ethan Are you asking about biplane vs monoplane or about the wing configuration of biplane (one low wing and one high wing)? $\endgroup$ – Manu H Aug 28 '15 at 12:55
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Mainly for strength. Up to about 1920, wings used very thin airfoils and were mainly made of fabric stretched over a wooden frame. The main spar was too thin and would have been prone to bending if it had had to support the whole weight of the aircraft in a single span. Only by using two wings, the upper as the compression member and the other as the tension member of a truss, the needed strength was possible.

Airfoil thickness comparison by D.R. Kirk, Florida Institute of Technology

Airfoil thickness comparison by D.R. Kirk, Florida Institute of Technology (picture source)

In your photo, you can just about see the wires running from the top wing (near the outboard struts) to where the bottom wing joins the fuselage. These wires take most of the strain in flight, spreading the load on the wings. It's the same principle as using triangles in the construction of electricity pylons or bridges.

Monoplanes of that time needed even more wire bracing. Look at the picture of a replica of the Etrich Taube, a very popular plane of the pre-WW I period.

Etrich Taube in flight

Etrich Taube in flight. Note the truss below the wing and the many wires keeping it in shape (picture source).

The shorter wingspan of a biplane also reduces the load on the wings. Biplanes tend to have a lot of lift (for their size) because of the large wing area, but also a lot of drag, so they're quite inefficient overall. There were also triplanes, with three mainplanes for even more lift in the same wingspan. WW I pilots demanded the highest rate of roll, which could best be achieved with biplanes.

In the time between the World Wars, the use of high-strength aluminium for aircraft and the monocoque airframe both allowed airframes to become stronger. This meant that monoplane designs became more practical, even though the older aviators didn't believe a monoplane could be strong enough. As soon as monoplanes were practical, the improvements in efficiency and flight performance saw them replace bi- and tri-planes.

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  • $\begingroup$ So they needed 2 wings to hold up the aircraft better, but in ww2 the had stronger wings to hold the body of the aircraft better. Is this basically what you are saying. $\endgroup$ – Ethan Aug 26 '15 at 22:44
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    $\begingroup$ I'm not saying you are wrong but there are plenty of wooden monoplanes that could do with being explained. $\endgroup$ – RedGrittyBrick Aug 27 '15 at 9:27
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    $\begingroup$ @RedGrittyBrick The Goose had wings about ten feet thick and only flew once. It doesn't really need much explaining. $\endgroup$ – Dan Hulme Aug 27 '15 at 22:01
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    $\begingroup$ @DanHulme: And what about the Mosquito, the "wooden wonder"? Your explanation is half right, but you omit that airfoils initially were very thin, so they could not carry the needed structure internally. Using steel, btw., would had not made a difference. $\endgroup$ – Peter Kämpf Aug 28 '15 at 12:37
  • $\begingroup$ I understand how the upper wing experiences compression(because the flying wire is at an angle),but what I don't get is how and why the lower wing of a biplane is the tension member?..@Dan Hulme @Peter Kämpf $\endgroup$ – Jessica Ham Jul 8 at 8:44
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In the early days, it really looks as if people hadn't really worked out what worked best

enter image description here
Horatio Frederick Phillips, 1904 Multiplane. Longest flight 50 feet (15 metres) enter image description here
Gianni Caproni, Ca.60. 1921. Crashed on second flight

A wire-braced box girder structure can probably provide better strength and rigidity than a single, and much longer, wooden spar.

An advantage of multiple wings is that you can have shorter wings, and this can improve maneuverability in combat

enter image description here
Fokker triplane. 1917

Maybe a better question is why people opted for only two wings when they could have had twenty?

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    $\begingroup$ Again, your answer is the best of the pack. It's too bad that so few people can appreciate this. $\endgroup$ – Peter Kämpf Aug 28 '15 at 12:40
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    $\begingroup$ "honey, come to dinner." "In a minute, soon as I troubleshoot this 'flaps disagree' indicator..." $\endgroup$ – Harper - Reinstate Monica Apr 23 '18 at 16:30
  • $\begingroup$ What is the airplane name of picture 1 and picture 2? Were they really fly? $\endgroup$ – AirCraft Lover Sep 17 at 13:25
  • $\begingroup$ @AirCraftLover: I have added captions. They flew briefly. See recent models of multiplane $\endgroup$ – RedGrittyBrick Sep 17 at 14:45
  • $\begingroup$ Thank you for the info. $\endgroup$ – AirCraft Lover Sep 17 at 14:53
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The main reason for having multiple wings in the initial years of the aviation was the lack of availability of materials with sufficient strength.

The main advantage of the biplane is that the wings could be shorter for a given lift. During the initial stages of aviation, the materials available for manufacture of aircrafts were timber, doped fabric etc. The structural members made of these did not have enough strength, limiting the wing size.

The early aircraft also had less powerful engines, resulting in lower speed. In this case, the lower stall speed of the biplanes was advantageous.

The biplanes had very good maneuverability (better roll rate) compared to the monoplanes. This is the reason triplanes were developed during the WWWI.

Sopwith Camel
"SopTri3" by http://www.earlyaviator.com/archive/a/images/tripe_peggy.jpg. Licensed under Public Domain via Commons. However, with the advent of better materials like Aluminum alloys, the aerodynamic disadvantages of biplanes became apparent and monoplanes rapidly became the preferred design choice.

As the material strength improved, the aspect ratios have become ever bigger, with civil aircrafts facing operational (airport) limitations rather than strength limitations in wing length.

However, the biplanes are still used. The better maneuverability and low stall speed make them popular aerobatic aircraft.

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    $\begingroup$ aeroalias ever since you started on this website you have had the best answers. So keep giving other people good answers too. Thanks for answering my questions. $\endgroup$ – Ethan Aug 27 '15 at 2:30
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    $\begingroup$ @Ethan, don't be afraid of waiting a few hours, or even a couple of days to select the answer that works best for you. You don't always have to accept the first answer. (Not saying that was the wrong choice this time, but something to keep in mind.) $\endgroup$ – FreeMan Aug 27 '15 at 13:53
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    $\begingroup$ To explain my downvote: The materials were not the problem. At the speeds possible 100 years ago, a thick wooden wing would have had plenty enough strength. No, it was the prevalence of very thin, highly cambered airfoils which simply did not have the space for an internal spar. Sometimes your answers are spot on, but sometimes you are just guessing. This time you guessed wrong. Also look at @RedGrittyBrick's answer for some valid reasons. $\endgroup$ – Peter Kämpf Aug 27 '15 at 22:31
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    $\begingroup$ @PeterKämpf While a thick wooden wing can have enough strength it's too massive to be used in those low powered planes. Note: wooden strength relates more less to it density so I cannot see a massive ebony or hickory wing working here $\endgroup$ – jean Jun 20 '18 at 11:22
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Actually, there were quite a few monoplanes around even in the early days:

enter image description here enter image description here
1909: Bleriot XI crosses the English Channel
1910: The Fokker Spider flies around the Haarlem church tower. Young Anthony is 20 years old.

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1912: Deperdussin Monocoque, the first design with a stressed-skin fuselage.
1913: Morane-Saulnier H

enter image description here enter image description here
1915: Junkers J1
1915: Fokker E II: Max Immelmann

enter image description here enter image description here
1915: Fokker E III
1918: Junkers D.1

enter image description here enter image description here
1919: Junkers F-13
1921: Fokker F.III

enter image description here enter image description here
1924: Fokker F.VII
1926: Ford Trimotor

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1933: Boeing 247
1934: DC-2

All images from Wikipedia

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  • $\begingroup$ Nice collection! A good addition would be the Deperdussin racer from 1912, one of the first designs with a stressed-skin fuselage. $\endgroup$ – Peter Kämpf Aug 26 '17 at 7:33
  • $\begingroup$ @PeterKämpf Done. $\endgroup$ – Koyovis Aug 26 '17 at 7:37
  • $\begingroup$ Antonov three. Tee hee! $\endgroup$ – Harper - Reinstate Monica Apr 23 '18 at 16:34
  • $\begingroup$ Mind that that's an F.VII-3M, the 3 engine derivative of the F.VII. It was license built by Ford who developed it into their Trimotor. $\endgroup$ – jwenting Nov 23 '18 at 4:58
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Even though some early aircraft were designed with cantilever wings which did not need wire supports, there were still a number of reasons why the wire truss biplane design remained in use for a long time. Like any new technology, development takes time. Fokker designer Reinhold Platz' designs during WW1 is a great example. His thick wing, box spars were a sound design, yet glue was not necessarily very strong, and/or the skilled labor for assembly was poor which created problems, and deaths. The wood is not the problem. High quality spruce actually has about the same strength to weight ratio as aircraft aluminum, though the AL is more isometric, wood proved itself worthy of aircraft construction when used correctly. When the Fokker box spars failed, word got around fast, and generated fear and loathing by Pilots who liked to know their wings would not fold. Pilots didn't dislike biplanes, and wanted "what we know already works". The triplane had box spars and were true cantilever wings without braces and struts, yet pilots veered away from it in great fear, and refused to fly it. So, Platz added the interplane struts to tie all three wings together at the ends purely for the impression it made on pilots. Wires were however eliminated. The Fokker D8 monoplane had a true cantilevered wing which also had some quality issues early on which of course usually ends in a death, and pilots didn't like that. Wired monoplanes had no advantages, High drag, lower lift and weaker than biplanes. Pilots, passengers and investors liked biplanes. Designers eventually proved their knowledge to pilots and to the public, but like anything else there is an enormous psychological element- A not altogether invalid fear that slowed progress. Military people are the worst at accepting new ideas, with a long history of accepting only the tried and true, with no real desire to put pilots at unnecessary risk. Plus there were two distinct advantages to biplanes, adequate lift with shorter wing span, plus the associated high roll rate, and strength was, and is- very good, they were in fact hard to beat.

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Structural strength was not the main reason for early bi-planes. Otto Lilenthal did his glider experiments in a monoplane like structure. The wright brothers were following his research but they wanted to create powered flight. The earliest engine didn't have sufficient power. A Biplane could generate more lift at slower speed. A monoplane couldn't. That was the primary reason for choosing a biplane design.

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  • $\begingroup$ By the 1930's, powered aircraft were much faster than Lilienthal's gliders. When we revisited underpowered aircraft (human- or solar-powered), we reverted to monoplanes. So the claim that biplanes make more lift at slow speeds is both false and irrelevant. -1. $\endgroup$ – Camille Goudeseune Sep 18 at 5:40

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