Were the second wings on biplanes necessary to keep the planes aloft?
Would the Wright Flyer, for instance, have flown without the upper
wing?

mccook-ga:

A good website that discusses this very topic can be found within the
materials posted at the US Centennial of Flight online exhibition:

US Centennial of Flight Commission:
http://www.centennialofflight.gov/

The Monoplane - US Centennial of Flight Commission
http://www.centennialofflight.gov/essay/Evolution_of_Technology/Monoplane/Tech13.htm

(an aside: if you haven't had the opportunity to visit the Smithsonian
Institute Museum of Flight, it is definitely worth planning into any
trip that takes you near Washington DC)

To summarize, yes, the second wing was necessary to keep the earliest
successful flying machines aloft. If we think of flying as simply
being pushed up by air pressure at the same rate, or higher, than the
rate at which you are falling, then it is really a balance between the
downward force due to gravity on the aircraft, and the upward force
generated by the air pressure differential created by the wing
surface. That upward force depends on the speed of the air passing
over/under the wing, and the surface area of the wing.

For the early planes, and most especially true for the Wright Flyer,
the limiting factor was the engine. This was the reason that many of
the Wright Brothers' comteporary rivals failed:

Power: The Final Ingredient 
http://wings.avkids.com/Book/History/instructor/power-01.html

Given how relatively weak the early engines were, this meant that
maximum thrust was limited. In turn, this meant that wings either had
to have greater surface area or more extreme angles of attack to
create greater lift. Too extreme an angle of attack, though, meant too
much drag and the need for an even more powerful engine. So, the focus
was on maximizing surface area. As the above website points out, the
early planes were made from the lightest materials then available
(wood and cloth), so the only way to achieve enough surface area while
maintaining structural integrity, was to use a box-beam type of
construction; hence the bi- and tri-planes were born.

Dual wings were actually first used in the development of successful
gliders. In particular, Otto Lilienthal developed a bi-wing glider
that, in 1895, allowed him to glide for about a quarter of a mile:

http://www.centennialofflight.gov/essay/Prehistory/lilienthal/PH6.htm
http://www.centennialofflight.gov/essay/Prehistory/lilienthal/PH6G2.htm

The advantage of a bi-wing in a wearable glider such as the Lilienthal
design is that it provides a lot more lift without increasing the
overall span of the wing; the downside is the increased drag from the
rigging and supports that necessarily run between the two wings.
Modern gliders use a much wider wingspan (and stronger materials) in a
mono-wing design that optimizes for drag while sacrificing span.

The importance of the "Monoplane" website, though, is to point out the
means by which the first successful monoplane was constructed. The
idea of using tension wires to pre-load the wings has been in use
since then to modern-times; these days, often the pre-loading is down
inside the wing, out of sight.

In either case, though, it is ultimately structural strength that
determines whether the plane design is successful or not. Bi-plane
wings can fail under enough load just like mono-plane wings fail.
Unfortunately, Lilienthal found this out the hard way, as he perished
when the upper wing on one of his bi-plane glider prototypes failed
causing him to fall about ninety feet to his death:

http://aerostories.free.fr/precurseurs/lilien/page2.html

Even today, aircraft designed for slow-speed flight while needing to
retain agility are designed as biplanes. For everything else, a
monoplane design is used to minimize drag, maximize airspeed, and
improve fuel efficiency.

Incidentally, if you are interested in other aspects of the history of
flight, a couple of useful online resources are the Centennial of
Flight site referenced above, and an online text hosted by NASA:

Quest for Performance: The Evolution of Modern Aircraft 
http://www.hq.nasa.gov/office/pao/History/SP-468/contents.htm

I hope this helps!

aht-ga
Google Answers Researcher


=========================================
Search Strategy:

biplane +why wings
://www.google.com/search?q=biplane+%2Bwhy+wings

wright flyer design
://www.google.com/search?q=wright+flyer+design

First-class work. Thanks for the clear and thorough answer, plus all
the websites for further investigation