From: jazzbo@leland.Stanford.EDU (Scott Raymond Harris)
Subject: Re: Why not a bi-winged sailplane??
Organization: DSG, Stanford University, CA 94305, USA
Distribution: usa
Date: Thu, 20 Aug 92 07:53:08 GMT

In article <1554@wtcp.DaytonOH.NCR.COM> mpeters@wtcp.DaytonOH.NCR.COM (Michael Peters) writes:
>
>bullocAwatson.bms.com writes:
>>In article <1992Aug18.164424.13691@eplrx7.es.duPont.com>, Mark Milbourne 
>><milbouma@esvax.dnet.dupont.com> writes...
>>> 
>>>I have been wondering if a bi-winged design would be feasable for a
>>>sailplane.   
>>..
>>>Adequate separation of wings might be a problem, since modern sailplanes
>>>typically have minimal fusealage height.  
>>
>>Interesting concept.  I'm not an aerodynamicist - not even a glider pilot, but 
>>intuition says you may have answered your own question above.
>>
>>However, why not a _tandem_ biplane?  As in a canard and it's cousins.
>>
>>OK, you aerodynamics people, as a fellow netter is fond of saying, inquiring 
>>minds want to know.
>
>I'm not an aerodynamics person, but a bi-winged design as described by
>the original poster, would seem to have a significant amount of drag as
>compared to a traditional sailplane.  There is much care taken by the
>sailplane designer both at the wing/fuselage connection and wingtips to
>eliminate drag and the inefficiencies at the wing tip area.  A bi-wing
>design would have twice as many problem areas as the traditional glider,
>as well as the problems mentioned above.

	The reson that biplanes exist is that way back when, the wrights
and others tested airfoil designs in very low reynolds number tunnels, and
they got airfoil ideas from birds.  At low reynolds numbers, thin airfoils
are better than thick ones.  So, to get enough structure to hold the wing
together they built biplanes with external bracing since the wings were
too thin for internal spars.

	For a sailplane, there are a couple of ideas why a biplane is not
such a good idea.

	1.  Each wing sits in the downwash of the other--as a result each wing
has the induced drag it would normally have, but the combination also has an
 interference drag--the total induced drag is more than 2 times the induces
 drag from an isolated wing.

	2.  The chord of a biplanes wings would probably be smaller than
that of a monoplane--as a result the wing would operate at a lower
reynolds number.  Skin friction drag coefficients go down as reynolds
number goes up, not to mention problems with separation.  This is one of
the reasons why model sailplanes can't even come close to getting the L/D's
 the full size airplanes get. 


	The main idea of all this is that a wing sheds most of its vorticity
near the tips, a high aspect ratio wing sheds the vorticity farther from the
bulk of the wing than a shorter span biplane--as a result the monoplane
has lower induced drag.

	A biplane is lighter, however, so its induce drag is reduced somwhat 
because it is producing less total lift.

	Overall, the biplane wins a little on weight, but looses a lot in
induced drag.  The result is almost certainly that a biplane
has lower performance, but I should do a little more work and check
it for sure...

							-Scott Harris	
Newsgroups: rec.aviation