Dave Hyde wrote in message ...

Thanks for taking the time to make insightful comments on the
discussion.

Because a single wing of equivalent area but longer span will
be more efficient in terms of drag. Biplanes are a simple,
but inefficient, way of getting more lift from wing area
when an increase in span is not feasible. The are not, nor
in general are they intended to be, "low drag."

You must understand that when I say "biplane" I'm not talking about a
Jenny or Spad, I just mean an airplane that meets the requirement of
having 2 lifting surfaces. I understand those early designs were
optimized for the heavy powerplants and weak construction materials of
the era, and had high drag wings that developed a lot of lift at low
speeds.

*negligible?* Some *large* fraction of the span. At a minimum.
Some airplanes are able to use the interaction for benefit,
but it's usually for things like lift improvement at high
AOA. Drag reduction requires doing things at the tips to
make the wings 'think' they are longer and thus have a higher AR.
Just slapping another wing on there ain't gonna do it.

Okay, I think you nailed the departure of my logic from yours. I
don't believe that span is in the formula (at least not in high
order). I think its a function of the airfoil dimensions (chord,
thinkness, shape) and stagger. I do realize that near the
fusalage/tip there is disturbance but this diminishes as you move away
on the span. Imagine that you're an air molecule; how do you know if
you're 5' or 10' along the wing? You don't, when the wing comes
along, you just move along the bottom or zip across the top.

I know that the rule of thumb is higher aspect, higher efficiency
(L/D), but this is only part of the story. That rule makes an
assumption of a single wing. That is to say, assuming you only have a
single wing, and you need to decide how you can distribute your square
feet of area, you'd pick a long skinny wing.

And knowing where to depart from the maze requires either a
foundation in basic principles or blind luck. Given the well-
known relationship between drag and aspect ratio, these principles
lead most people *away from*, not *to* biplanes for drag reduction.

Thats the problem with rules of thumb, often the people using them
forget the assumptions that went into the rule.

How 'bout a challenge: I can show you mathematically and using physical
relationships why (without aerodynamic treatments like winglets or
conjoined
wings) two wings will produce more drag than a single wing of equivalent
area but higher aspect ratio. Your challenge: Prove the physics wrong.
Show how a second wing will result in less drag. Show me the math.

That sounds like a fun challenge. I think we're going to have to
speak in realtionships instead of mathematic expression because we're
using the usenet as our white board. Okay, why don't you start off by
showing me how span comes into the relationship of air moving over a
wing's airfoil.

Dave 'usenet wind tunnel' Hyde


There was someone that commented that if 2 lifting surfaces made
sense, you'd see the 777 with 2 wings because they're Boeing and have
lots of money and super human engineers. I've worked for lots of
companies like Boeing (but not them because they tried to low ball me)
and they're made up of regular guys like you and me. Many of them
have interests and responsibility outside of designing the best
aircraft ever, and really just want to pay their bills and go home and
have a beer. You work as one guy in a huge machine where decisions
are often made on what's politicaly the best answer rather than what's
technically best. You get one tiny componant of this huge project.
These kinds of organizations often punish risk taking in that there is
no upside pay-off if you're right. But if you're wrong, and it was
because you did something different than before, you get hammered. So
the larger the project, the more conservative the approach tends to
be. Remember, bean counters hate risk of any kind.