Fast glass biplanes

Apart from the Quickie group of planes why aren't there more fast
biplanes? The quickies aren't exactly biplanes I know. It would
appear that 2 short wings can be built lighter than one long one since
the moment arm is half as long for the shorter wing pair. No struts
used because of drag, just short cantilever wings. You'd have a more
compact airplane that way with less weight that had the same drag as
an equal wing area monoplane.

Jay wrote:

It would appear that 2 short wings can be built lighter
than one long one since the moment arm is half as long
for the shorter wing pair.

There are a lot of benefits to higher aspect ratio
wings that far outweigh the structural advantage of low AR
wings. Reduced drag is but one. Ever wonder why you
don't see any biplane sailplanes?

You'd have a more compact airplane that way with less
weight that had the same drag as an equal wing area monoplane.

'Fast' and 'biplane' just don't go together. Fast "biplanes"(*)
like the Quickie, Mong, etc. do not get their speed and low
drag from the fact that they have two wings, but rather in
spite of it. Induced drag decreases as aspect ratio increases,
so a longer span wing of equivalent area and wing section will
have less induced drag than two wings with a lower AR. Two wings
will also have at least double the interference drag of one,
regardless of whether or not they use interplane struts.

Dave 'dragster' Hyde


(*) Jay has already stated that he knows the
Quickie is not a true biplane. The same principles apply, however.

Dave Hyde wrote in message
There are a lot of benefits to higher aspect ratio
wings that far outweigh the structural advantage of low AR
wings. Reduced drag is but one. Ever wonder why you
don't see any biplane sailplanes?

You bring up a good point about sailplane wings having the best L/D
ratios. But why not take each of those sailplane wings and put one
over the top of the other? You mentioned the interference drag, so
how far do wings need to be vertically separated for a given airfoil
and stagger for this effect to be negligable?

The fact that you don't see something commonly done says more about
the methods of development starting with what currently works, and
trying to make incremental improvement on it than anything else.
Sometimes the rat maze requires the rats (RAH) to back up and choose
another path, which in the short term means he is actually retreating
from the cheese (speed).

It's generally accepted to draw circle with a diameter of the wingspan,
centered at the middle of the wing. Do this once for every wing. Where
the circles intersect, they are interacting. I know of none that interact
positively. Some less negatively than others.

Another way to look at this is the aspect ratio. Aspect ratio is span
squared divided by the total area. More aspect ratio is better than less.

Anything less than 6 is not very efficient. Decent gliders are above 20.
I've flown one powered plane that was 10. It did very well. Flew like it
had a lot more area than it really had.



In article ,
(Jay) wrote:
Dave Hyde wrote in message
There are a lot of benefits to higher aspect ratio
wings that far outweigh the structural advantage of low AR
wings. Reduced drag is but one. Ever wonder why you
don't see any biplane sailplanes?

You bring up a good point about sailplane wings having the best L/D
ratios. But why not take each of those sailplane wings and put one
over the top of the other? You mentioned the interference drag, so
how far do wings need to be vertically separated for a given airfoil
and stagger for this effect to be negligable?

The fact that you don't see something commonly done says more about
the methods of development starting with what currently works, and
trying to make incremental improvement on it than anything else.
Sometimes the rat maze requires the rats (RAH) to back up and choose
another path, which in the short term means he is actually retreating
from the cheese (speed).

Jay wrote:

You bring up a good point about sailplane wings having the best L/D
ratios. But why not take each of those sailplane wings and put one
over the top of the other?

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 mentioned the interference drag, so how far do wings
need to be vertically separated for a given airfoil
and stagger for this effect to be negligable?

*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.

Sometimes the rat maze requires the rats (RAH) to back up and choose
another path, which in the short term means he is actually retreating
from the cheese (speed).

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.
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.

Dave 'usenet wind tunnel' Hyde

In article , Dave Hyde says...

Don't bet any big money cause Dave (usenet wind tunnel) Hyde is right.
Just think, if he was wrong we'd be seeing Biplane Boeing 777's,these guys spend
millions to get a couple percent increased efficiency on their transports.
Better believe if a biplane was more efficient they'd be doing it.
No if's, ands, or buts. :-)

Chuck(Lewis 10X10 wind tunnel) S




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.

Dave 'usenet wind tunnel' Hyde

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.

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.

Those molecules are smarter than you might expect. G

There can be significant spanwise flow of the air. Like most things in nature
air finds the path of least resistance and sometimes this is not where it was
headed when the wing bounced into it.

Even if you take the same 40 ft high aspect ratio wing, saw it into 2 halves
and manage to attach it to the fuselage with no increase in interference drag
it's going to be less efficient than the 1 long wing - because of the spanwise
flow. Winglets help, flow fences help, joined wing tips help, elliptical
planform helps.

Look up W. Kaspar and his work on tip vortices.

Earlier, (Jay) wrote:

Okay, why don't you start off by
showing me how span comes into the
relationship of air moving over a
wing's airfoil.

That's generally covered in what they call "Theory of Finite Wings."
There's a chapter on it in Abbott and Doenhoff. This Desktop Aero page
is a good introduction to the topic:

http://www.desktopaero.com/appliedae...ingmodels.html

Bob K.

Jay wrote:

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).

The generally accepted definition of the induced drag coefficient
is:

CDi=CL^2/pi/e/AR,
where CL is the wing lift coefficient at the conditions under
consideration,
pi=3.14159...
e = Oswald's efficiency factor (typically 0.8 or so)
AR = aspect ratio

The _definition_ of aspect ratio is chord/span, or span^2/aero (they're
equivalent), so as area remains the same but aspect ratio increases,
induced drag decreases by 1/span^2. That's what I call a primary
effector.
If you add wing treatments like winglets, fences, etc, you can increase
the
effective AR, but the big effects are gained by working at the tips,
not across the span, as another wing typically does.

Look at the lift side. The formula becomes messier, but for a finite
wing:

CL,finite ~= CL,infinite*(1/(1+(dCL,inf/daoa)/pi/AR))

As span increases through increased aspect ratio, the finite
wing lift coefficient gets closer to the infinite wing CL.

Can we agree that this is a good thing?

In the lift case, there is *some* easily realizable benefit.
A forward surface like a canard can be used as a big vortex
generator to keep flow attached over the 'main wing' and
increase lift/delay stall. That's why you see a lot of close-coupled
canards on fighters these days.

There's also the trim drag benefit of another surface if
that surface can be configured to reduce the total downforce
required to trim. That's another reason for canards and
relaxed stability airliners. This benefit is usually
not as pronounced as the high AR benefit.

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.

Um...you might want to review some finite wing theory.
There can be quite a bit of spanwise flow at the root _or_
the tip. When subsonic you make a bow wake. The air is moving
before you hit it, and it's not just front-to-back.

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's not a rule of thumb, that's physics. All other things being
equal, the highger AR wing *will* have less drag.

Okay, why don't you start off by
showing me how span comes into the relationship of air moving over a
wing's airfoil.

Done and done. Your turn.

I've worked for lots of companies like Boeing...

Have you ever worked in conceptual design and/or
aerodynamics? Most of your risk aversion comments
were way off the mark. A trip to the Air Force museum
to see the Bird of Prey or the X-36 could be illuminating.

Dave 'misconceptual design' Hyde