Tandem-wing Airplanes

Here's another question for you engineers out there. Traditional
airplane design has the tail pressing down, so the tail is fighting
the work that the main wing is doing. A tandem-wing airplane in which
both front and rear wings are lifting upward is a more efficient
design, which is one reason Bert Rutan chose the canard configuration
for so many of his designs. But in the canard configuration, the
front wing is smaller than the rear wing. This is what I don't
understand. It seems to me that a design in which the front wing was
larger and the rear wing was smaller would be more stable in pitch.
The smaller rear wing would automatically damp pitch excursions like
the fins of an arrow. So why is the canard the most successful tandem-
wing design flying?

Phil J wrote in news:e5efb14f-c2c5-41c7-a127-
:

Here's another question for you engineers out there. Traditional
airplane design has the tail pressing down, so the tail is fighting
the work that the main wing is doing. A tandem-wing airplane in which
both front and rear wings are lifting upward is a more efficient
design, which is one reason Bert Rutan chose the canard configuration
for so many of his designs. But in the canard configuration, the
front wing is smaller than the rear wing. This is what I don't
understand. It seems to me that a design in which the front wing was
larger and the rear wing was smaller would be more stable in pitch.
The smaller rear wing would automatically damp pitch excursions like
the fins of an arrow. So why is the canard the most successful
tandem-
wing design flying?


Well, it isn't a tandem wing, for one thing. It's a canard. It's front
"wing" is called a canard and not a wing. You could say it's a tomato
tomato thing, but that's the definition. A Bleriot could also be called
a tandem wing aircraft if you used the same standard. It's tail lifts.
So do most free flight models. These airplanes have very large stabs (or
wings, if you prefer), and very far aft CGs as compared to a a
"conventional" aircraft and usually very long fuselages.
Aircraft like the Bleriot were not very stable in pitch, and RC
conversions of old time free flight airplanes with the original FF CG
are very twitchy in pitch if elevator is used.

The basic principle is that more of the horizontal surface ( multiplied
by it's arm) has to be behind the CG to get the thing going in the
direction you want it to. Think horizontal weather vane.
That's pretty simplistic, but basically it's the way it works. The
horizontal weather vane principle also explains why conventional
aircraft get nasty when their CG is moved aft. Never mind any rubbish
Jepeson might tell you about the elevators making lift the wrong way.
..
I'm not exactly sure what the definition of a tandem wing is, percentage
wise, but basically if it looks like one then it is one. That is, the
wings should be in the neighborhood of each other area wise.
The Flying Flea would be a good example.

Bertie

On Feb 2, 1:59*pm, Bertie the Bunyip wrote:

Well, it isn't a tandem wing, for one thing. It's a canard. It's front
"wing" is called a canard and not a wing. You could say it's a tomato
tomato thing, but that's the definition. A Bleriot could also be called
a tandem wing aircraft if you used the same standard. It's tail lifts.
So do most free flight models. These airplanes have very large stabs (or
wings, if you prefer), and very far aft CGs as compared to a a
"conventional" aircraft and usually very long fuselages.
Aircraft like the Bleriot were not very stable in pitch, and RC
conversions of old time free flight airplanes with the original FF CG
are very twitchy in pitch if elevator is used.

*The basic principle is that more of the horizontal surface ( multiplied
by it's arm) has to be behind the CG to get the thing going in the
direction you want it to. Think horizontal weather vane.
That's pretty simplistic, but basically it's the way it works. The
horizontal weather vane principle also explains why conventional
aircraft get nasty when their CG is moved aft. Never mind any rubbish
Jepeson might tell you about the elevators making lift the wrong way.
.
I'm not exactly sure what the definition of a tandem wing is, percentage
wise, but basically if it looks like one then it is one. That is, the
wings should be in the neighborhood of each other area wise.
The Flying Flea would be a good example.

Bertie

OK. But why on the canards flying these days is the little wing in
front of the CG, and the big wing behind it. It seems like it would
be more stable in pitch if the little wing was behind the CG. Putting
the little wing in front of the CG seems like it would make the
airplane inherently unstable in pitch. Looking at Rutan's designs, it
looks like he countered this by using a swept main wing. But that
would have been unnecessary if he had put the smaller wing in back.
The only reason I can think of to put the smaller wing out front would
be for pilot visibility, so maybe that's the explanation?

Phil

Phil J wrote in news:75220ca0-969d-4a58-8dac-
:



OK. But why on the canards flying these days is the little wing in
front of the CG, and the big wing behind it. It seems like it would
be more stable in pitch if the little wing was behind the CG.


Then it wouldn't be a canard.


Putting
the little wing in front of the CG seems like it would make the
airplane inherently unstable in pitch. Looking at Rutan's designs, it
looks like he countered this by using a swept main wing. But that
would have been unnecessary if he had put the smaller wing in back.
The only reason I can think of to put the smaller wing out front would
be for pilot visibility, so maybe that's the explanation?


No, it's because he wanted a canard. I explained above that having a
lifitng stab, even a great big one, makes for a twitchy airplane. I'm
sure that could be managed if you wanted, but it's not ever going to be
a very happy airplane. The smaller "wing",on a canard is called a
canard. It's primarily a stabilsation surface that also contributes to
overall lift. It is not a wing
There are probably several reasons that Rutan elected to sweep the wing.
One, it gives good stability without sacrificing manueverability. two,
it expands the CG limits and in the case of this aricraft, allows a
shorter fuselage than would be the case if the weren't swept. .
So, tu summarise, if you put the "smaller wing" (sic) in the back, it';s
a tail unless it;s lifting. If it's lifting it needs to be fairly large
to be useful. make it large enough and you have problems with handling,
one solutuion for this problem is to reduce the sizre of the forward
wing and move the CG aft. Voila! you have a canard!


Bertie

On Feb 2, 2:21 pm, Bertie the Bunyip wrote:
So, tu summarise, if you put the "smaller wing" (sic) in the back, it';s
a tail unless it;s lifting. If it's lifting it needs to be fairly large
to be useful. make it large enough and you have problems with handling,
one solutuion for this problem is to reduce the sizre of the forward
wing and move the CG aft. Voila! you have a canard!

Bertie

Seems to me that lifting tails are, and have been, illegal for
long time. The regs call for the aircraft to automatically settle into
a glide if the power should fail, to prevent stalling. A lifting tail
just won't do this. As the airplane slows it will drop, raising the
nose, and the airplane will stall, and almost certainly enter an
unrecoverable spin. If the pilot does manage to establish a glide, the
nose will drop further as glide speed increases, opposite to what we
know in our airplanes, and totally unstable. Some early airplanes were
built this way, and after they'd killed enough pilots the designers
decided to make things differently.
See FAR 23 (U.S.) or CAR 523 (Canadian) for the details.

Dan

wrote in news:ac796136-f9af-4608-a9ad-
:

On Feb 2, 2:21 pm, Bertie the Bunyip wrote:
So, tu summarise, if you put the "smaller wing" (sic) in the back,
it';s
a tail unless it;s lifting. If it's lifting it needs to be fairly
large
to be useful. make it large enough and you have problems with
handling,
one solutuion for this problem is to reduce the sizre of the forward
wing and move the CG aft. Voila! you have a canard!

Bertie

Seems to me that lifting tails are, and have been, illegal for
long time.


Well, i wouldn't have thought so.

The regs call for the aircraft to automatically settle into
a glide if the power should fail, to prevent stalling. A lifting tail
just won't do this. As the airplane slows it will drop, raising the
nose, and the airplane will stall, and almost certainly enter an
unrecoverable spin. If the pilot does manage to establish a glide, the
nose will drop further as glide speed increases, opposite to what we
know in our airplanes, and totally unstable. Some early airplanes were
built this way, and after they'd killed enough pilots the designers
decided to make things differently.
See FAR 23 (U.S.) or CAR 523 (Canadian) for the details.


Wel, yeah, if you just put one one on a 172 and move the CG back you're
going to have problems. , but lots of free flight airplanes use this
system with no problems. They do get very twitchy n pitch if you put a
radio elevator on them though.
this is a typicl old time FF airplane with a lifting stab.

http://www.rcgroups.com/forums/attac...mentid=1670799

These don't spin even with elevator and rudder (wel, not easily) and as
you can see are arranged very different from a modern lightplane. They
are also extremley sensitive to CG variations which would make this
arrangemetn undesirable on full size aircraft.


Bertie

"Phil J" wrote in message
...


OK. But why on the canards flying these days is the little wing in
front of the CG, and the big wing behind it. It seems like it would
be more stable in pitch if the little wing was behind the CG. Putting
the little wing in front of the CG seems like it would make the
airplane inherently unstable in pitch. Looking at Rutan's designs, it
looks like he countered this by using a swept main wing. But that
would have been unnecessary if he had put the smaller wing in back.
The only reason I can think of to put the smaller wing out front would
be for pilot visibility, so maybe that's the explanation?

Phil

The pitch and yaw stability of an aircraft have to do with the relationship
of the center of gravity and the center of pressure. The farther ahead of
the CP the CG is, the more stable an aircraft will be.

The swept wing on rutan designs serves 2 purposes. First, it moves the
center of lift (and center of pressure) aft in order to accomodate the aft
CG caused by the rear mounted engine. Second, sweeping the wing places the
vertical surfaces farther aft, giving them more authority.

KB

Seems to me that lifting tails are, and have been, illegal for
long time. The regs call for the aircraft to automatically settle into
a glide if the power should fail, to prevent stalling. A lifting tail
just won't do this. As the airplane slows it will drop, raising the
nose, and the airplane will stall, and almost certainly enter an
unrecoverable spin. If the pilot does manage to establish a glide, the
nose will drop further as glide speed increases, opposite to what we
know in our airplanes, and totally unstable.

A good reference for this subject is "See How it Flies". It's not true that a
tail with positive lift will necessarily lead to instability:

"The biggest contribution to angle of attack stability is decalage. The thing
in back flies at a lower angle of attack than the thing in front. The thing in
back may, but need not, fly at a negative angle of attack."
http://www.av8n.com/how/htm/aoastab....oastab-summary

The pitch stability discussion is at:
http://www.av8n.com/how/htm/aoastab....ch-equilibrium

Phil J wrote:
Here's another question for you engineers out there. Traditional
airplane design has the tail pressing down, so the tail is fighting
the work that the main wing is doing. A tandem-wing airplane in which
both front and rear wings are lifting upward is a more efficient
design, which is one reason Bert Rutan chose the canard configuration
for so many of his designs. But in the canard configuration, the
front wing is smaller than the rear wing. This is what I don't
understand. It seems to me that a design in which the front wing was
larger and the rear wing was smaller would be more stable in pitch.
The smaller rear wing would automatically damp pitch excursions like
the fins of an arrow. So why is the canard the most successful tandem-
wing design flying?

There are a couple of "tandem wing" designs I can think of:

1) Samuel Pierpont Langley's "Aerodrome"
http://home.att.net/~dannysoar2/Langley.htm

2) The Mignet Pou du Ciel (Flying Flea)
http://www.flyingflea.org/

The Aerodrome was made to fly briefly after heavy modification by Glen
Curtiss. The Pou du Ciel, which is now a generic term covering a family
of designs using similar configuration, has been flying since the 1930s.

I'm not an aeronautical engineer (I wish!) but from my reading of your
post you are implying that the canard configuration is less stable in
pitch. The way I figure it is that while having a lifting surface ahead
of the CG is dynamically unstable (movement away from a stable "neutral"
configuration will produce forces which accelerate the movement away
from neutral) there is still the main wing behind the CG which will
produce a much greater force and return the aircraft to the neutral
position.

This sentence: "The smaller rear wing would automatically damp pitch
excursions like the fins of an arrow" applies to a normal
"downward-pushing" elevator too. However I don't think you can just make
a blanket statement along the lines of "this is less efficient because
it's working against the wing". I suspect other considerations such as
stability and control come into play here. One only has to look at the
design of modern airliners to see that those other considerations
override the potential efficiency gains of eliminating the
downward-pushing elevator.

On Feb 2, 11:59*am, Bertie the Bunyip wrote:
Phil J wrote in news:e5efb14f-c2c5-41c7-a127-
:



Here's another question for you engineers out there. *Traditional
airplane design has the tail pressing down, so the tail is fighting
the work that the main wing is doing. *A tandem-wing airplane in which
both front and rear wings are lifting upward is a more efficient
design, which is one reason Bert Rutan chose the canard configuration
for so many of his designs. *But in the canard configuration, the
front wing is smaller than the rear wing. *This is what I don't
understand. *It seems to me that a design in which the front wing was
larger and the rear wing was smaller would be more stable in pitch.
The smaller rear wing would automatically damp pitch excursions like
the fins of an arrow. *So why is the canard the most successful
tandem-
wing design flying?

Well, it isn't a tandem wing, for one thing. It's a canard. It's front
"wing" is called a canard and not a wing. You could say it's a tomato
tomato thing, but that's the definition. A Bleriot could also be called
a tandem wing aircraft if you used the same standard. It's tail lifts.
So do most free flight models. These airplanes have very large stabs (or
wings, if you prefer), and very far aft CGs as compared to a a
"conventional" aircraft and usually very long fuselages.
Aircraft like the Bleriot were not very stable in pitch, and RC
conversions of old time free flight airplanes with the original FF CG
are very twitchy in pitch if elevator is used.

*The basic principle is that more of the horizontal surface ( multiplied
by it's arm) has to be behind the CG to get the thing going in the
direction you want it to. Think horizontal weather vane.
That's pretty simplistic, but basically it's the way it works. The
horizontal weather vane principle also explains why conventional
aircraft get nasty when their CG is moved aft. Never mind any rubbish
Jepeson might tell you about the elevators making lift the wrong way.
.
I'm not exactly sure what the definition of a tandem wing is, percentage
wise, but basically if it looks like one then it is one. That is, the
wings should be in the neighborhood of each other area wise.
The Flying Flea would be a good example.

Bertie

I was gonna mention the Flea. The Frogs seam to like the Fleas.

Wil