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

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

On Feb 2, 4:22*pm, wrote:
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

OK, this makes sense. Since a small lifting tail would be a long way
from the CG (compared to the main wing), it would experience a much
higher angle of attack when the aircraft pitched up. It would be very
difficult to make the main wing stall before the tail.

Phil

On Feb 2, 3:21*pm, Bertie the Bunyip wrote:
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

OK, it's a canard if its primary function is stability rather than
lift. I guess Rutan's Quickie is more like what I was thinking
about. On that airplane the front wing contributes 60% of the lift,
so it's a true wing. And there you have the larger wing in front and
the smaller wing in back. I don't know much about the stall
characteristics of that airplane, but it definitely seems to be an
efficient design. With a 64-horsepower engine it has a 140-mph
cruise.

Phil

Phil J wrote in news:ad6eb94e-4e9e-46a8-b608-
:

On Feb 2, 3:21*pm, Bertie the Bunyip wrote:
Phil J wrote in news:75220ca0-969d-4a58-8dac-
:



OK. *But why on the canards flying these days is the little wing in



OK, it's a canard if its primary function is stability rather than
lift. I guess Rutan's Quickie is more like what I was thinking
about. On that airplane the front wing contributes 60% of the lift,
so it's a true wing.


OK, that's right. I had forgoten about them. Yeah, That's a true tandem
wing.


And there you have the larger wing in front and
the smaller wing in back. I don't know much about the stall
characteristics of that airplane, but it definitely seems to be an
efficient design. With a 64-horsepower engine it has a 140-mph
cruise.

Yeah, I kow someone who built a Q200 and it cruises at about 160 mph on
100 hp! Superb.
The decalage, as with the canards, is arranged so that the front wing
reaches critical angle first thus lowering the nose. So, they don't
realy stal in the same way that a conventinal airplane does. I've never
flown one, though I have a standing offer to fly this one if I ever get
into his neighborhood. I would have thought that the rear wing was
taking more of the load, though, but i can't find any info on that on
the net. The thing that always made me nervous about those things is the
pounding the front wing must take on the ground..

Bertie