Tandem-wing Airplanes

Phil J wrote in
:

On Feb 3, 12:31*pm, Bertie the Bunyip wrote:
"patrick mitchel" wrote in news:fo50mi$2bl2$1
@zook.lafn.org:

*Now about that Piaggio avanti....... Pat

Canard, . And I think the reason you're confused about Canards is
this: Th
e
canard is not there to provide stability. Not in the same way as a
stab on

a stadard airplane. It's there to provide control. The wing provides
the stability in the way you understand it.

Bertie

I think you are referring to me rather than Pat, but I understand what
you are saying. On a canard design, the main wing functions like a
stab since it is behind the CG. OTOH, trying to figure out the
Piaggio seems like a good way to get a headache. It's amazing what
they were able to do with compound curves in aluminum on that
airplane, though.

Yes, that's pretty much it. The Piaggio is pretty, allright!

Bertie

"Marc J. Zeitlin" wrote in news:47a60ac9$0
:


Dan_Thomas wrote:

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

This is not true. There are no regulations that prohibit lifting
tails, or in fact in any way prescribe which way the lift vector needs
to point on any lifting surface.

... The regs call for the aircraft to automatically settle into a
glide if the power should fail,to prevent stalling.

What reg would that be? What airplane does NOT glide when the power
fails? Your plane is trimmed for whatever speed you're flying - chop
the power, and you'll continue to fly at that speed, albeit
descending. There may be some trim changes on aircraft that have
their engines mounted substantially above the CG (Lake Buccaneer, or
the equivalent) or that have substantial thrust angles, but the
principal holds.

Stalling and gliding are independent activities - one is not related
to the other. You can stall while gliding or under power, and you can
glide while unstalled or stalled.

... 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.

That's incorrect. In a conventional tailed aircraft, the tail may
have a downforce, no force at all, or an upforce, depending upon the
design of the aircraft, the relationship of the aerodynamic center to
the CG, and the flight regime. There is no intrinsic reason why a
lifting tail (or a tail that in a given situation is producing no
lift) would cause the aircraft to do what you state.

... 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.

Incorrect. The pitch stability of an aircraft is not determined by
whether or not the tail lifts - it's determined by the relative
position of the aerodynamic center and the CG (CG always ahead of AC),
and the relative angles of the front and rear wings. See:

http://www.av8n.com/how/htm/aoastab.html

As long as the front wing is operating at a higher angle of attack
than the rear wing - tail, wing, 2nd wing of a tandem; makes no
difference - which is usually achieved by correct settings of the
angle of incidence and ensuring that the front wing has a higher lift
curve slope than the rear wing, then the aircraft will be statically
stable in pitch.

The above web site has some VERY good explanations and pictures of
these situations, including canards. Consider a tandem aircraft to
merely be a canard or conventional aircraft that has it's tail/canard
to be about the same size as the other wing.

... Some early airplanes were built this way, and after they'd
killed enough pilots the designers decided to make things
differently.

Hmmm. I wonder how my COZY MKIV canard aircraft maintains pitch
stability, then, and in fact is extremely stall resistant. Scaled
Composites' "Proteus" aircraft is essentially a tandem wing airplane -
the front and rear wings are very close in size. The Piaggo Avanti
has both a canard flying surface AND a tail.

There are numerous examples of tandem, canard, and three surface
aircraft that are completely stable in pitch, and have benign stall
characteristics.

See FAR 23 (U.S.) or CAR 523 (Canadian) for the details.

Yes, see 23.302, which specifically references the canards and tandem
wing aircraft. If they were "illegal", as you claim, there shouldn't
be any reference to the rules with which they must comply. There are
no other part 23 FARs that state what you claim.


Phil J wrote:

OK, this makes sense.

No, actually very little of what's been written in this thread makes
any sense. There's a lot of misinformation about what makes aircraft
work. The web site I pointed to above, while not perfect, has some
excellent explanations of many aerodynamic points about which there is
much misinformation, confusion, and clears up a lot of things that
people know that just isn't true.

Kyle Boatright was just about the only one to write something that
made aerodynamic sense.


Beg yer pardon, but I posted pretty muc the same thing as you but in my
native tongue..

Bertie

On Feb 3, 11:43 am, "Marc J. Zeitlin"
wrote:
Dan_Thomas wrote:
Seems to me that lifting tails are, and have been, illegal for
long time.

This is not true. There are no regulations that prohibit lifting
tails, or in fact in any way prescribe which way the lift vector needs
to point on any lifting surface.

... The regs call for the aircraft to automatically settle into a
glide if the power should fail,to prevent stalling.

What reg would that be? What airplane does NOT glide when the power
fails? Your plane is trimmed for whatever speed you're flying - chop
the power, and you'll continue to fly at that speed, albeit
descending. There may be some trim changes on aircraft that have
their engines mounted substantially above the CG (Lake Buccaneer, or
the equivalent) or that have substantial thrust angles, but the
principal holds.

Stalling and gliding are independent activities - one is not related
to the other. You can stall while gliding or under power, and you can
glide while unstalled or stalled.

... 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.

That's incorrect. In a conventional tailed aircraft, the tail may
have a downforce, no force at all, or an upforce, depending upon the
design of the aircraft, the relationship of the aerodynamic center to
the CG, and the flight regime. There is no intrinsic reason why a
lifting tail (or a tail that in a given situation is producing no
lift) would cause the aircraft to do what you state.

... 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.

Incorrect. The pitch stability of an aircraft is not determined by
whether or not the tail lifts - it's determined by the relative
position of the aerodynamic center and the CG (CG always ahead of AC),
and the relative angles of the front and rear wings. See:

http://www.av8n.com/how/htm/aoastab.html

As long as the front wing is operating at a higher angle of attack
than the rear wing - tail, wing, 2nd wing of a tandem; makes no
difference - which is usually achieved by correct settings of the
angle of incidence and ensuring that the front wing has a higher lift
curve slope than the rear wing, then the aircraft will be statically
stable in pitch.

The above web site has some VERY good explanations and pictures of
these situations, including canards. Consider a tandem aircraft to
merely be a canard or conventional aircraft that has it's tail/canard
to be about the same size as the other wing.

... Some early airplanes were built this way, and after they'd
killed enough pilots the designers decided to make things
differently.

Hmmm. I wonder how my COZY MKIV canard aircraft maintains pitch
stability, then, and in fact is extremely stall resistant. Scaled
Composites' "Proteus" aircraft is essentially a tandem wing airplane -
the front and rear wings are very close in size. The Piaggo Avanti
has both a canard flying surface AND a tail.

There are numerous examples of tandem, canard, and three surface
aircraft that are completely stable in pitch, and have benign stall
characteristics.

See FAR 23 (U.S.) or CAR 523 (Canadian) for the details.

Yes, see 23.302, which specifically references the canards and tandem
wing aircraft. If they were "illegal", as you claim, there shouldn't
be any reference to the rules with which they must comply. There are
no other part 23 FARs that state what you claim.

Phil J wrote:
OK, this makes sense.

No, actually very little of what's been written in this thread makes
any sense. There's a lot of misinformation about what makes aircraft
work. The web site I pointed to above, while not perfect, has some
excellent explanations of many aerodynamic points about which there is
much misinformation, confusion, and clears up a lot of things that
people know that just isn't true.

Kyle Boatright was just about the only one to write something that
made aerodynamic 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.

Not so. The distance of a lifting surface from the CG has no bearing
whatsoever on the AOA that the surface sees in a static situation.
During the dynamic rotation, there would be a slight difference, but
it's not particularly meaningful. See the web site listed above for a
great description/explanation of pitch stability (among many other
things).

... It would be very difficult to make the main wing stall before
the tail.

Not so. See the explanations above. I have a canard aircraft. The
canard wing (in front) has a higher incidence angle and lift curve
slope than the rear wing. As long as I keep the CG ahead of the
rearmost CG position, the aircraft will be stable in pitch, and the
front wing will ALWAYS stall first. When this happens, the nose
drops, and the rear wing maintains an appropriate AOA and doesn't stall.

Same thing happens on tandem wing aircraft, or conventional aircraft
with lifting tails.

Hope this helps clear things up.

--
Marc J. Zeitlin
http://www.cozybuilders.org/
Copyright (c) 2008 http://www.mdzeitlin.com/Marc/


Here are two conflicting statements you made:

In a conventional tailed aircraft, the tail may
have a downforce, no force at all, or an upforce, depending upon the
design of the aircraft, the relationship of the aerodynamic center to
the CG, and the flight regime. There is no intrinsic reason why a
lifting tail (or a tail that in a given situation is producing no
lift) would cause the aircraft to do what you state.


The pitch stability of an aircraft is not determined by
whether or not the tail lifts - it's determined by the relative
position of the aerodynamic center and the CG (CG always ahead of AC),
and the relative angles of the front and rear wings.

I was talking about airplanes that appear conventional, like
the lightplanes we have now. With a lifting tail the CG must be behind
the AC, not ahead of it, and stability would be negative. I was not
talking about canards like the Cozy or any other Rutan-inspired
design, or the tandems. Some early conventional-layout airplanes used
lifting tails and were deadly.
Which "conventional tailed aircraft" would have any upforce
in level flight?

Dan

wrote in news:f886c681-9425-48a9-831d-
:


Here are two conflicting statements you made:


They arent conflicting. They're the same statement made two different ways.


In a conventional tailed aircraft, the tail may
have a downforce, no force at all, or an upforce, depending upon the
design of the aircraft, the relationship of the aerodynamic center to
the CG, and the flight regime. There is no intrinsic reason why a
lifting tail (or a tail that in a given situation is producing no
lift) would cause the aircraft to do what you state.


The pitch stability of an aircraft is not determined by
whether or not the tail lifts - it's determined by the relative
position of the aerodynamic center and the CG (CG always ahead of AC),
and the relative angles of the front and rear wings.

I was talking about airplanes that appear conventional, like
the lightplanes we have now. With a lifting tail the CG must be behind
the AC, not ahead of it, and stability would be negative.



Nope, because th estab is part of the AC and the stab on a lifting stab
arrangement is significantly larger thna it's conventional counterpart.



Bertie

On Feb 3, 10:11*am, Phil J wrote:
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. *

So now you want both wings behind the CG generating lift? How do you
think it will balance?

Cheers

On Feb 3, 10:45*pm, WingFlaps wrote:

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. *

So now you want both wings behind the CG generating lift? How do you
think it will balance?

Cheers

No. I was talking about the larger wing being in front of the CG, and
the smaller wing behind the CG.

Phil

On Feb 3, 9:05 pm, "Marc J. Zeitlin"
wrote:

Any aircraft in which the CG can be ahead of the AIRCRAFT AC, but
behind the main wing AC. There are many aircraft that when flown at
rear CG positions and low speeds will have the tail producing lift.
Gliders, in particular. Off the top of my head, I don't know of any
in particular, although I remember being told that a C-172 will have
an upforce on the tail at low speeds and rear CG. Can't cite it, though.

Even with the CG at its most aft position, a 172's AC is still
behind the CG. CG range is typically 25-33% of MAC, while AC is around
40%.


The web site I indicated:

http://www.av8n.com/how/htm/aoastab.html

explains all this.

That's a good site that I've used for more than four years. I
think we're talking about two different things here and there's a
misunderstanding that leads to argument.
I find a fairly widely-held opinion that the aft CG can be
(legally) at or behind AC. This isn't true for any "modern"
lightplane. FAR 23.173 requires that the airplane return to trimmed
speed after being slowed or accelerated using pitch inputs only and
releasing them; this won't happen in a CG-behind-AC situation. As the
wing slows its AC moves forward due to the breakup of the boundary
layer toward the trailing edge, the CG therefore ends up even farther
behind the AC, the situation gets worse as the nose rises with the
forward-moving AC, and it eventually stalls. This is what I meant by
the illegality of a lifting tail.
Here's an example of some common miperceptions:
http://answers.yahoo.com/question/in...9225927AAfYZDU
Even the first answer, the one the voters liked, says that the CG is
12" ahead of the AC even in the most-aft position, This is extreme;
it's a lot less than that, but it's still forward. Other posters think
that with the CG at the aft position the tail must produce lift.

Dan

Dan

wrote in
:

On Feb 3, 9:05 pm, "Marc J. Zeitlin"
wrote:

Any aircraft in which the CG can be ahead of the AIRCRAFT AC, but
behind the main wing AC. There are many aircraft that when flown at
rear CG positions and low speeds will have the tail producing lift.
Gliders, in particular. Off the top of my head, I don't know of any
in particular, although I remember being told that a C-172 will have
an upforce on the tail at low speeds and rear CG. Can't cite it,
though.

Even with the CG at its most aft position, a 172's AC is still
behind the CG. CG range is typically 25-33% of MAC, while AC is around
40%.


We're not talking about a 172.


Bertie

On Feb 4, 8:48 am, Bertie the Bunyip wrote:
wrote :

On Feb 3, 9:05 pm, "Marc J. Zeitlin"
wrote:

Any aircraft in which the CG can be ahead of the AIRCRAFT AC, but
behind the main wing AC. There are many aircraft that when flown at
rear CG positions and low speeds will have the tail producing lift.
Gliders, in particular. Off the top of my head, I don't know of any
in particular, although I remember being told that a C-172 will have
an upforce on the tail at low speeds and rear CG. Can't cite it,
though.

Even with the CG at its most aft position, a 172's AC is still
behind the CG. CG range is typically 25-33% of MAC, while AC is around
40%.

We're not talking about a 172.

Bertie

OK. I'll leave now.

Dan

On Feb 4, 6:53 am, wrote:
On Feb 3, 9:05 pm, "Marc J. Zeitlin"
wrote:

Any aircraft in which the CG can be ahead of the AIRCRAFT AC, but
behind the main wing AC. There are many aircraft that when flown at
rear CG positions and low speeds will have the tail producing lift.
Gliders, in particular. Off the top of my head, I don't know of any
in particular, although I remember being told that a C-172 will have
an upforce on the tail at low speeds and rear CG. Can't cite it, though.

Even with the CG at its most aft position, a 172's AC is still
behind the CG. CG range is typically 25-33% of MAC, while AC is around
40%.



The web site I indicated:

http://www.av8n.com/how/htm/aoastab.html

explains all this.

That's a good site that I've used for more than four years. I
think we're talking about two different things here and there's a
misunderstanding that leads to argument.
I find a fairly widely-held opinion that the aft CG can be
(legally) at or behind AC. This isn't true for any "modern"
lightplane. FAR 23.173 requires that the airplane return to trimmed
speed after being slowed or accelerated using pitch inputs only and
releasing them; this won't happen in a CG-behind-AC situation. As the
wing slows its AC moves forward due to the breakup of the boundary
layer toward the trailing edge, the CG therefore ends up even farther
behind the AC, the situation gets worse as the nose rises with the
forward-moving AC, and it eventually stalls. This is what I meant by
the illegality of a lifting tail.
Here's an example of some common miperceptions:http://answers.yahoo.com/question/in...9225927AAfYZDU
Even the first answer, the one the voters liked, says that the CG is
12" ahead of the AC even in the most-aft position, This is extreme;
it's a lot less than that, but it's still forward. Other posters think
that with the CG at the aft position the tail must produce lift.
Dan

My experiments with models vindicate Dan's
explanation where a conventional planform is
concerned. Setting the CG = AC can cause a
screwy stall, such as nose pitch-up.
Ken