A dumb doubt on stalls

"Jim Macklin" wrote
[...] If the wing stalled, the center of
pressure would not be creating a moment arm to drop the
nose, the tail must loose lift (stall) to cause the stall
break which causes the recovery from the approaching stall.

The stall break occurs when the wings have already stalled, not when merely
approaching a stall. When the wings stall and then abruptly produce less
lift, the plane's flight path abruptly turns downward. The plane then
weathervanes into the relative wind, thus pitching downward.

It's true that the weathervaning itself can be explained in terms of a
reduction of (downward) lift from the elevator. However, contrary to your
explanation, the reduction of lift is not due to the tail stalling, but
rather just the opposite: the change in relative wind moves the tail's angle
of attack *away from* the (negative) critical angle (rather than toward it,
as would be necessary to stall). And also contrary to your explanation, it's
the stall (of the wings) that causes the tail to lose some lift
(because the relative wind changes), rather than vice versa.

http://www.faa.gov/library/manuals/a...83-25-1of4.pdf

You keep referencing this 111 page document, but you don't
reference where in it you found what you mention above.

I did not reference or even read the FAA Handbook when I
posted my answer. [...]
I referenced the [FAA Handbook] only to allow those who
asked the question to find a reference.

Ok, but if you'd read the book that you referenced but didn't reference,
you'd have seen that it doesn't say what you thought it said.

--Gary

"Matt Whiting" wrote in message
...
Jim Macklin wrote:
If the wing stalled, the center of pressure would not be creating a
moment arm to drop the nose, the tail must loose lift (stall) to cause
the stall break which causes the recovery from the approaching stall.

What?? The weight of the airplane is what creates the rotation once the
lift from the wing is greatly reduced after the wing stalls.

Jim's right that if the CP is aft of the CG, then the wings' lift applies a
nose-down moment, and a reduction in that lift rotates the nose upward,
other things being equal. But other things are not equal (in particular, the
reduction of lift changes the flight path and the relative wind), so the
rest of Jim's explanation is wrong.

--Gary

Unless the wing is producing lift, there is no rotation to
reduce the angle of attack. If the wing was really stalled,
the airplane would fall flat, if it was spinning it would be
a flat spin, but if was fully stalled, it would be a deep
stall and would not rotate the nose down and it would stay
in the stall.

Under normal conditions, neither the wing or the tail fully
stalls. Stall strips, wing twist or air foil changes along
the span keep the wing from reaching the critical angle of
attack at one moment in time. Also the tailplane is usually
a different airfoil and more heavily loaded and is designed
to begin shedding lift [down-force] before the wing. The CG
range is set so that a certified airplane will have that
stable pattern.

The weight of the airplane does not cause the stall break
rotation, it is the lift moment. If it was not for the wing
lift, the airplane's mass as concentrated on the CG would
simply fall as a unit in the same attitude as it was in at
the moment.


If the center of pressure was located at the same location
as the CG, there would be no moment or force to cause
rotation.



--
James H. Macklin
ATP,CFI,A&P

--
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"Matt Whiting" wrote in message
...
| Jim Macklin wrote:
|
| I did not reference or even read the FAA Handbook when I
| posted my answer. If the wing stalled, the center of
| pressure would not be creating a moment arm to drop the
| nose, the tail must loose lift (stall) to cause the
stall
| break which causes the recovery from the approaching
stall.
|
| What?? The weight of the airplane is what creates the
rotation once the
| lift from the wing is greatly reduced after the wing
stalls.
|
|
| I referenced the "book" only to allow those who asked
the
| question to find a reference.
|
| To find a reference that is wrong.
|
|
| BTW, stall behavior changes drastically with the center
of
| gravity and to a lesser amount with weight.
|
| Sure does.
|
| Matt

"Jim Macklin" wrote in message
news:jvdmg.49354$ZW3.19866@dukeread04...
Unless the wing is producing lift, there is no rotation to
reduce the angle of attack. If the wing was really stalled,
the airplane would fall flat,

A "really stalled" wing (that is, one that is past the critical angle of
attack) still produces lift. But even if it didn't, the tail could still
weathervane the plane nose-down into the relative wind.

if it was spinning it would be
a flat spin, but if was fully stalled, it would be a deep
stall and would not rotate the nose down and it would stay
in the stall.

Under normal conditions, neither the wing or the tail fully
stalls.

When you execute a normal stall in many typical GA planes, the wing can
certainly stall fully (that is, it reaches and exceeds the critical angle of
attack). That's why you lose altitude rapidly in a stall, even in a plane
that is maintaining level flight when just slightly above stall speed. The
tail, however, does not stall when you execute a normal stall.

If the center of pressure was located at the same location
as the CG, there would be no moment or force to cause
rotation.

That's true if you're referring to the CP of the whole plane (not just to
the CP of the wings). But when the wings stall and the plane descends,
shifting the relative wind, the tail's (upward) lift increases, shifting the
plane's CP backward and pitching the nose downward. (See John Denker's
excellent explanation of decalage and angle-of-attack stability:
http://www.av8n.com/how/htm/aoastab.html#sec-teeter)

--Gary

Gary Drescher wrote:
When you execute a normal stall in many typical GA planes, the wing can
certainly stall fully (that is, it reaches and exceeds the critical angle of
attack). That's why you lose altitude rapidly in a stall, even in a plane
that is maintaining level flight when just slightly above stall speed. The
tail, however, does not stall when you execute a normal stall.

Note also that once you start to descend, the relative wind is now
coming from below the airplane, further increasing the AOA and driving
the wing further into stall (assuming you continue to hold the same
pitch attitude).

Roy Smith wrote in news:roy-650C74.07172621062006
@reader2.panix.com:

Skywise wrote:

I've been seriously thinking of getting an account [on Wikipedia] so I
can make changes as I see the need.

In about the same amount of time it took you to write that sentence, you
could have made your account. Just got to http://tinyurl.com/6fvtg, type
in a user name and a password, and you're done.

Making the account may not take much time, but editing articles
does take time and that time adds up, over time. As it is, I'm
having difficulty keeping up with what I do now. To put it
another way, my plate is full, and I'm concerned about piling
on another helping of potatoes.

Brian
--
http://www.skywise711.com - Lasers, Seismology, Astronomy, Skepticism
Seismic FAQ: http://www.skywise711.com/SeismicFAQ/SeismicFAQ.html
Quake "predictions": http://www.skywise711.com/quakes/EQDB/index.html
Sed quis custodiet ipsos Custodes?

"Peter Duniho" wrote in news:129hnvrn4upe5e8
@corp.supernews.com:

Snipola
The real problem comes when a person blindly trusts any source of
information, as if they can just throw out their own responsibility to know
and understand the basis for that source of information and the
characteristics that affect its reliability.

Absolutely. No one source is error free. I have found wikipedia to
be a good "reference" to remind me of something I already know but
jsut can't remember. When accuracy of details are important, I cross
reference multiple sources, both online and written.

Brian
--
http://www.skywise711.com - Lasers, Seismology, Astronomy, Skepticism
Seismic FAQ: http://www.skywise711.com/SeismicFAQ/SeismicFAQ.html
Quake "predictions": http://www.skywise711.com/quakes/EQDB/index.html
Sed quis custodiet ipsos Custodes?

"T o d d P a t t i s t" wrote in message
...
(Roy Smith) wrote:

Note also that once you start to descend, the relative wind is now
coming from below the airplane, further increasing the AOA and driving
the wing further into stall (assuming you continue to hold the same
pitch attitude).

True. This reduces the wing lift even more, accelerating
the path of the aircraft towards the ground.

Yup. In fact, it's that positive feedback that defines a stall in the first
place. Short of the critical angle of attack, you get negative feedback: if
the plane accelerates downward a bit, the increased angle of attack
increases the coefficient of lift, counteracting the acceleration. But past
the critical angle of attack, the increased angle of attack *decreases* the
coefficient of lift, which amplifies the acceleration instead of
counteracting it.

The tail,
however, is going from a negative AOA towards a positive AOA
at which point it begins producing upward lift.. As the
flight path angles down, the tail and wing are both lifting
up, and that lift acts entirely on one side of the CG
forcing the plane to rotate nose down.

Yup. Jim is focusing exclusively on the wings' CP being aft of the plane's
CG (typically). If that were the only factor in play, he'd be right that a
stall of the wings couldn't result in the nose-down rotation that occurs (in
typical GA planes) at the stall onset. But he persists in ignoring the
stall's effect on the relative wind and the AOA, even though that's been
explained by several people in this thread.

--Gary

Jim Macklin wrote:
I did not reference or even read the FAA Handbook when I
posted my answer. If the wing stalled, the center of
pressure would not be creating a moment arm to drop the
nose, the tail must loose lift (stall) to cause the stall
break which causes the recovery from the approaching stall.

Two mistakes: An assumption that there is no center of
pressure if the wing stalls, and that the tail must lose lift (note the
spelling of "lose") to drop the nose.
Even a stalled wing generates lift, since there is still air
impacting its lower surface. The center of pressure moves forward
toward the CG as the wing's angle of attack increases, and with the
stall beginning to form it moves forward some more, but never ahead of
the CG. That's because the decreasing lift over the aft portion of the
wing, caused by the boundary layer breakup, leaves the forward part of
the wing to do much of the lifting. As the stall becomes complete the
center of pressure moves aft again, as the lift being generated comes
pretty much all from the lower surface, and that CP movement causes a
pitch-down force. Look at any good textbook and see it. Our Canadian
texts show it well (Aeroplane Flight Training Manual, 4th Edition, page
6). I couldn't find a good diagram on the 'net. There are plenty of
airfoil performance graphs that show CP movement with AOA.
The tail has only so much authority to raise the nose. As speed
decreases, it loses some of that authority and the pilot must either
pull back more, if he can, or watch the nose drop if he has no more
travel. It's not dropping because the tail is stalling; it just doesn't
have any more down-force to give, because it's too slow and because its
AOA is getting too negative.
In some airplanes, the elevator travel was limited to
prevent having enough authority to achieve a stall, in the hope of
avoiding the deadly stall/spin that killed so many pilots in the past.
The Ercoupe comes to mind as one of those "safe" airplanes. You could
try to hold the nose up but as speed decreased it would fall before the
wing stalled. Those airplanes killed people by getting them into rapid
sink situations instead.

Dan

Any treatise on aircraft stalls is going to be complicated if it gets
into detail. I just teach that the wind stops generating lift and
therefore the aircraft descends. Complicating it with (possible)
simultaneous tail stalls unecessarily confuses things. A stall is all
about angle of attack to the relative wind. When an airfoil reaches the
critical angle of attack to the relative wind, the airfoil stalls.

Now as to why the nose drops, that IS complicated. But the student
should know that a forward center of gravity is a good thing to have in
stall recovery. It is rear cg that invites the plane to be difficult to
recover from a stall.

Also, a stall is not necessarily followed by a nose drop. Take for
instance an aerobatic plane recovering from a steep dive with excessive
pull back on the stick. It stalls, but does not have a nose drop. The
plane just mushes down in a stall (or near stall). It is not nose high
to the horizon either.