A dumb doubt on stalls

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

"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).

Jim Macklin wrote:

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.

The lift from the wing doesn't go to zero during a 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.

I'm now really curious to hear your definition of what stall means.


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.

No it won't because there is still some lift from the wing, however, it
is now less than the weight of the airplane so the imbalance in forces
causes the airplane to both descend and rotate. It would only fall
downward in a flat attitude of the lift (and drag) of the wing and tail
and fuselage went either completely to zero or remained perfectly equal
forward and rearward of the CG.



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

Sure, there is still the force from the tail.

Are you really a CFI and ATP as your .sig advertises?

Matt

"Matt Whiting" wrote in message
...
Jim Macklin wrote:

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

Sure, there is still the force from the tail.

No, Jim is right if he's referring here to the plane's CP (not just the
wings' CP).

--Gary

Gary Drescher wrote:
"Matt Whiting" wrote in message
...

Jim Macklin wrote:


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

Sure, there is still the force from the tail.


No, Jim is right if he's referring here to the plane's CP (not just the
wings' CP).

True, and the odds of that happening are infinitesimal.

Matt

"Matt Whiting" wrote in message
...
Gary Drescher wrote:
"Matt Whiting" wrote in message
...

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

Sure, there is still the force from the tail.

No, Jim is right if he's referring here to the plane's CP (not just the
wings' CP).

True, and the odds of that happening are infinitesimal.

You're referring to the odds of the plane's CP and CG coinciding? There's
nothing unlikely about that--it's what happens whenever the plane is *not*
changing pitch.

--Gary

Gary Drescher wrote:
"Matt Whiting" wrote in message
...

Gary Drescher wrote:

"Matt Whiting" wrote in message
...


Jim Macklin wrote:

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

Sure, there is still the force from the tail.

No, Jim is right if he's referring here to the plane's CP (not just the
wings' CP).

True, and the odds of that happening are infinitesimal.


You're referring to the odds of the plane's CP and CG coinciding? There's
nothing unlikely about that--it's what happens whenever the plane is *not*
changing pitch.

The topic is stalling the airplane. That isn't a steady-state situation
as is straight and level and unaccelerated flight.

Matt