Accident avoided despite the efforts of the crew

[Evan]

Please explain me the "rudder to get the nose down" part.

No joke. The AA587 final report has some interesting information on the unusual attitudes recovery procedures for large dihedral aricraft. Of course, the first action in approach to stall is to get the pitch down with forward yoke, but they also point out that at extreme AoA the wings may be experiencing airflow separation while the rudder is rarely aerodynamically stalled. If the aileron and elevator response is degraded, Airbus had recommended rolling to 90° bank using the rudder to quickly lower the nose.

THE KEY HERE IS MODERATION: They also caution that using the rudder at high AoA is extremely dangerous and that too much slideslip can throw the aircraft into an uncontrollable spiral dive, and that the phase lag of the roll inducing effect can also confuse pilots and lead to corrective (fin-shearing) reversals.

But, put simply, in extreme AoA situation approaching or entering stall, the rudder may be the best control surface you've got. But also perhaps the most dangerous one.

[3WE]

I concede I have only circumstantial evidence, but people has been convicted based on circumstantial evidence alone if there is enough of it:



But the most important thing is that you don't realize that, other things left equal, the nose will go lower if you keep it coordinated than if you leave it slip. On the other hand, if you put too much rudder and make a skid, the nose will go lower than if coordinated (again, everything else left equal).

Take this sentence and think a bit about it.


(Note: coordinated means already weathervaned).
(Note 2: Imagine you are in a coordinated left turn an you apply right rudder. Are you now slipping or skidding? Does the nose go down or up?)

You are trying to drag me into some kind of trap here

And before we go any further- I need to remind you that our whole discussion here of "coordinated" turns and proper control inputs has tended to not mention the elevator. In the vast majority of cases, you need nose-up input because you will need to increase lift to maintain altitude when you bank the wing.



By the way- since you are going back and digging up quotes to try to trap me, don't forget the quote where I said,

It's all related Mr ISO 9001 QC Manager Black and White Thinker...

Indeed, ITS manipulates ailerons, elevators and the rudder* with genious to make turns of perfect coordiantion and amazing beauty without even thinking about it! (*Might not use the rudder on the 7X7, but he also owns that tail dragging tiolet paper shredder.

[Gabriel]

And before we go any further- I need to remind you that our whole discussion here of "coordinated" turns and proper control inputs has tended to not mention the elevator. In the vast majority of cases, you need nose-up input because you will need to increase lift to maintain altitude when you bank the wing.

Yes, and I'm trying to figure out the vast minority where you don't.

As a side note, as the bank steepens, the bank angle becomes a more effective mean to control pitch than the elevator, because most of your "pull up" effort would go "sideways" and tighten the turn instead of raising the nose.

Example: At 70° you need to pull 2.92G to keep 1G in the vertical direction. Say that a sink rate has developed and you want to generate an additional 0.1G vertical to have 1.1G vertical to kill the sink rate. Well, you need to bring the load factor to 3.22G. As an alternative, keep your 2.92G and reduce the bank a bit to just to 68° and you get your 1.1G vertical.

[3WE]

So, if the airplane is moving laterally (whether it be slip or skid or wind gust, or there's a rope tied on the main gear pulling laterally), a lot of things are going to happen.

The tail will cause it to "weathervane" in the direction it's going.

If you really are banked in a slipping turn (Banked is key), the weather-vaning is going to point the plane's nose more towards the ground. However, it is not going to do a true "pitch down/AOA reduction"

The dihedral angle is going to initiate a bank away from the sliding motion. If you really are in a steep, slipping turn (or skidding turn) a standard, well-designed airplane will actually fix itself up into a relatively coordinated turn- after a little wallow.

Also, as you are slipping the air doesn't flow across the wing "as intended" but at an agle. That reduces lift (more descent)

Finally, there is some added parasite drag as there's a bunch of surfaces that don't normally "hit the wind" and aren't designed to "hit the wind" that are "hitting the wind".

Cause and effect? I dunno- it's all an inter-related, and a woven, self-correcting analogue system...I hope TeeVee is MY lawyer and I will be presenting this post at my trial.

I also want the record to show that I changed my signature FIRST.

[3WE]

BS, Aeronautical Engineering.
Chief Instructor, Sweet Monkey River Flight School, Del Sur.
CEO - Sweet Monkey River Flight School Group.
QC Manager for Cheap Composites

Fixed.

[Gabriel]

There are a lot, LOT of misconceptions regarding the physics of the lateral-directional motion of the airplane that range.

You've probably have heard of the P-factor as the reason why, when flying at high AoA and high power, the airplane tends to yaw left and you need right rudder to compensate (you know, the down-going blade of the prop, which is the right-hand one, has a larger AoA then the other one and hence pulls stronger, generating an asymmetric thrust).

I never bought that. I didn't explain why the Tomahawk wanted to go left in the take-off roll, where the AoA was zero.

However, those misconceptions are benign because the reaction of the plane and the input required by the pilot are right (for the wrong reasons).

Slow and high power, the airplane wants left, use right rudder.

If you really are banked in a slipping turn (Banked is key), the weather-vaning is going to point the plane's nose more towards the ground and remove the slip angle.

The nose will go exactly to where it would have already been if you had kept it coordinated from the start, assuming that you do nothing different with the elevator. I'm playing with words here, but it's not the slip itself but the removing of said slip (be it by actively keeping it coordinated or by letting the directional stability a.k.a. weathwervane kick-in) what lowers the nose.

The dihedral angle is going to initiate a bank away from the sliding motion. If you really are in a steep, slipping turn (or skidding turn) a standard, well-designed airplane will actually fix itself up into a relatively coordinated turn- after a little wallow.

Not really, or not always, or not the full story.

Unlike what happens in the longitudinal (pitch) and directional (yaw) axes, the airplane has neutral STATIC stability in the lateral (roll) axis because there is no restoring rolling moment that opposes a bank. But what you are talking is not about static but dynamic stability: The dynamic response of a plane when disturbed in bank.

What you say is correct but incomplete. Lets start from straight and level and suddenly there is a bank disturbance, say 5 degrees left. Will this angle increase or diminish over time? Well, as you've said, the first thing that will happen is that the airplane will start to move sideways left (but initially no change in heading) due to the lateral component of the lift. This sideways motion (slip) will initially cause the two things that you've said: The dihedral effect will start to roll the wings back level, but the directional stability will make the plane weathervane to align itself with the relative wind. Note that this will initiate a left turn rate. Somewhere in the middle of this, somewhere before the wings reached back to level, the sideslip will be zero and there will be no more dihedral effect or weathervaning. The plane is now in a coordinated turn, at some bank angle between 0° and 5° of bank. End of story.

End of story? I'm afraid not. As the airplane turns, several things happen. When one say "the plane is aligned with the wind", which part of the plane are we talking about. Let's say the CG. This means that, because of the turn rate, the ind will hit a bit from the right in the nose and a bit from the left in the tail (imagine a plane yawing with no forward speed, then vectorially add the forward speed). So, in fact, the plane will tend to align its tail with the wind, and with a plane turning, this means that the plane as a whole will be in a slip. This again will trigger the dihedral effect that would tend to level the wings. Also, the outer wing will go faster than the inner wing. So it will generate more drag, thus increasing the slip (and the levelling-effect of the wings) even more. But on the other hand, the outer-faster-going wing also generates more lift, which will tend to increase the bank.

The end result? In typical airplanes, when you calculate the "spiral mode" (that's the name of this mode of motion) you get values of several seconds (say 10) to double (unstable) or to half (stable) the original bank disturbance. This is basically an "indifferent" result, and what really happens is affected by not only by modeling errors and even small rounding errors, but even by small manufacturing imperfections and small asymmetrical load distribution (for example, the guy in the left seat is heavier than the one on the right seat). The idea is that the pilot will keep the desired bank without even noticing whatever very small stabilizing or unstabilizing effect is there. And the model gets more complex as the bank increases, because now you have longitudinal effects too. More on this later, but just think of this: Say that you hold a plane from its wingtips with two fingers. Put the plane "straight and level". Now bank say 30 degrees. Now, holding your fingers in that position, and with your other hand stat to lower the nose (push down, reduce the pitch). What is the limit pitch angle that you'll achieve? 60°. What will be the bank angle at this point? 90°. You've just used the elevator to increase the bank from 30 to 90°.

Another question: Do you know why is it necessary to pull back on the yoke during a turn to prevent the nose from going down?

That you need to increase AoA / the total lift / load factor so the vertical component of the lift remains equal to the weight, right?
Yes. Partially right. But that's not all the truth. Let me ask you a questions:
What happens with the AoA / load factor if you establish a coordinated turn but don't touch the elevator?

Also, as you are slipping the air doesn't flow across the wing "as intended" but at an agle. That reduces lift (more descent)

Again, true, but much, MUCH less that what you think.

Why do you think that a wing flying at an angle produces less lift?
Yes, the span shortens a little bit, but the chord increases just by the same little bit to keep the wing area and wing loading unaffected.
There is a little bit of loss of lift because a small zone of the trailing wing being under the "shadow" of the fuselage.

The reason for the "more descent" part is the increase in drag that you describe below, which is the same thing that increase the descent when you lower the gear and even when you extend the flaps (even when they reportedly increase lift)

Finally, there is some added parasite drag as there's a bunch of surfaces that don't normally "hit the wind" and aren't designed to "hit the wind" that are "hitting the wind".

Right, mainly the fuselage.



Cause and effect? I dunno- it's all an inter-related, and a woven, self-correcting analogue system...I hope TeeVee is MY lawyer and I will be presenting this post at my trial.

I also want the record to show that I changed my signature FIRST.[/QUOTE]

[3WE]

Why do you think that a wing flying at an angle produces less lift?

1) I belive I heard that in ground school and/or from an instructor.

2) Engineers like you and NASA spend hours upon hours designing the perfect wing shape for optimum lift and minimal drag and polite stall behavior...and then you run the air crooked over it...the shape is "all" wrong- your effeciency is blown.

3) I exaggerated the thought process with the blue font- but it made sense to the aggie.

Conversely, and, over the years, I have come to belive that a deliberate cross control altitude burning slip probably got most of it's effectiveness by dragging the fuselage sideways through the air- because if the wing simply became ineffecient, the energy from your falling would have to manifest itself in some other way...but no, in a side slip your airspeed doesn't go crazy, and while you can get a good sink rate, it's pretty stable and nothing like a "pure dive" as though the wing quit making lift....you were burning energy- and flying the wing crooked isn't really gonna make that much drag....Like so many things- we don't want to over simplify this stuff to a single process.

4) Holy crap- a Gabriel post...might make a good paperback- "Aerodynamic misconceptions of parlour talking ass hat pilots, or even Flyboy."

I'm sure I have plenty of misconceptions and partial misconceptions...and not sure I am up to replying (and showing my partial stupidity) on all the things you are getting into.

I just know that speed control is important, pulling hard on the yoke has some dangers (especially when you are going slow), and if the stall horn honks, I might at least think about lowering the nose a little...Steep banks when you are low are not a great idea either.

[MCM]

First day back - my head hurts