Thanks for finding that. Brings back memories of "weather days" in UPT when they would roll out the projector and show these now-obsolete things called "films" and "slides".
I was going to link you directly to a .wav sound effect of an old movie projector that you could run while you watch the movie, just to make it a little more realistic.
I'm up to my butt in chores today so you get to go find it yourself. It's on this linked page somewhere.
I think I was pretty clear in describing how a vert stab without a rudder adds to yaw stability. Kind of like an arrow with fins is more stable than one without.
You are correct. Nobody disagrees that a vertical stabilizer adds to stability. Stability is different than control. There is simply no aerodynamic facts to back up the premise that you lose control and crash if you lose the vertical stabilizer. None.
You talked previously about Dutch Roll. In the old 727 days, we trained in recovery from severe Dutch Roll with the Yaw Damper inoperative. The recovery technique involves the use of aileron only - you never want to touch the rudder.
Carl
__________________
I'm an airline pilot - so I got that goin for me....which is nice.
You are correct. Nobody disagrees that a vertical stabilizer adds to stability. Stability is different than control. There is simply no aerodynamic facts to back up the premise that you lose control and crash if you lose the vertical stabilizer. None.
You talked previously about Dutch Roll. In the old 727 days, we trained in recovery from severe Dutch Roll with the Yaw Damper inoperative. The recovery technique involves the use of aileron only - you never want to touch the rudder.
Carl
That recover was a piece of work bu it DID work. Thanks for reminding me of it.
You are correct. Nobody disagrees that a vertical stabilizer adds to stability. Stability is different than control. There is simply no aerodynamic facts to back up the premise that you lose control and crash if you lose the vertical stabilizer. None.
You talked previously about Dutch Roll. In the old 727 days, we trained in recovery from severe Dutch Roll with the Yaw Damper inoperative. The recovery technique involves the use of aileron only - you never want to touch the rudder.
Carl
It is because you had a vert stab that was naturally dampening the yaw, the ailerons were used to dampen the roll.
If you had no vert stab, there would have been no (or a lot less) tendency for the nose to stop traveling in one direction and start traveling back to neutral. Also, without a vert stab, there would have been little or no tendency for the nose to stop yawing as it approached and went through neutral. The very reason you didn't touch the rudder was because of the natural stability provided by the vert stab. With a vert stab, the positive stability in the yaw axis would eventually dampen the movements - without it, the yaw stability would be very close to neutral or even negative. Without the vert stab, you would have lost control.
By the way, stability is natural - control is what you provide - and without stability you lose control (ask any F-117 pilot especially what would happen to their control if they lost the computer provided their positive stability). Without one you don't have the other, you aren't that fast.
Last edited by LivingInMEM : 07-04-2009 at 10:51 AM.
It is because you had a vert stab that was naturally dampening the yaw, the ailerons were used to dampen the roll.
You obviously never flew the 727. With the Yaw Damper inop, one push of the rudder resulted in increasing deviations from neutral until you were rolling +or- 80 degress, and yawing back and forth far worse than your original rudder input.
Quote:
Originally Posted by LivingInMEM
If you had no vert stab, there would have been no (or a lot less) tendency for the nose to stop traveling in one direction and start traveling back to neutral.
That's correct, there would be no (or a lot less) tendency for the nose to stop traveling. But again, you're talking about stability versus control. Yaw movements can be dampened with differential thrust. It's hard, but you can still control it. The airplane will not instantly become uncontrollable and crash.
Quote:
Originally Posted by LivingInMEM
By the way, stability is natural
No it's not. Stability must be designed into all 3 axes.
Quote:
Originally Posted by LivingInMEM
control is what you provide
That's correct.
Quote:
Originally Posted by LivingInMEM
without stability you lose control
No you don't - without control...you lose control. Stability characteristics (both static and dynamic) are judged during flight testing. Some designs of the past turned out to have terrible stability during flight tests - and the aircraft returned to base to be tweaked by the engineers.
Quote:
Originally Posted by LivingInMEM
(ask any F-117 pilot especially what would happen to their control if they lost the computer provided their positive stability). Without one you don't have the other, you aren't that fast.
Not just the F-117, but the F-16, F-22, etc. These aircraft were designed with extreme aft CG's so that they have extreme maneuverability. These specific aircraft will lose control because their Fly By Wire CONTROL systems are also what provides stability. When you lose your flight control system, you lose control. No argument there.
Carl
__________________
I'm an airline pilot - so I got that goin for me....which is nice.
Funny - I am qualified in the 727. I think your characterization of the aircraft's response to one rudder input with the yaw damper inop is exaggerated. First, the roll is due to difference in chord-length exposure of the forward going swept-wing as compared to the aft-going swept wing - the forward wing is more effective and producing more lift. Second, the resultant yawing that is WORSE than the original input is defined as negative stability - and I don't think anyone is under the impression that the 727 is negatively stable in pitch with the yaw damper inop (admittedly, the flight characteristics of a non-yaw damper 727 at high altitude are not great, but Negatively Stable?).
By saying that stability is natural, I meant inherent to the aircraft design and independent of a pilot's inputs. Stability is largely independent from axis to axis. An aircraft with long wings with a large dihedral will be inherently stable in roll - if combined with a fuselage that is longer in front of the CG than in back and a small or non-existent vert stab, the result will be an aircraft that is very positively stable in roll but negatively stable in yaw.
Admittedly, this is getting drawn-out. But, it's a worthwhile academic discussion for many who may be new to aviation. Please describe how you will reliably maintain control of an aircraft that has negative stability, especially in an environment complicated by turbulence, etc. The negative stability analogy of trying to balance a marble on top of a bowling ball is fitting - you may be able to do it for a short time, but eventually the magnitude of the excursions from neutral and the speed and magnitude of correction required to return to neutral would become excessive and the marble would fall off.
Every transient of the aircraft will result in an input from the pilot, the aircraft will respond to the input by transitioning to the other side of neutral (due to the negative stability), which will result in another input from the pilot continuing the cycle. Eventually, the end result will be a PIO that ends in catastrophe.
The F-117 is not an aircraft with an aft CG, it is an aircraft with no natural stability at all - without the computers reacting much faster than any human ever could, the result would be disastrous even if only level flight were attempted. Even if the pilot had full control of all available flight-control surfaces (albeit with no computer controlled fly-by-wire assistance), the results would be disastrous.
By the way, all an aft CG does is reduce the tendency of an aircraft nose to drop with a reduction in airspeed and a resultant reduction in downward force created by the horizontal stab. An aft CG is dangerous in a stall (because the nose won't naturally drop) and upon landing because the tendency will be for the nose to rise with a reduction in airspeed approaching the flare and stall.
What you were referring to is the reduction in the moment arm between the CL and the CG - decreasing that arm decreases the positive pitch stability of an aircraft. The shorter the arm, the more sensitive in pitch and pitch-responsive the aircraft is - assuming that the appropriate elevator/horizontal stab authority is maintained (a function of a combination of horizontal stab/elevator effectiveness and the length of the arm between CL of the stab and the CL of the wing).
From the fighter point of view, an aft CG does not increase maneuverability. The only regime where an aft CG increases pitch response is when wings-level and upright and pulling the nose up in the pure vertical when gravity is assisting with the movement of the tail with respect to the CL (and nose). Transitioning from nose-high flight downward would be degraded when wings-level due to that same aft CG - a sluggishness that can result in a lost fight. Also, when wings-level upside down and trying to pull the nose through the horizon, the nose response would be degraded because you are trying to pull the aft CG upwards (relative to the horizon and CL) against gravity. Fighters need maneuverability in all planes of motion irrespective of where the horizon and gravity are - that's why the short arm vs the long aft arm.
Last edited by LivingInMEM : 07-04-2009 at 02:23 PM.
Funny - I am qualified in the 727. I think your characterization of the aircraft's response to one rudder input with the yaw damper inop is exaggerated. First, the roll is due to difference in chord-length exposure of the forward going swept-wing as compared to the aft-going swept wing - the forward wing is more effective and producing more lift. Second, the resultant yawing that is WORSE than the original input is defined as negative stability - and I don't think anyone is under the impression that the 727 is negatively stable in pitch with the yaw damper inop (admittedly, the flight characteristics of a non-yaw damper 727 at high altitude are not great, but Negatively Stable?).
By saying that stability is natural, I meant inherent to the aircraft design and independent of a pilot's inputs. Stability is largely independent from axis to axis. An aircraft with long wings with a large dihedral will be inherently stable in roll - if combined with a fuselage that is longer in front of the CG than in back and a small or non-existent vert stab, the result will be an aircraft that is very positively stable in roll but negatively stable in yaw.
Admittedly, this is getting drawn-out. But, it's a worthwhile academic discussion for many who may be new to aviation. Please describe how you will reliably maintain control of an aircraft that has negative stability, especially in an environment complicated by turbulence, etc. The negative stability analogy of trying to balance a marble on top of a bowling ball is fitting - you may be able to do it for a short time, but eventually the magnitude of the excursions from neutral and the speed and magnitude of correction required to return to neutral would become excessive and the marble would fall off.
Every transient of the aircraft will result in an input from the pilot, the aircraft will respond to the input by transitioning to the other side of neutral (due to the negative stability), which will result in another input from the pilot continuing the cycle. Eventually, the end result will be a PIO that ends in catastrophe.
The F-117 is not an aircraft with an aft CG, it is an aircraft with no natural stability at all - without the computers reacting much faster than any human ever could, the result would be disastrous even if only level flight were attempted. Even if the pilot had full control of all available flight-control surfaces (albeit with no computer controlled fly-by-wire assistance), the results would be disastrous.
By the way, all an aft CG does is reduce the tendency of an aircraft nose to drop with a reduction in airspeed and a resultant reduction in downward force created by the horizontal stab. An aft CG is dangerous in a stall (because the nose won't naturally drop) and upon landing because the tendency will be for the nose to rise with a reduction in airspeed approaching the flare and stall.
What you were referring to is the reduction in the moment arm between the CL and the CG - decreasing that arm decreases the positive pitch stability of an aircraft. The shorter the arm, the more sensitive in pitch and pitch-responsive the aircraft is - assuming that the appropriate elevator/horizontal stab authority is maintained (a function of a combination of horizontal stab/elevator effectiveness and the length of the arm between CL of the stab and the CL of the wing).
From the fighter point of view, an aft CG does not increase maneuverability. The only regime where an aft CG increases pitch response is when wings-level and upright and pulling the nose up in the pure vertical when gravity is assisting with the movement of the tail with respect to the CL (and nose). Transitioning from nose-high flight downward would be degraded when wings-level due to that same aft CG - a sluggishness that can result in a lost fight. Also, when wings-level upside down and trying to pull the nose through the horizon, the nose response would be degraded because you are trying to pull the aft CG upwards (relative to the horizon and CL) against gravity. Fighters need maneuverability in all planes of motion irrespective of where the horizon and gravity are - that's why the short arm vs the long aft arm.
Like that line in the movie: What we have here is failure to communicate. You are responding to points that I haven't made, while not understanding other points. That's OK, because we've gone very far off subject. The subject is as follows:
Does the loss of a vertical stabilizer mean immediate loss of control and an aircraft crash? I say no, you say yes.
The film of the B-52 that lost almost all of its vertical stabilizer and made a safe landing is not germain to you (and some others) because there was still a small stub of metal sticking up. Your point is that even a small stub of metal is the difference between making a safe landing, and immediately losing control. Guess we'll just have to disagree on that one.
On AA 587, the aircraft was in the middle of a major upset when the vertical stabilizer separated. Shortly afterwards, both engines separated from the wings. Shortly afterwards, an unknown amount of the left wing tip separated from the wing. With all of those failures, it would have taken a great deal of skill to recover from - and it would have taken much more than 2,700 feet of altitude to accomplish.
Carl
__________________
I'm an airline pilot - so I got that goin for me....which is nice.
To the few remaining folks that think outsourcing is a good idea, the Wall Street Journal and others are reporting the following:
Boeing Co. is in negotiations to purchase operations from one of its main suppliers, as part of an effort to gain more control over the supply chain of its troubled 787 Dreamliner program, according to a person familiar with the matter.
The company is close to announcing that it will buy a facility from Vought Aircraft Industries that makes sections of the 787 fuselage. The facility is located in North Charleston, S.C. The person familiar with the matter said negotiations have been under way for months....
This has been correctly characterized as "Backtracking" on the oursourcing strategy that Boeing attempted. Outsourcing has been a disaster for Boeing, but unlike some true believers on this thread, Boeing is starting to make the proper corrections.
Carl
__________________
I'm an airline pilot - so I got that goin for me....which is nice.
EVERETT -- The 787 may still be weeks to months away from taking its first flight, but the new plane did get to stretch its legs a bit Tuesday.
Test pilots were taking the 787 for a low-speed roll around Paine Field's runway and taxi lanes. No word yet on what exactly they were testing.
The 787 program has been beset by delays. Initially supposed to be delivered in the first quarter of 2008, the delivery schedule has been pushed back five times. There were high hopes the plane would take its first flight in June, but the company announced it needs to reinforce small areas near the connection of the wings and fuselage before conducting the test flight. A revised schedule for the flight, as well as first deliveries to customers, will not be announced for several weeks.
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