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LivingInMEM · 07-04-2009, 02:03 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.

07-04-2009, 02:03 PM	#87 (permalink)
LivingInMEM Gets Weekends Off Joined APC: Dec 2007 Posts: 545	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.