I’ll try this one more time…
Originally Posted by Graybeard
The ventral fins (strakes) were not new on the MD-80. They were first fitted on the DC9-50, for stability at cruise. All DC-9 are subject to deep stall. The DC9-80 was fitted with a stick pusher after it became uncontrollable in a deep stall in flight test, and they had to deploy the parachute to recover.
1) The “strakes” mounted on the DC-9-50 were not mounted for stability at cruise. They were added to the DC-9-50 to improve airflow up and over the fuselage and around the vertical stabilizer to improve directional control during approaches. Up to the addition of the strakes, the DC-9 had a “rudder limiter” system installed that dropped a bracket around the rudder actuator in the tail that prevented the rudder from being displaced more than plus/minus 8 degrees either side of center (as I recall) when the flaps were retracted. On the other end, when flaps were extended, this “rudder limiter” was retracted out of the way, allowing the rudder surface to be displaced plus/minus 30 degrees either side of center (again, as I recall). The pilot annunciator panel had a “blue” advisory light that illuminated saying “Rudder Unrestricted” that had to be illuminated prior to initiating the approach, or the approach had to be flown at substantially higher approach speeds because of the potential limitation of the rudder. This system is still on the DC-9s - 10, 20, 30, and 40 series.
2) While the DC-9 was initially prone to the same malady that befell the French Caravelle – that of deep stall blanking out the tail sufficiently that elevator authority was lost. Therefore, McD widened the horizontal stabilizer, including the ends of the elevators, allowing both to extend out into the airflow that did exist beyond the “blanked out” areas in a deep aerodynamic stall. They also added a 3000 psi assistance to place the elevators into a full nose down position when either flight crew member pushed the control column full forward. Not only was this system successful, it was
so successful that the first few pilots landing on contaminated runways and desiring firm nosewheel contact with the surface and pushed full forward on the control column to get it, found themselves “wheelbarrowing” with only the nosewheel on the runway – that is, they actually had so much lift on the tail they actually lifted the main gear off the runway surface!
3) Any “stick pusher” that may have been installed would have been for flight testing only, and I'm not at all sure that ever happened. I am not aware that there was ever a production DC-9 delivered with a stick pusher – they relied on the wider horizontal stabilizer and elevator and the 3000 psi hydraulic system to ensure the elevators were properly positioned to “nose down.” You can still see flight crews testing this system today when taxiing out behind any DC-9 series airplane.