HotDog,
The following is from the Nov/Dec 1973 L-1011 TriStar Service Digest:
"Occasionally during ground operations of the L-1011, airframe viberations, thumpings or fuselage oscillations may occur which could cause cause concern to your flight crews, leading to possible delays until the cause has been determined. These conditions can be caused by two operational situtions as noted below:
1. Impingment of exhaust turblance:
During static engine run-up at N1 greated than 60% in certin cross wind conditions, the exhaust from the up-wind wing engine may impinge on the stabilizer and the resulting turbulences my generate a buffeting.
2. Moving the Stabilizer abruptly on the ground:
A. Rapid movement of the control column. The resultiing sudden acceleration of the large stabilizer mass may initiate a vertical oscillation of the fuselage.
B. The gain characteristic of the stabilizer servo input linkage is such that for a control column input motion, the stabilizer moves eight times as far/fast at the -14 degree leading edge down position (AC nose up) as at the +1 degree leading edge up position (AC nose down). This relationship allows for generation of an oscillation (similar to "A" above) at an approximate rate of 5 CPS.
These occasional vibrations/oscillations occue only on the ground; they are now a known peculiarity and should not cause concern once it has been determined that one of the above conditions induced the vibration. These conditions do not affect normal aircraft operation"
Last edited by glhcarl; 23rd August 2007 at 17:01.