Originally Posted by
172_driver
FCeng84,
Judging from your location one could think you're involved in Boeing aircraft design? One day I might find myself in a 45 tonnes 737-600 and the next in a 75 tonnes 737-800. There sure is differences in how the two planes handle, a light 737 is not far from a C172. In turbulence you're thrown about like a leaf and you have to resist the temptation of your bum, as it wants you to kick some rudder to keep her straight.
Are the flight control systems identical on all 737 models (-600 to -900) or are parts (in the feel system) modified to account for different stability and inertia properties? Does the elevator feel computer have any idea of the current weight of the aircraft?
172_driver - while I have not been involved in the 737 design I am familiar enough with the system to take a run at your questions.
The 737 variable pitch feel on a specific derivative is a function of airspeed and stabilizer position on all 737 derivatives. Between 737 derivatives the dependencies on speed and stab have been modified as needed to address handling characteristics of each derivative. On the more recent derivatives (starting with NG I believe) the variable feel has been updated to include: (1) features to increase forces at high AOA to provide better approach to stall awareness, and (2) an upper limit on feel setting for takeoff to avoid higher than desired takeoff rotation forces when heavy and taking off at higher speeds.
The variation in feel with stabilizer position provides compensation for the variation in pitch response to elevator with changes in CG. The stab tends to trim more airplane nose up at fwd CG where airplane maneuvering requires larger elevator displacements. Correspondingly the variable pitch feel is softer there. As CG moves aft, the stabilizer trims more and more airplane nose down and the variable feel stiffens.
The 737 variable feel does not have knowledge of airplane weight and thus does not vary on a given derivative with weight provided the trim stabilizer position is constant. (See paragraph above.) There is, however, some variation in feel schedule between derivatives in an effort to make the feel similar across the fleet.
When comparing airplane response between derivatives of a given model it is interesting to consider the geometric, inertia, and aerodynamic differences between a shorter / lighter derivative and a longer / heavier one. As length / weight is increased pitch inertia will go up, but so will the tail moment arm (distance from CG to elevator). Having a longer moment arm will partially compensate for the increased inertia such that similar handling qualities are usually achievable with the same sized elevator and the same gearing from column to elevator. It is to be expected that a light weight 737-600 will have snappier response than a heavy 737-800. I would be interested in your comments on how the variation in response on a 737-600 between when it is heavy shortly after a MTOW departure and when it is lighter during approach at the end of that flight compares with the range of characteristics you find on a 737-800 over its weight range. Along the same lines, how do you find the difference in response between a 737-600 and a 737-800 if both depart at their respective MTOWs?
FCeng84