Originally Posted by
safetypee
Dave, similar forces but not necessarily the same. Consider the aerodynamic influences of wing tip changes, and the 'MCAS issues' of thrust, engine cowl and wing-pylon interface.
If an improvement in tip losses occurs from the upper and lower wing tip devices, (which would be consistent with studies on sections such as flaps) then there is a slight increase in the effective aspect ratio, and the L/D curve would be slightly steeper. That effect doesn't affect to any significant extent the downwash that impinges on the horizontal stabiliser, but the engine geometry will. The effect would be for an increase in downwash to the tailplane, but disregarding the MCAS issue, that would be a constant differential to the forces acting at any time on the aircraft for a given cg/mach/AoA, for normal range AoA. For higher AoA, the effect of the nacelle interaction with the inner wing area, would be to increase downwash at higher AoA, which for a given stab setting would result in an increased resultant trim downforce giving a pitch up tendency. in itself, that is tending towards a higher pitch attitude and reduction in speed which reduces the load that would exist on the manual trim system. Yes, that would result in non linear elevator forces, which is undesirable to the certification for HQ, but the system itself washes out the stab forces so would not have been a problem to manual trim in the absence of MCAS. May sound odd, but thats how the forces would play out.
Originally Posted by
safetypee
Whilst there should not be any mechanical increase, small but significant changes can arise from revised cable runs and pulleys, pressure bulkhead seals, etc.
True, but there is apparently not much if any difference between the flavors considered here. Not sure that TBC has any drivers towards alteration of the control runs in the stab itself, or with the pressure bulkhead seals etc. Roll control architecture was altered in respect to the spoilers.
Originally Posted by
safetypee
If the NG was marginally acceptable, then a small change in the Max might not meet the certification requirements.
Maybe, but as far as the manual stab trim use goes, in the absence of MCAS, it probably was slightly better due to the slight changes in the wing downwash.