Dudes, & Fire Wall
Here is a bit more of an explanation. It was kindly provided by aerospace.org and I recommend them to you all.
Quote:
The 757 uses a supercritical airfoil which is the Boeing TR-51 series. More information on these airfoils can be found at the
following locations:
http://www.aerospaceweb.org/question...ls/q0073.shtml
http://www.aerospaceweb.org/question...s/q0054b.shtml
As you probably know, the reason an aircraft would need a mach speed trim device is because the wing center of pressure (Cp) moves aft at transonic speeds, usually starting at about Mach 0.6 or 0.7. As the Cp moves aft, the moment arm between it and the elevator decreases making the elevator less effective in providing pitch control. The difference in location between the Cp and the center of gravity (located in front of the Cp) causes the aircraft nose to pitch down, so more elevator trim is required to keep the aircraft level.
Now that we know what mach speed trim is, the question becomes why would one aircraft need it and the other not.
What this question suggests is that there is no movement of the Cp on the 767, or at least much less than there is on the 757.
Cp travel is usually associated with the wing airfoil section, but I believe that both aircraft use the same airfoils, or ones similar enough that their Cp movement should be similar. Another method of reducing Cp movement is to use wing sweepback.
This is indeed one major difference between the wings of the two aircraft. The 757 wing is swept 25° while the 767 wing is swept 31.5°. That 6.5° difference seems pretty small, but I think it likely plays a major role.
The 767 also has a thicker wing, greater wing chord, and larger horizontal tail than the 757. A combination of some or all of these variables may have the effect of making the aircraft
less sensitive to the Cp movement.
Enquote.