Mach tuck revisited

[Uptrim Disable]

Mach trim compensators trim the nose up to counter the effects of shock wave formation over the wings, right? So why is it that the B737 M. trim compensator kicks in at just above M .615 ? http://www.b737.org.uk/flightcontrols.htm
Are there shock waves forming on all passenger aircraft's wings? I thought the wings would shock stall if local Mno exceeded 1.0? What am I missing here?

[m&v]

My understanding is that prior to reaching M1,with the increase in speed the C/pressure moves aft on the wing allowing the nose to pitch/move down-hence the requirement for the Pitch trim Comp'....Used as such on the DC8/Learjet,comes into play above Mo.78,but the airflow isn't at M1,hence no 'shock'buffet.
cheers

[john_tullamarine]

Keep in mind that flow phenomena depend on local flow velocities .. not freestream such as the pilot reads. While the aircraft may be less than sonic velocity by a considerable margin, various areas of the wing and fuselage may be very close to running into shock problems ...

Compensation programming will be determined by the nature of the problem's onset and effect on aircraft handling.

[ATPMBA]

The aerodynamic center of airfoils in subsonic flight is located about 25% chord point. At supersonic flow the aerodynamic center changes to 50% chord point. An aircraft in transonic flight can experience large changes in the aerodynamic center thus causing the mach trimmer to “kick in”. (ref. “Aerodynamics for Naval Aviators”, that’s U.S. Naval Aviators)

[m&v]

to follow up the accelerated 'flow'issue,over what two parts of the airframe does the 'flow'reach M1 first(mid Atlantic Question-with the sun in ones eyes)
"over the top of the wing,and ??????

[john_tullamarine]

M&V,

Fat bits is a good place to look ... like where the initial nose taper blends into the main fuse diameter ... .. cockpit/fuse section changes .. that sort of thing ...

Thought I had some links to a couple of good FJ pix showing transonic shock patterns in humid air but I can't locate them just at the moment .. if I do I will post them later ... unless someone else beats me to the punch ..

I have a nice one of a transonic FJ with a clear shock at the back of the cockpit if you'd like it by email ....

[jungly]

Concur: apart from the wing & horizontal stabiliser a shockwave often forms just behgind the cockpit and at the base of the vertical fin.
Apart from the 'Fat bits' large angle changes (eg: cockpit to fuselage) accelerate the airflow.
In supersonic engines these angle changes are often used to create shockwaves - on purpose - to slow the airflow into the engine.

Try here for a good simple brief:

http://142.26.194.131/aerodynamics1/...ed/Page2d.html

[m&v]

Jungly'
Your close it developes over the 'low'pressure region over the Windshield-is that where the paint wears off???
If one flew an aircraft once around the world(great circle)what part of the A/craft 'travels'furthest??
Cheers

[Uptrim Disable]

I know that the AC/CP moves aft when flying through the transonic speed regime, but I thought this was because of shock wave formation? If shock waves do not cause the AC/CP to move aft to about 50% chord, then what does?