737 Mach Crit. Over what part of airframe does shock wave first appear.
 

Mach Crit in the 737. Trivial question I know - but on which part of the airframe does a shock wave first appear?
One opinion is that it is the airflow over the cockpit.

That is what I have been told but I never went outside to check.

I would have put my money on the wing root - or rather at the point of highest camber?

Generally we can "see" shock waves when air conditions are right either to see the normal light refraction due to density variations or moisture condensation.

YouTube has plenty of clips which let us all see these sorts of phenomena.

For instance, in the at around 1:50 we can see a number of shocks during a high speed pass, including at the back of the canopy discontinuity .. although one needs to look closely to see the latter. The SR71 model test shows a number of shocks along the aircraft's profile.

shows a spanwise shock over the top surface of a typical jet transport.

As to exactly where shocks initially form on a given aircraft will depend on the particular aircraft profile.

.. I make no comment about the dreadful background music which some seem convinced is mandatory for YouTube clips ...

Very much dependent on aircraft type.
Used to get it a lot in the Classic 737 whenever getting closer to Coffins Corner and pulling the slightest bit of G at the same time. Buffetting through the yoke.

Hi Tee Emm,

There's an interesting explanation about why a shock wave can form early over the flight deck window / fuselage area from 42nd AIAA Aerospace Sciences Meeting and Exhibit 5 - 8 January 2004, Reno, Nevada:
http://www.aer.ita.br/~bmattos/downl...elage_aiaa.pdf

Page 3
"However, this apparently simple nature of the flow leads to complex consequences. As an example, the flow over the cockpit region can develop shock waves. The main reason behind this phenomenon is related to the shorter path that the flow follow from the standstill state in front of windshield to the freestream level at the beginning of the central fuselage. The shorter the path, the higher the speed of the flow along the cockpit curvature that eventually leads to shock wave formation depending on the fuselage shape and freestream Mach number."

From what I can remember after reading "Fly the wing" and "Turbine pilot's manual" usually it forms on the wing first(usually). Always wondered where this first happens on the 737 airframe.

With mach trim inoperative our mach number is limited to .74, so I assume its somwhere around .73..(Think I read it somewhere before, maybe even in 737tech site or Eric Parks' notes, just can't be sure enough to confirm). :O

I seem to recall that on the 747 it first appeared on the top of the "hump" as that had a profile that accelerated the airflow to Mcrit before the wing. The 737 doesn't have that so my guess is that it would be at the wing root which has a higher incidence than the tip (washout). Don't shoot me if I'm wrong! ;)

I've often seen a small shock wave just inboard of the nacelle on a 737NG, just aft of the leading edge on top of the wing.

Thank you all very much for these replies.
TM:ok:

Well it is a critical Mach nr. We are talking about your wing developing lift, no?

It's actually an interesting question, especially relating to the mach trim. The mach trim on the 737 NG starts to function at M 0.615, but if it fails the restriction is 280 kts. / M 0.82. In other words it restricts the amount of knots (max is 340) but not Mach (max is 0.82).

I've assumed there isn't any restriction on the mach due to relatively low mach tuck tendencies and that this system exists only due to regulatory compliance, but I don't know why there's a restriction on the amount of knots.

Any smart heads in here that can enlighten me/us?

737NG VMO/MMO is limited by the shape of the rear of the fuselage. NG wing is based on the 757 so can go a lot faster than .82. 737 Classic had fatter wing so was probably limited by the wing?

VMO/MMO is limited by the shape of the rear of the fuselage

Interesting. Do you have any amplification on that ?

Hi John,

Boeing factory instructors did our line conversion when we first got the NG and that was the information they passed along. The NG really is a hybrid, that fuselage shape was originally designed for the slower -100. That's also the reason the NG cruises economically so close to MMO.

Thanks for that. I can see a reason for Mmo but not Vmo. Perhaps others reading may have further knowledge of the matter ?

I was told that the -NG's wing was actually a scaled down 777 wing when I did a conversion course in Seattle. The engineer went on to tell us that R&D funds were fairly tight as it wasn't envisaged that the -NG would be a big seller, but that in the engineer's opinion a bit more time and money spent on the -NG's wing would probably have yielded 777 speeds! I guess we may never know.

I also remember reading somewhere that the tailcone section is an area of high drag on the original 737, hence the proposed re-design on the 737 MAX.

I recall flying with a Capt. years ago who would fly at the max CI (200?) on the classics on the last flight to get home. You could certainly feel a "rumble" from the wings, was that Mcrit over the wing being reached? I don't know!

One would hope the management would soon `rumble` to that trick and either sack the bastard or fine him for loss of revenue:mad:

I'd be interested to know how the VMO/MMO was determined on the 737-800. Anyone know?

I thought most of the forward & aft fuselage design owed more than a little to the 707, what sort of MMO was that limited to?
If you have a look up & forward at the cockpit section of a B52 from below & behind the cockpit window you will also see the same shape. One size fits all for Boeing :ok:
Certainly the radical difference in the wing between the Classic & NG would lend support to that area not being limiting.