B737 - MD80 - Airbus330

[Crossunder]

What is the approximate Mcrit for the B737, MD80 and Airbus 330?

[Old Aero Guy]

To make sure the reply is appropriate, do you really want Mcrit or Long Range Cruise Mach?

And, what version of the 737? Classic or Next Gen.?

Given the size disparity, what conclusion are you going to draw?

[Crossunder]

Just want an approximate figure as an example for my Principles of Flight classes. Any version B737 (or twin jet) will do. I know Mcrit varies with mass, but a ballpark figure should suffice.

[Old Aero Guy]

Sorry, but I need to ask another clarifying question.

Mcrit is one of those terms which has been applied to a number of areas.

One definition refers to the Mach number at constant lift coefficient where the drag rise is a certain level above the drag level at .7M
The assumption is that .7M represents the incompressible level.

This definition is related to Long Range Cruise Mach (LRC) on the real airplane and is weight related.

Another definition is associated with the airplane's defined Mmo/Md speeds. These are in fact also weight related, but to keep things simple are given a single value.

Which one interests you?

[Crossunder]

I'm not refering to the drag divergence mach number! Mcrit is the speed at which the airflow reaches M 1.0 somewhere on the aircraft. More specifically, I'd like to know at what free stream Mno / TAS (at maximum certified take-off mass and at an altitude of you choice)) does the B737 (any version) experience the first little shock wave on the wings? Is that specific enough ;) ?

[Old Aero Guy]

If your definition of Mcrit is where a shock forms anywhere on the airplane, then I can't help you.

I don't think anyone else can either unless they have a set of surface pressure or velocity distributions for the wing, body, horizontal tail, or nacelles/struts.

In cruise, you can have a shock in the vicinity of the cab maximum curvature areas before any form on the lifting surfaces.

Another potential place is the nacelle/struct/wing junctures. More than one airplane as come to grief because to enough attention was paid to this area.

You also shouldn't neglect the low speed flight arena. Shocks can be present on slats as the airplane approachs a stall.

In summary, shock formation and drag rise isn't only a function of airfoil technology and wing sweep.

To answer your question in terms of approximate LRC Mach:

737 Classic .74M to .75M
737 Next Gen. .80M
MD80 .76M
A330-300 .82M
A340-300 .805M

Since the last two have the same wing, do you have any guess as to why the A340-300 is slower?

[Crossunder]

Hm. The 340 is heavier and needs a greater AoA which will bring it closer to Mcrit / high-speed buffet?

Also: if the Mach trim compensators of the B737 kicks in at just above M .6 , is this because shock waves form and start to affect the movement of the CP/AC or are there other factors that I'm not aware of?

[Old Aero Guy]

I think the lower LRC of the A340 has more to do with outboard nacelle integration difficulties than wing loading.
If it were higher AoA (i.e. higher lift coefficient), flying at a lower altitude would provide relief.

I can't shed any light on your question about 737 Mach trim.