Cubdriver

RJ, AoA does affect Mcrit. Think of the "coffin corner" example- AoA goes up with altitude due to the reduced pressure available from thin air. However speed of sound also goes down. So while Mcrit goes down due to higher AoA, speed of sound goes down too. This leads to a crisis between what the airplane needs to fly in terms of AoA that high, and the reduced speed of sound at higher altitude. The upshot of it is, the airplane can be on the verge of a stall at the same time it is going supersonic at some places on the top of the wing. The latter is a bad situation, drag goes high and the airplane falls out of flight into a dive. Continuing the story- an uncontrolled dive can perpetuate the nonflying situation by putting the airplane in a regime it is unable to accommodate aerodynamically and is unable to recover from no matter how good the pilot. This was what happened in WWII to some single engine fighters- they exceeded their max speed for recovery due to SS flow over the tail. The solution was all moving tails as well as a host of other high speed aerodynamic techniques (sweep, flatter airfoils, area rule, delta wings, etc.).

Another quick thought here- sweep is often thought of as the only way to make an airplane capable of a higher Mcrit. However, sweep is chiefly motivated by the need for wing volume. You can obtain higher Mcrit by tucking the wings behind the bow shock. This was how the early SS aircraft (Bell X-1 and other early NASA experimental airplanes) were designed: a flat, thin, short, unswept straight wing tucked in tightly behind the bow shock formed by the nose. It worked fine but as mentioned the wing was very flat, short, and thin. The fuel space was minimal, no place for guns bombs or wheels, which is what fighters need to carry. The practical solution was to simply sweep a fat wing to get ample space inside the wing with drag levels of a short low AR wing.

RJcursed4life

Thanks for the response. I think that helps. My original question was pertaining to Mcrit of a certain airplane. I understand the concept of Mcrit and how engineers and aircraft builders can use certain design techniques as you have mentioned (sweep, chamber, etc) to raise Mcrit. I as trying to determine what, if anything, can change Mcrit for a certain airplane (not withstanding swing-wing types ie F-14), such as AoA, altitude, temperature, pressure, etc....

I think I have determined with the help of you knowledgeable folks that Mcrit varies inversely to AoA, thus as altitude increases, Mcrit decreases on a certain airplane.

I think my original confusion for all of this was assuming the upper limit of the barber pole (Mmo) had a defined relationship to what Mcrit was (ie my plane's Mmo is .80 mach so, therefore, Mcrit was .80). I now see that Mmo and Mcrit is most likely not the same in a swept wing turbojet airplane and is a design limit imposed to ensure the airplane remains controllable in this flight regime.

Cubdriver

A source of confusion over the AoA-Mcrit discussion is grouped around the fact that AoA's for typical jets in cruise flight are fairly small, a degree or two, and variation with altitude is minor as well. The focus goes to critical mach number while holding AoA constant. You can even use an AoA of zero if you want. That's smart because a meaningful comparison is aided by removing the variables to allow direct comparisons. AoA, dynamic pressure, sometimes even speed is eliminated. It's sort of like trying to find a short circuit in your house, you need to figure out which circuits are not affected before you can determine those that are.