MACH Flight & Shockwave questions

[NYCPilot]

Okay, I’m trying to gain a better understanding of mach flight and the negative aerodynamic effects encountered when shockwaves are formed. Please add to or correct anything. Thanks.

Here it is as I understand it.

At speeds encountered during transonic flight (MACH .75 -1.20), compressibility effects begin to manifest themselves which in turn produce shockwaves to form.

Effects of compressibility are:
Shockwaves
Airflow separation
Drag increase
Control & stability problems
Buffeting

The critical MACH number is the speed that the airplane is traveling when a portion of the airflow over the wing first reaches MACH one.

Jet aircraft are most efficient at speeds within 5-10% of MACH one.

A shockwave is the dividing point between undisturbed and compressed air. Air ahead of the wing traveling at Mach 1 is compressed and has a higher pressure and density than the undisturbed air ahead of it. When this compressed air meets the undisturbed air, it forms a shockwave. A shockwave that is perpendicular to the airflow would be called a “normal” shockwave.

When this shockwave occurs on a portion of the wing, airflow traveling through it cause the supersonic air ahead of the shockwave to penetrate the shockwave and slow down. This air, now behind the shockwave has slowed down and increased in temperature, density and pressure. This disturbed air has now caused boundary layer separation on the wing which causes airflow separation. The separation has the effect of decreasing lift which leads to less downwash from the wing which causes turbulence at the trailing edge and causes the center of pressure to move aft. The turbulence formed at the trailing edge alongwith the C.P. moving aft has the combined effect of creating “MACH TUCK” which is basically a pitching down of the nose.



Also, what about coffin corner.

As altitude increases, TAS increases but the speed of sound decreases as temperature decreases. This causes the limiting Vmo or Mmo to decrease for a given % of MACH. In other words, the IAS for M.O. speed will decrease as altitude is increased.

How does this affect the stall speed and how does it create a corner you don’t want to get painted into. Stall speed increases but MACH IAS decreases??

 

[great cornholio]

How does this affect the stall speed and how does it create a corner you don’t want to get painted into. Stall speed increases but MACH IAS decreases??

Eventually the 2 speeds...Vmo/Mmo and Vs meet each other. So If you go 1KT too fast you hit Vmo/Mmo and if you go 1 Kt too slow you hit Vs and you stall. So that means you go 1 Kt too fast you get Mach buffet which could lead to out of control or pieces falling off of the plane and if you go 1 KT too slow then you hit Vs and stall. Lots of people have died from this so thats why the call it coffin corner.

I might be thinking too much here...but I think the real speed you are worried about in Coffin Corner is Mcrit not Vmo/Mmo...Cause I think Mcrit is where you get the Mach Buffet and Vmo/Mmo is a speed set up by the manufacturer to keep us just below Mcrit....but its early in the morning for me...so I might be wrong.

 

[NYCPilot]

How do you recover from such a condition.

Maintain airspeed and go lower?

 

[rickair7777]

How do you recover from such a condition.

Maintain airspeed and go lower?

If you're in the corner, a VERY gradual descent with precise airspeed control would be the only way out I think. This would move Vs down, and Mmo up.

Vmo is predicated on overstressing the aircraft due to dynamic air pressure or possibly other considerations (engine, etc). Violating Vmo should not normally produce any immediate catastrophe due to the extra 50% buffer added to design load (ultimate load). Unless you're in an A380...

Mmo is to keep wing airflow subsonic and avoid mach tuck. Mmo you really don't want to exceed...The sudden change of of CL caused by a shockwave shifting on top of the wing could seriously unbalance the aircraft and cause a MASSIVE pitch excursion...possibly breaking major structural components.

Some military recon planes are purposely flown in the corner...not easy to do for hours on end.