Aerodynamics Question

[CesnaCaptn]

Is there a forum on here to ask technical questions? Well here's one:

Does critical mach number change with altitude?

 

[Bjetset2]

Yep

Yes it does vary by altitude. Remember the speed of sound is related to temperature. Thus for a given TAS the MACH number will be much higher at altitude than at sea level. Thus compressibility effect due to supersonic airflow will be encountered at slower speeds at high altitudes versus low altitudes.

The Lear 60 for example has an MMO of .81 from 26,750 feet to 37,000. Above 43,000 the Mmo is .78. The Falcon 2000EX, a much better aerodynamic design, only varies from .862 to .85

Hope that helps.

 

[b82rez]

no...

 

[Bjetset2]

Better Put

The wing's Critical Mach is what it is, the indicated Mach you fly to avoid it changes.

I missed the intent of the question. My bad.

 

[GCD]

A "critical Mach number" is the speed of an aircraft (below Mach 1) when the air flowing over some area of the airfoil has reached the speed of sound. For instance, if the air flowing over a wing reaches Mach 1 when the wing is only moving at Mach 0.8, then the wing's critical Mach number is 0.8.

 

[Bjetset2]

Correct

That is correct GDC. Thus I will defer to my first response that yes the critical mach number does vary with altitude.

 

[Razorback one]

Someone has a Delta interview coming up..... Best of luck, I finally decided I was going to flip a coin on that question. It never came up.

R1

 

[sg737]

I think....

Critical mach is critical mach is critical mach... regardless of altidude. It occurs at a different AIRSPEED at different altitudes... but mach is mach....

 

[Truckdriver]

Keep going guys, I have the same problem with this issue. I think it stays the same, but the airspeed at which you reach critical mach will decrease. I also see how you could go the other way so I'm not sure here. Nothing in Aerodynamics for Naval Aviators really answers the question.

 

[macdu]

I think....

Critical mach is critical mach is critical mach... regardless of altidude. It occurs at a different AIRSPEED at different altitudes... but mach is mach....

This is what I have always used as the definition also. You want to avoid critical mach for various efficiency, safety reasons, and the simple fact the commiercial wing/tail design is not meant to go Mach 1. Up high you need to be cognizant of your gross weight, CG, ISA, turbulence. You end up backing the wing in to a corner, so to speak, of narrow performance capability of stall and mach buffet I believe. Remember you are actually "flying" the wing, not the plane if you follow me.