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.

 

[Amish RakeFight]

Critical mach remains the same regardless of altitude. Mach crit is expressed as a percentage of mach. This percentage remains the same for a specific wing. Only mach itself differs depending on temperature and hence, altitude.

For instance, mach is achieved at a higher speed at lower altitudes and as you ascend in altitude (decreasing temp) mach will be attained at a lower speed. However, the percentage of mach at which mach crit occurs is fixed in relation to the mach number itself which will vary.

 

[LearLove]

Nothing in Aerodynamics for Naval Aviators really answers the question.

thats because that book is nothing but a glorified cookbook on aerodynamics.

off the cuff I'd say the above post is best but i'd have to look at something wrt shockwave formation before laying down my opinion/answer.

All of my research and windtunnel exp. (done 10 years ago now) was in the area of low speed aerodynamics, Reynolds number around 500,000 and less.

Only the smart grad students/kids (China/Korean and Russians) got to play with the high-speed wind tunnels. I was only a C average white kid engineer.

 

[SkiFishFly]

Mach number stays the same. It is a percentage of Mach 1. Critical mach number stays the same for a specific wing design.
The indicated/true airspeed at which you reach a specific mach number decreases with altitude.

 

[BEECH-SLAPPED]

It is aircraft dependant

Critical Mach will change slightly with altitude depending on the design of the wing. It is tempting to say "no, it doesn't change" because Mach is Mach, regardless of altitude. However, very few things are linear with regard to flow around bodies. The local speed of flow around at any given point depends on several factors, not just freestream velocity. Two of those factors are density and temperature, both which change with altitude. To compare two conditions you need to keep the same Reynolds number, which is dependant on density, velocity, and viscosity of the air. Calculating the local velocity at a given point on a wing is very difficult and even with sophisticated compututational fluid dynamics software, the result is never exact with what is measured.

Now without all the engineering terms. Let's take a hypothetical jet with a published Critical Mach of .87. Let's say at M 0.87 at 30,000 ft, you have Mach 1 flow at a certain point on the wing. You measure and calculate the local SPEED of the flow at that point to be 589 kts. At M 0.8 at 30000, your TRUE airspeed of the aircraft is 512 kts (assuming standard atmosphere). Now, let's climb to 37,000 and hold a Mach of .87. Your true airspeed at this altitude is now 499 kts. Let go back to the the same point on the wing and measure and calculate the speed of the air. We get a value of 557 kts, or M .97. Wait! Shouldn't this value be 573 kts, which is the speed of sound at 37,000 ft? What this means is that you will not necessarily get supersonic flow at the same point on the wing at the same aircraft Mach # at every altitude. It depends on many factors! So basically the aircraft manufacturer publishes the most conservative number and that is what we use to fly by.

Few people also realize that the indicated stalling speed of a wing actually changes with altitude as well, due to the changes in density and viscosity of the air. I'm not talking about low-speed buffett at high altitudes either. A Cessna 172's stalling caliberated airspeed at Sea Level is not the same as at 8000 feet. It's just that the difference is very small.

 

[Bjetset2]

Yep

And that is why the Mmo for airplanes vary by altitude, some more than others. Good explanation sir.