Aerodynamics Question

[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.

To quote my earlier post, critical mach is fixed in relation to mach itself which fluctuates based upon the temperature which varies upon altitude.

Technically, one could argue that critical mach changes with altitude and this is correct, if you understand that it changes along with mach itself. Critical mach rides the coat tails (if you will) of mach.

But as a limitation or milestone, Mcrit is recognized as a percentage of mach and only that. It does not vary on its own, independant of mach itself. It's variance is dependant upon the present mach's variance that which a specific wing is traveling at within a certain altitude.

 

[PHXFLYR]

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.

So that's why you're a pilot ......

PHXFLYR

 

[Amish RakeFight]

So that's why you're a pilot ......

PHXFLYR

Not that I'd necessarily agree, but I once read somewhere that a good pilot is one who isn't too bright. Apparently you CAN be too smart to fly a plane. I believe it had something to do with overanalyzing and thinking too much in a perfectionist sort of way.

 

[luckytohaveajob]

A long time ago I remember reading the DC8 was actually dive tested above Mach 1 in flight testing. I can not remember the details. Does anyone have any good stories about the 8 going past Mach 1?

How about some stories about other aircraft in high speed flight.

Anyone have a B737 past .82? How about a B747 past Mmo? Or an old B727 past .92? And what about the Citation X?

 

[Amish RakeFight]

I can't recall which aircraft (L1011?), but the curvature just above the cockpit would cause flow to go supersonic before mach crit made an appearance on the wings.

 

[XJ-spartacus]

My quess is that a wings Mcrit is as fixed as it's Critical AOA. There are several variables that can change these values, but all are insignificant. With any value, there will be variances that must be averaged out. One of my profs used to say you measure with a micrometer and cut it with an axe. As pilots, we use a maul. The engineers know this, and write our manuals to reflect it. They don't want us trying to take the square root of the hypotenuse of an isosceles triangle and dividing it by one while were trying to make a crossing restriction. I like it when they keep it simple. I really like it when they install coffee holders next to where I lay my arm.

-Spartacus

 

[Amish RakeFight]

Thumbs up Spartacus!

 

[27 driver]

My quess is that a wings Mcrit is as fixed as it's Critical AOA. There are several variables that can change these values, but all are insignificant. With any value, there will be variances that must be averaged out. One of my profs used to say you measure with a micrometer and cut it with an axe. As pilots, we use a maul. The engineers know this, and write our manuals to reflect it. They don't want us trying to take the square root of the hypotenuse of an isosceles triangle and dividing it by one while were trying to make a crossing restriction. I like it when they keep it simple. I really like it when they install coffee holders next to where I lay my arm.

-Spartacus

Spartacus, I have to agree. The last thing I want to do when trying to maintain speeds, make a restriction, and read the news paper is do quantum physics calculations. Besides, Cmach means nothing until you exceed it, and to do this you are doing something you shouldn't be. Performance books are in the plane for a reason. Keep is simple stupid is a great rule. Just my .02