Why is the speed of sound significant aerodynamically?

[PeteCO]

Why is it that there are significant changes in aerodynamics at the speed of sound, with stuff like Mach tuck occurring? I don't see why the speed sound moves and airflow over an airfoil would be related. Seems kind of like the relation between the volume of dog turds in my back yard and the gas mileage my car gets - one is the speed of sound, the other is simply airflow. What/why is the connection?

 

[labbats]

The dog turds create methane which inhabits the atmosphere of air that your car intakes to mix with the fuel. Thus, the dogturds are in essence supercharging your car. Thanks fido! Oh, wait...

 

[PeteCO]

The dog turds create methane which inhabits the atmosphere of air that your car intakes to mix with the fuel. Thus, the dogturds are in essence supercharging your car. Thanks fido! Oh, wait...

Nobody likes a wiseacre. You better check your backyard for dogturds, labbats, I'm bringing a truckload of them over to your house right now.

 

[Axel]

http://www.pilotsbooks.com/aerodynamics_for_naval_aviators.htm

[font=Arial, Helvetica, sans-serif]

[/font]

[font=Arial, Helvetica, sans-serif]The classic book on aerodynamics and flight engineering from the pilot's perspective; written by the US Navy as a primary textbook for naval pilots. This book covers the basic principles of flight theory in both low and high speed regimes. Airflow theory, airfoil design, high lift devices, induced and parasitic drag, stall patterns, climb and sink performance, thrust and power, control & stability are all covered in subsonic, transonic, and supersonic conditions. Somewhat technical, although mathematics is kept to the minimum level required to understand the topics from an operational point of view.[/font]
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[labbats]

I like wiseacres. Fields and fields of wiseacres. Glistening in the sun.

 

[enigma]

PeteCO, good question. I don't know the answer off the top of my head, but I do have a few thoughts. First, we know that sound travels in pressure waves. We also know that pressure waves build up in airflow as an object reaches the speed of sound. It can't be coincidental that the two are related. Hopefully someone with a better memory than I can put the two together.

I can tell you that Mach Tuck is not directly related to your question. Mach Tuck is just a RESULT of the fact that airflow changes as an airfoil approaches Mach 1. Basically, mach tuck is what happens when a subsonic airfoil if forced into trans-sonic speed and the center of pressure moves aft. Mach tuck can be overcome by good aircraft design. Things like proper airfoil selection, horizontal stab planform, stab airfoil etc, can be designed to minimize tuck effects.

There is a Gulfstream test pilot on this forum who knows a heck of a lot about these issues. Maybe if I post something that could be construed as a slight to the GV winglets, he'll see it and enter this string. (just kidding with ya, Gulfstream testpilot)

enigma

 

[FN FAL]

I like wiseacres. Fields and fields of wiseacres. Glistening in the sun.

as far as the eye can see.

 

[Mr. Cole]

Well, without getting too deep into the physics. Think of air as a fluid such as water. In fact air is classified as a fluid. Just as wave propagate in water, they do the same in air. What we hear as sound is due to pressure waves that move through the air. The speed at which these disturbances move is the speed of sound, and is a function of a number of factors, one of which is temperature. As you approach the speed of sound, the plane is actually moving faster than the air that it disturbs. Typically the pressure waves created by an object would move away from the object, leaving the object behind. At the speed of sound and greater the object moves faster than the pressure waves, causing the pressure waves to bunch up and compress. This creates shock waves that have very different pressure distributions in front of the wave than behind the wave. Therefore the air in front of and behind the wave can have different velocities. If you have a wing with a shock wave forming on it, the differential in pressure in front of the wave versus behind the wave can be so great as to cause a backflow of the air over the wing. This results in a loss of lift and a shifting of the center of pressure, which is a cause of Mach tuck.

Dave

 

[TonyC]

There is a Gulfstream test pilot on this forum who knows a heck of a lot about these issues. Maybe if I post something that could be construed as a slight to the GV winglets, he'll see it and enter this string. (just kidding with ya, Gulfstream testpilot)

enigma

Dear enigma,

Thanks for the PM. No, I do not think Gulfstream winglets are a waste of epoxy.


Talk to ya later!


-Tony









Think that'll do it?









.

 

[typhoonpilot]

Why is it that there are significant changes in aerodynamics at the speed of sound, with stuff like Mach tuck occurring? I don't see why the speed sound moves and airflow over an airfoil would be related.

Why did Maverick say he was in a flat spin going out to sea ? I'm not sure why, strictly speaking, if you are in a flat spin the only place you are going is straight down.

Okay, in all seriousness ( is that a word Tony ? ) please turn to page 202 of Aerodynamics for Naval Aviators. The analogy of surface waves on the water may help clarify the question. Since a surface wave is simply the propagation of a pressure disturbance, a ship moving at a speed much less than the wave speed will NOT form a " bow wave ". As the ship's speed nears the wave propagation speed the bow wave will form and become stronger as the speed is increased beyond the wave speed.


Gillegan, little buddy. Are you reading this ? Answer from a true aerodynamicist please.



TP

P.S. Another important question for the day. Why hasn't qmaster been banned yet ?

 

[Kream926]

P.S. Another important question for the day. Why hasn't qmaster been banned yet ?

ask and you shall recieve

 

[TonyC]

Okay, in all seriousness ( is that a word Tony ? ) ...

Why, yes! And a very good noun at that!













.

 

[A Squared]

To put what Mr Cole said in very simple terms:

Below the speed of sound, the air starts to move out of the way a little bit before the airplane actually arrives there. Above the speed of sound, it doesn't. This isn't quite techincally correct, but it's useful for understanding in a comic-book sot of way.

 

[typhoonpilot]

ask and you shall recieve

Amazing that took so long, Mods were all out partying over the holiday weekend I guess. Had me ROFLMAO though.


TP

 

[PastFastMover]

PeteCo, another interesting point is when the aircraft is moving supersonically ,the airflow into the engines is subsonic. It is achieved by ramps, spikes, and various other type of devices. A few of the more exotic aircraft don't hold to this though.