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?
Welcome to Flightinfo.com
- Register now and join the discussion
- Friendliest aviation Ccmmunity on the web
- Modern site for PC's, Phones, Tablets - no 3rd party apps required
- Ask questions, help others, promote aviation
- Share the passion for aviation
- Invite everyone to Flightinfo.com and let's have fun
Why is the speed of sound significant aerodynamically?
- Thread starter PeteCO
- Start date
Welcome to Flightinfo.com
- Register now and join the discussion
- Modern secure site, no 3rd party apps required
- Invite your friends
- Share the passion of aviation
- Friendliest aviation community on the web
http://www.pilotsbooks.com/aerodynamics_for_naval_aviators.htm
[font=Arial, Helvetica, sans-serif]
[/font]
[font=Arial, Helvetica, sans-serif]
[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]
enigma
good ol boy
- Joined
- Nov 26, 2001
- Posts
- 2,279
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
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
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
Dave
typhoonpilot
Daddy
- Joined
- Feb 24, 2002
- Posts
- 1,381
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 ?
A Squared
Well-known member
- Joined
- Nov 26, 2001
- Posts
- 3,006
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.
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
Daddy
- Joined
- Feb 24, 2002
- Posts
- 1,381
PastFastMover
Well-known member
- Joined
- Aug 21, 2004
- Posts
- 93
EagleRJ
Are we there yet?
- Joined
- Nov 27, 2001
- Posts
- 1,490
The water analogy is useful for visualizing airflow, but it starts to make it confusing when you start talking about transsonic flow. Remember that unlike water, air is compressible, so its motion around an airfoil begins to look different from water as you approach the speed of sound.
The speed of sound in air is an important measurement in aerodynamics because sound is simply a compression that is propogating through the air. Knowing the speed of that compression in ambient air is key to knowing when the compression will start to have problems when it is asked to move faster than M 1.0.
We all know that air flows over an airfoil by accelerating as it passes the widest part of the chord. Airfoils on high-subsonic and supersonic aircraft have less camber than slower airplanes, but the air still accelerates, creating our lift. As the airplane reaches a certain speed (Mcrit), the air reaches M 1.0 as it flows over the wing. When that happens, a shock wave starts to form, since the air can no longer smoothly make the trip over the wing. As seen from the side, the shock wave stands straight up from the wing, and as airspeed increases, it starts to increase in intensity and move aft on the chord. Another shock wave begins to form on the underside of the chord, too. These shock waves moving aft are the cause of Mach Tuck, as the center of pressure is moving aft along with the shock waves.
The same shock waves will form on the stabilator, which is why all supersonic aircraft have stabilators, not elevators. Early test aircraft with horizontal stabilizers would lose elevator effectiveness as the shock waves moved onto the elevator, but engineers learned that having a full-flying stabilator would allow the pilot to maintain pitch control in supersonic flight.
Shock waves also begin projecting from the fuselage, canopy, and other areas of accelerated flow. That's the cause of the rise in drag immediately before an aircraft reaches M 1.0.
As the aircraft continues accelerating, the airflow converts from local areas of sonic and supersonic flow, to supersonic flow around the entire aircraft. That's when the only shock wave is a cone-shaped wave that originates at the nose of the aircraft. Since the shock wave is linear instead of localized, it propogates away from the aircraft, and is interpreted by people on the ground as a sharp, loud noise (a sonic boom).
Current supersonic aircraft with turbine engines need to slow the air to subsonic speeds before it enters the engines, since supersonic flow would play havoc inside the engine. The shock waves would create turbulent flow, and the engine would flame out since the air would be moving faster than the rate of propogation of a flame in the air/fuel mixture.
Some experimental Scramjets can generate thrust with supersonic flow through the entire engine. They are not turbine engines, but modified ramjets, where compression is caused by the aircraft's forward motion instead of a compressor section. How they can keep the flame lit with supersonic flow through the engine is beyond me!
Experimental models like the Hyper-X use hydrogen for fuel, so maybe that burns faster than Jet-A!
The speed of sound in air is an important measurement in aerodynamics because sound is simply a compression that is propogating through the air. Knowing the speed of that compression in ambient air is key to knowing when the compression will start to have problems when it is asked to move faster than M 1.0.
We all know that air flows over an airfoil by accelerating as it passes the widest part of the chord. Airfoils on high-subsonic and supersonic aircraft have less camber than slower airplanes, but the air still accelerates, creating our lift. As the airplane reaches a certain speed (Mcrit), the air reaches M 1.0 as it flows over the wing. When that happens, a shock wave starts to form, since the air can no longer smoothly make the trip over the wing. As seen from the side, the shock wave stands straight up from the wing, and as airspeed increases, it starts to increase in intensity and move aft on the chord. Another shock wave begins to form on the underside of the chord, too. These shock waves moving aft are the cause of Mach Tuck, as the center of pressure is moving aft along with the shock waves.
The same shock waves will form on the stabilator, which is why all supersonic aircraft have stabilators, not elevators. Early test aircraft with horizontal stabilizers would lose elevator effectiveness as the shock waves moved onto the elevator, but engineers learned that having a full-flying stabilator would allow the pilot to maintain pitch control in supersonic flight.
Shock waves also begin projecting from the fuselage, canopy, and other areas of accelerated flow. That's the cause of the rise in drag immediately before an aircraft reaches M 1.0.
As the aircraft continues accelerating, the airflow converts from local areas of sonic and supersonic flow, to supersonic flow around the entire aircraft. That's when the only shock wave is a cone-shaped wave that originates at the nose of the aircraft. Since the shock wave is linear instead of localized, it propogates away from the aircraft, and is interpreted by people on the ground as a sharp, loud noise (a sonic boom).
Current supersonic aircraft with turbine engines need to slow the air to subsonic speeds before it enters the engines, since supersonic flow would play havoc inside the engine. The shock waves would create turbulent flow, and the engine would flame out since the air would be moving faster than the rate of propogation of a flame in the air/fuel mixture.
Some experimental Scramjets can generate thrust with supersonic flow through the entire engine. They are not turbine engines, but modified ramjets, where compression is caused by the aircraft's forward motion instead of a compressor section. How they can keep the flame lit with supersonic flow through the engine is beyond me!
Experimental models like the Hyper-X use hydrogen for fuel, so maybe that burns faster than Jet-A!
"The speed of sound in air is an important measurement in aerodynamics because sound is simply a compression that is propogating through the air."
Perfect! I figured that the compressibility of the air increased somewhat linearly as speed increased. In other words, I had thought that the compressibility was not at some significant transition (in behavior) point right at the speed of sound, and therefore something else must be responsible for the fact that the speed of sound is significant. ("If compressibility of the air would gradually increase, why would it be related to the actual point where the speed of sound occurred?", asked I)
I see that the compressibility (as speed increases) is in fact not linear, and due to the shockwaves, a significant change takes place at the transonic transition. Sort of.
Thanks for all the replies.
Perfect! I figured that the compressibility of the air increased somewhat linearly as speed increased. In other words, I had thought that the compressibility was not at some significant transition (in behavior) point right at the speed of sound, and therefore something else must be responsible for the fact that the speed of sound is significant. ("If compressibility of the air would gradually increase, why would it be related to the actual point where the speed of sound occurred?", asked I)
I see that the compressibility (as speed increases) is in fact not linear, and due to the shockwaves, a significant change takes place at the transonic transition. Sort of.
Thanks for all the replies.
Last edited:
100LL... Again!
youwantapieceofme??
- Joined
- Dec 19, 2002
- Posts
- 1,533
Airplanes fly because they make loud noises.
Once you exceed the speed of sound, the airplane can no longer hear the noise it makes, and it thinks it lost all of its engines, and therefore it won't keep flying.
Supersonic aircraft get around this by employing gigantic speakers outside near the engines, where they play thundering jet noises at full volume. This keeps the airplane happy and willing to produce lift for us.
These speakers are the reason you hear sonic booms from supersonic aircraft. Which just goes to show that pilots of the first supersonic aircraft were actually the predecessors of those annoying kids with their stereos booming while you wait for the light to turn green.
If you doube the science behind this explanation, do the following:
Take a Cherokee up and dive until you go supersonic. You will hear one really big noise and then - blissful, eternal silence! You've just outrun the speed of sound.
Once you exceed the speed of sound, the airplane can no longer hear the noise it makes, and it thinks it lost all of its engines, and therefore it won't keep flying.
Supersonic aircraft get around this by employing gigantic speakers outside near the engines, where they play thundering jet noises at full volume. This keeps the airplane happy and willing to produce lift for us.
These speakers are the reason you hear sonic booms from supersonic aircraft. Which just goes to show that pilots of the first supersonic aircraft were actually the predecessors of those annoying kids with their stereos booming while you wait for the light to turn green.
If you doube the science behind this explanation, do the following:
Take a Cherokee up and dive until you go supersonic. You will hear one really big noise and then - blissful, eternal silence! You've just outrun the speed of sound.
The Learjet made the mach tuck famous. In reality it was because of poor airframe and wing design. We had to start somewhere and the Lear had WWII technology. The airframes that we have now are awesome and come from the fact that we can computer generate them before we even put them in the wind tunnel.
Share this page
Latest resources
-
-
-
-
-
AC 90-89C - Amateur-Built Aircraft and Ultralight Flight Testing HandbookAmateur-Built Aircraft and Ultralight Flight Testing Handbook
- Neal
- Updated:
