Why is the speed of sound significant aerodynamically?

[EagleRJ]

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).

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.

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!

 

[PeteCO]

"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.

 

[100LL... Again!]

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.

 

[TurboS7]

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.