Swept Wing, Spanwise Flow, and MCrit

[adk]

I've been reading and trying to understand why a swept wing delays MCrit. None of the books are helping. Would anybody care to clarify?

 

[seagull]

I've been reading and trying to understand why a swept wing delays MCrit. None of the books are helping. Would anybody care to clarify?

Ignoring the usual "vector" explanation, do it as a "thinking" exercise in a very simplified way. As a wing is moving forward, there is a "bow wave" in front of it if you are under the speed of sound, just like a wave develops in front of an object moving through the water. As you approach the speed of sound, you essentially catch up with that wave, and, as you move faster, you actually get ahead of the wave. The lack of the "bow wave" results in the air not being able to start moving in advance of the wing (remember the airflow subsonic the relative move is being pulled upwards before the wing gets there). The sudden presence of the wing with no flow developed results in a shock wave.

The wave propagates radially from any given point If you sweep a wing, the more forward portions create a "bow wave" ahead of the more aft portions of the wing, essentially delaying the "shock". The air is then able to start flowing upwards before the wing arrives as it would in normal subsonic flow.

Simplified, yes.

 

[derg]

Ignoring the usual "vector" explanation, do it as a "thinking" exercise in a very simplified way. As a wing is moving forward, there is a "bow wave" in front of it if you are under the speed of sound, just like a wave develops in front of an object moving through the water. As you approach the speed of sound, you essentially catch up with that wave, and, as you move faster, you actually get ahead of the wave. The lack of the "bow wave" results in the air not being able to start moving in advance of the wing (remember the airflow subsonic the relative move is being pulled upwards before the wing gets there). The sudden presence of the wing with no flow developed results in a shock wave.

The wave propagates radially from any given point If you sweep a wing, the more forward portions create a "bow wave" ahead of the more aft portions of the wing, essentially delaying the "shock". The air is then able to start flowing upwards before the wing arrives as it would in normal subsonic flow.

Simplified, yes.

You know, two classes in Aero at Riddle and now I finally understand.

Thanks brotha!

 

[ScoutFlyer]

I've seen a series of high speed aerodynamics videos produced by Shell in the 70s or 80s. They were cheesy and old, but they used some high tech wind tunnel imagery and really made a lot of the high speed aerodynamics concepts crystal clear. I can't find them reproduced online anywhere though. If I come across them, I'll post them.