Transonic Airflow on Stall-Speed

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Zipper730

Chief Master Sergeant
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Nov 9, 2015
It's strange in that I felt like I'd asked this before but I can't find the post. Regardless, I have two questions

1. Is buffet-boundary the same thing as the decrease in critical AoA as a function of transonic flight (and the resulting pressure gradients produced by shockwaves forming across the wings making it easier for the airflow to separate) and, if not what do you call this?

2. Are there general rules of thumb that indicate the degree to which this tends to vary on aircraft designs: For example the F-4 had a variation of around 15 knots in stall speed as a function of flying at high subsonic speed versus lower speed.


BTW: I've tagged the following because most people here are quite knowledgeable and I figure somebody might have an answer
A Admiral Beez , davparlr davparlr , drgondog drgondog , fannum fannum , fubar57 fubar57 , G Glider , GreenKnight121 GreenKnight121 , Greg Boeser Greg Boeser , GTX GTX , johnbr johnbr , michaelmaltby michaelmaltby , M MIflyer , Motocar Motocar , P pbehn , S Shortround6 , Simon Thomas Simon Thomas , swampyankee swampyankee , T ThomasP , T tyrodtom
 
It's strange in that I felt like I'd asked this before but I can't find the post. Regardless, I have two questions

1. Is buffet-boundary the same thing as the decrease in critical AoA as a function of transonic flight (and the resulting pressure gradients produced by shockwaves forming across the wings making it easier for the airflow to separate) and, if not what do you call this?

2. Are there general rules of thumb that indicate the degree to which this tends to vary on aircraft designs: For example the F-4 had a variation of around 15 knots in stall speed as a function of flying at high subsonic speed versus lower speed.


BTW: I've tagged the following because most people here are quite knowledgeable and I figure somebody might have an answer
A Admiral Beez , davparlr davparlr , drgondog drgondog , fannum fannum , fubar57 fubar57 , G Glider , GreenKnight121 GreenKnight121 , Greg Boeser Greg Boeser , GTX GTX , johnbr johnbr , michaelmaltby michaelmaltby , M MIflyer , Motocar Motocar , P pbehn , S Shortround6 , Simon Thomas Simon Thomas , swampyankee swampyankee , T ThomasP , T tyrodtom
This should help

1. Transonic shock wave formation causes separation (all things equal) to the airflow aft of the shock wave boundary. The adverse pressure gradient has the effect of not only major separation but also the resulting increase in pressure drag due to the 'new' frontal area that the freestream 'sees' over the wing.

2. No. Not as a general template where 'wing of airfoil section A with planform B will behave thusly at transonic speeds'. That said, I may often be wrong, but rarely uncertain.

Note:
Total Drag due to Viscous contribution = Totald Drag due to Friction + Total Drag due to 'Shape' 'Shape' largely Pressure Drag, which increases dramatically with boundary layer separation over the 'geometric form', causing the increased Pressure Drag.

FWIIW - the existance of Laminar flow on WWII airfoils is a matter of inches from the stagnation point at the LE where freestream velocity is brough to 'zero' then accelerates through the Reynolds Number of apprximately 500,000 (a very low speed over a wing) and transitions to a Turbulent flow with larger Boundary Layer (but orderly with respect to energy layers within the BL) while remaining attached to the wing (or fuselage) surface - until disruped by an adverse pressure gradient causing major separation.

Within the separation region is chaotic distributions of turbulent flow which causes increase in both pressure drag AND friction drag. The one 'B' I got in graduate school was a Boundary Layer curriculum which included Chaos Theory with a lot of complex statistic theory. If anybody has figured out predictive BL behavior via Chaos Theory, he is WAY Smarter than I ever was, and I have gone downhill ever since.
 

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