Transonic buffet happens at mach 0.95. The correlated true airspeed varies with altitude. Does the transonic buffet magnitude also vary with altitude, and if so, how?
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$\begingroup$ If an aircraft (fighter jet) is in transonic regime then the buffet occurs. Yes or no? But transonic speed region's is definitely dependent on the altitudes. Yes or no? So my question: is this different altitudes gives buffet intensity( during maneuver) differently any times? Now you understand? $\endgroup$– George GeoOct 24, 2019 at 12:17
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$\begingroup$ I don't know about fighter jets; never flown or designed one. But for commercial aircraft, you cannot have buffet during cruise up to your maximum operating Mach number. Forbidden by regulations. $\endgroup$– JZYLOct 24, 2019 at 12:22
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1$\begingroup$ Recommend editing your question to the first comment. Some punctuations and explanation (for example, what motivates the question, rationale for your assumptions) would help. $\endgroup$– JZYLOct 24, 2019 at 12:34
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1$\begingroup$ @Jpe64 "Transonic buffet happens always at mach 0.95"? That's misleading borderline incorrect. $\endgroup$– JZYLOct 24, 2019 at 14:14
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1$\begingroup$ @Jimmy that's not my statement, I merely tried to edit the question so that it would be comprehensible. You can check out the edit history to see the original question and edit further should you wish to do so. $\endgroup$– Jpe61Oct 24, 2019 at 14:40
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1 Answer
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Transonic buffet, or Mach induced buffet, or high speed buffet, is a flow separation due to the combined presence of shockwave and "high" angle of attack. The boundary layer separation results in random high frequency vibrations. As the AOA increases, the separation and the associated vibrations worsen, up to the point where the crew can no longer read the instrumentation and perform normal flight duties; this point is called deterrent buffet.
For a single Mach number, the AOA (or $C_{L}$) associated with buffet onset is generally constant; there will be some differences due to CG position, the associated trim and Reynolds number effects. During aircraft design and flight testing, the buffet onset boundary similar to the one below is mapped out. As you can see, the general trend is a decrease in buffet onset boundary with Mach (though the trend can reverse depending on the airplane).
(Image modified from https://www.researchgate.net/figure/Typical-buffet-onset-curve-and-separation-mechanisms-defining-the-boundaries_fig2_270086469.)
Flying at the same Mach number at a higher altitude means that it will have smaller dynamic pressure than flying at a lower altitude. This translates to a higher AOA at the higher altitude, which means that the buffet intensity will be higher. In other words, for the same Mach and weight, the maneuver margin is decreased the higher it flies, independent of its climb capability.
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$\begingroup$ Thanks "This translates to a higher AOA at the higher altitude, which means that the buffet intensity will be higher" is the answer to the question. Because of the AOA at higher altitudes the buffet is more aggressive than the lower altitudes. $\endgroup$ Oct 24, 2019 at 20:10