Low Altitude Performance

[KJ_Lesnick]

Rickshaw

I got a question: Does this formula accurately describe temperature build up to mach number at low altitude (mach^2 x 100F)+(Temperature F)?


tahsin

Well the thing about the shockwave being ingested by the inlet is an argument I heard before, I'm not sure if it's actually true: It wouldn't be the first time somebody made something up about the A-12/SR-71

• Mach 3.2 doesn't produce temperatures of 900-degrees: I think there was a mathematical formula for determining mach number based on sea-level which I think was (mach-number^2 x 100)+(air-temperature)= Skin temperature; this produces either (3.2^2+100)+(-70) = 954 F
  
• Stagnation temperatures are based on mach number, altitude, and air-temperature: The compression should theoretically produce the same mach number but the skin will absorb/reflect a certain amount of heat, if the air is thinner (higher altitude) there are less molecules of air able to impart their kinetic energy onto the aircraft.  The aircraft heats-up slower and heats-up less at higher altitudes.
  
• The colder air takes away heat and so on.
  

As for shockwave angles

• Theoretically the angle is Arcsin 1/M: In reality it's not exactly perfect as a needle and a fairly wide cone produce different angles, and a blunt surface gets a detached wave
  
• The fuselage lengthens, moving the nose further from the inlet
  
• Inlets work by basically exploiting the effects of either diverging, converging, or converging; then diverging the airflow: Diverging airflow when subsonic produces a deceleration in airflow; converging airflow when supersonic slows it down.  This is why subsonic inlets basically look like a bell-mouth (excepting a blunt-mouth), and supersonic ones range from a subsonic one (the lip abruptly slows down the flow) to a convergent/divergent flow with the flow slowed to subsonic speed, then diverged to 0.3 - 0.5 mach
  
• The fuselage technically provides a change in area in the aircraft (cross section)
  

[tahsin]

This is the problem you repeatedly encounter in these forums. People simply can not have all the answers. I can only assume you have the right stuff that people can correct where necessary but it's far too much math for me. Can't say anything.