Mass flow and pressure recovery

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Well-Known Member
Feb 26, 2019
Vienna, Austria
Hi! I am simulating the air inlet ducts (scoop type) for my small jet right now which is approximately the same category as the BD-5 or JSX-2.

I read the chapter in "Raymer: Aircraft Design - A conceptual approach" where inlet geometry is discussed (also for the specific BD-5). Speed has to be reduced to Mach 0.5 --> As the max speed is right around this value, the inlet does not need to increase in area. But it is recommended by multiple sources to have the area of the duct around 2x the area of the turbine inlet to account for the boundary layer within the duct.

Raymer writes that for subsonic aircraft a NACA duct can be used (as on the BD-5), but this will reduce the pressure recovery to 92%, so maximum thrust will even be reduced to ~89% (factor of 1.3 for each lost percent). It is preferred to use a scoop inlet, as this ensures 100% pressure recovery.

I would simulate the inlet by adding a mass flow inlet constraint at the turbine and ambient pressure arriving at the inlet at cruise speed.

Now I have the questions:
  • If the mass flow at maximum thrust is 1.2kg, does this mean that it is 0.9kg at 75% thrust?
  • Is the pressure recovery calculated correctly if measure the local pressure at the inlet of the turbine and divide it by the ambient pressure?
  • If I want to optimize the inlet for cruise speed, can I calculate the volume of 0.9kg and divide it by the cruise speed to get the desired area?
Thanks in advance!