In aircraft design, “flat engines” typically refer to horizontally opposed engines with a flat or boxer configuration, similar to the discussion in previous responses. To calculate the mass flow rate of air or fuel through a flat engine, you can use the formula for mass flow rate
ṁ is the mass flow rate in kilograms per second (kg/s).
ρ is the density of the fluid (air or fuel) in kilograms per cubic meter (kg/m³).
A is the cross-sectional area of the component through which the fluid flows in square meters (m²).
V is the velocity of the fluid in meters per second (m/s).