Flight Condition

About

Airspeed conversions (true/calibrated/equivalent/Mach), atmospheric data, and more with built-in unit checking. Specific sub-modules include:

• flightcondition: input altitude to compute common flight condition data. Easily swap between true airspeed, calibrated airspeed, equivalent airspeed, and Mach number. Includes atmospheric data.
• atmosphere: input altitude to compute atmospheric data. Many relevant, derived quantities are included. The upper limit is 86 km for the 1993 International Standard Atmosphere model and 10,000 kilometers for the NRL MSIS model.
• units: built-in unit-checking and conversion using the pint package.

Author

Matthew C. Jones matt.c.jones.aoe@gmail.com

Installation

Install Commands

Install using the pip package-management system. The easiest method is to open the terminal and run:

pip install flightcondition

Alternatively, manually download the source code, unpack, and run:

pip install <path/to/flightcondition>

Dependencies

• numpy: package for scientific computing.
• pint: package for dealing with units.
• pymsis: package for NRL MSIS atmospheric model.

Usage

Import all utilities with,

from flightcondition import *

or more explicitly as shown in the following examples.

Flight Condition

The FlightCondition class is used to compute and interact with common flight condition data. Inputs include altitude, velocity in some format, and an optional length scale.

Input Arguments

Input arguments include:

1. Altitude: h (aliases include: alt, altitude)
2. Velocity (pick one):
   • True airspeed: TAS (aliases include: tas, true_airspeed, U_inf, V_inf)
   • Calibrated airspeed: CAS (aliases include: cas, calibrated_airspeed)
   • Equivalent airspeed: EAS (aliases include: eas, equivalent_airspeed)
   • Mach number: M (aliases include: mach, Mach, M_inf, mach_number)
3. Length-scale (optional): L (aliases include: ell, bylen, length, length_scale, l)

Input quantities must be dimensionalized - see the usage below. Alternatively use KTAS, KCAS, or KEAS for convenience. For example, KCAS=233 is equivalent to CAS=233*unit('knots').

Output Quantities

The following tables list the quantities and the variables used to access them. Quantities may be accessed by either (a) their abbreviated variable, e.g. .TAS, or (b) by their full names, e.g. byname.true_airspeed. They may also be accessed through their particular sub-category: byalt, byvel, or bylen, e.g. .byvel.TAS or .byvel.byname.true_airspeed.

Example Usage

from flightcondition import FlightCondition, unit

# Compute flight condition at 3 km, Mach 0.5
fc = FlightCondition(h=3*unit('km'), M=0.5)

# Uncomment to print summary of flight condition quantities:
#print(f"{fc}")

# Uncomment to print abbreviated output in US units:
#print(f"\n{fc.tostring(full_output=False, units="US")}")

# Convert true, calibrated, equivalent airspeeds
KTAS = fc.TAS.to('knots')
KCAS = fc.CAS.to('knots')
KEAS = fc.EAS.to('knots')
print(f"Flying at {KTAS.magnitude:.4g} KTAS,"
      f" which is {KCAS.magnitude:.4g} KCAS,"
      f" or {KEAS.magnitude:.4g} KEAS")
# >>> Flying at 319.4 KTAS, which is 277.7 KCAS, or 275.1 KEAS

# Access atmospheric data (see Atmosphere class for more)
h, p, T, rho, nu, a = fc.h, fc.p, fc.T, fc.rho, fc.nu, fc.a
print(f"The ambient temperature at {h.to('km'):.4g} is {T:.4g}")
# >>> The ambient temperature at 3 km is 268.7 K

# Change airspeed to 300 KEAS and altitude to 12 kft
fc.EAS = 300 * unit('knots')
fc.h = 12 * unit('kft')
#print(f"{fc}")  # uncomment to print output

# Recompute for a range of altitudes at 275.14 knots-equivalent
# airspeed with a characteristic length scale of 10 meters
fc = FlightCondition(h=[0, 9.8425, 20]*unit('kft'),
                    EAS=275.14*unit('kt'),
                    L=10*unit('m'))

# Compute additional derived quantities - explore the class for more!
print(f"\nThe dynamic pressure in psi is {fc.q_inf.to('psi'):.3g}")
# >>> The dynamic pressure in psi is [1.78 1.78 1.78] psi
print(f"The Reynolds number is {fc.Re:.3g}")
# >>> The Reynolds number is [9.69e+07 8.82e+07 7.95e+07]
h_yplus100 = fc.wall_distance_from_yplus(100)
print(f"The wall distance where y+=100 is {h_yplus100.to('in'):.3g}")
# >>> The wall distance where y+=100 is [0.0126 0.0138 0.0153] in

# Alternatively access quantities by their full name
print(fc.TAS == fc.byname.true_airspeed)
# >>> [ True  True  True]

# Or by their sub-categories: `byalt`, `byvel`, or `bylen`
print(fc.byvel.TAS == fc.byvel.byname.true_airspeed)
# >>> [ True  True  True]

Atmosphere

The Atmosphere class can be used to compute and interact with common standard atmosphere data and derived quantities. See the list of output quantities in the FlightCondition documentation above. See also layer for layer properties such as layer.name for the layer name.

Note that all Atmosphere properties can be accessed through the FlightCondition class, however, this class stands on its own if the additional velocity and length-scale quantities are not desired.

Input Arguments

The input argument is geometric altitude h. Aliases include alt and altitude. Note that these geometric altitude must be input as dimensional length quantities - see the usage below. Alternatively input un-dimensionalized numbers using h_kft or h_km for kilofeet and kilometers respectively.

Example Usage

from flightcondition import Atmosphere, unit

# Compute atmospheric data for a scalar or array of altitudes
h = [0.0, 44.2, 81.0] * unit('km')
atm = Atmosphere(h)

# Uncomment to print all atmospheric quantities:
#print(f"\n{atm}")

# Uncomment to print while specifying abbreviated output in US units:
#print(f"\n{atm.tostring(full_output=False, units="US")}")

# See also the linspace() function from numpy, e.g.
# h = linspace(0, 81.0, 82) * unit('km')

# Access individual properties and convert to desired units: "
p, T, rho, nu, a, k = atm.p, atm.T, atm.rho, atm.nu, atm.a, atm.k
print(f"\nThe pressure in psi is {p.to('psi'):.3g}")
# >>> The pressure in psi is [14.7 0.024 0.000129] psi

# Compute additional properties such as mean free path
# Explore the class data structure for all options
print( f"\nThe mean free path = {atm.MFP:.3g}")
# >>> The mean free path = [7.25e-08 4.04e-05 0.00564] yd

Units

Conveniently input, output, and convert units using pint units.

from flightcondition import unit, printv

h = 33 * unit('km')
print(h.to('kft'))
# >>> 108.26771653543307 kft
printv(h, to='kft')
# >>> h = 108.27 kft

U_inf = 20 * unit('knots')
rho_inf = 1.225 * unit('kg/m^3')
q_inf = 0.5*rho_inf*U_inf**2
printv(q_inf, to='psi')
# >>> q_inf = 0.0094042 psi

Note that pint does not support conflicting unit registries so avoid interactions between flightcondition.unit and a separate pint.UnitRegistry.

Command Line Interface

A command line interface (CLI) is included for convenience but with limited functionality. Run flightcondition -h for help.

An example call is given for the flight condition of 233 knots-equivalent-airspeed at 23 kilofeet with a length scale of 4 feet and abbreviated output:

flightcondition --h 23 kft --EAS 233 knots --L 4 ft --no-full-output

===========================================================
   Flight Condition (units=US, full_output=False)
===========================================================
------------------  Altitude Quantities  ------------------
geometric_altitude  h       = 23 kft
pressure            p       = 857.25 lbf/ft²
temperature         T       = 436.74 °R
density             rho     = 1.1435×10⁻³ slug/ft³
sound_speed         a       = 1024.5 ft/s
kinematic_viscosity nu      = 2.8509×10⁻⁴ ft²/s
------------------  Velocity Quantities  ------------------
mach_number         M       = 0.55344
true_airspeed       TAS     = 335.93 kt
calibrated_airspeed CAS     = 238.14 kt
equivalent_airspeed EAS     = 233 kt
reynolds_per_length Re_by_L = 1.6573×10⁵ 1/in
------------------   Length Quantities   ------------------
length_scale        L       = 4 ft
reynolds_number     Re      = 7.9551×10⁶

Alternatively use the --KEAS 233 syntactic sugar to omit the knots unit. See also --KTAS and --KCAS.

Web Application

See the web application here: https://flightcondition.streamlit.app/

Assumptions

• When using model=standard, Atmospheric quantities follow the 1993 International Standard Atmosphere model.
• When using model=msis, atmospheric quantities follow the NRL MSIS model. Note that model=msis is automatically enabled for altitudes higher than 80 kilometers.
• Velocity computations include varying degrees of the following assumptions. Note that several assumptions break down for hypersonic flow.
  • Continuum flow (mean free path is much smaller than the characteristic length scale)
  • Ideal gas
  • Thermally perfect gas
  • Calorically perfect gas
  • Adiabatic
  • Reversible (CAS, q_c, p_0)

Unit Testing

flightcondition is maintained using unit tests to maintain functionality and accuracy. Even so, see the Disclaimer below.

License

flightcondition is licensed under the MIT LICENSE. See the LICENSE document.

Disclaimer

The software is provided "as is", without warranty of any kind, express or implied, including but not limited to the warranties of merchantability, fitness for a particular purpose and noninfringement. In no event shall the authors or copyright holders be liable for any claim, damages or other liability, whether in an action of contract, tort or otherwise, arising from, out of or in connection with the software or the use or other dealings in the software.