Source code for RCAIDE.Library.Mission.Segments.Descent.Constant_CAS_Constant_Rate
# RCAIDE/Library/Missions/Segments/Descent/Constant_CAS_Constant_Rate.py
#
#
# Created: Jul 2023, M. Clarke
# ----------------------------------------------------------------------------------------------------------------------
# IMPORT
# ----------------------------------------------------------------------------------------------------------------------
# RCAIDE imports
import RCAIDE
from RCAIDE.Framework.Core import Units
# package imports
import numpy as np
# ----------------------------------------------------------------------------------------------------------------------
# Initialize Conditions
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[docs]
def initialize_conditions(segment):
"""
Initializes conditions for constant calibrated air speed descent at fixed rate
Parameters
----------
segment : Segment
The mission segment being analyzed
- descent_rate : float
Rate of descent [m/s]
- calibrated_air_speed : float
Calibrated air speed to maintain [m/s]
- altitude_start : float
Initial altitude [m]
- altitude_end : float
Final altitude [m]
- sideslip_angle : float
Aircraft sideslip angle [rad]
- temperature_deviation : float
Temperature offset from standard day [K]
- state:
numerics.dimensionless.control_points : array
Discretization points [-]
conditions : Data
State conditions container
- analyses:
atmosphere : Model
Atmospheric model for property calculations
Returns
-------
None
Notes
-----
This function sets up the initial conditions for a descent segment with constant
calibrated air speed (CAS) and constant descent rate. It handles the conversion
between CAS and true air speed accounting for pressure and density variations
with altitude. Updates segment conditions directly with velocity_vector [m/s], altitude [m],
and position_vector [m].
**Calculation Process**
1. Discretize altitude profile
2. Get atmospheric properties at each altitude
3. Convert CAS to true air speed using:
- Pressure ratio (δ)
- Compressibility effects
- Equivalent air speed (EAS) conversion
4. Decompose velocity into components using:
- Fixed descent rate
- Sideslip angle
- Computed true air speed
**Major Assumptions**
* Constant calibrated air speed
* Constant descent rate
* Standard atmosphere model with temperature deviation
* Small angle approximations
* Quasi-steady flight
See Also
--------
RCAIDE.Framework.Mission.Segments
RCAIDE.Library.Mission.Common.Update.atmosphere
"""
# unpack
descent_rate = segment.descent_rate
cas = segment.calibrated_air_speed
alt0 = segment.altitude_start
altf = segment.altitude_end
beta = segment.sideslip_angle
t_nondim = segment.state.numerics.dimensionless.control_points
conditions = segment.state.conditions
# check for initial altitude
if alt0 is None:
if not segment.state.initials: raise AttributeError('initial altitude not set')
alt0 = -1.0 * segment.state.initials.conditions.frames.inertial.position_vector[-1,2]
# discretize on altitude
alt = t_nondim * (altf-alt0) + alt0
# determine air speed from calibrated air speed
RCAIDE.Library.Mission.Common.Update.atmosphere(segment) # get density for air speed
alt_data = segment.analyses.atmosphere.compute_values(alt,segment.temperature_deviation)
density = alt_data.density[:,0]
pressure = alt_data.pressure[:,0]
MSL_data = segment.analyses.atmosphere.compute_values(0.0,segment.temperature_deviation)
pressure0 = MSL_data.pressure[0]
if cas is None:
if not segment.state.initials: raise AttributeError('initial equivalent air speed not set')
air_speed = np.linalg.norm(segment.state.initials.conditions.frames.inertial.velocity_vector[-1,:])
else:
kcas = cas / Units.knots
delta = pressure / pressure0
mach = 2.236*((((1+4.575e-7*kcas**2)**3.5-1)/delta + 1)**0.2857 - 1)**0.5
qc = pressure * ((1+0.2*mach**2)**3.5 - 1)
eas = cas * (pressure/pressure0)**0.5*(((qc/pressure+1)**0.286-1)/((qc/pressure0+1)**0.286-1))**0.5
air_speed = eas/np.sqrt(density/MSL_data.density[0])
# process velocity vector
v_mag = air_speed
v_z = descent_rate # z points down
v_xy = np.sqrt( v_mag**2 - v_z**2 )
v_x = np.cos(beta)*v_xy
v_y = np.sin(beta)*v_xy
# pack conditions
conditions.frames.inertial.velocity_vector[:,0] = v_x
conditions.frames.inertial.velocity_vector[:,1] = v_y
conditions.frames.inertial.velocity_vector[:,2] = v_z
conditions.frames.inertial.position_vector[:,2] = -alt[:,0] # z points down
conditions.freestream.altitude[:,0] = alt[:,0] # positive altitude in this context
