From 9ab0125d7ded16345d18a85d53d60e7b786f5e8c Mon Sep 17 00:00:00 2001
From: Andrew Tridgell <andrew@tridgell.net>
Date: Sat, 2 Apr 2022 09:43:42 +1100
Subject: [PATCH] added eas2tas for airspeed tuning
---
mavextra.py | 33 +++++++++++++++++++++++++++++++++
1 file changed, 33 insertions(+)
diff --git a/mavextra.py b/mavextra.py
index 2b22c96a..ee7515a5 100644
--- a/mavextra.py
+++ b/mavextra.py
@@ -623,6 +623,39 @@ def EAS2TAS(ARSP,GPS,BARO,ground_temp=25):
tempK = ground_temp + 273.15 - 0.0065 * GPS.Alt
return sqrt(1.225 / (BARO.Press / (287.26 * tempK)))
+SSL_AIR_DENSITY = 1.225
+C_TO_KELVIN = 273.15
+ISA_LAPSE_RATE = 0.0065
+ISA_GAS_CONSTANT = 287.26
+SSL_AIR_TEMPERATURE = 288.15
+SSL_AIR_PRESSURE = 101325.01576
+
+def SimpleAtmosphere(alt_km):
+ REARTH = 6369.0
+ GMR = 34.163195
+
+ # geometric to geopotential altitude
+ h = alt_km*REARTH/(alt_km+REARTH)
+
+ if (h < 11.0):
+ # Troposphere
+ theta = (SSL_AIR_TEMPERATURE - 6.5 * h) / SSL_AIR_TEMPERATURE
+ delta = pow(theta, GMR / 6.5)
+ else:
+ # Stratosphere
+ theta = 216.65 / SSL_AIR_TEMPERATURE
+ delta = 0.2233611 * exp(-GMR * (h - 11.0) / 216.65)
+
+ sigma = delta/theta
+ return (sigma, delta, theta)
+
+def eas2tas(alt_m, groundtemp=25.0):
+ '''eas2tas from altitude in meters AMSL'''
+ (sigma, delta, theta) = SimpleAtmosphere(alt_m*0.001)
+ pressure = SSL_AIR_PRESSURE * delta
+ tempK = groundtemp + C_TO_KELVIN - ISA_LAPSE_RATE * alt_m
+ eas2tas_squared = SSL_AIR_DENSITY / (pressure / (ISA_GAS_CONSTANT * tempK))
+ return sqrt(eas2tas_squared)
def airspeed_ratio(VFR_HUD):
'''recompute airspeed with a different ARSPD_RATIO'''
--
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