Traditionally the industry has never used AoA because for any specific weight there is a direct, corresponding relationship between CAS and AoA regardless of altitude. "For every air speed - as indicated on the Air Speed Indicator - there is a corresponding angle of attack at which level flight can be maintained (provided the weight of the aeroplane does not change)" - Kermode, Mechanics of Flight, 1962.
That's the part in red I'm not sure about, but I don't have the knowledge so I'll keep it quiet, maybe someone would explain more ... ?
The best would be to flight test these lower speeds during a ferry at FL something, but I still need/like my job.
PJ2/CONF iture
within reason, (3 decimal places @ representative crz AoA, α ...) D.P.Davies was more or less correct in his statement.
Angle of attack is linearly correlated to section lift coefficient, Cl below separation angles of attack. The correlation of section lift coefficient,
a, is dependent on Re, and also ~Mcrit.
Reynolds is dependent on density & viscosity...
Re=(ρ.V.L)/μ;
= (density x mean fluid velocity x characteristic linear dimension)/dynamic viscosity of the fluid.
Total lift is also dependent on density;
L=Cl.0.5ρ.V^2.S
Density is a numerator in both cases, so is not self cancelling with variations...
At low AoA, there is almost no difference in the Cl for Re from 1,000,000 to 8,000,000, but at high AoA, the higher Re results in higher Cl for a given AoA.
At high subsonic MNo, the compressibility effects alter drag (drag divergence) but also alter Cl, for a given AoA, but the outcome is dependent on geometry of the foil.
remaining at normal cruise AoA and below drag divergence, D.P.Davies statement is reasonable that altitude doesn't effect AoA for a given CAS (restated). Humidity does have an effect but is minimal for normal operational conditions.
.................
Ideal Gas Law:
ρ = p / (R * T)
where: ρ = density kg/m3
p = absolute pressure Pa, N/m2
R = individual gas constant J/kg K
T = absolute temperature K
D=((P/(Rd*T))*(1-0.378*Pv/P)
where: D = density, kg/m3
Pd = pressure of dry air (partial pressure), Pascals
Pv= pressure of water vapor (partial pressure), Pascals
P = Pd + Pv = total air pressure, Pascals ( multiply mb by 100 to get Pascals)
Rd = gas constant for dry air, J/(kg*degK) = 287.05 = R/Md
Rv = gas constant for water vapor, J/(kg*degK) = 461.495 = R/Mv
R = universal gas constant 8314.32 (in 1976 Standard Atmosphere)
Md = molecular weight of dry air 28.964 gm/mol
Mv = molecular weight of water vapor 18.016 gm/mol
T = temperature, deg K deg C + 273.15
Links:
http://www.efm.leeds.ac.uk/CIVE/CIVE3400/stvenant.pdf
Equations - Air Density and Density Altitude
luizmonteiro - Altimetry Calculations / E6B Emulator
JavaFoil
Dynamic, Absolute and Kinematic Viscosity
References:
Batchelor, G. (2000). Introduction to Fluid Mechanics
Clancy, L.J. (1975), Aerodynamics, Pitman Publishing Limited, London. ISBN 0 273 01120 0
Kundu, P.K., Cohen, I.M., & Hu, H.H. (2004), Fluid Mechanics, 3rd edition, Academic Press
Ockendon, H. & Ockendon J. R. (1995) Viscous Flow, Cambridge University Press. ISBN 0521458811
Reynolds, Osborne (1883). "An experimental investigation of the circumstances which determine whether the motion of water shall be direct or sinuous, and of the law of resistance in parallel channels". Philosophical Transactions of the Royal Society 174: 935–982