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JSBSim Thrusters: Difference between revisions
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→Notes: details about propeller tables
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=== Notes === | === Notes === | ||
* Two tables are needed. One for coefficient of thrust (Ct) and one for coefficient of power (Cp). | * Two tables are needed. One for coefficient of thrust (Ct) and one for coefficient of power (Cp). | ||
The independent variable for the tables is Advance Ratio (J) and the dependent variable is the coefficient (Ct or Cp). [http://www.mh-aerotools.de/airfoils/pylonprops_1.htm Propellors for F3D Models explains the theory] and has [http://www.mh-aerotools.de/airfoils/pylonprops_2.htm formulas] and [http://www.mh-aerotools.de/airfoils/pylonprops_3.htm many graphs] showing the relationship between J, Ct, and Cp. | |||
Relevant formulas: | |||
* Thrust: T = Ct/(rho * n^2 * D^4) | |||
* Power: P = Cp/(rho * n^3 * D^5) | |||
* Advance Ratio: J = v/(n*D) | |||
* Efficiency: eta = Ct/Cp or, equivalently eta = v/(n*D) | |||
In the formulas | |||
* Ct = coefficient of thrust | |||
* Cp = coefficient of power | |||
* v = velocity of aircraft (m/s) | |||
* D = diameter of propeller disk (m) | |||
* n = rotations per second (1/s) (note RPS, not RPM) | |||
* rho = density of air (kg/m^3) | |||
* P = power (W) | |||
* T = thrust (N) | |||
For a typical propeller, both Cp and Ct are downward sloping curves that reach 0 near J=1. Typically Ct is slightly greater than than Cp for small J, crosses (is equal to) Cp at some point (typically, when J is between 0.6 and 0.8), and then remains smaller than Cp for larger J. Cp and Ct can be negative (typically for J > 1, approximately); this indicates the drag induced by the prop when the airspeed is relatively fast compared with prop RPM. | |||
Ct/Cp gives the efficiency (eta), and propeller shape and general design give each propeller a distinctive [http://www.mh-aerotools.de/airfoils/pylonprops_3.htm efficiency curve]. For fixed-pitch propellers, the propeller is generally designed to reach peak efficiency either at climb velocity & RPM, cruise velocity and RPM, or some compromise between the two. [http://en.wikipedia.org/wiki/Controllable_pitch_propeller Variable pitch propellers] and [http://en.wikipedia.org/wiki/Constant_speed_propeller constant speed propellers] bring different factors into play. | |||
* Two tables are optional. They apply a factor to Ct and Cp based on the helical tip Mach. | * Two tables are optional. They apply a factor to Ct and Cp based on the helical tip Mach. | ||
* Several references were helpful, here: | * Several references were helpful, here: | ||
**Barnes W. McCormick, "Aerodynamics, Aeronautics, and Flight Mechanics", Wiley & Sons, 1979 ISBN 0-471-03032-5 | **Barnes W. McCormick, "Aerodynamics, Aeronautics, and Flight Mechanics", Wiley & Sons, 1979 ISBN 0-471-03032-5 | ||
**Edwin Hartman, David Biermann, "The Aerodynamic Characteristics of Full Scale Propellers Having 2, 3, and 4 Blades of Clark Y and R.A.F. 6 Airfoil Sections", NACA Report TN-640, 1938 (?) | **Edwin Hartman, David Biermann, "The Aerodynamic Characteristics of Full Scale Propellers Having 2, 3, and 4 Blades of Clark Y and R.A.F. 6 Airfoil Sections", NACA Report TN-640, 1938 (?) | ||
**Various NACA Technical Notes and Reports | **Various NACA Technical Notes and Reports | ||
== FGRotor == | == FGRotor == | ||
FGRotor moodels a helicopter rotor. | FGRotor moodels a helicopter rotor. | ||