These example tables are from FlightGear's C172P aircraft: This is useful for seeing how the settings and tables play out under various conditions and fine-tuning the settings. Note that several of the values mentioned above can be viewed while FG is running, in the property tree under /fdm/jsbsim/propulsion/engine. Variable pitch propellers and constant speed propellers bring different factors into play. 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. At higher pitch angles Ct may have a positive slope or be flat in the lower J range.Ĭt/Cp gives the efficiency (eta), and propeller shape and general design give each propeller a distinctive efficiency curve. Cp and Ct can be negative this indicates the drag induced by the prop when the airspeed is relatively fast compared with prop RPM. n = rotations per second (1/s) (note RPS, not RPM)įor a typical propeller, both Cp and Ct are downward sloping curves that reach 0 when J is somewhere in the range 0-4 (depending on blade angle and other factors).Efficiency: eta = Ct/Cp * v/(n*D) (or equivalently, eta = Ct/Cp * J ).Relevant formulas relating the variables in the tables (and metric system units): Propellors for F3D Models explains the theory and has formulas and many graphs showing the relationship between J, Ct, and Cp. The dependent variable is the coefficient of thrust (Ct) for the C_THRUST and the coefficient of power (Cp) for C_POWER.įor variable pitch propellers, it is possible to give a two-dimensional table, showing Ct and Cp for different J and different pitch angles of the propeller. The independent variable for both tables is Advance Ratio (J). The C_THRUST and C_POWER tables are required. This allows to specify different sense and P factor values for each propeller of the model while using the same definition file for all the propellers. The parameters and must be specified at the parent level i.e. It is specified in the parent tag of the propeller.Ī multiplier for the coefficients of thrust (multiplies the dependent variable in the C_THRUST table by this factor).Ī multiplier for the coefficients of power (multiplies the dependent variable in the C_POWER table by this factor). See important note below regarding a JSBSim bug affecting sense and the direction of the resulting gyroscopic moment. Sense is specified in the parent tag of the propeller. Maximum rpm target for constant speed propeller.ġ = constant speed mode, 0 = manual pitch mode.ĭirection of rotation (1= clockwise as viewed from rear, -1=counter-clockwise as viewed from rear). Minimum rpm target for constant speed propeller. See the Moments of inertia reference page and list of conversion factors for different units for moment of inertia.) This can be english units, slug & feet^2:įor a thin rod of mass m (kg) and diameter D (meters) spinning about its center, the formula is m*D^2/12.
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