2011/6/19 Jon S. Berndt <jonsber...@comcast.net>: > Hal, > > Oops! Sorry I attributed code to you that came from somewhere else. :-) I > now seem to recall experimenting with your model at that time - actually > replacing the older one in JSBSim cvs with yours, then adding the estimated > propwash effects. > > Hal wrote: > > These were never in any of the code I worked with and were > removed before I started working on the FDM. My current Cmde function looks > like this: > > <function name="aero/coefficient/Cmde"> > <description> Pitch_moment_due_to_elevator </description> > <product> > <property> aero/qbar-psf </property> > <property> metrics/Sw-sqft </property> > <property> metrics/cbarw-ft </property> > <property> fcs/elevator-pos-rad </property> > <table> > <independentVar> velocities/mach </independentVar> > <tableData> > 0.0000 -0.9 > 0.66 -0.6 > 0.74 -0.4 > 1.0000 -0.05 > </tableData> > </table> > </product> > </function> > > This is using the qbar-psf which is not influenced by prop > wash. The Cmde function Jon has above has a lookup table that goes from MACH > 0 to MACH 2 in a linear fashion. This looks like something intended for a > supersonic aircraft and is not what I would expect from a subsonic aircraft. > The table I am using goes from MACH 0 to MACH 1 and has a strong inflection > at MACH 0.74 which is unlike the one in Jon's function since it is > non-linear. > > Yes, the above mach effects table looks better. I can't remember where that > came from. > > ... > > Jon thanks for the above code. I will look into integrating > this into the current P-51D. Also shouldn't the same sort of thing happen > with the rudder? > > Yes - in fact, in the version I have in JSBSim cvs, the modified qbar is > used for: CLde, Cldr, Cmde, and Cndr. I haven't tested any of these for > validity, though. > > And Jon do you have any ideas on how to go about writing a > function to implement downwash pitch moment affects? > > Yes, that's another thing that could be done. I've thought about that > sometimes, too. When the wing is generating lift, the airflow is being > deflected downward behind the wing, and so the alpha that the tail sees is > affected, since the normal airflow has been given an additional component in > the body Z direction (downward from the pilot perspective). Also, if there > is an appreciable pitch rate, the alpha at the tail is affected. Finally, if > the propeller is producing thrust, then there is that affect, too. So, how > do we calculate the alpha at the horizontal tail? > > I'm making this up right here as we go, but here are my thoughts. First, > calculate the wind velocity that the H. tail sees: > > U_adjusted = U + U_prop > > Next, the Z axis velocity that the H. tail sees: > > W_adjusted = W + q*ht_arm - i_wing > > Where W is the aircraft Z axis wind velocity at the CG, q is the pitch rate, > ht_arm is the distance from the CG to the horizontal tail, and i_wing is the > induced velocity produced by the wing. At zero lift i_wing would be zero, > and at lift=weight (such as at cruise) the i_wing value would be some value > - and it might not be trivial to calculate what that would be, though you > might be able to estimate it to produce a plausible qualitative effect. > There is a NASA paper that might be helpful in calculating this: "The > Calculation of the Induced Velocity Field of a Wing". It is the translation > of a technical paper by Klaus Gersten. It is NASA Technical Translation TT > F-12, 436, and you can download it at this URL: > http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19690023298_1969023298. > pdf. > > Anyhow, then, of course, the calculation is just this: > > alpha_tail = atan2(W_adjusted, U_adjusted) > > Maybe I've gone wrong somewhere here, but something similar might work. > Also, in situations like a flat spin or tail slide this probably falls > apart! >
To improve our bibliography I suggest to add this paper "Prediction of the effects of propeller operation on the static longitudinal stability of single-engine tractor monoplanes with flaps retracted" - J. Weil, W. C. Sleeman - NACA Report 941 available at this URL: http://hdl.handle.net/2060/19930092006 Figures 12 and 13 are of particular interest : you can see the influence of propwash on CL and Cm. On figure 12, the slopes of CL and Cm for a tail incidence of 0 deg are roughly increased by 30% and 60% respectively. Bertrand. ------------------------------------------------------------------------------ EditLive Enterprise is the world's most technically advanced content authoring tool. Experience the power of Track Changes, Inline Image Editing and ensure content is compliant with Accessibility Checking. http://p.sf.net/sfu/ephox-dev2dev _______________________________________________ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
