I have taken a bit of time to test the CVS Cessna 172P, and have made a few observations. The observations were made flying v1.0.0 on Windows XP, but with last week's CVS aircraft. I do not have FG compiled on Windows, and my Linux machine doesn't have nearly the performance as my Windows machine--therefore someone (John Denker or Ron Jensen?) might want to reproduce these findings.
1) The approved engine for the C172P is the 160-horsepower Lycoming O-320, not the IO-320. The late model 172R and 172S have Lycoming IO-360 engines, but these are essentially the only *injected* Lycoming-powered 172 aircraft I know of (besides the Cutlass). I checked the XML file for the engine and the horsepower is correct, but the name of the engine file should still be changed in my opinion--simply in the name of accuracy. The current file name is "eng_io320.xml" but this is simply incorrect, as documented by the TCDS. 2) The static RPM on my Windows XP system (running v1.0.0 with the latest CVS aircraft) is right about 2204 RPM. However the FAA Type Certificate Data Sheet (TCDS) for the aircraft shows the acceptable range as 2300-2420 RPM. Again, I feel that this should be adjusted in the current model. I don't see 2300 RPM until the aircraft reaches about 50-55 knots IAS, while still on the runway. 3) I find that either the torque or P-factor (or both) modeling for this emulation is unrealistic. When climbing at 75 knots with the Turn Coordinator (TC) ball centered, the aircraft must be banked to the right in order to maintain heading. This is unrealistic, and I recall that I had previously solved the problem via adjustment of the aileron control values in the c172p.xml files. Unfortunately I cannot find my notes, but I see now that the <gain> field for the RIGHT aileron is set to -0.01745, while the LEFT aileron is set to 0.01745. I believe herein lies the problem, as I am confident that my previous work (in 2007) resulted in values that yielded much more accurate climb performance. The discussion about climb performance for this emulation has been quite interesting of late. Several people have made excellent points, and I share some of these concerns. However I would caution folks that in order for the data from the model to be comparable to that of the real aircraft, the climb must be coordinated. In the real aircraft, climb performance can be as much as 300-400 feet per minute off, simply by allowing the aircraft to skid in the climb. As P-factor tends to yaw the aircraft to the left in the climb, one must step on the right rudder to correct the resulting skid--otherwise climb performance suffers dramatically (in my experience). Thus anyone testing these models should insure that the climb is indeed coordinated with the TC ball centered. 4) The tachometer markings for the FG C172P are incorrect. As has been pointed out, the red line for the O-320 engine in this installation is 2700 RPM--thus there should be a marking at this value, and the green arc should extend to that point. Again, this does not affect performance, but I only mention it to point out the inconsistency in the current model. 5) Level in cruise flight at 2000 feet MSL after departing KSFO, my system is showing me that the engine is turning just over 2850 RPM at full throttle. This is 150+ RPM over red line, and obviously incorrect, as previously mentioned by myself and others. Part of the discrepancy in performance we are seeing in this emulation is due to the the "over-powered" nature of the model. There are various equations for calculating horsepower, but if one considers this version... HP = [(2pi)*(Torque)*(RPM)] / 33000 ...one can easily see that the only factor we can do anything about once the aircraft is in the air, is the engine RPM. Thus it should be apparent that increasing the RPM by any significant amount will indeed increase the horsepower. And while there isn't a horsepower indication that I could find in the property tree for the C172P, there is an indication of the thrust. At 2851 RPM the thrust produced is just over 306 pounds; while at about 2700 RPM the thrust is just under 275 pounds. So this represents about a 10-11% increase in thrust from a 150 RPM (5-6 percent) increase in engine speed--so it shouldn't be hard to imagine that the emulation isn't going to perform as the real aircraft does. Finally, I apologize for the lengthy post, but the point is that I think that many of the perceived issues with the C172P can be (at least partially) resolved by fine-tuning the engine model to give a more realistic representation of the actual aircraft. I don't feel that we are far off here though, and I have been working with Ron Jensen a bit on this very issue. TB ------------------------------------------------------------------------------ SF.Net email is Sponsored by MIX09, March 18-20, 2009 in Las Vegas, Nevada. The future of the web can't happen without you. Join us at MIX09 to help pave the way to the Next Web now. Learn more and register at http://ad.doubleclick.net/clk;208669438;13503038;i?http://2009.visitmix.com/ _______________________________________________ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
