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Re: [Jsbsim-devel] C-172 Performance --- More comments
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From: Curtis L. O. <cur...@fl...> - 2004-10-14 19:45:50
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Let me just chime in here with a comment. If users complain about the difference between 436 and 437 mph, then that shows a fundamental misunderstanding of your users. There are way too many variables in real life. Do you seriously believe that if you take up any real P-51 on any day to 25,000' you will get exactly 437 mph? Consider the weather variations (temp/press). Consider differences in engine adjustment/configuration and performance. Consider differences in fuel or weapons load. Consider small errors in instrumentation and sensors which can affect the indicated performance in the cockpit. There are enough variables here to easily make a difference of a couple percent in performance. Even taking the same plane up on two different days can yield substantial performance differences. This does not mean that accuracy doesn't count for anything, but nailing cruise speed to 10 significant digits doesn't necessarily mean much about the overall accuracy of the model. When the FAA evaluates the flight dynamics of advanced training simulators, they look at things like: (Oh and don't forget these test are all performed at specific altitudes, specific pressure, specific temperate, specific aircraft weight, and specific balance point.) - Time to accelerate on the runway from 0 to rotation speed. - Given a specific speed, what rate of climb does that yield. - What speed triggers the stall horn (clean, and landing configuration) - Engine rpm acceleration and deceleration rates - setup in trimmed level flight, increase throttle to full power, how much yoke pressure is required to maintain the original indicated airspeed. - similar test only go from zero flaps to full flaps and visa versa - trim for level flight at some particular altitude and speed. What values do you get for pitch angle, elevator deflection, trim tab deflection, rpm? - same thing but in landing configuration - How's your phugoid? Is the period and amplitude within a couple percent of the real aircraft? - Do a full aileron deflection roll. Does the roll rate match the real thing? - Do the same thing but in landing configuration. - roll overshoot, spiral stability, rudder response, dutch roll ... - set up a full rudder deflection steady state side slip at a particular altitude, heading, and speed. What angle wheel (aileron) input does that require? What is your beta angle? What is your roll angle? These are just a subset of what the FAA requires for Level 3 FTD certification. As you move up the food chain to higher levels of certification, they require many additional tests. Arguable, even if you pass all these tests you could still be way off in many areas, and under all the conditions that aren't tested, so ... It doesn't hurt to nail down cruise speed to microscopic tolerances, but if you do that without addressing basic performance in many of these other areas, you are really just fooling yourself into thinking you have something that is a lot better than it really is ... especially considering the amount of real life variability you are going to encounter. Regards, Curt. Bill Galbraith wrote: >Bravo! > >Bill > > > >>-----Original Message----- >>From: jsb...@li... >>[mailto:jsb...@li...] On Behalf >>Of Gregory Pierson >>Sent: Thursday, October 14, 2004 12:19 PM >>To: jsb...@li... >>Subject: Re: [Jsbsim-devel] C-172 Performance --- More comments >> >> >>David >> >> >> >>No offense, but this attitude will doom JSBSim to failure. >> >> >> >>When you start using excuses to lower the quality of your >>input data, doesn' t it invalidate all the high quality >>equations in JSBSim all of you have worked so hard on? >> >> >> >>Realistically, 90% of JSBSim's input parameters effect less >>than +/- 1% of the aircraft's performance. So why use them then? >> >> >> >>Both Ron and myself have created a lot of FS flight models. >>We found it really does pay off if you try to carry as high a >>precision as possible. The end product will really show this >>when your user flies it. We've also seen products with a "we >>don't need that much precision" that have been total disasters. >> >> >> >>Ron and myself go to extreme lengths to make the input data >>as accurate as possible. The people that fly our FS aircraft >>can tell the difference. If I release a P-51 that does 436mph >>instead of 437 at 25,000ft the users will scream. My >>competition uses Cessna stability derivatives for all their >>fighters, we try to calculate the aircraft's true stability >>derivatives. The users can and do feel the difference. But >>the real pay dirt is when they fly against each other online >>and start complaining about the same issues the real pilots >>experienced when flying these aircraft. >> >> >> >>The point I'm trying to make is the JSBSim developers need to >>aim high. If they want JSBSim to gain wider acceptance, >>possibly create a cottage industry for JSBSim consultants or >>used in a retail product, their attitude has to always be to >>go that extra mile with their data. >> >> >> >>Just my two cents from the cheap seats >> >> >> >>Gregoryp >> >> >> >> >> >>----- Original Message ----- >>From: "David Megginson" <dav...@gm...> >>To: <jsb...@li...> >>Sent: Thursday, October 14, 2004 4:28 AM >>Subject: Re: [Jsbsim-devel] C-172 Performance --- More comments >> >> >> >> >>>On Wed, 13 Oct 2004 21:12:00 -0500, Ron Freimuth >>><ron...@at...> >>> >>> >>wrote: >> >> >>>> As far as 'accuracy goes', I'd disagree that ones flight model >>>> >>>> >>doesn't >> >> >>>>have to be that accurate, 'since real AC vary all over >>>> >>>> >>the place'. >>Seems >> >> >>>>to me that a good flight model should represent the ideal >>>> >>>> >>AC. One >> >> >>>>fresh >>>> >>>> >>out >> >> >>>>of the factory. Not one misrigged, with inaccurate instruments, >>>>etc. Ideally, a pilot could compare an accurate PC AC >>>> >>>> >>with what he >> >> >>>>flies. >>>> >>>> >>And >> >> >>>>see how much has real AC has deteriorated from a factory fresh >>>>model. >>>> >>>> >>>My point was slightly different -- I'm suggesting that we try to >>>choose an appropriate precision for our sample. For example, my >>>height probably changes by as much as 5 mm from morning to >>> >>> >>evening, so >> >> >>>there's no point measuring it to within 0.0001 mm, or even >>> >>> >>0.1 mm. Two >> >> >>>brand-new planes straight off the assembly line are still going to >>>handle differently, so even in the best case, there's no point >>>measuring more accurately than the likely variation between them. >>> >>>As a pilot, you must notice this problem with flight training -- >>>student pilots have to plan a cross-country predicting our exact >>>cross-wind correction, to the degree, our exact groundspeed, to the >>>knot, and our exact ETA at each checkpoint, to the second. That's >>>good practice in using the E6B, of course, and it's a good >>> >>> >>reminder to >> >> >>>be aware of winds aloft for fuel management, but it has >>> >>> >>very little to >> >> >>>do with actually flying a plane cross-country. I've never seen the >>>winds aloft *exactly* the same as forecast, and as you >>> >>> >>mentioned, the >> >> >>>plane becomes lighter as it goes anyway, so your TAS at the >>> >>> >>end of a >> >> >>>3-4 hour leg is going to be a few knots higher than your TAS at the >>>start. >>> >>> >>>All the best, >>> >>> >>>David >>> >>>-- >>>http://www.megginson.com/ >>> >>> >>> >>> -- Curtis Olson http://www.flightgear.org/~curt HumanFIRST Program http://www.humanfirst.umn.edu/ FlightGear Project http://www.flightgear.org Unique text: 2f585eeea02e2c79d7b1d8c4963bae2d |
