Re: [Flightgear-devel] Autopilot VOR-Tracking Algorithms
At 05:31 PM 7/10/2002 -0400, you wrote: Curt and I have been having a discussion offline about algorithms for NAV mode on a simple autopilot (like those typically found in a light Cessna or Piper). The current autopilot does not have a working NAV mode -- it was just a quick kludge, with ability to correct for a crosswind. What algorithms are commonly used to get and keep the CDI centered in a simple AP? It should be easy enough to start with the rate and direction of CDI deviation from center. I'd say the simplest to implement, while still being reasonably usable, would be a proportional plus derivative law. Command a heading equal to the course, plus a factor times the CDI deflection, plus a factor times the time derivative of the deflection; with the right coefficients, that will damp out any overshoot. Provide an intercept mode that will follow a pilot-selected heading until the CDI comes off the peg. I flew a Cherokee back around 1970 that had an unbelievably crude nav mode: it was a single-axis autopilot that did nothing but command a bank angle proportional to the CDI deflection, truncated at about 15 degrees. If the needle was on the peg, the airplane would fly in lazy circles forever -- but if you did the intercept manually, it worked surprisingly well. rj ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] DC-3 takeoff roll: partial solution
At 02:47 PM 5/22/2002 -0700, you wrote: David Megginson wrote: 1. According to the author, at least, differential braking is bad form while taxiing the DC-3; you should use differential power instead except for very tight turns. I'll buy that. But working dual throttles during the takeoff and landing rolls can't possibly be a good idea, right? In that regime, you're still stuck with rudder and braking only. During the landing roll (with no significant prop wash), you're stuck with braking only. 2. Maintaining a straight heading is hard during the early part of the takeoff roll, but the text describes S-curves rather than violent spinning as the problem for inexperienced pilots. Is that with or without braking being applied? I can confirm that I execute lots of S curves during takeoff in the DC-3 when using the brakes method. It only spins violently when you try to correct yaw divergence with a flapping rudder. Just to clarify what I said earlier: the reason that it looks like a rudder problem is that turning the plane a little bit with the rudder is possible. But once it is pointed little bit away from the velocity vector, it begins turning *farther* away very rapidly. If you don't correct this immediately, the aircraft will rapidly be so far out of whack that the rudder is incapable of correcting the yaw. Thus, what started out as a tiny rudder input diverges into a ground loop. But it's caused by a *lack* of rudder authority to correct the problem, not by too much authority causing it. Does that make more sense? [snip] Differential braking should be kept to a minimum in any airplane, for two reasons: (1) An airplane is a really lousy automobile. It has about as little undercarriage as it can get away with (one has only to look at pictures of an airplane and a truck scaled to the same size to realize this), and every brake application is hard on its pitiful little brakes. (2) Differential braking tends to scrub rubber off some very expensive tires. So differential power becomes the steering method of choice in airplanes that have it available. Light taildraggers generally have steerable tailwheels, and being single-engined, they always have some prop blast over the tail; consequently they're not very hard to steer in the takeoff roll. Larger taildraggers don't have steerable tailwheels because the steering forces would require powered controls which were not in use when they were designed. In the Gooney Bird one must line up on the runway, lock the tailwheel, and hold the wheel firmly back until there is full tail surface control. Prior to that point, you aren't really steering a heading: you're just holding yaw rate to a minimum. The airplane will turn somewhat in a crosswind; this can be dealt with to some extent by judiciously positioning and aiming the airplane before starting the roll. The divergence you mention is present in a real taildragger; it's just a basic instability in the yaw axis resulting from most of the weight being supported in front of the cg. When the fuselage is misaligned with the direction of motion, the side force on the wheels is destabilizing. I don't know exactly how the tailwheel lock is implemented in the DC-3; in the AT-6, the last couple of inches of aft stick travel center and lock the wheel. It's an ideal arrangement, because if you don't have the stick back the tailwheel won't do you any good anyway. rj ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
RE: [Flightgear-devel] FDMs and external atmosphere
At 07:36 AM 5/15/2002 -0400, you wrote: Jon Berndt writes: Yes. We've got hooks in JSBSim to add in the effects of turbulence, but the math model driver for turbulence can be complicated. It's being worked on, albeit slowly. When you're ready, let me know, and I'll add a normalized turbulence value (0:1) to FGEnvironment. Ditto for YASim (i.e. I'll add it as soon as *any* FDM supports turbulence). By the way, one of my more immediate goals is adding variable winds as a complement to gusting winds. I might also add variability for up/down drafts. It would, indeed, be nice to have a vertical velocity model for simulating soaring flight. I'm still trying to run down stability derivatives for my sailplane! rj ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] Cessna 310 flaps?
At 09:13 PM 2/28/2002 -0500, you wrote: Can someone describe to me where the flaps are located on the body of a Cessna 310? I haven't been able to see them clearly in any of the photos I've found on the Web, and the two sets of 3-views I have don't indicate them. The ailerons are on the outside of the trailing edge of the wings, ending at the wingtip fuel tanks; the flaps might be beside them, ending at the nacelles, they might be tucked in between the nacelles and the body, or there might even be leading-edge flaps. The C310, like the DC-3, has split flaps. Only the bottom part of the airfoil hinges down; from above, you can't see them at all. They extend from the fuselage to the aileron. rj ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] Autonomous helicopter
At 12:17 PM 2/6/2002 -0800, you wrote: http://web.mit.edu/newsoffice/nr/2002/robochopper.html Their ground station seems to have telemetry but no visualization ... They would seem to be a bit behind Georgia Tech: http://avdil.gtri.gatech.edu/AUVS/IARCLaunchPoint.html rj ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] DME FYI
At 05:39 PM 1/25/2002 -0500, you wrote: Alex Perry writes: Also, most aircraft make a noise when seriously uncoordinated (FGFS does not). We can, though -- what kind of a noise should it be? Kind of a fluttering noise; lots of burbling. Hey, how about dual air jets to blow air on one cheek in an open cockpit...;-) rj ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] ILS/Runway Heading
At 07:28 PM 1/22/2002 -0800, you wrote: Ralph Jones wrote: snip The setting of the OBS is immaterial on an ILS, because it does not work like a VOR... snip Actually (and much to my surprise) in some airplanes (for example the 737) it apparently does matter. I don't recall the details now, but I know of a case where a mis set instrument caused lots of confusion on an autopilot coupled ILS approach (in a full motion simulator, on a check ride), resulting in a missed approach, and a second check ride! The autopilot turned to the selected heading and departed the already captured LOC as I recall hearing it. (This still seems very strange to me too!!) If you are interested I can try to get the details for you. Come to think of it, that does make sense with a coupled autopilot: it would be the only way for the system to know what the base course is. The A/P would turn to the OBS course first and then start watching drift. It might wind up tracking eventually, by computing a gigantic wind correction angle! rj ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
RE: [Flightgear-devel] Why not use FS2002 aircraft files?
At 10:49 PM 1/4/2002 -0600, Jon S. Berndt wrote: This assumes that MS is doing things correctly and/or the way things should be done. This is an invalid assumption. This is one of the reasons I, personally, wanted to begin writing an FDM. A heavy assumption indeed. The MSFS flight model is crude at the logic level, and no amount of knob-twisting in the .air files can make it work right. Uncoordinated flight is a good example. It seems to work like this: heading gets incremented based on a coordinated turn at the existing airspeed and bank angle. Then the rudder position is compared to a correct position, and the nose is yawed off the flight path by an angle proportional to the error. The correct position seems to be a pulse of size determined by the roll rate -- in other words, a canned sequence. Lateral lift component is not modeled -- i.e., no matter how much uncoordinated rudder is applied, the airplane continues along the flight path determined by the bank angle history. It is impossible to make a skidding turn (which includes fine heading adjustments with rudder on an ILS) or to do a slip. Applying rudder without aileron pressure will turn the airplane, but only because another canned sequence applies a bank input; this bank actually controls the turn. Worse yet, as far as I can tell, variation of induced drag with angle of attack isn't modeled. And the seaplane model is beyond belief. An airplane on the water sits in a fixed position and heading regardless of wind and power setting until the throttle setting exceeds a threshold; then it starts to accelerate. Only then can it be turned. In other words, all they did was to take the ground model and add a gentle bobbing motion in the vertical axis. On the plus side, there are some lovely exterior graphics and some pretty good panels. But I don't think any part of the MSFS models could be used to determine flight performance of the aircraft. rj ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] Skyglow
At 04:22 PM 12/14/2001 +0200, you wrote: Once upon a time, you were sitting and writing: Would it be possible to model skyglow in FlightGear? That way you could see when you're approaching a city, even when you can't see it for mountains etc. Interesting question. You could either try volumetric fog rendering (yeah sure- on my GeForce9000), or seek for the right OpenGL technique. I'm not a big expert in such phenomena (so correct me if I'm wrong), but I guess fog and particles (and clouds) are the reason for the sky glow. One can render a thin yellow cloud layer to create a nice illusion. The city itself is a huge lightsource which should be used to illuminate the terrain (and objects) around. Hope it helps to define a more accurate problem :) You don't need clouds or fog; even clear air has enough scattering to make skyglow visible over the horizon. Quite a lovely effect, and it would be worth some programming effort. As you say, it would take some ultra-streamlined rendering. I'm doing an animation project on a .45 auto pistol, and the rendering times on the muzzle blast are pretty hefty. rj ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] Stall horn question
At 03:10 PM 11/28/2001 -0600, you wrote: David Megginson writes: Andy Ross writes: On the ground, gravity holds it down (open), so the horn is off. Now there's a good practical joke -- stick the horn tab on with a bit of duct tape. As long as they remember their pilot training and don't hurt themselves ... They won't if they follow the preflight inspection routine...flicking the tab is part of it. One reason Cessna changed from the tab to the passive horn is that a bug impact could gum up the tab. rj ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
