Re: [Flightgear-devel] DC-3 ground handling. Fixed?
Charlie Hotchkiss wrote: So, just for discussion's sake and noting that nobody with detailed knowledge of and experience with this aircraft has weighed in, I have questions. Isn't the L/D ratio at high angles of attack different (poorer) when in ground effect? Actually, it's better. Depending on which reference you look at, either induced drag is lower or lift is higher while in ground effect (YASim models the latter, FWIW). Either effect will have the effect of increasing the L/D ratio. If you can accelerate to above stall speed rolling on all three wheels, what happens when you start to lift. Wouldn't you expect the aircraft tend to nose up into stall? Be careful here. :) Stall speed is a myth. Aircraft stall at a given AoA, not speed. The reason stall speed is useful to talk about is that an AoA has a 1:1 correspondence with a speed required to keep the aircraft in level flight at that AoA. But that's only true in the air; aircraft on the ground have an AoA determined by wheel geometry, not elevator force, so stall speed is kinda meaningless in this context. Basically, as the DC-3 accelerates down the runway (assuming the tail wheel stays on the ground) it will eventually reach a speed where the lift generated by the wings at the 10 degree on ground AoA exceeds the weight, and the gear will lift off. At that point, the aircraft is in the air and will assume whatever AoA the elevator trim (or pilot) tells it to. Does this help? Andy -- Andrew J. RossNextBus Information Systems Senior Software Engineer Emeryville, CA [EMAIL PROTECTED] http://www.nextbus.com Men go crazy in conflagrations. They only get better one by one. - Sting (misquoted) ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
re: [Flightgear-devel] DC-3 ground handling. Fixed?
Andy Ross writes: I tried that, and it's an improvement, but the tailwheel seems to slide sideways too easily. Really? When I tried it, I couldn't turn against the tail wheel at all, even with full braking applied. The tail wheel didn't move until 50 kts or so when it lifted; then the plane became squirrely, as might be expected. Under what conditions did you see that behavior? If it's sliding at speed, are you absolutely sure that's wrong? I saw it in a slow, taxiing turn at around 10kt or less, but I had done the modification myself before you posted yours. I'll try it with exactly your suggestion. I noticed that too -- this will be harder to implement. There's no feedback for the steering direction right now -- castering is implemented by simply assuming that steer == velocity. The code would need to calculate a steer angle, test for forward, and set the castering bit to false only if that is true *and* the wheel lock is on. That ties a bunch of code sections together in a kinda messy way. For now, perhaps locking the tailwheel could automatically snap it to 0 deg steering angle. We could even handle that in the input bindings, if there were a pseudo-steering property for the tailwheel. All the best, David -- David Megginson, [EMAIL PROTECTED], http://www.megginson.com/ ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] DC-3 ground handling. Fixed?
Andy, Bingo. This is a bug in the propeller code; I apparently introduced a FPU explosion when the thing is going backwards. It has nothing to do with the aero or ground modelling, so I was looking in the wrong place. I'll get this fixed. (I'm pretty sure it used to work -- I remember doing hammerhead stalls in the A-4 when testing). Sorry I only realized this just before I actually mentioned it. I used to test in a from-the-back view, in which the window goes black when the divergence happens, I had to test it again with the normal out-of-the-cockpit view to see the 1000kt+. Really? When I tried it, I couldn't turn against the tail wheel at all, even with full braking applied. The tail wheel didn't move until 50 kts or so when it lifted; then the plane became squirrely, as might be expected. Under what conditions did you see that behavior? If it's sliding at speed, are you absolutely sure that's wrong? I can confirm this, the plane moves straight whatever you do as long as the tailwheel stays on the ground. This is definitely far closer to reality than the free swinging we had before. Modeling skidding and sliding friction would be more realistic, but not that important. Thanks for sorting this out so quickly, I will investigate the problems more thoroughly next time... Andras === Major Andras e-mail: [EMAIL PROTECTED] www:http://andras.webhop.org/ === ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] DC-3 ground handling. Fixed?
I wrote: Major A wrote: I think the main problem really is the rapid increase in airspeed, which is unnatural, and doesn't occur if both engines are used. Bingo. This is a bug in the propeller code [...] (I'm pretty sure it used to work -- I remember doing hammerhead stalls in the A-4 when testing). Don't say it. I know. The A-4 doesn't have a propeller. OK, I found an explosion case in the propeller code which could be triggered by a propeller turning backwards very slowly. That's pretty much consistent with a aircraft with a stopped engine, so I'm hopefully this is it. It's been checked in (this was obvious enough to check in based on code inspection only), so see if this fixes your problem. I make no promises about actually getting off the ground successfully on one engine, however. :) Andy -- Andrew J. RossNextBus Information Systems Senior Software Engineer Emeryville, CA [EMAIL PROTECTED] http://www.nextbus.com Men go crazy in conflagrations. They only get better one by one. - Sting (misquoted) ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] DC-3 ground handling. Fixed?
Andy Ross wrote: [I combined a bunch of responses... Once the tailwheel leaves the ground, it's squirrely but controllable. This is not doubt bad form, but I found that holding the stick back to keep the wheel firmly on the ground during the takeoff roll until the aircraft took off on its own was the safest way to take off. Andy, Perhaps I'm showing some ignorance here (I'm certainly not a pilot, much less an expert), but isn't the induced drag in that situation so large as to preclude reaching flying speed? The wings acting at that angle much like a drag brake? I read somewhere that pilots deliberately pushed forward on the column to lift the tail at a specific speed in order to reduce drag and allow speed to build up. Regards, Charlie H. - C makes it easy to shoot yourself in the foot; C++ makes it harder, but when you do, it blows away your whole leg. - Bjarne Stroustrup ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] DC-3 ground handling. Fixed?
Charlie Hotchkiss wrote: Perhaps I'm showing some ignorance here (I'm certainly not a pilot, much less an expert), but isn't the induced drag in that situation so large as to preclude reaching flying speed? The wings acting at that angle much like a drag brake? I read somewhere that pilots deliberately pushed forward on the column to lift the tail at a specific speed in order to reduce drag and allow speed to build up. In the DC-3 as modelled, the pitch angle of the wings while resting on the ground is only something like 10 degrees. That's high, but significantly lower than the stall angle of 14 degrees*. If the aircraft can fly at that AoA under its own power, it can certainly accelerate to that speed. It won't get there very quickly, and will use more runway doing it (a big problem, unless you happen to be at SFO), but it definitely works. Now, that number comes ruler and protractor work on a 3-view diagram. It's possible that I missed a measurement, and that the ground AoA should be much larger. It also assumes a build in wing incidence of 0, which may not be true on the real aircraft (I couldn't find data on this stuff). But if it's really 10 degrees, then the behavior is pretty much correct. I suspect the real reason that this isn't done in practice is the runway length issue, along with the fact that the tail wheel isn't as strong as the mains and probably doesn't like being dragged along as much. Anyone have a good reference? * This number is a guess, too. Andy -- Andrew J. RossNextBus Information Systems Senior Software Engineer Emeryville, CA [EMAIL PROTECTED] http://www.nextbus.com Men go crazy in conflagrations. They only get better one by one. - Sting (misquoted) ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] DC-3 ground handling. Fixed?
OK, I found an explosion case in the propeller code which could be triggered by a propeller turning backwards very slowly. That's pretty much consistent with a aircraft with a stopped engine, so I'm hopefully this is it. It's been checked in (this was obvious enough to check in based on code inspection only), so see if this fixes your problem. It does. I make no promises about actually getting off the ground successfully on one engine, however. :) I managed to do it (once), but I wouldn't like to be a passenger on that plane. Also, I can't reproduce the spinning problem, seems like your prop patch did the job. Thanks a lot! Andras === Major Andras e-mail: [EMAIL PROTECTED] www:http://andras.webhop.org/ === ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] DC-3 ground handling. Fixed?
Andy Ross writes: Now, that number comes ruler and protractor work on a 3-view diagram. It's possible that I missed a measurement, and that the ground AoA should be much larger. It also assumes a build in wing incidence of 0, which may not be true on the real aircraft (I couldn't find data on this stuff). But if it's really 10 degrees, then the behavior is pretty much correct. The tech drawings at http://www.douglasdc3.com/dc3tec/dc3tec.htm (especially the big one at the bottom) suggest to me an angle of incidence of 2-3 degrees, but I haven't printed them out and measured. All the best, David -- David Megginson, [EMAIL PROTECTED], http://www.megginson.com/ ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] DC-3 ground handling. Fixed?
* [EMAIL PROTECTED] (David Megginson) [2002.05.23 13:59]: Andy Ross writes: The tech drawings at http://www.douglasdc3.com/dc3tec/dc3tec.htm (especially the big one at the bottom) suggest to me an angle of incidence of 2-3 degrees, but I haven't printed them out and measured. As far as measuring aircraft dimensions and such, I use Gimp's measuring tool to measure things. Measure a known distance and record the length in pixels, then you can measure whatever else you want and do the pixel to feet conversions without haven't to print anything out. Pythagoras is your friend. :-) -- Cameron Moore [ Why is the man who invests all your money called a broker? ] ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] DC-3 ground handling. Fixed?
Andy, I'm really just exploring near stall modeling issues so that our simulation can be improved. Anyway you can get a bird in the air is reasonable, especially if it shows up weaknesses in modeling. So, just for discussion's sake and noting that nobody with detailed knowledge of and experience with this aircraft has weighed in, I have questions. Isn't the L/D ratio at high angles of attack different (poorer) when in ground effect? If you can accelerate to above stall speed rolling on all three wheels, what happens when you start to lift. Wouldn't you expect the aircraft tend to nose up into stall? Regards, Charlie H. Cameron Moore wrote: * [EMAIL PROTECTED] (David Megginson) [2002.05.23 13:59]: Andy Ross writes: The tech drawings at http://www.douglasdc3.com/dc3tec/dc3tec.htm (especially the big one at the bottom) suggest to me an angle of incidence of 2-3 degrees, but I haven't printed them out and measured. As far as measuring aircraft dimensions and such, I use Gimp's measuring tool to measure things. Measure a known distance and record the length in pixels, then you can measure whatever else you want and do the pixel to feet conversions without haven't to print anything out. Pythagoras is your friend. :-) -- Cameron Moore [ Why is the man who invests all your money called a broker? ] ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel -- C makes it easy to shoot yourself in the foot; C++ makes it harder, but when you do, it blows away your whole leg. - Bjarne Stroustrup ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
