Re: [Flightgear-devel] Aerobatics using flight gear and JSBSim
Looks like a spin, does not it ? ;) That is with stock JSBSim FDM ... attachment: spin.jpg ___ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
Re: [Flightgear-devel] Aerobatics using flight gear and JSBSim
Very cool. I guess you survived, or you would not be writing this. ;-) Did the spin seem to enter and exit normally? Jon -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Behalf Of flying.toaster Sent: Saturday, June 17, 2006 10:04 AM To: FlightGear developers discussions Subject: Re: [Flightgear-devel] Aerobatics using flight gear and JSBSim Looks like a spin, does not it ? ;) That is with stock JSBSim FDM ... ___ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
Re: [Flightgear-devel] Aerobatics using flight gear and JSBSim
Well actually I did not survive that one, I was too busy taking the shot ;) But I did survive many others Entry and exit is by the book : - stall the aircraft with wings level using gentle full back stick and apply full rudder to enter - release the stick and rudder, let the nose point down and catch un speed + pullout to exit The spin may be more or less flat depending on engine power and roll at the begining of the spin. After 2 to 3 turns it seems to be willing to go flat (as you can see on the picture alpha was fairly close to 90º) Snap rolls are also accomplished in a similar manner by using brisk full aft stick at high speed (full throttle) and full rudder. You get a very important roll rate but speed bleeds very quickly and after one turn you'd rather release the stick or you end up loosing a lot of height (turns into a spin). Inverted spins are tougher to perform though because the aircraft wants to get upright into a regular spin (takes a lot of aileron to keep the spin inverted) Message du 17/06/06 à 17h21 De : Jon S. Berndt [EMAIL PROTECTED] A : [EMAIL PROTECTED], FlightGear developers discussions flightgear-devel@lists.sourceforge.net Copie à : Objet : Re: [Flightgear-devel] Aerobatics using flight gear and JSBSim Very cool. I guess you survived, or you would not be writing this. ;-) Did the spin seem to enter and exit normally? Jon ___ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
Re: [Flightgear-devel] Aerobatics using flight gear and JSBSim
Jon S. Berndt wrote: ..another big deal to dig up: Downwash. There may end up being some things that don't get specifically addressed, but must be defined by the user with functions and tables. What I'd like most to do is to provide all the basic building blocks needed so that the user is free to create the model they'd like to create. Isn't downwash just a matter of a high angel-of-attack? Erik -- http://www.ehtw.info (Dutch)Future of Enschede Airport Twente http://www.ehofman.com/fgfs FlightGear Flight Simulator ___ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
Re: [Flightgear-devel] Aerobatics using flight gear and JSBSim
On Fri, 16 Jun 2006 09:54:39 +0200, Erik wrote in message [EMAIL PROTECTED]: Jon S. Berndt wrote: ..another big deal to dig up: Downwash. There may end up being some things that don't get specifically addressed, but must be defined by the user with functions and tables. What I'd like most to do is to provide all the basic building blocks needed so that the user is free to create the model they'd like to create. Isn't downwash just a matter of a high angel-of-attack? ..think downwash for a hovering chopper, to support its weight, it pushes down a similar mass of air, give and take some for manouvering, fixed wings just move in straighter lines. -- ..med vennlig hilsen = with Kind Regards from Arnt... ;o) ...with a number of polar bear hunters in his ancestry... Scenarios always come in sets of three: best case, worst case, and just in case. ___ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
Re: [Flightgear-devel] Aerobatics using flight gear and JSBSim
Isn't downwash just a matter of a high angel-of-attack? Erik When air passes over the wing, an incremental downward redirection is induced, so the horizontal tail sees a different airflow than the wing. Jon ___ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
Re: [Flightgear-devel] Aerobatics using flight gear and JSBSim
Jon S. Berndt wrote: Isn't downwash just a matter of a high angel-of-attack? When air passes over the wing, an incremental downward redirection is induced, so the horizontal tail sees a different airflow than the wing. Oh, downwash of the aircraft itself. I was thinking because of wind but I think that's called windshear? If I understand it correctly it should be possible to simulate the effects of downwash by creating a function of flap-settings and airspeed (and possible alpha), no? Erik -- http://www.ehtw.info (Dutch)Future of Enschede Airport Twente http://www.ehofman.com/fgfs FlightGear Flight Simulator ___ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
Re: [Flightgear-devel] Aerobatics using flight gear and JSBSim
Jon S. Berndt wrote: Isn't downwash just a matter of a high angel-of-attack? When air passes over the wing, an incremental downward redirection is induced, so the horizontal tail sees a different airflow than the wing. Oh, downwash of the aircraft itself. I was thinking because of wind but I think that's called windshear? If I understand it correctly it should be possible to simulate the effects of downwash by creating a function of flap-settings and airspeed (and possible alpha), no? Erik Maybe. I haven't thought about that much, yet. Try it. ;-) Jon ___ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
Re: [Flightgear-devel] Aerobatics using flight gear and JSBSim
Jon S. Berndt wrote: If I understand it correctly it should be possible to simulate the effects of downwash by creating a function of flap-settings and airspeed (and possible alpha), no? Maybe. I haven't thought about that much, yet. Try it. ;-) I did try something like this, but for prop-wash (over the vertical stabilizer) for the DHC-6 but at that time there was no easy way to get the engine output power. Erik -- http://www.ehtw.info (Dutch)Future of Enschede Airport Twente http://www.ehofman.com/fgfs FlightGear Flight Simulator ___ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
Re: [Flightgear-devel] Aerobatics using flight gear and JSBSim
Did you get the Sukhoi ? If you want I can send you a slightly updated model... rgrds Enrique ___ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
Re: [Flightgear-devel] Aerobatics using flight gear and JSBSim
Just came across this video: http://www.angelfire.com/il2/aphs/what_up_dog.wmv Are we ready to model this kind of aerobatics in FG? Andras ___ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
Re: [Flightgear-devel] Aerobatics using flight gear and JSBSim
On Jun 14, 2006, at 8:23 PM, Jon S. Berndt wrote: Maybe said a different way, imagine your wing is riding on the edge of the amount of air it can push down without stalling. Now you deflect the aileron down and try to push the air down even more. Stupid me. I forgot something. OK, deflecting an aileron is like deflecting a flap. If you look at a lift curve from a wing section you can see that deflecting a flap (aileron) increases the lift coefficient, but you also reduce your stall angle. That would be enough to do it for that portion of the airfoil. Jon There tends to be another issue here as well. And it might relate to what Curt experienced. When teaching students to do slow flight (near the edge of stall) the temptation is to use aileron to correct. If they do that, what can happen is that the down wing, that they want to raise, will get down aileron, more lift, and an increase in induced drag. That increase in drag will pull the wing back. Thus there is a yaw induced, that can have the same affect as using rudder...a spin! As an additional complication, wings don't stall all at once, instead it is a progressive event. When a wing is on the edge of stall the trailing edge is where the stall will start. So it is possible that a wing that is nearly stalled will have have some flow separation at the trailing edge. That is also where the ailerons are. Down aileron can cause flow separation and a decrease in lift, with an increase in drag, and that wing can drop on you. When in slow flight students have to be taught to use rudder instead of aileron, or the CFIs get nervous ;-) --Adam ___ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
Re: [Flightgear-devel] Aerobatics using flight gear and JSBSim
On Jun 14, 2006, at 8:33 PM, Josh Babcock wrote: Jon S. Berndt wrote: Snap roll: This is indeed the recipe for a snap roll: starting from a speed slightly above the stall, apply a sudden yaw with the rudder, apply opposite aileron, and pull back on the yoke. SNAP! --- One wing stalls and the plane rolls over. [I liked the clever use of the word, recipe with the phrase snap *roll*] This would be hard to model using lookup tables, but it might be possible using JSBSim functions and a table or tables, together. Could be fun. I need to think about this one. The first idea that comes to mind is that if the aircraft speed minus the yaw rate times some characteristic lateral length (span/2?) falls below the stall speed, then a rolling moment would be generated - maybe a yawing moment, too. Jon ___ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel My understanding is that a snap roll is initiated by yaw-roll coupling. The lower wing is put into the turbulent flow behind the fuselage by the hard yaw. This imparts a strong roll moment, and the result is that the AOA of the upper wing goes down, while the AOA of the lower wing goes up into the stall region. At that point the partial loss of lift on the down wing becomes almost complete, while the upper wing only loses a small amount of lift. If it were done at a low AOA you would only get roll damping as the low wing would go into a high AOA high lift regime, while the upper wing would go into a low AOA low lift regime. You need to be close enough to stall that the lower wing goes past the high lift regime and into the stall regime. I may be wrong about that. If the roll were initiated on the back side of the lift curve, the upper wing would actually gain lift in the roll, and the lower one would lose it as it goes into stall. I'm not sure which is right, but I'm pretty sure that to get a roll going fast enough to get only one wing into a stall you have to have the yaw-roll coupling. Otherwise roll damping would limit you to a mere barrel roll. So for JSBSim, you would need to add another dimension to your lookup tables that indicates the loss of lift as an airfoil goes through the turbulent wake of other elements like the fuselage. Not a bad idea really, but it's a lot of data and probably pretty hard to find. You would also need separate R/L wing elements. Josh I don't believe so. Remember that a snap roll is a type of spin. So probably this discussion should focus on simulating spins more generally. It is easy to enter a spin. For a simple spin the aircraft is just put in slow flight, near stall, and then some yaw is added, but a lot is generally not necessary (depends on the aircraft). I don't believe that it is enough to blanket the lower wing behind the fuselage. I believe that the main effect is just that the yaw rate is enough that the two wings are flying at different speeds. Essentially, like a helicopter, there is a retreating wing and an advancing wing. If both wings are near stall, then that retreating wing , going a little slower (or effectively a higher AOA), will stall, and you will end up in a spin. A snap roll is only different in that the aircraft is generally at a higher airspeed, so a high pitch rate is necessary to get the aircraft to that stall AOA, instead of just causing a climb, and a slow airplane. But the dynamics are still just high AOA, then yaw rate causing a wing to stall. It might be possible to simulate spins (and snap rolls) by just adding a new term to JSBSim that is essentially loss of lift due to yaw rate (or something similar...maybe normalize to Cl difference?). But it would have to be a table that would look like a Cl_Alpha curve, so that it would capture the stall on the wing. I believe, as Jon mentioned, that it could be done with a table, showing wing lift difference, from average alpha and yaw rate. And, just to complicate things, inverted spins are fun to do, and there could be some sign issues to keep in mind for the above table to also work that way as well. --Adam ___ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
Re: [Flightgear-devel] Aerobatics using flight gear and JSBSim
Adam Dershowitz wrote: I don't believe so. Remember that a snap roll is a type of spin. So We are probably both right. If the wing were to be split into several sections, the effect that you describe would be approximated (a-la X-plane), or you could add another table which could be much more accurate. I still think that the turbulent shadow effect (I cannot remember the proper name) is another important one to pay attention to. In low wing aircraft both these contribute to yaw-roll coupling, which is the source of many interesting phenomena including dutch rolls. They can also be a factor in higher frequency PIOs. Josh ___ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
Re: [Flightgear-devel] Aerobatics using flight gear and JSBSim
On Thu, 15 Jun 2006 18:08:25 -0500, Jon wrote in message [EMAIL PROTECTED]: Hmm, so how would JSBSim model low speed control reversals like this? Could it be done in a way that would also be able to model transonic control reversals that are caused by shockwaves near the control surface hinge? Josh I've got several ideas in mind. One is that the wing could be split up into sections for tip, aileron, interspan, flap, root. Alpha and spanwise velocity would be calculated for each section. Then, lateral effects due to the wing would be calculated. I think that would be rolling moment due to wing effects, and yawing moment due to wing effects. It would incorporate rolling moment due to aileron deflection, roll moment damping, roll due to yaw rate. Maybe other things as well. On the yaw side: yaw moment due to aileron deflection, yaw damping, and yaw moment due to roll rate. Another idea would be to just add additional incremental aerodynamic effects based on carefully calculated or devised functions or tables, etc., instead of the whole approach given above. Another approach might be to detect a stall at a specific wing location based on some given parameters, then manufacture an incremental aerodynamic effect based on that. There are a lot of ways to do this, but it should be done so as to not break the existing paradigm, but instead to fit within it. ..another big deal to dig up: Downwash. -- ..med vennlig hilsen = with Kind Regards from Arnt... ;o) ...with a number of polar bear hunters in his ancestry... Scenarios always come in sets of three: best case, worst case, and just in case. ___ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
Re: [Flightgear-devel] Aerobatics using flight gear and JSBSim
..another big deal to dig up: Downwash. -- ..med vennlig hilsen = with Kind Regards from Arnt... ;o) There may end up being some things that don't get specifically addressed, but must be defined by the user with functions and tables. What I'd like most to do is to provide all the basic building blocks needed so that the user is free to create the model they'd like to create. Jon ___ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
Re: [Flightgear-devel] Aerobatics using flight gear and JSBSim
On Thu, 15 Jun 2006 18:45:22 -0500, Jon wrote in message [EMAIL PROTECTED]: ..another big deal to dig up: Downwash. There may end up being some things that don't get specifically addressed, but must be defined by the user with functions and tables. What I'd like most to do is to provide all the basic building blocks needed so that the user is free to create the model they'd like to create. ..aye, I have a multi-fdm idea, that needs both downwash and collision detection turned off, to show how they respond differently to gusts etc, say a red 3d model for JSBSim, orange for yasim, yellow for LaRCSim, green for UIUC, cyan for UIUC-tail ice, blue for UIUC-wing ice, violet for UIUC-all-ice etc, and match those colors so they merge as one white. -- ..med vennlig hilsen = with Kind Regards from Arnt... ;o) ...with a number of polar bear hunters in his ancestry... Scenarios always come in sets of three: best case, worst case, and just in case. ___ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
[Flightgear-devel] Aerobatics using flight gear and JSBSim
Using derivations very similar to those described by Jon in his latest paper, I have managed having my Su-26 alpha model do most of these figures : http://aerobatics.ws/acro_figures.html The ones that still are a little dirty for me are the tailslide and sided loops (the former because I always seem to have some sideslip even with engine shut, the latter because I am plain bad as a pilot ;o) ). Cuban eigths on the other hand instance are piece of cake. Inverted spins require a lot of aileron work whereas normal spins are pretty easy. All this to say that it looks very good. Now for a few questions : - Are both half wings treated separately in JSBSim ? That can be important for snap rolls, even though I do them day in day out now - Has anybody given thought to have wing flex in 3D models ? That would be interesting effect for aerobatics in a sailplane, some undercarriages (like humm, say humm, the su-26) and modern airliners for which wings are quite flexible (look at a picture of any of them taking off) When I release the Su as beta, I would like some feedback of people who have flown aerobatics (so that I can tweak some more) Cheers ___ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
Re: [Flightgear-devel] Aerobatics using flight gear and JSBSim
Hey toaster :-), I love your idea of trying to simulate aerobatics correctly and I'd want to try to help you once you're able to release the first version. I'm not too familiar with flightgear (I'm only using it as a flight data display) so far (especially not JSBSim, can't help you with that), but I've been flying aerobatics for quite a while (I'd rather have other people judge my skill than myself :-) and I'm quite familiar with the flight dynamics of different aerobatic maneuvres. In my opinion simulating the wings seperately would be the key feature for simulating aerobatics. No simulator I know of has this ability so far, but accurate (inverted) snaps and spins won't work without it. You'll also have to include the torque effect since in modern aerobatics many figures rely on engine torque influence, and vice versa many figures simply won't work with or without it. In the end, I don't believe we even have an aerobatic plane for Flightgear yet (I'm loading MS Flight Simulator MDL-models). I don't know if I can help you out, but I'll try! Good luck on your design, TH flying.toaster wrote: Using derivations very similar to those described by Jon in his latest paper, I have managed having my Su-26 alpha model do most of these figures : http://aerobatics.ws/acro_figures.html The ones that still are a little dirty for me are the tailslide and sided loops (the former because I always seem to have some sideslip even with engine shut, the latter because I am plain bad as a pilot ;o) ). Cuban eigths on the other hand instance are piece of cake. Inverted spins require a lot of aileron work whereas normal spins are pretty easy. All this to say that it looks very good. Now for a few questions : - Are both half wings treated separately in JSBSim ? That can be important for snap rolls, even though I do them day in day out now - Has anybody given thought to have wing flex in 3D models ? That would be interesting effect for aerobatics in a sailplane, some undercarriages (like humm, say humm, the su-26) and modern airliners for which wings are quite flexible (look at a picture of any of them taking off) When I release the Su as beta, I would like some feedback of people who have flown aerobatics (so that I can tweak some more) Cheers ___ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel ___ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
Re: [Flightgear-devel] Aerobatics using flight gear and JSBSim
On Wed, 14 Jun 2006 22:12:31 +0200 (CEST), flying.toaster wrote in message [EMAIL PROTECTED]: Now for a few questions : - Are both half wings treated separately in JSBSim ? ..AFAIK, no, yasim yes. ..2 option for JSBSim, cut your Su-26 in 2, Su-26Left and Su-26Right, or rewrite JSBSim with 2 half wings per full wing. That can be important for snap rolls, even though I do them day in day out now - Has anybody given thought to have wing flex in 3D models ? ..just a wee bit, short term solution is look-up tables off FEA models, then hope Microsoft survives long enough to lure box vendors like Dell and Lenovo into shrinking Hollywood render farms into desktops. -- ..med vennlig hilsen = with Kind Regards from Arnt... ;o) ...with a number of polar bear hunters in his ancestry... Scenarios always come in sets of three: best case, worst case, and just in case. ___ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
Re: [Flightgear-devel] Aerobatics using flight gear and JSBSim
Using derivations very similar to those described by Jon in his latest paper, I have managed having my Su-26 alpha model do most of these figures : http://aerobatics.ws/acro_figures.html The ones that still are a little dirty for me are the tailslide and sided loops (the former because I always seem to have some sideslip even with engine shut, the latter because I am plain bad as a pilot ;o) ). Cuban eigths on the other hand instance are piece of cake. Inverted spins require a lot of aileron work whereas normal spins are pretty easy. This is fascinating to me - I'll be interested to try it out. All this to say that it looks very good. Now for a few questions : - Are both half wings treated separately in JSBSim ? That can be important for snap rolls, even though I do them day in day out now Yes, I know. The aerodynamics of a snap roll are ... interesting. We have talked about splitting up various surfaces on and off for years. I think David Megginson first suggested that approach. It's definitely a possibility. I suppose that ideally the wing would be split up into four or five parts on each side. Alpha would be calculated for each section given the rotational and translational state of the aircraft. But, I'm also wondering if there is a way to obtain the same effect with a three-dimensional table. Can someone give a detailed describption of a snap roll? Jon ___ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
Re: [Flightgear-devel] Aerobatics using flight gear and JSBSim
Jon S. Berndt wrote: But, I'm also wondering if there is a way to obtain the same effect with a three-dimensional table. Can someone give a detailed describption of a snap roll? My understanding of a snap roll is that at some speed (probably well above traditional stall speed) you command a large nose up elevator deflection -- if you have enough elevator authority you can quickly force the wing to a high alpha so that the wing stalls (at a much higher than normal speed.) What happens next is very similar to a spin: one wing stalls before the other leading to a rapid roll. But in this case you have so much forward momentum that the result looks more like a traditional aileron roll. I can do this in many of my R/C planes. Just pull back the elevator to full deflection and the plane rolls almost instantly. Let go of the elevator and the plane stops rolling and recovers. From the ground it looks *very* similar to a more traditional aileron roll. I have an aerobatic sea plane with the engine mounted on a pylon above the wing. There's one move that's fun and freaky to fly it through. First I accelerate to full speed and pull the aircraft into a vertical climb, then I induce a snap roll as I'm going straight up by pulling the elevator back to maximum deflection. The result is that I'm in a snap roll/spin but heading straight *UP*. If I do this at full throttle and then feed in some extreme aileron/rudder deflection, the airplane will continue to spin upwards until it runs out of momentum and then continue tumbling very strangely in mid air before it begins to drop and the maneuver transitions into a more traditional spin. It's very freaky to watch because the engine is on a pylon above the wing so you have a strange off axis thrust line that makes the plane tumble more strangely. http://www.flightgear.org/~curt/Models/Current/Mariner40/ I should point out that I'm an average R/C pilot at best so there are a *lot* of guys that can do a lot fancier and wilder stuff than I know how to do. Curt. -- Curtis Olsonhttp://www.flightgear.org/~curt HumanFIRST Program http://www.humanfirst.umn.edu/ FlightGear Project http://www.flightgear.org Unique text:2f585eeea02e2c79d7b1d8c4963bae2d ___ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
Re: [Flightgear-devel] Aerobatics using flight gear and JSBSim
Jon S. Berndt wrote: But, I'm also wondering if there is a way to obtain the same effect with a three-dimensional table. Can someone give a detailed describption of a snap roll? My understanding of a snap roll is that at some speed (probably well above traditional stall speed) you command a large nose up elevator deflection -- if you have enough elevator authority you can quickly force the wing to a high alpha so that the wing stalls (at a much higher than normal speed.) What happens next is very similar to a spin: one wing stalls before the other leading to a rapid roll. But in this case you have so much forward momentum that the result looks more like a traditional aileron roll. Curt. Partially right: http://www.av8n.com/how/htm/snaps.html Rudder is involved. Jon ___ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
Re: [Flightgear-devel] Aerobatics using flight gear and JSBSim
Snap roll: This is indeed the recipe for a snap roll: starting from a speed slightly above the stall, apply a sudden yaw with the rudder, apply opposite aileron, and pull back on the yoke. SNAP! --- One wing stalls and the plane rolls over. [I liked the clever use of the word, recipe with the phrase snap *roll*] This would be hard to model using lookup tables, but it might be possible using JSBSim functions and a table or tables, together. Could be fun. I need to think about this one. The first idea that comes to mind is that if the aircraft speed minus the yaw rate times some characteristic lateral length (span/2?) falls below the stall speed, then a rolling moment would be generated - maybe a yawing moment, too. Jon ___ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
Re: [Flightgear-devel] Aerobatics using flight gear and JSBSim
Sorry about that, prematurely hit send. Here is the link:http://forums.x-plane.org/index.php?showtopic=13050st=0Might be interesting, or maybe even relevant to modelling things like snap rolls in JSBSim. Regards,Hugo Vincent.On 6/15/06, Hugo Vincent [EMAIL PROTECTED] wrote: I came across this discussion about adding a new open source FDM toX-Plane, using CFD methods to get really really high fidelity models.On Wed, 2006-06-14 at 20:32 -0500, Jon S. Berndt wrote: Snap roll: This is indeed the recipe for a snap roll: starting from a speed slightly above the stall, apply a sudden yaw with the rudder, apply opposite aileron, and pull back on the yoke. SNAP! --- One wing stalls and the plane rolls over. [I liked the clever use of the word, recipe with the phrase snap *roll*] This would be hard to model using lookup tables, but it might be possible using JSBSim functions and a table or tables, together. Could be fun. I need to think about this one. The first idea that comes to mind is that if the aircraft speed minus the yaw rate times some characteristic lateral length (span/2?) falls below the stall speed, then a rolling moment would be generated - maybe a yawing moment, too. Jon ___ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel ___ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
Re: [Flightgear-devel] Aerobatics using flight gear and JSBSim
This would be hard to model using lookup tables, but it might be possible using JSBSim functions and a table or tables, together. Could be fun. I need to think about this one. The first idea that comes to mind is that if the aircraft speed minus the yaw rate times some characteristic lateral length (span/2?) falls below the stall speed, then a rolling moment would be generated - maybe a yawing moment, too. Jon We might also calculate alpha at a few points along the wing and key off of that (partially) for roll and yaw moment deltas... Jon ___ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
Re: [Flightgear-devel] Aerobatics using flight gear and JSBSim
On Wed, 14 Jun 2006 20:22:05 -0500, Jon wrote in message [EMAIL PROTECTED]: Jon S. Berndt wrote: But, I'm also wondering if there is a way to obtain the same effect with a three-dimensional table. Can someone give a detailed describption of a snap roll? My understanding of a snap roll is that at some speed (probably well above traditional stall speed) you command a large nose up elevator deflection -- if you have enough elevator authority you can quickly force the wing to a high alpha so that the wing stalls (at a much higher than normal speed.) What happens next is very similar to a spin: one wing stalls before the other leading to a rapid roll. But in this case you have so much forward momentum that the result looks more like a traditional aileron roll. Partially right: http://www.av8n.com/how/htm/snaps.html Rudder is involved. ..rudder _may_ be involved, any assymmetry will do the job. -- ..med vennlig hilsen = with Kind Regards from Arnt... ;o) ...with a number of polar bear hunters in his ancestry... Scenarios always come in sets of three: best case, worst case, and just in case. ___ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
Re: [Flightgear-devel] Aerobatics using flight gear and JSBSim
I came across this discussion about adding a new open source FDM to X-Plane, using CFD methods to get really really high fidelity models. On Wed, 2006-06-14 at 20:32 -0500, Jon S. Berndt wrote: Snap roll: This is indeed the recipe for a snap roll: starting from a speed slightly above the stall, apply a sudden yaw with the rudder, apply opposite aileron, and pull back on the yoke. SNAP! --- One wing stalls and the plane rolls over. [I liked the clever use of the word, recipe with the phrase snap *roll*] This would be hard to model using lookup tables, but it might be possible using JSBSim functions and a table or tables, together. Could be fun. I need to think about this one. The first idea that comes to mind is that if the aircraft speed minus the yaw rate times some characteristic lateral length (span/2?) falls below the stall speed, then a rolling moment would be generated - maybe a yawing moment, too. Jon ___ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel ___ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
Re: [Flightgear-devel] Aerobatics using flight gear and JSBSim
My understanding of a snap roll is that at some speed (probably well above traditional stall speed) you command a large nose up elevator deflection -- if you have enough elevator authority you can quickly force the wing to a high alpha so that the wing stalls (at a much higher than normal speed.) What happens next is very similar to a spin: one wing stalls before the other leading to a rapid roll. But in this case you have so much forward momentum that the result looks more like a traditional aileron roll. Partially right: http://www.av8n.com/how/htm/snaps.html Rudder is involved. ..rudder _may_ be involved, any assymmetry will do the job. Sounds like it. I've now read several accounts. There are at least two ways to enter it. See this, for instance: https://cnatra.navaltx.navy.mil/cnatra/folder5/T45/P-1287.PDF Jon ___ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
Re: [Flightgear-devel] Aerobatics using flight gear and JSBSim
Jon S. Berndt wrote: Partially right: http://www.av8n.com/how/htm/snaps.html Rudder is involved. The link you quote describes a situation where you get into a snap roll/spin when you don't want to. I had something similar happen when I was looping my R/C cub and tried to tighten up the bottom side of the loop a little too much. But as far as I know, rudder isn't required. It's just that rudder (or the lack of it (or compensating for change in rudder with aileron deflection)) can get you into trouble a lot quicker than you might realize. Also note that if your left wing is dropping due to being on the edge of a stall and you try to compensate with right aileron, that will cause the left side aileron to deflect down. This effectively increases the angle of attack a bit and can hasten the arrival of your spin. But if you don't want to spin, you might consider correcting with rudder next time when you are at these low speeds. :-) Curt. -- Curtis Olsonhttp://www.flightgear.org/~curt HumanFIRST Program http://www.humanfirst.umn.edu/ FlightGear Project http://www.flightgear.org Unique text:2f585eeea02e2c79d7b1d8c4963bae2d ___ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
Re: [Flightgear-devel] Aerobatics using flight gear and JSBSim
Curt, I can do this in many of my R/C planes. Just pull back the elevator to Ah, how come I haven't until now realized that you're into model aircraft...? What a great collection of models you have, too. First I accelerate to full speed and pull the aircraft into a vertical climb, then I induce a snap roll as I'm going straight up by pulling the elevator back to maximum deflection. The result is that I'm in a snap roll/spin but heading straight *UP*. If I do this at full throttle and That's often called a Lomcevak, I think, or at least one of the millions of variations thereof. I should point out that I'm an average R/C pilot at best so there are a *lot* of guys that can do a lot fancier and wilder stuff than I know how to do. This is a video I've just come across and it displays some of the best flying I've ever seen, it's great fun to watch. Warning: you might want to get one for yourself after seeing this (especially since the kind of plane shown here isn't very expensive): http://www.rcgroups.com/forums/showatt.php?attachmentid=831005 :) Andras (building a 74 EDGE 540, perfect for snap rolls etc.) ___ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
Re: [Flightgear-devel] Aerobatics using flight gear and JSBSim
Also note that if your left wing is dropping due to being on the edge of a stall and you try to compensate with right aileron, Right aileron as in trying to roll to the right? that will cause the left side aileron to deflect down. Left aileron TED follows from right aileron TEU. The pilot causes the left aileron TED movement. I'm not sure what you mean. This effectively increases the angle of attack a bit Why? I'm trying to picture the mechanics of that and can't quite. Seems to me like deflecting the left aileron down would cause the airflow to deflect down and reduce the angle of attack - all other things remaining constant. The hinge moment might also tend to reduce the angle of incidence of the outer length of the wing, thus reducing alpha. Jon ___ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
Re: [Flightgear-devel] Aerobatics using flight gear and JSBSim
This would be hard to model using lookup tables, but it might be possible using JSBSim functions and a table or tables, together. Could be fun. I need to think about this one. The first idea that comes to mind is that if the aircraft speed minus the yaw rate times some characteristic lateral length (span/2?) falls below the stall speed, then a rolling moment would be generated - maybe a yawing moment, too. There are two simulators out there that model all kinds of weird flight situations remarkably well -- Reflex XTR and Aerofly Pro Deluxe. Unfortunately, both are payware, but they both have a reputation among R/C modellers that they are as realistic as it gets. I'm not sure what kind of physics they use, but maybe we can learn something from them in one way or another. Just a thought. Andras ___ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
Re: [Flightgear-devel] Aerobatics using flight gear and JSBSim
Disclaimer: my degree is in computer science, I only walk through the aerospace engineering department on they way to my driving simulator lab. :-) Jon S. Berndt wrote: Also note that if your left wing is dropping due to being on the edge of a stall and you try to compensate with right aileron, Right aileron as in trying to roll to the right? Yes, that's what I meant. Left aileron TED follows from right aileron TEU. The pilot causes the left aileron TED movement. I'm not sure what you mean. TED = trailing edge down? By right aileron I mean turning the wheel to the right and commanding the aircraft to roll right. This effectively increases the angle of attack a bit Why? I'm trying to picture the mechanics of that and can't quite. Seems to me like deflecting the left aileron down would cause the airflow to deflect down and reduce the angle of attack - all other things remaining constant. The hinge moment might also tend to reduce the angle of incidence of the outer length of the wing, thus reducing alpha. I'm probably mixing up my terms here. Imagine some cross section of the wing (ie. airfoil.) This could be some complex shape, especially if it includes the aileron in a deflected state. To compute wing incidence at that cross section, you need to come up with some sort of average zero incidence line fit through the airfoil shape. There's probably a name for that and an official way to determine this zero incidence line. If you look at the cross section of the wing (through a point that includes the aileron) when you deflect the aileron down (TED), you are increasing the angle of that average zero incidence line relative to the wind stream. If you deflect the aileron up (TEU) you are reducing the average incidence of that section of the wing. So now, take an airplane that is flying at a high angle of attack where the wing is struggling to stay ahead of a stall. Now deflect one aileron down (TED). You have just slightly (or perhaps more than slightly) increased the incidence of the wing across the area covered by the aileron. All other things remaining equal which it will be in the short term, you have just increased the aoa on a portion of your wing, and if you are riding the edge already, it might be just enough to push you over into a snap roll. Maybe said a different way, imagine your wing is riding on the edge of the amount of air it can push down without stalling. Now you deflect the aileron down and try to push the air down even more. For what it's worth, I experienced this first hand in my Piper Cub (R/C) model (so I was safely on the ground.) I was attempting to do a loop, but in retrospect I started too low and too slow. I got really slow over the top and due to my low altitude, I tried to tighten up the backside of the loop on the way down by feeding in some additional elevator. But the cub snapped hard on me. I released the elevator and got some speed and then pulled back again to avoid the ground and she started to snap again. But I somehow managed to find some sort of middle ground with the elevator to keep pulling out of the dive while maintaining just enough aileron authority to somehow save it. Both wing tips were literally inches from the ground at various points in the manuever and I was still flying right on the ragged edge of the stall. I was one tremble short of another full snap roll. The spectators claimed that ground effect saved me. :-) Somehow in the end I was back flying with no vegetation in the gear or wing tips. WHEW! BUT! Had I known then what I know now and steered with the rudder rather than the ailerons, it probably wouldn't have been nearly such a close call. Curt. -- Curtis Olsonhttp://www.flightgear.org/~curt HumanFIRST Program http://www.humanfirst.umn.edu/ FlightGear Project http://www.flightgear.org Unique text:2f585eeea02e2c79d7b1d8c4963bae2d ___ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
Re: [Flightgear-devel] Aerobatics using flight gear and JSBSim
BUT! Had I known then what I know now and steered with the rudder rather than the ailerons, it probably wouldn't have been nearly such a close call. There are a few very spectacular inadvertent stalls and spins and suchlike in this video as well. It's actually quite funny to watch: http://www.rusjet.ru/video/krach.wmv #2 is very much what happened to you, I think, with a slightly different outcome. Andras ___ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
Re: [Flightgear-devel] Aerobatics using flight gear and JSBSim
Maybe said a different way, imagine your wing is riding on the edge of the amount of air it can push down without stalling. Now you deflect the aileron down and try to push the air down even more. Stupid me. I forgot something. OK, deflecting an aileron is like deflecting a flap. If you look at a lift curve from a wing section you can see that deflecting a flap (aileron) increases the lift coefficient, but you also reduce your stall angle. That would be enough to do it for that portion of the airfoil. Jon ___ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
Re: [Flightgear-devel] Aerobatics using flight gear and JSBSim
Jon S. Berndt wrote: Snap roll: This is indeed the recipe for a snap roll: starting from a speed slightly above the stall, apply a sudden yaw with the rudder, apply opposite aileron, and pull back on the yoke. SNAP! --- One wing stalls and the plane rolls over. [I liked the clever use of the word, recipe with the phrase snap *roll*] This would be hard to model using lookup tables, but it might be possible using JSBSim functions and a table or tables, together. Could be fun. I need to think about this one. The first idea that comes to mind is that if the aircraft speed minus the yaw rate times some characteristic lateral length (span/2?) falls below the stall speed, then a rolling moment would be generated - maybe a yawing moment, too. Jon ___ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel My understanding is that a snap roll is initiated by yaw-roll coupling. The lower wing is put into the turbulent flow behind the fuselage by the hard yaw. This imparts a strong roll moment, and the result is that the AOA of the upper wing goes down, while the AOA of the lower wing goes up into the stall region. At that point the partial loss of lift on the down wing becomes almost complete, while the upper wing only loses a small amount of lift. If it were done at a low AOA you would only get roll damping as the low wing would go into a high AOA high lift regime, while the upper wing would go into a low AOA low lift regime. You need to be close enough to stall that the lower wing goes past the high lift regime and into the stall regime. I may be wrong about that. If the roll were initiated on the back side of the lift curve, the upper wing would actually gain lift in the roll, and the lower one would lose it as it goes into stall. I'm not sure which is right, but I'm pretty sure that to get a roll going fast enough to get only one wing into a stall you have to have the yaw-roll coupling. Otherwise roll damping would limit you to a mere barrel roll. So for JSBSim, you would need to add another dimension to your lookup tables that indicates the loss of lift as an airfoil goes through the turbulent wake of other elements like the fuselage. Not a bad idea really, but it's a lot of data and probably pretty hard to find. You would also need separate R/L wing elements. Josh ___ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
Re: [Flightgear-devel] Aerobatics using flight gear and JSBSim
On Wed, 2006-06-14 at 19:34 -0500, Jon S. Berndt wrote: All this to say that it looks very good. Now for a few questions : - Are both half wings treated separately in JSBSim ? That can be important for snap rolls, even though I do them day in day out now Yes, I know. The aerodynamics of a snap roll are ... interesting. We have talked about splitting up various surfaces on and off for years. I think David Megginson first suggested that approach. It's definitely a possibility. I suppose that ideally the wing would be split up into four or five parts on each side. Alpha would be calculated for each section given the rotational and translational state of the aircraft. But, I'm also wondering if there is a way to obtain the same effect with a three-dimensional table. Can someone give a detailed describption of a snap roll? Jon Jon, I'm not an aeronautical engineer by any stretch of the imagination, but I am remembering something you said on this forum awhile back about modelling the effect vs. modelling the structures... So, what are the effects of one wing stalling? An uneducated guess on my part would be lift reduced by about 1/2 and AERORP moving towards the (middle of the) unstalled wing. A quick read of many of the posts here suggests looking at some function of yaw rate, pitch rate, aileron deflection and alpha into tables to move AERORP and reduce lift. Ron ___ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
