Re: [Flightgear-devel] JSBSIm, aeromatic, crosswind taxiiing, et cetera
On Sunday 19 June 2011 10:50:01 John Denker wrote: On 06/19/2011 06:46 AM, Jon S. Berndt wrote: Maybe I've gone wrong somewhere here, but something similar might work. Also, in situations like a flat spin or tail slide this probably falls apart! Let's postpone discussion of exotic flight conditions such as flat spins and tail slides. There are much more prosaic situations that need to be addressed. Let's start by getting the aircraft to behave properly when a) _taxiing_ with a crosswind and/or tailwind, and b) _landing_ and _taking off_ with a crosswind and/or tailwind, These seem like basic and fundamental features. Snip speculation and rumor The value of these features can hardly be exaggerated. For example, according to page 4-3 of the POH the maximum demonstrated crosswind for a C-172 is 15 knots. Snip long ramble and speculation c) _engine out_ (asymmetric thrust) in a twin, We already model asymmetric thrust. d) simple _inverted flight_ ... not an inverted flat spin, just plain old inverted flight, such as people routinely do in a Cessna 150 Aerobat. Aerodynamically, inverted flight is already possible. e) The effect of propwash on trim and on elevator authority. This is a big deal in some aircraft, including the 152/172/182 family. Snip more long off-topic rambling and speculation So, I set up a soft field take off in JSBSim stand-alone with a 15 knot crosswind using the c172p that is in FGFS git: Rotation 13 seconds @41 knots 375 feet Lift off 19 seconds @58 knots 800 feet Distance over 50' 2150 feet Heading Error 45 degrees Then I added the induced thrust which was the topic of this thread in the first place: Rotation 1 seconds @3 knots 3 feet Lift off 20 seconds @55 knots 900 feet Distance over 50' 2550 feet Heading Error 3 degrees Rudder deflection 34% Thanks, Ron -- EditLive Enterprise is the world's most technically advanced content authoring tool. Experience the power of Track Changes, Inline Image Editing and ensure content is compliant with Accessibility Checking. http://p.sf.net/sfu/ephox-dev2dev ___ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
[Flightgear-devel] JSBSIm, aeromatic, crosswind taxiiing, et cetera
On 06/19/2011 06:46 AM, Jon S. Berndt wrote: Maybe I've gone wrong somewhere here, but something similar might work. Also, in situations like a flat spin or tail slide this probably falls apart! Let's postpone discussion of exotic flight conditions such as flat spins and tail slides. There are much more prosaic situations that need to be addressed. Let's start by getting the aircraft to behave properly when a) _taxiing_ with a crosswind and/or tailwind, and b) _landing_ and _taking off_ with a crosswind and/or tailwind, These seem like basic and fundamental features. As far as I can tell, none of the existing FG aircraft that use the JSBSim FDM behave properly under these conditions. (FWIW the Pitts and the Comanche use YASim). The value of these features can hardly be exaggerated. For example, according to page 4-3 of the POH the maximum demonstrated crosswind for a C-172 is 15 knots. It is important for pilots to know what happens if they use soft-field takeoff procedure with a 15-knot crosswind. We do not want them to discover this the hard way, in a real airplane. It would be extremely valuable to have a simulator that faithfully models the real behavior. Et cetera. For more perspective and motivation, see appendix below. Returning to the technical issues: AFAICT the most fundamental issues are not JSBSim issues strictly speaking, but rather aeromatic issues. The aeromatic output I have seen is 100% predicated on the assumption of small alpha and small beta. The entire strategy of the aeromatic- based aircraft.xml file is predicated on this. For example: -- The idea that there would be lift due to alpha and then some delta lift due to flap extension is absurd. Near the stall, extending the flaps (at constant pitch attitude, and constant direction of flight) will make a /negative/ contribution to the lift in the real airplane. -- Forsooth, the whole idea of lift due to alpha is absurd, since the lift of the real airplane depends in a nonlinear way on alpha _and beta_. Specifically, for an unswept wing we expect the lift of the wing to go to zero when beta is 90 degrees. Few if any of the existing FG aircraft model this beta-dependence. A faithful model of this would require a major reorganization of the aircraft.xml file AFAICT. Small changes will not suffice. That leads to an other rather fundamental issue: Let's talk about lift. Lift is a vector. It is defined to be perpendicular to the wind, and perpendicular to the Y axis. Axes are defined here: http://www.av8n.com/how/htm/motion.html#fig-axes Specifically, if W is the relative wind velocity (directed toward the airplane, not toward the wind-source) then lift is in the direction W × Y. The component of lift along the W × Y direction is positive, for not-too-large positive alpha. -- Minor point: This can be confusing to non-experts there is a tailwind, since W × Y is downward in that case. -- This is undefined when there is a direct crosswind, since in that case W × Y is zero and does not define a direction. For an unswept wing it doesn't matter, since the magnitude of the lift of the wing is zero ... but for a swept wing this is an utterly nontrivial issue. Remark: Here is an item that is *not* on the list of fundamental issues. I mention it just for perspective. The last time I checked, in all the aeromatic aircraft, the lookup tables for coefficient-of-lift versus alpha were defined over a severely limited domain of alpha values. This is not a fundamental issue, because it is so straightforward to fix. It will of course need to be fixed, but it will be nowhere near sufficient. Constructive suggestion: According to the JSBSim manual, the wind axis system (LIFT, DRAG, and SIDElift) is not the only choice; the body axis system (X, Y, and Z) is also supported. Alas, the last time I checked, precisely none of the FG aircraft used the XYZ axis system in their JSBSim configuration (aircraft.xml). I suggest that the first step toward getting an aircraft to behave properly during crosswind taxiing would be to convert to the XYZ axis system. I am quite aware that this conversion requires a large investment per aircraft. However, AFAICT the investment will pay for itself very soon. I for one am not interested in re-arranging the deck chairs on the Titanic, and I am not interested in making minor tweaks on an aircraft.xml file that is mathematically unsound. Another constructive suggestion: While we are reorganizing the aircraft.xml file, we should get rid of the notion of lift due to alpha et cetera. I suggest a more faithful model would work with things like force due to wing and force due to elevator. As a first step, compatible with the existing approach, we can treat the wing as a whole. Then, later, as a second step we can model the wing in four parts: port outboard (with aileron), port inboard (with flap),
Re: [Flightgear-devel] JSBSIm, aeromatic, crosswind taxiiing, et cetera
2011/6/19 John Denker j...@av8n.com: On 06/19/2011 06:46 AM, Jon S. Berndt wrote: Maybe I've gone wrong somewhere here, but something similar might work. Also, in situations like a flat spin or tail slide this probably falls apart! Let's postpone discussion of exotic flight conditions such as flat spins and tail slides. There are much more prosaic situations that need to be addressed. Let's start by getting the aircraft to behave properly when a) _taxiing_ with a crosswind and/or tailwind, and b) _landing_ and _taking off_ with a crosswind and/or tailwind, These seem like basic and fundamental features. As far as I can tell, none of the existing FG aircraft that use the JSBSim FDM behave properly under these conditions. (FWIW the Pitts and the Comanche use YASim). The value of these features can hardly be exaggerated. For example, according to page 4-3 of the POH the maximum demonstrated crosswind for a C-172 is 15 knots. It is important for pilots to know what happens if they use soft-field takeoff procedure with a 15-knot crosswind. We do not want them to discover this the hard way, in a real airplane. It would be extremely valuable to have a simulator that faithfully models the real behavior. Et cetera. For more perspective and motivation, see appendix below. Returning to the technical issues: AFAICT the most fundamental issues are not JSBSim issues strictly speaking, but rather aeromatic issues. The aeromatic output I have seen is 100% predicated on the assumption of small alpha and small beta. The entire strategy of the aeromatic- based aircraft.xml file is predicated on this. For example: -- The idea that there would be lift due to alpha and then some delta lift due to flap extension is absurd. Near the stall, extending the flaps (at constant pitch attitude, and constant direction of flight) will make a /negative/ contribution to the lift in the real airplane. -- Forsooth, the whole idea of lift due to alpha is absurd, since the lift of the real airplane depends in a nonlinear way on alpha _and beta_. Specifically, for an unswept wing we expect the lift of the wing to go to zero when beta is 90 degrees. Few if any of the existing FG aircraft model this beta-dependence. A faithful model of this would require a major reorganization of the aircraft.xml file AFAICT. Small changes will not suffice. That leads to an other rather fundamental issue: Let's talk about lift. Lift is a vector. It is defined to be perpendicular to the wind, and perpendicular to the Y axis. Axes are defined here: http://www.av8n.com/how/htm/motion.html#fig-axes Specifically, if W is the relative wind velocity (directed toward the airplane, not toward the wind-source) then lift is in the direction W × Y. The component of lift along the W × Y direction is positive, for not-too-large positive alpha. -- Minor point: This can be confusing to non-experts there is a tailwind, since W × Y is downward in that case. -- This is undefined when there is a direct crosswind, since in that case W × Y is zero and does not define a direction. For an unswept wing it doesn't matter, since the magnitude of the lift of the wing is zero ... but for a swept wing this is an utterly nontrivial issue. Remark: Here is an item that is *not* on the list of fundamental issues. I mention it just for perspective. The last time I checked, in all the aeromatic aircraft, the lookup tables for coefficient-of-lift versus alpha were defined over a severely limited domain of alpha values. This is not a fundamental issue, because it is so straightforward to fix. It will of course need to be fixed, but it will be nowhere near sufficient. Constructive suggestion: According to the JSBSim manual, the wind axis system (LIFT, DRAG, and SIDElift) is not the only choice; the body axis system (X, Y, and Z) is also supported. Alas, the last time I checked, precisely none of the FG aircraft used the XYZ axis system in their JSBSim configuration (aircraft.xml). I suggest that the first step toward getting an aircraft to behave properly during crosswind taxiing would be to convert to the XYZ axis system. I am quite aware that this conversion requires a large investment per aircraft. However, AFAICT the investment will pay for itself very soon. I for one am not interested in re-arranging the deck chairs on the Titanic, and I am not interested in making minor tweaks on an aircraft.xml file that is mathematically unsound. Another constructive suggestion: While we are reorganizing the aircraft.xml file, we should get rid of the notion of lift due to alpha et cetera. I suggest a more faithful model would work with things like force due to wing and force due to elevator. As a first step, compatible with the existing approach, we can treat the wing as a whole. Then, later, as a second step we can model the wing in four parts:
Re: [Flightgear-devel] JSBSIm, aeromatic, crosswind taxiiing, et cetera
From: John Denker [mailto:j...@av8n.com] On 06/19/2011 06:46 AM, Jon S. Berndt wrote: Maybe I've gone wrong somewhere here, but something similar might work. Also, in situations like a flat spin or tail slide this probably falls apart! Let's postpone discussion of exotic flight conditions such as flat spins and tail slides. John, I agree, but the comment I made was more of a disclaimer than anything. The value of these features can hardly be exaggerated. For example, according to page 4-3 of the POH the maximum demonstrated crosswind for a C-172 is 15 knots. It is important for pilots to know what happens if they use soft-field takeoff procedure with a 15-knot crosswind. We do not want them to discover this the hard way, in a real airplane. It would be extremely valuable to have a simulator that faithfully models the real behavior. Et cetera. For more perspective and motivation, see appendix below. Returning to the technical issues: AFAICT the most fundamental issues are not JSBSim issues strictly speaking, but rather aeromatic issues. I agree, again. One problem with having a tool such as Aeromatic is that some people use it to generate the template and then don't modify it. Ideally, Aeromatic would be modified to produce better output in these circumstances. I might look into that at some point, but I'll need to find a 26 hour day lying around somewhere... ;-) Another constructive suggestion: While we are reorganizing the aircraft.xml file, we should get rid of the notion of lift due to alpha et cetera. I suggest a more faithful model would work with things like force due to wing and force due to elevator. As a first step, compatible with the existing approach, we can treat the wing as a whole. Then, later, as a second step we can model the wing in four parts: port outboard (with aileron), port inboard (with flap), starboard inboard (with flap), and starboard outboard (with aileron). This is AFAICT the only reasonable way to model the effect of flaps near the stall, the effect of flaps in inverted flight, the loss of aileron authority near the stall, et cetera. It is also the only reasonable way AFAICT to model swept wings. There is soon going to be a donation of JSBSim code mods that *may* be useful in implementing this approach. Stay tuned. I would be happy to discuss the details with anybody who wants to contribute in this area. If you do a writeup for this, it would be helpful. Jon -- EditLive Enterprise is the world's most technically advanced content authoring tool. Experience the power of Track Changes, Inline Image Editing and ensure content is compliant with Accessibility Checking. http://p.sf.net/sfu/ephox-dev2dev ___ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
