Re: [Flightgear-devel] blue angel
* [EMAIL PROTECTED] (Jon S Berndt) [2002.06.18 09:31]: On Tue, 18 Jun 2002 09:07:04 -0500 (CDT) Curtis L. Olson [EMAIL PROTECTED] wrote: Melchior FRANZ writes: Wow, what a beautiful model. Shiny blue steel, perfectly animated. Many thanks! I'd be interested to know what is being talked about. Screen shot? Me too, but I think they are probably talking about the YASim A-4. Somebody* made changes to it yesterday, and it's the only aircraft in FG that the Blue Angels used to fly. (*) Any chance we can get the fgfs-base cvs server to tell us who commits changes? -- Cameron Moore / Beauty is skin deep, but ugly goes right to the bone. If \ \ that's the case, I pity the person who's only skin and bones. / ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] blue angel
Curtis L. Olson wrote: I have a lot of problems flying it well from the mouse. It doesn't seem to respond well to elevator input ... you get an initial bump and then pitch oscillations ... I don't know if that's realistic or not. I'm sure Andy can provide a suitable explanation for why it does this from a physics perspective, but I don't think the real airplane behaves like this in real life. I'm not sure I understand. A given stick position corresponds very closely to a given angle of attack. If you change the stick position, the aircraft will seek to the new AoA. If you change the stick position very rapidly, it will seek rapidly, overshoot, and oscillate. This is pretty normal behavior for all aircraft; but some baby (mostly military) consumer simulators aren't like this -- they map stick position to pitch *rate*, which is unphysical. Well, not quite unphysical: rather, coming up with a control system that did this would require gyros and fancy computers. The A-4 doesn't have those -- stick positions in traditional aircraft act like trim settings, they select for AoA. The differences from other YASim aircraft are that the A-4, as modelled, has a lot of elevator authority. So if you just yank the stick back, you'll end up in a nasty pitch oscillation with the nose tipping up well past 20 degrees. Just don't do that. Be gentle, and the aircraft won't oscillate. The added elevator authority is needed to effect high AoA combat maneuvers, but you can't use it all at once. Yanking on the stick in a 172 works just fine -- you're only transiting through a 5-10 degrees of pitch. Yanking the stick back on the A-4 tries to rotate the aircraft by 20-30 degrees, and is much less forgiving. I rather like this property, honestly. I've never tried to fly it with the mouse, but I could easily believe it would be difficult. But flown correctly, the aircraft moves smoothly and does nice things. You just can't yank it around the sky like a 172 -- I'm fairly confident that this is true of the real aircraft, too. Here's a really quick overview of my flight ops procedures in the current A-4, for those who want to try it. These are culled from tidbits I've found on the web, in books and on my own experience with YASim. They certainly shouldn't be taken as official Navy procedures. :) Takeoff: + No flaps. Full throttle. + At 145-160 knots*, apply about 1/2 elevator and raise the nose to 15-20 degrees. The aircraft will lift off. + Gear up. + Do not try to climb yet. Accelerate to ~300 knots** with no more than 5 degrees of climb, and then pull the nose up to 20 degrees or so. Trying to climb while slow requires high AoA's and bleeds energy rapidly. [* Depending on gross weight. I have a trial version at home that models the external stores, absolutely refuses to leave the ground sustainably at less than 210 knots when fully loaded, and handles like a royal pig. Dunno if this is correct or not, but it makes physical sense and is awfully fun to fly. You can map joystick buttons to drop the bombs and actually feel them release.] [** Military jets don't usually follow FAA speed regulations. They aren't built to fly that slow.] Landing: + Fly over the field at ~350 knots and ~1000ft AGL. + Chop throttle. Roll into a level, 2G turn to the downwind leg, decelerating to ~200 knots or so. This is called the brake, because the aircraft will only decelerate quickly at high AoAs. Trying to line up for an approach at 350 knots and fly it like a civilian plane is a recipe for an overshoot. + Drop gear. Add full flaps gradually as the aircraft decelerates to 13.5 degrees AoA. The Navy doesn't fly approaches to speed, they track AoA. Add throttle to keep the aircraft level at 600-1000 ft. AGL. Try to get the aircraft trimmed at 10-12 degrees or so, to avoid having to push on the stick during final. + Turn onto final in one 180 degree turn, and land. Be *very* careful with the throttle. The aircraft decelerates quickly as you reduce thrust, and you can rapidly find yourself falling out of the sky at 110 kias. If you see speed dropping past 130 knots, jam the throttle to full. I find that at 20% fuel, the aircraft needs about 40-50% throttle to sustain 130 knots at 13.5 degrees. But speed management on the approach is really hard. It's really easy to get trapped in a throttle oscillation between too fast (pushing on the nose to keep the aircraft from balooning) and too slow (yikes! help! eject!). It may be that the flap drag is overmodelled. Another possibiliy is that the engines are spooling too slowly. I know that real skyhawks usually fly the approach with speed brakes out, so that the throttle is at a higher setting and can spool faster to maximum thrust. YASim doesn't have speedbrakes yet (although adding them would be really simple if someone wants them). Andy -- Andrew J. RossNextBus Information Systems Senior
Re: [Flightgear-devel] blue angel
Andy Ross writes: I'm not sure I understand. A given stick position corresponds very closely to a given angle of attack. If you change the stick position, the aircraft will seek to the new AoA. If you change the stick position very rapidly, it will seek rapidly, overshoot, and oscillate. For anyone who'd like further reading on phugoid oscillations, see http://www.monmouth.com/~jsd/how/htm/aoastab.html#toc106 In a C172, these usually show up (at least for me) as a very gentle up-and-down drift, like the bow of a boat over light swells -- still very annoying if you're trying to hold altitude or airspeed and don't get on top of them fast. For a fast jet trainer, I can easily imagine that things are more dramatic. Yanking on the stick in a 172 works just fine -- you're only transiting through a 5-10 degrees of pitch. Yanking the stick back on the A-4 tries to rotate the aircraft by 20-30 degrees, and is much less forgiving. That might be overstating the case. Smooth inputs are necessary on a C172 as well, especially if you're trying to stay within small tolerances (i.e. +-5kt airspeed or +-50ft altitude). 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] blue angel
Andy Ross writes: I'm not sure I understand. A given stick position corresponds very closely to a given angle of attack. Nope, only for a given airspeed. The balance between tailplane and main wing, for a given elevator position, is speed dependent. Thus phugoids. If you change the stick position, the aircraft will seek to the new AoA. If you change the stick position very rapidly, it will seek rapidly, overshoot, and oscillate. That statement is only true if rapidly means faster than 5 minutes. Basically, any elevator or trim change will cause a phugoid on any conventional aircraft. You need active pilot participation, an autopilot or considerable patience (waiting for the decay) to avoid it. For anyone who'd like further reading on phugoid oscillations, see http://www.monmouth.com/~jsd/how/htm/aoastab.html#toc106 Yup. ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] blue angel
David Megginson wrote: For anyone who'd like further reading on phugoid oscillations, see Alex Perry wrote: Nope, only for a given airspeed. The balance between tailplane and main wing, for a given elevator position, is speed dependent. Thus phugoids. I think I should clarify. First off, this is not the phugoid oscillation -- that is a very long-period oscillation that has to do with the coupling between speed and lift. The aircraft speeds up, which causes more lift, which pitches the velocity vector up, which causes the aircraft to lose speed due to climbing up hill, which causes a loss of lift and a pitch down. That's the phugoid, it happens with periods in the tens of seconds (sometimes minutes) and is not what I meant at all. What I mean is the short term, 1-2 second oscillations about the trim AoA causes by an instantaneous change of control configuration. There's no deep magic to this one, it's just the weathervane behavior you get from any statically stable object. If you change the control configuration, you change the AoA for which the aircraft is trimmed. Since the aircraft is not actually pointed in that direction, it feels a torque to put it there. The torque is not completely damped, so you get an oscillation. With the 172, the magnitude of the oscillation caused by a full back-stick at takeoff speed is small, and damps out quickly as the aircraft lifts off. Doing the same thing to the A-4 causes a wild, scary pitch-up well past the stall, and the aircraft plops ungracefully back down on the ground (well, or crashes violently). But the effect is the same in both cases. Alex is absolutely right that the *value* of the AoA change is speed dependent (for a trivial example: wiggling the elevator on a stopped aircraft at the top of a hammerhead stall obviously doesn't change the AoA at all), but that wasn't my point. Curt was complaining that yanking the stick back caused a pop to a state where the aircraft was oscillating about the new AoA. In this circumstance, there isn't time for the speed to change appreciably; it might as well be constant. The stick change changes the trimmed AoA, and the aircraft seeks very rapidly to that new AoA. Thus, it looks like there was a pop between two states. But it's real, physical behavior. David Megginson wrote: That might be overstating the case. Smooth inputs are necessary on a C172 as well, especially if you're trying to stay within small tolerances (i.e. +-5kt airspeed or +-50ft altitude). True enough; graceful control input is always important. But yanking the stick back during takeoff on the little Skyhawk isn't likely to kill you the way it can on the big one. :) 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] blue angel
Andy Ross writes: David Megginson wrote: That might be overstating the case. Smooth inputs are necessary on a C172 as well, especially if you're trying to stay within small tolerances (i.e. +-5kt airspeed or +-50ft altitude). True enough; graceful control input is always important. But yanking the stick back during takeoff on the little Skyhawk isn't likely to kill you the way it can on the big one. :) Probably not, but it could trash the plane pretty easily when you stall and nose down from 50 ft above the runway. For the record, I just tried the A4 with the mouse. It handles fine, but is *extremely* sensitive to control input, as one would expect. During the takeoff roll, I gently eased the mouse back just until the nose wheel started to move a bit off the ground, then held it still and waited for the plane to start climbing on its own; once it was in the air, I needed to push the mouse back forward a bit to keep the climb from getting too steep. In both cases, the amount of mouse movement would be best measured in millimeters. Note that some fighter aircraft, like (I think) the F-4, are inherently unstable, and if they're modelled correctly we won't be able to fly them at all by direct controls: we'll need to work though a fairly sophisticated FCS. 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] blue angel
On Tue, 18 Jun 2002 17:04:04 -0400 David Megginson [EMAIL PROTECTED] wrote: Note that some fighter aircraft, like (I think) the F-4, are inherently unstable, and if they're modelled correctly we won't be able to fly them at all by direct controls: we'll need to work though a fairly sophisticated FCS. Fighters, by nature, are generally less stable (= more nimble) than GA aircraft which are more stable (= less nimble). The F-4, IIRC is dynamically stable. The only aircraft which I am personally aware of that is neutrally stable (longitudinally, at least) is the F-16, which is of course controlled by a sophisticated FCS (there are undoubtedly others). Typically, the closer the CG is to the aerodynamic center, the quicker and easier you can yank the plane around (and possibly break your neck). It wouldn't surprise me that the A-4 is so maneuverable. It would be nice to get input from a real A-4 driver or find some old aero test data for short and long period oscillatory characteristics. We have yet to find a good way to validate our aero models for the various FDMs. Jon ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] blue angel
David Megginson wrote: Note that some fighter aircraft, like (I think) the F-4, are inherently unstable, and if they're modelled correctly we won't be able to fly them at all by direct controls: we'll need to work though a fairly sophisticated FCS. The F-4 is stable. It's actually much older than you think -- it started life as the McDonell F4H before the 1962 unification of aircraft identifications. The idea of using statically unstable aircraft wasn't practical until the advent of cheap computers in the late 70's. I believe the F-16 was the first operational unstable aircraft (it was certainly the first in US service, anyway). This feature gets hyped up more than it should, IMHO. The advantages to having an unstable aircraft are that you can hold it at a much higher peak AoA. Stable aircraft tend to run out of elevator authority somewhere around the stall. If you want to hold a higher AoA, you need a bigger elevator, which adds weight and drag and is generally a pain for the designer to deal with. If you have a computer that can wiggle the elevator for you, you can move the wings forward (ahead of the c.g.) to the point where only a comparatively tiny elevator is needed. Basically, it's a weight/drag optimization, nothing more. And the FCS doesn't have to be too terribly complicated. To first approximation, you would just simulate stability by computing a target AoA from the stick position, trim, and airspeed and using elevator deflections to seek to that. I strongly suspect this is how the F-16A works, although I know it has a bunch of modes for safety and usability reasons. There really wasn't too much computer available in the mid 1970's to do much else. [Random aside: I was recently reading a description of the E-3 Sentry from the 1970's when it was introduced. They described the on-board IBM computer in glowing terms: 760 KHz (yes, kilohertz) cycle times, with 111 kwords of core memory. Wrist watches toast that this these days. The F-16 was from about the same generation, and I'd be really shocked if its computer was bigger than this one. There's only so much software complexity you can fit in something like that.] 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] blue angel
On Tue, 18 Jun 2002 17:04:04 -0400, David Megginson [EMAIL PROTECTED] wrote: snip Note that some fighter aircraft, like (I think) the F-4, are inherently unstable, and if they're modelled correctly we won't be able to fly them at all by direct controls: we'll need to work though a fairly sophisticated FCS. Not, AFAIK, the F4 - F16 and many later aircraft. The FCS on the F16 keeps you within 'safe' limits, giving 'carefree' handling in rapid combat maneuvers. It does _not_ however give _careless_ handling - a number of aircraft were lost when a 'gap in the coverage' was found. 90 deg of bank at low level and letting the nose drop towards the ground is not good in any aircraft. Aircraft with this sort of FCS can be spotted by the continuous movement of control surfaces during the taxi - the FCS is trying to 'fly the bumps out of the concrete'. Military aircraft can be handful at times, for example in the Lightning (UK Jet) the turn onto finals was done in the pre-stall buffet in order to keep landing speeds within limits. Traits like this gave it its nickname 'Frightening'. Rick -- David Farrent and Dougie O'Hara on the Cold War role of the ROC: 'What a world of sorrow is hidden in those few words - [Post attack] crew changes would have been based on crew availability.' ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] blue angel
Jon S Berndt wrote: ... Typically, the closer the CG is to the aerodynamic center, the quicker and easier you can yank the plane around (and possibly break your neck). It wouldn't surprise me that the A-4 is so maneuverable. It would be nice to get input from a real A-4 driver or find some old aero test data Nimble. Hmm. Wasn't the F16 so responsive that it became the first fighter to put its pilot to sleep if he yanked to hard on the controls. I recall a video made during research after an unexpected crash. The g force load up is supposedly so much faster than the F4 that experienced F4 pilots in the F16 pulled right past the tunnel vision point to blackout before realizing what was happening. Anyone else remember this with details? Also, A means attack, not fighter. The A4 was Douglas' hot rod nuke bomber. Its primary design goal was delivering a largish H-bomb using an interesting attack sequence. Alternately it had hard points for fuel tanks, bombs and missiles for conventional ground attack. Dog fighting was contemplated, but more in terms of self defense as it was strictly sub-sonic. 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] blue angel
On Tue, 18 Jun 2002 14:37:16 -0700 Andy Ross [EMAIL PROTECTED] wrote: The advantages to having an unstable aircraft are that you can hold it at a much higher peak AoA. IIRC, the F-16 is neutrally stable throughout much of its flight envelope. The main advantage for having a neutrally stable or unstable fighter aircraft is agility, quickness in manueverability. And the FCS doesn't have to be too terribly complicated. To first approximation, you would just simulate stability by computing a target AoA from the stick position, trim, and airspeed and using elevator deflections to seek to that. I strongly suspect this is how the F-16A works, although I know it has a bunch of modes for safety and usability reasons. There really wasn't too much computer available in the mid 1970's to do much else. It's quite a nice piece of work, the F-16 DFlCS for Block 40 and subs. It includes little things like gun compensation. There are structural filters, scheduled filters, etc. It's not unsophisticated. Jon ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] blue angel
Jon S. Berndt wrote: IIRC, the F-16 is neutrally stable throughout much of its flight envelope. The main advantage for having a neutrally stable or unstable fighter aircraft is agility, quickness in manueverability. It's a chicken an the egg problem. Any aircraft can have quickness in maneuverability with large enough control surfaces. But you can't make the control surfaces too large and still intercept nuclear bombers at Mach 2. So the fighter aircraft of the late 1960's ditched pitch control in exchange for speed. With the advent of computers, it turned out that designers could play tricks to get the maneuverability back by reducing the stability of the main wing. Thus, at some level this allows designers to put back elevator authority that they had to remove to get the drag performance they wanted. The idea of having agile aircraft able to pitch to very high AoA's has never required a stability augmentation system. Many WWII fighters were extraordinarily nimble in pitch. 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] blue angel
C. Hotchkiss writes: Nimble. Hmm. Wasn't the F16 so responsive that it became the first fighter to put its pilot to sleep if he yanked to hard on the controls. People can pass out at as little as 6Gs, can't they? It takes 4Gs to start a loop in an aerobatic plane, so it shouldn't be that hard; I'd imagine that even a Spitfire could manage 6Gs in some maneuvers. 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] blue angel
On Tue, 18 Jun 2002 16:37:27 -0500, Jon S Berndt [EMAIL PROTECTED] wrote: On Tue, 18 Jun 2002 17:04:04 -0400 David Megginson [EMAIL PROTECTED] wrote: Note that some fighter aircraft, like (I think) the F-4, are inherently unstable, and if they're modelled correctly we won't be able to fly them at all by direct controls: we'll need to work though a fairly sophisticated FCS. Fighters, by nature, are generally less stable (= more nimble) than GA aircraft which are more stable (= less nimble). The F-4, IIRC is dynamically stable. The only aircraft which I am personally aware of that is neutrally stable (longitudinally, at least) is the F-16, which is of course controlled by a sophisticated FCS (there are undoubtedly others). Typically, the closer the CG is to the aerodynamic center, the quicker and easier you can yank the plane around (and possibly break your neck). It wouldn't surprise me that the A-4 is so maneuverable. It would be nice to get input from a real A-4 driver or find some old aero test data for short and long period oscillatory characteristics. We have yet to find a good way to validate our aero models for the various FDMs. Practically every modern fighter is unstable. The absolute classic is the F117 which is a horrible shape aerodynamically and seems to have had several fatal crashes before they finally sorted the FCS - one of its nicks was 'The Wobblin Goblin'. Other Unstable production aircraft include: F-22 ATF Eurofighter Typhoon JAS-39 Grippen Su-27 Flanker Family FA-18 Hornet Super Hornet F-35 JSF (http://www.aoe.vt.edu/aoe/faculty/Mason_f/JSFHosder.pdf has some aero data) B-2 Spirit Rick -- David Farrent and Dougie O'Hara on the Cold War role of the ROC: 'What a world of sorrow is hidden in those few words - [Post attack] crew changes would have been based on crew availability.' ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] blue angel
Charlie Hotchkiss wrote: Nimble. Hmm. Wasn't the F16 so responsive that it became the first fighter to put its pilot to sleep if he yanked to hard on the controls. Certainly not the first. GLOC has been an known issue from the very early days of aviation. There was an experimental fighter design in the 40's (someone help me out here with the name...) that had the pilot lying on his stomach to increase the allowable G loads. A pilot in a normal seat without a G suit can lose conciousness in as few as 10 seconds at 7G (I'm making those numbers up, but they're in the ballpark). You can do this in a Cessna 172 in a dive, if the wings stay on the plane. But certainly the F-16, owing to the neutral stability, was capable of pulling this hard through a much larger portion of its flight envelope. Most aircraft don't have the elevator authority to do this except at very high speeds (and at the AoA's you need to pull for those G's, you don't stay at those speeds very long). Also, A means attack, not fighter. The A4 was Douglas' hot rod nuke bomber. Its primary design goal was delivering a largish H-bomb using an interesting attack sequence. The original design actually was for a fighter. But the Navy had already bought into the supersonic world with the F8U Crusader project, and didn't want a subsonic jet (despite the fact that the A-4 could fly rings around any production fighter at the time). So they told Douglas to submit it for the carrier based nuclear attack role that you mention. Actual deployed A-4's, though, ended up being used primarily for vanilla ground interdiction roles in Vietnam. Later, they got picked up by the Marines for ground based light attack and close air support. The Blue Angles flew it for ~15 years, until they replaced it with the Hornet. The Air Force bought a bunch for use as aggressor training aircraft, and the 2-seat A-4J still serves in a few squadrons as an advanced jet and carrier qualification trainer (although these are all being replaced by T-45 Goshawks, I believe). Very few A-4's every served in the nuclear deterent role, as it happens. Alternately it had hard points for fuel tanks, bombs and missiles for conventional ground attack. Dog fighting was contemplated, but more in terms of self defense as it was strictly sub-sonic. Actually, by virtue of the Air Force employment as agressor planes, the Skyhawk probably has as much dogfighting experience as any other in-service aircraft in the world. It's one of those designs that Just Worked for pretty much everything it was used for, which is why I like it so much. It's a good, simple, unambitious aircraft. If it were a programming language, it would be C to the F-16's C++. :) 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] blue angel
On Tue, 18 Jun 2002 18:53:41 -0400, David Megginson [EMAIL PROTECTED] wrote: C. Hotchkiss writes: Nimble. Hmm. Wasn't the F16 so responsive that it became the first fighter to put its pilot to sleep if he yanked to hard on the controls. People can pass out at as little as 6Gs, can't they? It takes 4Gs to start a loop in an aerobatic plane, so it shouldn't be that hard; I'd imagine that even a Spitfire could manage 6Gs in some maneuvers. There are two issues here, blackout through pulling to many Gs (or redout though -ve Gs). IIRC pilots can remain conscious at c. 6G continuous. From memory G-Induced Loss of Consciousness (GLOC) is the 'new' problem - this is caused by the rapid onset of G. Blackout is progressive and therefore gives a warning. GLOC is sudden and occurs 4 to 6 seconds after the manoeuvre. Its insidious as short periods of rapidly applied G don't trigger it - but rapid onset G, with the G sustained for c.5 sec or more will cause it Proper straining procedures by the aircrew can avoid it. http://www.vnh.org/FSManual/02/02SustainedAcceleration.html Rick -- David Farrent and Dougie O'Hara on the Cold War role of the ROC: 'What a world of sorrow is hidden in those few words - [Post attack] crew changes would have been based on crew availability.' ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] blue angel
On Tue, 2002-06-18 at 15:53, Andy Ross wrote: Jon S. Berndt wrote: IIRC, the F-16 is neutrally stable throughout much of its flight envelope. The main advantage for having a neutrally stable or unstable fighter aircraft is agility, quickness in manueverability. It's a chicken an the egg problem. Any aircraft can have quickness in maneuverability with large enough control surfaces. But you can't make the control surfaces too large and still intercept nuclear bombers at Mach 2. So the fighter aircraft of the late 1960's ditched pitch control in exchange for speed. With the advent of computers, it turned out that designers could play tricks to get the maneuverability back by reducing the stability of the main wing. I think the big deal with the F-16 was not that it was neutrally stable, that had been done before. The big deal was that it's FCS was all electric and fly-by-wire. Something I'm still not so sure is a good idea ... there's something comforting about a flight control system that can work with just muscle and aerodynamics. Thus, at some level this allows designers to put back elevator authority that they had to remove to get the drag performance they wanted. The idea of having agile aircraft able to pitch to very high AoA's has never required a stability augmentation system. Many WWII fighters were extraordinarily nimble in pitch. 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 -- Tony Peden [EMAIL PROTECTED] We all know Linux is great ... it does infinite loops in 5 seconds. -- attributed to Linus Torvalds ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] blue angel
Rick Ansell wrote: From memory G-Induced Loss of Consciousness (GLOC) is the 'new' problem - this is caused by the rapid onset of G. Blackout is progressive and therefore gives a warning. GLOC is sudden and occurs 4 to 6 seconds after the manoeuvre. Its insidious as short periods of rapidly applied G don't trigger it - but rapid onset G, with the G sustained for c.5 sec or more will cause it Proper straining procedures by the aircrew can avoid it. From my reading of the link (which is great, btw, many thanks), these are actually the exactly same effect. The distinction based on rapid onset has to do with the eyeball. :) Apparently, the inner pressure of the eye (the thing that goes nuts when you have glaucoma) causes the retina to lose blood flow at a slightly lower G load than the brain. So, if you are increasing the G load slowly, you can pull until your eyesight starts failing, and then let up. This was the technique used in the old days, before the Falcons, Hornets and Sukhois arrived that could snap into a 9G turn from 1G within a second or two. The problem is that the tissue (both retina and brain, which are both just nerve bundles) contains enough oxygen to function for about 5 seconds without oxygen. So if the G load is not increasing slowly, you could pull past the eye limit and into the GLOC arena at essentially the same time. Now, your brain is failing at exactly the same time as your eyesight, and you don't have a chance to notice anything before you lose it. Game over. But the effect isn't any different -- it's just that the old trick for noticing the GLOC before it happens doesn't work as well with modern aircraft. 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] blue angel
On Tue, 18 Jun 2002 16:36:54 -0700, Andy Ross [EMAIL PROTECTED] wrote: Rick Ansell wrote: From memory G-Induced Loss of Consciousness (GLOC) is the 'new' problem - this is caused by the rapid onset of G. Blackout is progressive and therefore gives a warning. GLOC is sudden and occurs 4 to 6 seconds after the manoeuvre. Its insidious as short periods of rapidly applied G don't trigger it - but rapid onset G, with the G sustained for c.5 sec or more will cause it Proper straining procedures by the aircrew can avoid it. From my reading of the link (which is great, btw, many thanks), these are actually the exactly same effect. The distinction based on rapid onset has to do with the eyeball. :) snip But the effect isn't any different -- it's just that the old trick for noticing the GLOC before it happens doesn't work as well with modern aircraft. This is my reading to, but the two are usual treated/described as separate and 'GLOC' was certainly heralded as a new hazard in the 80's. (Back when I religiously read Flight International from cover to cover each week!) Now who's going to write this lot into the visuals and control interface? :) (Says he, no substantive contribution yet) Rick -- David Farrent and Dougie O'Hara on the Cold War role of the ROC: 'What a world of sorrow is hidden in those few words - [Post attack] crew changes would have been based on crew availability.' ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] blue angel
Robert Detmers wrote: Actually the F-4 is unstable, but only marginally. It just means that the plane would eventually diverge if the pilot did nothing to stop it. Not in pitch, certainly? An aircraft that is unstable in pitch, if you pulled the stick a little bit and got the nose going up just a teeny bit, would *continue* diverging from zero AoA all the way up into a tumbling spin. Once in the spin, it wouldn't be humanly recoverable to a forward-pointing state (at wacky AoA's the controls don't do what you expect them to). If you got the nose rotating quickly and then looked down at your engine gauge for a second, you'd lose it completely. Maybe you mean that it was unstable in roll? That's true of many aircraft, including GA ones. This means that given time, the plane will roll over into a tight turn on its own. The negative dihedral on the F-4's tail would cause this kind of effect, for example. This is a relatively benign effect, since a simple autopilot can correct it when the pilot isn't paying attention (and in a dogfight, you're hardly worried about a minor roll divergence). The instability that this discussion focuses on is pitch instability. Roll stability isn't very important for combat aircraft. And conversely, many aircraft that are non-stable in pitch are quite stable in yaw (there's no great advantage to having high yaw agility). 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] blue angel
It's a chicken an the egg problem. Any aircraft can have quickness in maneuverability with large enough control surfaces. But you can't make the control surfaces too large and still intercept nuclear bombers at Mach 2. True .. though not so much Chicken and Egg as balanced design tradeoffs. Thus, at some level this allows designers to put back elevator authority that they had to remove to get the drag performance they wanted. The idea of having agile aircraft able to pitch to very high AoA's has never required a stability augmentation system. Many WWII fighters were extraordinarily nimble in pitch. Hmm. I'm thinking of high alpha as something over 20 degrees. Even 50 or 60 degrees. Of course, this does not require a SAS, or a DFlCS. All you have to do is cut the engine and yank back on the stick :-) In any case, agility is being able to roll quicker than your opponent, or pitch up quicker. High alpha is not so much the goal as change in direction is. One interesting high alpha film clip I saw recently was the (Sukhoi?) forward swept wing demonstrator. The thing could almost hover in midair. The high alpha maneuvers were amazing. I was discussing it with a former F-18 driver and we agreed that it would be an easy target during those kinds of maneuvers. Not very useful for combat at the low speeds demonstrated given the current engagement practices... The point I was making in the paragraph I wrote (above) was that it was the neutral stability that gives you the agility in today's fighter aircraft (given the design trade-offs) but it requires computer control. Here's a small portion of the technical doc I wrote some years ago for inclusion into the F-16 DFlCS simulation model for Link: A flight control system of sorts has been present on all aircraft since the earliest attempts to fly were made. It is a human. The pilot commands a turn, for instance, by applying force to the stick. If the turn is not quick enough the pilot increases the turn rate. The pilot has received feedback information from the aircraft instruments, and has compensated for the slower than desired turn rate by commanding additional turn rate by applying additional force to the stick. Modern flight control systems use the same technique: comparing the pilot's command to actual aircraft dynamics, and driving the control surfaces to produce zero error between these two quantities Recently [note: this was written ca. 1990], fighters such as the F-16 have been produced which wander between neutral stability and instability. These aircraft are extremely agile. They require a highly capable, fail-safe flight control computer to keep from tumbling out of control. In such a system the pilot inputs a command through the stick, the flight control computer processes that command, and the computer then sends an electrical signal to an actuator at the control surfaces to implement the command. This is called fly-by-wire. The responsibilities of the modern flight control system are wide ranging. During straight and level flight, for instance, the F-16 flight control computer keeps the aircraft straight and level by taking the air data information supplied to it by the Air Data Converter (ADC) and aircraft dynamics data from rate gyros and feeding it back into the flight control loop. During this straight and level flight - assuming no pilot inputs are present - the aircraft basically flies itself, sending commands to the aerodynamic control surfaces based on air data and rates. Because the aircraft is statically unstable, and is traveling through a random and imperfect atmosphere, the control surfaces are in motion constantly. Another responsibility of the FLCS is to process pilot commands, and to drive the surfaces accordingly. When the pilot pulls back on the stick with a force of X pounds, for instance, he/she is commanding a pull up of Y g's [note: the F-16 FLCS pitch channel is a normal acceleration command type]. The flight control system takes the command, limits it if necessary, compares it to the actual current g force, and drives the tail surface until the actual and commanded values are equal. Jon smime.p7s Description: application/pkcs7-signature
Re: [Flightgear-devel] blue angel
Rick Ansell wrote: This is my reading to, but the two are usual treated/described as separate and 'GLOC' was certainly heralded as a new hazard in the 80's. (Back when I religiously read Flight International from cover to cover each week!) I hadn't realized this was a new(ish) term. I've been abusing it all these years to mean generic blackout, apparently. Learn something new every day... Now who's going to write this lot into the visuals and control interface? :) Actually, it wouldn't be too terribly hard. Write some filter code that reads /accelerations/z-g or whatnot and sets /pilot/gloc-norm between 0 (no effect) and 1 (out) based on the 5 second rule and a few recovery heuristics. Then the controls stuff can filter down joystick axis effectiveness based on that number, and refuse to accept any input when it's at 1.0. Easy enough. Add an alpha-blended tunnel over the screen -- just a black quad with low alpha in the center and blend it based on the gloc property to simulate the impending blackout. This burns a lot of fill rate, so maybe we should make it configurable for low-end systems. Then again, low FPS during blackout might be a feature!. For extra credit, record a pilot grunting or huffing sound and play it at high G's. One of the sims I've played did this. It was cool. :) 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] blue angel
For extra credit, record a pilot grunting or huffing sound and play it at high G's. guffaw smime.p7s Description: application/pkcs7-signature
Re: [Flightgear-devel] blue angel
Andy Ross writes: Actually, it wouldn't be too terribly hard. Write some filter code that reads /accelerations/z-g or whatnot and sets /pilot/gloc-norm between 0 (no effect) and 1 (out) based on the 5 second rule and a few recovery heuristics. It's been a while, but I think that Battle of Britain already models this -- if you pull too many G's, things go blurry and red. For extra credit, record a pilot grunting or huffing sound and play it at high G's. One of the sims I've played did this. It was cool. :) Yes, I think that was BoB. 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] blue angel
- Original Message - From: Andy Ross [EMAIL PROTECTED] To: [EMAIL PROTECTED] Sent: Tuesday, June 18, 2002 7:05 PM Subject: Re: [Flightgear-devel] blue angel Robert Deters wrote: Actually the F-4 is unstable, but only marginally. It just means that the plane would eventually diverge if the pilot did nothing to stop it. Not in pitch, certainly? An aircraft that is unstable in pitch, if you pulled the stick a little bit and got the nose going up just a teeny bit, would *continue* diverging from zero AoA all the way up into a tumbling spin. Once in the spin, it wouldn't be humanly recoverable to a forward-pointing state (at wacky AoA's the controls don't do what you expect them to). If you got the nose rotating quickly and then looked down at your engine gauge for a second, you'd lose it completely. Yes in pitch. Besides, I think you are confusing static stability and dynamic stability. The F4 is statically stable, but dynamically unstable. Just because the aircraft is unstable, doesn't mean that it is uncontrollable. Give the F4 a pitch input and it will oscillate and diverge, if the pilot does nothing to stop it. It is not that unstable, so the pilot can easily control it. The longitudinal characteristics of the F4 has what is generally called Tuck. It is when the roots of the phugoid mode (from the longitudinal transfer functions of the aircraft) become real and one goes unstable. Maybe you mean that it was unstable in roll? That's true of many aircraft, including GA ones. This means that given time, the plane will roll over into a tight turn on its own. The negative dihedral on the F-4's tail would cause this kind of effect, for example. This is a relatively benign effect, since a simple autopilot can correct it when the pilot isn't paying attention (and in a dogfight, you're hardly worried about a minor roll divergence). I think you mean the spiral mode is unstable. It is sort of a roll-yaw coupled effect (not to be confused with dutch roll). You can sort of think of the instability in the pitch the same way as the spiral (qualitatively not quantitatively of course). Just because the aircraft has spiral instability doesn't mean it can't be flown. The instability that this discussion focuses on is pitch instability. Roll stability isn't very important for combat aircraft. And conversely, many aircraft that are non-stable in pitch are quite stable in yaw (there's no great advantage to having high yaw agility). Actually roll stability is important and is a requirement for military certification. Rob Deters Department of Aeronautical and Astronautical Engineering University of Illinois at Urbana-Champaign ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
RE: [Flightgear-devel] blue angel
Robert Deters wrote: Actually the F-4 is unstable, but only marginally. It just means that the plane would eventually diverge if the pilot did nothing to stop it. Rob: I think most people, when thinking of stability think: If I made an exact paper airplane of the aircraft in question and threw it, would it fly straight - like the quintessential dart paper airplane - or would it go head over heels? I think I have a feel for it (and I really should *know* it) but because I am lazy and tired at the moment and because I have a hunch you'd do justice to the topic, for the benefit of the us all, would you differentiate and explain the difference between static and dynamic stability? Jon smime.p7s Description: application/pkcs7-signature
Re: [Flightgear-devel] blue angel
Robert Deters wrote: Andy Ross wrote: Robert Deters wrote: Actually the F-4 is unstable, but only marginally. Not in pitch, certainly? Yes in pitch. Besides, I think you are confusing static stability and dynamic stability. Er, normally one interprets an unqualified use of stable as referring to static stability. The rest of the conversation has been about stability augmentation systems (which have to do only with static stability -- phugoid damping can be done with an autopilot). I even went out of my way (two paragraphs worth!) to point out that I wasn't talking about the phugoid when we got into this. Apologies if I didn't get that across; and apologies for trying to explain to you stuff that you already knew. :) For those who are wondering what the difference is: static stability refers to weathervane behavior. If you point the aircraft forward (for whatever definition of forward you are using) in an airstream and then perturb it a little bit, does the aircraft feel a torque that tries to return it to the forward direction? Dynamic stability is a different beast. I described the phugoid earlier --- this is the pitch up, slow down, pitch down, speed up, rinse, repeat process. It turns out that, because of an interaction (a coupling in the strict sense) between the period of this oscillation and the regular weathervane static oscillation, some aircraft are unstable in the phugoid. Each time they pitch up, the pitch up a bit farther, and then lose more energy and pitch down farther, and the process diverges over a period of minutes. One way to imagine it is that the aircraft always lags the gravity effect. It has to wait a bit at the top of the curve to point downward again, so it loses a bit more energy than it should and slows down farther than it should. When it gets back to the bottom of the curve, it's going faster, *and* it takes a bit longer to pitch back up to the right value. As you decrease the rate at which it changes its pitch to match its velocity (i.e. as you decrease the static stability) you eventually reach a point where the phugoid oscillation is no longer damped, or is actually augmented. So, the aircraft always tries to point toward its velocity vector, so it meets the criteria for being statically stable. But if you leave it alone, it's velocity doesn't approach any single asymptotic value. So it's not stable, either. This kind of effect is called dynamic instability. In principle, there are other kinds of dynamic instabilities. In practice, this is the only one people mean when they say an aircraft is dynamically unstable. As Rob points out, it's usually treated in textbooks as a set of differential equations in variables like: height, Vx, Vy and pitch angle. That is: the time derivative of Vy is gravity plus thrust times the sin of pitch angle, etc... It turns out that after a series of variable substitutions that make my eyes cross*, you can characterize the solution as a polynomial. The difference between a convergent and divergent solution turns out to be equivalent to the existence of real or complex roots to the polynomial. But the effect is qualitatively understandable without the math. 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
