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Nerobro posted:This has definitely gotten beyond the "you can't fly a wing without computers". For practical purposes that is true though, no matter how much you hurf durf about semantics. Yes there are alternate ways of achieving yaw stability and even yaw control without a vertical stabilizer or a conventional rudder, and yes pitch stability without a conventional elevator isn't that hard to do, but in order to make an aircraft with neither a horizontal nor a vertical stabilizer controllable with the stick-and-rudder system a pilot of conventional aircraft will actually understand, you need something significantly more sophisticated than just direct manipulation of the flight control surfaces. It might not have to be a digital computer, and it might not have to be full fly-by-wire either, but you do need some kind of processing or translation of the pilot's control inputs. Even plain old tailless deltas also tend to have some pretty annoying oscillation issues so even many of the late 1950's ones have some kind of analog dampening autopilot going on. e: tangentially related: without having any insight into any (probably classified) stability curves for real military aircraft with relaxed stability, it seems to me like calling them "unstable" isn't really the right term. They're almost certainly as close to neutrally stable as they can be (that is, disturbances do not lead to a correcting moment, but it doesn't lead to a reinforcing moment either), since if they were statically unstable they'd just have the same issues as a statically stable aircraft except in the other direction. You can also have local instabilities at certain angles of attack (or sideslip) without that restricting the entire flight envelope by those angles. TheFluff fucked around with this message at 00:15 on May 3, 2019 |
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May 2, 2019 23:39
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Ola posted:Maybe it belongs best in a different thread, but flying military history goes well here and I think you will enjoy this animated map video of the battle of Midway from the japanese perspective. I really enjoyed this video, very interesting and easy to understand. I've subscribed and look forward to more!
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May 3, 2019 00:26
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Ola posted:Maybe it belongs best in a different thread, but flying military history goes well here and I think you will enjoy this animated map video of the battle of Midway from the japanese perspective. The Algorithm has been pimping this hard for the past few days, glad to hear it's actually worthwhile.
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May 3, 2019 02:15
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TheFluff posted:hurf durf You too are confusing impractical with impossible, given that flying wings without computers did actually fly. The one that started the discussion is one example.
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May 3, 2019 03:54
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Ola posted:You too are confusing impractical with impossible, given that flying wings without computers did actually fly. The one that started the discussion is one example. The one that started this fell out of the sky
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May 3, 2019 03:56
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Jesus Mary and Joseph, obviously from the first use of "impossible" in this conversation it has been as an exaggeration for "difficult" or "impractical" If a sense of an ambiguous word is nullified by a clear example openly acknowledged by the writer, then maaaaaaybe it's not the intended sense.
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May 3, 2019 04:02
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hobbesmaster posted:The one that started this fell out of the sky Yes, after flying for years. Impractical or impossible?
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May 3, 2019 04:26
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Sometimes I feel like communication is impossible 
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May 3, 2019 04:28
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vessbot posted:Sometimes I feel like communication is impossible If words have too ambiguous meanings, it's certainly impractical.
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May 3, 2019 04:31
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Ola posted:If words have too ambiguous meanings, it's certainly impractical. They're not too ambiguous though. Exaggeration is a very common manner of speech. And in any case, he clarified his intent many posts ago (as if it wasn't already clear enough by virtue of the other sense saying that the impossible has been occurring for decades.)
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May 3, 2019 04:36
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Hyperbole is literally impossible to understand.
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May 3, 2019 04:42
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vessbot posted:They're not too ambiguous though. Exaggeration is a very common manner of speech. And in any case, he clarified his intent many posts ago (as if it wasn't already clear enough by virtue of the other sense saying that the impossible has been occurring for decades.) I was responding to a post on this page by a different poster. Language isn't that hard.
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May 3, 2019 04:44
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TheFluff posted:e: tangentially related: without having any insight into any (probably classified) stability curves for real military aircraft with relaxed stability, it seems to me like calling them "unstable" isn't really the right term. They're almost certainly as close to neutrally stable as they can be (that is, disturbances do not lead to a correcting moment, but it doesn't lead to a reinforcing moment either), since if they were statically unstable they'd just have the same issues as a statically stable aircraft except in the other direction. You can also have local instabilities at certain angles of attack (or sideslip) without that restricting the entire flight envelope by those angles. Seems like a good time to post the YF-16's first flight. https://www.youtube.com/watch?v=UR-48Kri0Tw The voiceover is overly dramatic...the pilot intended to take off a little bit and test stick response over the runway, then land a few hundred feet down the runway. But the engine nozzle wouldn't open to reduce thrust, and the roll response was crazy, so he ended up powering up, then letting it fly itself in a slow left turn then landed.
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May 3, 2019 05:49
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TheFluff posted:For practical purposes that is true though, no matter how much you hurf durf about semantics. Yes there are alternate ways of achieving yaw stability and even yaw control without a vertical stabilizer or a conventional rudder, and yes pitch stability without a conventional elevator isn't that hard to do, but in order to make an aircraft with neither a horizontal nor a vertical stabilizer controllable with the stick-and-rudder system a pilot of conventional aircraft will actually understand, you need something significantly more sophisticated than just direct manipulation of the flight control surfaces. It might not have to be a digital computer, and it might not have to be full fly-by-wire either, but you do need some kind of processing or translation of the pilot's control inputs. Even plain old tailless deltas also tend to have some pretty annoying oscillation issues so even many of the late 1950's ones have some kind of analog dampening autopilot going on. No they are dynamically unstable, the flight control computers make them flyable. The instability makes them more maneuverable. Ex: https://en.wikipedia.org/wiki/General_Dynamics_F-16_Fighting_Falcon#Negative_stability_and_fly-by-wire
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May 3, 2019 07:10
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Ola posted:You too are confusing impractical with impossible, given that flying wings without computers did actually fly. The one that started the discussion is one example. Also, the XB-35 and YB-49 both had severe oscillation issues that they were hoping to resolve with a dampening autopilot. I will admit though that the yaw control on those things was more clever than I thought.
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May 3, 2019 07:25
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By the same criteria, and the modern safety expectation which is the subtext in all of this, the Sopwith Camel is also impractical to fly without computer control, but it's pretty absurd to assert that.
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May 3, 2019 08:09
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Ola posted:By the same criteria, and the modern safety expectation which is the subtext in all of this, the Sopwith Camel is also impractical to fly without computer control, but it's pretty absurd to assert that. WW I rotary airplane engines "The best ideas they'd had yet." Or possibly " Yeah, hindsight will do that to you." 
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May 3, 2019 08:31
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e.pilot posted:No they are dynamically unstable, the flight control computers make them flyable. The instability makes them more maneuverable. I skipped over that part, but you are right. The whole point is to give the tail as much leverage to point the nose as you dare. An intuitive way to look at it is to imagine balancing a broomstick in your palm. If you add a big weight to its bottom end, it's very easy to balance but it takes more force to push it over. If you add a weight to the top, it's very hard to balance it, but it tips over super fast when you want it to. Another intuition is suspending a model aircraft from two strings. It's easy to balance it when the strings are far apart, but harder to move (rotate about the axis perpendicular to the strings). The closer the strings are, the harder it is to balance but the easier it is to move.
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May 3, 2019 08:33
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e.pilot posted:No they are dynamically unstable, the flight control computers make them flyable. The instability makes them more maneuverable. I guess it depends on what you mean by negative stability or "unstable", and a more precise definition is exactly what I'm after here. Actual negative stability in the pitch axis to me means that any increase in the AoA leads to a pitch-up moment that works to further increase the AoA and further increase the pitch-up moment, which means that you have a positive feedback loop that works to very rapidly stall the aircraft at any disturbance. This is also what Ola's describing. The problem with this is that it's not actually beneficial. The reason you go for relaxed or neutral stability in the first place is that you want to get less drag from the control surfaces. If you have positive stability and you want to make a turn, you first need a bunch of control surface deflection just to counteract the increasing pitch-down moment that comes with increasing AoA. If you have static negative stability (pitch-up moment increases with increasing AoA) then you haven't actually gained anything - you have the same problem but in reverse. What you actually want (and what I understand the F-16 to have) is neutral stability (or relaxed stability, if you want). If you have a better source than Wikipedia that says I'm wrong I'd love to see it, and I'd especially like to see a static stability diagram (pitching moment as a function of AoA).
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May 3, 2019 11:00
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Here is a little bit of Aeronautical Insanity for you; https://www.youtube.com/watch?v=TaR7Xr3Nf0k One of his earlier attempts https://www.youtube.com/watch?v=sguCEWhXtWY The actual plane he used to fly to Kenia In the early 2000's a Dutch artist called Joost Conijn build an airplane himself and flew it (in a few stages during 4 months) all the way from the Netherlands to Kenia.
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May 3, 2019 11:12
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I don't have any literature on hand, but you are describing only static stability with convergence/divergence and there are more kinds than that. This says something about how sensitive it is, how easily a small control input can lead to a change in AoA. An F-16 gets more stable at speed because the center of lift moves backwards so the two strings are further apart.
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May 3, 2019 11:16
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Ola posted:I don't have any literature on hand, but you are describing only static stability with convergence/divergence and there are more kinds than that. This says something about how sensitive it is, how easily a small control input can lead to a change in AoA. An F-16 gets more stable at speed because the center of lift moves backwards so the two strings are further apart. I... am not sure if you understood what I said. I may have to MSpaint some stability diagrams. e: what you're describing with the two strings close together is neutral stability btw TheFluff fucked around with this message at 11:24 on May 3, 2019 |
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May 3, 2019 11:21
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Q. Why did Lech Kaczyński's plane crash? A. It was full of Poles, and they were all way out on the Right.
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May 3, 2019 11:21
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TheFluff posted:I... am not sure if you understood what I said. I may have to MSpaint some stability diagrams. You said "any increase in the AoA leads to a pitch-up moment that works to further increase the AoA" which is not the correct flavor of stability the F-16 is renowned for having little of. "Stability" means more than simply a tendency to converge or diverge. For instance, wanting to increase a pitch up is negative static stability, but you can for instance have positive static and negative dynamic, where the plane wants to return to center but does so at steeper and steeper angles. TheFluff posted:e: what you're describing with the two strings close together is neutral stability btw No, all planes have these two virtual strings, it's just the center of lift and the center of gravity. What the F-16 designers wanted to exploit is what happens when the two strings get closer to one another.
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May 3, 2019 11:38
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Ola posted:You said "any increase in the AoA leads to a pitch-up moment that works to further increase the AoA" which is not the correct flavor of stability the F-16 is renowned for having little of. "Stability" means more than simply a tendency to converge or diverge. As far as I know dynamic instability isn't really interesting at all in this discussion, since it doesn't really have any benefits to maneuverability. I don't know why the F-16 (or any aircraft) would be renowned for being dynamically unstable - that just means it wants to oscillate really bad and that it needs some kind of dampening. Ola posted:No, all planes have these two virtual strings, it's just the center of lift and the center of gravity. What the F-16 designers wanted to exploit is what happens when the two strings get closer to one another. Stop confusing things with these analogies if you know the appropriate terminology. Of course every plane has a CoL and a CoG. If the center of lift and the center of gravity are very close together then the aircraft is probably going to have a neutral (ish) static longitudinal stability (but of course things are complicated), which is why I said that your example with two strings close together describes an aircraft that is close to having neutral static stability. Again, the entire point of my argument is that this is desirable, while actual negative static stability isn't. I don't think we disagree really, I'm just hurfing durf about terminology here (because of course). TheFluff fucked around with this message at 13:27 on May 3, 2019 |
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May 3, 2019 13:17
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Post a forward swept flying wing.
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May 3, 2019 13:34
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Had to go look up some actual citations because people are wrong on the internet, of course, and I believe I'm right (at least in terms of what is desirable, althoug the term neutral stability doesn't seem commonly used, so I guess I'll have to concede that one). I found a paper on the most unstable plane that's ever flown: X-29 flight control system: lessons learned, where the conclusion is that excessive static instability actually makes you less maneuverable because it's hard to make the control surfaces react fast enough to the plane's strong desire to depart from controlled flight. The X-29 had like half the pitch rate (pitch change in degrees per second) of the F-18 below the transonic region, despite being touted as super-maneuverable. It would've needed control surfaces that were twice as fast to reach parity. Dynamic instability is also just a nuisance that further slows down the control system response. quote:The X-29 pilots consistently found the maximum pitch rate capability of the airplane inadequate. Figure 12 shows the pre- dicted maximum nose up and nose down pitch rates of the X-29 as a function of Mach number (altitude varied from 10,000 to 20,000 ft). Several flight data points (both nose up and nose down) from the maneuver shown in figure 11 are also included as well as F-18 pitch rate data for comparison purposes. The simulation maneuvers consisted of two types of maneuvers: a full aft stick step input and a doublet type input which consisted of a full aft stick input followed by a full forward stick input timed to try to force the control surfaces to maximum rates.
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May 3, 2019 14:04
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TheFluff posted:Of course I'm talking about static stability Yeah, and I'm saying that's not the kind of stability relevant to the F-16s maneuverability. "Short coupled" would be a good description of the F-16, like having a short wheelbase. Which General Dynamics marketing genius came up with "relaxed stability" I don't know, it was probably to calm down some Air Force generals who didn't like the sound of "unstable". TheFluff posted:As far as I know dynamic instability isn't really interesting at all in this discussion... It was just an example of different flavors of stability. You're stuck on this static stability thing where any plane called "unstable" exhibits diverging characteristics, i.e. a dive wants to be a steeper dive, but that is not the case. There are different kinds of stability. But a plane with relaxed stability will probably have negative dynamic stability, even if it has positive static stability. TheFluff posted:If the center of lift and the center of gravity are very close together then the aircraft is probably going to have a neutral (ish) static longitudinal stability No not at all. They can be very far apart and still have any flavor of static longitudinal stability. Negative is for instance a plane where increasing speed from diving causes a steeper dive, and slowing speed from climbing causes a steeper climb. Typically this is more of a thing in the roll axis than in pitch, where a moment's distraction lets a slight bank run away into a steep one.
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May 3, 2019 14:10
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Ola posted:Yeah, and I'm saying that's not the kind of stability relevant to the F-16s maneuverability. "Short coupled" would be a good description of the F-16, like having a short wheelbase. Which General Dynamics marketing genius came up with "relaxed stability" I don't know, it was probably to calm down some Air Force generals who didn't like the sound of "unstable". Ola posted:It was just an example of different flavors of stability. You're stuck on this static stability thing where any plane called "unstable" exhibits diverging characteristics, i.e. a dive wants to be a steeper dive, but that is not the case. There are different kinds of stability. Ola posted:But a plane with relaxed stability will probably have negative dynamic stability, even if it has positive static stability. Ola posted:No not at all. They can be very far apart and still have any flavor of static longitudinal stability. Ola posted:Negative is for instance a plane where increasing speed from diving causes a steeper dive, and slowing speed from climbing causes a steeper climb. Typically this is more of a thing in the roll axis than in pitch, where a moment's distraction lets a slight bank run away into a steep one. None of these are relevant to this discussion. I don't think stability around the roll axis even is a thing that exists at all. You're confusing a whole bunch of different dynamic modes here that don't really have anything to do with the discussion. Like, sure, speed instability is a thing that exists, sure - the Draken for example is very hard to fly on final approach because any reduction in speed leads to an increase in AoA which leads to more drag which leads to a speed reduction, so you can call it "unstable in airspeed", but it's not what I'm talking about and it's not relevant to maneuverability. Like, you're using some words that have meanings but almost nothing you're saying makes any sense. TheFluff fucked around with this message at 14:47 on May 3, 2019 |
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May 3, 2019 14:44
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Ola posted:Yeah, and I'm saying that's not the kind of stability relevant to the F-16s maneuverability. And in saying that you're incorrect, the F-16 has reduced static stability, that's the whole thing about it. quote:It was just an example of different flavors of stability. You're stuck on this static stability thing where any plane called "unstable" exhibits diverging characteristics, i.e. a dive wants to be a steeper dive, but that is not the case. quote:
How so? E: aaaaand beaten E2: interesting fact: early in the F-16 design they left themselves the ability to move the wing one bulkhead aft and thereby have positive static stability like a normal plane, in case the FBW didn't work out. vessbot fucked around with this message at 14:57 on May 3, 2019 |
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May 3, 2019 14:48
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I'm not going to do the massive quotathon thing. Here is a summary of static/dynamic: https://www.boldmethod.com/learn-to-fly/aerodynamics/3-types-of-static-and-dynamic-stability-in-aircraft/ Have fun.TheFluff posted:I don't think stability around the roll axis even is a thing that exists at all. Oh c'mon...
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May 3, 2019 14:59
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blugu64 posted:Post a forward swept flying wing. When they're tumbling, the wing sweep is forward about half the time.
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May 3, 2019 15:13
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Ola posted:. Here is a summary of static/dynamic: https://www.boldmethod.com/learn-to-fly/aerodynamics/3-types-of-static-and-dynamic-stability-in-aircraft/ Have fun. It would help support your point if you say what you're addressing with this, because right now that's not clear.
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May 3, 2019 15:13
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Ola posted:I'm not going to do the massive quotathon thing. Here is a summary of static/dynamic: https://www.boldmethod.com/learn-to-fly/aerodynamics/3-types-of-static-and-dynamic-stability-in-aircraft/ Have fun. Okay, I'll do an effortpost to explain why you're wrong and what static stability analysis is when I get home because I'm clearly not getting through. Ola posted:Oh c'mon... No I'm serious, I don't think any aircraft exist that are anything other than neutrally stable around the roll axis (that is, neither stable nor unstable). I believe you may be thinking of the natural tendency to spiral, but that's actually a yaw coupling. As far as my understanding of aerodynamics goes, a disturbance that causes movement around the roll axis cannot on its own cause any further movement around that same axis - it will neither lead to a further roll nor return to the initial state. TheFluff fucked around with this message at 15:26 on May 3, 2019 |
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May 3, 2019 15:16
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TheFluff posted:Okay, I'll do an effortpost to explain why you're wrong and what static stability analysis is when I get home because I'm clearly not getting through. Not as in "suddenly falls away to the left" but: https://en.wikipedia.org/wiki/Aircraft_dynamic_modes#Spiral_divergence vessbot posted:It would help support your point if you say what you're addressing with this, because right now that's not clear. His original post basically calls out General Dynamics et al for being wrong: TheFluff posted:e: tangentially related: without having any insight into any (probably classified) stability curves for real military aircraft with relaxed stability, it seems to me like calling them "unstable" isn't really the right term. They're almost certainly as close to neutrally stable as they can be (that is, disturbances do not lead to a correcting moment, but it doesn't lead to a reinforcing moment either), since if they were statically unstable they'd just have the same issues as a statically stable aircraft except in the other direction. You can also have local instabilities at certain angles of attack (or sideslip) without that restricting the entire flight envelope by those angles. But either he has got the terms mixed up or he's putting the cart before the horse. Having the center of lift near the center of gravity means the aircraft responds quickly, but as a side effect it gets undesirable characteristics that the computer can handle.
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May 3, 2019 15:29
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I didn't catch the edit: Yes it was spiral divergence I was thinking of. If it's yaw coupling, then yaw coupled to what? Roll. There are three axes, longitudinal, lateral and directional and stability can pertain to any one or any combination. Sloshing fuel tanks and bad load management can cause roll instability, if not an aerodynamic instability.
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May 3, 2019 15:35
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Ola posted:I didn't catch the edit: TheFluff fucked around with this message at 15:50 on May 3, 2019 |
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May 3, 2019 15:45
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No offense but you sound like someone that plays a lot of flight sims that knows enough to sound like they know what they’re talking about, but have literally no idea what you’re talking about. Aircraft, specifically militarily aircraft (and I guess the MAX8  ) can and are built to be unstable and are only flyable with computers making adjustments faster than a human can to keep the instability in check and make them fly like normal airplanes. There are advantages to maneuverability that come with doing that. I don’t know what’s so hard to understand about this.
e.pilot fucked around with this message at 15:56 on May 3, 2019 |
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May 3, 2019 15:54
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e.pilot posted:No offense but you sound like someone that plays a lot of flight sims that knows enough to sound like they know what they’re talking about, but have literally no idea what you’re talking about. I'm not gonna lie, most of what I've learned about aerodynamics and stability analysis came from Kerbal Space Program, or were at least initiated by Kerbal Space Program. If you disagree with what I'm saying though you're welcome to make an argument any time you like. I know we do have some actual knowledgeable people in here and I'm certainly not claiming to be an expert, but I'm pretty sure I know what I don't know. e: e.pilot posted:Aircraft, specifically militarily aircraft (and I guess the MAX8 I guess I must be really bad at making my point, or maybe nobody except vessbot understands the words I'm using, because this is orthogonal to the entire discussion. The only thing I'm talking about is that I find "unstable" to be a misleading term that really doesn't help understanding of how or why that maneuverability advantage exists. TheFluff fucked around with this message at 16:02 on May 3, 2019 |
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May 3, 2019 15:56
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Imagine you’ve built some silly rocket in Kerbal that flies beautifully and is amazingly stable, but turns like garbage because it is so stable and is next to useless. Now imagine you’ve built one that you can turn, but turns too easily and is a handful to fly and frequently loses control, but it is controllable for a little bit you just can’t physically keep up with the control inputs to keep it flying straight. Now imagine you put a computer in charge of making it fly straight, it makes all the little adjustments for you so it appears to fly like the normal stable rocket you built before, but when you want to turn, it relaxes all those hundreds of little inputs it had been doing and allows the rocket to turn really well in the direction you’ve asked the computer to turn, taking advantage of the instability. When you decide you’re done with the turn and return the controls back to normal the computer resumes making the constant adjustments to make it fly straight. Now let’s take this a step further: You’ve built an airplane that’s amazingly stable, the best, let’s call it an F4 Phantom. It’s not a bad plane, but man it really sucks at turning, especially at speed, what can be done to fix that? Bigger control surfaces maybe? Now it weighs more and the stresses on the control surfaces are greater. Less payload and less reliability. And since it weighs more, now it needs bigger engines to have the same performance, those weigh more too, and use more fuel so now the plane needs to carry more fuel. Now the handling characteristics are even worse than what they were before since it’s so much heavier and bigger. So what do you do? You build a plane that turns so well it normally wouldn’t be able to fly without losing control because the plane would want to turn itself if not for making control inputs to counter the turning tendencies. But that would be fatiguing to fly, and if it were made really unstable maybe even impossible to fly, because control inputs couldn’t be put in fast enough. So what if we put one of those newfangled computers in charge of keeping it stable? These computers can keep the oscillations of the instability in check faster than the pilot can even notice them. In fact the computer will be so important to keeping the plane flying we’ll put four of them in for redundancy. We’ll call this one the F-16. It works great and is very maneuverable but flies intuitively like a normal airplane because of the computers, even though it is an unstable design. e.pilot fucked around with this message at 16:26 on May 3, 2019 |
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May 3, 2019 16:24
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