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Evan
Evan
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Last Activity: Today, 02:39
Joined: 2008-01-19
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  • You are confusing me. Yes, the philosophy should be 'fail gracefully', or more to the point, 'fail survivably'. So the gear strut should be fused to fail before transferring catastrophic loads to the wing spar, either by detaching or by penetrating the wing without damaging the wing spar or fuel tanks. And yes, the scenario that caused both accidents was foreseeable and not of such low probability that it could be disregarded. (The pilot actions that caused AA587 were beyond pilot error. They were bizarre, abusive, wrongly trained and reckless, which I would concede is unforeseeable, but I like the idea of fusing the rudder anyway). But I lost you here:



    They happened because the gear resisted against the wing spar, which couldn't resist that load. RIght?...
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  • That's what I'm getting at. If something has to give, and the gear strut is designed not to give, and the ground isn't giving, isn't that something sort of obvious? And isn't it obvious that, if that something is the core wing structure, it is going to spill some fuel, specifically all of the fuel, as the opposing wing rolls the fuselage inverted? I suppose they considered the possibility of gross pilot error and PIO an unreasonable case, and I think that's where the problem begins....
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  • The ADS-B data has it approaching around 120kts and landing between 110-103kts (ground speed).



    Indeed.










    While I share your concerns, I am wondering why the design certification standards focus on ensuring the gear fails in a way that does not compromise the fuel tanks but not on the vulnerability of the wing spar, which, if compromised, has repeatedly resulted in a very dramatic compromising of the fuel tanks.



    This should certainly be a focus of the investigation....
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  • Category G is altitude. I is speed. K is vertical speed. The outlined portion exceeds stable criteria. Why no go-around?




    The winds at that general time (172000Z 27027G35KT) were reportedly exceeding landing criteria as well. Was there windshear involved?...
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  • Apparently, there is video of the landing but I haven’t seen a link. Does the final approach look stabilized and it’s just a late flare or lack of flare?
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  • First impressions:

    - Getthereitis.
    - CRJ900 not rated for carrier landings.
    - Possibly a gear collapse and broken wing spar leading to roll over. Where is the main strut attachment in relation to the wing spar of the CRJ?...
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  • No, the original point was that there is a minimum speed at which an airplane can fly with aerodynamic lift alone, and thus you cannot stall below this speed. Now, I don't know if this is true or not.

    Vmu is a reference speed, the minimum speed that the airplane will fly at MTOW (in ground effect with flaps extended) at the maximum mechanically-limited AoA. The data I have shows tail-strike attitude for the B777-200 at just above 12deg and clean maximum angle-of-attack at a bit under 13deg. I mention this only as a baseline reference.

    So, if we can fly the same airplane at 60kts, either something other than airspeed and AoA is augmenting the lift force or something is greatly reducing the load factor?



    Let me put it this way: It is commonly agreed that 'flying' in the airplane context began with the Wright brothers. Yet, people had been 'flying' long before this. In airships. Being hurled in trebuchets or shot out of cannons. Or just...
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  • The pitch angle of the aircraft is not directly controlled by the pilots, but is a secondary result of other control inputs. During level flight the wings produce an upward lift, while the tail-mounted elevators produce a small downward component of lift in order to maintain a desired pitch angle. When flight conditions change, such as when the pilots deflect the elevators down during freefall, the relative forces from these two surfaces change. Since their moment arms are different, this causes a shift in the center of lift so that it no longer coincides with the aircraft center of gravity, resulting in a torque that causes a pitch rotation of the aircraft....
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  • Aerodynamic lift = lift force generated by the wing surface moving through the airstream.

    Aerodynamic stall = a loss (stall) of that lift force due to the separation of the airstream from the wing surface.

    Centrifugal lift (this is where I get creative): = a reduction in load factor provided by the mass of the aircraft moving about a radius.

    I don't know much about the forces involved in aerobatics. Explain to me precisely how a 650,000lb airliner could have a load factor low enough to enable it to remain under 60kts in flight? I don't know what Vmu is on the 777 but it's well above that. It can be descending in an unstalled condition but it must not be descending at a rate that would generate an airspeed above 60kts. So what I'm assuming here is that a force must be greatly reducing the load factor, and I assume it is centrifugal force....
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  • Who said anything about 'intentional'? The point is that the airplane cannot be stalled if it's not generating any meaningful lift to begin with. There's nothing to stall.
    If only you had worked that out 4 pages ago....
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  • Okay, wait a sec... my one condition is that an airplane cannot be stalled if it's not flying to begin with. Meaning, it cannot be aerodynamically stalled if it's not aerodynamically flying, that is, if the lifting force needed to fly is being provided by something other than aerodynamic force. Centrifugal force or momentum, for example.

    As I'm envisioning it (and your use of the Vomit Comet example) you are referring to a state where insufficient aerodynamic lift is being supplanted with centrifugal force, so that, while the airfoil cannot generate aerodynamic lift needed to keep the aircraft from falling out of the sky, the CF augments this. Is that fairly considered 'flying' in the context of stall? This goes from aerodynamics to ballistics.

    If so, can you stall it? If you could pitch the aircraft up above critical AoA near the top of a parabola, would it make any difference when CF is doing most (or all) of the work?

    Meanwhile, is the airplane...
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  • But can you get there? When you are generating less lift than weight, gravity will increase your airspeed until the ground abruptly reduces it....
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  • Well, fortunately, there's better answers to this "simply stupid question". I found this very detailed description of parabolic flight that does also make this "nonsense assumption".



    https://pmc.ncbi.nlm.nih.gov/articles/PMC2598414/...
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  • Hmmm… To stall, the wings must first be generating lift. Then they must exceed critical AoA. Keep in mind the difference between flying and falling....
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  • Ah... are you bringing in an example of flying level below the 1G stall speed? What I'm saying is that the airplane cannot be stalled below the speed at which it would stall in a given condition, so, in this given condition, there is no possibility of stall at the top of the parabola, therefore there is no stall speed, but it is still moving at about 140kts indicated....
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  • In order to stall, you must first be flying, unstalled. How do you fly a 777 down to 60kts?...
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  • Sure. It flies a parabola that creates a 0G (or near 0G) freefall condition. What does that have to do with stall? What is the 0G stall speed of a KC-135?

    Oh not again... make that: What is the 0G stall speed of a KC-135?...
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  • Fixed....
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  • I’m talking about ANY attitude or condition, but not any speed.

    (sigh) This statement is technically untrue:



    It is commonly used in training to emphasize the fact that you can stall at any speed above the nominal 1G stall speed, as Gabriel pointed out. I am referring to speeds well below that, speeds at which the airplane cannot be flying in the first place. For example, if your Cessna 172 has a flaps-down minimum speed of 40kts, you can still stall it at 140kts but you can’t stall it at 20kts. If the airplane isn’t flying, it can’t stall. Yet these speeds are still speeds, so you can’t stall every airplane at every speed.

    That’s the riddle. If you don’t get it now, I guess you never will....
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  • Now I'm curious. If you try to slow a 777 down to 60kts in level flight, it will stall long before you get there. If you keep the AoA below critical angle, it will steeply descend, and thus the speed will increase long before it drops to 60kts. So how could you possibly stall a 777 at 60kts if you can't even get to 60kts? Or any such speed?

    You could let the clutch out too quickly. That's the winning answer unless you've got a better one....
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