Here is the best example of all - a B-757
http://www.aerospaceweb.org/question/conspiracy/q0274.shtml
http://www.aerospaceweb.org/question/conspiracy/q0274.shtml
Why Flight 93?
- Thread starter PhantomWolf
- Start date
OK, those charts are a pain to read, so I'll take some time to do that tomorrow. Beachnut passed me some additional AoA parameters last evening so we'll discuss those and resolve it. The numbers he passed DO NOT correlate to reasonable or proper AoA for any aircraft and the AoA range didn't correlate. It's possible that Gravy is not the culprit here.
Assuming Mr. Morten @ pprune is correct with a -20 degree AoA they were pushing some pretty hefty -G at impact. Based on 1-2 degrees at cruise I'd estimate at least 2-3 or more negative G's to achieve that kind of negative AoA.
ADD:I have found the AoA parameter on the FDR printout and although it says degrees on the printout, that simply can not be. The aircraft is cruising at a -16 on the charts, but we know to generate a positive lift component the AoA must be positive. Based upon the Boeing statement that the B-757 cruises at 1-2 degrees AoA and assuming the scale is linear the aircraft would have had -3 to -4 degrees AoA at impact. That's a guess as some type of correction factor must be applied to the FDR data to achieve the correct number of degrees.
ADD: That's what beachnut said several posts ago. Why did I waste my time doing this?
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Gravy
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I would go over the charts myself if I thought this whole issue was anything but an attempt by Gregory to get lost in details, to find smaller and smaller "anomalies," in order to avoid the mountains of evidence that prove beyond all doubt that flight 93 crashed there, killing real people in the real world.
I will not waste my time on 9/11 "no-planers."
I will not waste my time on 9/11 "no-planers."
An excellent suggestion! I just glanced at a couple of other parameters and find no difference to what Mr. Morten listed. Heck, you can even see the hijacker pitching the aircraft as the passengers were attempting to enter the cockpit. I'm sure you can see the roll too, but I didn't look. That gave me cold chills anyway, so I'll leave it be.....
Gravy
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I don't want to suggest that it's not interesting, or that Gregory is being dishonest about not knowing the answer. It's tricky, and I have great respect for people who have the intellectual curiosity and ability to solve problems like this. I just don't think that's what's driving Gregory in this case. Just my $.02.
Isn't the "indicated AoA" on 757s actually some sort of an interpreted value? It doesn't show the actual AoA but a calculated suggestion, an envelope, boundary, where one could still fly at current parameters (airspeed) without going in a stall? Could the -19° actually mean "Hey, we're going so fast and generating so much lift, we could raise our pitch by 19° and still not stall." 19 degrees below stall? Positive values would indicate stall? Is that it? FDR data appears to support this.
The only instance where the iAoA went into the positive, was shortly after the autopilot was disengaged and Jarrah (or whoever the hijacker pilot was) took the plane into +20° pitch, pulling over 3 g (@ 09:59:53).
AA77's data doesn't go positive at all, right until the last recorded value, where it hits a whopping +71.5°. Have any CT[]s latched onto this anomaly yet?
The only instance where the iAoA went into the positive, was shortly after the autopilot was disengaged and Jarrah (or whoever the hijacker pilot was) took the plane into +20° pitch, pulling over 3 g (@ 09:59:53).
AA77's data doesn't go positive at all, right until the last recorded value, where it hits a whopping +71.5°. Have any CT[]s latched onto this anomaly yet?
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The AOA of -20 in a high speed dive going fast, is relative to a cruise AOA of -16 and a takeoff AOA of -6 on this 757. Think about being on a 757 at cruise the AOA on this model with this AOA would read -16. Think about your pitch angle on the cruise it would be only 3 degrees. I do not know what units the AOA is in on the FDR. "Pilots" are use to having the AOA presented in a form of approach speed bug, or a normalized instrument to 1, with .2 as cruise and .6 as approach speeds and a red zone of stall near .9. The neat thing about AOA, it covers all weights, I mean a pilot has to know the weight of an aircraft to calculate speeds for landing and approach, etc. But the AOA reading shows the pilot the speeds, just check the AOA as you slow to a approach speed and it indicates on speed with a reading. You set .6 AOA in a KC-135, and that is the approach speed for your configuration, you do not have to look it up in Charts. (but you pilots need to know the speed incase the instrument is bad, a good check both ways)
The -20 AOA is relative and could be used to calculate a velocity vector from pitch, as speed increases the angle of AOA would decrease. As seen the high speed dive is -20, cruise is -16. The AOA could be in degrees, but you need to know where zero angle with the cord line is. Good thing the aircraft does not care about all this crap. It would not be too out of line for AOA to be in degrees, but you must think about relative to what. So it may not be far off to think the AOA would be -4 degrees from cruise for a high speed dive.
So at take off you pitch up to lift off, the AOA is -6, then you level off at cruise and it is -16. Not bad, take off pitch is about 10 degrees, and cruise is about 10 degrees less. AOA angle in the text books would be smaller at high speed. At a real fast speed, in level flight, I would not be surprised if the cabin deck would appear to be nose down slightly. As a plane slows down with out flaps the AOA would approach -6, and the deck of the aircraft would appear to be nose up. The AOA can not be added or subtracted from the pitch to get the velocity vector unless you know some more information.
http://www.aerospaceweb.org/question/aerodynamics/q0165.shtml Shows the AOA stuff. Have I messed this up enough?
Gregory is not worth it. If he was a "real" engineer, he could do this himself.
The -20 AOA is relative and could be used to calculate a velocity vector from pitch, as speed increases the angle of AOA would decrease. As seen the high speed dive is -20, cruise is -16. The AOA could be in degrees, but you need to know where zero angle with the cord line is. Good thing the aircraft does not care about all this crap. It would not be too out of line for AOA to be in degrees, but you must think about relative to what. So it may not be far off to think the AOA would be -4 degrees from cruise for a high speed dive.
So at take off you pitch up to lift off, the AOA is -6, then you level off at cruise and it is -16. Not bad, take off pitch is about 10 degrees, and cruise is about 10 degrees less. AOA angle in the text books would be smaller at high speed. At a real fast speed, in level flight, I would not be surprised if the cabin deck would appear to be nose down slightly. As a plane slows down with out flaps the AOA would approach -6, and the deck of the aircraft would appear to be nose up. The AOA can not be added or subtracted from the pitch to get the velocity vector unless you know some more information.
http://www.aerospaceweb.org/question/aerodynamics/q0165.shtml Shows the AOA stuff. Have I messed this up enough?
Gregory is not worth it. If he was a "real" engineer, he could do this himself.
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rwguinn
Penultimate Amazing
You are likely correct. If the cruise at -16 as quoted by beechnut is correct, then they are referenced to something other than degrees.
I know that some flat-bottom airfoils will generate lift at negative AoA (The Clark-Y springs to mind), but -16 degrees is way outside the positive lift range. I also seriously doubt that any commercial carrier aircraft has the pitch authority to induce a +20 degree AoA at 500Kt (Which would, without a doubt stall the wing). It would require very rapid and very large elevator deflections to accomplish. With the way most commercial aircraft are set up, any attempt would result in a "zoom" climb, trading airspeed for altitude, culminating in a stall when the airspeed bled of.
Sorry I started this mess--AoA is not particularly important, other than to determine stall status. Pitch angle will be important, if by "pitch" we mean "angle of the aircraft X axis with respect to the gravity vector".
This is the best possibility that's been suggested. This is very possible and a cursory glance at the numbers it seems to be in the ballpark. I'd guess the wing would stall at approximately +16-18 degrees AoA, so the -16 at cruise with this interpolation would roughly correlate to the Boeing published cruise AoA for a B-757. Using this same interpolation, a -6 for TO would also be in the ballpark as well. For the climb the AoA would be near Max lift over drag (Max L/D), but it's so compressed by time in the chart it's difficult to determine numbers, but it is still in the ballpark as well.
Remember this is not what the pilot is seeing, it is merely the FDR data. There is likely some sort of light display in the cockpit, not a actual AoA indicator of real numbers.
This idea also accounts for the range of +5 to -25, as well. I'd say this is very close to what the FDR is showing.
ADD - I don't want to speculate what the FDR for AA77 shows as I haven't looked at it. As I understand they were made by different manufacturers, so may show parameters differently.
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This is the best idea of all.
I think we need to put this to bed and leave it be. One thing is for sure. If the FDR data were real AoA in degrees we'd be seeing POSITIVE numbers for the entire flight except for the level off at altitude and final decent. We can not determine the real AoA numbers in degrees for the final descent prior to impact until we know what the FDR numbers are.
Celestrin's suggestion is the best, so far, as it does correlate to reality. Until such time as someone knowledgeable of this FDR model stops by we don't know for certain.
While steering clear of a discussion of UA93's FDR AoA indications it might be worthwhile for those interested to learn more about these modern computerized (fly-by-wire) flight control systems with regard to a stall condition.
In about 1981-82 the F-111 flight control system was modified to NOT allow the aircraft to stall in a conventional manner. In other words, it would not exhibit the traditional wing drop, post stall gyration or other typical stall characteristics of traditional aircraft. The fly-by-wire system would not allow the AoA to reach a stall point (out of control). The aircraft would merely enter a high rate of descend (sink rate) still in a very controllable attitude. These same modifications were included in the flight control system in the F-16. Even tho' these aircraft were manufactured by General Dynamics, I suspect Boeing used similar technology in the 757 and 767 fly-by-wire flight controls. Consequently, the 757 and 767 likely will not stall in a conventional sense. Of course, the high sink rate will still allow the aircraft to crash, but (if you will) in a controllable manner. Just a $ 0.01 or two for those interested.
In about 1981-82 the F-111 flight control system was modified to NOT allow the aircraft to stall in a conventional manner. In other words, it would not exhibit the traditional wing drop, post stall gyration or other typical stall characteristics of traditional aircraft. The fly-by-wire system would not allow the AoA to reach a stall point (out of control). The aircraft would merely enter a high rate of descend (sink rate) still in a very controllable attitude. These same modifications were included in the flight control system in the F-16. Even tho' these aircraft were manufactured by General Dynamics, I suspect Boeing used similar technology in the 757 and 767 fly-by-wire flight controls. Consequently, the 757 and 767 likely will not stall in a conventional sense. Of course, the high sink rate will still allow the aircraft to crash, but (if you will) in a controllable manner. Just a $ 0.01 or two for those interested.
Probably true, but the problem is, this is not about Gregory at all. I've been casually wondering about AoA for months, never bothering to really get to the bottom of it. It's time to clear it up once and for all
I've found a Boeing article about the AoA.
http://www.smartcockpit.com/pdf/flightops/aerodynamics/0039/
It would appear that our mystery FDR values are actually a part of what's called the Pitch Limit Indicator (PLI).
That's the behaviour displayed in the FDR data, so I'm pretty sure that this is what "indicated AoA" actually is. The numbers are not the AoA itself, it's the difference between the actual AoA and the calculated stall AoA, which is Mach number dependent.
AA77's data appears to follow the same principles, so it's even more likely to be a Boeing feature rather than a feature of the FDR manufacturer. And it would be measured in degrees.
Although not "fly by wire", the flight control system were advanced and help making flying the jets easier.
Correct! Fly-by-wire was a poor choice of words on my part when applied to 757/767. Let's change it to computerized flight control system and then we'd be correct.
I guess the military paves the way as fly-by-wire has been a viable concept since the 60's, 70's at the latest. I did not realize Commercial Airline design was so far behind the technology curve.
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By George, you've got it!
Everything now fits near perfectly with the FDR Data. So, in conclusion the actual AoA of UA93 near impact was as I said previously, in the ballpark of -2 to -4 degrees, not -20 degrees as previously stated.
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Gravy
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The Concorde used analog FBW computers, and the A320 was introduced with digital FBW in 1984. I think all the modern Airbus planes are FBW, as is the Boeing 777. As Apathoid points out in this excellent paper, the 757/767 engines are essentially FBW.
Probably true, but the problem is, this is not about Gregory at all. I've been casually wondering about AoA for months, never bothering to really get to the bottom of it. It's time to clear it up once and for all
I've found a Boeing article about the AoA.
http://www.smartcockpit.com/pdf/flightops/aerodynamics/0039/
It would appear that our mystery FDR values are actually a part of what's called the Pitch Limit Indicator (PLI).
That's the behaviour displayed in the FDR data, so I'm pretty sure that this is what "indicated AoA" actually is. The numbers are not the AoA itself, it's the difference between the actual AoA and the calculated stall AoA, which is Mach number dependent.
AA77's data appears to follow the same principles, so it's even more likely to be a Boeing feature rather than a feature of the FDR manufacturer. And it would be measured in degrees.
Good find, Celestrin. I completely forgot about this and I think that solves our little AoA mystery. I forgot about it because you hardly ever see it on the display(on the 757 - it shows up as a horizontal yellow bar on the EADI, over the various pitch reference lines in the "sky") . I wonder if there is another AoA parameter for vane angle? If not, I guess a stall active/inactive discrete would tell investigators what they wanted to know if there was a suspected high AoA...
Mince
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Why flight 93? Why flight 93? Why flight 93? Sheesh, always with the flight 93, you guys. Look, you stick to operations and let us handle planinng, mmmmk? When you need to know something, we'll tell you. If you don't know it, you don't need to know it.
K?
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BTW, there is a vane on the 757 as one of the parameters on the FDR is for AoA heat on/off. (I believe you were in one the previous discussion about the source of the AoA data).
I see no need for measuring vane angle. What purpose would that serve when the differential data is already recorded on the FDR?