Originally Posted by
Smythe
1. I am looking at what was stated, and provide the aerodynamic reasoning behind it. Forget about stick feel.
2. If, at high angle of attack, or low speeds, if, due to certain areas of the wing the airflow is going supersonic, the resulting lack of laminar airflow over the wing (as stated by the Boeing wind tunnel tests) induces a stall...
3. The aerodynamics of the engine and the wing are creating stall conditions at lower AoA and other conditions than the system is programmed for.
4. Look at what they tried to do, adding vortex tabs, changing the wing design....
5. Changing the wing design?? Do you change the wing design to make the stick pressure the same, or to prevent stall?
The differential pressure on the yoke is a RESULT of the stall.
6. Stick pressure? I feel that is a half baked response by Boeing to mask the problems with the aerodynamics of the wing/engine design, and simply does not make sense. Maybe that is how is was presented to the FAA, but I dont think that is reality. Boeing will never admit that the aircraft was not aerodynamically stable.
7. Is MCAS operational in AP? While I keep hearing the mantra, it only operation with AP off, it appears it is operational according to several reports that show turning off the AP resolves the problem. Wasnt it the case with the last crash, that when they turned AP back on, MCAS engaged again?
8. In one incident, an airline pilot reported that immediately after engaging the Max 8’s autopilot, the co-pilot shouted “DESCENDING,” followed by an audio cockpit warning, “DON’T SINK! DON’T SINK!”
9. “I immediately disconnected AP (Autopilot) (it WAS engaged as we got full horn etc.) and resumed climb,” the pilot writes in the report, which is available in a database compiled by NASA. “Now, I would generally assume it was my automation error, i.e., aircraft was trying to acquire a miss-commanded speed/no autothrottles, crossing restriction etc., but frankly neither of us could find an inappropriate setup error (not to say there wasn’t one).
10. In reality, MCAS is anti-stall.
1. "I am looking at what was stated, and provide the aerodynamic reasoning behind it. Forget about stick feel."
stick force is a consequence of aerodynamics, control system architecture and modifiers.
2. (A) "...going supersonic..."
High AOA is inconsistent with high mach numbers in general, wings tend to fall off Par 25 planes when pulling high AOA at high mach, available AOA is constrained by buffet in such cases, so the terminology used is vague... At low speeds, there is no transonic flow on a BAC447-450 type section at any AOA. The BAC airfoil section may have gone supersonic in the Silk air bingle, and possibly Adam Air splash, but otherwise it is rather unlikely to get to a point where the section is supersonic, e.g., has an oblique shock formation. It will usually have a normal shock at around 0.5c-0.6c in cruise, at say M0.78 and above, at 2.3 AOA.
(B) "...lack of laminar flow...";
irrespective of what is being smoked, and how much was spent on the design, there is no laminar flow worth noting on any RPT jet transport. If you want laminar flow, go and look at a standard sail plane. the slat TE destroys laminar flow, as does the first rivet head, screw head etc that exists on the slat. For the first 1% of the chord, which more or less is in the radius of the LE, around the Kutta point, there may be laminar flow dependent on when the plane was last washed. FYI, the slat TE eats up the boundary layer, it is a discontinuity in the surface, and causes separation in the presence of a shear, and that always gives an initial span-wise vortex structure which is highly unstable as is any flow behind an aft facing step... That vortex structure starts to shed with a Strouhal number that is identifiable as a harmonic of the how frequency vibration that arises from the instability of the normal shock and the associated SBLI foot, which is observable oscillating on the wing in steady flight. Look out the window with the sun aligned down the mid chord span and you will have a Schlieren iamge of the shadow of the densty change in the foot of the shock. In essence, at any time, your Boeing, or Bus doesn't have laminar flow anywhere except in the idealised models of the airfoil in simplified CFD modelling. Sorry. That is not to say it can't be improved...
(C ) "...induces a stall..."
not the way flow works. Sorry. A turbulent BL has one benefit over laminar and that is it can cope with adverse pressure gradient perturbations before becoming messy. Flow behind a shock is separated near the surface, but that is not a stall as such, which is defined in the regs... as a number of specific conditions that occur. separation may be a pain, particularly geographic ones, but it is a normal part of life. Stalls are stalls... per the regs. Taking your comment to an extreme, pulling say 10 AOA at M0.82 will still not end up in a stall, it will take the wings right off the aircraft, but before you get to that point, flow separation will have resulted in severe buffet, and reducing Cl/AOA, which put you back toward the beginning...
3. "...The aerodynamics of the engine and the wing are creating stall conditions at lower AoA and other conditions than the system is programmed for..." what is your evidence for that statement? If that was so, then the aircraft would have rather high V speeds, as they are a function of Vs.
There is an interaction between the nacelle and the wing, and was ever thus. If you look on the side of a B737 Classic, NG A320, 330, B777 B787 etc etc etc, you will find, there is a strake on the side of the nacelle around the location of 10 to 2 o'clock, more often on the inner side of the nacelle, but it can be on both sides. The strakes are agnostic, they are on CF6, RR, GE, CFMI engines etc, and they are there as the standard, pre Max and the GTF designs did indeed have an interaction between the nacelle and the wing abaft of the nacelle that suppressed Cl/AOA at modest to high AOAs, and that caused performance decrements. The strake develops a very powerful vortex filament that passes over the top of the LE HLD's and controls the extent of the interaction. It is a thing of beauty, and pretty to watch on a high humidity day. The Vortex itself is not doing the work, it stops the flow adjacent to the filament from being discombobulated.
The Max issue is entirely different, in fact, it goes in the opposite direction, and it increases lift of the section effectively, or at least in the aggregate results in an reduction in the component Cm of the span-wise region behind the nacelle, which is exhibited as a reduction in stick force per g... If you have a back of an envelope you will see that in order to achieve a reduction in Cm, for the location on the planform there is a high likelyhood that the design actually increased local CL, at the same time as moving the Cp forward on that section, and that gives the Cm outcome. Now, Bill B could cure the deal by reducing or removing the strake that is on the max engine, but that comes with a performance penalty, the Max had better performance than it would have otherwise due to the nacelle design, it just came with a side effect that the regs would need a SAS input to meet longitudinal control reqts. Had that been done well, then the Max would have been acknowledged as a great design. Heck, even now, it can be, if Boeing corporate management grows a spine and get their house in order on ethical matters. The design of the MCAS and its changes did not happen in a vacuum, they occurred as a result of the attitude of the beancounters and the ethics that flow down the sewer that is the board room to the poor sod at the coal face.
4. Look at what they tried to do, adding vortex tabs, changing the wing design....
That is what happens with almost all designs, including Busses etc. The dog tooth on the Bus arises from a surprise in testing... VG's are a good tool for curing issues of separation and shock issues, they don't cook means well, but they have their uses. I would be happier if they did do more work on the wing, there is a lot of room for improvement on all of these designs which are the industrial engineers mass produced product rather than the ASW-21 or other embodiment of elegant design.
5. "...Changing the wing design?? Do you change the wing design to make the stick pressure the same, or to prevent stall? The differential pressure on the yoke is a RESULT of the stall..."
There are a number of manners that the design could be tweaked in testing to attempt to achieve compliance with the stick force gradient requirements. As commented above, removing the strake would get rid of the issue, but comes at a cost, whereas MCAS had no cost had it been implemented competently. There are designs out there that have specific application of VGs to meet the same requirement. Almost every aircraft on the ramp has flow modifiers on them, and they are all specific in their application, what the defect was that was underlying their implementation. Each conventional VG has a drag count penalty, according to NASA research of around 0.0002Cd, so they are used sparingly and only when the gain from their use offsets the Cd increase from their installation. Not sure what a differential pressure on a yoke refers to, I only speak english, but if you are referring to the stick force gradient non linearity, that is not caused by stall. Consider that any premature stall around the nacelle will actually improve handling qualities of any aircraft; it will ensure that the lateral control requirements are met, that the aircraft will pitch down, that buffet on the airframe from impingement of wake on the stab and elevators is more likely to be encountered. These are good things. The GTF nacelle doesn't cause a premature stall, it does the opposite... Now if the wing tips stall, then you get great entertainment, and that is not the problem, nor would it arise from the nacelle.... Think of why simple wings have wash out, and why dog tooths, VG's vortilons etc are used on swept wing aircraft. I don't think I agree with your paragraph on that matter...
6. Stick pressure? I feel that is a half baked response by Boeing to mask the problems with the aerodynamics of the wing/engine design, and simply does not make sense. Maybe that is how is was presented to the FAA, but I dont think that is reality. Boeing will never admit that the aircraft was not aerodynamically stable.
If the aircraft was unstable at any point, which is pretty hard to see how that would be achievable, given the margins that exist between the normal aft envelope and the neutral point, then in any case, no TP would have signed off on the acceptability of the aircraft. It is a very straight forward matter to ascertain if the design is or was unstable, and for the record the data that has been published already is sufficient to show that the aircraft was statically and dynamically stable. Long ago I looked at an RPT aircraft that was on the limit of stability in flight, and it is not hard to detect in the data. THE MAX IS NOT UNSTABLE. PERIOD. Don't take my word for it, look at the public domain data on the control inputs. It had a trim issue, and that is all.
7. Is MCAS operational in AP? While I keep hearing the mantra, it only operation with AP off, it appears it is operational according to several reports that show turning off the AP resolves the problem. Wasnt it the case with the last crash, that when they turned AP back on, MCAS engaged again?
Again, as the aircraft IS NOT UNSTABLE, an autopilot doesnt need MCAS to meet any stick force per g compliance matters. That is the first big clue that the issue is and has always been about the force gradient compliance. MCAS would be redundant, a double negative, nonsense for an autopilot engaged condition.
8. "....In one incident, an airline pilot reported that immediately after engaging the Max 8’s autopilot, the co-pilot shouted “DESCENDING,” followed by an audio cockpit warning, “DON’T SINK! DON’T SINK!”
Autopilots are born of man (or woman etc... )per the bard and fail. The reported issue was a pitch down excursion which is an event that is required to be covered in certification, being an aspect that is considered in the minimum engagement and disengagement heights for autopilots. Had the MCAS not resulted in public misinformation and hysteria, then this particular matter would have been kept in its proper place, that being an APFD anomaly, with no association to the MCAS issue.
9. “I immediately disconnected AP (Autopilot) (it WAS engaged as we got full horn etc.) and resumed climb,” the pilot writes in the report, which is available in a database compiled by NASA. “Now, I would generally assume it was my automation error, i.e., aircraft was trying to acquire a miss-commanded speed/no autothrottles, crossing restriction etc., but frankly neither of us could find an inappropriate setup error (not to say there wasn’t one).
The crews getting into the Max deserved more than an Ipad briefing. However, the comment on the APFD anomaly has nothing to do with MCAS.
10. In reality, MCAS is anti-stall.
Nope, not even close. Refer preceding.
cheers,