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WPvsMW
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MAX nacelle, AOA, center of lift, and C/G

Sat Mar 16, 2019 7:13 pm

The premise: In the MAX design, there is a major aerodynamic problem with the impact of the airflow over the tops of the nacelles onto the wing during high AOA, which conditions move the center of lift, and therefore the C/G to center of lift relationship. What was stable becomes unstable, and MCAS version 1 doesn't solve the problem. I don't think the problem that MCAS is trying to solve has been properly characterized, and therefore, remains unsolved except for a small part of the operating window. It's like sRGB colorspace... is MCAS v. 1 covering 50% of the color space?

Thoughts?
 
GalaxyFlyer
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Re: MAX nacelle, AOA, center of lift, and C/G

Sat Mar 16, 2019 9:13 pm

MCAS addresses a aerodynamic problem in the small part of the envelope near stall AOA in clean configuration. It’s not a major problem, only a problem in that small regime. The operational problem that testing didn’t reveal was the case of sole source sensor failing or providing incorrect sensor data to the MCAS. Or, in operation crews are poorly trained in handling these failures.

MCAS is probably a fairly simple elegant solution, it just needed more robust sensor input. In service without the sensor failure(s) it would likely never be encountered by crews. The alternative was likely a stick pusher.

GF
 
stephanwintner
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Re: MAX nacelle, AOA, center of lift, and C/G

Mon Mar 18, 2019 5:59 pm

GalaxyFlyer wrote:
MCAS addresses a aerodynamic problem in the small part of the envelope near stall AOA in clean configuration. It’s not a major problem, only a problem in that small regime. The operational problem that testing didn’t reveal was the case of sole source sensor failing or providing incorrect sensor data to the MCAS. Or, in operation crews are poorly trained in handling these failures.

MCAS is probably a fairly simple elegant solution, it just needed more robust sensor input. In service without the sensor failure(s) it would likely never be encountered by crews. The alternative was likely a stick pusher.

GF


As far as understand what's happened, I agree. But, there's this thing called a Failure Mode Analysis.... in one variant or another (FMEA, FMECA, FMECACA, etc.), Boeing should have considered sensor failures and provided for them. Maybe they erred in counting on the crew knowing to turn it off....or maybe their analysis wasn't sufficient ? Operational testing shouldn't uncover fundamental errors in the failure mode analysis.... shouldn't.

Stephan
 
tealnz
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Re: MAX nacelle, AOA, center of lift, and C/G

Mon Mar 18, 2019 6:47 pm

There’s also now reason to believe there were shortcuts in certification. See the Gates piece in the Seattle Times: https://www.seattletimes.com/business/boeing-aerospace/failed-certification-faa-missed-safety-issues-in-the-737-max-system-implicated-in-the-lion-air-crash/ Looks as if Boeing chose not to share with foreign regulators and possibly FAA the big increase in MCAS command authority implemented as a result of flight testing – on the face of it making it much harder for pilots to respond effectively to faulty MCAS activation immediately after takeoff.
 
WPvsMW
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Re: MAX nacelle, AOA, center of lift, and C/G

Mon Mar 18, 2019 7:21 pm

From the systems design and methodology I worked under in my software years, MCAS should have had logic that clearly alerted the pilots to its intervention, and also disabled its intervention (with alert to pilots)j upon certain conditions, e.g., ground proximity, sensor disagreement, sensor failure. Also, from SDM, a 3 sensor network in view of MCAS' control authority. I think the coming shitstorm of litigation will produce discovery evidence of designers and engineers who advocated that SDM approach being overruled by management.

Side note: I think the MAX models will be recertified as a separate type, and lose commonality with the NG and earlier variants. I also think the impact of the nacelles on center of lift during high AOA will be reexamined.
 
stephanwintner
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Re: MAX nacelle, AOA, center of lift, and C/G

Mon Mar 18, 2019 11:00 pm

I cannot imagine Boeing deliberately withheld data. Certification and the related paperwork processes are (shouldn't be, but are) too complex, I'm sure it's a late change that got missed. And while management may have overridden certain engineers, I again cannot imagine they did so without some supporting analysis. Things get missed, sometimes due to sloppy management and poor processes.

That is not to excuse Boeing - it's the firms job, management's job, to have processes in place to allow engineering - all levels and functions - to deliver high quality work. If the late change paperwork didn't get properly filed and analyses didn't get updated, that's on Boeing. It sounds like the review and approval process should have rejected the high authority. Again, that's on Boeing and the guys who signed it.
 
pugman211
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Re: MAX nacelle, AOA, center of lift, and C/G

Mon Mar 18, 2019 11:37 pm

2 loop holes in MCAS logic,

1, if it got interrupted, it would stop/reset and go again for the full amount (2.5 degrees?)

2, no maximum travel limit was built in. It could command travel to the end stops.

I'm sure once the software update is completed, all will be good with the Max again
 
WPvsMW
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Re: MAX nacelle, AOA, center of lift, and C/G

Tue Mar 19, 2019 12:20 am

pugman211 wrote:
I'm sure once the software update is completed, all will be good with the Max again


After Boeing properly characterizes the movement of the center of lift during high AOA. I would not be surprised if the nacelle tops were flattened, like the nacelle bottoms of some previous B737 variants. We might even see sensors deployed on the top surface of the wings aft of the nacelles. I think the "huge fan hung forward" concept is here to stay, and longer landing gear struts won't be the answer.
 
747Whale
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Re: MAX nacelle, AOA, center of lift, and C/G

Tue Mar 19, 2019 3:55 am

tealnz wrote:
There’s also now reason to believe there were shortcuts in certification.


That's not "reason to believe." That's an article in popular media. The cited article is misquotes, use out of context, and very misleading, as the original material does not state what the article imports.

Some tend to give credence to anything found in writing, on popular media, in the news, or on TV. Just because someone put it print does not make it so. The use of statements in that article actually say the opposite of what they were written to say by their original source.
 
WPvsMW
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Re: MAX nacelle, AOA, center of lift, and C/G

Tue Mar 19, 2019 4:03 am

stephanwintner wrote:
I cannot imagine Boeing deliberately withheld data.


I can't either. It's a felony. I don't know any engineers or executives who want that kind of "free room and board."
 
tealnz
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Re: MAX nacelle, AOA, center of lift, and C/G

Tue Mar 19, 2019 4:59 am

747Whale wrote:
tealnz wrote:
There’s also now reason to believe there were shortcuts in certification.


That's not "reason to believe." That's an article in popular media. The cited article is misquotes, use out of context, and very misleading, as the original material does not state what the article imports.

Some tend to give credence to anything found in writing, on popular media, in the news, or on TV. Just because someone put it print does not make it so. The use of statements in that article actually say the opposite of what they were written to say by their original source.

You’re making a series of negative assertions without citing specifics, let alone offering facts. You can’t expect to be taken seriously. Gates’s article is deeply reported. If you want to challenge his reporting then give us specifics and explain where and why he is wrong.
 
frmrCapCadet
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Re: MAX nacelle, AOA, center of lift, and C/G

Tue Mar 19, 2019 12:48 pm

As I read Gate's article, there was some ambiguity regarding the .6 versus 2.5. FAA did get some information about it, but not all FAA people.
Buffet: the airline business...has eaten up capital...like..no other (business)
 
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GEUltraFan9XGTF
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Re: MAX nacelle, AOA, center of lift, and C/G

Tue Mar 19, 2019 1:07 pm

The Boeing <> FAA certification issue aside, the whole MCAS system screams of being sloppy, rushed, half-baked work. Everything mentioned here and elsewhere is design 101.

-MCAS activation should depend on agreement of 2-3 sensors. If there is disagreement, a warning light should appear in the cockpit.
-AOA sensors themselves should be "smart" enough to realize they are reset to "0" on the ground. If not, a warning light should appear in the cockpit.
-MCAS activation should be indicated clearly in the cockpit.
-The fact that there is discrepancy in the degree MCAS activation (.6 vs. 2.0 degrees) screams more sloppy, rushed project management than corporate malfeasance to me. Neither side knew what was going on and both sides (Boeing and FAA) let it all slip. Whether intentionally or not is the question. If so, I think it's criminally negligent.
-Ditto in terms of the system resetting versus constantly working away, exacerbating the problem. Sloppy work. Bad project management and oversight.

A lot of Boeing heads should roll:
-MCAS project manager
-737 MAX project manager
-Boeing CEO
© 2020. All statements are my own. The use of my statements, including by journalists, YouTube vloggers like "DJ's Aviation", etc. without my written consent is strictly prohibited.
 
stratclub
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Re: MAX nacelle, AOA, center of lift, and C/G

Tue Mar 19, 2019 7:16 pm

GEUltraFan9XGTF wrote:
The Boeing <> FAA certification issue aside, the whole MCAS system screams of being sloppy, rushed, half-baked work. Everything mentioned here and elsewhere is design 101.

-MCAS activation should depend on agreement of 2-3 sensors. If there is disagreement, a warning light should appear in the cockpit.
-AOA sensors themselves should be "smart" enough to realize they are reset to "0" on the ground. If not, a warning light should appear in the cockpit.
-MCAS activation should be indicated clearly in the cockpit.
-The fact that there is discrepancy in the degree MCAS activation (.6 vs. 2.0 degrees) screams more sloppy, rushed project management than corporate malfeasance to me. Neither side knew what was going on and both sides (Boeing and FAA) let it all slip. Whether intentionally or not is the question. If so, I think it's criminally negligent.
-Ditto in terms of the system resetting versus constantly working away, exacerbating the problem. Sloppy work. Bad project management and oversight.

A lot of Boeing heads should roll:
-MCAS project manager
-737 MAX project manager
-Boeing CEO

Could you give it a rest? What we actually know.
    *Boeing screwed the pooch on this one.
    *Boeing is taking full responsibility for this one.
    *Boeing will engineer a fix.
    *The fix will be reviewed and approved for validation testing by the FAA.
    *The fix will be validated by Boeing through testing.
    *Once Boeing has final FAA approval all delivered max aircraft will be modified at Boeing expense.
    *The fix will be incorporated into production.

I don't think that Boeing or the FAA have a requirement for social media approval............
 
kalvado
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Re: MAX nacelle, AOA, center of lift, and C/G

Tue Mar 19, 2019 10:14 pm

stratclub wrote:
GEUltraFan9XGTF wrote:
The Boeing <> FAA certification issue aside, the whole MCAS system screams of being sloppy, rushed, half-baked work. Everything mentioned here and elsewhere is design 101.

-MCAS activation should depend on agreement of 2-3 sensors. If there is disagreement, a warning light should appear in the cockpit.
-AOA sensors themselves should be "smart" enough to realize they are reset to "0" on the ground. If not, a warning light should appear in the cockpit.
-MCAS activation should be indicated clearly in the cockpit.
-The fact that there is discrepancy in the degree MCAS activation (.6 vs. 2.0 degrees) screams more sloppy, rushed project management than corporate malfeasance to me. Neither side knew what was going on and both sides (Boeing and FAA) let it all slip. Whether intentionally or not is the question. If so, I think it's criminally negligent.
-Ditto in terms of the system resetting versus constantly working away, exacerbating the problem. Sloppy work. Bad project management and oversight.

A lot of Boeing heads should roll:
-MCAS project manager
-737 MAX project manager
-Boeing CEO

Could you give it a rest? What we actually know.
    *Boeing screwed the pooch on this one.
    *Boeing is taking full responsibility for this one.
    *Boeing will engineer a fix.
    *The fix will be reviewed and approved for validation testing by the FAA.
    *The fix will be validated by Boeing through testing.
    *Once Boeing has final FAA approval all delivered max aircraft will be modified at Boeing expense.
    *The fix will be incorporated into production.

I don't think that Boeing or the FAA have a requirement for social media approval............

Outlined above is the very best case scenario.
Social media approval may not be required, but approval by other regulators may be a bit more interesting story. Not to mention that FAA may reject either implementation of the patch as presented or entire idea of software patch.
Worst case scenarios may include a lot of fun things. How about avionics replacement on built frames (a-la Toyota unintended accelerations)? Buyback of frames already sold a-la VW dieselgate?
I don't quite believe in those, but best case scenario is not fully believable as well. Hopefully something in between.
 
stratclub
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Re: MAX nacelle, AOA, center of lift, and C/G

Wed Mar 20, 2019 3:38 am

kalvado wrote:
stratclub wrote:
GEUltraFan9XGTF wrote:
The Boeing <> FAA certification issue aside, the whole MCAS system screams of being sloppy, rushed, half-baked work. Everything mentioned here and elsewhere is design 101.

-MCAS activation should depend on agreement of 2-3 sensors. If there is disagreement, a warning light should appear in the cockpit.
-AOA sensors themselves should be "smart" enough to realize they are reset to "0" on the ground. If not, a warning light should appear in the cockpit.
-MCAS activation should be indicated clearly in the cockpit.
-The fact that there is discrepancy in the degree MCAS activation (.6 vs. 2.0 degrees) screams more sloppy, rushed project management than corporate malfeasance to me. Neither side knew what was going on and both sides (Boeing and FAA) let it all slip. Whether intentionally or not is the question. If so, I think it's criminally negligent.
-Ditto in terms of the system resetting versus constantly working away, exacerbating the problem. Sloppy work. Bad project management and oversight.

A lot of Boeing heads should roll:
-MCAS project manager
-737 MAX project manager
-Boeing CEO

Could you give it a rest? What we actually know.
    *Boeing screwed the pooch on this one.
    *Boeing is taking full responsibility for this one.
    *Boeing will engineer a fix.
    *The fix will be reviewed and approved for validation testing by the FAA.
    *The fix will be validated by Boeing through testing.
    *Once Boeing has final FAA approval all delivered max aircraft will be modified at Boeing expense.
    *The fix will be incorporated into production.

I don't think that Boeing or the FAA have a requirement for social media approval............

Outlined above is the very best case scenario.
Social media approval may not be required, but approval by other regulators may be a bit more interesting story. Not to mention that FAA may reject either implementation of the patch as presented or entire idea of software patch.
Worst case scenarios may include a lot of fun things. How about avionics replacement on built frames (a-la Toyota unintended accelerations)? Buyback of frames already sold a-la VW dieselgate?
I don't quite believe in those, but best case scenario is not fully believable as well. Hopefully something in between.


What I laid out, it is not a "best case scenario", it is exactly how the system works. Every step of the process has FAA oversight and approval. If the FAA does not approve the fix, then Boeing has to address the FAA's issues before proceeding on to modification and validation testing.

Besides the FAA the NTSB what other government agencies have to sign off on the resolution of this issue? The Dept of the interior or the IRS maybe? Sure Boeing will be eaten alive on liability, but that has nothing to do with the actual resolution of the issue.
 
WPvsMW
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Re: MAX nacelle, AOA, center of lift, and C/G

Wed Mar 20, 2019 4:12 am

Recertification ... certainly, but I don't think anyone at Boeing has solved the complex aerodynamics problem (huge fans hung forward with nacelles disrupting lift during high AOA) for which MCAS was a patch. Enough disruption and a MAX stalls at an AOA at which an NG does not stall ... preventing that is the purpose of MCAS. The disrupted airflow necessarily impacts airflow over the stabilizers... what if the stabilizers behave erratically, or even stall? Max nose-down can't be healthy.

IMO, MCAS may be the biggest kludge in history, certainly in dollars (recertification, recalls, product liability,...). Bigger than dieselgate or air bags, and with the same motivation, greed. The plaintiffs' litigators first premise: MCAS causes loss of control
 
kalvado
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Re: MAX nacelle, AOA, center of lift, and C/G

Wed Mar 20, 2019 11:28 am

stratclub wrote:
What I laid out, it is not a "best case scenario", it is exactly how the system works. Every step of the process has FAA oversight and approval. If the FAA does not approve the fix, then Boeing has to address the FAA's issues before proceeding on to modification and validation testing.

Besides the FAA the NTSB what other government agencies have to sign off on the resolution of this issue? The Dept of the interior or the IRS maybe? Sure Boeing will be eaten alive on liability, but that has nothing to do with the actual resolution of the issue.

In case you missed it, EASA announced it is going to do its own certification process, so is Canada - and I expect China will join the party.
How deep that will go is anyone's guess at this point, depends on the amount of leniency Boing got from the FAA.
I can fully envision going back to MAX allowed in the US, but not elsewhere, at least for some time.
 
stratclub
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Re: MAX nacelle, AOA, center of lift, and C/G

Wed Mar 20, 2019 2:18 pm

Well duh. Those countries have their own regulatory agencies. For the most part with some small exceptions most foreign regulatory agencies pretty much mirror what the FAA does. Boeing's fix will have to pass approval with those countries as well.

Everyday Boeing delivers aircraft that meet airworthiness requirements with the regulatory agency where they will be operated. The fix being written to meet FAA requirements will also be written to meet the requirements of the country they are operated in. If the fix meets FAA requirements, most likely other regulatory agencies will review the engineering data and approve it based on the FAA's findings during validation testing etc.

What leniency's are you referring to? Are you implying the FAA might allow a fix that is not airworthy?

I have no clue why you believe that this is some kind of soap opera. In any case the media is famous for trying to sensationalize things, miss quote people and in many cases just plane old get things wrong all with the purpose of increasing circulation.
 
GalaxyFlyer
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Re: MAX nacelle, AOA, center of lift, and C/G

Wed Mar 20, 2019 3:00 pm

Bring back D.P Davies of the U.K. CAA certification branch. He insisted on better yaw stability for the B707 when the FAA didn’t require it and refused the B727-200 which has very similar behavior nearing the stall as the MAX. Listen to his podcasts.

GF
 
kalvado
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Re: MAX nacelle, AOA, center of lift, and C/G

Wed Mar 20, 2019 3:03 pm

stratclub wrote:
Well duh. Those countries have their own regulatory agencies. For the most part with some small exceptions most foreign regulatory agencies pretty much mirror what the FAA does. Boeing's fix will have to pass approval with those countries as well.

Everyday Boeing delivers aircraft that meet airworthiness requirements with the regulatory agency where they will be operated. The fix being written to meet FAA requirements will also be written to meet the requirements of the country they are operated in. If the fix meets FAA requirements, most likely other regulatory agencies will review the engineering data and approve it based on the FAA's findings during validation testing etc.

What leniency's are you referring to? Are you implying the FAA might allow a fix that is not airworthy?

I have no clue why you believe that this is some kind of soap opera. In any case the media is famous for trying to sensationalize things, miss quote people and in many cases just plane old get things wrong all with the purpose of increasing circulation.

Following FAA didn't prevent rest of the world from grounding max while it was still flying in US. I suspect at least 2/3 of current fleet will be grounded until EASA gives their OK. Welcome the new trendsetter.
 
stratclub
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Re: MAX nacelle, AOA, center of lift, and C/G

Wed Mar 20, 2019 3:46 pm

GalaxyFlyer wrote:
Bring back D.P Davies of the U.K. CAA certification branch. He insisted on better yaw stability for the B707 when the FAA didn’t require it and refused the B727-200 which has very similar behavior nearing the stall as the MAX. Listen to his podcasts.

GF

Yes, the CAA airworthiness requirements are more stringent in some areas than the FAA's requirements. So, what is the point? Boeing has been building aircraft to CAA requirements for decades. Between different regulatory entities there always will be requirements that differ and that is why the fix will be designed to meet CAA's specific requirements as well if Britain has carriers that fly the MAX.
 
stratclub
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Re: MAX nacelle, AOA, center of lift, and C/G

Wed Mar 20, 2019 4:00 pm

kalvado wrote:
stratclub wrote:
Well duh. Those countries have their own regulatory agencies. For the most part with some small exceptions most foreign regulatory agencies pretty much mirror what the FAA does. Boeing's fix will have to pass approval with those countries as well.

Everyday Boeing delivers aircraft that meet airworthiness requirements with the regulatory agency where they will be operated. The fix being written to meet FAA requirements will also be written to meet the requirements of the country they are operated in. If the fix meets FAA requirements, most likely other regulatory agencies will review the engineering data and approve it based on the FAA's findings during validation testing etc.

What leniency's are you referring to? Are you implying the FAA might allow a fix that is not airworthy?

I have no clue why you believe that this is some kind of soap opera. In any case the media is famous for trying to sensationalize things, miss quote people and in many cases just plane old get things wrong all with the purpose of increasing circulation.

Following FAA didn't prevent rest of the world from grounding max while it was still flying in US. I suspect at least 2/3 of current fleet will be grounded until EASA gives their OK. Welcome the new trendsetter.

Very unfortunate. I personally think that the MAX should have been grounded after the Lion Air crash. Bad on the FAA for that. I don't think that EASA's just prudence in this matter makes them any kind of trendsetter. In aviation, we try to always error on the side of safety and EASA's do diligence led them to make the decision to ground the MAX.

Until we get at least a preliminary report from Ethiopia's regulatory agency, We have no proof that MCAS contributed to the crash. I thought I read something that stated that there have been "similarities" in flight characteristics between the 2 crashes as found from data extracted from the FDR. I don't have a citation to the article so I can't say how reliable the source is.
Last edited by stratclub on Wed Mar 20, 2019 4:09 pm, edited 1 time in total.
 
kalvado
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Re: MAX nacelle, AOA, center of lift, and C/G

Wed Mar 20, 2019 4:01 pm

stratclub wrote:
GalaxyFlyer wrote:
Bring back D.P Davies of the U.K. CAA certification branch. He insisted on better yaw stability for the B707 when the FAA didn’t require it and refused the B727-200 which has very similar behavior nearing the stall as the MAX. Listen to his podcasts.

GF

Yes, the CAA airworthiness requirements are more stringent in some areas than the FAA's requirements. So, what is the point? Boeing has been building aircraft to CAA requirements for decades. Between different regulatory entities there always will be requirements that differ and that is why the fix will be designed to meet CAA's specific requirements as well if Britain has carriers that fly the MAX.

in theory passing EASA certification should be a breeze. In practice, there may be a bit more to handle. For example:
As far as I understand, Boeing rated MCAS problems as serious accidents which would not lead to loss of life for certification purposes. Now with actual events leading to major crashes, a higher rating and lower event probability will be required. Boeing may be required to demonstrate significantly higher reliability of involved software and hardware, to go from 1e-7 probability they clamed and 1e-5 they actually got to 1e-9 required for catastrofic events. Which may lead to electronics requirements being upgraded from "manual mode is always there" to basically FBW standards. That would mean new set of flight computers (probably transplant from 787) and a full new software base. That is lots of work and tons of money for just a single word in certification papers.

I am not saying this will happen as outlined, this is what I see on a worst case scenario side - but these is the type of issues that may come once certification agency is inclined to have their own opinion and not just trust OEM .
 
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QuarkFly
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Re: MAX nacelle, AOA, center of lift, and C/G

Wed Mar 20, 2019 4:24 pm

The grounding should not be lifted when just a new MCAS software patch is installed. Any flight manual or training that requires the electric-trim system be powered off when nothing is wrong with it -- e.g. the problem is some sensor or software, is unacceptable. Powering off a properly working trim system should never have been certified.

The pilots should not have to deal with an obsolete manual-trim wheel during 737 high AOA or other conditions where their attention should be elsewhere. This means the new MAX engine-wing aerodynamic envelope should be dealt with in different ways...Perhaps a larger HZ-stabilizer or aerodynamic adjustments to the nacelles or wing...or maybe a FBW pitch control system similar to the way Boeing added a FBW roll axis to the 747-8.

MCAS functionality belongs in a well implemented flight control system, ideally FBW...not kludge software patches to 1960's era manual flight controls. The 737-Max should never have been certified and should be grounded for a long time...until it is fixed properly, no more patching.
Always take the Red Eye if possible
 
stratclub
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Re: MAX nacelle, AOA, center of lift, and C/G

Wed Mar 20, 2019 4:26 pm

kalvado wrote:
stratclub wrote:
GalaxyFlyer wrote:
Bring back D.P Davies of the U.K. CAA certification branch. He insisted on better yaw stability for the B707 when the FAA didn’t require it and refused the B727-200 which has very similar behavior nearing the stall as the MAX. Listen to his podcasts.

GF

Yes, the CAA airworthiness requirements are more stringent in some areas than the FAA's requirements. So, what is the point? Boeing has been building aircraft to CAA requirements for decades. Between different regulatory entities there always will be requirements that differ and that is why the fix will be designed to meet CAA's specific requirements as well if Britain has carriers that fly the MAX.

in theory passing EASA certification should be a breeze. In practice, there may be a bit more to handle. For example:
As far as I understand, Boeing rated MCAS problems as serious accidents which would not lead to loss of life for certification purposes. Now with actual events leading to major crashes, a higher rating and lower event probability will be required. Boeing may be required to demonstrate significantly higher reliability of involved software and hardware, to go from 1e-7 probability they clamed and 1e-5 they actually got to 1e-9 required for catastrofic events. Which may lead to electronics requirements being upgraded from "manual mode is always there" to basically FBW standards. That would mean new set of flight computers (probably transplant from 787) and a full new software base. That is lots of work and tons of money for just a single word in certification papers.

I am not saying this will happen as outlined, this is what I see on a worst case scenario side - but these is the type of issues that may come once certification agency is inclined to have their own opinion and not just trust OEM .


Kalvado. In the end, Boeing will have to jump through all the required hoops to meet all of the regulatory requirements of all of the counties the MAX are registered in. Your understanding does seem to be spot on. Time line? hard to say at this point.
 
kalvado
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Re: MAX nacelle, AOA, center of lift, and C/G

Wed Mar 20, 2019 4:34 pm

stratclub wrote:
Kalvado. In the end, Boeing will have to jump through all the required hoops to meet all of the regulatory requirements of all of the counties the MAX are registered in. Your understanding does seem to be spot on. Time line? hard to say at this point.

Time and cost are the main question. Absolutely worst case scenario - MAX is not cost effective to bring to compliance, going back to NG and paying a fortune for everything. Followed by chapter 7-11.
I definitely not want to see that, but IMHO this is not an impossible - just very unlikely - outcome. But this is not unprecedented either - see Theranos story.
Sometimes it is better to be a pessimist and enjoy the bright future when it comes...
 
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SheikhDjibouti
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Re: MAX nacelle, AOA, center of lift, and C/G

Wed Mar 20, 2019 5:09 pm

GalaxyFlyer wrote:
Bring back D.P Davies of the U.K. CAA certification branch. He insisted on better yaw stability for the B707 when the FAA didn’t require it and refused the B727-200 which has very similar behavior nearing the stall as the MAX. Listen to his podcasts.

GF

wikipedia wrote:
Re Boeing 707; The British Air Registration Board refused to give the aircraft a certificate of airworthiness, citing insufficient lateral control, excessive rudder forces, and the ability to over-rotate on takeoff, stalling the wing on the ground (a fault of the de Havilland Comet 1). Boeing responded by adding 40 inches to the vertical stabilizer, applying full instead of partial rudder boost, and fitting an underfin to prevent over-rotation. These modifications except to the fin under the tail became standard on all 707 variants and were retrofitted to all earlier 707s.


Of course, in those days (long long ago) the British had their own thriving aircraft industry, and a real need for qualified professionals and a strong independent CAA.
These days, I suspect they sub-contract the job out to - basically anybody. Just as long as they don't have to pay for it.

Up until 1996 they even operated aircraft of their own (C.A.F.U.) for runway/radio/radar calibration and other services. (Just like the FAA)
That seems to have ended too.

Photo on left shows the good old days with a UK registered HS748 helping the Canadians out at YUL.
Photo on the right shows where they are at today (in a museum)

Nothing to see here; move along please.
 
stratclub
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Re: MAX nacelle, AOA, center of lift, and C/G

Wed Mar 20, 2019 6:55 pm

kalvado wrote:
stratclub wrote:
Kalvado. In the end, Boeing will have to jump through all the required hoops to meet all of the regulatory requirements of all of the counties the MAX are registered in. Your understanding does seem to be spot on. Time line? hard to say at this point.

Time and cost are the main question. Absolutely worst case scenario - MAX is not cost effective to bring to compliance, going back to NG and paying a fortune for everything. Followed by chapter 7-11.
I definitely not want to see that, but IMHO this is not an impossible - just very unlikely - outcome. But this is not unprecedented either - see Theranos story.
Sometimes it is better to be a pessimist and enjoy the bright future when it comes...

If you mean the Elizabeth Holmes/Theranos story, I don't even remotely see the parallel. You have to remember that this isn't Boeing's first rodeo and complete resolution might be quicker than many people think. I try to be neither pessimistic or optimistic. I just wait for actual facts. I think a lot of people are looking for a smoking gun that does not exist..........
 
kalvado
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Re: MAX nacelle, AOA, center of lift, and C/G

Wed Mar 20, 2019 8:46 pm

stratclub wrote:
kalvado wrote:
stratclub wrote:
Kalvado. In the end, Boeing will have to jump through all the required hoops to meet all of the regulatory requirements of all of the counties the MAX are registered in. Your understanding does seem to be spot on. Time line? hard to say at this point.

Time and cost are the main question. Absolutely worst case scenario - MAX is not cost effective to bring to compliance, going back to NG and paying a fortune for everything. Followed by chapter 7-11.
I definitely not want to see that, but IMHO this is not an impossible - just very unlikely - outcome. But this is not unprecedented either - see Theranos story.
Sometimes it is better to be a pessimist and enjoy the bright future when it comes...

If you mean the Elizabeth Holmes/Theranos story, I don't even remotely see the parallel. You have to remember that this isn't Boeing's first rodeo and complete resolution might be quicker than many people think. I try to be neither pessimistic or optimistic. I just wait for actual facts. I think a lot of people are looking for a smoking gun that does not exist..........

One thing we can agree upon: if we wait for some time, we will see...
 
WPvsMW
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Re: MAX nacelle, AOA, center of lift, and C/G

Thu Mar 21, 2019 12:37 am

Boeing will survive, I'm sure. The feds will never let it go under. Basis: bailout of the US auto industry. If there's a silver lining, it's that the NSA will be finished faster and some really special pricing offered to MAX owners... Whether the MAX is recertified or not.
 
dakota123
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Re: MAX nacelle, AOA, center of lift, and C/G

Thu Mar 21, 2019 12:58 am

GalaxyFlyer wrote:
Bring back D.P Davies of the U.K. CAA certification branch. He insisted on better yaw stability for the B707 when the FAA didn’t require it and refused the B727-200 which has very similar behavior nearing the stall as the MAX. Listen to his podcasts.

GF


Thanks for that, looking forward to giving a listen. (It’s found by looking for AeroSociety.)
“And If I claim to be a wise man, well surely it means that I don’t know”
 
WPvsMW
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Re: MAX nacelle, AOA, center of lift, and C/G

Thu Mar 21, 2019 10:37 pm

Guys, can be get back to the aerodynamic problems attendant with the MAX design, specifically whether the solution of the "huge fan hung forward" concept (which is how the NG became the MAX) is economically possible for the MAX. Economically possible depends upon full characterization of the problem, i.e., what happens to the center of lift (and the C/L to C/G relationship) during high AOA? I suspect Boeing has engineers who knew that MCAS ver. 1 was a kludge, and the "hazardous" category during certification understated the risk. IMO, recertification of the MAX will require not just more AOA sensors, but computation of the C/L, C/L to C/G, corresponding control actuation, explicit reporting on EICAS, and ground proximity cutout.
 
stratclub
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Re: MAX nacelle, AOA, center of lift, and C/G

Thu Mar 21, 2019 11:52 pm

I don't think the C/L or C/G would change unless Boeing relocated the engine or changed their weight in some way. The problem isn't either of those, it is the tendency of the aircraft to pitch up when power is applied and pitch down when power is reduced.

Something they may have considered in the MAX original design is changing the angle if incidence of the engines. If they did consider it, it may not have proven to be something viable so we have MCAS instead.
 
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7BOEING7
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Re: MAX nacelle, AOA, center of lift, and C/G

Fri Mar 22, 2019 12:50 am

stratclub wrote:
I don't think the C/L or C/G would change unless Boeing relocated the engine or changed their weight in some way. The problem isn't either of those, it is the tendency of the aircraft to pitch up when power is applied and pitch down when power is reduced.

Something they may have considered in the MAX original design is changing the angle if incidence of the engines. If they did consider it, it may not have proven to be something viable so we have MCAS instead.


We don't have MCAS because the airplane pitches up or down, we have MCAS because at one very small area of the flight envelope if the pilot is not pushing the nose over to recover from a stall MCAS helps him out. And by the way the pitch up/down differences between the NG and the MAX probably are no where near the differences between the Jurassic and the Classic.
 
dakota123
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Re: MAX nacelle, AOA, center of lift, and C/G

Fri Mar 22, 2019 1:16 am

7BOEING7 wrote:
stratclub wrote:
I don't think the C/L or C/G would change unless Boeing relocated the engine or changed their weight in some way. The problem isn't either of those, it is the tendency of the aircraft to pitch up when power is applied and pitch down when power is reduced.

Something they may have considered in the MAX original design is changing the angle if incidence of the engines. If they did consider it, it may not have proven to be something viable so we have MCAS instead.


We don't have MCAS because the airplane pitches up or down, we have MCAS because at one very small area of the flight envelope if the pilot is not pushing the nose over to recover from a stall MCAS helps him out. And by the way the pitch up/down differences between the NG and the MAX probably are no where near the differences between the Jurassic and the Classic.


True as far as it goes. What I’ve read (wish i could find the reference now) is that the stick force gradient lessens when you get to the point where MCAS is useful because of lift developed by the larger engine cowlings. It doesn’t go negative, it just lessens, which in any case is not ideal (and not allowable) because one would want it to become ever more difficult to get to a stall AOA, not less.
“And If I claim to be a wise man, well surely it means that I don’t know”
 
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atcsundevil
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Re: MAX nacelle, AOA, center of lift, and C/G

Fri Mar 22, 2019 5:16 pm

Please keep the thread on topic. This isn't a discussion about VW or other companies. Those discussions belong in Non Av.

✈️ atcsundevil
 
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trpmb6
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Re: MAX nacelle, AOA, center of lift, and C/G

Fri Mar 22, 2019 5:38 pm

I'm not sure where the discussions of lift being produced by the engine are coming from. It has far more to do with the fact that the engine was relocated forward and up in relation to where it previously was, resulting in a different thrust vector orientation than the NG. This creates different pitching moments with which the tail must generate an up or down force to counter act. It only becomes a problem at certain parts of the flight envelope (becoming increasingly noticeable as you approach stall speeds). The first instinct when you approach stall is to immediately increase thrust, but this has an unintended consequence because the pitching moment at that point may cause your AOA to increase faster than the horizontal stabilizer is able to account for. Thus MCAS was implemented to trim the Horizontal stab such that it nosed the plane down as you were approaching that portion of the flight envelope.
 
WPvsMW
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Re: MAX nacelle, AOA, center of lift, and C/G

Fri Mar 22, 2019 7:09 pm

Hypothetical: how do you mount "junior GenX" engines on a B737? That's where the "huge fan hung forward" concept is going. Better performance in NB frames means bigger fans. The MAX sales success is based on its performance numbers, so to preserve that performance the "huge fan hung forward" issues have to be solved if not in the MAX, then in the NSA.
Last edited by WPvsMW on Fri Mar 22, 2019 7:20 pm, edited 1 time in total.
 
WPvsMW
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Re: MAX nacelle, AOA, center of lift, and C/G

Fri Mar 22, 2019 7:16 pm

Duplicate. Slow server.
 
PStechPaul
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Re: MAX nacelle, AOA, center of lift, and C/G

Fri Mar 22, 2019 7:20 pm

Perhaps it would be more cost effective, and safer, for Boeing to revise their present manufacturing of the 737-MAX to the NG by using the original smaller engines, and keeping the larger ones for a new clean-sheet design. The existing MAX versions should be able to be retrofitted with the smaller engines and thus eliminate the MAX version altogether. That might result in faster return to a full fleet, and would jump start the design of the complete new model. Personally, I would be reluctant to fly in an aircraft that had additional software patches to correct a fundamental design problem, and it seems the only problem with the NG was less fuel efficiency, and Boeing jumped on the MAX too quickly in order to boost profits in the face of competition.
 
stratclub
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Re: MAX nacelle, AOA, center of lift, and C/G

Fri Mar 22, 2019 9:19 pm

trpmb6 wrote:
I'm not sure where the discussions of lift being produced by the engine are coming from. It has far more to do with the fact that the engine was relocated forward and up in relation to where it previously was, resulting in a different thrust vector orientation than the NG. This creates different pitching moments with which the tail must generate an up or down force to counter act. It only becomes a problem at certain parts of the flight envelope (becoming increasingly noticeable as you approach stall speeds). The first instinct when you approach stall is to immediately increase thrust, but this has an unintended consequence because the pitching moment at that point may cause your AOA to increase faster than the horizontal stabilizer is able to account for. Thus MCAS was implemented to trim the Horizontal stab such that it nosed the plane down as you were approaching that portion of the flight envelope.

I'm not sure I saw anyone post that. Because of the lever moment the engine exerts on the wing, thrust level would certainly change pitch attitude. Lift is determined by the wings AOA. Thrust vector is mostly optimized for cruise and could be optimized for specific flight phases but still changes in thrust would still cause pitch changes in the Max design.

The best fix would be for the engines thrust component to be centered on the same longitudinal axis as the wings chord. I hope that makes sense. That would be impossible unless Boeing went back to Turbo jets.

Like this one:
Image
 
WPvsMW
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Re: MAX nacelle, AOA, center of lift, and C/G

Fri Mar 22, 2019 9:53 pm

Love the square windows. No problem.
Back on track, today's Comet would be a flying wing but we're not going to see that because pax want views.
I'm not an aerospace engineer, and I have invited a few to join the thread to comment on this complex issue of larger nacelles reducing the boundaries of the safe flight envelope.
 
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Starlionblue
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Re: MAX nacelle, AOA, center of lift, and C/G

Fri Mar 22, 2019 10:43 pm

stratclub wrote:
trpmb6 wrote:
I'm not sure where the discussions of lift being produced by the engine are coming from. It has far more to do with the fact that the engine was relocated forward and up in relation to where it previously was, resulting in a different thrust vector orientation than the NG. This creates different pitching moments with which the tail must generate an up or down force to counter act. It only becomes a problem at certain parts of the flight envelope (becoming increasingly noticeable as you approach stall speeds). The first instinct when you approach stall is to immediately increase thrust, but this has an unintended consequence because the pitching moment at that point may cause your AOA to increase faster than the horizontal stabilizer is able to account for. Thus MCAS was implemented to trim the Horizontal stab such that it nosed the plane down as you were approaching that portion of the flight envelope.

I'm not sure I saw anyone post that. Because of the lever moment the engine exerts on the wing, thrust level would certainly change pitch attitude. Lift is determined by the wings AOA. Thrust vector is mostly optimized for cruise and could be optimized for specific flight phases but still changes in thrust would still cause pitch changes in the Max design.

The best fix would be for the engines thrust component to be centered on the same longitudinal axis as the wings chord. I hope that makes sense. That would be impossible unless Boeing went back to Turbo jets.

Like this one:
Image


You'd introduce a host of other problems too. The engines hung way up front limit wing twisting moment, they're way easier the service than engines embedded in the wing, and there's less risk of collateral damage if you lose a blade.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
YYZYYT
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Re: MAX nacelle, AOA, center of lift, and C/G

Fri Mar 22, 2019 11:49 pm

I've been wondering how the change in engine placement / size would result in an increased tendency yo pitch up (it's counter-intuitive, as the larger engine, farther forward would suggest CG shifted forward). The comment re thrust vector orientation makes sense.

I also recall seeing reference to air flow over the wing being disturbed by the changed engine size / placement (kinda like on the the original DC 10?) in one of the many, long threads over in Civ Av. Is there any possibility that this could play a role, ie high AOA = disrupt air flow = loss of lift ?
 
Elshad
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Re: MAX nacelle, AOA, center of lift, and C/G

Fri Mar 22, 2019 11:53 pm

The best solution is to retrofit the airframe with new structures e.g. tail. Would be expensive but much better than a software patch which doesn't address the underlying aerodynamic problem.
 
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Starlionblue
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Re: MAX nacelle, AOA, center of lift, and C/G

Fri Mar 22, 2019 11:55 pm

YYZYYT wrote:
I've been wondering how the change in engine placement / size would result in an increased tendency yo pitch up (it's counter-intuitive, as the larger engine, farther forward would suggest CG shifted forward). The comment re thrust vector orientation makes sense.

I also recall seeing reference to air flow over the wing being disturbed by the changed engine size / placement (kinda like on the the original DC 10?) in one of the many, long threads over in Civ Av. Is there any possibility that this could play a role, ie high AOA = disrupt air flow = loss of lift ?


The CG probably hasn't been moved much at all compared to the NG because moving it forward would reduce efficiency. Move the engines forward, but something else went backwards, or the fuselage was stretched more behind the wing or something.

The issue here is that big nacelles contribute significant lift, especially at higher AoA. You have a situation where at high AoA, the center of lift moves forward, inducing a pitch up moment.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
WIederling
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Re: MAX nacelle, AOA, center of lift, and C/G

Sat Mar 23, 2019 12:03 am

YYZYYT wrote:
I've been wondering .......................
Is there any possibility that this could play a role, ie high AOA = disrupt air flow = loss of lift ?


Think of the engines as 2 big draggy bottles fitted ahead of the wings.

Think about what happens to that drag vector when you change wing incidence from 0° ( when that vector still points parallel but below the wing line ) to higher incidence / AoA ( when that vector origin moves up and its direction follows airflow i.e. more upwards pointing over the wing.

engine thrust effects:
Origin of the thrust vector hasn't changed ( slightly shorter lever than on the NG) and it does not change with changing AoA
as the engine is fixed relative the airframe.
Murphy is an optimist
 
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trpmb6
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Re: MAX nacelle, AOA, center of lift, and C/G

Sat Mar 23, 2019 4:08 am

stratclub wrote:
trpmb6 wrote:
I'm not sure where the discussions of lift being produced by the engine are coming from. It has far more to do with the fact that the engine was relocated forward and up in relation to where it previously was, resulting in a different thrust vector orientation than the NG. This creates different pitching moments with which the tail must generate an up or down force to counter act. It only becomes a problem at certain parts of the flight envelope (becoming increasingly noticeable as you approach stall speeds). The first instinct when you approach stall is to immediately increase thrust, but this has an unintended consequence because the pitching moment at that point may cause your AOA to increase faster than the horizontal stabilizer is able to account for. Thus MCAS was implemented to trim the Horizontal stab such that it nosed the plane down as you were approaching that portion of the flight envelope.

I'm not sure I saw anyone post that. Because of the lever moment the engine exerts on the wing, thrust level would certainly change pitch attitude. Lift is determined by the wings AOA. Thrust vector is mostly optimized for cruise and could be optimized for specific flight phases but still changes in thrust would still cause pitch changes in the Max design.

The best fix would be for the engines thrust component to be centered on the same longitudinal axis as the wings chord. I hope that makes sense. That would be impossible unless Boeing went back to Turbo jets.

Like this one:
Image


The best fix is not always to have the thrust vector thru the chord of the wing. There are far more parameters to consider. What I stated in my original post is 100% correct. You must consider weights, CG due to said weights, lifting bodies, control surfaces, etc. Many parameters that factor in to the design at hand. Requirements may drive you to other designs.
 
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trpmb6
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Re: MAX nacelle, AOA, center of lift, and C/G

Sat Mar 23, 2019 4:11 am

Starlionblue wrote:
YYZYYT wrote:
I've been wondering how the change in engine placement / size would result in an increased tendency yo pitch up (it's counter-intuitive, as the larger engine, farther forward would suggest CG shifted forward). The comment re thrust vector orientation makes sense.

I also recall seeing reference to air flow over the wing being disturbed by the changed engine size / placement (kinda like on the the original DC 10?) in one of the many, long threads over in Civ Av. Is there any possibility that this could play a role, ie high AOA = disrupt air flow = loss of lift ?


The CG probably hasn't been moved much at all compared to the NG because moving it forward would reduce efficiency. Move the engines forward, but something else went backwards, or the fuselage was stretched more behind the wing or something.

The issue here is that big nacelles contribute significant lift, especially at higher AoA. You have a situation where at high AoA, the center of lift moves forward, inducing a pitch up moment.



Again.. the nacelles aren't generating much more lift than before. It's the change in moment arm to the thrust vector by movinCNN the engines up and forward.

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