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

Mon Apr 08, 2019 2:27 am

So, now the FDR is suspect? What will you believe? How does your “theory” explain irrecoverable loss of control at high AOA when the facts show both mishap aircraft were at normal AOA for IAS at speeds exceeding Vmo?

You really need some study on aerodynamics, IMHO.

GF
Last edited by GalaxyFlyer on Mon Apr 08, 2019 2:33 am, edited 1 time in total.
 
OldAeroGuy
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Re: MAX nacelle, AOA, center of lift, and C/G

Mon Apr 08, 2019 2:29 am

WPvsMW wrote:
I am skeptical... the AOA-L always fails and the AOA-R never fails?


If there is going to be a random AoA signal failure, the probability of it happening twice in a row on the same side would be 1 in 4 (25%).

Would you be skeptical if you flipped a coin and it came up heads two times in a row?

This doesn't deny an AoA-L failure bias, that is an area that should be investigated. It doesn't prove there is an AoA-L failure bias either.

Edit: Judgement should also be withheld depending on the maintenance resolution of the AoA vane signal failure on the Lion Air flight prior to JT610. Was that also an AoA-L signal failure?
Last edited by OldAeroGuy on Mon Apr 08, 2019 2:52 am, edited 1 time in total.
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
 
planecane
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Re: MAX nacelle, AOA, center of lift, and C/G

Mon Apr 08, 2019 2:51 am

WPvsMW wrote:
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.


You don't. Anything larger than what is on there will require a new airframe (or very modified 737 with longer gear). If anything larger could have fit in the more forward position, the MAX would have larger diameter engines.

The NSA will be high enough off the ground to fit the engines necessary. Also, the NSA will be full FBW. If the 737 was full FBW, none of this would be an issue.
 
justloveplanes
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Re: MAX nacelle, AOA, center of lift, and C/G

Mon Apr 08, 2019 7:32 am

planecane wrote:
WPvsMW wrote:
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.


You don't. Anything larger than what is on there will require a new airframe (or very modified 737 with longer gear). If anything larger could have fit in the more forward position, the MAX would have larger diameter engines.

The NSA will be high enough off the ground to fit the engines necessary. Also, the NSA will be full FBW. If the 737 was full FBW, none of this would be an issue.


Thank for the interesting post. I do have one question:

Why would FBW have prevented this? Bad MCAS logic is bad MCAS logic. I get a more modern system could deconflict inputs better in theory... but FBW in and of itself might not be a guarantee.
 
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SheikhDjibouti
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Re: MAX nacelle, AOA, center of lift, and C/G

Mon Apr 08, 2019 9:12 am

OldAeroGuy wrote:
WPvsMW wrote:
I am skeptical... the AOA-L always fails and the AOA-R never fails?

If there is going to be a random AoA signal failure, the probability of it happening twice in a row on the same side would be 1 in 4 (25%).

Would you be skeptical if you flipped a coin and it came up heads two times in a row?

You are being too generous. :lol:

Assuming WPvsMW would make the same argument if both failures had been on the right hand side; for the first case, the failure could occur on either the left or the right.

The chance of the second failure being the same side is then just 50%.

At that rate you could make the same skeptical argument if both failures were not on the same side!

[JT043 aside...]
Nothing to see here; move along please.
 
WPvsMW
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Re: MAX nacelle, AOA, center of lift, and C/G

Mon Apr 08, 2019 3:11 pm

My skepticism about AOA vanes and downstream processing in a nutshell: why believe any AOA data from a MAX during an "AOA divergence event" is accurate? The pitch attitude and G traces are a roller coaster in ET302. Based on the the FDR, R-AOA was essentially ZERO degrees while the L-AOA indicated +75 degrees for 99% of the event time, then L-AOA goes to 60 degrees negative, while the L-AOA stays at or near zero degrees, but goes to about 7.5 degrees negative when the L-AOA goes to 60 degrees neg. Please explain that. How can AOA be zero when the ride was a roller coaster?

IMO, AOA data is unreliable based on conflicting pitch attitude and G force data. Ergo, very skeptical.
https://leehamnews.com/wp-content/uploa ... cifics.png

Note that MCAS is active while flaps are at 5 after liftoff.
 
WPvsMW
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Re: MAX nacelle, AOA, center of lift, and C/G

Mon Apr 08, 2019 3:42 pm

typo: ...while the R-AOA stays at or near zero degrees...

My skepticism about AOA vanes and downstream processing in a nutshell: why believe any AOA data from a MAX during an "AOA divergence event" is accurate? The pitch attitude and G traces are a roller coaster in ET302. Based on the FDR traces, R-AOA was essentially ZERO degrees while the L-AOA indicated +75 degrees for 99% of the event time, then L-AOA goes to 60 degrees negative, while the R-AOA stays at or near zero degrees, but goes to about 7.5 degrees negative when the L-AOA goes to 60 degrees neg. Please explain that. How can AOA be zero when the ride was a roller coaster?

IMO, AOA data is unreliable based on conflicting pitch attitude and G force data. Ergo, very skeptical.
https://leehamnews.com/wp-content/uploa ... cifics.png

Note that MCAS is active while flaps are at 5 after liftoff.

ADDED: Boeing says that MCAS is disabled except at Flaps Zero. More evidence that MCAS v.1 is buggy, and IMO there are more "MCAS gone wild" incidents among the majors that have been unpublicized and will be discovered.

As for MAX being certifiable if all FBW... maybe. If recovery requires 5,000 ft of altitude AGL, even FBW won't save the MAX.

Can the MAX be re-certified with taller gear and lower hung engines? Of course... that would normalize it.
 
Elshad
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Re: MAX nacelle, AOA, center of lift, and C/G

Mon Apr 08, 2019 3:54 pm

Taller gear would be a real problem in terms of space. Just look at what they had to do for the MAX 10 to fit the gear in the wheel well, with the lever action etc. Keep in mind that even the MAX 10 landing gear only provides additional height on rotation, not all the time.

It would be easier (in my opinion) to retrofit a larger horizontal stabiliser to solve the issue.
 
OldAeroGuy
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Re: MAX nacelle, AOA, center of lift, and C/G

Mon Apr 08, 2019 5:04 pm

WPvsMW wrote:
My skepticism about AOA vanes and downstream processing in a nutshell: why believe any AOA data from a MAX during an "AOA divergence event" is accurate? The pitch attitude and G traces are a roller coaster in ET302. Based on the FDR traces, R-AOA was essentially ZERO degrees while the L-AOA indicated +75 degrees for 99% of the event time, then L-AOA goes to 60 degrees negative, while the R-AOA stays at or near zero degrees, but goes to about 7.5 degrees negative when the L-AOA goes to 60 degrees neg. Please explain that. How can AOA be zero when the ride was a roller coaster?


The problem is that the scale of the graph, 15 deg per minor division, is too small to show AoA deviations in detail. For instance, Flaps Up Stall AoA is probably around 12 - 14 deg. The entire 737 MAX Flaps Up operation range from Vmo to Stall can be contained in the first 15 deg minor division of the plot. As the R-AoA reading was never far from Zero, it shows that the airplane was never near stall.

R-AoA vane activity matches that of the "G" trace. At the final nose over where the "G" loading is about -0.4, R-AoA appears to be -10 deg to -12 deg.

When flying at Vmo, very little change in AoA is needed cause a significant airplane load change. The "bumpiness" shown in the L-Column position and the "G" trace is probably due to the Left Stick Shaker moving the L-Column. The R-Column is smoother. The Stick Shaker induced L-Column movement is also moving the elevator, causing AoA variations and resulting in "G" trace "bumpiness"

WPvsMW wrote:
Note that MCAS is active while flaps are at 5 after liftoff.

ADDED: Boeing says that MCAS is disabled except at Flaps Zero. More evidence that MCAS v.1 is buggy, and IMO there are more "MCAS gone wild" incidents among the majors that have been unpublicized and will be discovered.


You're mistaking stab trim movements due to STS (Speed Trim System) for MCAS action. STS functions for all 737 MAX Flap positions. Note the short duration of the STS commands compared to the long duration MCAS command that occurs after the Flaps are retracted. There is even a short Nose Up STS command immediately prior to Flaps Up. After Flaps Up, you see MCAS activation due to the L-AoA vane signal failure.

I think you'll find that MCAS.v1 only goes "wild" when there is an AoA vane signal failure. As I have said previously, this would have manifested itself as erroneous Stick Shaker triggering at liftoff when Takeoff Flaps are set. As I understand MCASv2, this will still happen if there is an AoA vane signal failure.

Nuisance warnings of this type will disrupt airline operations and put strong pressure on resolving the poor AoA vane signal failure rate issue.
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
 
WPvsMW
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Re: MAX nacelle, AOA, center of lift, and C/G

Mon Apr 08, 2019 5:13 pm

Thanks, AOG. I was not aware that STS and MCAS would show up on the same trace.
 
IADFCO
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Re: MAX nacelle, AOA, center of lift, and C/G

Mon Apr 08, 2019 6:36 pm

Elshad wrote:
Taller gear would be a real problem in terms of space. Just look at what they had to do for the MAX 10 to fit the gear in the wheel well, with the lever action etc. Keep in mind that even the MAX 10 landing gear only provides additional height on rotation, not all the time.

It would be easier (in my opinion) to retrofit a larger horizontal stabiliser to solve the issue.

Obviously the true cure for the MAX problem would be a higher landing gear and the repositioning of the engines. I doubt that it would be hard to do from an engineering point of view. The problem is that it would probably cost a lot of money. Judging from the detailed photos available on the web, indeed there is no room for a longer gear, everything is absolutely packed in there. There doesn't seem to be any room to move the hinge outboard because of the pylon. It would require some major surgery at the bottom of the aircraft. There would be few if any changes to the cruise loads (the landing gear just sits there and does nothing in flight), but redoing the attachment points would be simple but not cheap.

As to the larger elevator, we would have to know how big of a problem nacelle-induced flow separation is, and I think that nobody outside Boeing has the quantitative data from analysis and/or test. Maybe not even Boeing, if it's true that the MAX design was rushed. I wonder whether any of this will surface during litigation or recertification.
 
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Starlionblue
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Re: MAX nacelle, AOA, center of lift, and C/G

Mon Apr 08, 2019 10:44 pm

IADFCO wrote:
Elshad wrote:
Taller gear would be a real problem in terms of space. Just look at what they had to do for the MAX 10 to fit the gear in the wheel well, with the lever action etc. Keep in mind that even the MAX 10 landing gear only provides additional height on rotation, not all the time.

It would be easier (in my opinion) to retrofit a larger horizontal stabiliser to solve the issue.

Obviously the true cure for the MAX problem would be a higher landing gear and the repositioning of the engines. I doubt that it would be hard to do from an engineering point of view. The problem is that it would probably cost a lot of money. Judging from the detailed photos available on the web, indeed there is no room for a longer gear, everything is absolutely packed in there. There doesn't seem to be any room to move the hinge outboard because of the pylon. It would require some major surgery at the bottom of the aircraft. There would be few if any changes to the cruise loads (the landing gear just sits there and does nothing in flight), but redoing the attachment points would be simple but not cheap.

As to the larger elevator, we would have to know how big of a problem nacelle-induced flow separation is, and I think that nobody outside Boeing has the quantitative data from analysis and/or test. Maybe not even Boeing, if it's true that the MAX design was rushed. I wonder whether any of this will surface during litigation or recertification.


Not only would it cost a lot of money to design, tool up for and so on. It would cost operators a lot of money since it would probably result in a different type certificate. And now you need separate fleets.

If you're going to perform a major redesign, you might as well start from scratch.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
WPvsMW
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Re: MAX nacelle, AOA, center of lift, and C/G

Tue Apr 09, 2019 1:03 am

The World outside (the US) will, I predict, either (i) deny airworthiness for the MAX, or (ii) give the MAX a new type designation which will force Boeing to deliver the B797, all FBW.

Which will aggregate a greater net loss for the manufacturer: the A380 or the MAX?
 
prebennorholm
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Re: MAX nacelle, AOA, center of lift, and C/G

Tue Apr 09, 2019 3:27 am

justloveplanes wrote:
Why would FBW have prevented this? Bad MCAS logic is bad MCAS logic. I get a more modern system could deconflict inputs better in theory... but FBW in and of itself might not be a guarantee.

There will never be anything like MCAS on a FBW plane. On a FBW plane the pilot commands a roll rate and a pitch angle, or rather vertical acceleration. All within sensible aerodynamic limits - soft or hard protection.

If we imagine that the 737NG and MAX had both been FBW planes, then the MAX would have had some slightly different parameters in the otherwise identical pitch control software which controls the hinged elevator. Just slightly different numbers to compensate for the increased and forward shifted lift from the nacelles.

Nobody would ever had gotten the idea to involve the trim function on the horiontal stab. NEVER!
Always keep your number of landings equal to your number of take-offs
 
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Starlionblue
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Re: MAX nacelle, AOA, center of lift, and C/G

Tue Apr 09, 2019 3:45 am

prebennorholm wrote:
justloveplanes wrote:
Why would FBW have prevented this? Bad MCAS logic is bad MCAS logic. I get a more modern system could deconflict inputs better in theory... but FBW in and of itself might not be a guarantee.

There will never be anything like MCAS on a FBW plane. On a FBW plane the pilot commands a roll rate and a pitch angle, or rather vertical acceleration. All within sensible aerodynamic limits - soft or hard protection.

If we imagine that the 737NG and MAX had both been FBW planes, then the MAX would have had some slightly different parameters in the otherwise identical pitch control software which controls the hinged elevator. Just slightly different numbers to compensate for the increased and forward shifted lift from the nacelles.

Nobody would ever had gotten the idea to involve the trim function on the horiontal stab. NEVER!


Indeed. You don't need a bandaid if you've designed the entire flight control system as FBW. All the parameters are already there to be adjusted as needed.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
WPvsMW
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Re: MAX nacelle, AOA, center of lift, and C/G

Tue Apr 09, 2019 2:55 pm

One caveat on FBW for the MAX: if there is a HFHFCC as I posit, FBW could keep the pilot from entering the HFHFCC voluntarily, but if wind shear, wake, mountain wave, CAT, and similar "instantaneous extremely turbulent" events can put the a/c into HFHFCC and there's not enough altitude to recover (which FBW may or may not be able to do once out of Normal law), then even FBW can't save the MAX.
 
dakota123
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Re: MAX nacelle, AOA, center of lift, and C/G

Tue Apr 09, 2019 3:12 pm

prebennorholm wrote:
justloveplanes wrote:
Why would FBW have prevented this? Bad MCAS logic is bad MCAS logic. I get a more modern system could deconflict inputs better in theory... but FBW in and of itself might not be a guarantee.

There will never be anything like MCAS on a FBW plane. On a FBW plane the pilot commands a roll rate and a pitch angle, or rather vertical acceleration. All within sensible aerodynamic limits - soft or hard protection.

If we imagine that the 737NG and MAX had both been FBW planes, then the MAX would have had some slightly different parameters in the otherwise identical pitch control software which controls the hinged elevator. Just slightly different numbers to compensate for the increased and forward shifted lift from the nacelles.

Nobody would ever had gotten the idea to involve the trim function on the horiontal stab. NEVER!


Are you 100% positive about that (serious question). Ever read the A-380 Special Conditions on controllability? I mean, there are systems to warn the pilot when control surface deflection is nearing maximum, it stands to reason, or at least is not inconceivable, that at some point in the process trim might be employed to provide additional authority.

General observation, the Special Conditions read like one long MCAS-like description of adjusting for “this” and ensuring “that” doesn’t happen. People are appalled at MCAS, but don’t realize how much goes into FBW to save the pilot’s and pax ass from situations that would otherwise be undetectable to a pilot until it’s too late. Not arguing FWB better or worse, just a fact.
“And If I claim to be a wise man, well surely it means that I don’t know”
 
BravoOne
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Re: MAX nacelle, AOA, center of lift, and C/G

Thu Apr 11, 2019 3:48 pm

FWIW, Two days ago a Boeing VP did an internal briefing for some Boeing employees and predicted the MAX would be back in service by mid June. That sounds vey optimistic, but the again I don't have access to all the information he has.
 
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SheikhDjibouti
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Re: MAX nacelle, AOA, center of lift, and C/G

Thu Apr 11, 2019 4:33 pm

I found this image in one of the Civ Av threads

It may be that this photo displays some wide-angle distortion, but the degree to which the MAX engines are more forward and higher than before is abundantly clear.

And assuming this is landing phase, we are witnessing a some AoA (i.e. not high, but not zero either) with vortices streaming off the nacelle vane.

I don't pretend to understand everything happening in this photo, but if it gives anyone else here something to talk about, then it's not entirely wasted. :D

Image
Thx to Amiga500

p.s. if somebody jumps in and tells me the wing configuration is set for taking-off, I'll take that too..... :D
Nothing to see here; move along please.
 
greg85
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Re: MAX nacelle, AOA, center of lift, and C/G

Thu Apr 11, 2019 6:28 pm

SheikhDjibouti wrote:
I found this image in one of the Civ Av threads

It may be that this photo displays some wide-angle distortion, but the degree to which the MAX engines are more forward and higher than before is abundantly clear.

And assuming this is landing phase, we are witnessing a some AoA (i.e. not high, but not zero either) with vortices streaming off the nacelle vane.

I don't pretend to understand everything happening in this photo, but if it gives anyone else here something to talk about, then it's not entirely wasted. :D

Image
Thx to Amiga500

p.s. if somebody jumps in and tells me the wing configuration is set for taking-off, I'll take that too..... :D


The picture is of an A340 wing. Not sure it’s relevant to the conversation about the 737.
 
stephanwintner
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Re: MAX nacelle, AOA, center of lift, and C/G

Thu Apr 11, 2019 7:06 pm

In case anyone missed it :

https://aviationweek.com/commercial-avi ... detachment

I will leave commenting to the experts.
 
OldAeroGuy
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Re: MAX nacelle, AOA, center of lift, and C/G

Thu Apr 11, 2019 7:49 pm

It does look like takeoff.

It also does not look like a 737 MAX. This airplane has inboard wing slats. All 737's have inboard leading edge Krueger Flaps.

Kind of shows that the 737 MAX engine location is not so unique.
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
 
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SheikhDjibouti
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Re: MAX nacelle, AOA, center of lift, and C/G

Thu Apr 11, 2019 9:22 pm

greg85 wrote:
SheikhDjibouti wrote:
I found this image in one of the Civ Av threads

It may be that this photo displays some wide-angle distortion,.....

The picture is of an A340 wing. Not sure it’s relevant to the conversation about the 737.

Ok, I'll have take that on the chin. I guess the wide angle distortion was more than I fully appreciated.... :lol:

Kind of shows that the 737 MAX engine location is not so unique.

Like I said - if it prompts a discussion, then that is surely a good thing.

So.…. why are the engines on an A340 positioned where they are?
The A342/A343 had tiny CFM56 hairdryers.
The A345/346 had much larger Trent 500s

Were these larger RR engines positioned further forward in the same way as on the MAX?
Nothing to see here; move along please.
 
WPvsMW
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Re: MAX nacelle, AOA, center of lift, and C/G

Thu Apr 11, 2019 9:24 pm

There are significant differences.
The CFM56 engines on the A342 and A343 (and the B737NG) have a 61.5" diameter and the A340 nacelle bottoms are round, are not known to generate lift. Does FBW do anything re: "nacellle lift"? Not that I can find.
The CFM LEAP 1B engines on the B737 MAX fans have a 69.4" diameter and the nacelle bottoms are flat, and generate lift that requires MCAS.

The HUGE difference from NG to MAX is stunning, esp. the decreased distance from the wing chord.
scroll down: https://leehamnews.com/2018/11/14/boein ... he-pilots/

See discussion of the effect of the change in C/G in the link.
 
OldAeroGuy
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Re: MAX nacelle, AOA, center of lift, and C/G

Fri Apr 12, 2019 3:25 am

WPvsMW wrote:
There are significant differences.
The CFM56 engines on the A342 and A343 (and the B737NG) have a 61.5" diameter


Except the picture shown is the Trent 500 on the A340-500/600. The Trent 500 fan is 97.4" in diameter. The Trent 500 fan is 58% bigger than the CFM56. Meanwhile, the LEAP1B fan is 69.4", only 12.8% larger than the CFM56.

Furthermore, because lift of the nacelle "ring wing" is dependent on the nacelle frontal area, an individual Trent 500 nacelle has 2.5 times the frontal area (and lift) for a given angle of attack compared to the CFM56.

The LEAP1B has only 1.27 times as much lift as the CFM56 at a given AoA.

WPvsMW wrote:
...the A340 nacelle bottoms are round, are not known to generate lift.

The CFM LEAP 1B engines on the B737 MAX fans have a 69.4" diameter and the nacelle bottoms are flat, and generate lift .


You should go back and read the "ring wing" aerodynamics characteristics reference that was identified early in this topic. Both the Trent 500 and the LEAP1B will provide lift when set at a positive AoA. The flat bottom of the LEAP1B nacelle will have a minor effect on the amount of lift generated. The proportional lift change due to new engines on the A340-500/600 relative to the A340-300 will be higher than the new engines on the 737 MAX. relative to the 737 NG.

WPvsMW wrote:
....that requires MCAS


Neither one of us knows if the larger Trent 500 engines didn't create the need for the A340-500/600 FBW equivalent of MCAS to control airplane pitch rate during stall entry. It would be buried in the FBW software. We won't know until Airbus releases the relevant Cm vs AoA data.

I will bet that neither the A340-500/600 or the 737 MAX has an unstable stall entry although relaxed stability ie "stick lightening" is possible.

Where Boeing really screwed up was not stopping MCAS operation with a AoA vane signal failure.
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
 
WPvsMW
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Re: MAX nacelle, AOA, center of lift, and C/G

Fri Apr 12, 2019 4:16 am

OAG, you know A340 variants better than I do.
I agree that what the FBW in the A345 and A346 does to avoid the "MAX problem" is buried in the FBW code, thus my observation that FBW might solve the "MAX problem", but if it does, a new type is likely (IMO) outside the US, and maybe even in the US.

The nagging fact is the ET302 had MCAS turned off (or more accurately, STAB TRIM), and the pilots could not recover.
 
OldAeroGuy
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Re: MAX nacelle, AOA, center of lift, and C/G

Fri Apr 12, 2019 5:25 am

A problem was that electrical power was restored to the STAB and the flaws of MCAS.v1 started to work again.

Wait until the ET302 accident report is published for more detail on the controllability issues.
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
 
WPIAeroGuy
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Re: MAX nacelle, AOA, center of lift, and C/G

Fri Apr 12, 2019 3:11 pm

SheikhDjibouti wrote:
I found this image in one of the Civ Av threads

It may be that this photo displays some wide-angle distortion, but the degree to which the MAX engines are more forward and higher than before is abundantly clear.

And assuming this is landing phase, we are witnessing a some AoA (i.e. not high, but not zero either) with vortices streaming off the nacelle vane.

I don't pretend to understand everything happening in this photo, but if it gives anyone else here something to talk about, then it's not entirely wasted. :D

Image
Thx to Amiga500

p.s. if somebody jumps in and tells me the wing configuration is set for taking-off, I'll take that too..... :D


The vortices are coming off the inboard strake because it designed to energize the flow coming around the nacelle to prevent separation. Lots of aircraft have these strakes, because pretty much any round object in front of the wing will tend to do funky things at high AoA.

WPvsMW wrote:
OAG, you know A340 variants better than I do.
I agree that what the FBW in the A345 and A346 does to avoid the "MAX problem" is buried in the FBW code, thus my observation that FBW might solve the "MAX problem", but if it does, a new type is likely (IMO) outside the US, and maybe even in the US.

The nagging fact is the ET302 had MCAS turned off (or more accurately, STAB TRIM), and the pilots could not recover.


Fly-by-wire simply means the pilots control inputs are converted to an electronic signal before being sent to the control surfaces. You can't just say "FBW will fix this problem" because the specific implementation of FBW could have vastly differing amount of logic and redundancy. An R/C airplane is fly by wire but in most cases simply transmits stick position to servo travel, usually on a relatively linear scale.

MCAS makes stick force linear with AoA increases. You could have a force-feedback system built in to the airplane, but (I'm not a 737 pilot) I believe this would be an all-new system (including hardware) which could bring in a host of unknowns. If you had a bad AoA signal, its going to make the elevator more difficult to move at low AoA. I don't like the idea of primary control being limited in such a fashion, although you could probably design it such that the maximum feedback provided could always be overcome by the pilot.

Without force feedback like an Airbus, most likely the pitch command would simply be limited at high AoA so that pitch rate remained proportional to pilot input (correct me if I'm wrong on this implementation). However, as above, you're now limiting the the pilots direct control of the primary control surfaces, but you can't "fight through it". It would require an extremely robust, multiply-redundant control system, and require the pilots to recognize when something does go wrong (as in the Lufthansa incident) and give them the ability to override the system.

What Boeing did, from an aerodynamic standpoint, is not unusual. They built the force feedback into the stabilizer, which can be operated independently to the elevator.

The Piper Cherokee also has a mechanism to build force-feedback into elevator travel using the stabilizer. The Cherokee has an all-moving stabilator that has an anti-servo tab on the back that moves in the direction of stabilator travel. The purpose of the tab is to make sure stick forces increase as stabilator angle increases. Note this is opposite of airplane like the MD-80 which use flying control surfaces and servo tabs. In the case of the 737, the non-linear stick forces were only present at a high AoA, therefore they could make use of the existing stabilizer trim system (piggybacking on STS) to provide the additional control feel only when needed. Having been in many similar design meetings, I can completely see how Boeing came to the conclusion that if the electric stab trim started doing something against the pilots command, it would simply be disconnected.

Where Boeing screwed up, and I don't believe they did this for nefarious reasons, is that they 1. assumed pilots would recognize intermittent uncommanded stab trim as runaway trim, and 2. they did not include any indication of MCAS or AoA disagree. I can sort of see why on the latter - AoA is simply not a common flight instrument outside of aerobatics and military fighters. It's becoming more common, but I've read that its use is limited because AoA indicators (at least in the general aviation world) generally have poor accuracy and significant lag and thus are only useful mainly as an approach-to-stall warning. They won't be taking the place of the basic six-pack for primary flight instrumentation.
-WPIAeroGuy
 
OldAeroGuy
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Re: MAX nacelle, AOA, center of lift, and C/G

Fri Apr 12, 2019 5:31 pm

Well written WPIAeroGuy.
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
 
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Starlionblue
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Re: MAX nacelle, AOA, center of lift, and C/G

Fri Apr 12, 2019 6:10 pm

In an FBW Airbus, pitch input is proportional to load factor demand, not pitch rate.

In Alpha Prot, it becomes proportional to AoA.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
WPIAeroGuy
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Re: MAX nacelle, AOA, center of lift, and C/G

Fri Apr 12, 2019 8:10 pm

Starlionblue wrote:
In an FBW Airbus, pitch input is proportional to load factor demand, not pitch rate.

In Alpha Prot, it becomes proportional to AoA.


Thank you, I knew it wasn’t proportional to elevator deflection. Is aileron control proportional to roll rate? That may be what I’m confusing it with.
-WPIAeroGuy
 
WIederling
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Re: MAX nacelle, AOA, center of lift, and C/G

Fri Apr 12, 2019 8:30 pm

greg85 wrote:
SheikhDjibouti wrote:
Image
Thx to Amiga500

p.s. if somebody jumps in and tells me the wing configuration is set for taking-off, I'll take that too..... :D


The picture is of an A340 wing. Not sure it’s relevant to the conversation about the 737.



Take off :: YES
Source. somebody had an eye on airflow and exposing vortexes via condensation.
https://frank.itlab.us/photo_essays/wra ... _sq22.html
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OldAeroGuy
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Re: MAX nacelle, AOA, center of lift, and C/G

Fri Apr 12, 2019 10:41 pm

Starlionblue wrote:
In an FBW Airbus, pitch input is proportional to load factor demand, not pitch rate.

In Alpha Prot, it becomes proportional to AoA.


But that's what happens via the FBW controls operating in Normal mode. In effect, the FBW has the potential to provide an MCAS like function.

In Direct mode, "stick lightening" might be present but the airplane stalling characteristics (or lack there of) would be acceptable in Normal mode due to the longitudinal stability augmentation of FBW.

It would be interesting to see a comparison of the A340-300 vs the A340-500 "stick fixed" Cm vs AoA curves, up to and through stall.
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
 
WPvsMW
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Re: MAX nacelle, AOA, center of lift, and C/G

Fri Apr 12, 2019 11:40 pm

FBW is far more than an electromechanical interface, it is mostly "a computer that in turn determines how to move the control surfaces to best achieve what the pilot wants in accordance with which of the available Flight Control Laws is active."
https://www.skybrary.aero/index.php/Fly-By-Wire

FBW dates from NASA's F-8 test plane in 1972. The technology was there for Boeing to use.
https://www.nasa.gov/vision/earth/impro ... _wire.html

The A345 frame is 223 ft long, 4 engines. The B739 MAX is 138 ft long, 2 engines.

Are the A340 nacelle tops as close to the wing chord as nacelle tops on a MAX? Visually, it appears that the MAX is closer. Plus, very different frame lengths, 4 engines vs. 2, and FBW vs. kludge. I think such a severe word, "kludge", is appropriate. The consequences may have been unknown to Boeing, but the kludgeness is massive.
 
OldAeroGuy
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Re: MAX nacelle, AOA, center of lift, and C/G

Sat Apr 13, 2019 12:46 am

Sorry, my troll warning just went off.
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
 
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Re: MAX nacelle, AOA, center of lift, and C/G

Sat Apr 13, 2019 12:57 am

OldAeroGuy wrote:
Sorry, my troll warning just went off.


Agreed, I’ve stopped posting as his is trollware.

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

Sat Apr 13, 2019 2:42 am

WPIAeroGuy wrote:
Starlionblue wrote:
In an FBW Airbus, pitch input is proportional to load factor demand, not pitch rate.

In Alpha Prot, it becomes proportional to AoA.


Thank you, I knew it wasn’t proportional to elevator deflection. Is aileron control proportional to roll rate? That may be what I’m confusing it with.


Lateral stick deflection is proportional to roll rate in Normal Law.



WPvsMW wrote:
FBW is far more than an electromechanical interface, it is mostly "a computer that in turn determines how to move the control surfaces to best achieve what the pilot wants in accordance with which of the available Flight Control Laws is active."
https://www.skybrary.aero/index.php/Fly-By-Wire

FBW dates from NASA's F-8 test plane in 1972. The technology was there for Boeing to use.
https://www.nasa.gov/vision/earth/impro ... _wire.html

The A345 frame is 223 ft long, 4 engines. The B739 MAX is 138 ft long, 2 engines.

Are the A340 nacelle tops as close to the wing chord as nacelle tops on a MAX? Visually, it appears that the MAX is closer. Plus, very different frame lengths, 4 engines vs. 2, and FBW vs. kludge. I think such a severe word, "kludge", is appropriate. The consequences may have been unknown to Boeing, but the kludgeness is massive.


And the tech for the F-8 FBW program was initially developed for the Apollo CSM and LM.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
WPIAeroGuy
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Re: MAX nacelle, AOA, center of lift, and C/G

Sat Apr 13, 2019 12:23 pm

WPvsMW wrote:

Are the A340 nacelle tops as close to the wing chord as nacelle tops on a MAX? Visually, it appears that the MAX is closer. Plus, very different frame lengths, 4 engines vs. 2, and FBW vs. kludge. I think such a severe word, "kludge", is appropriate. The consequences may have been unknown to Boeing, but the kludgeness is massive.


You should tell Boeing, who pioneered early CFD development during the design of the 737-300 that they could have saved millions on expensive CFD runs by simply eyeballing photos on Airliners.net.

If you’re so keen on FBW you should read up on the Gripen pilot who had to crashed twice, ejecting once due to software issues with the FBW control. Or the B-2 crash caused by water in the air data system. FBW is just as susceptible to design failures or to errors induced by erroneous sensor data. Neither mechanical control nor fly-by-wire is wholly better than the other. It depends on the individual aircraft and what the stability and control requirements are.
-WPIAeroGuy
 
WPvsMW
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Re: MAX nacelle, AOA, center of lift, and C/G

Sat Apr 13, 2019 2:32 pm

It's not that I'm keen on FBW, it's that FBW is inevitable in transport category a/c (less so in GA a/c). I worked in software development for years. I know about bugs, crashes, and worst of all, trying to patch legacy systems to support new modules, esp., reformatting data to exchange data between legacy and new. New grads in comp sci don't know legacy systems, make mistakes, and you hope the older coders and testers catch the mistakes.

There have been 4 generations of B737, but there have probably been more than 4 generations of software. "Massive kludgeness".
 
WIederling
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Re: MAX nacelle, AOA, center of lift, and C/G

Sat Apr 13, 2019 2:51 pm

OldAeroGuy wrote:
It would be interesting to see a comparison of the A340-300 vs the A340-500 "stick fixed" Cm vs AoA curves, up to and through stall.


Interesting : yes.
but the scaling is all wrong. proportionally the Leap1Bs are much bigger in relation to the airframe.
( shorter tail, smaller stab ..)
Murphy is an optimist
 
WPIAeroGuy
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Re: MAX nacelle, AOA, center of lift, and C/G

Sat Apr 13, 2019 3:10 pm

WIederling wrote:
OldAeroGuy wrote:
It would be interesting to see a comparison of the A340-300 vs the A340-500 "stick fixed" Cm vs AoA curves, up to and through stall.


Interesting : yes.
but the scaling is all wrong. proportionally the Leap1Bs are much bigger in relation to the airframe.
( shorter tail, smaller stab ..)


What does that matter? You still may find that the derivative of the curve goes negative, indicating non-linearly increasing control forces. Dealing with it may have just been baked into the flight control system. I'd be curious about the KC-135 too, going from JT3Cs (39in diameter) to CFM56 (68 in).
-WPIAeroGuy
 
WIederling
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Re: MAX nacelle, AOA, center of lift, and C/G

Sat Apr 13, 2019 3:48 pm

WPIAeroGuy wrote:
WIederling wrote:
OldAeroGuy wrote:
It would be interesting to see a comparison of the A340-300 vs the A340-500 "stick fixed" Cm vs AoA curves, up to and through stall.


Interesting : yes.
but the scaling is all wrong. proportionally the Leap1Bs are much bigger in relation to the airframe.
( shorter tail, smaller stab ..)


What does that matter? You still may find that the derivative of the curve goes negative, indicating non-linearly increasing control forces. Dealing with it may have just been baked into the flight control system. I'd be curious about the KC-135 too, going from JT3Cs (39in diameter) to CFM56 (68 in).


The planes must be stable in direct law. so : probably not.
Murphy is an optimist
 
WPIAeroGuy
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Re: MAX nacelle, AOA, center of lift, and C/G

Sat Apr 13, 2019 5:04 pm

WIederling wrote:
WPIAeroGuy wrote:
WIederling wrote:

Interesting : yes.
but the scaling is all wrong. proportionally the Leap1Bs are much bigger in relation to the airframe.
( shorter tail, smaller stab ..)


What does that matter? You still may find that the derivative of the curve goes negative, indicating non-linearly increasing control forces. Dealing with it may have just been baked into the flight control system. I'd be curious about the KC-135 too, going from JT3Cs (39in diameter) to CFM56 (68 in).


The planes must be stable in direct law. so : probably not.


Stability just requires the pitching moment is always a restoring force, (typically negative) using normal conventions. What I was saying was look at the rate of change of that curve, the curve should become increasingly more negative for increased stick forces. If it reverses, you can still have a stabile pitching moment but with decreasing stick force, which is where MCAS is required.
-WPIAeroGuy
 
OldAeroGuy
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Re: MAX nacelle, AOA, center of lift, and C/G

Sat Apr 13, 2019 5:11 pm

Exactly correct WPIAeroGuy.

If you read the Aviation Week article on MCAS.v2, you'll see they had the evaluating pilots stall the MAX in the simulator at the critical condition (aft cg) with MCAS off. The pilot comments were that the stick lightening was barely noticeable.

Still requires MCAS to pass cert though.
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis

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