[Sutripta]

Originally Posted by ads11

Well I thought the C series deal that morphed into the A220 line was more a result of using that Airbus US assembly facility to get around the sanctions the US were putting on Bombardier. And last I checked, Bombardier had already offloaded their entire civilian air division prior to the virus outbreak.

Precisely because of that people have their doubts. A fire sale, but with Bombardier I think still holding 49% will always think they have been shortchanged.

https://www.team-bhp.com/forum/comme...ml#post4430654 (Boeing, Airbus struggling to deliver promised jets)

Regards
Sutripta

[MegaWhat]

Originally Posted by Jeroen

Next time when you board a plane remember this:

https://www.scientificamerican.com/a...ay-in-the-air/

Lots of people will tell you how lift works, but unfortunately none of that is the true, full understanding of lift. We simply do not know how planes stay up!

Excellent article! Thanks for sharing this Jeroen! It took me back to my airfoil studying days in college.

One of the contributors to that article, Prof. Holger Babinsky had written an article in 2003 in "Physics education" on how wings work. I feel that comes close to physically describing the probable theory behind lift. It considers the curvature of objects and the effect it has on the centripetal force on a fluid element. It then considers fluid viscosity around that element and how that leads to curvature of the flow overall and then on to local pressure differences and hence forces.

It's explained quite well, much better than how I attempted to put it. You can find it here:

http://www3.eng.cam.ac.uk/outreach/P...wwingswork.pdf

The only two things missing in that article are:
1. In the basic explanation, viscosity is not considered, but it's role has been acknowledged by prof. Babinsky. He had mentioned a while back that his team is working on it.
2. The math. The equations are still in the works.

But it's still a good theory to be in the mix with the rest.

Originally Posted by Jeroen

To your point below, the pitch moment is related to the distance between the CG of the engine and wing (or hull/wing). In this image the vertical distance. How far apart the CG are horizontally so to speak (e.g. mounting the same engine further forward) does not have any effect on the pitch moment.

Although I am not a hundred percent sure, the total centre of gravity of the Max is not any different from the previous versions.

A lot of people assume moving the CG of the engine forward (away from the CG of the rest of plane) creates a pitch moment. Simply not true. The Max, apparently does have a bit of this pitch upward tendency, but it is not related to thrust (pitch moment) forces, but to aerodynamic forces.

Makes sense. I've been baffled as to how an engine that's moved ahead and up could cause a pitch up moment. Scraped through a few other forums and looked at the other differences between the 737-800 and the 737-8 MAX and here's my two cents:

1. An engine has its own aerodynamic forces acting on its exterior. At high angles of attack - like during a climb - these forces can be quite strong. And if the engine is placed further away from the CG, like it is in the 737 MAX, these forces will generate a relatively stronger pitch up moment as compared to if the engine was closer to the CG. Essentially like canard that comes into play only at high angles of attack. Just like the good folks on the other forum mentioned.

An engine that's mounted ahead of the wings will definitely take the CG ahead with it but in any case, the CG has to be near the aerodynamic center - defined primarily by the position and shape of the wing and the tail. And that's mostly unchanged between the 737 NG and the MAX I believe, even if the wing has been updated a bit.

2. In all the new passenger aircraft, Boeing has used what it calls load alleviation on its wings. Basically you allow the wings to bend under the lift load a little bit instead of keeping it rigid and transferring all that stress to the wing box. Remember how much the wings of the 787 flex in flight? This allows for a lighter wing box and hence lower weight.

Now, under high lift conditions - like in climb - the wings will flex upwards more than they would flex while in cruise. So basically the CG and the aerodynamic center is going to shift upwards, away from the engine, though only marginally. This will increase the moment arm for the aerodynamic loads acting on the engine with respect to the CG and further increase the pitch up tendency.

I believe both of these aspects will increase the pitch up tendency only marginally, and only at high angles of attack. Maybe because it's only marginal and temporary, Boeing thought that it could be handled by software instead of hardware. And that complicated things further with the AoA sensor and MCAS as we all now know. Like any Aviation accident, it's tough to blame a single aspect, it's always a series of events.

In any case, this is also a speculation. Unless you're a Boeing engineer who has worked on the aircraft, it's tough to get any particular design details. Maybe I should just wait for more information than speculate here.

Cheers!

[Jeroen]

Originally Posted by MegaWhat

But it's still a good theory to be in the mix with the rest.

Thanks, you might enjoy this little video too:

https://www.youtube.com/watch?v=QKCK4lJLQHU

[Sutripta]

Originally Posted by MegaWhat

Makes sense. I've been baffled as to how an engine that's moved ahead and up could cause a pitch up moment.

Originally Posted by Jeroen

1 The Max has a tendency to pitch up due to the engines being mounted further forward.This is simply incorrect.

Originally Posted by Sutripta

Have you posted this on any of the more professional aviation forums? What replies did you get?

As I said, Jeroen's is a very unambiguous statement. Would have been interesting if it had been posted on professional aviation forums. (exact words, not couched in different language. Because the strong view would have elicited spirited discussions, from a more professional aviation group than on an automobile forum).

Regards
Sutripta

[Jeroen]

Originally Posted by Sutripta

As I said, Jeroen's is a very unambiguous statement. Would have been interesting if it had been posted on professional aviation forums. (exact words, not couched in different language. Because the strong view would have elicited spirited discussions, from a more professional aviation group than on an automobile forum).

Nothing unambiguous about pitch moment, not aviation nor automobile specific. Simple mechanics, but many people get confused how it really works.

MegaWhat, going by his profile is a mechanical engineer, and does understand it and thus says

I've been baffled as to how an engine that's moved ahead and up could cause a pitch up moment

I just came across this site. I had visited it before, many moons ago. But it has done a very good job of updating the information regarding the 737 MAX and MCAS in particular. He/she must have been trawling the aviation forums and come to the same conclusion. Have a look:

http://www.b737.org.uk/mcas.htm

It does a good job of summarising what MCAS is all about and what the problem with the larger engines really is;

MCAS is a longitudinal stability enhancement. It is not for stall prevention (although indirectly it helps) or to make the MAX handle like the NG (although it does); it was introduced to counteract the non-linear lift generated by the LEAP-1B engine nacelles at high AoA and give a steady increase in stick force as the stall is approached as required by regulation.

The LEAP engine nacelles are larger and had to be mounted slightly higher and further forward from the previous NG CFM56-7 engines to give the necessary ground clearance. This new location and larger size of nacelle cause the vortex flow off the nacelle body to produce lift at high AoA. As the nacelle is ahead of the C of G, this lift causes a slight pitch-up effect (ie a reducing stick force) which could lead the pilot to inadvertently pull the yoke further aft than intended bringing the aircraft closer towards the stall. This abnormal nose-up pitching is not allowable under 14CFR §25.203(a) "Stall characteristics".

Of course, as with any and all information on the internet, everybody needs to make up their own mind as to how credible this particular explanation is.

Jeroen

[Jeroen]

This one just popped up in my Youtube feed. Mentions viscosity too!

https://youtu.be/aa2kBZAoXg0

[MegaWhat]

Originally Posted by MegaWhat

I've been baffled as to how an engine that's moved ahead and up could cause a pitch up moment.

Okay, I did say this, but then I had followed it up with the below as well :

Originally Posted by MegaWhat

Scraped through a few other forums and looked at the other differences between the 737-800 and the 737-8 MAX and here's my two cents:

1. An engine has its own aerodynamic forces acting on its exterior. At high angles of attack - like during a climb - these forces can be quite strong. And if the engine is placed further away from the CG, like it is in the 737 MAX, these forces will generate a relatively stronger pitch up moment as compared to if the engine was closer to the CG. Essentially like canard that comes into play only at high angles of attack. Just like the good folks on the other forum mentioned.

So yes, I was baffled initially as to how an engine mounted ahead could lead to a pitch up tendency, but after some reading up, I stand corrected. There is a way by which such a pitch up moment could happen by such a forward mounted engine as the engine generates it's own lift, away from the CG of the aircraft.

We should look at the aerodynamic forces created by the engine rather than the CG of the engine itself.

There is one thing I would like to correct in the above explanation though. I spoke to a friend who is in the aircraft design business and he mentioned that the flow over an engine cannot be as independent as that over a canard. The flow over the wing and the engine are highly coupled. But he does agree that considering that the engine for the 737 MAX is moved further ahead of the CG, it could be affecting the flow - over itself and over the wing - to generate a nose up tendency at high Angles of attack.

So at certain conditions, at high angles of attack, the aircraft could have a pitch up tendency due to the forward mounted engines.

Originally Posted by Jeroen

http://www.b737.org.uk/mcas.htm

..This new location and larger size of nacelle cause the vortex flow off the nacelle body to produce lift at high AoA. As the nacelle is ahead of the C of G, this lift causes a slight pitch-up effect..

This is a good summary and makes sense. It aligns with what I and my friend believe seems to be happening on the aerodynamics front. The rest of the story is another part of the puzzle. Thanks for sharing, it's very comprehensive.

Cheers!

[MegaWhat]

Apologies, I missed my editing window of 30 minutes. Found something more to add though.

Check out the shape of the nacelles below:

Originally Posted by MegaWhat

So at certain conditions, at high angles of attack, the aircraft could have a pitch up tendency due to the forward mounted engines.

The engine nacelle has been designed to be flat towards the bottom to maintain ground clearance like the 737 NG. However, such an asymmetry (flat bottom and curvy top) could potentially generate more lift from the engine nacelle as compared to a symmetric curvy nacelle (both top and bottom either curvy) which is usually the case.

This could further add to the lift generated by the engine nacelle at high angles of attack.

[Jeroen]

Originally Posted by MegaWhat

Okay, I did say this, but then I had followed it up with the below as well :

Yes, absolutely. My original point was more about the fact that many people take it for granted that moving the engine forward would cause a pitch up tendency. And that is because they get it wrong, which length, the arm, matters. (Also, interesting to note that nobody mentions the thrust. All of what we are discussing is only valid for identical thrust.

The pitch up tendency, as you mentioned is due aero dynamic forces on the engine nacelle. Which are (in theory) 90 degrees different from the force of engine thrust. So the arm for the aero dynamic force on the engine trust is related to the distance between the engine nacelle en the aircraft CG.

Originally Posted by MegaWhat

Now, under high lift conditions - like in climb - the wings will flex upwards more than they would flex while in cruise. So basically the CG and the aerodynamic center is going to shift upwards, away from the engine, though only marginally. This will increase the moment arm for the aerodynamic loads acting on the engine with respect to the CG and further increase the pitch up tendency.

I am not sure I follow this. When the wing flexes I would image it actually shortens the moment arm? The engine moves upwards towards the CG? There might be some rotation as well, not sure, but even so.

Jeroen

[MegaWhat]

Originally Posted by Jeroen

I am not sure I follow this. When the wing flexes I would image it actually shortens the moment arm? The engine moves upwards towards the CG? There might be some rotation as well, not sure, but even so.

The engines don't move upwards as much as the rest of the wing. The inboard part of the wing is a little more rigid. Here's an image of the 787 flexing:



My guess was that as the wings flex upwards, the CG moves a little upwards too (from the blue dot to the purple dot). That means the arm between the engine thrust and the CG will increase, leading to a slightly higher moment. Looking at the thrust here instead of the external aerodynamic forces.

In any case, I think this hypothesis is dead on arrival. I searched around a bit more and it seems the structure for the 737 MAX was not changed significantly as compared to the NG. The wings do not flex as much as the 787. So this hypothesis is out of the window.

[Jeroen]

Originally Posted by MegaWhat

The engines don't move upwards as much as the rest of the wing. The inboard part of the wing is a little more rigid. Here's an image of the 787 flexing:
)

Nice image!

The engine on a 737 weighs just a few tonnes, compared to its maximum take off weight of well over 80 tonnes. So moving a few tonnes, compared to the total isn’t going to shift the CG by much. But the arm causing the pitch up effect does get shorter of course, but again not by much, especially as the engines are pretty close to the body where the actual movement of the wings due to flexing is relatively low.

But irrespective, to your point as well, both effects are likely to be very small indeed.

Although I am sure the designer do take those things into consideration.

Where as the moment changes due to these effects are likely to be small in absolute terms, the aerodynamic effects of wing flexing and therefor slightly different orientation of also the engines, could be substantial. Aerodynamics and lift is a finicky science. Small bits can make big differences!

[MegaWhat]

Originally Posted by Jeroen

But irrespective, to your point as well, both effects are likely to be very small indeed.

Agreed. The shifts caused by these changes would be quite small. My line of thought was that with the wings flexing that much, maybe the fuel inside the wings moves up that much as well, taking the CG upwards with it. But I'm not a structures man and I don't know where exactly the fuel tanks are within the wings, so me commenting on that isn't ideal. But in any case, it won't make much difference, that's for sure.

Originally Posted by Jeroen

Aerodynamics and lift is a finicky science. Small bits can make big differences!

Indeed! That's what makes it fun to study and work on!
In all the organizations I've worked with, contour changes of 0.5 - 1mm are quite standard to get upto 10% variation in lift/forces. The slightest changes in contour in the critical regions of the airfoil can make all the difference!

[fhdowntheline]

Originally Posted by MegaWhat

Agreed. The shifts caused by these changes would be quite small. My line of thought was that with the wings flexing that much, maybe the fuel inside the wings moves up that much as well, taking the CG upwards with it. But I'm not a structures man and I don't know where exactly the fuel tanks are within the wings, so me commenting on that isn't ideal. But in any case, it won't make much difference, that's for sure.



Indeed! That's what makes it fun to study and work on!
In all the organizations I've worked with, contour changes of 0.5 - 1mm are quite standard to get upto 10% variation in lift/forces. The slightest changes in contour in the critical regions of the airfoil can make all the difference!

Meanwhile, this article on aerodynamics further queers the pitch:

https://www.scientificamerican.com/a...ay-in-the-air/

[Sutripta]

Originally Posted by MegaWhat

There is one thing I would like to correct in the above explanation though. I spoke to a friend who is in the aircraft design business and he mentioned that the flow over an engine cannot be as independent as that over a canard. The flow over the wing and the engine are highly coupled. But he does agree that considering that the engine for the 737 MAX is moved further ahead of the CG, it could be affecting the flow - over itself and over the wing - to generate a nose up tendency at high Angles of attack.

Laymans views on canards
https://www.team-bhp.com/forum/comme...ml#post4742469 (Boeing 737 Max crashes and grounding)

BUT
effects of canards (or canard like artifices) will, or should be speed dependent, not thrust dependent. And everyone talks of increasing the thrust causes the nose to pitch up.

First question should be validating the accepted belief that 'increasing thrust on the Max causes the nose to pitch up'. If so, then one can get into the whys. If not, question doesn't arise.

Any really good (gaming) simulators availlable for the 737 and the 737 Max?

Regards
Sutripta

[Jeroen]

Originally Posted by Sutripta

First question should be validating the accepted belief that 'increasing thrust on the Max causes the nose to pitch up'. If so, then one can get into the whys. If not, question doesn't arise.

Any really good (gaming) simulators availlable for the 737 and the 737 Max?

Just about any aircraft with engine underslung of the wings will have a pitch up tendency when thrust is increased. (Note, this is a different situation as to what we have been discussing so far. MCAS is first and foremost MCAS a longitudinal stability enhancement. Which, as per the regulations, assumes constant thrust!

The reason for nose up pitch due to increase thrust is back to basic mechanics. Moment is Force times arm. The arm is constant, but if you increase the force (by increasing thrust) you will increase the moment and thus have some pitch up tendency.

There is also an aerodynamic effect. At least on most airplanes. At level flight, properly trimmed when thrust is increased, speed increases. Which means the lift produces by the wings, compared to the lift produced by the stabiliser increases. Simply put, most airplanes are designed in such a way that the stabiliser always produces a negative lift compared to the wings, As speed increases the result of the variations in wing lift compared to stabiliser lift, it makes for a pitch up tendency. (or rather a shallower AoA).

In practices it is a little more complicated, with all sorts of other effects, (e.g. downwash, AoA versus pitch rotation), above is just a simplified version the way I recall. I will need to look up a more detailled explanation.

So as with the MCAS discussion, it is not just about the effect of the engine, it is also about the effects of the overall aerodynamics of the plane and what a change in speed (i.e. thrust) does.

I am not sure about (gaming) simulators for the 737/Max. But as with most gaming simulators (including the various Microsoft variant, the flight model tends to be very limited.

Jeroen