737 max nacelle lift generation
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737 max nacelle lift generation
I have tried to find an explanation online but I have not found a good explanation of how the nacelles on the max series are capable of generating lift (and why doesn’t it happen on the a320 neo?) thus necessitating mcas? Is it because the bottom is flattened vs the a320?
I have tried to find an explanation online but I have not found a good explanation of how the nacelles on the max series are capable of generating lift (and why doesn’t it happen on the a320 neo?) thus necessitating mcas? Is it because the bottom is flattened vs the a320?
A spinning sphere can produce lift as every curve ball thrown in baseball proves. Even low rates of spin at high enough airspeed are enough to generate some.
Given that Airbus is essentially in 100% MCAS mode full time, they may be covering for it just fine. Also, the engine on the 737 is closer to the wing, so interactions will be greater.
Given that Airbus is essentially in 100% MCAS mode full time, they may be covering for it just fine. Also, the engine on the 737 is closer to the wing, so interactions will be greater.
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The official story, as I understand it, is that the short undercarriage on the B737 required the engines to be lifted up higher relative to the wing than on the A320 and it's this engine position that's causing the adverse lift at high angles of attack.
The issue is not the nacelle lift at high alpha but the stick force gradient towards a stall that is not conforming to FARs that demand increasing force.
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Let's look at it in a different way.
The thing of interest here normally is called "the normal lift force". If you have something - anything - which turns the airflow, there must be a force arising normal (ie at right angles) in the plane of the airflow's motion inbound and outbound. This is all about force being equivalent to the change in momentum.
We see this with a wing's generating life, in the most familiar example to a pilot.
A propeller, with the aircraft at high alpha, does much the same sort of thing. The incoming airflow comes more or less from ahead of the aircraft, but then is turned quite significantly as it goes through the propeller disc. This gives a force oriented along the plane of the propeller disc but in a direction perpendicular to the aircraft's fore and aft axis. Normally, the propeller is back near the wing and the resulting nose up pitching moment is taken care of in the basic aircraft design process, so it doesn't result in any major problem.
However, with light aircraft piston to turboprop conversions, we have a big hunk of an engine spinning a propeller somewhere back near the wing replaced by a little hunk of an engine (but, generally, with a heap more thrust/power). Considerations of static balance usually lead to the new engine's being shoved well out forward. This, then, puts this normal force (now considerably bigger due to the bigger output of the new engine) well out forward. The new configuration's nose up pitching moment can create a bit of a problem at lower speeds/higher alpha. Indeed, for some conversions, this can give rise to a situation where the static longitudinal stability can reduce to a point where it is unacceptable and, on occasion can lead to static longitudinal instability which is grossly unacceptable for certification. Typically, this problem arises in the missed approach situation.
A common fix in this case is to incorporate a variable elevator down spring arrangement into the design so that the pilot is fooled into seeing a statically stable situation. This modification often goes by the name of SAS (stability augmentation system). If the SAS is U/S, a short term fix is to limit the maximum power and thrust in the missed approach.
With the underwing jet, we have gone from relatively short, small diameter nacelles to the monsters we see in modern aircraft. Two problems can arise. First, the shed nacelle airflow now goes over the leading edge whereas, in the older types, it passed under the leading edge. Fix for this is the nacelle chine VG to keep wing trailing edge separation problems at bay.
Second, in this configuration, we end up with a nice aerodynamic profile stuck well out forward so that, in the low speed, high thrust configuration, we have a vertical (nose up) pitching moment due to the normal force, this time associated with the nacelle leading edge.
As I read the MCAS story, the TP folk found an unacceptable handling bit in a corner of the low speed envelope and the design folk figured to fix it with MCAS. That possibly might have gone well, had it not been for a lack of redundancy in the event of system glitches.
The issue is not the nacelle lift at high alpha
.. but that gives rise to the stick force problems ...
The thing of interest here normally is called "the normal lift force". If you have something - anything - which turns the airflow, there must be a force arising normal (ie at right angles) in the plane of the airflow's motion inbound and outbound. This is all about force being equivalent to the change in momentum.
We see this with a wing's generating life, in the most familiar example to a pilot.
A propeller, with the aircraft at high alpha, does much the same sort of thing. The incoming airflow comes more or less from ahead of the aircraft, but then is turned quite significantly as it goes through the propeller disc. This gives a force oriented along the plane of the propeller disc but in a direction perpendicular to the aircraft's fore and aft axis. Normally, the propeller is back near the wing and the resulting nose up pitching moment is taken care of in the basic aircraft design process, so it doesn't result in any major problem.
However, with light aircraft piston to turboprop conversions, we have a big hunk of an engine spinning a propeller somewhere back near the wing replaced by a little hunk of an engine (but, generally, with a heap more thrust/power). Considerations of static balance usually lead to the new engine's being shoved well out forward. This, then, puts this normal force (now considerably bigger due to the bigger output of the new engine) well out forward. The new configuration's nose up pitching moment can create a bit of a problem at lower speeds/higher alpha. Indeed, for some conversions, this can give rise to a situation where the static longitudinal stability can reduce to a point where it is unacceptable and, on occasion can lead to static longitudinal instability which is grossly unacceptable for certification. Typically, this problem arises in the missed approach situation.
A common fix in this case is to incorporate a variable elevator down spring arrangement into the design so that the pilot is fooled into seeing a statically stable situation. This modification often goes by the name of SAS (stability augmentation system). If the SAS is U/S, a short term fix is to limit the maximum power and thrust in the missed approach.
With the underwing jet, we have gone from relatively short, small diameter nacelles to the monsters we see in modern aircraft. Two problems can arise. First, the shed nacelle airflow now goes over the leading edge whereas, in the older types, it passed under the leading edge. Fix for this is the nacelle chine VG to keep wing trailing edge separation problems at bay.
Second, in this configuration, we end up with a nice aerodynamic profile stuck well out forward so that, in the low speed, high thrust configuration, we have a vertical (nose up) pitching moment due to the normal force, this time associated with the nacelle leading edge.
As I read the MCAS story, the TP folk found an unacceptable handling bit in a corner of the low speed envelope and the design folk figured to fix it with MCAS. That possibly might have gone well, had it not been for a lack of redundancy in the event of system glitches.
The issue is not the nacelle lift at high alpha
.. but that gives rise to the stick force problems ...
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The formal FAR requirement must be met. EASA and FAA test flew the MAX 7 with and without MCAS (2.0) and permitted its return to service. It is obviously safe to fly.
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I have tried to find an explanation online but I have not found a good explanation of how the nacelles on the max series are capable of generating lift (and why doesn’t it happen on the a320 neo?) thus necessitating mcas? Is it because the bottom is flattened vs the a320?
The 737 has a traditional flight control system, control runs of cables and push/pull rods that operate hydraulic actuators, but also with some manual redundancies in the event of hydraulic loss. Pilot commands are not modified by digital computers. The launch customer of the MAX did not want to buy an aircraft that would need re training for their pilots. They wanted it to fly in the same way as the NG. The engine nacelles, being larger, further forward and higher than the NG, in certain high AoA conditions would produce a nose up pitching force that would need the pilots to be trained effectively on a new type as it's operating flight envelope is different. Southwest didn't want this so Boeing introduced MCAS to apply an input to the horizontal stabiliser negating the pitch up force created by the nacelles. The rest is history.
At least that's the way I understand it. I'm sure someone will be along shortly to correct my errors. 👍
The need for MCAS to be safe wasn't redundancy in reading sensors, it was for the sensors to be self-validating, which isn't that difficult to do (putting on a fireproof suit, but I have done related design work.)
1) Put mechanical stops on the vanes so that any reading outside the mechanical stops invalidates the output. There are already internal stops to protect the mechanism; they stay and are outside the range of the vane stops.
2) Put direct drive motors on the shafts that produce a small, moderate frequency variation for the SMYD to detect that the vanes can move, and can also perform Built-in Test (BIT) at startup by driving the vane from one stop to the other.
The system can then also validate that the readings at the stops make sense. If a motor fails, then the BIT will fail; if the motor drive won't shut off then the BIT will fail. If the motor fails in flight, then the oscillation will stop and the reading is invalid. If the motor driver won't shut off then the reading won't oscillate and the reading is invalid. If the vane is removed, then the internal counterweight will eventually drive the reading past the stop and the reading is invalid. If the vane slips or is bent, then one direction or the other and it will not reach the stop at the correct angle during BIT and the reading is invalid. If ice builds up and prevents the vanes from moving in response to the motor oscillations, the reading is invalid.
Any case of invalid AoA sensor reading causes the related SMYD to be invalid and the software in the flight computer switches to the back up. At no time is the MCAS software forced to figure out what the sensors are doing and, more importantly, the SMYD is not going to produce false stall warnings. The only warning needed is that the primary SMYD has failed.
I am unsure what would happen if both SMYDs fail now, but as far as triple redundancy being a fix, if a second non-self validating sensor goes wonky after one of the three (for example) has already been voted out I see no way for the software to decide which of the discrepant two should be believed unless the remaining one is self-validating.
1) Put mechanical stops on the vanes so that any reading outside the mechanical stops invalidates the output. There are already internal stops to protect the mechanism; they stay and are outside the range of the vane stops.
2) Put direct drive motors on the shafts that produce a small, moderate frequency variation for the SMYD to detect that the vanes can move, and can also perform Built-in Test (BIT) at startup by driving the vane from one stop to the other.
The system can then also validate that the readings at the stops make sense. If a motor fails, then the BIT will fail; if the motor drive won't shut off then the BIT will fail. If the motor fails in flight, then the oscillation will stop and the reading is invalid. If the motor driver won't shut off then the reading won't oscillate and the reading is invalid. If the vane is removed, then the internal counterweight will eventually drive the reading past the stop and the reading is invalid. If the vane slips or is bent, then one direction or the other and it will not reach the stop at the correct angle during BIT and the reading is invalid. If ice builds up and prevents the vanes from moving in response to the motor oscillations, the reading is invalid.
Any case of invalid AoA sensor reading causes the related SMYD to be invalid and the software in the flight computer switches to the back up. At no time is the MCAS software forced to figure out what the sensors are doing and, more importantly, the SMYD is not going to produce false stall warnings. The only warning needed is that the primary SMYD has failed.
I am unsure what would happen if both SMYDs fail now, but as far as triple redundancy being a fix, if a second non-self validating sensor goes wonky after one of the three (for example) has already been voted out I see no way for the software to decide which of the discrepant two should be believed unless the remaining one is self-validating.
To answer the OP, this is for two reasons.
Because the B737 has such short main gear legs, they limit the diameter of engine that can be mounted under the wing - larger fan sizes are not possible because there would not be enough clearance between the nacelle and the ground.
Boeing initially got around this by moving the main engine gearboxes from underneath the engines to the sides, and you might have seen those ridiculous looking engines with flat bottomed triangular nacelles.
To go to engines with yet bigger diameter fans, Boeing needed to mount the engines higher up, but because the wing was in the way; they had to mount the engines much further forward to allow the higher mounting, and this introduced extra leverage. So by mounting the engines so far forward, they introduced an extra pitch-up moment caused when the AoA of the engine nacelles passed a certain pitch value.
The second reason is that as I understand it; regulations state that aircraft control feel should be broadly linear, i.e., it should not markedly change over its range. But because of the engines now being so far forward, the pitch feel at high AoAs did change. The Boeing is a conventional aircraft - it has a mechanical connection and relationship between the yokes and the control surfaces. Therefore, to make the pitch feel consistent, the only way Boeing could introduce an automatic correction was by using the THS trim motor to introduce a countering nose-down force at high AoAs. Unfortunately this workaround was "engineered" in a really terrible way, and relied on just a single sensor. The inevitable happened, and more than 300 people died as a result.
Airbus designed a much better aircraft, (I have flown both on the line), with triple, voting sensors and among other things, longer undercarriage legs and therefore plenty of under-wing clearance for larger diameter engines, e.g. the NEOs. They also provided comprehensive fly-by-wire with 5 fly-by-wire computers, any one of which can control the 'plane. So Airbus can easily tweak minor handling issues by adjusting numerical values in the FBW computers, then test flying to confirm results.
It is far better to slightly adjust a parameter in an extensively proven and many-tested system as Airbus can do, than having to crowbar in an unproven work-around in a mechanical aircraft, as Boeing did.
Because the B737 has such short main gear legs, they limit the diameter of engine that can be mounted under the wing - larger fan sizes are not possible because there would not be enough clearance between the nacelle and the ground.
Boeing initially got around this by moving the main engine gearboxes from underneath the engines to the sides, and you might have seen those ridiculous looking engines with flat bottomed triangular nacelles.
To go to engines with yet bigger diameter fans, Boeing needed to mount the engines higher up, but because the wing was in the way; they had to mount the engines much further forward to allow the higher mounting, and this introduced extra leverage. So by mounting the engines so far forward, they introduced an extra pitch-up moment caused when the AoA of the engine nacelles passed a certain pitch value.
The second reason is that as I understand it; regulations state that aircraft control feel should be broadly linear, i.e., it should not markedly change over its range. But because of the engines now being so far forward, the pitch feel at high AoAs did change. The Boeing is a conventional aircraft - it has a mechanical connection and relationship between the yokes and the control surfaces. Therefore, to make the pitch feel consistent, the only way Boeing could introduce an automatic correction was by using the THS trim motor to introduce a countering nose-down force at high AoAs. Unfortunately this workaround was "engineered" in a really terrible way, and relied on just a single sensor. The inevitable happened, and more than 300 people died as a result.
Airbus designed a much better aircraft, (I have flown both on the line), with triple, voting sensors and among other things, longer undercarriage legs and therefore plenty of under-wing clearance for larger diameter engines, e.g. the NEOs. They also provided comprehensive fly-by-wire with 5 fly-by-wire computers, any one of which can control the 'plane. So Airbus can easily tweak minor handling issues by adjusting numerical values in the FBW computers, then test flying to confirm results.
It is far better to slightly adjust a parameter in an extensively proven and many-tested system as Airbus can do, than having to crowbar in an unproven work-around in a mechanical aircraft, as Boeing did.
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Obviously not, but it is not like the aircraft is lifting its nose into a stall. It is the stick feel "only" that is not compliant to formal requirements. Not defending Boeing in any way.
One thing people are really bad at is estimating linear increases in force. Add one pound to two and they notice right away. Have them pick up a 10 pound weight, put it down, and then an 11 pound weight and they are likely to say there is a difference, but don't know how much it is. In addition, people will pull as hard as they can if they think pulling as hard as they can will resolve the situation, so increasing the control force to the pilot doesn't stop them from leaving the envelope unless is is more than the pilot can possibly pull, in which case they may ask the other pilot to help pull as hard as they can as well.
AFAIK the MCAS low speed addition was for a condition found by instrumenting the control column and the aircraft reaction and checking for conformance by plotting them, not because a test pilot said it was too light at the top end. If so, that's why it was only near the end of development that it became noticed, when all the gross handling was done and lots and lots of instruments runs take place.
What's in the FAR is easy to codify and difficult to check and yes, there does need to be some oversight for it. But I am sure if a FBW system were to be programmed with some strange curve that wiggled it's way up, that pilots would not experience the feel of the plane much differently, particularly in case like this for an area of the flight envelope commercial pilots should not be operating in and should not be familiar by repetition with.
No one should be saying "Hey I stalled an NG and it was tough, but the MAX stalls slightly more easily" because they stalled them enough to get a good feel for it.
AFAIK the MCAS low speed addition was for a condition found by instrumenting the control column and the aircraft reaction and checking for conformance by plotting them, not because a test pilot said it was too light at the top end. If so, that's why it was only near the end of development that it became noticed, when all the gross handling was done and lots and lots of instruments runs take place.
What's in the FAR is easy to codify and difficult to check and yes, there does need to be some oversight for it. But I am sure if a FBW system were to be programmed with some strange curve that wiggled it's way up, that pilots would not experience the feel of the plane much differently, particularly in case like this for an area of the flight envelope commercial pilots should not be operating in and should not be familiar by repetition with.
No one should be saying "Hey I stalled an NG and it was tough, but the MAX stalls slightly more easily" because they stalled them enough to get a good feel for it.
Because for each new generation there was this question of whether it was a new type. Commercial team to airlines it was "Yes, trade in your old fleet for this new one". Certification team to FAA it was "No, it's a 737, grandfather rules apply".
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it is not like the aircraft is lifting its nose into a stall. It is the stick feel "only"
If the stick forces and gradients (OK, "feel") get a bit too far away from a nice region, then the aircraft can, indeed, lift its nose into a stall. That's one of the reasons why we have requirements for static stability. In this case the nacelle lip normal force "helps" the aircraft to become less statically stable.
not because a test pilot said it was too light at the top end.
The TP does, indeed, measure these things in a quantified way. A preliminary qualitative assessment may raise a concern, but the data has to be quantified if anyone is to believe the story.
that's why it was only near the end of development that it became noticed
These programs take considerable time for all the test card points of interest to be flown. As I read the story, this came up a concern when it was checked because that was when the program got around to that test point. So, what's new in the flight test game ?
This could be warming up to an interesting thread ?
If the stick forces and gradients (OK, "feel") get a bit too far away from a nice region, then the aircraft can, indeed, lift its nose into a stall. That's one of the reasons why we have requirements for static stability. In this case the nacelle lip normal force "helps" the aircraft to become less statically stable.
not because a test pilot said it was too light at the top end.
The TP does, indeed, measure these things in a quantified way. A preliminary qualitative assessment may raise a concern, but the data has to be quantified if anyone is to believe the story.
that's why it was only near the end of development that it became noticed
These programs take considerable time for all the test card points of interest to be flown. As I read the story, this came up a concern when it was checked because that was when the program got around to that test point. So, what's new in the flight test game ?
This could be warming up to an interesting thread ?