737 max nacelle lift generation
BugBear, Farther up I explained what AoA sensors and the SMYD should have. It's rather inexpensive compared to the costs and potential flaws in depending on redundancy.
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John Tullamarine
'...Nacelle airflow and forces are considerably more complex than your assertion suggests, I'm afraid. Indeed, perhaps run some research into, say, Concorde's story on the subject which is in threads in this site. SR-71 Blackbird has a similar tale. At the end of the day, the nacelle can produce quite a good deal of lift force, not to mention thrust force..."
above the center of lift. (My text)
By this I mean the vertical of the pylons upforce. The engine/pylon is a whole system. The net energy is an AXIAL line, to show the twisting motion the pylon exerts on the bottom of the wing. Pushing up on the front of the pylon, pulling downward on the back. The Max has a larger (Curved) line, expressing its additional power and more forward location from the earlier model... Its additional force lifting up the leading edge of the wing (and NOSE) CAUSES THE PROBLEM expressed in lighter NU control feel...imo
I don't believe it intentional, but parsing the problem into separate vectors and causations diminishes and distracts from the net effect on the airworthiness of the MAX. I think people understand the nature of the issue. The fact is the new engines caused an unacceptable problem with handling of the new jet.
above the center of lift. (My text)
By this I mean the vertical of the pylons upforce. The engine/pylon is a whole system. The net energy is an AXIAL line, to show the twisting motion the pylon exerts on the bottom of the wing. Pushing up on the front of the pylon, pulling downward on the back. The Max has a larger (Curved) line, expressing its additional power and more forward location from the earlier model... Its additional force lifting up the leading edge of the wing (and NOSE) CAUSES THE PROBLEM expressed in lighter NU control feel...imo
I don't believe it intentional, but parsing the problem into separate vectors and causations diminishes and distracts from the net effect on the airworthiness of the MAX. I think people understand the nature of the issue. The fact is the new engines caused an unacceptable problem with handling of the new jet.
Last edited by BugBear; 26th Jun 2024 at 17:00.
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Moderator
I suspect the basics of your suggestion is aligned with what we have been saying previously. My reading of the public arena information is that there was a small region of the envelope which was a bit iffy so it needed to be addressed. Unfortunately, the fix, perhaps, could have been thought through a little bit better than it was ?
Quite agree...
I suspect the basics of your suggestion is aligned with what we have been saying previously. My reading of the public arena information is that there was a small region of the envelope which was a bit iffy so it needed to be addressed. Unfortunately, the fix, perhaps, could have been thought through a little bit better than it was ?
Respect Sir
Moderator
At day's end it is not about a nice, lively response. Rather, stick loads need to be reasonable (ie acceptable in the FT results) so that the average pilot is able to handle situations which might arise.
The cert rules require X, Y, Z. If the bird doesn't comply, in this case with stick loads, then the problem needs to be fixed, or a concession sought.
Been that way for a long, long, time.
The cert rules require X, Y, Z. If the bird doesn't comply, in this case with stick loads, then the problem needs to be fixed, or a concession sought.
Been that way for a long, long, time.
From what I learned, the stick force gradient started to flatten out at a certain AOA, that is force per additional AOA I unit increase became less than earlier. Not unstable, not divergent, just became lighter. The certification standard is that not happen, so the force required is always the same as the last unit added or greater.
Memory has faded but I don’t remember the -38 being tricky in rudder use. The F-100 certainly required judicious and correct rudder at a certain AOA.
Memory has faded but I don’t remember the -38 being tricky in rudder use. The F-100 certainly required judicious and correct rudder at a certain AOA.
Some opinion published in Air Current suggest that the difference between MCAS and non MCAS handling were minimal. There is also reference to pitch up at the stall for both the NG and the Max. The "break" (I assume this means G break) is described as a pitch up. How can this comply with any stall handling requirements, not considering any longitudinal stability aspects approaching the stall?
Who knows?
As an ex certification TP the information that I have sought since day one of MCAS is:
What, exactly, are the flight conditions required to trigger MCAS operations; and
Where are a set of deceleration time histories, which show exactly what happens in both MCAS and non MCAS aircraft?
Some information about the actual destabilising effect of intake momentum drag in the situations under investigation would have been helpful.
A lot of conjecture without adequate data.
Who knows?
As an ex certification TP the information that I have sought since day one of MCAS is:
What, exactly, are the flight conditions required to trigger MCAS operations; and
Where are a set of deceleration time histories, which show exactly what happens in both MCAS and non MCAS aircraft?
Some information about the actual destabilising effect of intake momentum drag in the situations under investigation would have been helpful.
A lot of conjecture without adequate data.
Some opinion published in Air Current suggest that the difference between MCAS and non MCAS handling were minimal
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I suspect the basics of your suggestion is aligned with what we have been saying previously. My reading of the public arena information is that there was a small region of the envelope which was a bit iffy so it needed to be addressed. Unfortunately, the fix, perhaps, could have been thought through a little bit better than it was ?
I wonder why a fence or spoiler could not be fitted to the bottom of each nacelle to cancel or reduce lift at high AOA, surely better than the fix which eventually gave the software more control power than the pilot.
I am not about to delve into the 737 Max certification in this area - impossible anyway without having attended any of the certification meetings between Boeing and the FAA, or having any access to the documents showing the agreed certification standard, interpretations and procedures.
It has been stated here that the Max should demonstrate linear stick forces during deceleration to the stall. This is generally not true, some nonlinearity is acceptable below Vsw subject to test pilot qualitative acceptance. However, there is little guidance in FAR 25 or AC 25-D on longitudinal stability and control at low speed, high deceleration rates, possibly approaching the stall. This situation will invariably result in different interpretations of the intent of the standard in question and application of test pilot qualitative opinion. If this was the case, it should not be a surprise that opinions about the necessity of the MCAS system varied between certification authorities.
As I've previously posted, Boeing did try various aero fixes (strakes, fins, etc.) and not only did they all add to aircraft drag (and hence fuel burn), none really had the desired effect.
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The 737MAX engines are enormous, quite far forward and closely coupled to the wing. At high alphas the nacelle generates lift and probably increases the local alpha at the wing, which causes a pitch up.
Last edited by BugBear; 28th Jun 2024 at 21:59.
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Torque has nothing to do with it. There is not much swirl in the flow leaving a turbofan due to stators and thanks to conservation of angular momentum not much torque is passed through the pylon. Maybe you're talking about thrust, remember that the 737MAX doesn't have much more thrust than the 737NG, and that stall tests are started trimmed longitudinally with constant thrust.
You don't need an airfoil to create lift, the nacelle is just pushing air downwards.
You don't need an airfoil to create lift, the nacelle is just pushing air downwards.
OK, I'll try one more time...
The thrust induced pitch-up due to the engines below the wing centerline is nothing new. It's been there ever since people started hanging engines below the wing. That thrust induced pitch-up is why the max rated thrust is less on the shorter 737s than it is on the longer ones - on the 737-3/4/500 it was limited by a physical stop in the strut that limited how far forward you could advance the thrust lever - on the NG and MAX it's done with the engine ratings plugs.
This is something completely different - it's an aerodynamic force created by those big engine nacelles out in from of the wing that - at high angles of attach - tries to push the nose further up, reducing the 'stick force' needed to pull into a stall (which violates the regulations). That happens even if the engine is at idle and not producing meaningful thrust because it's an aero effect. Boeing tried various aerodynamic doodads to reduce that aero pitch-up effect but none worked satisfactorily. That's why they ended up with MCAS.
The thrust induced pitch-up due to the engines below the wing centerline is nothing new. It's been there ever since people started hanging engines below the wing. That thrust induced pitch-up is why the max rated thrust is less on the shorter 737s than it is on the longer ones - on the 737-3/4/500 it was limited by a physical stop in the strut that limited how far forward you could advance the thrust lever - on the NG and MAX it's done with the engine ratings plugs.
This is something completely different - it's an aerodynamic force created by those big engine nacelles out in from of the wing that - at high angles of attach - tries to push the nose further up, reducing the 'stick force' needed to pull into a stall (which violates the regulations). That happens even if the engine is at idle and not producing meaningful thrust because it's an aero effect. Boeing tried various aerodynamic doodads to reduce that aero pitch-up effect but none worked satisfactorily. That's why they ended up with MCAS.
violator, et al,
When considering aerodynamics, and control and stability, the dynamic combination of lift, drag/thrust, pitching moment, trim force must all be considered for the aircraft configuration, speed and altitude.
These aspects were discussed at length in past threads about the MAX and MCAS; so too the relevance of matching the control and stability characteristics of the MAX with previous variants to minimise training requirements.
cf / search as required
also, see the thread on 'roller coaster' manoeuvre for trim runaway - failure cases requiring pilot recognition and intervention.
Ref OP, most aerodynamic surfaces can generate lift; thus the nacelles on the A320 could produce similar magnitude of changes in the parameters above.
The difference between aircraft is the manner in which these are compensated for, particularly for Airbus with the highly redundant sensors and control system computation - the engineered solution .
.
When considering aerodynamics, and control and stability, the dynamic combination of lift, drag/thrust, pitching moment, trim force must all be considered for the aircraft configuration, speed and altitude.
These aspects were discussed at length in past threads about the MAX and MCAS; so too the relevance of matching the control and stability characteristics of the MAX with previous variants to minimise training requirements.
cf / search as required
also, see the thread on 'roller coaster' manoeuvre for trim runaway - failure cases requiring pilot recognition and intervention.
Ref OP, most aerodynamic surfaces can generate lift; thus the nacelles on the A320 could produce similar magnitude of changes in the parameters above.
The difference between aircraft is the manner in which these are compensated for, particularly for Airbus with the highly redundant sensors and control system computation - the engineered solution .
.
Last edited by PEI_3721; 28th Jun 2024 at 21:39.