AF 447 Thread No. 6
I believe that Lyman was trying to say somewhat less than concisely, was that with the aircraft established in the stall and dropping at over 100 kts, that the g could have been less than .75g leading to the THS software logic locking in position.
or to put it another way
If the software driving the THS uses g sensimg
and moves the THS with the intent of maintaining 1 g then
0.9 g would be enough for the software to continue commanding full NU.
or to put it another way
If the software driving the THS uses g sensimg
and moves the THS with the intent of maintaining 1 g then
0.9 g would be enough for the software to continue commanding full NU.
For once an for all, the THS followed the PF demand applied for NU. Once the elevators were consistently held NU the integrating function that looks at the demand, determined a constant NU offset was being applied to the controls and followed the demand in an attempt to off-load the elevator (as designed, trim, hello!). I don't know what else to say? The PF asked for NU, the system delivered. The PF also decided that TO/GA was a good idea, again assisting in NU.
BTW - Owain Glyndwr - nice work, helps understand things and also reaffirms the point that the THS was NOT the 500lb gorilla in the room here. The elevator demand alone was enough to put the aircraft in the ocean.
I can't help but equate the THS sysetem to the power steering assistance on most cars - yes, if you turn the wheel to point the car over the edge of the precipice the car will dive over the edge to it's doom, the power steering just makes it less hard to do the pointing, but over you will go, whether the power steering works or not, if that is where you point the car in the first place. In fact the THS is LESS intrusive than the power steering analogy, but you get my point...
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A little off topic
Lone, I noticed this, "Alt 2 latched does not mean "flight control locked and no longer able to be moved."" before I noticed it was to bear and ignored the message.
I have an idle question that does not bear on this crash. Why might Alt 2 latch in and not be able to be escaped back to normal once the aircraft is flying normally again? Permanently latching out the aircraft safety systems for half of a long (10 hours?) flight seems counter productive.
I have an idle question that does not bear on this crash. Why might Alt 2 latch in and not be able to be escaped back to normal once the aircraft is flying normally again? Permanently latching out the aircraft safety systems for half of a long (10 hours?) flight seems counter productive.
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DozyWannabe
Actually full motion simulators are useful to teach pilots NOT to trust their somatic senses when flying purely on instruments. It is extraordinarily easy to fool somebody into thinking they are accelerating forward simply by tipping them back. This is done in amusement park simulator rides all the time. Heck, even simple manipulation of the seat's configuration can lead to a feeling of (slight) upwards acceleration.
A full motion simulator can drive that point home to pilots who do not quite get it. PF apparently did not get it. He remarked about high speed. He was tilted back and drew the wrong confusion from his senses.
Actually full motion simulators are useful to teach pilots NOT to trust their somatic senses when flying purely on instruments. It is extraordinarily easy to fool somebody into thinking they are accelerating forward simply by tipping them back. This is done in amusement park simulator rides all the time. Heck, even simple manipulation of the seat's configuration can lead to a feeling of (slight) upwards acceleration.
A full motion simulator can drive that point home to pilots who do not quite get it. PF apparently did not get it. He remarked about high speed. He was tilted back and drew the wrong confusion from his senses.
Quote from Owain Glyndwr:
I didn't think I said that in quite that way, but yes, I agree
Whoops, I take your point. Apologies for putting words into your mouth.
I didn't think I said that in quite that way, but yes, I agree
Whoops, I take your point. Apologies for putting words into your mouth.
Originally Posted by HarryMann
Oh yes, another thing...
whether this is still taught, explicitly, or left to the control laws to sort out today, would be interesting.
Quote=D.P.Davies
So be gentle with the aeroplane at high altitude . Make control movements slowly and smoothly and recover from divergences in the same manner.
one good reason for this, is the reduced aerodynamic damping at altitude.
So be gentle with the aeroplane at high altitude . Make control movements slowly and smoothly and recover from divergences in the same manner.
one good reason for this, is the reduced aerodynamic damping at altitude.
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'Nuther silly question --
A33Zab, does the BUSS report a line of flight component of airspeed or does it report the square root of the sum of the squares of the three dimensional velocity vector components? (Or ideally, does it report both?)
A33Zab, does the BUSS report a line of flight component of airspeed or does it report the square root of the sum of the squares of the three dimensional velocity vector components? (Or ideally, does it report both?)
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Trim is not implicated in the climb into the stall, so wouldn't change that.
To get out of stall, the first step is the pilots recognizing it. That didn't happen, and trim (or lack of) wouldn't change that.
It might have made for a faster recovery, but there was no recovery attempt, and had there been a timely recovery attempt, the trim wouldn't have been full up anyway.
In contrast, the behaviour you suggest (autotrim drops out) is a contributing factor in several fatal accidents and near-accidents, on both A & B. Particularly when the automatics have trimmed the a/c up into the stall in the first place. One more thing too many for the pilot to remember in a recovery.
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Hi
My guess: having encountered once a HARD failure (i.e. not intermittent), the system cannot be sure its sensors are back to normal.
OK, we (human) may analyse that a priori the speed (pitots) are back to normal.
But a computer cannot be sure of that (unless implementing complicated/prone to failure cross checks). So, as long as a real check of the once faulty part(s) (here: the pitots/ADR) has been done (on the ground, that is), the computers considers their functionnality (here: airspeed values) as unreliable ; or, at last, not enough reliable to revert back to NORMAL LAW and its associated protections.
Just an (educated?) guess, though. If anyone knowledgable can/may confirm or infirm, I'll be glad.
I have an idle question that does not bear on this crash. Why might Alt 2 latch in and not be able to be escaped back to normal once the aircraft is flying normally again? Permanently latching out the aircraft safety systems for half of a long (10 hours?) flight seems counter productive.
OK, we (human) may analyse that a priori the speed (pitots) are back to normal.
But a computer cannot be sure of that (unless implementing complicated/prone to failure cross checks). So, as long as a real check of the once faulty part(s) (here: the pitots/ADR) has been done (on the ground, that is), the computers considers their functionnality (here: airspeed values) as unreliable ; or, at last, not enough reliable to revert back to NORMAL LAW and its associated protections.
Just an (educated?) guess, though. If anyone knowledgable can/may confirm or infirm, I'll be glad.
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The PF asked for NU, the system delivered. The PF also decided that TO/GA was a good idea, again assisting in NU.
On the power steering analogy, I find myself helpless not to comment, as I find too that has completely destroyed many driver's ability to steer accurately, or hold the wheel properly with two hands (at 10 to 2 O'Clock) such that in a moment of crisis, any meaningful avoiding action could ever be taken; some drivers, seemingly bunched up behind the wheel, with a laissez faire one-handed grip in the most strange of places (even right down the bottom) are unlikely to be able to input more than 10 or 15 degrees wheel input... even after 2 to 3 whole seconds of reaction time
NB. The steering wheel will save your life easily as often as the brakes, sometimes more so...
NB2. IMHO, this is 'on' topic.. witness the likely appalling stick handling skills... treating the stick as just another appendage, is somewhat akin to the modern power steering driver's complete failure to honour the steering wheel as a vital life & death control (usually though its not them in their massive heavy & expensive energy absorbing steel box who suffers, but a pedestrian or occupant of an older or smaller car !)
Last edited by HarryMann; 26th Aug 2011 at 13:33.
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IF789 I am NOT suggesting AutoTrim 'dropout'. That is a bad idea. It should not have begun in the first place, but once present, an immediate loss of AoI aspect could be deadly.
If THS dialed in NU to accommodate PF input, there is a big problem with that, and the main reason recovery most likely was impossible. The fact that PF did not recover the flight Path is not relevant for the general discussion.
If THS was loading NU to re-gee the airframe, same story, it was out of its program, functionally. The a/c had to have ND, and just cause the PF didn't grok that, it is ok for the THS to blow it also?
At the onset of climb, PF had elevators loaded with airstream, and THS was unnecessary. He was barred from over loading the tail via 'g' protection, but that wasn't necessary either. He was allowed (or input himself) nibbles of NU sufficient to STALL. Because it took so long to STALL, he had zippo energy (~ or, 'not enough') left to allow a rapid break, and the MUSH brigade began. Now without cue, no-one gets that the airframe is STALLED, and TOGA is still proper (in the BOOK) per procedure, Bob's yer uncle. Done, done, done.
REPEAT... I do not advocate 'dumping' trim in the guise of recovery.
per LATCH AL2. Why do we have to go back to NORMAL LAW from AL2?
How about just a simpler version of it, like an autopilot. Is that to Ludditic?
If THS dialed in NU to accommodate PF input, there is a big problem with that, and the main reason recovery most likely was impossible. The fact that PF did not recover the flight Path is not relevant for the general discussion.
If THS was loading NU to re-gee the airframe, same story, it was out of its program, functionally. The a/c had to have ND, and just cause the PF didn't grok that, it is ok for the THS to blow it also?
At the onset of climb, PF had elevators loaded with airstream, and THS was unnecessary. He was barred from over loading the tail via 'g' protection, but that wasn't necessary either. He was allowed (or input himself) nibbles of NU sufficient to STALL. Because it took so long to STALL, he had zippo energy (~ or, 'not enough') left to allow a rapid break, and the MUSH brigade began. Now without cue, no-one gets that the airframe is STALLED, and TOGA is still proper (in the BOOK) per procedure, Bob's yer uncle. Done, done, done.
REPEAT... I do not advocate 'dumping' trim in the guise of recovery.
per LATCH AL2. Why do we have to go back to NORMAL LAW from AL2?
How about just a simpler version of it, like an autopilot. Is that to Ludditic?
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JD-EE, re yr #460 -
From Joelle Barthe's text:
Leaving out the more subtle details for readability, this is how I think it works in principle:
If one knows the airplane's aerodynamic characteristics (in particular, lift coefficient versus AoA), and the airplane's mass and airspeed, one can calculate the normal load factor for each AoA. I think the BUSS essentially uses the same equation in reverse, solving for airspeed from sensed LF and AoA.
P.S. For two reasons this doesn't work very well at high altitude. Firstly, the slope of AoA vs airspeed becomes rather shallow at high airspeed. Secondly, the aerodynamic characteristics vary with Mach number, i.e. (pressure) altitude.
From Joelle Barthe's text:
This (the BUSS - HN39) indication is based on angle of attack (AOA) sensor information, and is therefore not affected by erroneous pressure measumements.
If one knows the airplane's aerodynamic characteristics (in particular, lift coefficient versus AoA), and the airplane's mass and airspeed, one can calculate the normal load factor for each AoA. I think the BUSS essentially uses the same equation in reverse, solving for airspeed from sensed LF and AoA.
P.S. For two reasons this doesn't work very well at high altitude. Firstly, the slope of AoA vs airspeed becomes rather shallow at high airspeed. Secondly, the aerodynamic characteristics vary with Mach number, i.e. (pressure) altitude.
Last edited by HazelNuts39; 26th Aug 2011 at 14:05. Reason: P.S.
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Hi HN39,
It's not that clever.
It simply displays a sensible angle of attack as the "green" sector. Other angles of attack as orange and red. It is orientated so that the red sector (with high angle of attack) is at the bottom of the scale, etc.
Hi JD-EE,
Pilots can only sense accelerations - not speed. The sensation of high speed was due to the unusual air noise around the cockpit sounding like they were going very fast.
I think the BUSS essentially uses the same equation in reverse, solving for airspeed from sensed LF and AoA.
It simply displays a sensible angle of attack as the "green" sector. Other angles of attack as orange and red. It is orientated so that the red sector (with high angle of attack) is at the bottom of the scale, etc.
Hi JD-EE,
He remarked about high speed. He was tilted back and drew the wrong confusion from his senses.
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HarryMann,
Your reference to power steering, brings forward the "feedback question again". As one that spends time racing cars at speeds that are well above those that normal drivers do, the steering feedback is a very important sensory element in terms of wheel adhesion, or lack of adhesion - the lightness/hardness of the steering is an adhesion indication - if no other sensing mechanism is included in the system, to replace it.
Your reference to power steering, brings forward the "feedback question again". As one that spends time racing cars at speeds that are well above those that normal drivers do, the steering feedback is a very important sensory element in terms of wheel adhesion, or lack of adhesion - the lightness/hardness of the steering is an adhesion indication - if no other sensing mechanism is included in the system, to replace it.
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Qwain Glyndwr,
Thank you for constructing the graph, and curves, illustrating, and explaining what appears to be the science behind some of the interpretations and assumptions made based on the existing BEA Report graphs in earlier posts .
Your post is substantial, while also dense, and at a quick reading - I don't have much time now - I can react with some quick questions, but I may have more later....
1. Regarding the Elevator to THS 1:1.5 ratio, what is the ratio between the surface/area of the Elevator versus surface/area of the THS?
2. Is this 1:1.5 ratio the reason (or are there more?) behind qualifying the THS as the "most powerful control surface on the plane" - paraphrasing text from some earlier posts on this thread, related to the THS?
The explanation above seems to emphasize the role of the Stall Warning as being by far the most important element present among the Stall Warning mechanisms at the time, and thus the need for it to be reliable.
The THS reached the 13 degree NU when the airplane was already in STALL, so the effect of the max 13 degree deflection can be counted on from that moment, and not earlier - I would think.
3. At what AoA, and Elevators max NU, would the THSmaxNU pitching moment switch into becoming a NU moment?
While at 40 degrees AoA, the THSmaxNU ND momentum (in spite of its NU position) opposed the Elevators NU momentum; at this switching point, it would start helping the Elevators NU pitching moment, or opposing the Elevators ND pitching momentum, if Elevators were moved to be deflected ND.
You seem to have taken in consideration a first important element regarding the THS contribution to the pitching momentum, which is the THS angle in itself.
But there is a second element, which I think may be, or is important, which depends on the question:
Is the Elevator swing action relative to the THS surface, that is, Elevator max NU and max ND angle relative to the THS surface?
If the answer is yes, than in that case, in absolute value relative to the airplane longitudinal axle (absolute - for shortness):
a) Elevators max NU (absolute) angle is at
- maximum at THSmax NU
- minimum at THSmax ND
and
b) the Elevators max ND (absolute) angle is
- minimum at THS max NU,
and
- maximum at THS max ND.
This implies a difference between the absolute Elevators ND angle relative to the a/c longitudinal axle:
- Elevators max ND angle with THS max NU, versus
- Elevator max ND with THS at 3 degree NU (call it Neutral), and further
- Elevator max ND with THS max ND.
4. I would be interested in reading your comments extended to these above elements.
The combination of answers to 3. and 4. would explain further, I think, your conclusion above.
5. At AoA 40 degrees, are the THS + Elevator, Thrust, (and Cg) the only (force) factors contributing to the pitching momentum?
I am confused. Is perhaps my first reading. I read "first chart", but I don't see more than one chart, so I must be missing something.
Or are you perhaps referring to first curve? Also you mentioned in your text "traces", or "trace" which I read as "curves" or "curve" on the graph, but I want to make sure my reading was correct.
Overall things are very complex, but it is nice to peal off one by one the layers involved.....
Without having a Normal Acceleration in the picture, is there any indication that in order to being able to quickly and effectively react and have full control of the control surfaces at Stall, would have been better achieved by being in Direct Law?
Thank you for constructing the graph, and curves, illustrating, and explaining what appears to be the science behind some of the interpretations and assumptions made based on the existing BEA Report graphs in earlier posts .
1. Regarding the Elevator to THS 1:1.5 ratio, what is the ratio between the surface/area of the Elevator versus surface/area of the THS?
2. Is this 1:1.5 ratio the reason (or are there more?) behind qualifying the THS as the "most powerful control surface on the plane" - paraphrasing text from some earlier posts on this thread, related to the THS?
So when we talk of the PF not recognising that he was stalled, one of the classic indications was missing.
The other criterion is buffet. ... a 5 Hz component. .... but it seems to me to be perhaps expecting too much for a pilot to distinguish a change in frequency content of cockpit motion when he has been experiencing turbulence and was expecting more severe turbulence to come shortly. I note that the AI Chief Test Pilot has said that it is very difficult, even for experienced test pilots, to distinguish the transition into stall.
The other criterion is buffet. ... a 5 Hz component. .... but it seems to me to be perhaps expecting too much for a pilot to distinguish a change in frequency content of cockpit motion when he has been experiencing turbulence and was expecting more severe turbulence to come shortly. I note that the AI Chief Test Pilot has said that it is very difficult, even for experienced test pilots, to distinguish the transition into stall.
...
For virtually the whole of the event, and certainly for the whole time the aircraft was stalled, the THS had a positive AOA so that it was generating upwards lift and a nose down pitching moment despite the fact that it was set at -13.5 degrees!. This is consistent with it being a stable aeroplane as shown by that pitching moment curve.
For virtually the whole of the event, and certainly for the whole time the aircraft was stalled, the THS had a positive AOA so that it was generating upwards lift and a nose down pitching moment despite the fact that it was set at -13.5 degrees!. This is consistent with it being a stable aeroplane as shown by that pitching moment curve.
3. At what AoA, and Elevators max NU, would the THSmaxNU pitching moment switch into becoming a NU moment?
While at 40 degrees AoA, the THSmaxNU ND momentum (in spite of its NU position) opposed the Elevators NU momentum; at this switching point, it would start helping the Elevators NU pitching moment, or opposing the Elevators ND pitching momentum, if Elevators were moved to be deflected ND.
Of course the net HS lift was negative and the net pitching moment positive (nose up), but this was made up of a very large downwards lift from the elevators partly offloaded by the positive lift from the THS itself. If the elevator had been returned to neutral the THS lift would have given a ND pitch and attitude reduction. Look at the traces - that is exactly what happened!
But there is a second element, which I think may be, or is important, which depends on the question:
Is the Elevator swing action relative to the THS surface, that is, Elevator max NU and max ND angle relative to the THS surface?
If the answer is yes, than in that case, in absolute value relative to the airplane longitudinal axle (absolute - for shortness):
a) Elevators max NU (absolute) angle is at
- maximum at THSmax NU
- minimum at THSmax ND
and
b) the Elevators max ND (absolute) angle is
- minimum at THS max NU,
and
- maximum at THS max ND.
This implies a difference between the absolute Elevators ND angle relative to the a/c longitudinal axle:
- Elevators max ND angle with THS max NU, versus
- Elevator max ND with THS at 3 degree NU (call it Neutral), and further
- Elevator max ND with THS max ND.
4. I would be interested in reading your comments extended to these above elements.
The combination of answers to 3. and 4. would explain further, I think, your conclusion above.
The nose was being held up by the application of elevator.
Of course, the THS setting made the elevator’s job easier, and if the THS had been (sensibly in my view) restricted to 3 deg the eventual AOA would have been lower, How much lower you can get from the first chart – with 3 deg THS and 30 deg elevator you could expect to arrive at 35 deg AOA – big deal! – you are still well stalled and although the descent would have been shallower the end would have been the same unless he had recognised early on that he was in a stall.
Or are you perhaps referring to first curve? Also you mentioned in your text "traces", or "trace" which I read as "curves" or "curve" on the graph, but I want to make sure my reading was correct.
Overall things are very complex, but it is nice to peal off one by one the layers involved.....
Without having a Normal Acceleration in the picture, is there any indication that in order to being able to quickly and effectively react and have full control of the control surfaces at Stall, would have been better achieved by being in Direct Law?
Last edited by airtren; 26th Aug 2011 at 22:53.
OwainG;
Great post, thank you - it is taking me some time to absorb and think it through. I would like to offer what I can by way of response.
Yes, I think that is a very good point. I think it is probably expecting too much to distinguish between the character of turbulence and buffet under the circumstances described. Confirmation bias may play a role in this, for as you say, "more turbulence" was expected. I have asked and I think simulators, even D-Level, do not replicate turbulence and either high or low speed buffet differently. Perhaps that will be one of many changes that will emerge in the learning.
Again yes, I think that is a good place to start the change in the teaching/checking of stalls in all conditions. Despite the fact that simulators do not currently replicate the aerodynamics of the actual stall, (once past the approach to the stall), they could certainly replicate these two symptoms and training stall recovery techniques as per the revised "Stall Warning or Aerodynamic Stall Recover Procedure" (Airbus, Boeing, Bombardier, Embraer document), revised published procedures could be scripted in recurrent training. In fact, I would expect such training and checking are already being implemented as is the UAS procedure (including clarifications on how to do the drill and checklist).
I'm sure that the various regulatory authorities are examining all this and will change their PPC (Pilot Proficiency Check) mandatory requirements concerning the approach to the stall from "minimum loss of altitude" to the current wisdom. Currently in Canada even teaching/checking the approach to the stall, and the stall itself, are not required if the PPC is for FBW aircraft, (not sure if this includes the B777 or not - anyone?).
The one question I have on this then, is about the elevator which clearly had aerodynamic authority all the way down. If, after the stall was fully-developed and even with the THS at -13.5deg, if the SS had been placed in the full ND position and held there, depending upon when this was done, (earlier the better of course!), and given the ND pitching moment afforded by the THS, would such elevator position be sufficient to eventually get the nose down or would it partially/fully stall given the already-positive AoA of the THS, and lose all effectiveness thereby?
Fascinating post, thank you sir.
Great post, thank you - it is taking me some time to absorb and think it through. I would like to offer what I can by way of response.
I will bow to any pilot opinion that differs, but it seems to me to be perhaps expecting too much for a pilot to distinguish a change in frequency content of cockpit motion when he has been experiencing turbulence and was expecting more severe turbulence to come shortly. I note that the AI Chief Test Pilot has said that it is very difficult, even for experienced test pilots, to distinguish the transition into stall.
If there is no pitch break and the buffet is difficult to interpret, he would have been thrown back on symptoms that do not appear in the JAR description of stalls – high attitude and inability to arrest the rate of descent – but over to pilots to evaluate those.
I'm sure that the various regulatory authorities are examining all this and will change their PPC (Pilot Proficiency Check) mandatory requirements concerning the approach to the stall from "minimum loss of altitude" to the current wisdom. Currently in Canada even teaching/checking the approach to the stall, and the stall itself, are not required if the PPC is for FBW aircraft, (not sure if this includes the B777 or not - anyone?).
One other thing that is hardly rocket science either but will probably surprise many people is the THS behaviour. THS AOA is just the body AOA less downwash and THS setting. The downwash is usually expressed as a value at zero AOA plus a downwash gradient relating downwash and AOA. I have used 1deg and 0.4 if anyone cares.
The aircraft first went into the final stall at about 02:11:55 at which time the AOA was around 10 deg and the THS at -3.4. Downwash was 5 deg, so the THS angle of attack was 1.6 degrees positive. The THS arrived at -13.6 at 02:12:27, when the AOA was over 40 deg. With the above assumptions the downwash would have been 17 deg and the THS angle of attack 9.5 deg positive. There are arguments to suggest that this is an overestimate of downwash behind a stalled wing, so the THS AOA would have been at least this positive value.
For virtually the whole of the event, and certainly for the whole time the aircraft was stalled, the THS had a positive AOA so that it was generating upwards lift and a nose down pitching moment despite the fact that it was set at -13.5 degrees!. This is consistent with it being a stable aeroplane as shown by that pitching moment curve.
The aircraft first went into the final stall at about 02:11:55 at which time the AOA was around 10 deg and the THS at -3.4. Downwash was 5 deg, so the THS angle of attack was 1.6 degrees positive. The THS arrived at -13.6 at 02:12:27, when the AOA was over 40 deg. With the above assumptions the downwash would have been 17 deg and the THS angle of attack 9.5 deg positive. There are arguments to suggest that this is an overestimate of downwash behind a stalled wing, so the THS AOA would have been at least this positive value.
For virtually the whole of the event, and certainly for the whole time the aircraft was stalled, the THS had a positive AOA so that it was generating upwards lift and a nose down pitching moment despite the fact that it was set at -13.5 degrees!. This is consistent with it being a stable aeroplane as shown by that pitching moment curve.
Fascinating post, thank you sir.
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Air noises
rudderrudderrat
The sensation of high speed was due to the unusual air noise around the cockpit sounding like they were going very fast.
The sensation of high speed was due to the unusual air noise around the cockpit sounding like they were going very fast.
Just an idea.
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Hello,
What about his experience seeing full blue area in the artificial horizon
The pilots had a very good horizon indication .. it's even the last words of the captain ....
Do you think it might be possible that these sounds reminded him of the sounds heard in a glider going at high speed with which he was probably familiar, and that in this respect his glider experience to some extent misled him?
Just an idea.
Just an idea.
The pilots had a very good horizon indication .. it's even the last words of the captain ....
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airtren
The elevators are full span and the hinge line is at 70% chord, so the elevator area is 30% of the exposed HS area
I know this has been said, but I think it is based on a misinterpretation of the facts (as is quite a lot said here
)
The THS limits are 14 deg NU to 2 deg ND and the elevator limits are 30 deg NU to 15 deg ND. Taking my 1.5:1 power ratio the elevator/THS power would be 30/21 and the ND power 15/3. I rather suspect people have been thinking of the ability of the elevator to overcome the THS when they are in opposition, but since the THS is slaved to the elevator it is difficult to see how this could in any way be relevant particularly when you consider the main thrust of my arguments on THS behaviour.
So far as I can see, never. To give a NU moment the THS would have to experience a negative AOA. It might get this in cruise at fwd CG, but then the elevators would be neutral, since that is why the THS would be so positioned. As soon as full NU elevator was applied the AOA would go up and you would be back where we started.
To be honest I am not sure what you are asking here.
Elevator deflection is always quoted relative to the THS chordline.
For the rest it is probably easiest just to give the numbers:
Maximum NU deflection relative to body axis is (14+30) = 44 deg
Maximum ND deflection relative to body axis is (2+15) = 17 deg.
It is usual to take moments around the CG, so that doesn't figure in the calculation of moments except through variations in the relevant moment arms. The pitching moment is then affected by THS+elevator, Thrust and pitch rate, although this last is a minor term compared to the others.
Sorry - minor typo - my first version of the post included two charts, the second of which was THS AOA vs aircraft AOA. For "first chart" read "the chart above"
I don't think it would stall, because even at 40 deg aircraft AOA the THS AOA was only about 10 deg. After all, you don't very often see wings stalling at that sort of AOA with plain flaps deflected do you?
1. Regarding the Elevator to THS 1:1.5 ratio, what is the ratio between the surface/area of the Elevator versus surface/area of the THS?
2. Is this 1:1.5 ratio the reason (or are there more?) behind qualifying the THS as the "most powerful control surface on the plane" - paraphrasing text from some earlier posts on this thread, related to the THS?
)The THS limits are 14 deg NU to 2 deg ND and the elevator limits are 30 deg NU to 15 deg ND. Taking my 1.5:1 power ratio the elevator/THS power would be 30/21 and the ND power 15/3. I rather suspect people have been thinking of the ability of the elevator to overcome the THS when they are in opposition, but since the THS is slaved to the elevator it is difficult to see how this could in any way be relevant particularly when you consider the main thrust of my arguments on THS behaviour.
3. At what AoA, and Elevators max NU, would the THS pitching moment switch into becoming a NU moment?
While at 40 degrees AoA, the THS ND momentum helped the Elevators ND, at this switching point, it would start opposing the Elevators ND pitching moment.
But there is a second element, which I think may be, or is important, which depends on the question:
Is the Elevator swing action relative to the THS surface, that is, Elevator max NU and max ND angle relative to the THS surface?
If the answer is yes, than in that case, in absolute value relative to the airplane longitudinal axle (absolute - for shortness):
a) Elevators max NU (absolute) angle is at
- maximum at THSmax NU
- minimum at THSmax ND
and
b) the Elevators max ND (absolute) angle is
- minimum at THS max NU,
and
- maximum at THS max ND.
This implies a difference between the absolute Elevators ND angle relative to the a/c longitudinal axle:
- Elevators max ND angle with THS max NU, versus
- Elevator max ND with THS at 3 degree NU (call it Neutral), and further
- Elevator max ND with THS max ND.
Is the Elevator swing action relative to the THS surface, that is, Elevator max NU and max ND angle relative to the THS surface?
If the answer is yes, than in that case, in absolute value relative to the airplane longitudinal axle (absolute - for shortness):
a) Elevators max NU (absolute) angle is at
- maximum at THSmax NU
- minimum at THSmax ND
and
b) the Elevators max ND (absolute) angle is
- minimum at THS max NU,
and
- maximum at THS max ND.
This implies a difference between the absolute Elevators ND angle relative to the a/c longitudinal axle:
- Elevators max ND angle with THS max NU, versus
- Elevator max ND with THS at 3 degree NU (call it Neutral), and further
- Elevator max ND with THS max ND.
For the rest it is probably easiest just to give the numbers:
Maximum NU deflection relative to body axis is (14+30) = 44 deg
Maximum ND deflection relative to body axis is (2+15) = 17 deg.
5. At AoA 40 degrees, are the THS + Elevator, Thrust, (and Cg) the only (force) factors contributing to the pitching momentum?
I am confused. Is perhaps my first reading. I read "first chart", but I don't see more than one chart, so I must be missing something.
Or are you perhaps referring to first curve? Also you mentioned in your text "traces", or "trace" which I read as "curves" or "curve" on the graph, but I want to make sure my reading was correct.
Or are you perhaps referring to first curve? Also you mentioned in your text "traces", or "trace" which I read as "curves" or "curve" on the graph, but I want to make sure my reading was correct.
The one question I have on this then, is about the elevator which clearly had aerodynamic authority all the way down. If, after the stall was fully-developed and even with the THS at -13.5deg, if the SS had been placed in the full ND position and held there, depending upon when this was done, (earlier the better of course!), and given the ND pitching moment afforded by the THS, would such elevator position be sufficient to eventually get the nose down or would it partially/fully stall given the already-positive AoA of the THS, and lose all effectiveness thereby?
OwainG;
I was in the process of working on all the relative angles (a la mm43) and this instantly cleared it up, thank you. QED!
I don't think it would stall, because even at 40 deg aircraft AOA the THS AOA was only about 10 deg. After all, you don't very often see wings stalling at that sort of AOA with plain flaps deflected do you?