Habsheim
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Originally Posted by Owain Glyndwr
As for the rest, you are distorting my words to suit your own purposes. I made no mention of Valphamax, those are your words.
The objective for an Airbus test pilot when making high AOA demonstrations with alphafloor switched off is to demonstrate the capacity of the airplane to safely maintain alpha max at the corresponding Valphamax.
- Trough the elevator the FCS controls speed by maintaining the proper alpha
- Trough the thrust the pilot controls altitude ... not speed
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Originally posted by Confiture
As far as I know YOU set the initial conditions in YOUR post #652
As far as I know YOU set the initial conditions in YOUR post #652
Although Airbus test pilots routinely made high AOA demonstrations with alphafloor switched off they invariably compensated for that by selecting a level of thrust commensurate with maintaining speed and altitude.
Last edited by Owain Glyndwr; 18th Mar 2014 at 19:01.
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Originally Posted by CONF iture
Trough the thrust the pilot controls altitude ... not speed
When flying at alphamax, thrust controls altitude through speed:
Increase thrust - speed goes up - lift goes up - airplane goes up
Decrease thrust - speed goes down - lift goes down - airplane goes down
Thread Starter
Windshear in the lee of the trees?
Quote from HN39, in reply to CONF_iture:
"I'm disappointed that you still don't seem to grasp a point I made more than once (see f.e. my post #520 on page 26)
When flying at alphamax, thrust controls altitude through speed:
Increase thrust - speed goes up - lift goes up - airplane goes up
Decrease thrust - speed goes down - lift goes down - airplane goes down"
You're not the only one to be disappointed! In fact, for the benefit of new readers, you had earlier made the point on page 24. It was picked up by others including myself, although the vertical component of thrust at high pitch-attitudes may assist the pilot somewhat.
The co-pilot, without any previous experience of the task, had been briefed to take control of the throttle levers if and when Capt Asseline found the task too onerous. Commenting on the knife-edge accuracy of control required by the co-ordinated efforts of the two pilots to maintain (even) the briefed height of 100 ft at alpha-max, I then wrote:
"Clearly, low flight at alpha-max is hazardous, for at least one reason: wind-shear, which is inevitable over and near trees and buildings, unless the wind at all levels is flat-calm (a rare event). Any loss of headwind or increase of tailwind leads to a loss of IAS. Recovery of IAS requires an increase in GS, i.e., kinetic energy. With no surplus of potential energy to convert, that increase in kinetic can only be supplied by an increase in thrust."
So was there any windshear on the day? A few days ago, I wrote:
Starting from level-off at TGEN 321 [t -13] and finishing with TGEN 334, the estimated W/Cs at one-second intervals are as follows:
-7, -4, -8, -9, -7, -7, -6, -4, -6, -5, -7, -7, -3, -3.
(TGEN 334 represents t -zero, the approximate second at which the a/c reached the treeline)
The Karsenty video shows a tall balloon on the west side of the airfield leaning slightly to the south, suggesting a light headwind for the accident a/c. The above wind-components are based on the usual comparison of the IAS-derived TAS, and the inertial ground-speeds (GSs). Any errors in the recorded GSs are likely to be consistent, but the IAS may have suffered increasing position-error in the last few seconds at higher AoAs.
The free-air W/C seems to be between -5 and -9. As the a/c CG was roughly the same height as the approaching treetops, it is likely that the loss of headwind component indicated in the above figures is genuine, and due to slight windshear in the lee of the trees.
Between TGENs 332 and 333 (t -2 and t -1), the recorded IAS drops by 4 kt (116 to 112), although there is no change in AoA, pitch, or GS. That would suggest a loss of wing-lift of the order of 7%, partly offset by the increasing vertical component of thrust as the N1 increased? (There may also be a slight reduction in HS downforce as the elevators unload slightly, one at a time.)
"I'm disappointed that you still don't seem to grasp a point I made more than once (see f.e. my post #520 on page 26)
When flying at alphamax, thrust controls altitude through speed:
Increase thrust - speed goes up - lift goes up - airplane goes up
Decrease thrust - speed goes down - lift goes down - airplane goes down"
You're not the only one to be disappointed! In fact, for the benefit of new readers, you had earlier made the point on page 24. It was picked up by others including myself, although the vertical component of thrust at high pitch-attitudes may assist the pilot somewhat.
The co-pilot, without any previous experience of the task, had been briefed to take control of the throttle levers if and when Capt Asseline found the task too onerous. Commenting on the knife-edge accuracy of control required by the co-ordinated efforts of the two pilots to maintain (even) the briefed height of 100 ft at alpha-max, I then wrote:
"Clearly, low flight at alpha-max is hazardous, for at least one reason: wind-shear, which is inevitable over and near trees and buildings, unless the wind at all levels is flat-calm (a rare event). Any loss of headwind or increase of tailwind leads to a loss of IAS. Recovery of IAS requires an increase in GS, i.e., kinetic energy. With no surplus of potential energy to convert, that increase in kinetic can only be supplied by an increase in thrust."
So was there any windshear on the day? A few days ago, I wrote:
Starting from level-off at TGEN 321 [t -13] and finishing with TGEN 334, the estimated W/Cs at one-second intervals are as follows:
-7, -4, -8, -9, -7, -7, -6, -4, -6, -5, -7, -7, -3, -3.
(TGEN 334 represents t -zero, the approximate second at which the a/c reached the treeline)
The Karsenty video shows a tall balloon on the west side of the airfield leaning slightly to the south, suggesting a light headwind for the accident a/c. The above wind-components are based on the usual comparison of the IAS-derived TAS, and the inertial ground-speeds (GSs). Any errors in the recorded GSs are likely to be consistent, but the IAS may have suffered increasing position-error in the last few seconds at higher AoAs.
The free-air W/C seems to be between -5 and -9. As the a/c CG was roughly the same height as the approaching treetops, it is likely that the loss of headwind component indicated in the above figures is genuine, and due to slight windshear in the lee of the trees.
Between TGENs 332 and 333 (t -2 and t -1), the recorded IAS drops by 4 kt (116 to 112), although there is no change in AoA, pitch, or GS. That would suggest a loss of wing-lift of the order of 7%, partly offset by the increasing vertical component of thrust as the N1 increased? (There may also be a slight reduction in HS downforce as the elevators unload slightly, one at a time.)
Last edited by Chris Scott; 18th Mar 2014 at 15:57. Reason: Last sentence added.
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This thread has been quite interesting to read, lots of factual information and explanations. But there are exceptions:
Say what? Since when?
with maintaining altitude, speed not being dependent on the level of thrust.
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Originally Posted by OK465
Folks it is not necessary that speed change to change flight path angle up or down with thrust.
You're making the assumption AOA stays constant which is not true.
Salute!
It's good for some to realize that you do not have to increase speed/AoA to climb or descend.
Now, ya gotta be gentle with the throttle(s), but a well-trimmed plane (or one with max AoA command in the FBW planes) will climb or descend nicely using only power changes.
Would be interseting if any of the "modern" planes with "speed" mode on the AP would allow throttle/power changes made manually to control climb rate.
It's good for some to realize that you do not have to increase speed/AoA to climb or descend.
Now, ya gotta be gentle with the throttle(s), but a well-trimmed plane (or one with max AoA command in the FBW planes) will climb or descend nicely using only power changes.
Would be interseting if any of the "modern" planes with "speed" mode on the AP would allow throttle/power changes made manually to control climb rate.
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Originally Posted by gums
Would be interseting if any of the "modern" planes with "speed" mode on the AP would allow throttle/power changes made manually to control climb rate.
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I'm glad to watch that old piloting debate about how controlling path and speed. If we stop to forget the shape modifications and to forget that the aircraft is mostly in unsteady transient dynamic everybody will agree to common equations. 
Thread Starter
Hello gums,
If you were asking about ops in the normal range of AoAs, the answer is yes on most current transports.
With the A/THR off, but the AFS in speed mode, the AFS controls speed with elevator. So, if (s)he wants to do it that way, the pilot can adjust VS by changing the thrust manually. The most common scenario might be in a descent at idle thrust, if the a/c is going low on the profile and the pilot doesn't want to reduce the IAS/Mach, but it would work the same in a climb.
if the AFS is in altitude-hold, VS, (or FPA) mode, the pilot would of course be controlling IAS/Mach with thrust. And, if the A/THR was restored, it would use thrust to achieve whatever IAS/Mach the pilot calls for.
Returning to topic, you say:
"It's good for some to realize that you do not have to increase speed/AoA to climb or descend."
I would argue that the only way to initiate an increase in the FPA is to increase total lift. We are considering a case where the AoA is already at the maximum permitted. In a steady atmosphere - without briefly increasing the IAS - total lift can only be increased by increasing the vertical component of thrust, and/or reducing the HS downforce.
Re the vertical component of thrust, the high attitude obviously provides some effect in this case.
Re reducing the elevator downforce, the pitch couple with increasing thrust on an aeroplane with under-slung engines means that - bearing in mind that the A320 is not a canard - the HS downforce has to be reduced to counteract the pitch-couple. So the elevators move down accordingly, which - as I pointed out yesterday - can be seen on the Habsheim DFDR.
Does that make sense?
If you were asking about ops in the normal range of AoAs, the answer is yes on most current transports.
With the A/THR off, but the AFS in speed mode, the AFS controls speed with elevator. So, if (s)he wants to do it that way, the pilot can adjust VS by changing the thrust manually. The most common scenario might be in a descent at idle thrust, if the a/c is going low on the profile and the pilot doesn't want to reduce the IAS/Mach, but it would work the same in a climb.
if the AFS is in altitude-hold, VS, (or FPA) mode, the pilot would of course be controlling IAS/Mach with thrust. And, if the A/THR was restored, it would use thrust to achieve whatever IAS/Mach the pilot calls for.
Returning to topic, you say:
"It's good for some to realize that you do not have to increase speed/AoA to climb or descend."
I would argue that the only way to initiate an increase in the FPA is to increase total lift. We are considering a case where the AoA is already at the maximum permitted. In a steady atmosphere - without briefly increasing the IAS - total lift can only be increased by increasing the vertical component of thrust, and/or reducing the HS downforce.
Re the vertical component of thrust, the high attitude obviously provides some effect in this case.
Re reducing the elevator downforce, the pitch couple with increasing thrust on an aeroplane with under-slung engines means that - bearing in mind that the A320 is not a canard
- the HS downforce has to be reduced to counteract the pitch-couple. So the elevators move down accordingly, which - as I pointed out yesterday - can be seen on the Habsheim DFDR.Does that make sense?
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Originally Posted by HN39
When flying at alphamax, thrust controls altitude through speed
The corresponding speed to that fixed alpha max value is called Valphamax and remains the same as well, be it in level flight descent or climb ... The amount of thrust will decide the V/S but the speed will remain at Valphamax.
Thread Starter
Quote from CONF_iture:
"[...] Alpha max is an AoA that the FCS maintains at the fixed value Airbus decided to adopt depending on the configuration.
The corresponding speed to that fixed alpha max value is called Valphamax and remains the same as well, be it in level flight descent or climb ... The amount of thrust will decide the V/S but the speed will remain at Valphamax."
The process of changing the FPA requires a force. Can you go on and explain how that force is produced?
"[...] Alpha max is an AoA that the FCS maintains at the fixed value Airbus decided to adopt depending on the configuration.
The corresponding speed to that fixed alpha max value is called Valphamax and remains the same as well, be it in level flight descent or climb ... The amount of thrust will decide the V/S but the speed will remain at Valphamax."
The process of changing the FPA requires a force. Can you go on and explain how that force is produced?
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Originally Posted by CONF iture
Alpha max is an AoA that the FCS maintains at the fixed value Airbus decided to adopt depending on the configuration.
The corresponding speed to that fixed alpha max value is called Valphamax and remains the same as well, be it in level flight descent or climb ...
The amount of thrust will decide the V/S but the speed will remain at Valphamax.
Last edited by HazelNuts39; 19th Mar 2014 at 18:59. Reason: clarification of load factor reduction for FPA
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CONF is correct in stating that Alpha Max "...is an AoA that the FCS maintains at the fixed value Airbus decided to adopt depending on the configuration", but before the value can be maintained, it must be *attained*, and, if I read things correctly, the rate of attainment of that value will clearly be affected by other aspects, for example the phugoid damping if already in High AoA Protection mode.
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OK465,
I don't disagree with what you are writing, but it does not change the physical law that increasing the V/S requires a vertical force that exceeds the weight of the airplane.
Secondly, the airspeed will increase or decrease at any instant during the transition at which the thrust is greater or less than that required to sustain the FPA at that instant. Therefore the FPA has to increase faster than the thrust to avoid any speed increase during the transition.
I don't disagree with what you are writing, but it does not change the physical law that increasing the V/S requires a vertical force that exceeds the weight of the airplane.
Secondly, the airspeed will increase or decrease at any instant during the transition at which the thrust is greater or less than that required to sustain the FPA at that instant. Therefore the FPA has to increase faster than the thrust to avoid any speed increase during the transition.
I do not disagree that lift must change for a flight path change.
The point is that the change in lift does not have to be due to even a tenth of a gee or a knot or two. And it only has to change for a few seconds to have an effect. Then the plane tries to resume trimmed condition with thrust equal to drag and so forth.
I fully understand the "point the nose and use power for speed" technique. I also understand the "Navy" technique where AoA is very important to get the plane on the boat. I personally used a blend of the techniques depending on the plane I was flying. The low aspect ratio wings and high drag suckers were easier to use the throttle for descent or climb changes. The cleaner ones with high aspect ratio wings and low induced drag were more like an Aeronica or Cessna.
Still a good, if not great discussion.
The point is that the change in lift does not have to be due to even a tenth of a gee or a knot or two. And it only has to change for a few seconds to have an effect. Then the plane tries to resume trimmed condition with thrust equal to drag and so forth.
I fully understand the "point the nose and use power for speed" technique. I also understand the "Navy" technique where AoA is very important to get the plane on the boat. I personally used a blend of the techniques depending on the plane I was flying. The low aspect ratio wings and high drag suckers were easier to use the throttle for descent or climb changes. The cleaner ones with high aspect ratio wings and low induced drag were more like an Aeronica or Cessna.
Still a good, if not great discussion.
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Originally Posted by gums
The point is that the change in lift does not have to be due to even a tenth of a gee or a knot or two. And it only has to change for a few seconds to have an effect.
To achieve a FPA of 10 degrees at 115 kTAS takes roughly:
10 seconds at one tenth of a gee
5 seconds at two tenths of a gee
3 seconds at three tenths of a gee
Last edited by HazelNuts39; 21st Mar 2014 at 12:38.
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gums didn't fly in wind tunnels, nor in perfect gas, nor in a totally rigid plane, always in complex transient combinations.
Perhaps other physical models than official models do the plane fly . ie viscoplastic model is never used, where the chains of molecules brake partially, or scratched molecules like Velcro patches, or ....!
Any math model is worthful only if it is confirmed by accurate experience. gums is speaking from 1/10 of gee ,and probably less, and one knot difference, a part of a second. We still have much to learn from our best military pilots ...
Perhaps other physical models than official models do the plane fly . ie viscoplastic model is never used, where the chains of molecules brake partially, or scratched molecules like Velcro patches, or ....!
Any math model is worthful only if it is confirmed by accurate experience. gums is speaking from 1/10 of gee ,and probably less, and one knot difference, a part of a second. We still have much to learn from our best military pilots ...
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Originally Posted by Chris Scott
The process of changing the FPA requires a force. Can you go on and explain how that force is produced?
The initial comment that started our discussion :
"Although Airbus test pilots routinely made high AOA demonstrations with alphafloor switched off they invariably compensated for that by selecting a level of thrust commensurate with maintaining speed and altitude."
had to be corrected as the FCS is dealing with speed through AoA and the pilot is dealing with altitude through thrust.
The later comment :
"They know that to make a correct demonstration of alphamax it is necessary to have enough thrust not only to maintain altitude but also to maintain speed. If that is not so the EFCS will apply a nose down corrective pitch command that will prevent achievement of alphamax."
had also to be corrected as the level of thrust is strictly to maintain altitude and not speed. You could be idle thrust and still maintain alpha max at Valphamax.