A320 Sidestick Forward
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Fair enough.
The reason I am asking the question is because when I was going through conversion to type, I forgot a couple of times and it appeared to make no difference to the handling at the lower speeds.
The reason I am asking the question is because when I was going through conversion to type, I forgot a couple of times and it appeared to make no difference to the handling at the lower speeds.
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Tourist:
I will caveat this reply by saying that I am not privy to any inside knowledge on the A320, however I am approaching this from a large-aircraft performance flight testing background.
In short, application of nose-down elevator input will permit an increase in directional nosewheel effectiveness, and a reduction in Vmcg, which in turn brings about beneficial runway performance figures.
Considering Vmcg in isolation, FAR 25.149 defines it as;
VMCG, the minimum control speed on the ground, is the calibrated airspeed during the take-off run, at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the aeroplane with the use of the primary aerodynamic controls alone (without the use of nose-wheel steering) to enable the take off to be safely continued using normal piloting skill.
Now, as you can see, you’re not allowed to use the nosewheel steering, however positive nosewheel contact provides additional stability (the straight nosewheel will resist yaw related sideloads). The amount of sideways force the nosewheel can react against is related to the friction it can generate, which simplistically will be the perpendicular force the tyre contacts the runway with, multiplied by the coefficienct of friction (F = mu*R). The more force pressing the tyre against the tarmac, the more sideload it can achieve before skidding, the more it can help you keep going straight down the runway.
Ideally for performance reasons you want your Vmcg figure to be as low as possible to minimize required runway length (assuming you’re not otherwise limited in some other regard). Considering the aircraft at high-speed on the runway with a take-off stab. trim setting, typically the nosewheel would ordinarily be light on the ground at speeds near Vr. This leads to a good natural take-off rotation, however you lose the benefit of the nosewheel’s ability to counteract any yaw force, and so your Vmcg speed would have to be declared at a much higher speed where a mix of rudder and residual nosewheel force can balance thrust asymmetry. It should also be noted that for most large aircraft, Vmcg is a real cliff-edge speed. One or two knots can make the difference between spearing off the runway edge to maintaining control of the aircraft.
But hang on…why not just drive the nosehweel into the ground to make up for it?
Well this is exactly what Airbus advocate in this instance. The next step is to then approach the certification authorities with this procedure to ask permission. In considering an operational procedure to supplement the aircraft’s performance, one overriding aspect in assessing the permissibility of such a proposal is the crew workload.
If you came up with a procedure where the crew start the take-off roll with neutral pitch input, and then at >60 KIAS applied nose down, and then washed it off again by rotate – I can imagine there would be concerns with workload. For a light aircraft using no flex or derates, that would make for a lot of handwaving in an otherwise busy phase of flight. So the answer is to simplify the technique – have our man at the controls hold pitch down from brakes release, and slowly wash it off by the time you get to rotate. It has absolutely no benefit at lower speeds, however in the crucial cliff-edge region where Vmcg occurs, it absolutely makes a difference.
So on the balance of it – the ‘cost’ of holding a nose-down input where it makes no difference, versus the simplification in workload and crucial enhancement of runway directional control – it’s an easy call.
I will caveat this reply by saying that I am not privy to any inside knowledge on the A320, however I am approaching this from a large-aircraft performance flight testing background.
In short, application of nose-down elevator input will permit an increase in directional nosewheel effectiveness, and a reduction in Vmcg, which in turn brings about beneficial runway performance figures.
Considering Vmcg in isolation, FAR 25.149 defines it as;
VMCG, the minimum control speed on the ground, is the calibrated airspeed during the take-off run, at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the aeroplane with the use of the primary aerodynamic controls alone (without the use of nose-wheel steering) to enable the take off to be safely continued using normal piloting skill.
Now, as you can see, you’re not allowed to use the nosewheel steering, however positive nosewheel contact provides additional stability (the straight nosewheel will resist yaw related sideloads). The amount of sideways force the nosewheel can react against is related to the friction it can generate, which simplistically will be the perpendicular force the tyre contacts the runway with, multiplied by the coefficienct of friction (F = mu*R). The more force pressing the tyre against the tarmac, the more sideload it can achieve before skidding, the more it can help you keep going straight down the runway.
Ideally for performance reasons you want your Vmcg figure to be as low as possible to minimize required runway length (assuming you’re not otherwise limited in some other regard). Considering the aircraft at high-speed on the runway with a take-off stab. trim setting, typically the nosewheel would ordinarily be light on the ground at speeds near Vr. This leads to a good natural take-off rotation, however you lose the benefit of the nosewheel’s ability to counteract any yaw force, and so your Vmcg speed would have to be declared at a much higher speed where a mix of rudder and residual nosewheel force can balance thrust asymmetry. It should also be noted that for most large aircraft, Vmcg is a real cliff-edge speed. One or two knots can make the difference between spearing off the runway edge to maintaining control of the aircraft.
But hang on…why not just drive the nosehweel into the ground to make up for it?
Well this is exactly what Airbus advocate in this instance. The next step is to then approach the certification authorities with this procedure to ask permission. In considering an operational procedure to supplement the aircraft’s performance, one overriding aspect in assessing the permissibility of such a proposal is the crew workload.
If you came up with a procedure where the crew start the take-off roll with neutral pitch input, and then at >60 KIAS applied nose down, and then washed it off again by rotate – I can imagine there would be concerns with workload. For a light aircraft using no flex or derates, that would make for a lot of handwaving in an otherwise busy phase of flight. So the answer is to simplify the technique – have our man at the controls hold pitch down from brakes release, and slowly wash it off by the time you get to rotate. It has absolutely no benefit at lower speeds, however in the crucial cliff-edge region where Vmcg occurs, it absolutely makes a difference.
So on the balance of it – the ‘cost’ of holding a nose-down input where it makes no difference, versus the simplification in workload and crucial enhancement of runway directional control – it’s an easy call.
Last edited by Another St Ivian; 8th Jan 2014 at 17:00.
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FYI Tourist, rudderrudderrat is currently a TRE on the A320 series, with an exceptional amount of experience on many types before he became one.
As others have alluded to, loading in the pitch axis has many more aspects to it than just aerofoil behaviour - non-pilot though I am, I expect the ethos to be "every little helps".
As others have alluded to, loading in the pitch axis has many more aspects to it than just aerofoil behaviour - non-pilot though I am, I expect the ethos to be "every little helps".
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Are you trying to suggest that we are trying to hold the nose down because the main wings generate lift sooner than the tail-plane
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Another St Ivian.
Thanks for that, that does sound reasonable. The trickiest bit of a high power engine failure is at slow speed, and the nose down force from stick forward is the best you are going to get, albeit that it only comes into real effect as the speed increases, and if you are going to push the stick forward, why not do it before you start moving to save adding it to the list of stuff before rotate.
Dozy.
I'm not interested in peoples cv, just an answer to my question.
Who says my cv isn't equally or more impressive? Then again I may be playing on MS Sim. (or whatever its called!) That's the joy of faceless forums...
underfire.
I know you think you understand the diagrams you put up, but trust me, you don't.
"Steering feels light way before v1, well, it may not be lift. it is the bird trying to push away from the ground, just in a different way."
"causing cavitation which will tend to bring the tail down (a downwash suction). "
Oh Dear.
Thanks for that, that does sound reasonable. The trickiest bit of a high power engine failure is at slow speed, and the nose down force from stick forward is the best you are going to get, albeit that it only comes into real effect as the speed increases, and if you are going to push the stick forward, why not do it before you start moving to save adding it to the list of stuff before rotate.
Dozy.
I'm not interested in peoples cv, just an answer to my question.
Who says my cv isn't equally or more impressive? Then again I may be playing on MS Sim. (or whatever its called!) That's the joy of faceless forums...
underfire.
I know you think you understand the diagrams you put up, but trust me, you don't.
"Steering feels light way before v1, well, it may not be lift. it is the bird trying to push away from the ground, just in a different way."
"causing cavitation which will tend to bring the tail down (a downwash suction). "
Oh Dear.
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Originally Posted by Tourist
If that is the case, why extra nose down for tailwind?
You only have tailwind until you hit 10kts GS, after that it is just a normal take-off as if you started at nil wind. The apparent wind vector very quickly moves forward.
If that is the case, why extra nose down for tailwind?
You only have tailwind until you hit 10kts GS, after that it is just a normal take-off as if you started at nil wind. The apparent wind vector very quickly moves forward.
Obviously the difference diminishes with increasing airspeed (64 units v 49 units at 80 kts in still air (i.e. 70 kts with 10 kt tail)).
It is the extra nose tyre adhesion which helps keep the aircraft straight with a crosswind and initially when an engine failure is recognised. If one method is approved for still air and all head winds, then it is prudent to modify the technique for tailwind take offs.
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Another St. Ivian,
The reason for establishing Vmcg with aerodynamic controls alone (without using nosewheel steering) is that Vmcg needs to cover wet and slippery runway condiions. Nosewheel steering is usually disconnected during Vmcg testing allowing the nosewheel to castor so that it cannot produce a sideforce.
The reason for establishing Vmcg with aerodynamic controls alone (without using nosewheel steering) is that Vmcg needs to cover wet and slippery runway condiions. Nosewheel steering is usually disconnected during Vmcg testing allowing the nosewheel to castor so that it cannot produce a sideforce.
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Tourist
You see this . . ?
"To give you positive yaw control on the ground until your rudder is effective and to stop the nose lifting prematurely with thrust on a tail heavy a/c"
Well, Touristy, that is why.
Plus all the `adhesion` posts + Another St. Ivian - these are all why.
I saw a video where they did not actually do this and I could not figure out why . . ?
You see when you have take off thrust? The nose wants to come up.
It will do this at a very slow speed. It will even do this with reversxers during the landing run, oh yes.
So, in order to stop this happening, we keep half down scale until 100kts then bring the Maltese cross thing, sidestick indicator to the artificial horizon at 100kts.
The affect of Elevator at this speed is . . .effective enough to keep the nose on the runway, whereas before it was not, but the thrust from the very powerful engines in the comparatively very light aircraft will get the nose up at very low speeds.
When we get to V-rotate speed it is just about flying anyway and the nose wheel does not want to be on the runway any more so blissfully we rotate smoothly (about 2 degrees a second ) so the nose wheel rises (under control of the elevator v the thrust) into the air and the aircraft lifts into the sky and flies off to Greece or Spain or Italy, Palma, Geneva, Turkey, Miami . . .
"To give you positive yaw control on the ground until your rudder is effective and to stop the nose lifting prematurely with thrust on a tail heavy a/c"
Well, Touristy, that is why.
Plus all the `adhesion` posts + Another St. Ivian - these are all why.
I saw a video where they did not actually do this and I could not figure out why . . ?
You see when you have take off thrust? The nose wants to come up.
It will do this at a very slow speed. It will even do this with reversxers during the landing run, oh yes.
So, in order to stop this happening, we keep half down scale until 100kts then bring the Maltese cross thing, sidestick indicator to the artificial horizon at 100kts.
The affect of Elevator at this speed is . . .effective enough to keep the nose on the runway, whereas before it was not, but the thrust from the very powerful engines in the comparatively very light aircraft will get the nose up at very low speeds.
When we get to V-rotate speed it is just about flying anyway and the nose wheel does not want to be on the runway any more so blissfully we rotate smoothly (about 2 degrees a second ) so the nose wheel rises (under control of the elevator v the thrust) into the air and the aircraft lifts into the sky and flies off to Greece or Spain or Italy, Palma, Geneva, Turkey, Miami . . .
Last edited by Natstrackalpha; 10th Jan 2014 at 20:32.
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Something I learned when drawing cross sections of the AI family THS is that it has a slight "negative" camber. How about factoring that into the discussion?
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When flying along, more power in an A320 should certainly push the nose up; however, on the ground the aircraft pivots not around somewhere in the middle above the wing box, but on the wheel-runway contact point; hence, more thrust should act to plant the nose down more firmly (* WRONG). As speed increases, and the higher-located drag builds, with the same thrust power, it should unload the nosewheel. * WRONG - it's just like a dragster.
Nevertheless, stick forward should keep most force on the nose wheel, giving the best ability to stay straight whatever happens.
On landing, more reverse thrust, everything else being equal, should indeed move to raise the nose; if it doesn't, then there's no backwards force from the engine, and it's not doing its job. * WRONG - it's just like a braking car.
* EDIT: Everything else isn't equal, as it's accelerating or decelerating.
Friday night shortcuts. Keeping the nose wheel planted firmly as the reaction from that wheel reduces under acceleration is important.
Nevertheless, stick forward should keep most force on the nose wheel, giving the best ability to stay straight whatever happens.
On landing, more reverse thrust, everything else being equal, should indeed move to raise the nose; if it doesn't, then there's no backwards force from the engine, and it's not doing its job. * WRONG - it's just like a braking car.
* EDIT: Everything else isn't equal, as it's accelerating or decelerating.
Friday night shortcuts. Keeping the nose wheel planted firmly as the reaction from that wheel reduces under acceleration is important.
Last edited by awblain; 11th Jan 2014 at 18:01. Reason: It was not correct
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Originally posted by awblain... however, on the ground the aircraft pivots not around somewhere in the middle above the wing box, but on the wheel-runway contact point; hence, more thrust should act to plant the nose down more firmly.
During braking, the C of G is above the drag line of the wheel contact point, so then there is a nose down couple to "plant the nose down more firmly".
Last edited by Goldenrivett; 11th Jan 2014 at 11:49.
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Goldenrivett, you are correct
Apologies for that Friday night-posted nonsense.
I was fixated on the main gear, with the aircraft almost being balanced - too late on a Friday night. If you have the brakes on and don't slide, then the nose wheel should plant more firmly, but without the brakes, it's not a static situation, and just like a car, acceleration in an A320 on the ground does indeed unload the front, while deceleration unloads the back.
I've tried to edit and fix the early post.
I was fixated on the main gear, with the aircraft almost being balanced - too late on a Friday night. If you have the brakes on and don't slide, then the nose wheel should plant more firmly, but without the brakes, it's not a static situation, and just like a car, acceleration in an A320 on the ground does indeed unload the front, while deceleration unloads the back.
I've tried to edit and fix the early post.
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Tourist im surprised that 50% of the people here can't understand your question. Talk about frustrating!
I understand completely what you are asking.
At application of Flex / TOGA Time =0s, even T=2s there is bugger all airflow over the THS.
You can go full up on the elevator and it wouldn't do anything.
In short the FCTM should say go elevator down at ~40/50kts. However us Brain Surgeon Pilots might forget. So we do it at the beginning .
Simples.
I understand completely what you are asking.
At application of Flex / TOGA Time =0s, even T=2s there is bugger all airflow over the THS.
You can go full up on the elevator and it wouldn't do anything.
In short the FCTM should say go elevator down at ~40/50kts. However us Brain Surgeon Pilots might forget. So we do it at the beginning .
Simples.
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betpump5
At the instant you move the T/L, almost true. Idle thrust will still produce some airflow over the THS. But with TO thrust there will be a lot of airflow .. See posts #9 and 18.
At application of Flex / TOGA Time =0s, even T=2s there is bugger all airflow over the THS.
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Simpler to do it from the start of T/O roll, which also avoids a change in rudder pedal deflection as PF applies full forward in a crosswind
Displace the white cross into wind by up to half its width.
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On this subject, why do we get full elevator deflection at half forward sidestick?
and why do we use full forward sidestick on a cross or tailwind takeoff, if we already get full down elevator at Half forward ?
and why do we use full forward sidestick on a cross or tailwind takeoff, if we already get full down elevator at Half forward ?
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“why do we get full elevator deflection at half forward sidestick?
and why do we use full forward sidestick on a cross or tailwind takeoff, if we already get full down elevator at Half forward ?“
Hi MD83FO, From FCOM Pitch control, ground mode:“Ground mode is a direct relationship between sidestick deflection and elevator deflection. ……
During the take off roll, the system may reduce the maximum up elevator deflection from 30 ° to 20 °, depending on weight and CG.“
Why do you think you get full elevator with half side stick?
and why do we use full forward sidestick on a cross or tailwind takeoff, if we already get full down elevator at Half forward ?“
Hi MD83FO, From FCOM Pitch control, ground mode:“Ground mode is a direct relationship between sidestick deflection and elevator deflection. ……
During the take off roll, the system may reduce the maximum up elevator deflection from 30 ° to 20 °, depending on weight and CG.“
Why do you think you get full elevator with half side stick?