A320 Sidestick Forward
 

Why do we do it on the take-off roll?


There is no prop wash, so it has very little aerodynamic effect in holding the nose-wheel firmly on the ground until just about when we remove it at 80-100kts.


With a tailwind, if anything it has a negative effect for the first part of the acceleration yet we double the input. Why?

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.

5LY


For the same reason that the rudder is ineffective, the elevator is ineffective.


There is initially very little airflow or even worse reverse airflow with a tailwind, and the rudder would be becoming usefully effective at about the same time as the elevator.


In the tailwind case, if I actually believed that the stab has any effect at low speed in the initial accel, which I don't, it would be the opposite of what is wanted! Until gnd speed at least equals tailwind, the negligible lift it is producing is working against holding the nosewheel down.


That's why I asked the question. Both rudder and elevator/horizontal stab require positive and significant airflow to have any effect. Try taxiing at 30kts and pull full back. Does the nose rise majestically??
I think not.


I will possibly accept that something usefully aerodynamic is starting to happen IAS >50kts, but that is also the case with a tailwind, obviously, so why the difference in procedures.


Is this left over from old prop-wash aircraft?!


So why do we do it?

Tourist...

Next time you see an Airbus depart at the holding point, have a look at the horizontal stab. Amazing the amount that it shakes when the engines wind up... I'm guessing it's not the wind doing that...

Loads of jets have the same effect of shaking the horizontal Stab early in the take off roll, perhaps the down elevators will catch some of the rising hot air from the engine efflux to provide some up force to stop the shake a bit?


Mind you, it could all just be some old fashioned airmanship that escaped the translation into French and back?

Hi Tourist,
By increasing the load on the nose wheel, you improve the nose steering capability with rudder bar deflection. Initially up to say 50kts, your ground speed is low and there is time to react before you run off the side of the runway after an engine failure. As you approach VMCG, the rudders will have sufficient control hence relax on the nose down bit.
With a tailwind, you'll need more even more elevator to provide the same nose wheel steering for the same ground speed.

Rudderrat

You are not reading what I have written.

I would fully understand the procedure if in fact it had the effect of increasing pressure on the nose wheel.

I don't think it does for the reasons I have stated twice.

Particularly in the case of a tailwind, where if there is any effect of stick down it would be to release pressure on the nose wheel for the critical early stage of the acceleration. Think about what reverse flow over a wing does! If you push forward you are increasing the downward force it is producing until flow over the tail is in the correct sense.

Fortunately, I don't think this is a problem because the forces created over a wing at low speed are negligible hence relying on nose wheel steering until some decent air flow is produced by forward acceleration.

Cough

That would be a good justification if true. Do you reckon that there is actually some "blown" effect that high up?

Tourist.

Simplistic way of showing it, but check this page. How does a piece of taxyway behind the engines lift to the level of the stab?

Tourist, good question. I've often thought the same myself. However looking at the following:
http://i1119.photobucket.com/albums/...Untitled-1.jpg

It seems the stab is very much under the influence of the engine airflow.

Hmm.


That pic only shows plan form and doesn't really show either way.


Cough, yes that does seem to suggest that there is some impingement, though flat items of pavement can behave oddly in airflow.




The tail-plane really is quite high, and the flaps work to direct the wash
downwards.



Ok, for the sake of argument, lets accept that the tailplane has some effect.


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.

Did anybody see a video with brave/stupid plane watchers blown away while trying to hold themself at the fence behind A320 (or may be 757) applying TOGA on take off at Princess Juliana?:eek:

Taken from the A320/21 FCTM:

On a normal takeoff, to counteract the pitch up moment during thrust application, the PF should apply half forward (full forward in cross wind case) sidestick at the start of the takeoff roll until reaching 80 kt. At this point, the input should be gradually reduced to be zero by 100 kt.

So I imagine Airbus have a different opinion on aerodynamic effects than you do.

Well done Jonty, you can read a book.

I didn't ask what to do, I asked why.

We know that in normal flight there is negligible engine "blown" effect over the tail plane, otherwise stall recovery would not be such a concern with thrust induced pitch up having not enough elevator control to counteract it.

Tourist
I also always wondered about this. I guess the elevator is down and when the blast goes under them it causes some airflow to create pressure on them from underneath. It doesn't need air to go over the elevators.

Ahh yes, the ever present misunderstanding of how an aircraft flies, rears its ugly head again.

Should you actually believe that the airfoil generates lift by the difference in pressures, ie Bernoullian physics, like this, then read no further
http://assets.openstudy.com/updates/...rplanewing.jpg

IF you believe in Newtonian physics, then you have a chance to understand why Airbus is much further ahead in airfoil design.

So, as the ac moves down the runway, the pressure wave underneath the wing builds as speed increases. Given the angle of the wing, the pressure wave will begin to build in front of the wing, causing the nose to lift, at the same time, the compressed air from the trailing edge, will be forced out at a rapid rate, causing cavitation which will tend to bring the tail down (a downwash suction).

While this illustration is closer, is does touch on the building of the pressure wave in front of the airfoil..think of that effect on takeoff.

http://www.clearwater.org/wp-content...11/AIRFOIL.gif

Note the high pressure forward of the airfoil! (ps, high pressure means compressed air, ie, the air had to come from somewhere, hence the 'low pressure' zone above, but still missing a bunch of components)


As you can sortof visualize, the pressure wave building in front of the airfoil has a rotational effect. This rotation effect exceeds the lift component of the ac to lift from the ground, ie the ac will rotate the nose up, before it can lift it from the ground.

That is why you hold the nose down, it is beacuse of the function of a much more effective lift of the wings, and especially the centerwing. 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.

Much the same effect on landing, which is why the A380 crosses the threshold at 120kts. (if you do decide to stand under the flightpath, (or measure the winds), you will find that you get blown back, not from the engines, but from the compressed pressure wave, for lack of a better analogy, compressing the air in front of the wing, and shooting it out the back as it moves forward, or a funneling effect)
I am probably not expaining the mechanics very well, but hopefullly, one can visualize what I am trying to illustrate.

EDIT: interesting, that 'ground effects' are understood for flare on landing, but not on takeoff...

Hi Tourist,
How big a tail wind do you accept on take off?
We are limited to only 10 kts - and the effect on the nose wheel from stationary to 10 kts ground speed is not significant.
You're not reading mine either.

underfire.


Yes, I understand Lift. From your post I have my doubts that you do....


In fact I have taught the subject to aviators.


"That is why you hold the nose down, it is beacuse of the function of a much more effective lift of the wings, and especially the centerwing. 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."


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 thus we have to hold the nose down to stop the nose rotating up until the tail-plane can take its share of the lifting load?


If so, then you need to do a bit more study about which way the tail-plane lift is acting.

Two thoughts, Tourist...
1. Jet efflux passes under the THS at more than 60kts.... this will entrain more air all over the THS at a speed high enough for the deflected elevator to produce an upward force.
2.Without the deflection a mis-set or mis-calculated THS might produce an undesirable downward force.
3. If FCOM said "from 50 to 100 knots apply half forward side-stick" it might need an extra call when PM should be concentrating elsewhere, and plenty of us would forget! 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...

Tyro


I think that is the conclusion I have been coming to, ie simplicity of operation rather than any real effect in the early stages.
Makes sense I suppose.

Whereas I am convinced that it does have an effect at low speeds, as well as being simpler.

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.

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.

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".

Vmcg is typically 105-109 knots (A320 CFM).
Airbus require the side-stick to be neutral in pitch by 100 kts.
So no effect on Vmcg.....

no, I am saying that the pressure wave in front of the wing is. This explains how the aircraft reacts in ground effect vs not.

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.

If the extra down load on the nose wheel, produced by the tail, is proportional to the airspeed squared, then at around 40 kts in still air (or 30 kts with a 10 kt tail wind) the tail loads are 16 units versus 9 units. Therefore you'd need almost double the amount of down elevator for the same effect in the tailwind case.
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.

Tourist,

Time to evolve my friend.

Not to worry, you are in good company, after all, 737-900 still have vortex tabs along the top of the wings.

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.

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 . . .

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?

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.

???? Surely the engine thrust line couple is below the C of G which will produce a nose up torque.
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".

It can take very little to make an A320 sit on it's tail. The application of power could be (has been?) the last straw.

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.

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.

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.

And a little into wind too . . ?

Displace the white cross into wind by up to half its width.

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 ?

“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?