B737 35 knot Crosswind take off. Which wing drops on lift off?
 

Simulator conundrum. B737-Classic. Runway 36. Wind 270 at 35 knots (steady 90 degree crosswind component).
Considerable right rudder needed initially to track the centre-line but as speed approaches VR of 130 knots, less rudder pressure needed. As the main wheels leave the runway, I would expect the aircraft would weather-cock into the wind and this manifests itself by the left wing dropping slightly which is easily countered by appropriate aileron (plus minor spoiler deflection depending on amount of control wheel offset). In other words the momentary lowering of the left wing conveniently counters the drift angle and the aircraft tracks the extended centre-line albeit with a bit of wing wobbling until you settle on the correct amount of drift angle.

At least that is what I thought should happen and in fact I am sure I have seen this many times.

But what if it is the right wing that goes down at lift off, and a considerable change of heading into wind is needed to not only get back on to the extended centre-line but to accurately track the centre-line once drift angle is established?

Trouble is I am now momentarily confused as to which wing (left or right or if you prefer, upwind or downwind wing) one could expect to momentarily drop at the point of lift off? Could this right wing drop be a simulator fidelity defect? Or would you rightly expect the right wing to drop slightly and that anything else is simply finger trouble by the pilot? :hmm:

Is this something you have seen in the sim? I would not expect either wing to 'drop' at the moment of lift-off providing appropriate aileron had been applied, but to commence thereafter with the yaw. The natural 'yaw' into wind at lift-off should produce a corresponding roll to the left (ie left wing 'drop'), both of which are 'good'. Any right-wing drop I suspect could possibly be the result of too much right rudder at lift-off causing yaw to the right as the wheels unstick? I assume the 'necessary' left aileron had also been applied whenever needed? Insufficient into-wind aileron, if not corrected as the wing rises, would cause the right wing to 'drop'.

Just remember that 'seat of the pants' is absent in the sim so inappropriate reactions could occur, and I was also very conscious of an evident 'yaw' with roll in all the 737 sims I flew, which was an 'unfaithfull' representation of the real.

Well Sir,
I have different experiences. I have noticed the plane would like to roll to the right in this case, as the wind hits the vertical stabilizers and "pushes" it. This will result in a left wing up momentum, wich has to be counteracted by aileron input.

Of course, with everything done right, there should be no wing dropping at all when leaving the ground.

Nic

Admiral - in steady conditions this fin rolling moment is there from brakes off, so you should have countered that with aileron already? Quite how you are able to distinguish between upwind wing lifting and fin rolling I do not know!

On the 747, you can say the required aileron input on rotation is twice what you had during the run. If while rotating you nicely increase aileron as well, no wing will drop. On the 737 it must be the same, only maybe a bit less pronounced.. And don't be afraid of the spoilers. I hung once out the window of a 737 while parked, and they came already up at 2.5 units. So you already quickly have the spoilers. You need that amount of aileron, so please disregard the spoiler deflection.

'As the main wheels leave the runway, I would expect the aircraft would weather-cock into the wind'

I was always told that the tendency to weathercock is principally confined to ground movements. Therefore as weight is transferred from wheels to wings the amount of rudder deflection required to counteract will decline.

If you can track the centerline until liftoff with a certain amount rudder input, should any of it be removed in the rotation? My question is how can you have too much rudder input because if you had would you not loose the centerline before liftoff?

IR

There are numerous threads on PPR about x-wind take-offs. Essentially you get airborne with 'crossed controls' - in this example, right rudder and left aileron. These controls are restored to neutral in a 'cordinated' smooth fashion at and after rotate. At lift-off the a/c starts to drift downwind, being no longer constrained by the runway surface, BUT it naturally 'weathercocks' into the relative 'wind' (slightly left of the nose) by yawing to the left and 'fine tuning' then takes place to track the centreline with no yaw or roll present. That's how it should be done - then, of course, there's my thrashing about of the controls:)

I have found with for example a crosswind from the left, with about 4 units on aileron into wind and tracking the centerline nicely once I get airborne that the left wing picks up. I am always asking myself is it too little aileron or too much rudder??

IR

In this scenario start with a bit of aileron (left wing down) and as you rotate increase the input in order to keep wings level. You will need quite a bit of left wing down aileron at 35kts across.

The OP has it wrong. The upwind wing will lift, or right wing drop. It's an indication that not enough into-wind aileron was applied at the moment of lift off. You find there is not a great requirement for rudder at lift off speeds- it's aileron that is far more critical, and the vast majority of pilots never apply enough. I put on much more than most and find it far better. The ones that apply hardly any soon start learning why! I once did a 747 take-off with a copilot who applied none in a 25Kt+ crosswind from the left. I thought it would be the only way to get him to learn. The resulting right wing drop was really scary.

I am now operating 737 in a very windy place. I'd almost say you can't apply too much aileron. Put loads on and leave it there!

I don't find that to be the best way of doing it.
I have a feeling you are a lot more experienced than me Rainbow so I am ready to learn something if need be. I find that the best way is to just put in a moderate amount on the roll and keep the nose straight with a a bit of rudder.....basically feels like you're sharing the job between the two....and as you rotate roll in that extra aileron that you know for sure is going to be needed to keep the wings level.
I agree that it is a lot of aileron during the rotation, I have heard people say that they double the cross wind and that will be the yoke angle ie 30kts then 60degrees of yoke angle.

Farmer: In this scenario start with a bit of aileron (left wing down) and as you rotate increase the input in order to keep wings level. You will need quite a bit of left wing down aileron at 35kts across.

correct me if I am wrong here but shouldn't it be the other way around.

I mean at very low airspeed the aileron applied should be more and gradually reduced as you accelerate down the runway - ie as the effectiveness of the control surface increases.

I do agree with you about the comment on allowing both rudder and aileron to share the work. I find that if the aircraft is fighting my rudder input.. ie doesn't want to stay on the centreline even with quite a lot of rudder ... it's usually that there is not enough aileron.

There really are some profound misconceptions about crosswind takeoffs in this thread! Understand the difficulty of keeping straight is unrelated to aileron. The crosswind is blowing the fin and weathercocking the aeroplane into wind. The only way to counter that is to hold the aeroplane on the centreline with rudder- aileron is ineffective at that. Consider the moment of lift off, the aeroplane's wheels leave the runway, and the aeroplane is then effectively in a yaw situation, with the wind blowing on one side and lifting the upwind wing, very powerfully. There is only one way to remain level. If you have applied not very much aileron, this is when you get a nasty surprise.

I won't repeat the previous stuff about how much aileron to apply. But if you want to keep life simple for yourself at a very busy difficult time, slap on a load of into-wind aileron at the start, and leave it there until after the wing drop. To assess how much you need, closely observe how much extra aileron you have to put on in a cross wind lift off. I have never heard or seen of too much in 38 years of jets, but you can so easily way underdo it.

Stand corrected....you are wrong.

It is such a dynamic event from zero knots on the ground to 170 airborne with all the factors/forces changing with every knot increase in speed and every gust of wind.
I can definately see your point there Rainbow. The way I look at it is that there is definately a prerfect amount of aileron for any snapshot in the roll. That amount will be just enough to stop the wing lifting and not so much as to create an extra "weight" on that side of the aircraft. If for example you take it to the extreme and used full left airleron in this scenario, then as the speed built and the aileron/spoiler had more effect, effectively extra weight would be felt on the left main gear/extra drag on left side of aircraft, and the a/c would want to turn slightly in that direction, the reaction to this, of the pilot, is to put in more right rudder to hold the center-line. So the pilot needed x units of aileron and x units of rudder but ended up using x+1 of both just to hold the centerline. A training captain I had once demonstrated an interesting technique to me aimed at getting the correct amounts of aileron and rudder and it results in a nice roll and lift-off. Basically he starts with fwd pressure and no aileron, as he rolls and needs rudder to keep it straight he feeds in aileron, just a little, if he needs more rudder he feeds in a little more airleron, baiscally like they are connected. You find that a requirement for rudder arrises, and as a little more aileron goes in the requirement for rudder reduces a fraction. It's a bit hard to explain here but in real life it seems to work very well. The result is that even in 30kts across you don't need a heck of a lot of either.....until rotation that is. I am pretty sure that a lot of guys I watch take-off are needing so much rudder because they have so much aileron.
I know I open myself up to getting shot down when putting opinions like that out there. Thats all good, might learn something :ok:

Farmer: Stand corrected....you are wrong.

It is such a dynamic event from zero knots on the ground to 170 airborne with all the factors/forces changing with every knot increase in speed and every gust of wind.

again.. I could be wrong with the previous statement about amount of aileron however I fail to see how your statement has "corrected" me.

I thought we were talking about a constant crosswind here..

rainboe: you are right . i was actually referring to the fact that the upwind wing could be jumping up and down along the runway (due not enought aileron) hence making the centerline keeping kind of wobbly.

My head is starting to ache here! Let´s deconstruct what you say. As the speed builds up, the upwind wing lifting effect gets stronger. The reason you are putting aileron on in the first place is to reduce lift on that wing to counter the upwind lifting effect. Who knows what extra drag or alleged wheel drag there is? You have no way of knowing, you just have to keep the thing balanced and straight into the air. Forget 'spoiler drag´, it´s a complete nonentity for spoilers slightly cracked open below 150kts. Hell, the things are hardly effective up to 250kts fully open.

You bang on aileron to stop the wing lifting (be generous- it´s never enough at liftoff), you put downwind rudder on to keep straight (you will find that requirement reduces as speed builds up and the rudder becomes more effective). There is no such thing as asymmetric thrust to keep straight. Keep it simple like that and forget about other complications. My qualification is about 15,000 hours of 737, 747, 757. They are all the same and behave just the same way. I am now flying in possibly the most notoriously heavy wind place in the world, with frequent 35 kt crosswinds. It works. Don´t come up with manufactured problems or theories that are not factors. Keep it simple requiring minimum brain effort and you have more capacity for deciding which crew meal you are going to have and which of the lovelies may strike lucky.

Something you may not have noticed in your flying, RB - and you need that rare down the runway KEF wind to try it - but you CAN steer with aileron on the ground, as Framer is saying. Try it one day (after your crew meal and before you 'strike lucky', of course). It was demonstrated to me years ago in Aunty Betty's flying club and it works.

Just to interrupt with something taken from my 738 FCTM:

"Large control wheel inputs can have an adverse effect on directional control near V1(MCG) due to the additional drag of the extended spoilers."

and

"Excessive control wheel displacement during rotation and liftoff increases spoiler deployment.... drag increases and lift is reduced.... results in reduced tail clearance..."

Yes, we know about that! Define 'large control wheel inputs'as seen by Boeing? Over 45 degrees? I would say something up to about 35 degrees maximum would not be considered 'large'. 45 degrees or more would be more than required up to limiting crosswind.Boeing make no attempt to define 'large'

I think it is one of those statements meant to absolve the authors of any blame in a subsequent accident enquiry, like in rear-view mirrors: 'caution, objects are closer than they appear in the mirror!' Boeing have the opportunity for a known crosswind to specify a maximum control wheel deflection- they haven't done anything apart from a wishy-washy statement that means little and is no real guide.