Downward turning effect during crosswind takeoff roll
 

Hello,

I have seen a post quite some time ago regarding this. I’m curious as to the aerodynamics. Boeing mention it in the FCTM. The effect is that during a crosswind takeoff initially the rudder is counter to that expected given the direction of the crosswind. For instance if there was a crosswind from the right then initially right rudder would be required before left rudder being required as usual at about 20 knots ground speed.
I’m not sure the previous thread got to the bottom of it. This counter intuitive rudder seems to be due to the airflow turning to be directed into the engine. I personally can’t quite see this so if anyone could explain then would be appreciated. Thank you.

In a crosswind from the right, IF the right engine has a bigger/better airflow, and IF this has an effect that is noticeable on thrust, then the right engine initially produces more thrust hence right rudder is required.
These are only "ifs", I never noticed or heard about such a thing before.

Is the directional static stability, on the ground, tending to rotate the aircraft around the mainwheel contact patch rather than the CG. The larger area ahead of the stability rotation point gets "blown" downwind.

On the four types I've flown (all Boeing and Airbus) I've never noticed having to apply anything other than left rudder in a right crosswind. Nothing in the very early stages, then appropriate rudder as the airspeed comes alive. Maybe it's more noticeable on aircraft types under 60 tons?

A handling characteristic noted in original HS125 AFM. With description - never personally noticed one any aircraft type.

I am not familiar with the term.

Might this be related ?

https://safetyfirst.airbus.com/app/t...y_first_27.pdf

Page 6 … fig 8 tailwind, cross wind

Re HS-125. The aircraft is 'short coupled', small distance between wheels and fin.
It is possible to steer the 125 on the runway with ailerons, depending on speed.
Surprisingly, the longer fuselage aircraft had greater effect; as I recall the extension forward of the wing was larger than behind

777 behaves like this and AFAIK it is something to do with the (big) engine nacelles and thrust/airflow increasing during the early stages of the ground run interacting with the crosswind. Once steady over 60-80kts it reverts to how you think it should work.

As you naturally apply rudder to keep the aircraft tracking straight, and this becomes an almost reflex action, it’s not until you are made aware of this particular phenomenon and think about what your feet are actually doing that it becomes obvious that this is what happens on most occasions.

I do notice this in the 73. My theory is that the vertical stab is huge and flat, so a direct crosswind pushing against it tends to cause the nose to drift into the wind when beginning the roll, so a little rudder input away from the wind helps keep centerline via the nosewheel steering system until the actual rudder becomes effective at around 60kts. At this point rudder input into the wind and very slight aileron into the wind is used to maintain centerline up to and after rotation and to crab for climb out. YMMV

The 787 also has this trait. I always put it down to the surface area of the large nacelles being blown downwind.

I had a briefing on this from Airbus flight test a few years ago.
At low speeds (< ~70kt) in high-bypass ratio engines and at high thrust, a yawing moment counteracts the natural weathercocking effect of the aircraft. The resulting downwind turning moment is overall smaller than the opposite upwind turning moment, with the downwind moments and upwind moments counteracting each other equally just below 70kt, and then beyond this speed, the upwind moment has an overall larger effect.
The phenomena were first noticed by Boeing during the 777 test campaign and was been observed by Airbus in their 350-900 testing, and then subsequently in the 320-NEO and 350-1000.

It can occur in any aircraft but is generally hidden by the upwind weathercocking effect.

So is it just the upwind engine getting cleaner air and making more thrust with it? Would have never thought of that until this thread, but now it seems like an easy (post-facto) explanation.

Or, like the nose up pitching moment which can be generated by a higher powered propeller machine at high incidence (which, in turn, may create some missed approach long stab problems requiring a variable downspring SAS to fix), it can be viewed a bit like an aerofoil, in that the incoming mass airflow is turned somewhat as it rounds the nacelle lip - this generates a change in momentum and a force in the downwind direction. Such effects strongly correlate with the level of forces generated in the particular aircraft under the microscope. I would expect to see much the same sort of effect with a higher powered piston machine - the airflow goings-on are going to be much the same ....

Can't say that I have ever noticed the effect but that may just be a reflection on my inadequate handling sensitivity competence ?

never personally noticed one any aircraft type.

Knowing zzuf well in a past life and holding his handling competence in the highest regard, I am surprised to hear that a chap of his background hasn't observed the effect ... makes me feel a little less inadequate ....

And you describe quite the opposite of what's discussed here. What 737 series are you referring to in your statement?

@all: The new effect is related to high bypass engines and them pushing a considerable higher mass of air than former models?

Sorry to disappoint JT - it wasn't for the lack of trying. As you know, I must have done a few hundred high crosswind, narrow runway takeoff tests, in more than a dozen aircraft types. Probably thinking too much about the upcoming engine failure, and knowing with such crosswinds Vmcg was a figment of ones imagination!

Ah, but not once did you fall into the trap of having the crosswind from the wrong side ....... (unless you were primed for the accel-stop).

Re vmcg, and HS 125-800.
As I recall the aileron effect for steering was sufficiently strong to discuss its use with the regulator to reduce vmcg.
Regulator's response was that pilot input could not be guaranteed; counter argument was the similarity with rudder input. Regulator muttered cross wind, wet runway, … end of discussion.

Re thread question: From those who have experienced the effect / need of rudder input, any difference with runway condition, dry - wet ?

It’s very obvious in the 787, but I never noticed it on the A320 series.

Can’t say I’ve noticed it in the 737, but then any initial peddling required to get straight and on the centre-line I’ve put down the Captain’s lousy line-up!

Yes, yes. During a crosswind takeoff, the airflow over the wings is affected by the crosswind component. The counterintuitive rudder input is a result of the asymmetric thrust produced by the engines as observed in the first 2-3 seconds of the A321 simulation.

After reading this thread, I did notice it the next cross-wind take-off I did. Wind 90’ off at 15kts. Needed 1/4 to 1/2 into wind rudder to keep straight initially, past about 40kts was downwind rudder as normal.