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Wow... This is crosswind...

Started by cagarini, Fri, 6 Oct 2017 08:59

skelsey

I do see the logic of what Jose is saying;  however, in practice I don't think it is really applicable to an aircraft the size of the 744. In a glider (or other light aircraft) one does need to 'fly' the aircraft on the ground because one only needs a relatively light wind to lift a wing (for example, if you are in an aircraft that can fly at an IAS of around 30kt, as is the case for many gliders and microlights, it's easy to see how any significant wind becomes a high percentage of flying speed).

In the Jumbo, however, the wind speed required to lift a wing is much higher. Boeing don't even really like you using in to wind aileron on the takeoff roll at all if you can possibly avoid it: the danger is that if you put too much control wheel deflection in you will start to crack the roll spoilers open and thus destroy your takeoff performance.

Large transport jets aren't generally known for sideslipping very well;  again the roll spoilers don't help matters, and as I alluded to earlier it is impractical to land a long wingspan quad jet with underslung engines with any significant amount of bank on as you risk bashing an outboard pod. Hence just fly it down in the crab, squeeze (don't kick!) the rudder to gently remove some of the crab in the flare and just enough opposite aileron to keep the wings level (after touchdown though, you can put in as much as you like).

Fundamentally though you want to be touching down wings level: reference https://youtu.be/HmdrYKIDbSQ or https://youtu.be/I_TgukaMc1U

United744

I agree - I think in an aircraft as large as the 744, initial roll and rudder inputs for wind might be a good idea, but once the aircraft gets any significant forward speed, the aileron can be returned to center. It's the rudder that really matters, to keep things aligned and the aircraft on the runway.

I was just playing with side-slipping to keep the aircraft aligned with the runway and the nose pointing in the same direction, but I think the sheer area of the fuselage prevents it. It feels like the slip angle is limited, which I suspect might be due to the fuselage "sail area".

Hardy Heinlin

Imagine you were able to side-slip at 5° bank against a 30 kt crosswind. Now imagine the wind is calm and you still keep this side-slip. Would this tiny 5° bank side-slip make the aircraft move sideways at 30 kt lateral velocity? No. You would need a 30 kt anti-vector against the 30 kt crosswind vector.

cagarini

#23
Quote from: Hardy Heinlin on Tue, 10 Oct 2017 00:59
Jose, a few rhetorical questions for you:

At what airspeed does a glider start to fly?
At what airspeed does a 747 start to fly?

When rolling on the ground at heading 360° and the wind is 210° at 50 kt ...
... and the groundspeed is 10 kt, do the wings get air from the front or from the aft?
... and the airspeed is 10 kt, do the wings get air from the front or from the aft?

In PSX, when parked, the ailerons are only effective if there is a strong headwind component. Unlike you thought, they don't lift anything when parked in tailwind. So there is nothing to reverse. Zero is zero.


|-|

Hardy,

at 80 km/h we usually are flying with the tug still on it's takeoff run. Depends on glider and ballast being used or not....

On the Phoebus, a tailwind of near 10 knot ( gusts ) will force me to start my takeoff run with the technique I describe, but as I gain speed, of course the relative wind becomes positive and we do have to apply the usual sort of corrections for x-wind.

I never had thought about the effect being felt on a 744, but in PSX, if I start a situ on rw with a x-wind of 30 knot or more I do see the effect in the tilt of the aircraft, it's upwind wing being up. And... I also found that as I start taxiing, above a certain taxi speed, not necessarily a huge one ( 15 knot G/S will do ) if I deflect the yoke I can see the resulting bank, and try to align the wings, so... I thought to myself that if this is possible at such low speeds, the effect of taxing or taking of with an unusually high cross-tailwind would probably be felt...

So I created SITUs with various wind intensities and tail angles, and started experimenting with it. If the effective axial component is superior to your G/S then there will be negative relative wind. Say you get such a speed up to 30 knot, and you're taxiing at 12 knot. Shouldn't then the aileron ( and rudder ) deflections be sufficient to create the same rolling moment they do with positive relative wind ?

It's a very picky question, I know.... but it just came to my mind :-) 

Will think more about the maths of it. And Hardy, I don't think about "lift" in this case, although it is of course necessarily a factor too, but the mechanical effect of the deflected ( down ) ailerons on the upwind wing, getting the blowing wind from behind, will be more of a factor under such circumstances, and do the opposite of what they would do should they be deflected Up, if the yoke was being deflected "into the wind"..

Addendum: Hmmm probably the effect of the dihedral and the way the wind interacts with it, lifting the upwind wing, is much more significant than any possible effect from the "tiny" ailerons....

Tried it in X-Plane 11, and the 744 does tilt like in PSX, and using opposite downwind aileron and upwind rudder Works, but... it's X-Plane, so... it's just another sim, not the real thing ....

Walktall


IefCooreman

Lot's of theory for something that is widely discussed but hardly ever executed properly... despite what many will claim :-). I didn't read everything above but...

Fly the aircraft the way the designers ask you to.

The Airbus problem is that the stick left/right command is a demand for a mathematical "roll RATE". Landing an Airbus in a crosswind and then compare it to a Cessna landing is likes apples and... you know. Basically you would need to apply the perfect "ticks" to put the aircraft in the correct slip and then don't touch anymore. Fly by wire make sure roll rate remains 0 as long as sidestick is central.

Boeing acts like a Cessna where any rudder application will create secondary roll so the landing remains a cross-controlled maneuver with constant rudder and aileron application. You also have to remember the application point of the rudder "lift" is above the aircraft cg, so any rudder force will make the aircraft roll. There is also the effect of the swept back wing to be taken into account during the "aligning with the centerline with rudder". But in general, during the slip, you will have to maintain some aileron to compensate the rudder force application point.

Also, modern jets with underwing engines (including the classic 737) can only be landed up to certain wind limits "wing low" because of pod strike danger. The values used to be in the FCTM, for a 73 I recall something like 17kts, 25kts depending on flap setting but don't quote me on that... so yes, it is adviced to land in crab, and us pilots end up doing a mixture of "timed rudder and aileron" hoping for the best result. And that best result appears to be harder to achieve on a fly-by-wire Airbus.

So please don't compare gliders, cessna's, fighters, Boeings and Airbusses... :-). And for the record, if you land a Boeing 747 like those Fedex guys, you will scare the shit out of your captain/first-officer. 5° bank max is more limiting than longitudinal axis aligned with centerline. The gear can manage crab. The engine cannot manage concrete, nor grass.

PS: the Airbus 380 video seems to be a case of not really taking care of acft inertia. Too late in, too much correction, too late out, too much correction again. Just my thought :-)

Phil Bunch

Quote from: Hardy Heinlin on Fri,  6 Oct 2017 16:36
The zoom lens exaggerates the angle, but the swing of the tail, in my opinion, is still enormous: the pax in the aft zone must have been thrown sideways over a distance of ca. 10 meters in less than 2 seconds. And then back. And then again. I'm not sure how much of the effect was induced by a crosswind gust and how much was caused by the pilot who tried aligning the aircraft before touchdown; that overshooting yaw-swing actually started when the rudders were kicked to the left. The rudders stayed in that position during the entire swing. It looks like the pilot didn't stop this turn by the rudders; the rudders rather supported it.


Regards,

|-|ardy
---------------------------------
A question:  how do the stresses on the tail of the aircraft in this crosswind landing compare with the stresses that broke up an Airbus A300-600 (American flight 587)?  The 587 crash was attributed to excessive use of the rudder.  Below are some excerpts from an interesting Wikipedia article on this flight.

----------------------------------
http://en.wikipedia.org/wiki/American_Airlines_Flight_587

http://en.wikipedia.org/wiki/American_Airlines_Flight_587#Findings

Aircraft tail fins are designed to withstand full rudder deflection in one direction when below maneuvering speed, but this does not guarantee that they can withstand an abrupt shift in rudder from one direction to the other.

From the NTSB report of the accident:

The National Transportation Safety Board determines that the probable cause of this accident was the in-flight separation of the vertical stabilizer as a result of the loads beyond ultimate design that were created by the first officer's unnecessary and excessive rudder pedal inputs. Contributing to these rudder pedal inputs were characteristics of the Airbus A300-600 rudder system design and elements of the American Airlines Advanced Aircraft Maneuvering Program (AAMP).

Since the NTSB's report, American Airlines has modified its pilot training program.[26] Previous simulator training did not properly reflect "the actual large build-up in sideslip angle and sideloads that would accompany such rudder inputs in an actual airplane", according to the NTSB final report.

Best wishes,

Phil Bunch

andrej

Andrej

United744

QuoteAircraft tail fins are designed to withstand full rudder deflection in one direction when below maneuvering speed, but this does not guarantee that they can withstand an abrupt shift in rudder from one direction to the other.
Correct!

To put it slightly more succinctly, only ONE flight control may be deflected ONCE to full deflection and returned to center, but NOT reversed, and flight control inputs may NOT be combined (e.g. full aileron AND full elevator could exceed either pitching and/or rolling g limits, and snap the wings off).

What destroyed the tail on the Airbus, was the REVERSAL of the rudder. The initial rudder deflection got the aircraft yawing one way, but then they reversed the controls, with the aircraft now in a slip, and the forces were so great (rudder + airflow acting the same way) that it exceeded the structural strength and snapped it off.

Landing can stress the tail in a similar manner, but the speeds are generally lower (landing speed or less), and I would hope the aircraft is well inside what it can handle at that point.

United744


Hardy Heinlin

Filmed from a seat at the CG, not from a seat in the aft zone.

And even when just watched from the CG position, the fast changing distance to that right, white side strip is impressive. Don't watch the wing, it doesn't depart from the camera; watch the side strip, that's what is changing its distance to the camera. Now extrapolate this side drift to the swinging tail ...

United744

Quote from: Hardy Heinlin on Tue, 10 Oct 2017 06:58
Imagine you were able to side-slip at 5° bank against a 30 kt crosswind. Now imagine the wind is calm and you still keep this side-slip. Would this tiny 5° bank side-slip make the aircraft move sideways at 30 kt lateral velocity? No. You would need a 30 kt anti-vector against the 30 kt crosswind vector.

Understood. I tried it at higher altitude without limits to roll angle, and still couldn't get it to track. It seems the aircraft just can't slip sufficiently.

United744

Quote from: Hardy Heinlin on Mon, 16 Oct 2017 03:49
Filmed from a seat at the CG, not from a seat in the aft zone.

And even when just watched from the CG position, the fast changing distance to that right, white side strip is impressive. Don't watch the wing, it doesn't depart from the camera; watch the side strip, that's what is changing its distance to the camera. Now extrapolate this side drift to the swinging tail ...

Good point. I did watch the how the runway disappeared! I'd find it rather disconcerting if I was a passenger...

Hardy Heinlin

Quote from: United744 on Mon, 16 Oct 2017 04:05
I tried it at higher altitude without limits to roll angle, and still couldn't get it to track. It seems the aircraft just can't slip sufficiently.

Yes, it can. Try this:

Wind calm.
Level flight at 200 KIAS at 4000 ft.
Deflect your rudder left 30%.
Control the aileron so that the compass rose is not turning. The bank will be ca. 10°.

Now check the difference between track and heading: It's about 3°. You are drifting sideways while maintaining track and heading in calm wind.


|-|

cagarini

Quote from: Hardy Heinlin on Mon, 16 Oct 2017 04:54
Quote from: United744 on Mon, 16 Oct 2017 04:05
I tried it at higher altitude without limits to roll angle, and still couldn't get it to track. It seems the aircraft just can't slip sufficiently.

Yes, it can. Try this:

Wind calm.
Level flight at 200 KIAS at 4000 ft.
Deflect your rudder left 30%.
Control the aileron so that the compass rose is not turning. The bank will be ca. 10°.

Now check the difference between track and heading: It's about 3°. You are drifting sideways while maintaining track and heading in calm wind.


|-|

With the necessary adaptation :) the reason why I always perform sideslips in gliders towards the wind, even if faint wind...

Jeroen Hoppenbrouwers

You can have one wing a bit lower than the other . . . unless there is a bank of snow next to the runway . . . then you need to keep both wings high  :-P

http://avherald.com/h?article=492eaf3a



Hoppie

cagarini

Quote from: Jeroen Hoppenbrouwers on Wed, 18 Oct 2017 12:20
You can have one wing a bit lower than the other . . . unless there is a bank of snow next to the runway . . . then you need to keep both wings high  :-P

http://avherald.com/h?article=492eaf3a



Hoppie

Banks of snow not simulated in PSX !  Ufff!