EFIS wind arrow behaviour during takeoff

[Hardy Heinlin]

Hi all,

here's another question for jet pilots in general, not only for 744 drivers. Perhaps someone can answer it?

First, the things that are obvious:

Imagine we are on the takeoff roll ...

- Wind data is displayed when TAS reaches 100 kts (in a Hurricane with 100% headwind, it would be displayed while parking on the apron).

- Assuming the wind is coming from 2 o'clock: With increasing IAS the aircraft gets lighter and the ground contact gets weaker; with strong crosswind the aircraft starts to drift sideways which must be compensated with the flight controls, resulting in an increasing difference between heading and track.

- Before heading and track start to differ, i.e. as long as the wheels keep the aircraft on track, the IRS detects only a forward motion, i.e. it detects no crosswind, just a headwind, negative or positive depending on the TAS-GS subtraction.

- Approaching VR, the wind arrow tail turns more and more towards 2 o'clock, but not fully, only after liftoff it eventually shows the actual wind.

- Gross weight and runway condition are, of course, a factor.

...

Can anybody give me some tips regarding typical wind arrow behaviour after reaching 100 KTAS? How noticeable are the effects of strong crosswinds? Is it noticeable at all or is the wind arrow vertical all the time until liftoff?


Thanks a lot!

|-|ardy

[John H Watson]

The wind arrow is computed from vectors of Groundspeed/Track and TAS/Heading. If the heading remains the same as track, the wind arrow must remain dead ahead.

Why would the heading be different from the track with the nosewheel still on the ground and tracking the runway centerline?

Cheers
JHW

[Will]

In the planes I flew: Rudder keeps the nose aligned with the runway (i.e. keeps heading and track the same) and aileron compensates for crosswind.  The heading and track are the same until rotation, at which time the crab angle is set.  That's when the wind arrow should move... essentially it would move just as fast as the nose moves to assume the crab, perhaps with a delay (how long?) as the difference is computed.

[Hardy Heinlin]

Why would the heading be different from the track with the nosewheel still on the ground and tracking the runway centerline?

Same effect as with a car on a stormy bridge with heavy crosswind: Drive slower to keep the cross-drift vector small, and also compensate the drift by steering into the wind. The friction force between the wheels and the ground is not infinitely high, especially on a non-dry surface. The friction also gets weaker with the rising lift force (which also depends on the gross weight and IAS).

If the transition from crosswind 0% to crosswind 100% wouldn't be smooth and gradual, the landing crab angled aircraft would immediately switch its track upon ground contact, like a flying train falling into offset angled rails (and falling apart due to its inertia and the sudden anti-force). A rubber wheel at 100 knots cannot have the same ground contact as a train in a rail, I would say.


Cheers,

|-|ardy

[Hardy Heinlin]

In the planes I flew: Rudder keeps the nose aligned with the runway (i.e. keeps heading and track the same) and aileron compensates for crosswind.

My understanding is that the aileron-into-the-wind rule (basic pilot rule) has the main purpose to not let any crosswind gusts under the wing (wing on the crosswind side) which may bank the aircraft leeward a bit (wings and undercarriage are flexible) and enforce the crosswind even more.

I bet, at strong crosswinds and high IAS, there are differences between aircraft heading and aircraft track up to 10 degrees or so (on dry runways) and that pilots normally don't notice because it is not very noticeable in the outside window, and because they are more busy with scanning the IAS and pitch on the instruments.

To take a more extreme, but still realistic example: Take a wet runway with average aquaplaning effect and approach VR with a crosswind of 15 knots: Physically it's impossible to keep the aircraft on the centerline solely with ailerons. I'm sure you'll have to face the nose a bit off the centerline into the wind with the rudders (supported by the ailerons to not let the wind go under the wing).


Cheers,

|-|ardy

[Will]

Nah, that's not right.  The heading and track are the same until rotation.  Any tendency for the aircraft nose to veer into the wind is countered by rudder, and then aileron holds the ground track true.  The aircraft lifts off in a slight bank into the wind, but with the nose aligned directly down the runway.  As soon as the wheels lift off, the pilot crabs into the wind to hold the track straight, and levels the wings.

You really don't want to have the nose pointing into the wind on the runway, because the aircraft is transitioning from a ground vehicle to a flight vehicle... and on the ground, hard braking with a crab angle would send you off the runway and into the weeds.

[Hardy Heinlin]

Remember, I'm talking about a few degrees only, about a rising trend when approaching VR, i.e. when approaching a point at which the aircraft almost lifts off.

If it were true that the wheels stick on the ground like a train on rails, then you could steer the heading with the nose wheel tiller until liftoff. But the truth is that this little nose wheel is useless at high speeds and you need to steer with the rudders, i.e. with air instead of asphalt, because the asphalt provides no friction force anymore -- even more so on water, snow or ice.

Vice versa on landing ... at strong crosswinds the slip (aileron/rudder crossed) is restricted by the bank angle limit and a slight crab angle may remain on touchdown. Not much. Just a little (think of Kai Tak). It takes a moment until the aircraft is in line with the runway heading. During that phase, if the wheels would behave like a train on a rail, the aircraft would in zero time change its track. Zero time is impossible in the laws of mass and inertia. The aircraft would fall apart.

In my opinion, liftoff is a gradual process, not a yes/no event. It doesn't go like this:

100 knots = 400 tons pressing on the asphalt
120 knots = 400 tons pressing on the asphalt
140 knots = 400 tons pressing on the asphalt
141 knots = 000 tons pressing on the asphalt

Rather like this:

100 knots = 400 tons pressing on the asphalt
120 knots = 200 tons pressing on the asphalt
140 knots = 010 tons pressing on the asphalt
141 knots = 000 tons pressing on the asphalt

[Tor]

Will try to have a look about the wind arrow tomorrow if there is any cross wind.

About track and heading, I would agree that is the same. If not if must be the flex that is inherent in the torque links, but actually I think that actively steering away from the weathercock effect will even this out. The ailerons will also "bank" the aircraft into the runway so there will be more friction on the upwind gear. It's too much physics for me to work out the net result of friction and aerodynamic forces (sideslip even, perhaps adverse yaw) :oops: , but for sure every takeoff looks like it's straight ahead down the runway. Not sure how much the human eye would pick up, but my gut feeling says that I would spot if the aircraft weathercocked even with less that 1 deg.

The feeling when the wheels lift off is quite violent with strong crosswind. In literally an instant the nose will swing over and you have the feeling of going sideways. Much like a car oversteering in a curve. If the aileron input was sufficient during the takeoff roll almost no input is necessary though.

By the way, on the 737 the ailerons also have a great steering effect once the aircraft picks up speed. I learned from an old captain to stop manipulating the rudder/nosewheel during the whole takeoff roll. The rudder is very uncomfortable for passengers and basically anyone but the guy who applies the inputs. Using rudder during the entire takeoff roll has the tendancy to "wickle" the aircraft from side to side with every constant correction.

A much better technique is to initially apply opposite rudder of the wind, as the aircraft starts to move. Then move the aileron aileron into the wind. Once you get a feel for it you know if the aircraft starts to go off the centerline to apply more of less aileron in to the wind and simultaneously easy off some of the initial pressure on the pedal. Result is that you never has to modulate the pressure on the rudder pedals. Instead it's one push and a gradual easy off. If the wind is gusty it may be necessary to modulate the rudder though and passenger confort will come second.  

Pilots (and this include seasoned captains) that didn't benefit from such wisdom from skilled old captains will apply a set and fixed amount of aileron and work a lot with the rudder. Flying a lot as a passenger when proceeding to active duty, or with captains modulating the rudder, I often think about this old captain during takeoff roll when being shaken from side to side. And am thankful for his "leave the pedals alone!" speech.  

[Will]

Tor is right, including about the dramatic swing of the nose in a heavy crosswind.  And Hardy, I'll give you 1 degree here and there, but by and large the track and heading are the same on the ground.

Also Hardy, you're only half-right that the aircraft gets progressively lighter.  It does, but the final transition to flight is NOT gradual.  The wing doesn't generate enough lift to fly the plane off the runway when the nose wheel is on the ground.  That's because the angle of attack is too small.  Rotation ends up increasing the angle of attack by tilting the wing into the relative wind, and all that extra lift pulls the plane up into the air.  Before rotation, at VR, there are still many, many tons pressing down on the runway.  I'd guess something like this:

100 knots = 400 tons pressing on the asphalt
120 knots = 380 tons pressing on the asphalt
140 knots = 360 tons pressing on the asphalt
141 knots = 000 tons pressing on the asphalt <-- rotation, generates much lift

If the pilot didn't rotate at all, you'd see something like this:

100 knots = 400 tons pressing on the asphalt
120 knots = 380 tons pressing on the asphalt
140 knots = 360 tons pressing on the asphalt
160 knots = 340 tons pressing on the asphalt
180 knots = 320 tons pressing on the asphalt
200 knots = 300 tons pressing on the asphalt
220 knots = 280 tons pressing on the asphalt
240 knots = 260 tons pressing on the asphalt

[Hardy Heinlin]

OK, I rely on you, Tor and Will, and if it later turns out to be wrong, you're responsible! :-)

Rotation, sure, I took that into account. Of course, the aircraft won't lift off at VR if  pitch is zero.

You start rotating at VR, rotation rate ca. 2 degrees per second, i.e. 4 seconds after VR you are at pitch 8.

 At pitch 8 you are in level flight attitude (0 tons, no friction anymore). Another 4 seconds later, at pitch 15, you're climbing, then you have a negative weight, so to speak.

But at pitch 8 already your weight and friction is zero, I would say.

So, do we agree if I say the biggest transition happens between VR and pitch 8, and that it is a gradual transition, say, from 90% grossweight at VR ... to 0% grossweight at pitch 8?


Thank your for the discussion.

|-|ardy

[Will]

So, do we agree if I say the biggest transition happens between VR and pitch 8, and that it is a gradual transition, say, from 90% grossweight at VR ... to 0% grossweight at pitch 8?

Agreed!  Although the figure of 90% was a guess; it's probably 80-95%, I dunno exactly.

[Hardy Heinlin]

My 90% were just symbolically meant.


P.S.: Don't forget the many Kai Tak photos and videos of crab angled landings. They prove that heading and track may differ on the ground with a pitch lower than level flight attitude. Not for long, but for a couple of seconds anyway.

[Will]

Yeah, Kai Tak is amazing.

[edit: shortened this post]

[Tor]

Thank you for good discussion as well. It's always nice to exercise the grey cells.

I think it's a fair point that track and heading may differ when the aircraft is skipping along the runway during rotation and after the flare when landing with a crab angle until inertia (and pilot input) aligns the aircraft with the runway.

Just a slight point about lift and weight. By weight negative or zero - I assume that you by weight mean "the force that act upon the wheels".  
And hence that the aircraft climbs as a result of excess thrust. And that "lift" and the "mass X gravity" of the aircraft are always equal

[effte]

Getting in a bit late, I think.

As soon as there is a lateral force applied to a moving wheeled vehicle, the heading and the velocity vector will differ. This will however rarely be due to the tyres skidding. What you are seeing is the slip angle in the tires. The contact patch between the tyre and the surface is rotated by the lateral force in combination with the rotation of the tire, creating forces in the carcass which are translated to the rim of the tyre as the lateral force needed to oppose the lateral force applied to the vehicle.

When the tyre starts to skid, you have put more stress on it than the carcass flexing can stand up to, thus increasing the shear force at the aft part of the contact patch (where the distortion of the carcass is the greatest) to beyond what the tyre and surface can support. Experienced racing drivers are able to sense this beginning skid and can thus drive right on the edge of the performance envelope of the tyres.

In bikes, where you have a round cross section tyre with a camber angle going from 0 to around 70 degrees as the vehicle manoeuvres, it all gets very interesting indeed! In fact, bikes get their lateral turning force through a quite different mechanism of deformation of the tyre than cars, as the individual parts of the tyre contact patch are forced to follow straight lines rather than their normal circular path around the axis of the wheel... but now I think I am drifting too far into a particular corner of my own nerdism!

The actual slip angle of a tyre depends on many factors, such as tyre profile and carcass construction. With the high sidewalls of aircraft tyres, it should be noticeable, but to quantify the angle is beyond my experience level and expertise.

To reiterate the main point, the nose will be pointing upwind by a number of degrees (or perhaps a fraction of a degree - I don't know) dictated by the crosswind and tyre construction.


Most transport category aircraft will not generate much lift prior to rotation, as they will be close to the zero-lift angle of attack on the ground. Hence the weight on the wheels will not change much to this point during the take off roll. If I remember correctly this is described anecdotally in an excellent publication released a long time ago, namely the manual for the Rolls Royce of heavy transport aircraft simulators; Aerowinx PS1.3. In the story I remember it is described how a 744 takes off on a snowy day, with the snow to the sides of the runway visualizing where the wingtip vortices begin (and with them, lift generation) as the aircraft rotates.

(Subtle kissing up, wasn't it? We DO get discount coupons for PSX based on our forum posts, right? )


Aileron into the wind as the wind will be your friend if you overdo it and lift the downwind wing (not likely to happen, I'd say), making compensation easy. In the opposite situation, with the upwind wing lifting, the wind will be quick to worsen the situation and aid your rapid diversion to the downwind side of the runway. Never played with it in jets, but almost got myself into trouble once getting sloppy in a light aircraft.

Cheers,
   Fred

[John H Watson]

Will try to have a look about the wind arrow tomorrow if there is any cross wind.

A 737 may be a bad example. From what I hear, some even crab during taxy )))

[Hardy Heinlin]

Just a slight point about lift and weight. By weight negative or zero - I assume that you by weight mean "the force that act upon the wheels".  

Correct. That's why I wrote "pressing on the asphalt" -- imagining the asphalt were a weight detector. I was too lazy to convert tons to Newton :-)


|-|

[Hardy Heinlin]

Thank you, Fred! All your three subjects are very interesting: Carcass flexing, Mel Ott's anecdote, discount coupons :-)

Inspiring indeed!

|-|

[Jamie]

Agree with Tor and Will. Agressive rudder input while roling: highly uncomfortable.

- Wind data is displayed when TAS reaches 100 kts

On the F70/100 its displayed just after lift-off. On Vr the nose swings into the wind, you keep the wings level, let the FD roll bar settle for a few seconds until it has calculated the wind and Nav mode captures.

Can anybody give me some tips regarding typical wind arrow behaviour after reaching 100 KTAS? How noticeable are the effects of strong crosswinds? Is it noticeable at all or is the wind arrow vertical all the time until liftoff?

We compensate with oposite rudder as Tor described well Aileron doesn't do much on the Fokker until just after VR.

Most transport category aircraft will not generate much lift prior to rotation, as they will be close to the zero-lift angle of attack on the ground. Hence the weight on the wheels will not change much to this point during the take off roll.

Most :mrgreen: then Fokker is a bit different: it wants to lift-off 10-20kts before VR and you have to keep it on the runway by gently pushing the yoke forward just a tad until V1, V2, VR is reached. Could be the safety margins, but colleages from other companies flying the Fokker confim this behaviour.

[effte]

I'd say it wants to rotate, rather than take off. No pushing on the yoke could make it stay on the ground if it was indeed generating enough lift in the ground level attitude to take to the air.

Cheers,
   Fred