Crosswind Cross Control effect on T/O distance?

[mscar]

Hello all,

From my understanding, crossing the controls on a Crosswind Takeoff effects the drag force acting on an aircraft on take-off and hence Take-off distance required by the aircraft. Whether this distance is large I don't know.
Does anyone know of any reports/research articles that investigate the prescribed effect or have any academic involvement in the subject?

Thanks for any possible feedback.

mscar

[Miserlou]

Being a conservative business, only half of the headwind component or one and a half times the tailwind component is used in the calculaton of take-off distance. Thus, the aircraft should at any time out perform the calculated figure.

It follows then that any extra drag must be so small as to be negligible and far less than running off the side of the runway. Consider also, the effect of tyre pressures.

[Rainboe]

I think the effect of crossed controls is negligible to take-off roll and has a very minor effect on total drag. The reason for the crossed controls is to stop one wing lifting prematurely and to stop the aircraft weathercocking into wind. So it's into wind aileron and downwind rudder. Small deflections. There is an appreciable effect on approach in a high drag, unclean and cross controlled situation. This is because the rudder creates yaw. To stop roll, aileron is used. The sustained yaw causes high form drag- drag is increased, not because of control surface deflection, but because of the larger shape of the aeroplane moving through the air. Don't be fooled by inflight control surface deflection causing unbalanced drag across the aeroplane and hence causing yaw- the drag itself is still quite low, but being caused by aileron deflection right out near the wingtips, the asymmetric effect has high moment. As proof, sudden high aileron deflection causes yaw, but no perceptible decceleration.

On a 737 simulator all-engines out drill, I ended up passing 1000' VERY high. You need to be about double the height of a powered approach- I was way above that. Full flap, gear, speedbrake- it was obvious we were still too high and going to overshoot. It was creating great mirth with the copilot and instructor. I said I had one more trick to pull out of the hat, and did full crossed controls- we came down like a lift and I completed the detail with my dignity intact! (But the underarm deodorant had failed totally)

[PW1830]

Several years ago we tried various scenarios in a 767 sim. (the sim performance was as per the aircraft perf manual in all other areas.).
Runway limited, max crosswind, V1 cut critical engine and consistently came up with a screen height about 10-15 feet less than the 35ft expected without the crosswind. Convinced us but it was just a Sim!

[mscar]

Thanks to all,
Sure enough the outcome of performing an unbalanced cross controlled landing is a high profile drag, and hence its use by barnstormers and as last attempt tricks in 737 sim checks.

Where I'm interested is at the takeoff stage. I can only assume that crossing controls must have a small influence on the aircraft drag polar on take-off as said above.
But where would I find this out for sure? Has anyone read any articles, performed the research, crunched the numbers to find this out?

All the best,

mscar

[Centaurus]

Colleague as first officer watched as captain began take off roll at Melbourne runway 27 in B737-400. Although the crosswind was only 25 knots the captain used full into wind aileron for the entire take off run. He later said he always used full aileron into wind above 15 knots. Around 115 knots (VR 135 knots) the acceleration slowed markedly and it was obvious the take off run was going to be critical. All the runway was used when the performance calculations showed ample runway was available.

The increased spoiler drag became readily apparent as the aircraft neared VR.
Regardless of "padding" designed into take off performance tables there is definately a serious increase in take off distance with full crosswind controls applied.

[Genghis the Engineer]

Previous discussion. (http://www.pprune.org/forums/showthread.php?t=169049)

G

[Rainboe]

Centaurus- the spoiler deflection due to high control wheel angle would be quite significant, and compared to it, the drag from the ailerons would be quite minor. Mscar is looking at this from an academic 'xx degrees control wheel deflection throughout the take-off roll'. I think there is no answer to his question because you may start off with full deflection at slow speed, but this will markedly decrease as speed increases, so at high speed there is little or no drag as both rudder and aileron control angles will have been largely washed out to almost zero. It is just not quantifiable.

[wiggy]

From the depths of the past I recall ( I think:confused: ) that when doing take-off calculations on the 747-100/200 you made an adjustment if the crosswind was above a certain value, to account for the extra drag of spoliers/rudder - am I imaging this?

Certainly don't do the same on the 744.

[OzExpat]

There's nothing wrong with commencing a take-off with full aileron applied, as a technique, if the conditions warrant it. Of course, as speed increases, the controls become more effective and, therefore, the amount of aileron and rudder usage needs to diminish accordingly. This is pretty much P of F 101 - effect of controls stuff.

I don't know what stage mscar is at but I suspect that the thing he/she is after will be found in the take-off performance charts for the particular aeroplane type. I recommend that he/she grabs the performance manual for any aeroplane type and run some calculations based on the same basic environmental factors but considering the following wind conditions :-

1. 20 knot headwind;
2. 20 knot tailwind; and
3. 20 knot crosswind.

I would expect that the results of this work, which should take no more than about 10 minutes or so, will yield the answers fairly definitively.

[Centaurus]

Surely full aileron applied at the start of the take off run would only apply to a light aircraft type where there is definate wing lifting at slow speed in a limiting crosswind? Certainly in something relatively heavy like a 737 you need very little control wheel down simply because the wing doesn't rise.

[wiggy]

Don't know about the 737 but in the 747 you certainly do use and need in to wind spoiler and aileron, otherwise the wing will rise. With 40 kts across (my outfits dry X-wind limit) you have just about full wheel applied ( and commensurate crossed rudder) .

[Blip]

mscar I can't answer your question but I hope someone can. It would be very interesting to see the data.

I would like to add my bit to the discussion regarding my own thoughts on crosswind technique, and possible performance penalties.

In my experience, it is best to use aileron in proportion to rudder imputs. If I have about half of full scale rudder imput, I would have the control wheel at approximately 45 degrees from horizontal.

I refine this too by looking at the ADI and simply keeping the wings level. If you look closely, you can actually see 1 or 2 degrees of roll as the upwind wing produces more lift. This is especially true for aircraft with sweepback wings. So if the crosswind is from the right, and the wings are rolling 1 degree to the left, I simply put in more right aileron and level the wings again.

With the correct aileron input as per above, the rotation can be done nice and slow (2.5 degrees per second) without any unexpected roll or yaw. The rudder and ailerons are already where they need to be. As a result, there is no urgency or need to rush the rotation and get the aircraft off the ground in a hurry.

Once airbourne and around 100 ft AGL, I simply slowly relax the rudder input over a 3 to 5 second period, and adjust the aileron input as required to keep the wings level. By the time the rudder input is back to zero, the aileron input is back to zero, and the aircraft is nicely tracking the runway centreline with the correct drift established. :)

Sometimes I get the feeling some pilots want to get the crosswind rotation over with as quickly as possible because they haven't got the correct cross-controls BEFORE the rotaion begins. They're just a tailscrape waiting to happen!

Regarding the performance penalty associated with cross-controls, especially aileron input and spoilers, I believe the performance penalties would be quite neglegable. For starters, flight control spoilers do not extend nearly as far as when they are used as ground spoilers even at full aileron deflection. Also the number of panels used as flight control spoilers is less than the actual number of panels present. Obviously they are all used when used as ground spoilers.

It has been argued that the raising of spoiler panels reduces the Vmcg speed. My answer to this is so what? Unless Vmcg is very close to V1 (which happens on very rare occasions) it makes no difference. Engine failure below V1, you close the thrust levers and remove the thrust asymmetry.

If you should have a V1 near Vmcg, you could reduce the aileron input for that take-off until well above V1 if you want to. But for the vast majority of take-offs, the benefits as mentioned above far outweigh the risks of negligable performance penalties.

Like I said, I love to see the performance data if there is any.

[Jonty]

Interesting question this. For the 757 the FCTM makes no mention of the effect of drag on takeoff run with cross controls. It does mention that "large control wheel inputs can have an adverse effect on directional control near V1 (MCG) due to the additional drag of extended spoilers"

It then goes on to mention that "Use of excessive control wheel may cause spoilers to rise which has the effect or reducing tail clearance"

In our airline we use a maximum of 2 units of control wheel displacement so as not to "pop" the spoilers, and this technique is taught throughout the fleet.

[Jambo Buana]

1.6 units control wheel deflection pops the spoilers on the NG and 4 units creates full spoiler pickup. Use as much spoiler/aileron as you need but as you lift the nosewheel at Vr allow the a/c to weathervane slowly by relaxing rudder then reducing aileron input to almost zero. If you get airborne cross controlled in the NG with a 40kt crosswind you will need way too much aileron input which aint good when the donkey fails and it aint good for accurate IAS readouts either.

[alatriste]

Trying to be more concise,YES crosswind does affect take off performance, for instance an A-320 in excess of 20kt X-wind, RTOM must be reduced in 300KG and ATM for TOflex must be increased 1ºC.
Reason for that penalty? Not sure.

[Ready]

Yeap!
If you don't have roll spoilers, yes it is acceptable to have full aileron into the wind in case of strong X-winds.
With roll spoilers all you need is enough aileron into the wind until the spoilers come out then release a bit to make them flush with the wing, no matter how strong the wind is.
It's about time that some of you guys admit that you were wrong all these years.......plus, read the Training Part of your airplane and I bet you that this is exactly what it says......not something issued by some of your own pilots, but Boeing or Airbus.
Full aileron on one side, with full roll spoilers on the other side, is lots of drag not accounted for, and poor poor technique......Sorry, drop it.

[Rainboe]

Well that's a bit sweeping! It's a bit difficult when you have no indication of popping the spoilers, or if you do have a spoiler indicator, you most certainly should not be looking at that on a take-off roll! I can't ever remember needing more than about half aileron on a crosswind take-off, and this quickly reduces as speed increases so the drag element is reduced and completely unquantifiable anyway.

So what sort of admission that we were all wrong are you after?

[Ready]

Rainboe,
I never said that everybody was wrong but some, yes. No I don't look if the spoilers are up during the takeoff roll but I know that if beyond so many degrees of control wheel deflection, it pops out the roll spoilers, so that's my limit at the start of the roll and I never go beyond that.

[JackOffallTrades]

I try to use the minimum amout of aileron (and therefore spoiler) to keep the wings level on take-off. I try to start spoilers fully down and a smidge of aileron. That's what I was taught to do and it seems like good practise.

[411A]

Yes, indeed so.

In the good 'ole B707, you needed quite a fistful of aileron/spoilers into the wind, just as the rotation was commenced.
Not much before.

Likewise with the Lockheed TriStar.

Those who rapidly charge down the runway from a standing start with full aileron from the get go are seemingly applying DC-3 procedures in a jet.

Incorrectly..........accept at rotation, as needed.

[Jambo Buana]

411A

Sorry but you are wrong, at least on Boeings. The modern wing "flies" from about 90 kts these days and you need to apply aileron at this point at the latest. I show guys in the sim what happens around the longtiudinal axis if you apply full aileron left and right as you barrel down the runway. Watch the sky pointer and you will see a few degrees of roll occur as the plane flies around the oleos.

[411A]

Well JB, I did mention...it depends on the type.

Newer designs may well require slightly different techniques, but in older designs (especially the long body 707) the spoilers came up rather soon and far with small inputs, which degraded performance quite a bit.
Combine this with heavy weights and limiting runways, and you could well become up close and personal with the far end of the runway...;)

[mscar]

Thanks to everyone for the advice and snippets of info that kinda show me what the general opinion of c/w effect is.
I'm going to go ahead and research this for a final project in college so if I find any interesting results ( which I hope to god I do!) then I'll keep you posted.

If anyone has any ideas or comments, please post them, no matter how outrageous or irrelevant you deem them ( with in reaso:)) as they might poimnt out things I might have missed.


thanks,

mscar