Cerulean From , joined Dec 1969, posts, RR: Posted (12 years 4 months 6 days 1 hour ago) and read 2019 times:
I ask this question for one very simple reason: Contrails.
We've all heard the talk about "minimum separation due to 'wake turbulence'" and not to get too close to aircraft while in flight.
But this is what bothers me:
If "wake turbulence" was as real and dangerous as made out to be, then why do contrails (the white streaks) form straight lines halfway across the sky?
If the air behind an airplane was so choppy as to be dangerous, then one would think that contrails would be very short or nonexistant as the turbulent air would dissipate the steam rather quickly. Yet the white line just hangs there for several minutes and seems to take its time to disperse, as though there was nothing more than a light 2 or 3 kt wind. Hardly enough to knock a plane out of the sky.
Cerulean From , joined Dec 1969, posts, RR:
Reply 2, posted (12 years 4 months 6 days 1 hour ago) and read 1977 times:
Does that mean that wake turbulence is only a problem at the ends of a flight? In other words, is the air behind a "clean" plane (flaps, slats, gear up) a lot smoother-hence the existence of the trails?
Tom in NO From United States of America, joined Nov 1999, 7194 posts, RR: 32
Reply 4, posted (12 years 4 months 6 days 1 hour ago) and read 1961 times:
I'm going to go out on a limb here and see if I can make some comments on the question. They may not be entirely accurate, but just what my thinking would be:
1) the air at cruising altitude is thinner, and much colder, than it is closer to the ground, therefore, not as much choppiness to wake turbulence. I remember flying on a UA plane from DEN-MSY, and flying perpendicular to, and directly across and through a recently made contrail. Barely even a small ripple.
2) obviously contrails are pushed along by the wind. I've seen contrails that maintain their original 'straightness' for miles, yet get pushed halfway across the sky by a uniform wind. Others get tossed about in nothing flat.
Just my thoughts.
Tom in NO (at MSY)
"The criminal ineptitude makes you furious"-Bruce Springsteen, after seeing firsthand the damage from Hurricane Katrina
FBU 4EVER! From Norway, joined Jan 2001, 998 posts, RR: 7
Reply 5, posted (12 years 4 months 6 days 1 hour ago) and read 1938 times:
Wake turbulence is a real and dangerous fact.The air behind an aircraft will move considerably and create turbulence,but it is the high-pressure air below the wings that will try to switch onto the low-pressure area above the wings that form wake turbulence in the form a high-rotation vortices.
Seen from behind,the right hand vortice will rotate counter-clockwise,while the left hand rotates clocwise.The vortices are much more powerful at situations where the wing produces maximum lift,that means during take-off and landing.At cruise altitude,the diff. between high and low pressure is not so pronounced,but they are still there,believe me!Next time you have a look at another plane producing contrails,you'll see it is really the vapor from the engines.Looking closely,for instance with a telescope or good binoculars,you'll see these contrails almost immediately they're behind the plane,they start to rotate very rapidly as they get in contact with the wing-tip vortices.As the rotational energy dissipates with time,the contrails will rotate more and more slowly,and fan out a bit to make wider stripes.How long they stay there depends on atmospheric conditions in that particular airmass,factors like pressure,temperature and humidity influence on the stability of the airmass.Clear-air turbulence will tend to dissipate the contrails more rapidly than in a stable airmass.Airmass might also be influenced by winds creating a wave-like condition and an airplane flying through this mass will produce contrails one moment,then nothing at all and finally contrails again as it enters air with the original conditions.
Basically,if your plane has a wingspan greater than the one in front of you,there isn't so much a problem with wake as if you're plane has a shorter span.In this case,you're completely within the turbulence area and ailerons are hardly effective,at all.
NiteRider30 From United States of America, joined Jul 2001, 150 posts, RR: 0
Reply 8, posted (12 years 4 months 6 days ago) and read 1891 times:
Actually, wake turbulence is most severe with a heavy aircraft, at slow speeds, clean configuration, and high angle of attack. Basically, this means takeoff. Wake turbulence is indeed a potentially dangerous thing if not properly avoided.
Areopagus From United States of America, joined Sep 2001, 1376 posts, RR: 1
Reply 9, posted (12 years 4 months 6 days ago) and read 1876 times:
Cerulean, the pictures you posted show the vortices turning the contrails completely over. Note how the contrails from two engines on the same wing appear to cross over each other and separate again. The vortex persists longer than any particular molecule stays in it. The contrail steam is thrown down, then out and over the top, but the vortex as a whole moves downward.
The airplane stays aloft by transferring downward momentum to the air as fast as gravity imparts it to the airplane. At cruising speed, a given amount of momentum is transferred to a much longer tube of air than when at slow speed, and of course, the aircraft is lighter than it was when taking off.
Ragousis From United States of America, joined Dec 2000, 30 posts, RR: 0
Reply 10, posted (12 years 4 months 6 days ago) and read 1856 times:
NiteRider30 is correct. Wake turbulence is most severe with a heavy aircraft, at slow speeds, clean configuration, and high angle of attack. The reason that clean configuration is more dangerous is that the vortices form at the end of the wing and are very strong. If the flaps are down the vortices form at the ends of the flaps and at the end of the wings, some of upwash of the flap vortices and the downwast from the tip vortices reduce the strength of the vortices. It's on the ATP pilot written exam.
MD88Captain From United States of America, joined Nov 2001, 1338 posts, RR: 20
Reply 11, posted (12 years 4 months 6 days ago) and read 1849 times:
Ever been almost turned upside down on final by the wake turbulence from the aircraft ahead of you? I have and it was ugly and scary. Ailerons didnt even slow the roll a bit. Rudder and power and then we were out of it. We probably went from straight ahead fat, dumb, & happy to rolling left through 45 degrees in seconds. Hoax? I don't think so. I've been in wake at altitude, after take off, and on approach hundreds of times. It is no exaggeration to say I saw it once a month when I was flying domestic legs. Everything from a few speed bumps to the described incident. I've experienced it in everything from a T34 to a MD11. It has been documented by NASA, the FAA, the US military, APLA, et al. It is no hoax and it is something I go out of my way to avoid.
Now if you want to talk hoax lets talk about that Apollo moon landing.
Yvr74 From Canada, joined Sep 2002, 52 posts, RR: 0
Reply 14, posted (12 years 4 months 5 days 22 hours ago) and read 1785 times:
There is a great place to plane spot at YVR. Planes pass overhead just seconds before touchdown. 10 to 20 seconds after some of the airplanes, most notably the large ones, pass overhead you can hear the wake turbulence.
It is a sound that is really hard to describe. The best way I can think is sort of a rushing, sucking vacuous sound with a bit of a cutting or sharp quality as it passes overhead. It usually lasts for perhaps 5 to 10 seconds. It's really an amazing thing, and sometimes I never hear it but on rare occasions I've heard it for up to 20 or 30 seconds after it starts.
MD88Captain From United States of America, joined Nov 2001, 1338 posts, RR: 20
Reply 18, posted (12 years 4 months 5 days 22 hours ago) and read 1759 times:
I was an FO in an MD88 going into ATL and we were probably 12 miles out and above 3000'. And I should explain that we would have flipped with no intervention, but we approached 60 degrees of bank before the recovery. And most of the recovery was due to just flying out of the wake. It was with no warning and happened very quickly. The speed of the event is what sticks in my mind. I've been rocked around often, but this was like a snap roll. I did not like it.
BTW. I pay attention to what I am following on approach and if the conditions are right for wake turb, I will fly high on the glideslope and sometimes will offset a bit to the up wind side (traffic permitting). Approach/departure will squeeze you everytime on wake separation.
Drewwright From United States of America, joined May 2001, 621 posts, RR: 1
Reply 20, posted (12 years 4 months 5 days 21 hours ago) and read 1688 times:
There are several rather popular photos that would illustrate wingtip votices very well...you may be able to find them on this board. There's a great picture of a learjet flying through some clouds and it's wake causes the clouds to curl around. Very cool.
Also, if you're ever outside on a humid day and you see those "mini contrails" streaming from behind a jet wing on final (especially a 757), that's fast moving, high pressure air cooling the surrounding air, causing the moisture in the air to condense and form a trail...yet another testimony to the presence of airfoil vortices. Before I became a pilot, I thought it was a plane dumping fuel.
Finally, i've seen a great video of a 747 landing at Kai Tak that kicked up a great dust storm in it's wake which lasted for several minutes.
One thing you may find interesting is that winglets help reduce the magnitude of wingtip vortices, but a 747-400 still produces lots of turbulance, winglets notwithstanding.
Ikarus From United Kingdom, joined Jan 2001, 3524 posts, RR: 2
Reply 21, posted (12 years 4 months 5 days 20 hours ago) and read 1667 times:
An aircraft wake.
ask yourself: How does drag happen? Air pulls at the plane? Not quite - but the plane loses energy (otherwise it wouldn't need its engines running). This energy goes into the air. There are 2 main effects: 1) a large proportion of air gets accelerated and follows the plane (much like you will feel a draft when a lorry or a train passes) and 2) trailing vortices form on the outer edges of the wings.
Both can take other aircraft out of the sky.
Now, on landing, the aircraft produces more lift coefficient than at any other time - this generally means a lot of induced drag, and a lot of choppy air behind it. Similarly on takeoff. And true, for a clean configuration, the most dangerous bits for a following plane will be the trailing vortices. For dirty configuration, the vorticity is better spread and less severe, but the large gust following the plane will still be dangerous...
On a related note: A few years ago in Germany there was a rather well-known court case: A father had been accused of manslaughtering his infant child. He denied the charges. What happened? He had placed his baby in its baby cart, and waited for a train at a station. He was standing behind the "yellow line". And he wasn't watching too carefully. A freight train sped past on the track adjacent to the platform. The wake of the locomotive grabbed the baby cart (incl baby) and threw it in between train and carriages, the baby (and cart) got smashed to bits. The prosecution believed he must have pushed the cart, but in a rather worrying demonstration, it was proven that a 15kg cart with baby WILL be picked up by the wake of a train and thrown onto the tracks. So, if a train moving at 100kph can do that, imagine the power in the wake of a heavy jet at 8-9 times the speed. (In case you're interested: The verdict was "not guilty" and it resulted in speed restrictions for trains passing through stations)
Jtamu97 From United States of America, joined Mar 2001, 658 posts, RR: 2
Reply 24, posted (12 years 4 months 5 days 19 hours ago) and read 1604 times:
Oh yeah it is real!! I was in a Cessna 152 and a AA 757 crossed over head on approach to DFW. All you can do is enjoy the ride and hope your seat belt does not fail. While wake turbulence is most dangerous behind a heavy, clean, high angle of attack plane (departure) it is real and dangerous to almost any GA aircraft. Also, the streaks seem straight from the ground but in forming off the wing it is usually several streams of air meeting in a swirling fashion. Bottom line, it is real and very dangerous but easily avoidable.
Propeller, we don't need no stinkin propeller
: Just think of it this way... Waterhas the same properties as air when disturbed. So picture yourself riding a small two-person fishing boat going abou
: Wake Turbulence is most at the induced end of the Drag curve. During cruise, wake turbulence is least because the aircraft is now at the parasitic end
: If you watch a contrail for a number of seconds after the 'plane has moved on, you will see that it will begin to curl and rotate as the vortex takes