Skywatch From United States of America, joined Mar 2005, 923 posts, RR: 5 Posted (8 years 9 months 1 week 2 days 15 hours ago) and read 6496 times:
I have heard some educated guesses, but have never had a sure answer about what makes an aircraft a Heavy? Is it weight, fluid displacement, ect....
If it is weight, is it based on empty weight or total payload weight? If it is based on payload weight, can an aircraft take off of the runway as a heavy
and land without the heavy status because of burned fuel?
Klaus From Germany, joined Jul 2001, 21346 posts, RR: 54 Reply 4, posted (8 years 9 months 1 week 2 days 15 hours ago) and read 6448 times:
Skywatch: I have not heard 757's refered to as heavies. Are you sure? Also, is a DC-8 a heavy?
It was discovered that the 757 produces much stronger wake turbulence than other narrowbody aircraft, so it enforces separation distances comparable to "heavy" widebodies. It´s a special exemption primarily due to aerodynamic effects, not actual weight.
QantasA332 From Australia, joined Dec 2003, 1500 posts, RR: 34 Reply 8, posted (8 years 9 months 1 week 2 days 12 hours ago) and read 6345 times:
Quoting 2H4 (reply 5): Think the upcoming winglets on 757's will alleviate that wake turbulence, or is it mainly a result of the trailing edge flap configuration?
Keep in mind that a large portion of winglet operation is the production of thrust through a forward vectoring of the resultant freestream-vortex flow at the wingtips, an effect which doesn't relate directly to wake turbulence strength at all. The reduction of vortex strength is the other (sometimes smaller) part of winglet operation, which does affect wake strength, but it is probably not major enough to significantly reduce the 757's wake turbulence strength. Having said that, the main source of the 757's disproportionately strong wake is indeed its general wing-flap geometry (as Klaus correctly stated), anyway, so with that in mind winglets would have an even smaller overall effect on the wake strength.
Whether the 757's wake really is so grossly out of proportion to its size and weight is another much-debated question...
WakeTurbulence From United States of America, joined Apr 2004, 1291 posts, RR: 18 Reply 9, posted (8 years 9 months 1 week 2 days 11 hours ago) and read 6324 times:
I have heard in this forum that UA also has or had some 752's that qualify as heavies over 255,000 lbs, in addition to ATA. I agree with Qantas332 that winglets won't reduce the 752's wake that much. I have read winglets help reduce it, but by minimal amounts.
Quoting QantasA332 (reply 8): Whether the 757's wake really is so grossly out of proportion to its size and weight is another much-debated question...
In 1993 a 757 caused a private jet to crash with the founder of In N Out on board during final to SNA, killing all on board. Separation was a key factor, but so was the 752's wake.
2H4 From United States of America, joined Oct 2004, 8955 posts, RR: 61 Reply 20, posted (8 years 9 months 1 week 14 hours ago) and read 5967 times:
AIRLINERS.NET CREW HEAD DATABASE EDITOR
Quoting Skywatch (Reply 19): I will ask yet again: Does the payload weight increase wake turbulence? Likewise, can, for example, 747A be full of pax, and 747B empty, and they still generate the same amount of wake turbulence?
No. The heavier an aircraft is, the more lift it has to produce to remain airborne. The more lift it produces, the stronger the wingtip vorticies become. Stronger wingtip vorticies create stronger wake turbulence.
The only part I'm not totally clear on is how the trailing-edge flap design contributes to the wake turbulence. Perhaps someone more knowledgeable than myself can explain things further.
Skywatch From United States of America, joined Mar 2005, 923 posts, RR: 5 Reply 22, posted (8 years 9 months 1 week 2 hours ago) and read 5921 times:
2H4: Flaps generate wake turbulence by a process called the Bernoulli Principle. This can be defined as "the sideward pressure exerted by a moving fluid decreases as the fluid's velocity increases." This is the reason for the airfoil design on wings. The reason this produces lift is because the air traveling over the top of the wing has to travel faster than the air on the bottom of the wing to meet up with the air from the bottom side of the wing. If you add flaps to this, you can imagine how this would upset this process. Lots of miniature vortices resembling whirlpools are formed, causing the extra wake. And, the faster it is going, with the given degrees of flaps, the more wake turbulence there will be.
Skywatch From United States of America, joined Mar 2005, 923 posts, RR: 5 Reply 23, posted (8 years 9 months 6 days 14 hours ago) and read 5851 times:
Wait a minute everyone! I looked up some stats on the 757 and 767, and their gross empty weights are below 255,000 lbs by a long shot! So what is the real weight criteria for a heavy? It can't be 255,000lbs! Or does that MGTOW mean something that I don't know about?
Newark777 From United States of America, joined Dec 2004, 9348 posts, RR: 31 Reply 24, posted (8 years 9 months 6 days 14 hours ago) and read 5843 times:
Quoting Skywatch (Reply 23): Wait a minute everyone! I looked up some stats on the 757 and 767, and their gross empty weights are below 255,000 lbs by a long shot! So what is the real weight criteria for a heavy? It can't be 255,000lbs! Or does that MGTOW mean something that I don't know about?
I'm pretty sure MGTOW stands for Maximum Gross Take Off Weight, which is quite different than empty weight.
Why grab a Heine when you can grab a Busch?
25 OPNLguy: In the begining, a "heavy" was defined as any aircraft that was capable of a max gross takeoff weight MGTOW) of 300,000 lbs. or more, irrespective of
26 Bri2k1: Please do a thorough search of this forum before posting anything like "the air has to travel faster to meet up with the air on the bottom." Air is no
27 Skywatch: Would some 752's have greater takeoff capacities than other 752's because of different engine manufacturers?
28 OPNLguy: >>>Would some 752's have greater takeoff capacities than other 752's because of different engine manufacturers? Can't say, as I'm not familar with tho
29 Starlionblue: As Bri2k1 says, this explanation for lift is flawed. Try this instead (keep reading a few pages on): http://travel.howstuffworks.com/airplane3.htm
30 CRJ200Mechanic: Okay that article just threw everything I learned in school out the window.
31 Skywatch: What the article said is basically what I had in mind, but I did not want to type that much! Thanks for keeping me in check.
32 Sprout5199: Same here. I guess its just PFM. (pure F$%king magic) Dan in Jupiter
33 Starlionblue: Yes, for example AAs RB211 powered 752s have higher MTOW than the Pratt powered ones. Isn't life fun
34 Bri2k1: It IS fun. Flying requires faith, because you certainly can't "see" lift - just it's effects. Humans don't necessarily understand every single thing a