Lehpron From United States, joined Jul 2001, 7027 posts, RR: 31 Posted (4 years 7 months 1 week 6 days 4 hours ago) and read 1449 times:
Imagine an A380 landing. Now imagine a much smaller airplane landing with A380's mass.
Would the smaller plane end up having a greater wake turbulence, or wake vortex spin?
What about cruise (for the purposes of this question assume they have the same speed and altitude), overall wake is smaller due to lower coeff of lift; but still, which is greater?
[Edited 2005-04-01 08:46:11]
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TripleDelta From Croatia (Hrvatska), joined Jul 2004, 815 posts, RR: 5 Reply 1, posted (4 years 7 months 1 week 6 days 2 hours ago) and read 1326 times:
Personally, I'd say that the wake turbulence would depend on the area of the wing of the smaller a/c. The wing, no matter how big, still has to create enough lift to counteract the weight of the a/c, which is the same in both cases, as the mass is the same and g is the same.
If the smaller aircraft had a wing of lower area than the A380 (assuming that the aspect ratios of that and the A380's wing are the same), it would have to produce a huge pressure difference above and below to produce enough lift and offset its lower area, leading to a greater coefficient of lift, thus also greater induced drag and greater (stronger) wake turbulence...
Lehpron From United States, joined Jul 2001, 7027 posts, RR: 31 Reply 2, posted (4 years 7 months 1 week 5 days 18 hours ago) and read 1264 times:
Initially it appeared to me as a contradiction, you're right though that it would depend on the wing area and would end up with a massive lift coefficient on the smaller plane.
The idea that it could have a stronger wake still throws me off, granted the smaller plane would have to be contructed of really dense material, I wonder how they can't be the same. I did do a quick mathematical and if as you've stated that the aspect ratios were the same it would simply be the ratio of the wing areas.
Weird idea though, thanks for replying.
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N60659 From United States, joined Mar 2005, 654 posts, RR: 48 Reply 3, posted (4 years 7 months 1 week 5 days 15 hours ago) and read 1231 times:
Quoting Lehpron (Reply 2): Initially it appeared to me as a contradiction, you're right though that it would depend on the wing area and would end up with a massive lift coefficient on the smaller plane
The implicit assumption there is that the spanwise cross-section or aerofoil is proportionately the same for both aircraft.
IIRC weight doesn't have any direct contribution to wake turbulence. Aircraft with high wake turbulence are classified as "heavies" because of the force of lift required to counter the weight of the aircraft is higher in heavier aircraft.
727EMflyer From United States, joined Mar 2005, 547 posts, RR: 1 Reply 4, posted (4 years 7 months 1 week 5 days 7 hours ago) and read 1181 times:
Wake turbulence is a function of many factors: Weight, speed, wingspan, and configuration. Weight is definitely the biggest factor, followed by speed and configuration as evidenced by the truth that the strongest wake turbulence is caused by heavy, slow aircraft in a clean configuration. In all reality we can dismiss wing span as a factor since a larger wing will be needed to support your weight, and unless you are a high speed delta wing type, that wing area will be realized as span.
Now, I would guess that your smaller aircraft might have a more concentrated effect on trailing aircraft since two vortices would theoretically be closer together. I imagine a light single being pushed directly down by both vortices hitting the tops of both wings rather than being carried in a circle by one of them.
Lehpron From United States, joined Jul 2001, 7027 posts, RR: 31 Reply 5, posted (4 years 7 months 1 week 4 days 16 hours ago) and read 1141 times:
Quoting 727EMflyer (Reply 4): Now, I would guess that your smaller aircraft might have a more concentrated effect on trailing aircraft since two vortices would theoretically be closer together
Quoting N60659 (Reply 3): Aircraft with high wake turbulence are classified as "heavies" because of the force of lift required to counter the weight of the aircraft is higher in heavier aircraft.
Hence the downwashes are going to be stronger and maybe ground-effect height could be higher.
Seems that the effect may be similar whether in landing or cruise config...?
The meaning of life is curiosity; we were put on this planet to explore opportunities.
ReguPilot From Puerto Rico, joined Jan 2004, 466 posts, RR: 41 Reply 6, posted (4 years 7 months 1 week 4 days 7 hours ago) and read 1104 times:
Quoting N60659 (Reply 3): Aircraft with high wake turbulence are classified as "heavies" because of the force of lift required to counter the weight of the aircraft is higher in heavier aircraft.
I'm am completely new in the subject (and the first time ever to visit this forum) so, is that the reason why the 757's are considered heavies (at least in Flight Simulator)???
TripleDelta From Croatia (Hrvatska), joined Jul 2004, 815 posts, RR: 5 Reply 8, posted (4 years 7 months 1 week 4 days 6 hours ago) and read 1096 times:
ReguPilot: wake turbulence categories are determined by weight, as N60659 stated. According to ICAO regulations (Annex 2 - Rules Of The Air), wake turbulence categories go like this:
LIGHT - below and including 7,000 kg
MEDIUM - 7,000 - 135,000 kg
HEAVY - 135,000 kg and more
These depend on the aircraft MTOW, not it's "current" weight for a specific flight - and none of the 757 variants have a MTOW above 122,000 kg if I'm not mistaken, so it may be a mistake of the FS model designer. Though, I gather that this division will be changed soon (there's a lot of talk around about that), since the A380 and the more and more frequent An-124 need a category of their own.
Newark777 From United States, joined Dec 2004, 8796 posts, RR: 40 Reply 11, posted (4 years 7 months 1 week 2 days 9 hours ago) and read 990 times:
Quoting TripleDelta (Reply 8): and none of the 757 variants have a MTOW above 122,000 kg if I'm not mistaken, so it may be a mistake of the FS model designer
The -300 and some -200's fall above the threshold, making them heavy aircraft.
TripleDelta From Croatia (Hrvatska), joined Jul 2004, 815 posts, RR: 5 Reply 14, posted (4 years 7 months 1 week 15 hours ago) and read 893 times:
Newark777: could be a local operating procedure, because technically the 757 does not fall into the ICAO heavy category... unless the FAA has it's own division (for domestic/internal flights maybe), though I must admit my lack of information about FAA regs
Yikes! From Canada, joined Oct 2001, 268 posts, RR: 2 Reply 15, posted (4 years 7 months 1 week 6 hours ago) and read 837 times:
The 757 -200 has always been classified as a heavy by ATC strictly for wake turbulence separation purposes. It was one of the first of the modern airliners to be able to use and perfect the managed drag style approach which has it, on occasion intercepting the glideslope from above at ranges of less than 10 miles to touchdown. Consequently, most other aircraft end up in its wake.
One very important mitigating factor in newer wing designs, adding to the discussion about wing area is aspect ratio. There is much less induced drag on the newer design of wing. Notwithstanding the massive amount of air displaced regardless by something as massive as an A380.
TripleDelta From Croatia (Hrvatska), joined Jul 2004, 815 posts, RR: 5 Reply 16, posted (4 years 7 months 6 days 17 hours ago) and read 796 times:
Yikes: hmm... didn't know that. I've heard of the 757's legendary climb performance only - but apparently its descent performance isn't far behind... Well then, if it is a special approach, it makes sense to separate it from the common folk then .
EDIT: But wouldn't then separating it from the rest of the flock be just a matter of distance, or do those special approaches really cause a lot of wake turbulence on account of the maneuvers involved?
Yikes! From Canada, joined Oct 2001, 268 posts, RR: 2 Reply 18, posted (4 years 7 months 6 days 12 hours ago) and read 777 times:
Quoting TripleDelta (Reply 16): Yikes: hmm... didn't know that. I've heard of the 757's legendary climb performance only - but apparently its descent performance isn't far behind... Well then, if it is a special approach, it makes sense to separate it from the common folk then .
EDIT: But wouldn't then separating it from the rest of the flock be just a matter of distance, or do those special approaches really cause a lot of wake turbulence on account of the maneuvers involved?
The Boeing product, at least the hi-bypass birds, do this type of approach really nicely. When done correctly, the power comes back to idle at altitude then spools up at about 500' AGL. Unfortunately it is sometimes compromised by STAR's that force the arrival into windows that are not fuel efficient.
And yes, distance is the way to separate the wheat from the chaff!
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