The A380 and wake turbulence

Originally posted by Affirmative Reaction

Originally posted by RichardPrice

Originally posted by Affirmative Reaction


Simply flying through the jet wash of another fighter won't put you into a flat spin as depicted.

The bad thing about jetwash as opposed to wake turbulence is that jetwash is preheated, and can cause huge problems within the engine if ingested, as it cant be expanded as much as cold air.

This actually causes huge problems with the Harrier, because if the engine ingests hot air it can cause engine failure or loss of power. The Boeing contender for the JSF had problems with this as well, as demonstrated on several occasions.

I can see where this might cause a problem with a possible compressor stall, but only if the trailing bird was flying directly in the jet wash of the lead bird for an extended period of time, at least several seconds. Simply passing through the wash would not be enough to cause a compressor stall, especially of both engines as depicted with the F-14. Single engine aircraft might have a slight "hiccup", but the minute time frame of quick fly through exposure would not cause a catastrophic engine failure.

Actually, ingestion of small amounts of jetwash has caused many harrier losses, and it did used to cause problems within several of General Electrics engines because of a design flaw. Im unsure as to whether the topgun scenario is possible or not, but Maverick was flying within the scope of Icemans diluted jetwash for the majority of that scene, and him flying directly through it could cause problems for the compressors if they were already pushed to limits.

Also another method of crashing the Harrier, a particular favourite with trainee pilots im told, its to cause a compressor stall by rotating the aircraft too quickly while in a hover, which caused a buildup of static air at the entrance of the intakes, blocking proper airflow. Tho I doubt this has anything to do with Hollywood physics

If you want to bash topgun, theres plenty more obvious stuff in it (like an A-6 being depicted as a F-14 lining up for catapult or in pretty much every one of Cruises in-cockpit scenes having the safety bolts still in the ejection seats).

Originally posted by COOL HAND

You forgot the part where the daughters got to get their hands on classified aerial recon. Not exactly the AF's best moment.

Not the AF's best moment??? HA!!! Not HOLLYWOOD's best moment...what a travesty...and how many sequels did they make? Iron Eagle XXXIV?

Originally posted by RichardPrice

Actually, ingestion of small amounts of jetwash has caused many harrier losses, and it did used to cause problems within several of General Electrics engines because of a design flaw. Im unsure as to whether the topgun scenario is possible or not, but Maverick was flying within the scope of Icemans diluted jetwash for the majority of that scene, and him flying directly through it could cause problems for the compressors if they were already pushed to limits.

I'll not argue the Harrier, as I know very little about that particular aircraft, but on the subject of the above scenario, it's false. A flat spin could not be induced in the way depicted, and simply flying through the jet wash of a lead aircraft, regardless of how close to performance limits the engine is pushed, will not cause a compressor stall. Demonstration teams do it on a daily basis, flying in echelon formation, rolling to inwardly opposing 90-degree banks, and crossing. The proof is in the pudding.

Originally posted by Affirmative Reaction
Not the AF's best moment??? HA!!! Not HOLLYWOOD's best moment...what a travesty...and how many sequels did they make? Iron Eagle XXXIV?

Wait a minute: are you telling me that Iron Eagle is not a accurate representation of life in the AF? Aggggg bubble bursted.. How about Dawns Early light? Apart from the bad acting, how about a B-52 that drops a a bomb to take out a persuing fighter, and then decides that thier orders are crap and turn around and not complete thier mission.

Originally posted by RichardPrice
Actually, ingestion of small amounts of jetwash has caused many harrier losses,

Harrier suffers from a major problem: it's extremely sensitive to engine problems. For example, if it's engine fails in a hover, you have to eject immediately unless you happen to be only a few feet off the ground. Add to the fact that's it's a single-engined plane..

British and american companies spend lots of money keeping the Pegasus engine as good as it is. It has to be due to the nature of the plane.

Originally posted by RichardPriceAlso another method of crashing the Harrier, a particular favourite with trainee pilots im told, its to cause a compressor stall by rotating the aircraft too quickly while in a hover, which caused a buildup of static air at the entrance of the intakes, blocking proper airflow.

That sounds weird. Harriers have huge intakes compared to the engine and have many (i forget the name but let's call them "side intakes" dotted around the front of the engine cowling). I wouldn't think lack of air would be an issue for a Harrier...

Originally posted by FredT
For the militant Airbus types, I have a question. How much wake turbulence does the A380 generate? Is it going to require greater separation for aircraft in the pattern? If so, won't that negate some of its advantages by reducing the number of planes that can land etc?

Getting back to the original question.

It's impossible to know "how much" wake turbulence a particular aircraft creates since there's no way to quanitify wake turbulence. The FAA does, however, provide separation requirements between aircraft based on their weight:

Light aircraft: Less than 41,000 Lbs
Large aircraft: 41,000 to 249,000 Lbs
Heavy aircraft: 255,000 Lbs and higher

ATC uses time intervals between similar vs disimilar aircraft for takeoff.

What's important to discuss about this is that the strongest wake turbulence may not necessarily be caused by the heaviest aircraft. An example of this is the B-757.

Approximately 5-10 years ago the FAA modified its definition of a "heavy" aircraft. It used to be 300,000 Lbs. But problems with other aircraft encounters with wake turbulence behind B-757's prompted the FAA to change the sepration requirements of aircraft following B-757's to the same as if they were following a heavy aircraft; even though the B-757 didn't fall into this category. Later, the FAA reduced the "heavy' criteria to 255,000 Lbs or greater.

So it's not only the weight of the aircraft that determines the strenghth of wake turbulence; but so too does the design of the wing. The Airbus 380 will come with winglets (those vertical fins located at the tips of the wings) which are designed to reduce the amount of air lost (spilled) over the wingtips and will therefore reduce wake turbulence. Thoug this is a positive effect of winglets, this is not their primary purpose. When less air is lost over the wingtips, the wing becomes more efficient as less induced drag is produced.

It may be interesting to know that if a hypothetical wing had no wingtips, it would be highly efficient as no induced drag would be created (and no wake turbulence). Of course, in order to accomplish this, this hypothetical wing's span would have to strech around the world and meet up with itself on the other side.

Originally posted by FredT

Wait a minute: are you telling me that Iron Eagle is not a accurate representation of life in the AF? Aggggg bubble bursted.. How about Dawns Early light? Apart from the bad acting, how about a B-52 that drops a a bomb to take out a persuing fighter, and then decides that thier orders are crap and turn around and not complete thier mission.

Well, not unless there is another Air Force that I HAVEN'T been serving in for the last 20 years!!!

I have never heard of Dawn's early light...I'll have to look on Netflix for it...or do I really want to????

Originally posted by Freedom_for_sum


It may be interesting to know that if a hypothetical wing had no wingtips, it would be highly efficient as no induced drag would be created (and no wake turbulence). Of course, in order to accomplish this, this hypothetical wing's span would have to strech around the world and meet up with itself on the other side.

While I agree with most of what you have posted, I must point out that there can be no such thing as a wing that produces no induced drag. The wingtip is not the only reason drag is produced, or where wake turbulence escapes the wing.

As a wing produces lift, it deflects the airflow along a curved path. The resulting force is known as �total reaction force�.

If the total reaction force were exactly at right angles to the direction of flight, then there would be lift with no induced drag. Due to the camber of the wing and the deflection of the airflow aft over the wing, this is not possible.

While winglets do decrease wake turbulence, on some aircraft as much as 60 or 70 percent, it is still a product of lift, and cannot be totally quashed on conventional aircraft.

Can't believe I still remember this stuff after all these years! Maybe someone will hire me after I retire, eh???

Originally posted by Affirmative Reaction
I have never heard of Dawn's early light...I'll have to look on Netflix for it...or do I really want to????

Do yourself a favor and just say no AR. It isn't worth your time to see it. I would recommend waiting to see it on cable, they play it all the time.

Originally posted by Affirmative Reaction

Originally posted by Freedom_for_sum


...The wingtip is not the only reason drag is produced, or where wake turbulence escapes the wing.

Really? Where else, besides the wingtip, does wake turbulence "escape" the wing?

Originally posted by Affirmative Reaction


As a wing produces lift, it deflects the airflow along a curved path. The resulting force is known as �total reaction force�.

If the total reaction force were exactly at right angles to the direction of flight, then there would be lift with no induced drag. Due to the camber of the wing and the deflection of the airflow aft over the wing, this is not possible.

You are correct that the lift vector (reaction force) is angled back to some degree and that this angle is the same angle as induced angle of attack. But what causes the lift vector to angle back? By definition, angle of attack is the angle between the chordline of the wing and the relative wind. Induced angle of attack is the angle between the downwash (created by the air lost over the wingtips) and the relative wind. A wing with no wingtips would produce no downwash and, therefore, would have 0 induced angle of attack; the lift vector would be 90 degrees to the relative wind; and there would be no induced drag.

I recommend "Aerodynamics for Naval Aviators" as a good reference for aerodynamic information.

I apologize for my clumsiness with the quotes on my previous post. I'm still trying to figure out how to use this system.

I decided to look up this aero stuff in my copy of "Aerodynamics for Naval Aviators". If you have a copy, look on pages 63-68. There it gives a very good explanation of induced AOA/Drag and lift associated with induced AOA.

From page 68: "If the aspect ratio were infinite, the induced angle of attack would be zero and the aerodynamic characterstics of the wing would be identical with the airfoil section properties."

An infinite aspect ratio is a wing with no wingtips.

Originally posted by Freedom_for_sum
I apologize for my clumsiness with the quotes on my previous post. I'm still trying to figure out how to use this system.

I decided to look up this aero stuff in my copy of "Aerodynamics for Naval Aviators". If you have a copy, look on pages 63-68. There it gives a very good explanation of induced AOA/Drag and lift associated with induced AOA.

From page 68: "If the aspect ratio were infinite, the induced angle of attack would be zero and the aerodynamic characterstics of the wing would be identical with the airfoil section properties."

An infinite aspect ratio is a wing with no wingtips.

The point is that this is not possible. Because the "wingtip-less wing" you are speculating about does not and cannot exist, it is a moot point. The fact is that the total reaction force cannot be exerted at right angles to the direction of flight, therefore there is induced drag.

Where else is wake turbulence created? While the wingtip vortices are the greatest factor in wake turbulence, it is created all along the wing as the relative wind slips over it and increases speed over the top of the wing. There are wing-root vortices, as well as fuselage vortices. These all contribute to wake turbulence, which is one reason the air force went to aft body strakes on the C-17. Besides the increased performance, handling and stability they lend to the aircraft, they have the effect of channeling the wing root and body vortices under the aircraft, eliminating separation, and reducing wake turbulence produced.

Again, winglets do a fantastic job of reducing wingtip vortices and thus wake turbulence, but if they didn't do much, much more than that, they most likely wouldn't be used. They increase climb performance dramatically, allowing greater fuel efficiency as climb power can be set earlier and lower, which translates to lower EGT, lending to increased turbine life and thus lower maintenance costs. Increased takeoff performance also translates into a greater ACL and payload range, thus increasing revenues.


Additionally, I would never rely on a Navy publication on aerodynamics�those crazies fly off BOATS!!!!

Originally posted by Affirmative Reaction
Additionally, I would never rely on a Navy publication on aerodynamics�those crazies fly off BOATS!!!!

Hey, don't knock it till you try it.

Most fun you can have without taking off clothes. The landings on the other hand...

Originally posted by COOL HAND


Hey, don't knock it till you try it.

Most fun you can have without taking off clothes. The landings on the other hand...

Hey, I had to fly back and forth between Clark AB and Subic Bay numerous times on an E-2 while I was stationed in the PI...worst flights I have EVER been on in my LIFE!!! The wings are so stiff, and yes, I know they have to be, but they have no give what so ever, and it's the most bone jarring ride in the world, especially when you fly over the mountain and hit the "Luzon Bump".....

Never again, thank you....

Originally posted by Affirmative Reaction

Originally posted by Freedom_for_sum
I apologize for my clumsiness with the quotes on my previous post. I'm still trying to figure out how to use this system.

I decided to look up this aero stuff in my copy of "Aerodynamics for Naval Aviators". If you have a copy, look on pages 63-68. There it gives a very good explanation of induced AOA/Drag and lift associated with induced AOA.

From page 68: "If the aspect ratio were infinite, the induced angle of attack would be zero and the aerodynamic characterstics of the wing would be identical with the airfoil section properties."

An infinite aspect ratio is a wing with no wingtips.

The point is that this is not possible. Because the "wingtip-less wing" you are speculating about does not and cannot exist, it is a moot point. The fact is that the total reaction force cannot be exerted at right angles to the direction of flight, therefore there is induced drag.

Where else is wake turbulence created? While the wingtip vortices are the greatest factor in wake turbulence, it is created all along the wing as the relative wind slips over it and increases speed over the top of the wing. There are wing-root vortices, as well as fuselage vortices. These all contribute to wake turbulence, which is one reason the air force went to aft body strakes on the C-17. Besides the increased performance, handling and stability they lend to the aircraft, they have the effect of channeling the wing root and body vortices under the aircraft, eliminating separation, and reducing wake turbulence produced.

Again, winglets do a fantastic job of reducing wingtip vortices and thus wake turbulence, but if they didn't do much, much more than that, they most likely wouldn't be used. They increase climb performance dramatically, allowing greater fuel efficiency as climb power can be set earlier and lower, which translates to lower EGT, lending to increased turbine life and thus lower maintenance costs. Increased takeoff performance also translates into a greater ACL and payload range, thus increasing revenues.


Additionally, I would never rely on a Navy publication on aerodynamics�those crazies fly off BOATS!!!!

I only used the example of the "wing-tip-less wing to illustrate the point that it is solely wingtips (and their vortices) that cause induced drag; without wingtips, there would be no induced drag.

The turbulence you describe along the wing root and fuselage; etc, is not wake turbulence as it applies to wingtip vortices and has nothing to do with induced AOA or induced drag. This is called interference drag and falls under the category of parasite drag. You are correct about the strakes (they're used on several aircraft) in that they streamline the turbulent airflow, created by interference drag, along the vertical fin to improve directional stability.

All jest and inter-service ribbing aside, because of the flying they do, the Navy boys have a viceral appreciation and understanding of aerodyamics. I almost hate to admit it as an Air Force trained pilot

Originally posted by Cjwinnit

Originally posted by RichardPrice
Actually, ingestion of small amounts of jetwash has caused many harrier losses,

Harrier suffers from a major problem: it's extremely sensitive to engine problems. For example, if it's engine fails in a hover, you have to eject immediately unless you happen to be only a few feet off the ground. Add to the fact that's it's a single-engined plane..

British and american companies spend lots of money keeping the Pegasus engine as good as it is. It has to be due to the nature of the plane.

Originally posted by RichardPriceAlso another method of crashing the Harrier, a particular favourite with trainee pilots im told, its to cause a compressor stall by rotating the aircraft too quickly while in a hover, which caused a buildup of static air at the entrance of the intakes, blocking proper airflow.

That sounds weird. Harriers have huge intakes compared to the engine and have many (i forget the name but let's call them "side intakes" dotted around the front of the engine cowling). I wouldn't think lack of air would be an issue for a Harrier...

The Harrier needs a massive amount of air to do what it does in a hover. When rotating the aircraft too quickly, a static 'bubble' of air builds up on the protected side of the aircraft during the turn, thus choking the airflow on that side. The springload intakes around the edge of the main intake wont work in this case because the clear intake is already ingesting as much as it can.

Originally posted by Freedom_for_sum



I only used the example of the "wing-tip-less wing to illustrate the point that it is solely wingtips (and their vortices) that cause induced drag; without wingtips, there would be no induced drag.

The turbulence you describe along the wing root and fuselage; etc, is not wake turbulence as it applies to wingtip vortices and has nothing to do with induced AOA or induced drag. This is called interference drag and falls under the category of parasite drag. You are correct about the strakes (they're used on several aircraft) in that they streamline the turbulent airflow, created by interference drag, along the vertical fin to improve directional stability.

I hate to correct you, as I really don't want to get into an argument about this, especially with a fellow Air Force guy, but I never said that wing root or fuselage drag did have anything to do with induced drag, as it applies to AOA. However, hypothetical lack of wing tips, regardless of the fact that it is not possible, would not eliminate wake turbulence. For that to be true, all air passing over the wing would have to escape at the wing tip. That is not the case. Air slips over the trailing edge of the wing all along its length, causing wake turbulence, albeit at a less violent rate, from wing tip to wing root, regardless of induced or other kinds of drag.

In addition, "interference drag" is not, as you stated, what is caused by the fuselage. Defined, interference drag is "the effect of an aerodynamic component on another: wing to body, wing to nacelle, vertical to horizontal tail, junctions in general, biplane, ground effect, free surface problems in hydro- dynamics, and more." Friction drag is the correct term, and it contributes to the creation of vortices along the wing root and fuselage. Interference drag and induced drag combine at the wing root to create vortices as well. The effect at the wing root is obviously less for swept wing aircraft than non or forward swept wings. All vortices created by an aircraft, regardless of where they are created or what type of drag creates them, contribute to the total amount of wake turbulence created by the aircraft.

These do fall under the heading of parasite drag, which is the sum of pressure drag and skin friction. The equation for parasite drag computation is, "Dp = CDp x S x � r V2", where Dp = Parasite Drag, CDp= Coefficient of Parasite Drag, S = Wing area, r = Air density, and V = Velocity.

Aft body strakes do much more than simply add stability, as I mentioned. Among other things, they redirect and combine airflow to reduce the severity of wake turbulence. Wind tunnel tests show where they can, along with winglets, significantly reduce the effects of wake turbulence.

I need to get up in the attic and see if my Advanced Aerodynamics books are still there.

Right now, it�s been too long. My brain hurts!!!

I am having dificulty following your logic. In the first paragraph you state:

Originally posted by Affirmative Reaction

...but I never said that wing root or fuselage drag did have anything to do with induced drag, as it applies to AOA.

The in the second paragraph you say:

Originally posted by Affirmative Reaction

Interference drag and induced drag combine at the wing root to create vortices as well.

In reference to the original post there is only one place on the wing where the higher pressure air beneath the wing migrates to the relatively lower pressure air above the wing and that is at the wingtips. This is what causes wake turbulence, or wingtip vortices, which are one in the same. The migration of air is the only thing that causes induced AOA and its corresponding induced drag. Other items which cause turbulent air (where the wing meets the fuselage; antenae; hangar rash, bird crap; etc) are parasite drag and do not fall under the classical deffinition of "wake turbulence", even though they do cause turbulent air.

I can't tell if we agree on theory or disagree on semantics. Perhaps we can at this point say we respecfully agree to diagree on some things.

Originally posted by Freedom_for_sum


In reference to the original post there is only one place on the wing where the higher pressure air beneath the wing migrates to the relatively lower pressure air above the wing and that is at the wingtips. This is what causes wake turbulence, or wingtip vortices, which are one in the same. The migration of air is the only thing that causes induced AOA and its corresponding induced drag. Other items which cause turbulent air (where the wing meets the fuselage; antenae; hangar rash, bird crap; etc) are parasite drag and do not fall under the classical deffinition of "wake turbulence", even though they do cause turbulent air.

I can't tell if we agree on theory or disagree on semantics. Perhaps we can at this point say we respecfully agree to diagree on some things.

Ok, wait a minute here...are you saying that the ONLY portion of the wing where air passes from higher pressure to lower pressure is the wingtip?

If that were true, the only portion of the wing that would create lift would BE the wingtip. The entire aircraft creates lift, but the wings create the majority. Air flows over every surface of the craft, and does NOT just escape the wings at the wing tips.

Wake turbulence and wingtip vortices are not one and the same, one is the produce of the other. Wingtip vortices are the main contributor to wake turbulence, but they are not the sole perpetrator. Simple aerodynamics, which point to turbulence being created by every part of the aircraft, lift being created by every part of the aircraft, and vortices being created by several points on the aircraft prove that conclusively. To say that wingtip vortices are the ONLY cause of wake turbulence is absolutely incorrect.

Originally posted by Affirmative Reaction

Ok, wait a minute here...are you saying that the ONLY portion of the wing where air passes from higher pressure to lower pressure is the wingtip?

YES

Originally posted by Affirmative Reaction

If that were true, the only portion of the wing that would create lift would BE the wingtip.

No. The wingtip is the ONLY portion where energy (lift) is being lost.

Originally posted by Affirmative Reaction

The entire aircraft creates lift, but the wings create the majority. Air flows over every surface of the craft, and does NOT just escape the wings at the wing tips.

In terms of induced lift/drag (and wake turbulence) the only important factor here is the wingtips and the airflow lost over them.

quote]Originally posted by Affirmative Reaction

For some reason, the system won't let me "quote" the following:

"Wake turbulence and wingtip vortices are not one and the same, one is the produce of the other. Wingtip vortices are the main contributor to wake turbulence, but they are not the sole perpetrator. Simple aerodynamics, which point to turbulence being created by every part of the aircraft, lift being created by every part of the aircraft, and vortices being created by several points on the aircraft prove that conclusively. To say that wingtip vortices are the ONLY cause of wake turbulence is absolutely incorrect."



Not to sound condescending: But I gotta say that at this point that I'm done kicking a dead horse. I'm crying UNCLE! You need to do some research on basic aerodynamics because you keep confusing the differences between induced drag and parasite drag. Regardless, we've both drifted away from the basic topic of this forum which, if I remember correctly, had to do with wake turbulence and separation limitations of the Airbus 380.

By the way: Airbus SUCKS!