Aerodynamics thread

[Air Force Brat]

This is what I call a self-contradictory argument. You argued not having lift is bad due to lack of maneuverability, at the same time that you are arguing having lift is bad due to drag.

At cruise, the best location to keep a canard is not a position at minimum lift and minimum drag, but at a position that maximizes the lift and minimizes the drag for the entire aircraft. That is also known as optimizing the lift/drag ratio. There is no requirement that canard has to create zero lift.

As to the canard having to move frequently, it is not unique on the J-20 as fighters with tailplane also face the same problem.

Please, Log in or Register to view URLs content!

. After all, the whole point about instability is to enable tiny control surface movement to cause large change in the aircraft's motion.

When it comes to wing loading, the metric is only useful to aircraft that purely rely on wing for lift such as airliners and military transports. The metric is nearly useless in gauging performance of modern fighters, starting from the F-16. The reason being is that modern fighters also employ the effects of vortex for lift. This is done by extending the root of the wing forward, resulting in a surface that slices the air at an angle, causing the air to swirl and creating more suction above the wing. The vortex effect increases as the angle-of-attack of the aircraft increases, in essence allowing the aircraft to get more lift when it needed the most.

In addition to the vortex effect I described above, the J-20 also generate another vortex system through the canard. This is achieved whenever the canard is in a non-zero angle-of-attack position. Unlike the vortex system earlier, the system from the canard is also tunable, resulting in many aerodynamic wonders that is not doable with a traditional configuration. The flat belly of J-20 also acts as a lifting surface, turning most of the fuselage into a wing, which creates more lift.

So, on J-20 there are three methods of generating lift in total. At cruise, most of the lift comes from the delta wing, minimizing drag. In a turn, the total amount of lift can be ramped up to 1.8 times of what the wing is capable of producing. So in your scenario, a J-20 pilot would have no problem making sharp turns to get behind an opponent's tail.

Yeah Sam, he does have you here buddy, the Eng understands the aero's of the J-20, even though he is an "ornery cuss",, funny to see you lads banished to the aerodynamics thread, always happens when we get into a great discussion of the J-20, that actually has something to do with the airplane, everyone likes the pretty pictures, but they don't give a rats rear end about what makes it great, or not???? the last post here was Aug of 2013 by me,,,,,, if we had any sense, we would make this THE preeminent thread on Sino Defense.

Hi Eng, sorry to refer to you sir as an "ornery cuss", but I did get a little chuckle, I'm surprised one of the mods didn't move that whole conversation over here, LOL

and Sam, as much as I love your presence on here to keep us honest, the Eng is right, the canards contribute to total overall lift and stabilators "always" create a downforce, or negative lift on the tail, as they say, "that's just the nature of the beast", but like you, I am still a fan of the "conventional configuration", but I do see the beauty of the canard????

don't worry Eng, I've got your six, and I liked your discussion! brat

 

[Deino]

I just moved about 40 complete-engine-unrelated posts from the Engine-development tread to this one ... therefore I beg You all no more post on aerodynamics and the pros and cons for canards or so in the engine tread ... continure here please:


Deino

 

[delft]

A historical note on vortex lift:
It becomes stable to a high angle of attack when the wing leading edge sweep angle is 57 degrees or higher. The first aircraft with 57 degrees wing leading edge sweep were MiG-19 and Su-7. The English Electric Lightning ( that's a historical name ) was given 60 degrees when in a test aircraft 50 degrees didn't cut the mustard, apparently with little insight on the side of the designers. That 57 degrees was told me as something new during a lecture organized by the NATO research arm AGARD in the early '70's.

 

[Air Force Brat]

A historical note on vortex lift:
It becomes stable to a high angle of attack when the wing leading edge sweep angle is 57 degrees or higher. The first aircraft with 57 degrees wing leading edge sweep were MiG-19 and Su-7. The English Electric Lightning ( that's a historical name ) was given 60 degrees when in a test aircraft 50 degrees didn't cut the mustard, apparently with little insight on the side of the designers. That 57 degrees was told me as something new during a lecture organized by the NATO research arm AGARD in the early '70's.

Lots of the science of aerodynamics have changed Master Delft, and there seem to be few hard and fast rules, the fact that the F-35A was taken to 73 degrees AOA and flown out is a bit of an amazement, though I have seen the Raptor taken well beyond that to the vertical, but it does have OVT, welcome aboard, no one stays here long, but this is where all the good conversations seem to end up. LOL brat out

 

[thunderchief]

This is what I call a self-contradictory argument. You argued not having lift is bad due to lack of maneuverability, at the same time that you are arguing having lift is bad due to drag.

At cruise, the best location to keep a canard is not a position at minimum lift and minimum drag, but at a position that maximizes the lift and minimizes the drag for the entire aircraft. That is also known as optimizing the lift/drag ratio. There is no requirement that canard has to create zero lift.

As to the canard having to move frequently, it is not unique on the J-20 as fighters with tailplane also face the same problem.

Please, Log in or Register to view URLs content!

. After all, the whole point about instability is to enable tiny control surface movement to cause large change in the aircraft's motion.

When it comes to wing loading, the metric is only useful to aircraft that purely rely on wing for lift such as airliners and military transports. The metric is nearly useless in gauging performance of modern fighters, starting from the F-16. The reason being is that modern fighters also employ the effects of vortex for lift. This is done by extending the root of the wing forward, resulting in a surface that slices the air at an angle, causing the air to swirl and creating more suction above the wing. The vortex effect increases as the angle-of-attack of the aircraft increases, in essence allowing the aircraft to get more lift when it needed the most.

In addition to the vortex effect I described above, the J-20 also generate another vortex system through the canard. This is achieved whenever the canard is in a non-zero angle-of-attack position. Unlike the vortex system earlier, the system from the canard is also tunable, resulting in many aerodynamic wonders that is not doable with a traditional configuration. The flat belly of J-20 also acts as a lifting surface, turning most of the fuselage into a wing, which creates more lift.

So, on J-20 there are three methods of generating lift in total. At cruise, most of the lift comes from the delta wing, minimizing drag. In a turn, the total amount of lift can be ramped up to 1.8 times of what the wing is capable of producing. So in your scenario, a J-20 pilot would have no problem making sharp turns to get behind an opponent's tail.

Few points here :

- Aerodynamics is an art of compromise . You have aircraft optimized for low speed (high lift) like A-10. You have aircraft optimized for high speed (low drag) like SR-71. Everything else falls in between, and you have to balance between maneuverability (or better said agility) and speed. Usually, they are mutually exclusive.

- Unfortunately you cannot force canard to generate high lift and low drag at the same time Therefore, it is usually kept at a position that keeps minimum drag (and oscillating a little around that position for unstable fighters ) .

- Vortex lift is not panacea, nor is something new and unusual . Delta shaped fighters used vortex lift to improve maneuverability since Mig-21 and similar. But vortex lift doesn't magically solve all of the problems of delta fighters and it doesn't negate wing loading. If you look at delta-fighters optimized for high maneuverability like Rafale or Tejas, they have relatively low wing loading and lower top speed then contemporary fighters .

- I never said J-20 could not turn sharply. I said, it could but it would loose energy rapidly. After that maneuver you would have slow aircraft that is not designed for slow speed. Worse, you would have aircraft that cannot accelerate rapidly because of relatively obsolete engines . Therefore, IMHO , J-20 would avoid knife-fights, at least until it gets better engines (WS-15, 117S or AL-31F M2 )

 

[Inst]

Engineer's sophistry is quickly getting old; this is a quick and easy loadout:

Price/Unit Cost: The F-22 is no longer in production. In 2007, the unit cost of the F-22A was $136.2 million ($148.7 million flyaway cost or $179.7 million incl. support costs). The airframe costs $87.74 million, the F119-PW-100 engine costs $10.03 million, and the avionics cost $28.36 million

Please, Log in or Register to view URLs content!

So tell me again that airframe costs make up relatively little of aircraft costs. Even if it were true, the fact is that the radar and engines tend to be scaled to airframe weight; larger aircraft require larger engines than smaller aircraft and can be designed to support more capable radars.

If you're going to bash me over that, also consider the B-2. It's listed as costing about $750 million per unit. The engines are 4 F118 non-afterburning engines derived from the F-16's F110, and the radar is known to cost $50 million for the array; the FCC accidentally sold its radar frequencies to a commercial user, which required a $1 billion project to redesign and install 20 new AESA arrays for 20 B-2s. In either case, when the airframe's total cost is about $750 million, the cost of both engines and avionics are thus minuscule.

 

[delft]

Lots of the science of aerodynamics have changed Master Delft, and there seem to be few hard and fast rules, the fact that the F-35A was taken to 73 degrees AOA and flown out is a bit of an amazement, though I have seen the Raptor taken well beyond that to the vertical, but it does have OVT, welcome aboard, no one stays here long, but this is where all the good conversations seem to end up. LOL brat out

It's not science, it's engineering. And that keeps changing. Long live vortex generators.

 

[delft]

Engineer's sophistry is quickly getting old; this is a quick and easy loadout:

Please, Log in or Register to view URLs content!

So tell me again that airframe costs make up relatively little of aircraft costs. Even if it were true, the fact is that the radar and engines tend to be scaled to airframe weight; larger aircraft require larger engines than smaller aircraft and can be designed to support more capable radars.

If you're going to bash me over that, also consider the B-2. It's listed as costing about $750 million per unit. The engines are 4 F118 non-afterburning engines derived from the F-16's F110, and the radar is known to cost $50 million for the array; the FCC accidentally sold its radar frequencies to a commercial user, which required a $1 billion project to redesign and install 20 new AESA arrays for 20 B-2s. In either case, when the airframe's total cost is about $750 million, the cost of both engines and avionics are thus minuscule.

B-2 and F-22 are of course much more expensive air frames than similar aircraft built from conventional materials. But you did make your point.

 

[Engineer]

Few points here :

- Aerodynamics is an art of compromise . You have aircraft optimized for low speed (high lift) like A-10. You have aircraft optimized for high speed (low drag) like SR-71. Everything else falls in between, and you have to balance between maneuverability (or better said agility) and speed. Usually, they are mutually exclusive.

Flight dynamics have gone a long way since those aircraft you spoke of were designed. Design has moved on from just wing shape to computational fluid dynamics, allowing once mutually exclusive design objectives to be met simultaneously.

- Unfortunately you cannot force canard to generate high lift and low drag at the same time Therefore, it is usually kept at a position that keeps minimum drag (and oscillating a little around that position for unstable fighters ) .

Not generate high lift does not equate to not generating any lift. Nice try with the strawman though.

- Vortex lift is not panacea, nor is something new and unusual . Delta shaped fighters used vortex lift to improve maneuverability since Mig-21 and similar. But vortex lift doesn't magically solve all of the problems of delta fighters and it doesn't negate wing loading. If you look at delta-fighters optimized for high maneuverability like Rafale or Tejas, they have relatively low wing loading and lower top speed then contemporary fighters .

The concept of vortex lift isn't new, but the full use of vortex lift is a much more recent phenomenon than the MiG-21. Comparing vortex lift of today to that used on a simple delta is like comparing modern wing design to that of Wright's flyer. Vortex lift of today can contribute significant amount to the total lift. Since lift no longer comes from the wing alone, this also renders wing loading as a meaningless metric in gauging modern fighters performance.

- I never said J-20 could not turn sharply. I said, it could but it would loose energy rapidly. After that maneuver you would have slow aircraft that is not designed for slow speed. Worse, you would have aircraft that cannot accelerate rapidly because of relatively obsolete engines . Therefore, IMHO , J-20 would avoid knife-fights, at least until it gets better engines (WS-15, 117S or AL-31F M2 )

Acceleration uses up energy, so by nature any object would loose energy rapidly through changing of velocity vector. This isn't an issue found only on the J-20. What matters is how fast J-20 gets behind an opponent's tail, which boils down to lift-to-weight ratio. With the help of vortex lift and body lift, the use of delta-wing actually becomes an advantage, since the lift doesn't have to overcome the structural weight of a large wing. That translates to a faster turn. The rest becomes rather moot after missile is away.

 

[F-15]

I read the other day that PAKFA can roll by using the deflection of the LEVCONS can some one explain me that?

I also read that the LEVCON re-attaches the vortex it generates without increasing the drag can some one explain me how it is posible it buffles me a lot

I watched a video where the PAKFA engineer says LEVCONS generate less drag

Does any one knows why?

[video=youtube;Li-xPxcA8tg]https://www.youtube.com/watch?v=Li-xPxcA8tg[/video]