Aerodynamics thread

[Engineer]

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

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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.

I will concede on the point of cost breakdown, since I mistakenly used airframe cost as total cost in my previous analysis. Regardless, it doesn't help your case at all. Looking at fiscal budget 2006, the unit cost of F-22A less advance procurement is around 129.76 millions. Accounting for inflation, that is 152.09 millions in 2014.

Looking at fiscal budget at 2014, the unit cost for F-35 less advance procurement is around 152.08 millions.

Your small aircraft is not that much cheaper than the larger aircraft.

 

[Air Force Brat]

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.


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


Vortex from the leading edge of a MiG-21's wing is merely an unavoidable side effect of a high swept wing at high angle-of-attack, rather than something the designers purposely built into the aircraft to boost performance. In fact, in the days of MiG-21, a main part of the aircraft design is about avoiding interference, which led to features like simple wing geometry, lack of wing-body blending, and nose intake. Purposely designed vortex lift is first utilized on the F-16 and the Su-27, solving the contradictory nature between level flight and flight at high angle-of-attack. 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. Since lift no longer comes from the wing alone, this also renders wing loading as a meaningless metric in gauging modern fighters performance.


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.

its only moot if its a one on one engagement, then it would depend on what the J-20 was engaged with and at what range, if you have multiple bogeys, you want to be high, supersonic, and stealthy for a BVR kill, with a multiple launch before being seen. If you are WVR, you want a helmet mounted sight, and supermaneuverability, OVT will help if you have a big heavy bird like a Flanker, Rapter, T-50, and it depends on what the bogey is targeting you with, so no it does not necessarily become moot..... Raptor drivers train and plan on meeting multiple bogeys, in the case of the Iranian F-4s, they gave them a visual and verbal warning, DON'T DO IT! Likely two Phantom jockeys had to go home for a change of diapers.

Aerodynamics are not only a science, but as you mentioned a "black art", sometimes we stumble on the secrets, sometimes we don't, and when your "friends" find the magic potion, you do what you can to find some of your own, or else "borrow" some of theirs???? what are "friends for"?????

 

[Air Force Brat]

I read the other day that PAKFA can roll by using the defection 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]

1.Levcons are independent of one another, by increasing vortex lift on one wing and not the other, the wing developing lift will roll over the top and continue on around the longitudinal axis. Its very simple, as that is all the ailerons do, although they are more effective due to a longer moment arm when located on the outboard trailing edge of the wing, slats or leading edge flaps will also do the same. So whether located on the leading or trailing edge, they increase the camber of the wing when deflected downward in the airflow, creating lift in the process causing a roll due to assymetric lift of the two main wings.

2. Its likely the vortex never completely detaches in the first place, but simply remains within the boundary layer over the top of the wing reducing pressure due to increased velocity caused by the turbulence, all the talk in the world likely won't help you visualize this phenomena, but a little blue smoke in a wind tunnel will show you the whole truth. In flight, yarn tufts have been used to visualize these effects, as with the F-18, when they were getting a lot of transonic buffeting on the vertical stabs????

 

[F-15]

1.Levcons are independent of one another, by increasing vortex lift on one wing and not the other, the wing developing lift will roll over the top and continue on around the longitudinal axis. Its very simple, as that is all the ailerons do, although they are more effective due to a longer moment arm when located on the outboard trailing edge of the wing, slats or leading edge flaps will also do the same. So whether located on the leading or trailing edge, they increase the camber of the wing when deflected downward in the airflow, creating lift in the process causing a roll due to asymmetric lift of the two main wings.

thanks but i knew that, but my question was more related to why canards are not used like roll control in modern fighter aircraft but only in missiles.
PAKFA seems to use the movable LEX as roll control

2. Its likely the vortex never completely detaches in the first place, but simply remains within the boundary layer over the top of the wing reducing pressure due to increased velocity caused by the turbulence, all the talk in the world likely won't help you visualize this phenomena, but a little blue smoke in a wind tunnel will show you the whole truth. In flight, yarn tufts have been used to visualize these effects, as with the F-18, when they were getting a lot of transonic buffeting on the vertical stabs????

In the video the engineer from Sukhoi says the LEVCON works as a drooping leading edge, i guess it re-attaches the vortex generated by the movable LEX "LEVCON" but that does not explains the drop in drag with respect the canards thanks any way

 

[Air Force Brat]

thanks but i knew that, but my question was more related to why canards are not used like roll control in modern fighter aircraft but only in missiles.
PAKFA seems to use the movable LEX as roll control

In the video the engineer from Sukhoi says the LEVCON works as a drooping leading edge, i guess it re-attaches the vortex generated by the movable LEX "LEVCON" but that does not explains the drop in drag with respect the canards thanks any way

Actually if you look at the picture of your PAKFA, the elavons are in full opposite deflection, the Levcons are just a part of that process of creating lift.....the canard are simply not used in that way in any application that I know of, because they are needed in the pitch process of generating lift, and they do contribute to total lift, working together with the aerodynamic forward fuselage. The LEVCONs generate less drag because they are smaller and integrated in the leading edge, and their deflection is not as extreme, the canards on the other hand generate parasite drag due to their increased surface area in addition to the leading edge of the wing, the LEVCONs are not part of the Pitch control, but their assymetric application will cause the aircraft to roll.

The LEVCONs do not create a drop in drag, their application will inevitable increase drag, but their application creates less drag than the canards, that's all they were saying, anything more is simply Bravo Sierra.

 

[F-15]

The LEVCONs generate less drag because they are smaller and integrated in the leading edge, and their deflection is not as extreme, the canards on the other hand generate parasite drag due to their increased surface area in addition to the leading edge of the wing, the LEVCONs are not part of the Pitch control, but their assymetric application will cause the aircraft to roll.

The LEVCONs do not create a drop in drag, their application will inevitable increase drag, but their application creates less drag than the canards, that's all they were saying, anything more is simply Bravo Sierra.

See the size and the canards on PAKFA and Su-30MKI are almost the same size

their deflection is like canards and on this picture you can see the vortex creation

you can see vortex creation on PAKFA

vortex system on PAKFA

I think the reason is because PAKFA does not have movable LEX down wash thus it does not generate as much drag as canards.

Any way as you said

and integrated in the leading edge

I think that is the real reason. Thanks

 

[Air Force Brat]

See the size and the canards on PAKFA and Su-30MKI are almost the same size

their deflection is like canards and on this picture you can see the vortex creation

you can see vortex creation on PAKFA

vortex system on PAKFA

I think the reason is because PAKFA does not have movable LEX down wash thus it does not generate as much drag as canards.

Any way as you said



I think that is the real reason. Thanks

Great Pictures of the PAKFA F-15, thanks for the good questions and thanks for these great pix, you can see the vortices in the vapor cloud and that is very helpful to "illustrate" their function, good post young man. brat

 

[thunderchief]

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.

Oh, yea Tell me again what is fastest military aircraft ever, what is fastest fighter ever, and when were they designed ? And, while we are at it, why are 5th gen fighters generally slower then their 4th gen counterparts ?

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

You sad maximum lift and minimal drag. Mutually exclusive. You could have either on of them , or compromise between the two.

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.

Only thing that is fundamentally changed since 60's and 70's is use of computers. Basically, today you could work with inherently unstable elements like cranked delta, you could move center of the mass aft etc. But basic laws of aerodynamics have not changed and you still cannot generate lift if you don't have wings (please don't attach pictures of rockets to prove me wrong ) . That is why even very modern fighters have large wings if their designers want superb low-speed handling (PAK-FA , F-22) .

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.

Any aircraft loses energy in turn, it is just the matter how much . Delta fighters are known to have good ITR and bad STR. Classical example are Mirage-2000 and F-16 in Hellenic Air Force. They had both aircraft. Mirage has better ITR so it would get a chance for a first shot . But if Mirage pilot failed to score a kill, F-16 would beat him with superior STR and power-to-weight ratio. Same thing could happen to careless J-20 pilot,and I don't think China would risk their new birds in such encounter (at least not until they get sufficient number of them to cover loses ) .

 

[latenlazy]

Any aircraft loses energy in turn, it is just the matter how much . Delta fighters are known to have good ITR and bad STR. Classical example are Mirage-2000 and F-16 in Hellenic Air Force. They had both aircraft. Mirage has better ITR so it would get a chance for a first shot . But if Mirage pilot failed to score a kill, F-16 would beat him with superior STR and power-to-weight ratio. Same thing could happen to careless J-20 pilot,and I don't think China would risk their new birds in such encounter (at least not until they get sufficient number of them to cover loses

May get off more than one shot if the J-20 has superior supercruising performance and enters the merge with more energy. May also get off more than one shot if the J-20 is able to employ tactics that strike the right balance between energy bleed and nose pointing.

Finally, I contest that the J-20 must necessarily have bad STR compared to its peers. STR isn't a number that stays fixed for all flight regimes. The J-20 could end up having better STR in certain regimes than its peers (perhaps say supersonic turn rates). Furthermore, we simply don't know that it's low speed handling MUST be inferior. All three 5th generation designs are very radical in the methods they employ to minimize performance compromises, and as has already been pointed out, the J-20's wing area is not small.

 

[thunderchief]

May get off more than one shot if the J-20 has superior supercruising performance and enters the merge with more energy. May also get off more than one shot if the J-20 is able to employ tactics that strike the right balance between energy bleed and nose pointing.

Finally, I contest that the J-20 must necessarily have bad STR compared to its peers. STR isn't a number that stays fixed for all flight regimes. The J-20 could end up having better STR in certain regimes than its peers (perhaps say supersonic turn rates). Furthermore, we simply don't know that it's low speed handling MUST be inferior. All three 5th generation designs are very radical in the methods they employ to minimize performance compromises, and as has already been pointed out, the J-20's wing area is not small.

Not very likely you going to get supercrusing performance with these engines. You would need at least something in the rank of 117S . That doesn't mean J-20 could not start a fight from superior position (higher speed and height) . Relatively low T/W would matter later in the combat when you need to regain energy quickly.

Also, if you get into prolonged air combat where STR matters, you are already subsonic 99% of the time. Simply saying, J-20 would start with supersonic speed, use excellent ITR to get first shoot. But in the process it would bleed lot of energy . If it misses, things would get ugly because it would get in a low-speed dogfight were delta-fighters with mediocre T/W don't have much chance.

As for J-20 low-speed handling, I admit it is enigma. We could only make conclusions based on what we saw, and what we know about similar aircraft in similar circumstances . If PLAAF records a demo of J-20 with superb low-speed handling (like Americans did with F-22, and Russians with PAK-FA) I would be willing to eat my words