Chengdu J-20 pictures, analysis and speculation Part I

[SteveO]

Just came across this fan art on my computer. Think it was posted on this forum a while back ???

 

[lantinian]

The Yf-23 FCS on the ground was not optimized and first taxis revealed the FCS made too many corrections;

Actually the reason it was making so many corrections was precisely because it was so optimized and sensitive that it detected the tiniest bumps on the runway. To "solve" the problem, Northrop just turned large part of it off until nose-wheel lift.

Even the F-18 uses its V tail for take off

Yes, but like the YF-23 it uses the V tails to PUSH the aft section of the aircraft down so the nose can lift up. Judging from the nice take off picture, J-20 V-tail are rotated so as to create positive lift - to lift the tail up. This is aerodynamically more efficient use of lift and it suggest the CG to be very close to the Aerodynamic Center.

How do you know where the CG is?

I don't know about Sundog, but I was just looking at the aircraft taxing and it seams to be tumbling a lot . So ether the Chinese runways are very bumpy, the engine power is fluctuating a lot or the CG is much less in front of the main wheel than say in the F-22.

So IMHO if CG=AC for take off, this aircraft will have an easier time taking of from a carrier.

 

[Ogami musashi]

Actually the reason it was making so many corrections was precisely because it was so optimized and sensitive that it detected the tiniest bumps on the runway. To "solve" the problem, Northrop just turned large part of it off until nose-wheel lift.

Isn't it exactly what i wrote?

The Yf-23 FCS on the ground was not optimized and first taxis revealed the FCS made too many corrections;

 

[PAK FA]

You can't conclude anything from that; Mig 1.44 was meant to be in the same TOW than the supposed one of this plane (40 tons) and was meant to be maneuverable and agile.
Weight is not a problem as long as lift is here to compensate it;

Weight is always a problem, and it will be, unless we use something like antigravity, but as long as we use jets and wings, weight always will be the main problem.
The whole thing is at this moment many wonder what is the main mission for this big aircraft?
Is it a good dogfighter? can it beat the F-22?
So the first thing people ask what are the first signs of a good dogfighter?
Now compare the differences it has with the Rafale or Eurofighter, there are several that show is not optimized as these fighters for air combat.
Even if you compared it to the MiG-1.44 there are several diferences that show is is not as optimized for air combat, the MiG has more control surfaces on its hinged ventral fins and on its tail booms flaps, that are absent on the J-20. besides the MiG was not as compromised by stealth considerations as the J-20.
Now the F-22 uses thrust vectoring, this improves roll rates, turn rates and AoA handling at high AoA.
Now you can not expect the F-22 to fly at its best at Max take off weight, it will fly the best at normal combat weight.
Some things are already known, for example Chines do help at high AoA handling in yaw and pitch, this was tested on F-5s and was applied on the YF-23 and F-22 in fact the YF-23 chines shed votices in the same way LERXs do.
Now the F-22 is considerable smaller than the J-20 and its fuselage considerably smaller, so you can not expect this jet will be light, first it carries all its fuel and weapons internally. second it is a long aircraft that will have ventral weapons bays located near the CG, so you can not expect this jet to be very agile after looking its sluggish take off and the lack of TVC nozzles.
That is why many analysts say its role is near the F-111 or MiG-25

 

[PAK FA]

To PAK FA:
This bird is weird ;D

Its wierd but up to a certain degree i like it

 

[Abraham Gubler]

Even if you compared it to the MiG-1.44 there are several diferences that show is is not as optimized for air combat, the MiG has more control surfaces on its hinged ventral fins and on its tail booms flaps, that are absent on the J-20. besides the MiG was not as compromised by stealth considerations as the J-20.

The J-20 is not wanting for attitude control surfaces. The all moving horizontal and vertical surfaces (canards and tails) will provide it with all the yaw and pitch control it could possibly need. Also it has full length slats and is designed for TVC so is going to be very agile in the air.

you can not expect the F-22 to fly at its best at Max take off weight, it will fly the best at normal combat weight.
Some things are already known, for example Chines do help at high AoA handling in yaw and pitch, this was tested on F-5s and was applied on the YF-23 and F-22 in fact the YF-23 chines shed votices in the same way LERXs do.
Now the F-22 is considerable smaller than the J-20 and its fuselage considerably smaller, so you can not expect this jet will be light, first it carries all its fuel and weapons internally. second it is a long aircraft that will have ventral weapons bays located near the CG, so you can not expect this jet to be very agile after looking its sluggish take off and the lack of TVC nozzles.
That is why many analysts say its role is near the F-111 or MiG-25

The F-22 is not considerably smaller than the J-20 it is just not as long. The weight and size of the J-20 is being constantly over predicted. Judging by the scale of the gear it is likely a 30 tonne MTOW aircraft similar to the FLANKER and F-22 and not in the 45 tonne league of the F-111 and FOXHOUND.

The J-20 appears to be optimised for low drag for high speed and agility. Like the T-50 it appears to be designed to compensate for its lack of LO with a narrow frontal arc of LO and high speed and agility. What you build when you don’t have the tech to build a real 5th generation fighter.

 

[Ogami musashi]

Weight is always a problem, and it will be, unless we use something like antigravity, but as long as we use jets and wings, weight always will be the main problem.

No. weight is a problem when for a given size you have weight increase; An heavy plane can have a high turn rate without any problem. Wing loading is the main parameter, so as long as the plane is larger with increased weight that doesn't prevent him to be agile and maneuverable.

Even if you compared it to the MiG-1.44 there are several diferences that show is is not as optimized for air combat, the MiG has more control surfaces on its hinged ventral fins and on its tail booms flaps, that are absent on the J-20. besides the MiG was not as compromised by stealth considerations as the J-20.

That means nothing; The turning performance of a plane is not dependant on number of control surfaces;

Now the F-22 uses thrust vectoring, this improves roll rates, turn rates and AoA handling at high AoA.

That again means nothing; Because F-22 uses TVC doesn't mean it will be superior.

Now you can not expect the F-22 to fly at its best at Max take off weight, it will fly the best at normal combat weight.

I fail to see the point of this sentence sorry;

Some things are already known, for example Chines do help at high AoA handling in yaw and pitch, this was tested on F-5s and was applied on the YF-23 and F-22 in fact the YF-23 chines shed votices in the same way LERXs do.

Chines behavior is not linear; They were canceled out the F-23 version because their behavior past a certain AOA destabilized the plane too much; As i told you many times, a plane is not a lego of design technologies but a blended integral aerodynamic formula.

Now the F-22 is considerable smaller than the J-20 and its fuselage considerably smaller, so you can not expect this jet will be light, first it carries all its fuel and weapons internally. second it is a long aircraft that will have ventral weapons bays located near the CG, so you can not expect this jet to be very agile after looking its sluggish take off and the lack of TVC nozzles.

That's the opposite; If the F-22 is so much smaller than you think, the fact it weights 19 tons is the parameter that in theory would make it less agile because of more wing loading...but hopefully and as i tell you since many post, things are not that simple and because of vortex lift, compression lift in supersonic, large torque moments the F-22 is very agile and aerodynamically efficient.

That is why many analysts say its role is near the F-111 or MiG-25

And that's why i laugh at that...

 

[RSF]

Even if you compared it to the MiG-1.44 there are several diferences that show is is not as optimized for air combat, the MiG has more control surfaces on its hinged ventral fins and on its tail booms flaps, that are absent on the J-20. besides the MiG was not as compromised by stealth considerations as the J-20.

The J-20 is not wanting for attitude control surfaces. The all moving horizontal and vertical surfaces (canards and tails) will provide it with all the yaw and pitch control it could possibly need. Also it has full length slats and is designed for TVC so is going to be very agile in the air.

you can not expect the F-22 to fly at its best at Max take off weight, it will fly the best at normal combat weight.
Some things are already known, for example Chines do help at high AoA handling in yaw and pitch, this was tested on F-5s and was applied on the YF-23 and F-22 in fact the YF-23 chines shed votices in the same way LERXs do.
Now the F-22 is considerable smaller than the J-20 and its fuselage considerably smaller, so you can not expect this jet will be light, first it carries all its fuel and weapons internally. second it is a long aircraft that will have ventral weapons bays located near the CG, so you can not expect this jet to be very agile after looking its sluggish take off and the lack of TVC nozzles.
That is why many analysts say its role is near the F-111 or MiG-25

The F-22 is not considerably smaller than the J-20 it is just not as long. The weight and size of the J-20 is being constantly over predicted. Judging by the scale of the gear it is likely a 30 tonne MTOW aircraft similar to the FLANKER and F-22 and not in the 45 tonne league of the F-111 and FOXHOUND.

The J-20 appears to be optimised for low drag for high speed and agility. Like the T-50 it appears to be designed to compensate for its lack of LO with a narrow frontal arc of LO and high speed and agility. What you build when you don’t have the tech to build a real 5th generation fighter.

I'm not so sure that using the gear to estimate the size of the J-20 is going to give us the real size of this beast. Has anyone made an attempt to estimate its size from one of the in-flight photos with the escort J-10A? The J-10A has a wingspan of around 32 ft (9.7 M) and is nose to tail just a bit over 50 ft long (15.5 M). Looking at these photos, the J-20 looks quite a bit larger then its escort.


http://www.ausairpower.net/PLA-AF/Chengdu-J-XX-VLO-Prototype-28S.jpg

http://www.ausairpower.net/PLA-AF/Chengdu-J-XX-VLO-Prototype-7S.jpg

 

[Abraham Gubler]

I'm not so sure that using the gear to estimate the size of the J-20 is going to give us the real size of this beast. Has anyone made an attempt to estimate its size from one of the in-flight photos with the escort J-10A? The J-10A has a wingspan of around 32 ft (9.7 M) and is nose to tail just a bit over 50 ft long (15.5 M). Looking at these photos, the J-20 looks quite a bit larger then its escort.

There is a big difference between size as in volume and size as in weight. As to estimating weight the gear is the best way to do it quickly and easily. As to the dimensional size there have been quite a few estimates made on different yardsticks that all pretty much align and are reasonable. These estimates are earlier on in this thread and its predecessor. In terms of using size to extrapolate capability the best measure is weight.

 

[PAK FA]

author=Abraham Gubler link=topic=11768.msg113774#msg113774 date=1295239112]
The J-20 appears to be optimised for low drag for high speed and agility. Like the T-50 it appears to be designed to compensate for its lack of LO with a narrow frontal arc of LO and high speed and agility.

The F-22 is considerably smaller than the J-20. In my opinion the J-20 it is heavier for the following reason, the F-22 has tails that are the farther tip aft, its Nozzles are few meters away ahead of the end of the tailplanes, now on the J-20 its booms are not long and do not protude very far from where the nozzles end, this reflects it has longer inlets ducts and engine bays, this means its volume is higher than the F-22.
Also see the canopies of the F-22 and J-20 are about the same size, see the difference in proportion between the J-20 and F-22

On a stealthy aircraft you need use internal volume, longer ducts that are surrounded by fuel and weapons bays. this will give a higher weight than a F-22 for sure. On the T-50 case, it has a longer fuselage thanthe F-22 but it is flatter so it is not so heavy specially it has a small forebody compared to the F-22, it has a weight similar to the F-22, J-20 has an F-22 fuselage type and it is longer than the F-22`s.
The J-20 might do okay on its current configuration but the MiG-1.44 had more control surfaces as a result of a very demanding high AoA behavior, the MiG-1.44 is not stealthy as the F-22 but it is for sure very agile for its size and weight, its inlet is designed to have more sideslip resistance and better high AoA behavior. you might say that the F-22 and J-20 have chines that stabilize them at high AoA and keeps them stable at yaw but the MiG.144 was desigend as a pure aerodynamic aircraft with little stealth applied.

 

[rousseau]

You can't conclude anything from that; Mig 1.44 was meant to be in the same TOW than the supposed one of this plane (40 tons) and was meant to be maneuverable and agile.
Weight is not a problem as long as lift is here to compensate it;

Now the F-22 uses thrust vectoring, this improves roll rates, turn rates and AoA handling at high AoA.

Just correct one thing: TVC does not necessarily improve the roll rate despite its deflected differentially.

 

[N22YF]

You can't conclude anything from that; Mig 1.44 was meant to be in the same TOW than the supposed one of this plane (40 tons) and was meant to be maneuverable and agile.
Weight is not a problem as long as lift is here to compensate it;

Now the F-22 uses thrust vectoring, this improves roll rates, turn rates and AoA handling at high AoA.

Just correct one thing: TVC does not necessarily improve the roll rate despite its deflected differentially.

Actually (getting off topic a bit) the F-22's roll rate benefits significantly from TVC at higher AoA even though it is not deflected differentially. This is because, without TVC, the aerodynamic control surfaces must keep the noise pointed in the right direction during a roll; however, with TVC, the aerodynamic control surfaces can be devoted entirely to roll, with the TVC providing the needed pitch control.

(Chart source: Barham, Robert W. "Thrust Vector Aided Maneuvering of the YF-22 Advanced Tactical Fighter Prototype," N94-34610, March 1994.)

 

[PAK FA]

An heavy plane can have a high turn rate without any problem.
That means nothing; The turning performance of a plane is not dependant on number of control surfaces;

Weight is a problem always is a problem, the Tu-160 is just rated for 2Gs, try to take it to 9Gs it will break down.
The number of controls are very important, the Su-37 has two dorsal vertical tails, two wings, two ventral fins, two tailplanes, two canards, two LERXs, and TVC, now tell me an aircraft that can do what the Su-37?, the only aircraft in that league use TVC, the F-22, the T-50, the F-16 and F-15 with TVC prototypes.
Can the Eurofighter do what the SU-37 does? no it can not, Rafale? no, if you are going to have post stall agility=supermaneouvrability either you have a configuration like the S-37 or Su-35 or TVC.
On the Su-27 tailplanes are used to stabilize the aircraft at 120 deg of AoA, on a triplane configuration like the Su-30MKI the canards are not responsive once the aircraft enters post stall, in fact it is its tailplanes that are used to return the aircraft to level flight, the MiG-1.44 uses those flaps at the end of the tailbooms in the same way as if they were tailplanes

 

[SOC]

if you are going to have post stall agility=supermaneouvrability either you have a configuration like the S-37 or Su-35 or TVC.

TVC is not necessarily required. Before Su-37 #711 crashed, it was doing some interesting things. Sukhoi had refitted the aircraft with non-TVC AL-31F engines and had modified the flight control software. The result was that they were able to emulate the Su-37's TVC-aided maneuverability using non-TVC engines.

the MiG.144 was desigend as a pure aerodynamic aircraft with little stealth applied.

Only because it was the aerodynamic proof-of-concept vehicle. The 1.42 production configuration featured a revised forward fuselage, a revised intake, and a revised wing planform.

 

[PAK FA]

TVC is not necessarily required. Before Su-37 #711 crashed, it was doing some interesting things. Sukhoi had refitted the aircraft with non-TVC AL-31F engines and had modified the flight control software. The result was that they were able to emulate the Su-37's TVC-aided maneuverability using non-TVC engines.

i agree but the reason TVC is used is to free some aerodynamic surfaces like tailplanes on the F-22, reduce their size like the T-50`s small vertical dorsal stabilizer and omit others like the canards on the Su-35BM, TVC becomes really handy for stealth aircraft for that reason.
I do not think the J-20 will do okay without it.

 

[Machdiamond]

Weight is a problem always is a problem, the Tu-160 is just rated for 2Gs, try to take it to 9Gs it will break down.

I am with Ogami on this one (and his other comments in the thread), weight is not a problem at all. Your example is not relevant since the Tu-160 was not designed to pull 9G. It is not because it is heavy that it cannot do it.

What you need to understand on aircraft weight is that in the design process there is an "aircraft sizing" stage where you try to meet a design mission. Once you have that weight nailed down, you design the structure and the flight controls to manoeuver and withstand whatever design loads you pick (9G for example), and you size the wing for proper wing loading and aerodynamic characteristics.

Weight may become a problem at a later stage when you loose control of it and it grows during the detailed design stage, that is where you are confused. The structure needs to be beefed up to cope with it and down the deadly weight spiral you go, and no engines available to keep the design thrust to weight ratio.

--Luc

 

[Sundog]

I do not understand why you arrive to the conclusion it is stable or near stable and say he is wrong when he says the aircraft is near stable by the fact the canard is used as a big balance for the very aft wing.

I say it based on the fact that I know how to calculate it and was just waiting for a good planfrom pic to the aerodynamic center of the vehicle. I'll get around to it soon. I'm just very busy right now.

How do you know where the CG is?

Design basics. If you know where the main landing gear is, you can figure out generally where the cg is located. Which, for a land based fighter, is usually around 12 degrees to 15 degrees ahead of the main landing gear. You want it far enough ahead that a wind or different loading doesn't cause it to fall on it's tail, but not too far forward to make it difficult to rotate on take-off. This is obviously different for Navy aircraft, as they have to to keep from tipping off the deck of a carrier. They're usually around 20 degrees ahead of the MLG. To see this difference in design, look at where the main landing gear wheels are located on the YF-17, then look where they are located on the F/A-18.

 

[sferrin]

Just came across this fan art on my computer. Think it was posted on this forum a while back ???

I posted it somewhere. Didn't see it in this thread though so maybe it was on another board. The picture is 3-5 years old.

 

[Ogami musashi]

Weight is a problem always is a problem, the Tu-160 is just rated for 2Gs, try to take it to 9Gs it will break down.

If the TU-160 is rated at 2gs is because of the wing loading; Again, as long as you size the plane according to the weight, there's no problem; The SU-35 was 2 tons heavier than SU-27..with the same engines (i talk about the SU-35 first verison) and it was a problem in inertial performance because of the added weight with the lack of increased engine thrust but was an improvement over tthe SU-27 in roll, turn and yaw rates.

As i strive to make you see...a plane is not a lego.

The number of controls are very important, the Su-37 has two dorsal vertical tails, two wings, two ventral fins, two tailplanes, two canards, two LERXs, and TVC, now tell me an aircraft that can do what the Su-37?, the only aircraft in that league use TVC, the F-22, the T-50, the F-16 and F-15 with TVC prototypes.

If you want to be fair and true to your logic, then i point at the X-31 with only canards+ailerons and later only a miniaturized rudder that was able to pull maneuver the SU-37 never did and will never do.

Similarly if you want to speak about AOA performance, the YAK-130 without TVC and only conventional layout can maintain 41° of AOA while the SU-35 was rated at 35°.


As for the S-37, the plane was limited to 5G's...so turning performance wise, the rafale and eurofighter were actually far better; in addition, except for max AOA, the plane was not the best controllable plane in the world because of many parameters (and again..a plane is not a lego, the berkut was one of the most obvious example of that).

On the Su-27 tailplanes are used to stabilize the aircraft at 120 deg of AoA
, on a triplane configuration like the Su-30MKI the canards are not responsive once the aircraft enters post stall, in fact it is its tailplanes that are used to return the aircraft to level flight, the MiG-1.44 uses those flapsat the end of the tailbooms in the same way as if they were tailplanes

It illustrates where i think you struggle..because the SU-30mki canards are not responsive doesn't mean all canards will be like that... A plane is an integral airframe; not a plug-this into thing.

 

[Mike Pryce]

Design basics. If you know where the main landing gear is, you can figure out generally where the cg is located. Which, for a land based fighter, is usually around 12 degrees to 15 degrees ahead of the main landing gear. You want it far enough ahead that a wind or different loading doesn't cause it to fall on it's tail, but not too far forward to make it difficult to rotate on take-off. This is obviously different for Navy aircraft, as they have to to keep from tipping off the deck of a carrier. They're usually around 20 degrees ahead of the MLG. To see this difference in design, look at where the main landing gear wheels are located on the YF-17, then look where they are located on the F/A-18.

I was taught that the CG should be at least 4 degrees ahead of the mainwheel's point of contact with the runway when the aircraft is at maximum nose-up, just avoiding a ground scrape. The picture in the textbook said it better than my words, but I can't find it at the moment. But 12-15/20 when level sounds about the same.

UPDATE: I decided to check by making my own 'drawing' using a bit of basic 'Rotate Image' and Microsoft Paint. Green lines show groundscrape/max rotation and 90 degrees perpendicular to that. Blue line shows 15 degrees forward from perpendicular. Yellow line shows 4 degrees forward from a notional line (not shown) perpendicular to the groundscrape line. The blue and yellow lines both line up, so the design rules of thumb Sundog and I mentioned do seem similar. Rough idea of CG will be about mid way up fuselage on blue/yellow line.

Note: Take all this with a pinch of salt. Sundog's 12 or 20 degrees would differ, and the rule I was taught was 'at least 4 degrees', so more would also change things. Plus this is based on a fuzzy, not fully aligned pic of static J-20 that may be empty - as it takes-off/lands the oleos are extended as lift reduces weight, but it may be full of fuel and weapons, so heavier, and the point at which the wheels touch may then differ, hence groundscrape line will change etc. Anyway, CG is easy compared to estimating the aerodynamic centre at different speeds/CL's etc. Knowing that and the CG would be MOST interesting! My own guesstimates indicate that with a load of weapons etc. the CG will move forward, and that the aerodynamic centre is somewhere between the mid and forward point of the main undercarriage doors.

 

[frank]

So, the USAF versions of the F-4 & A-7 would have different CGs than their USN counterparts? Same for other commonly used naval & land based designs?

I do not understand why you arrive to the conclusion it is stable or near stable and say he is wrong when he says the aircraft is near stable by the fact the canard is used as a big balance for the very aft wing.

I say it based on the fact that I know how to calculate it and was just waiting for a good planfrom pic to the aerodynamic center of the vehicle. I'll get around to it soon. I'm just very busy right now.

How do you know where the CG is?

Design basics. If you know where the main landing gear is, you can figure out generally where the cg is located. Which, for a land based fighter, is usually around 12 degrees to 15 degrees ahead of the main landing gear. You want it far enough ahead that a wind or different loading doesn't cause it to fall on it's tail, but not too far forward to make it difficult to rotate on take-off. This is obviously different for Navy aircraft, as they have to to keep from tipping off the deck of a carrier. They're usually around 20 degrees ahead of the MLG. To see this difference in design, look at where the main landing gear wheels are located on the YF-17, then look where they are located on the F/A-18.

 

[PAK FA]

If the TU-160 is rated at 2gs is because of the wing loading; Again, as long as you size the plane according to the weight, there's no problem;

To address this i will say that any mass can be accelerated, therefore you can acelerate or increase the G load of any aircraft that is true, however what you fail to see is that the structure by it self can not cope so easily as weight increases, a Tu-160 will break apart because the materials and structure used are not capable of resisting 9Gs as in an small aircraft due to the higher loads achieved.
To understand it simple G equals weight if i weigh 75kg at 1G, at 10Gs i am weighing 10 times my weight, so i will weigh 750kg , i can increse several Gs but around 50Gs i will badly injured or even dead (depending how long i sustain them).
A 180000 kg Tu-160 at 10Gs or 1800000kg will collapse its structure it will be too much weight for its structure imaging a Tu-22 at a 9Gs turn for 20 seconds.
Heavy aircraft under the current technology can not be flown as agile as fighters, that is the simple reason the Tu-160 is limited to 2Gs.
So if the J-20 is longer than the F-22 and has longer inlet ducts it surely weighs more, so it will be more difficult to make it fly as well as the a lighter aircraft.

 

[Ogami musashi]

If the TU-160 is rated at 2gs is because of the wing loading; Again, as long as you size the plane according to the weight, there's no problem;

To address this i will say that any mass can be accelerated, therefore you can acelerate or increase the G load of any aircraft that is true, however what you fail to see is that the structure by it self can not cope so easily as weight increases, a Tu-160 will break apart because the materials and structure used are not capable of resisting 9Gs as in an small aircraft due to the higher loads achieved.
To understand it simple G equals weight if i weigh 75kg at 1G, at 10Gs i am weighing 10 times my weight, so i will weigh 750kg , i can increse several Gs but around 50Gs i will badly injured or even dead (depending how long i sustain them).
A 180000 kg Tu-160 at 10Gs or 1800000kg will collapse its structure it will be too much weight for its structure imaging a Tu-22 at a 9Gs turn for 20 seconds.
Heavy aircraft under the current technology can not be flown as agile as fighters, that is the simple reason the Tu-160 is limited to 2Gs.
So if the J-20 is longer than the F-22 and has longer inlet ducts it surely weighs more, so it will be more difficult to make it fly as well as the a lighter aircraft.

Sorry but that is simply wrong...

And again, this is a question of stress (for mechanical loads) and aero loads which are both in Force/area units...which means what it means...

In addition the fact you are extrapolating an example with a Tu-160 shows well you take things to extreme to try and prove your point; We are dealing with a plane that is well within the current fighter's dimensions.

And i'm sorry, but from flight mechanics and fluids mecanics point of view, weight is not a problem as long as it is matched with according dimensions and thrust I.E the wing loading and thrust to weight ratio.

Take it from any angle you want, this is simple physics; F=ma ...a turn is an acceleration, so if m is higher only F has to be greater and you'll have the same acceleration.

 

[sferrin]

I do not understand why you arrive to the conclusion it is stable or near stable and say he is wrong when he says the aircraft is near stable by the fact the canard is used as a big balance for the very aft wing.

I say it based on the fact that I know how to calculate it and was just waiting for a good planfrom pic to the aerodynamic center of the vehicle. I'll get around to it soon. I'm just very busy right now.

How do you know where the CG is?

Design basics. If you know where the main landing gear is, you can figure out generally where the cg is located. Which, for a land based fighter, is usually around 12 degrees to 15 degrees ahead of the main landing gear. You want it far enough ahead that a wind or different loading doesn't cause it to fall on it's tail, but not too far forward to make it difficult to rotate on take-off. This is obviously different for Navy aircraft, as they have to to keep from tipping off the deck of a carrier. They're usually around 20 degrees ahead of the MLG. To see this difference in design, look at where the main landing gear wheels are located on the YF-17, then look where they are located on the F/A-18.

Okay, but that window is going to leave significant uncertainty as to the location of the CG and subsequent natural stability.

 

[Sundog]

So, the USAF versions of the F-4 & A-7 would have different CGs than their USN counterparts? Same for other commonly used naval & land based designs?

No they didn't move the landing gear, which is one of the reasons the USAF doesn't like using Navy aircraft, because they require more runway. That's why the USAF version of the F-4 had drag chutes added, because Navy aircraft can't hold the nose up on the runway for aero-braking as long as aircraft designed for land use can and they can't rotate as early in the take-off run as ground based aircraft. Of course, that isn't a problem for navy aircraft because they are catapulted off the deck and use a hook for deceleration when landing on the carrier.

It's also why one of the major redesigns for the F-111B was going to be to move the MLG back, because the first F-111B's would tip on their tails on the carrier due to it's, the carriers, pitching and rolling. Which is a serious problem since the Navy likes to park their planes with the tails hanging over the deck to maximize the available space. People tend to get angry when multi-million dollar aircraft tip off the edge of a carrier due to poor placement of the cg wrt the mlg wheel contact point.

 

[Sundog]

Okay, but that window is going to leave significant uncertainty as to the location of the CG and subsequent natural stability.

I understand what you're saying, but if it's that close, I would be willing to bet it's probably unstable at subsonic speeds and stable at supersonic speeds, like the F-16. If it's highly unstable, like the X-29 was, it should be easily noticeable. I'm not looking for exact, I'm looking for "ballpark."

 

[AeroFranz]

The whole argument of heavier vehicle = less maneuverable is easily solved. Take any design textbook (Raymer, Torrenbeek, Nicolai, McCormick, Roskam, etc...), go to the performance section, and look up turn rate or turn radius.

I attached the equation for maximum sustained load factor according to Nicolai. "n" is the load factor, q is dynamic pressure, K should be a lumping of Pi*oswald*aspect ratio, Tmax is max thrust available, S is wing area, Cdo is parasite drag coefficient. It says that the max sustained load factor is inversely proportional to wing loading, and nothing else.

In my experience, the tipback (or tail strike angle) is usually 15 degrees. In Nicolai it says 10 deg for land based and 15 for carrier-based, but that's the only place where the difference is spelled out, so i'm not sure. I am positive about the need for wider turnover angles (roughly the angle between landing gear contact point and cg, as seen head on) for navy planes. Requirements are 54deg for navy and 63 deg for Airforce.

 

[Avimimus]

Of course, structural strength is an issue and fluid dynamics change with the scale and flight regime (so equations may be more correct than real life phenomena involving complex flows of molecules etc.)

At a more basic level - I think it is important to consider the difference between maximum turn rates and sustained turn rates...

 

[Ogami musashi]

The whole argument of heavier vehicle = less maneuverable is easily solved. Take any design textbook (Raymer, Torrenbeek, Nicolai, McCormick, Roskam, etc...), go to the performance section, and look up turn rate or turn radius.

I attached the equation for maximum sustained load factor according to Nicolai. "n" is the load factor, q is dynamic pressure, K should be a lumping of Pi*oswald*aspect ratio, Tmax is max thrust available, S is wing area, Cdo is parasite drag coefficient. It says that the max sustained load factor is inversely proportional to wing loading, and nothing else.

The equation says the maximum sustained factor is inversely proportionnal to wing loading and proportionnal to lift/drag ratio but yes...the weight impact here is in the form of wing loading and nothing else...

 

[AeroFranz]

Of course, structural strength is an issue and fluid dynamics change with the scale and flight regime (so equations may be more correct than real life phenomena involving complex flows of molecules etc.)

You must be referring to the square-cube law with regards to structures, and Reynolds numbers effects for aero. I assumed that we'd be comparing fighters and the size difference would be relatively small - less than an order of magnitude (!)
But yeah, you have to simplify the problem while still capturing all the major effects. I did not think those were important enough.

 

[Steve Pace]

As far as I know there is no concrete set of rules for aerodynamicists throughout the world. What works for one might not work for another, what doesn't work for one might work for another. -SP

 

[Abraham Gubler]

As far as I know there is no concrete set of rules for aerodynamicists throughout the world. What works for one might not work for another, what doesn't work for one might work for another. -SP

Yep the nature of air, gravity and material science must be different in China… Of course there are different approaches to achieve the same end but since the J-20 doesn’t appear to have any radically different engineering or aerodynamic solutions to other contemporary aircraft one is safe to extrapolate understanding of it via comparative analysis.

 

[Sundog]

It says that the max sustained load factor is inversely proportional to wing loading, and nothing else.

I believe I said that at the beginning of this thread, or somewhere way back in the other. Which means I agree completely.
In fact, Interavia had a good article back in the 80's on why smaller fighters aren't usually better. Mainly due to their lack of growth potential; i.e.-no room to put more stuff. Therefore, any serious upgrades tend to be more costly. Of course, the flip-side is the larger aircraft costs more.

 

[Sundog]

As far as I know there is no concrete set of rules for aerodynamicists throughout the world. What works for one might not work for another, what doesn't work for one might work for another. -SP

Actually, I think you sort of mixed your metaphor's up there a bit. As AG points out, the physics is exactly the same. What tends to be different is design approaches, mission requirements, and each company usually has it's own set of reference data. For instance, Lockheed has a lot of data on the F-22, which they can now "plug" into their equations to make better analysis for similar aircraft and they aren't going to share that with their competitors. The same is true for the other companies. That's why when I was in school we we were allowed access to some excellent data pertaining to the HSCT from various companies but weren't allowed to make copies of it, as it was proprietary to each company.

It's also why I'm always in search of a good engine deck (An engine deck is a set of data for a specific engine/powerplant over the flight envelope) and is invaluable in making performance calculations and for reference when coding aircraft for MS Flight Simulator. ;D

 

[PAK FA]

In addition the fact you are extrapolating an example with a Tu-160 shows well you take things to extreme to try and prove your point; We are dealing with a plane that is well within the current fighter's dimensions.

I am not taking to the extreme, no i am taking to the real life conditions, no one will make a Tu-160 or B-1B a dogfighter, the real issues of drag, thrust and structure will simply render the aircraft unpractical.
Other issues are aircraft have AoA limits due to stall, some configurations are better to retain lift than others and the canard delta does not have anything special that one with tailplanes, LERX and trapezoidal wings would not offer, and the example still is the Su-27 that has post stall ability at an AoA of 120 deg while other designs with canards do not.
The J-20 also is within the size of strikers like Su-24, Su-34 and and F-111 and MiG-25BR

 

[PAK FA]

I understand what you're saying, but if it's that close, I would be willing to bet it's probably unstable at subsonic speeds and stable at supersonic speeds, like the F-16. If it's highly unstable, like the X-29 was, it should be easily noticeable. I'm not looking for exact, I'm looking for "ballpark."

But this does not shed any light about if it is a good dogfighter or not, it is obvious the chinese wanted a good supercruiser so less unstability and trim will be desired; first less trim for drag and stealth requierements however the J-20 has huge canards which obviously are not exactly the best for a good supercruiser, if you remember the Tu-144 only deployed its canards at landing and the Eurofighter has small canards well ahead of the wing, in the case of the Eurofighter this allows for high degrees of unstability at subsonic speeds and low drag at supersonic speeds, why the chinese added huge canards which basicly are not the best answer for stealth and low drag, and these canards are relatively close to the wing?

see the J-10 looks dwarfted and the J-20 canards look huge.
If canard deflections are not desired due to drag and higher RCS then why adding huge canards to an aircraft which already is big

 

[SOC]

the J-20 has huge canards which obviously are not exactly the best for a good supercruiser

Funny, they worked on the MiG 1.42/1.44, intended to supercruise in the Mach 1.5+ range.

Canards are not going to automatically give you a much higher RCS, either. Northrop made them work on the NATF-23. Various JSF/JAST/CALF configurations also made them work.

What I find personally amusing at this juncture is that the J-20 appears to illustrate a more thorough application of LO design than the T-50. 20 years ago China could barely get by with making various size MiG-21 ripoffs.

 

[PAK FA]

Funny, they worked on the MiG 1.42/1.44, intended to supercruise in the Mach 1.5+ range.

Canards are not going to automatically give you a much higher RCS, either. Northrop made them work on the NATF-23. Various JSF/JAST/CALF configurations also made them work.

What I find personally amusing at this juncture is that the J-20 appears to illustrate a more thorough application of LO design than the T-50. 20 years ago China could barely get by with making various size MiG-21 ripoffs.

The NATF-23 had different shape canards and the CALF also had the same rhomboid low aspect canards, on the MiG-1.44 the canards were also huge but it had TVC nozzles so as the F-22 canard deflections were not going to be the main way to stabilize the MiG and the MiG-1.44 also had flaps on the booms to work as tailplanes as pitch control, these are absent on the J-20.
The MiG also has dogtooth to re-energize the canard and create vortices, on the J-20 this is absent.
The T-50 has ommited completely the canard and the MiG 1.44 was cancelled deemded as not the best way to achieve stealth and supercruise

 

[SOC]

So basically you're assuming that because the J-20 lacks an extra set of flaps and is currently not fitted with TVC nozzles it cannot be maneuverable.

 

[PAK FA]

So basically you're assuming that because the J-20 lacks an extra set of flaps and is currently not fitted with TVC nozzles it cannot be maneuverable.

In my personal opinion, and i say in a humble way up to what i know but i am not an expert, the J-20 is not as optimized to high AoA as the original MiG was, the canards are not placed on the same way high performance fighters place them, and from shots on the video, the nozzles never moved and in any flight test these have to move, so in my opinion yes i do not think the J-20 will beat the MiG in maneuvrability, it will beat it only in stealth