DISCUSSION: THUNDER AND TEJAS

[Stephen Cohen]

The JF-17 would try to bait the LCA into sustained turns to bleed its energy and the LCA will try to point its nose at the JF-17 and get a lock.

Tejas will NOT enter into Sustained turns

Its strength lies in First lock and Kill which it can achieve by its HMDS +HOBS

The reason it will achieve First lock is because of better ITR
Relaxed Static stability And Low wing loading

 

[amardeep mishra]

@

Each pilot would try to use the advantages of each aircraft while trying to exploit the disadvantages of the other. The JF-17 would try to bait the LCA into sustained turns to bleed its energy and the LCA will try to point its nose at the JF-17 and get a lock.

@JamD
I agree without a doubt, that has always been the case and that's how pilots are trained in both these air forces. I think the STR of LCA is lower than JF17-although I confess, I've not really performed any simulation or calculated its actual value. The ITR though is appreciably higher than JF17. Also I'd like to bring your attention to the pitch rates of both these aircrafts,since LCA has RSS, it's pitch rate would be higher than that of JF17 that doesn't employ an unstable design.

 

[JamD]

1 What is the wing loading in air superiority fighters?
2 Is the wing loading of ground attack fighters lesser or more?
3 How would designers adjust the wing loading of a multirole aircraft which has to do both air superiority and ground attack missions?
4 Would the mutirole aircraft be still more tilted towards either air or ground missions based on the designers preferences?
5 Do delta fighters have higher instantaneous as well as sustained turn rates?
6 Why were canards needed on a simple delta fighter?
7 Why is the location of these canards different in euro , j10 and rafale?
8 Do LERX of jf-17 generate lift? I s this lift controllable or does it have another control on the back of the aircraft?
9 Do canards generate controlled lift?
10 Has the JF-17s wing loading been published?
11 Where would a fighter having higher sustained turn rate outclass a fighter with higher Inst. TR
12 Has IAF developed specially designed bombs with reduced frontal RCS to reduce the fronal RCS of a fully loaded LCA?

I'll take a shot if you permit

1. There is no generally agreed rule. It would depend on the design of the aircraft. Generally it is between 40-130 lb/sq-ft
2. Ground attack aircraft would generally have higher wing loading because they carry a lot of weight and you can't put as much wing area as you'd like as you are restricted by a bajiliion other constraints.
3. This is something done in conceptual design stage. You start off with a maximum takeoff weight and mission requirements and you try to balance all of these. You do optimizations and figure out the wing loading that would best satisfy your particular mission requirements.
4. It would really depend on the requirements.
5. As a rule of thumb deltas have higher instantaneous turn rates but slower sustained turn rates. But there are always exceptions to rules.
6, 7. Best explained here: https://en.wikipedia.org/wiki/Canard_(aeronautics)
8. Yes they generate lift. The JF-17 has no control surface on its LERX.
9. Depends, read 6,7.
10. No. It is not a constant number the weight of the aircraft changes drastically during its mission or from mission to mission. We usually correct wing loading for each phase of a mission during conceptual design. It is never just ONE number. You can calculate the takeoff worst possible wing loading by dividing the MTOW by the wing area.
11. Not necessarily. Different styles of flying. Would depend on the skill of the pilots and the kinds of weapons at their disposal.
12. I don't know.

@


@JamD
I agree without a doubt, that has always been the case and that's how pilots are trained in both these air forces. I think the STR of LCA is lower than JF17-although I confess, I've not really performed any simulation or calculated its actual value. The ITR though is appreciably higher than JF17. Also I'd like to bring your attention to the pitch rates of both these aircrafts,since LCA has RSS, it's pitch rate would be higher than that of JF17 that doesn't employ an unstable design.

How do you know the JF-17 is stable in pitch? I must admit I have seen no information in this regard.

 

[amardeep mishra]

How do you know the JF-17 is stable in pitch? I must admit I have seen no information in this regard.

@JamD
That is because JF17 from the outset was designed to be cost effective and besides why do you think the designers I.e CATIC would leave such a prominent aspect unadvertised especially when they're looking for international customers. Ofcourse, if KF17 had this inn it's design, the designer -CATIC,producer-PAC or the user PAF would have published or revealed it by now especially when Pakistan is aggressively pushing this jet for exports.

 

[Arsalan]

Tejas will NOT enter into Sustained turns

Its strength lies in First lock and Kill which it can achieve by its HMDS +HOBS

This is NOT what was being advocated by @amardeep mishra so perhaps we need to reconsider all that have been discussed?

 

[JamD]

@JamD
That is because JF17 from the outset was designed to be cost effective and besides why do you think the designers I.e CATIC would leave such a prominent aspect unadvertised especially when they're looking for international customers. Ofcourse, if KF17 had this inn it's design, the designer -CATIC,producer-PAC or the user PAF would have published or revealed it by now especially when Pakistan is aggressively pushing this jet for exports.

I really don't believe this is a "prominent feature". It is rather standard. Also, even if we agree that it is, absence of evidence isn't evidence. I would be honestly surprised if what you're saying is true. Till I see any evidence, this is just speculation.

And don't even get me started on what should have been revealed or not. That's a whole different can of worms.

 

[GURU DUTT]

This is NOT what was being advocated by @amardeep mishra so perhaps we need to reconsider all that have been discussed?

sir just a noob question if say in a air to air combat you have HMDS+HOBS combo and your enemy dose not have it what will you to switch off HMDS-HOBS combo and fight him the conventional way to show your fighting skills or as soon as he is in NEZ of your HOBS missile you will shoot and scoot ? just asking no intention of trolling .. thanks waiting for your response sir ?

 

[Manticore]

I'll take a shot if you permit

1. There is no generally agreed rule. It would depend on the design of the aircraft. Generally it is between 40-130 lb/sq-ft
2. Ground attack aircraft would generally have higher wing loading because they carry a lot of weight and you can't put as much wing area as you'd like as you are restricted by a bajiliion other constraints.
3. This is something done in conceptual design stage. You start off with a maximum takeoff weight and mission requirements and you try to balance all of these. You do optimizations and figure out the wing loading that would best satisfy your particular mission requirements.
4. It would really depend on the requirements.
5. As a rule of thumb deltas have higher instantaneous turn rates but slower sustained turn rates. But there are always exceptions to rules.
6, 7. Best explained here: https://en.wikipedia.org/wiki/Canard_(aeronautics)
8. Yes they generate lift. The JF-17 has no control surface on its LERX.
9. Depends, read 6,7.
10. No. It is not a constant number the weight of the aircraft changes drastically during its mission or from mission to mission. We usually correct wing loading for each phase of a mission during conceptual design. It is never just ONE number. You can calculate the takeoff worst possible wing loading by dividing the MTOW by the wing area.
11. Not necessarily. Different styles of flying. Would depend on the skill of the pilots and the kinds of weapons at their disposal.
12. I don't know.



How do you know the JF-17 is stable in pitch? I must admit I have seen no information in this regard.

Thankyou for your answer. I do not work in this field however I am an enthusiast. I posted these questions so that members here have some pointers to talk about and keep the thread interesting. Old members on PDF generally know about these points as these have been discussed to death so many times.

The point is that delta fighters typically need a stronger engine to overcome greater drag. JF-17 is better in sustained turn rate and at subsonic speeds if you follow the yf-18 tests. The canards are not a new thing rather they were introduced as early as the world wars. Huge LERX are not new either. Both of these elements were even introduced on the MIGs. I dont remember if the canard was fixed in this case or moveable.

The canard placement with respect to the wing makes the typhoon better for air superiority missions whereas in case for rafale, better for ground attack missions. The canards location is not at these two extremes in case of j10

The aircraft will not be flying without weapons and they will drastically increase RCS even if the aircraft is using composites

During design phase, the requirements of the fighter are chalked down and parameters are set down accordingly. If some aircraft has lower wing loading, so what? Its sacrificing some other capability in return.

 

[Arsalan]

sir just a noob question if say in a air to air combat iyou have HMDS+HOBS combo and your enemy dose not have it what will you to switch off HMDS-HOBS combo and fight him the conventional way to show your fighting skills or as soon as he is in NEZ of your HOBS missile you will shoot and scoot ? just asking no intention of trolling .. thanks waiting for your response sir ?

Again,
i seriously doubt that! It is just that you are trying to be a smart a55 and think you can fool everyone with such quirky sarcastic remarks. For one last time, refrain from that on this thread.

But to answer your question, it will quite stupid of that pilot to put that aircraft at risk just to show off his skills.

 

[Taygibay]

I'm jumping from the first page to answer quotes but found Manticore's post #67
partially answering ...

The real reason why canard is used in these aircrafts is because it produces vortices which are very strong immediately behind canard itself, and get progressively weaker, but they also produce a downwash. Properly positioned downwash creates a low pressure region on front part of the wing upper surface which has a significant contribution to lift. Imagine bernoulli's effect and that is a wing will higher lift if the pressure differential on both the surfaces of the wing is high. Apart from creating a higher pressure differential these "euro-canards" make the aircraft dynamically unstable- mind it - it is different from static instability.

Not all canards work the same. You are right about the vortices and other effects when closed-coupled!
The canards placement on the Typhoon so far from the wing is exactly chosen to minimize wing interaction
while instilling maximal moment on the nose authority. The Rafale placement does the exact opposite.
Just because of the control of airflows over the wing, a Raffy can land at lower speeds than traditional delta
designs. It was a design objective so the plane would be able to land on carriers since omnirole, Dassault dixit.

That is why whenever you hear someone talking about Sea Typhoon ( carrier adaptation ), you can conclude
lacks in their aeronautics knowledge. The Gripen is much more like Rafale and thus much more likely to fit
( though not easily ) "navalisation". The above also induces that using Euro-Canards anywhere but in market
grouping meant for sales is a mistake. Adding canards to a plane does not produce ISO results depending on
the design choices so there are no group results performance-wise either and thus no group to be derived.

Secondly,as i have always maintained,No one from PAF,PAC to CATIC have ever claimed that JF-17 is unstable in pitch plane.I would gladly correct myself if you can feed simple statement from the manufacturer,user or literature pertaining to the same.

Absence of proof is not proof of absence.
One cue is that the Thunder has FBW over the pitch axis, right?

• Flight Control System​
• Composite Flight Control System comprising
  conventional controls with stability augmentation in roll
  and yaw axis and fly by wire in pitch axis​
• Quad-redundancy in Fly By Wire System​
• http://www.pac.org.pk/jf-17​

Why? Why would a design team avoid a technology but then decide to use a bit of it?
Simple answer for good design teams : when necessary / required / can't be avoided.
So pitch change rate was deemed too fast for a traditional interface and pilot inputs ...
at the very least. If more control over it is required, it has to be difficult to control in the
first place if not impossible. As Oscar and even yourself explained, instability is exactly
that, the threshold past which the human piloting is overwhelmed and FBW required.

Just sayin' Tay.

 

[ghazi768]

1 What is the wing loading in air superiority fighters?
2 Is the wing loading of ground attack fighters lesser or more?
3 How would designers adjust the wing loading of a multirole aircraft which has to do both air superiority and ground attack missions?
4 Would the mutirole aircraft be still more tilted towards either air or ground missions based on the designers preferences?
5 Do delta fighters have higher instantaneous as well as sustained turn rates?
6 Why were canards needed on a simple delta fighter?
7 Why is the location of these canards different in euro , j10 and rafale?
8 Do LERX of jf-17 generate lift? I s this lift controllable or does it have another control on the back of the aircraft?
9 Do canards generate controlled lift?
10 Has the JF-17s wing loading been published?
11 Where would a fighter having higher sustained turn rate outclass a fighter with higher Inst. TR
12 Has IAF developed specially designed bombs with reduced frontal RCS to reduce the fronal RCS of a fully loaded LCA?


@amardeep mishra

If you are claiming to be a more knowledgeable member in this field, then you should cover all the bases for new members.

I do not want to delve in a debate of Tejas vs. Thunder which will undoubtedly lead to a lot of trolling from both sides. Let just discuss both as separate cases and discuss what design choices both airforces made and may be why, that I think will be a more productive discussion.

All design decisions for fighter aircraft are trade-offs and are based on what requirements they are setting and those requirements are set based on experiences those air forces already had and what they see as potential threats and how they are thinking of dealing with those.

For JF-17, PAF I think wanted first and foremost to have an excellent dog fighter, plane form wise they had lot of experience with Mirages, F-16s and had even got modified cranked alpha F-7s. I think the aim was to have a highly manoeuvrable jet but with energy conservation like in F-16 and with a significantly higher AOA like F-18 which may be some PAF guys had experience dog fighting against. All design decisions related to lift, drag, wing-loading were in this regards. And the resulting fighter I am very sure is 'satisfying'.

Wing loading:
Too low wing loading is not a desirable variable unless you want to build a bomber. It does result in inducing a lot of drag especially at lower altitudes and depletes energy faster that is why most tailless alphas and canard alphas prefer to fight at a higher altitude at higher super sonic speeds.
In case of thunder, I think the aim was to 'balance' it in regards to energy and be able to fight equally in transonic ranges.

ITR and STR
Canard alphas do have beter inst. turn rates than F-16 or JF-17 but not better sustained turn rates. Sustained turn rates mean maintaining your turn rate and speed and altitude, F-16 still beats all canard alphas in this regard whether at lower altitudes or higher altitudes. And you can well make your inferences for thunder, it is an excellent energy maintainer.
Also I'll mention one more thing about inst. turn rate. In dog fight it is not only the ability to break hard which is important but how quickly you can roll i.e. roll onset rates and also does your jet still have enough control left to even yaw or not. Also at high Gs roll on-sets decline pretty fast and yawing can be out of question. An excellent dog fighter should still have decent roll onset and some controllable yaw at higher AOAs, that can make a ton of difference.
It will be very hard to even try getting on tail of a fighter which has better roll onset and yaw at higher AOA even if you can occasionally get your nose to point in the direction of the fighter at cost of losing a lot of energy.

About canards:
Canards are not a panacea if they had been Russian would have kept them on their latest fighters even when they added canards to indian Su-30s on request. On European tailless deltas I think they included them to off-set lower turn rates at medium to low altitudes and to achieve higher AOA at such altitudes with unstable design. But canards do add drag and also interfere with vortices a lot (although french claim that their software can move them so that this effect is decreased), this will result in issues with roll and yaw at higher AOA.

LERX may generate lift depending upon design of it and even fuselage does generate lift, how much lift depends upon the design e.g. F-16's fuselage is touted to generate more than 35% lift (according to public sources, it may be more if you know more).

I do not think any one publishes publicly accurate lift and other data and should we.

Now about Tejas:
It looks to be a good light weight fighter which I think is more in tune with IAF's experience with Mirage 2k and should be a fine fighter with all the pros or cons of a tail less delta. But I am not fully sure why they chose a reverse compound alpha design. Logically they should have gone for a design more like F-16 based cranked arrow design, that would have resulted in higher turn rates. But may be their designers thought that this will result in a better performance at high altitude and super sonic ranges may be for BVR combat. But does not look to be a good design choice, you can go super sonic fast if your engine responds fast and handling may be an issue with such a design at high AOA at lower altitudes and subsonic speeds. But as they say the taste of pudding is in eating, I might be wrong.

 

[punit]

I am not an aerospace engineer, but the turn performance and of TEJAS and all Mirage derived aircraft is extremely poor. As a design, TEJAS is already obsolete. Some of our boys examined it in Bahrian and they couldn't hold back their laughter. Sigh of relief for Indians to be scrapping the Rafale and going for TEJAS.

this was an extremely professional reply with lot of technical details. Isnt it @Oscar ?

 

[ghazi768]

sir just a noob question if say in a air to air combat you have HMDS+HOBS combo and your enemy dose not have it what will you to switch off HMDS-HOBS combo and fight him the conventional way to show your fighting skills or as soon as he is in NEZ of your HOBS missile you will shoot and scoot ? just asking no intention of trolling .. thanks waiting for your response sir ?

Are you sure that your enemy does not have a HMS+HOBS combo, I think you should read the details of PL-5EII. And look for anywhere where it mentions 'all aspect IR homing' and than work out what all aspect means and what +- ranges it has.

 

[Omega007]

Now about Tejas:
It looks to be a good light weight fighter which I think is more in tune with IAF's experience with Mirage 2k and should be a fine fighter with all the pros or cons of a tail less delta. But I am not fully sure why they chose a reverse compound alpha design. Logically they should have gone for a design more like F-16 based cranked arrow design, that would have resulted in higher turn rates. But may be their designers thought that this will result in a better performance at high altitude and super sonic ranges may be for BVR combat. But does not look to be a good design choice, you can go super sonic fast if your engine responds fast and handling may be an issue with such a design at high AOA at lower altitudes and subsonic speeds. But as they say the taste of pudding is in eating, I might be wrong.

May be because the LCA was from its start was developed to replace the MiG 21 variants in the interceptor role, so the designers may have put greater emphasise on faster climb rate and better ITR than energy conservation.After all,it was never meant to be the front line fighter of the IAF!!

 

[Taygibay]

Changing doctrine to optimise on the advantages of a particular design is actually a very interesting subject.

Actually, it's routine, mate . . . every time the weapon provided to the forces improves
the performance enough over past products ( as was the case with the long bow ).
In other words, you adapt the tactical book if the new tool is revolutionary in some way.

Sir any idea regarding composite usage and the resultant decrease in frontal rcs of a loaded mission ready fighter?

No link between the two exists. To give you just one example, think of radar transparency.
If you have a huge metallic main structure inside the plane and cover it with composites
that are transparent to radar waves, you are showing this immense radar reflector more!
It is quite possible that the metal skin if properly shaped hid the aircraft better by reflecting
those waves in off-directions.

To quote Albert Einstein:
“If you can't explain it to a six year old, you don't understand it yourself.”

Yeap, and centuries before Boileau had rightly said :
"Ce qui se conçoit bien s'énonce clairement et les mots pour le dire arrivent aisément.
/ What is correctly thought is easily expressed and words to describe it come easily."

It's an excellent light to write by. Thanks mate!

And in general good day all, Tay.