There was a comment on Prof. Prodyut's site:
What I want to understand is if this sort of reversal is possible/advisable.
We have seen this kind of wing on UCAV and F-16XL but both these are not meant for high performance dogfights or at least I would believe will have a tough task keeping up.
For a dogfighter esp. the high ITR cut and slash kind, I think a wing should be the way it is currently for LCA. Low sweep closer in and attached to the fuselage at the longest possible axis and High sweep at the outside where the vortex strength would not put unduely higher forces on the extended wing.
F-16XL was also characterised by its high payload capacity. It did gain on the ITR but that I guess was because stock F-16 was poorer on the ITR to begin with. So for F-16XL it made sense to do that but for LCA it does not make as much sense because LCA is not expected to be a bomb truck.
Ah! I don't know. I think you guys need to weigh in.
Problem is the wily prof has succeeded in confusing more people than he intended!!!
In BR I was surprised to find this comment.
All the so called drag for not following Whitcomb's rule more rigidly comes into play only in super sonic flight regime, not in subsonic flight regime of WVR combat speeds.
So no drag issues here also.
The comment was from Indranil Roy,
What I meant was many people were using the
CEMILAC report which mentioned the not so gradual increase in cross section from 4 to 5 meter on fuselage length being the prime reason for the Tejas not achieving its design top super sonic speed at sea level(no brochure says tejas is super sonic at all levels)
as a diagnosis of general drag(which people allege ,ADA failed to notice during wind tunnel or computer modeling) which prevents Tejas from reaching its ASR specs on STR and G limit(which ASR is again no one knows, Tamil Mani Aero DG was last heard talking about 1994 ASR, i.e an ASR 5 years after design freeze).
What I wanted to point out was the general ITR and STR specs are not for supersonic speeds, but corner velocities below super sonic speeds.
In those speeds CEMILAC diagnosis does not hold water.
Good if some one from other forum, Indranil Roy himself, or any one from DFI can explain this .
ANother problem with prof das article is the comparison table, It is like comparing the anatomies of dinosaur and Tiger to reach a conclusion of physique needed for top speed and performance.
Why? Because all the fighter in the comparison table has relation to RSS 4.5 th gen compound delta Tejas. They are all historic fighters with mostly Positive stability and high wing loading design era, which was long left behind by designers Mirage-2000 onwards.
And the reason for the lower sweep angle near wing root is vortex generation to delay the on set of stall at high alpha,(the job of strakes in mirage-2000 and primary job of canards in eurocanards) which was graphically explained by design papers . SO reversing the sweep is some strange idea nowhere related to tejas airframe.
This is typical Das reply which I almost expected.
M/s Shakthivel and Pathak
You are obviously not reading what I have written and I find it tiresome to point out errors of your understanding.
I have taken clean full fuel weight ( 2400 kilos) BECAUSE IN ALL THE COMPARISONS IT WAS NOT POSSIBLE TO GET THE INTERNAL FUEL CAPACITY. THIS DOES NOT CHANGE THE FIGURES OF COMPARISON because identical parameters were used in all cases.
OBVIOUSLY YOU HAVE NOT READ OR UNDERSTOOD.
I am refraining from answering your other questions because I have failde to understand what you are driving at. Perhaps if we meet sometime.
BTW Whitcomb's area rule will change with the speed you are designing for. Many modern designers will ignore it e.g. Lee Begin did in the change from Northrop F5 E to F20 but it is too long to discuss here!
MR. Pathak in which course did you graduate out ?
SO Mr das "fails to understand" the points.And feel it is tiresome.
Giving source to all the basis behind calculation is a practice followed in college level projects!!!, which is something that can not be adhered by Das, who took the pains to suggest a "new wing design" for Tejas !!!
SO there is no need to take anything written by Mr. Das seriously ,
As far as Muthuk Pathak's last comment is concerned which goes on to say,
One of the biggest disadvantage of low wingloading is poor gust tolerance and if we have wing like LCA, it also means extremely poor CLmax so even with low wing loading, we end up having a high stall speed which defeats the very purpose of having low wing loading
.
low wing loading on tejas was proposed with the primary aim of high ITR not for high or low stall speeds(it is another matter no one knows these specs of tejas)
for this part,
LCA as a design is inefficient and it is for this reason that they are now applying fixes to it in the form of Levcons etc. The best is to just change the wing and instead of 50*-62.5* make it 62.5*-50* and add LERX. This will reduce the wing area to a more respectable 30sqm and also take the AR to 2.25 which is the ideal AR for high speed interceptors. This will also help increase the wing fuel load.
LEVCONS are no fixes !!! Even in 2001 article on tejas by none other than Air Marshal MSD Woolen LEVCONs for naval tejas was proposed for carrier landing requirements.
If we go by efficiency of a design, A wing-tail, unstable combo is best, canard-delta RSS is second best, a wing-tail, stable design is third best and last is tailless delta RSS/Stable. All combinations of canard-delta and tailless delta suffer from high landing speeds as they can't handle double slotted flaps at TE of the wing.
As far the part above without going into specs like a combo of TWR and low wing loading advantage making suggestion on which fighter wing design is best is a suicidal job. Does this mean the russians who designed SU-35 and PAKFA didn't know of these "facts"?
LCA as a design is inefficient and it is for this reason that they are now applying fixes to it in the form of Levcons etc. The best is to just change the wing and instead of 50*-62.5* make it 62.5*-50* and add LERX. This will reduce the wing area to a more respectable 30sqm and also take the AR to 2.25 which is the ideal AR for high speed interceptors. This will also help increase the wing fuel load.
If this suggestion is accepted who will do the job of vortex generation which is presently done by lower swept wing leading edge root near fuselage. Why this "great design " idea of reversing the wing sweep is not followed by Russians in PAKFA ?
In its present shape, LCA is still a capable ac and comparing it with JF-17/Mig-21 etc is wrong. this ac even today can easily take on these ac in combat. In air combat, there is a way in which you use a delta planform. today we have HMDS and agility has lost most of its meaning with missiles like R-73 but even in guns only fight which is a true dogfight, LCA will outperform the ac which you have listed but the very big question is, where will you go to fight with just 2.4tons of fuel? you will use up 200kgs in start up taxi & T/O. You will need another 200Kgs for landing circuit+400 kgs as standard reserve. That leaves you with just 1600 kgs to go to the combat zone and return. Mig-21 uses about 2000ltrs/hr and that gives it a range of 750 kms and is a turbojet engine, LCA has a turbofan, so we can assume a range of about 1000kms but than we need fuel for combat also, so we are down to a combat radius which is very easily equal to just about that of Mig-21.
LCA Mk2 will not offer much of a respite, ADA must redesign the wing and increase the length to be 14.5m atleast. that will give it more fuel and better agility.
combat radius of tejas mk1 without certification of center line fuselage was given as 500 Km for hot indian climatic condition which saps 10 percent engine thrust and
12 percent wing lift for any other fighter. SO why the fuss?
since I correctly guessed that Mr Pathak is Decklander here in DFI, I would welcome him again for a debate here, He left the forum on a big political fight with one of the prominent member here on account of BJP Vs AAP politics. It would be nice to have him here again.
owah!:
