LCA Tejas: News and Discussions

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Re: LCA Tejas: News and Discussions

Postby Gyan » 31 Mar 2015 08:28

USA had manufactered 50 F-22 prior to IOC. Similarly USA will have produced 500 F-35 prior to FOC

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Re: LCA Tejas: News and Discussions

Postby brar_w » 31 Mar 2015 08:36

^^F-22 @ IOC - 50, @ FOC 101
F-35 @ FOC (FOC for USAF, USMC): 336 delivered (A,B,C) with 200-250 in some stage of production.

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Re: LCA Tejas: News and Discussions

Postby rohitvats » 31 Mar 2015 08:52

brar_w wrote:^^F-22 @ IOC - 50, @ FOC 101
F-35 @ FOC (FOC for USAF, USMC): 336 delivered (A,B,C) with 200-250 in some stage of production.


Will it be possible for you to list down the set of milestones which needed to met at IOC and FOC stage? Some source which documents this? Thanks.

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Re: LCA Tejas: News and Discussions

Postby brar_w » 31 Mar 2015 09:12

Its a bit different for both platforms. I can post a more detialed set later but the basics were the air superiority mission with a specified software built that was completely tested and certified by the proper authorities. The full Operational Testing may or may not have been completed but the requirement is usually to have completed all test-points and received certification for the capability being sought at IOC. FOC usually means complete OT certification, fully installed, debugged and certified EMD contracted Software build and weapons envelope capability.

For the F-35 the IOC is defined by each operator differently. The Marines want software 2B, with internal weapons capability for a CAS mission i.e. 2 bombs and 2 missiles. The USAF wants a similar capability for a first day light strike. In addition you have to complete all concurrency changes that have been identified during testing. This means you have to have the logistical capability to incorporate those changes at the depot for a dozen or so F-35's at a time (10-11 for the USMC and a similar amount for the USAF) and also receive complete certification for that envelope.

FOC for the F-35 involves all OT testing certification, full SDD (system development) capability delivery, 3F software and all weapons clearance. Post IOC and Pre FOC there would be no hardware change to the jet (other than Marine Corps jets that will receive new ICP in 2016 but that is going to happen every other block irrespective of FOC as its a part of the growth roadmap) and the only change would be in the software addition. Weapons certification, envelope expansion for the weapons continue post IOC all the way till 2018 (2 years from USAF IOC). Beyond mid 2018 the only thing left would be software build delivery which is expected to face a 6 month delay. So FOC may come for the USAF/USMC around early 2019.

The USN has not defined a proper FOC but many think it is a block 4I configuration which is slated for 2021 or 2 years after they IOC.

Over the years, especially for large programs the services in the US have taken a more pragmatic view on IOC. Its a way to unshackle yourself from the developmental program that is usually slow and cautious. For example each and every sortie of the F-35 has to be video recorded on the ground (take offs, landings, taxis etc) and there are other restrictions that are controlled by the Joint program office. Basically the services want to be unshackle form these restrictions and fully utilize the jet for training and ( and to begin to absorb the jet in the larger force structure) are looking to get out as soon as the entire system is stable enough to allow them to do so. In July the Marines declare IOC...They'll take another 12 months to really get a hang of the jet and won't be deploying outside of the US till early 2017. The USAF IOC's in 2016 (August) and they'll take another 12-15 months to fully absorb the capability...IOC is very much a soft-induction of sorts and most are looking at full SDD capability which itself would come into the fleet in 2017 (thats when block 3F is delivered) but the certifications for that wont show up till a year later. The F-35 is likely to see at least a couple of red-flags before it sees FOC as the aim is to get it out fast and get the pilots, maintainers fully trained and proficient on the jet. They also want to integrate 5th and 4th generation jets as quickly as possible as they have to learn to live and fight together. All that happens in ernest once the Services have "full control" over their weapon systems which they only get post IOC. Even now (pre-IOC) The Marines have begun doing simulated CAS missions, and have been even training 2 ship and 4 ship flights with F-18E/F's against 'reds' out of Yuma but that for the moment is usually in support of tactic development or some other technicality that allows them to do so.

Many wrongly believe that IOC is a major event when its really not that big of a deal. It takes months if not years to absorb a new capability into your force structure. The one advantage the F-35 has is numbers so no one will have to wait for production to catch up but regardless its going to take time to get it fully integrated into the USAF and its not as easy one may think.

Here's a brief on the F-35 IOC criteria and requisites : http://breakingdefense.com/wp-content/u ... _FINAL.pdf
Last edited by brar_w on 31 Mar 2015 17:53, edited 3 times in total.

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Re: LCA Tejas: News and Discussions

Postby Yagnasri » 31 Mar 2015 09:22

It is not just how much built before or after FOC. How many are expected to be ordered is the question. We do not know how many LCAs will be ordered in near future. In this situation how the production etc can be planned?

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Re: LCA Tejas: News and Discussions

Postby agupta » 31 Mar 2015 09:26

^^

Simply plan what was promised in the contract for a known number. N_aircraft by 2XXX (so assumed yield, EMD execution) - that is supposed to be the advantage for a vertically integrated player.

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Re: LCA Tejas: News and Discussions

Postby Kartik » 31 Mar 2015 13:37

srin wrote:Basic aero pooch out of curiosity: for area rule, do they calculate cross-sectional area with the external stores and without ? Because, pylons and drop tanks seem to noticeably increase the cross-sectional quite a bit ...


you cannot design an aircraft around optional stores that it may carry - the reason being that those stores are optional and the aircraft with each store will give a different cross-sectional area..how do you optimize for that?

So it is done with the aircraft in a clean configuration only.

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Re: LCA Tejas: News and Discussions

Postby tsarkar » 31 Mar 2015 17:43

JE Menon wrote:...wings bent almost flexibly like a bird...
That is a deliberate design feature. Its a cranked anhedral and the only plane in the world to have it. Supposed to help with High AoA.

http://cdn-www.airliners.net/aviation-p ... 867325.jpg
http://newsonair.com/feature-image/LCA-tejas-header.jpg

In plain visual terms, when seen from the side, the inner wing is at an angle to the longitudinal axis of the aircraft.

However, this wing angle or anhedral gradually reduces as we move spanwise along the wing.

As we reach towards the outer span of the wing, angle becomes zero. The outer wing is actually parallel or in-line with the longitudinal axis of the aircraft.

In theory, when the inner wing starts to stall at progressively higher AoA, the outer wing will still be at a slightly lesser AoA and provide sufficient lift.

I'm not sure whether flight test results on the actual performance of this wingform have been published, but it'll be interesting reading.

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Re: LCA Tejas: News and Discussions

Postby JE Menon » 02 Apr 2015 09:48

Thank you doc & tsarkar. Yes shiv that's the picture or something very close to it. I found it very beautiful but only saw it once on BRF I think and then couldn't relocate.

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Re: LCA Tejas: News and Discussions

Postby Raman » 02 Apr 2015 17:43

tsarkar wrote:
JE Menon wrote:...wings bent almost flexibly like a bird...
That is a deliberate design feature. Its a cranked anhedral and the only plane in the world to have it. Supposed to help with High AoA.

http://cdn-www.airliners.net/aviation-p ... 867325.jpg
http://newsonair.com/feature-image/LCA-tejas-header.jpg

In plain visual terms, when seen from the side, the inner wing is at an angle to the longitudinal axis of the aircraft.

However, this wing angle or anhedral gradually reduces as we move spanwise along the wing.

As we reach towards the outer span of the wing, angle becomes zero. The outer wing is actually parallel or in-line with the longitudinal axis of the aircraft.

In theory, when the inner wing starts to stall at progressively higher AoA, the outer wing will still be at a slightly lesser AoA and provide sufficient lift.

I'm not sure whether flight test results on the actual performance of this wingform have been published, but it'll be interesting reading.


tsarkar,

I'm afraid that's not what anhedral is. Anhedral is the opposite of dihedral - it is, informally speaking, when the wing tips are lower than the wing roots when viewed head on. E.g., C-17 and IL-76 show pronounced anhedral. The primary reason for providing anhedral is lateral stability. Some high-wing swept-wing airplanes can be "too stable" and therefore difficult to maneuver because high wings and wing sweep both contribute to lateral stability. Anhedral decreases lateral stability. (In comparison, dihedral increases lateral stability.)

The phenomenon you are describing is called washout, when the sectional coefficient of lift is gradually reduced spanwise from root to tip. Washout can be geometric (where, the wing is actually "twisted" so the roots have higher angle of attack than the tips), or aerodynamic (where different airfoil sections are used along the span for the same effect), or (most often in today's airplanes) a combination of both.

Designers include washout for two reasons:
- As you correctly point out, it prevents the whole wing from stalling all at once.
- By having higher lift at the roots and lower lift at the tips, they are approximating an elliptical lift distribution along the span of the wing. The closer you can get the lift profile to match this elliptic lift distribution, the less your induced drag.

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Re: LCA Tejas: News and Discussions

Postby vardhank » 02 Apr 2015 17:48

^ Reminds me vaguely of the Stuka

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Re: LCA Tejas: News and Discussions

Postby JayS » 03 Apr 2015 01:33

Raman wrote:
tsarkar,

I'm afraid that's not what anhedral is. Anhedral is the opposite of dihedral - it is, informally speaking, when the wing tips are lower than the wing roots when viewed head on. E.g., C-17 and IL-76 show pronounced anhedral. The primary reason for providing anhedral is lateral stability. Some high-wing swept-wing airplanes can be "too stable" and therefore difficult to maneuver because high wings and wing sweep both contribute to lateral stability. Anhedral decreases lateral stability. (In comparison, dihedral increases lateral stability.)

The phenomenon you are describing is called washout, when the sectional coefficient of lift is gradually reduced spanwise from root to tip. Washout can be geometric (where, the wing is actually "twisted" so the roots have higher angle of attack than the tips), or aerodynamic (where different airfoil sections are used along the span for the same effect), or (most often in today's airplanes) a combination of both.

Designers include washout for two reasons:
- As you correctly point out, it prevents the whole wing from stalling all at once.
- By having higher lift at the roots and lower lift at the tips, they are approximating an elliptical lift distribution along the span of the wing. The closer you can get the lift profile to match this elliptic lift distribution, the less your induced drag.


You missed the main motivation behind the washout requirement: To keep aileron authority in stall. If tip stalls first, ailerons become useless.

In my opinion, LCA has neither anhedral (definitely not anhedral) nor conventional wing-twist. It seems to have conical camber. But I do not have hard proof from ADA publication mentioning this explicitly, though enough suggestive proof for me to work in this direction exists.

Anyway something about LEVCON on NLCA:

I studied a paper by ADA guys (AIAA-2008-336) on LEVCON. And also found one series of studies from NASA on this (few papers, yet to digest all) - they called it inboard LE vortex flap. But they really work on same principle viz. decoupling of inboard and outboard LE vortex flows by reducing the spanwise drift of inboard flow and thus reducing the mingling of inboard and outboard flows. Both of them are 'Wing Apex VOrtex Controller'. Looks like the concept is been around for sometime.

Digest this:
-- LCA's pitch-up tendency is due to both typical reasons: 1. Loss of lift on aft sections due to vortex burst 2. increased lift at apex region
-- LEVCON do not produce any leading edge vortex to enhance lift like LERX.
-- On contrary, it controls the wing apex LE vortex by suppressing it and keeping the flow over the inboard wing section attached. In fact there is no inboard vortex at all for +20deg deflection even for moderately large AoA. :-o
-- It does reduce the vortex-induced lift on the outboard but compensates for it by having attached flow on inboard part.
-- The vortex bursting is delayed and so is upstream movement of vortex bursting point.
-- Also at +20deg deflection it almost eliminates the pitch-up tendency. There is slight increase in L/D ratio giving little better efficiency (something like 5% is my guess).
-- So looks like all LEVCON does is improve handling of the LCA at high AoA and thus enable the pilot to fly it at higher AoA than w/o LEVCON limit. This is through reduction in pitch up moment.
-- There isn't any lift enhancement due to LEVCON so to speak, only a slight reduction in drag I think, which gives a slightly better L/D.
-- Some improvement in directional stability as well in high AoA regime.

The study was done w/o LE slats or TE flaps into consideration, but that should not change the things on the inboard sections and for wing as overall, qualitatively.

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Re: LCA Tejas: News and Discussions

Postby Indranil » 03 Apr 2015 03:05

nileshjr wrote:
In my opinion, LCA has neither anhedral (definitely not anhedral) nor conventional wing-twist. It seems to have conical camber. But I do not have hard proof from ADA publication mentioning this explicitly, though enough suggestive proof for me to work in this direction exists.

I am quickly coming over to the dark side of conical camber as well. :) Its effect near the wingtip LE is fairly visible. But I still don't understand the inboard section completely. Its too non-planar (if you know what I mean).

nileshjr wrote:
Anyway something about LEVCON on NLCA:

I studied a paper by ADA guys (AIAA-2008-336) on LEVCON. And also found one series of studies from NASA on this (few papers, yet to digest all) - they called it inboard LE vortex flap. But they really work on same principle viz. decoupling of inboard and outboard LE vortex flows by reducing the spanwise drift of inboard flow and thus reducing the mingling of inboard and outboard flows. Both of them are 'Wing Apex VOrtex Controller'. Looks like the concept is been around for sometime.

Digest this:
-- LCA's pitch-up tendency is due to both typical reasons: 1. Loss of lift on aft sections due to vortex burst 2. increased lift at apex region
-- LEVCON do not produce any leading edge vortex to enhance lift like LERX.
-- On contrary, it controls the wing apex LE vortex by suppressing it and keeping the flow over the inboard wing section attached. In fact there is no inboard vortex at all for +20deg deflection even for moderately large AoA. :-o
-- It does reduce the vortex-induced lift on the outboard but compensates for it by having attached flow on inboard part.
-- The vortex bursting is delayed and so is upstream movement of vortex bursting point.
-- Also at +20deg deflection it almost eliminates the pitch-up tendency. There is slight increase in L/D ratio giving little better efficiency (something like 5% is my guess).
-- So looks like all LEVCON does is improve handling of the LCA at high AoA and thus enable the pilot to fly it at higher AoA than w/o LEVCON limit. This is through reduction in pitch up moment.
-- There isn't any lift enhancement due to LEVCON so to speak, only a slight reduction in drag I think, which gives a slightly better L/D.
-- Some improvement in directional stability as well in high AoA regime.

The study was done w/o LE slats or TE flaps into consideration, but that should not change the things on the inboard sections and for wing as overall, qualitatively.

Thanks for finding out the literature on this. I have never read about the bold part before. You are on a spree here. Please keep unearthing.

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Re: LCA Tejas: News and Discussions

Postby Singha » 03 Apr 2015 07:11

however despite all this the certified AoA of the tejas so far is less than the far simpler Mirage2K config and less than EF and Rafale and F18

something does not add up...unless some other part of the design is so bad, the wing stuff above just brings it back to even keel.

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Re: LCA Tejas: News and Discussions

Postby negi » 03 Apr 2015 09:18

Well looks like the AOA numbers as per the manual have some safety margin look at the Page 10 on this article the table of AOA as a function of Mach number for Mig-21. I would take some of the observations with a pinch of salt but if the data is not fudged it looks impressive for such an old fighter.

http://www.slideshare.net/mishanbgd/mi- ... days-terms

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Re: LCA Tejas: News and Discussions

Postby negi » 03 Apr 2015 09:29

This was posted here long time back by one of the oldies

http://www.scribd.com/doc/78345390/Appr ... jas#scribd


From Page 5
"Currently the Tejas is flying to AoAlimits of 20 and 22 never exceed. Fortunately as shown in fig-6, the LCA hassignificant rudder authority (CYM-Del R) even up to 30 AoA that will allow artificial stabilization in yaw at high AoA"

Like all other things with Tejas we have a lot of safety margins built in , I think in next 5 years we will open up the flight envelope even more and get an idea about actual achievable AoA for a given Mach number and loading.

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Re: LCA Tejas: News and Discussions

Postby Indranil » 03 Apr 2015 11:55

There are quite a few things to explain here, so I will keep it short :) .

1. AoA is a very naive way of evaluating a fighters performance. A plane at 26 degree AoA may turn much faster than another at 28 degree AoA.
2. Mig the data is not smudged. It is the norm. LCA wing does not stall at 35 degree AoA either. The alpha limits of modern fighters are nit set by the critical angle of the wing.
3. The reason why modern planes have to be limited to a smaller AoA is a trade-off. Modern planes are much more instantaneous (crisp) in their response to pilot input. This requires "unstability" instead of the tendency to fly straight (hallmark of older designs). The result is that modern planes become too unstable beyond a certain AoA. It is difficult to maintain authority even with a computer at the helm.
4. I don't know where you guys are getting your data from. But FBW of most modern planes limit AoA around 25 degrees. F-16: 25 degrees, E/F: 25 degrees, Gripen:26, Rafale: 32

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Re: LCA Tejas: News and Discussions

Postby JayS » 03 Apr 2015 12:08

negi wrote:Well looks like the AOA numbers as per the manual have some safety margin look at the Page 10 on this article the table of AOA as a function of Mach number for Mig-21. I would take some of the observations with a pinch of salt but if the data is not fudged it looks impressive for such an old fighter.

http://www.slideshare.net/mishanbgd/mi- ... days-terms


Amazing. Its quite counter-intuitive to see Mig-21 could be better machine at low speeds (If whatever mentioned there is all true).

The safety margin would depend upon how well you know your machine. High AoA regime is still quite a shady region. In case of Su-27, the Russians did not realize the full potential of the wing initially. They even thought that its impossible for it to come out of spin (WT spin tests had showed so) and thus they restricted the AoA to quite less value. The standard procedure was to bail-out if it enters the spin. But during service, further test flights and with more investigations at TsAGI, they discovered unimaginable characteristics including much better high-AoA capability, super-maneuverability and ability to come out of spin even without manual intervention.

I am sure these machines are not explored to its full aerodynamic potential, because its too complicated, dangerous and time and money consuming. Apart from Su-27, F-16XL is the good example of what can be achieved with more work on same machine.

negi wrote:This was posted here long time back by one of the oldies

http://www.scribd.com/doc/78345390/Appr ... jas#scribd


From Page 5
"Currently the Tejas is flying to AoAlimits of 20 and 22 never exceed. Fortunately as shown in fig-6, the LCA hassignificant rudder authority (CYM-Del R) even up to 30 AoA that will allow artificial stabilization in yaw at high AoA"

Like all other things with Tejas we have a lot of safety margins built in , I think in next 5 years we will open up the flight envelope even more and get an idea about actual achievable AoA for a given Mach number and loading.


LCA could achieve 28deg AoA with little efforts I believe (26deg is done thing already). Every jet is explored slowly. With wider user base more and more things comes to light which were not seen in testing phase, as user has a different take on the system. Every jet is fine-tuned after it comes to service and user feedback is received. Even the best of the best of the best do not come out of factory free of bugs. Thats the reason I think IAF should have inducted LCA after IOC and started using it. It would have helped ADA big time in fine-tuning the machine even further to IAF's doctrine of use. Now this phase will come only after FOC. Doesn't make sense to give a machine Initial Operational Clearance and then not operationalise it at all. JMT.

Singha wrote:however despite all this the certified AoA of the Tejas so far is less than the far simpler Mirage2K config and less than EF and Rafale and F18

something does not add up...unless some other part of the design is so bad, the wing stuff above just brings it back to even keel.


"Wing Stuff" :lol:

Could you please list down the AoA limits with the respective flight conditions at which it is applicable for above mentioned jets?? I would love to see the numbers. BTW, the LEVCONs do not extend the allowable AoA, they just make the life much better at moderate to high AoA.

The max AoA regime is heavily restricted and is rarely reached in normal use. We start seeing a lot of coupling in lateral-directional motions and effectiveness of control surfaces reduce. There is nothing wrong with LCA wing so to speak, if you look at hard data (of coarse its not the best in the world but its not the worst of the lot either). All the delta jets suffer from the same issues, its how you manage the short-comings. We haven't really tried to improve/fine tune its characteristics. LCA still remains quite a clean configuration. F-18 for example showed pretty bad tail buffet issues at high AoA, after getting inducted. Its only after extensive studies and some modifications to air-frame that they satisfactorily work at higher AoA. LCA is not quite in that phase yet.

Aerodynamically speaking, LCA is not awesome (as compared to something like Su-30MKI). But I like it, because, its our baby.

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Re: LCA Tejas: News and Discussions

Postby Singha » 03 Apr 2015 12:33

^^ :rotfl: well I love it too, and I am all for producing and using 150 mk1 with primary A2G role and secondary CAP role if IAF feels with its warts its not suited to a offensive air to air role.
follow that up with another 300 Mk2.

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Re: LCA Tejas: News and Discussions

Postby JayS » 03 Apr 2015 13:35

Only way to know if LCA is worth its salt in A2A role is to fly it in 'Red vs Blue' scenario in some IAF exercise. Let the bird prove its mettle. :mrgreen:

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Re: LCA Tejas: News and Discussions

Postby negi » 03 Apr 2015 14:04

I remember the 26 deg AoA part iirc Saurav Jha had tweeted that. Just curious at what altitude and airspeed ?

I also had one question about spin recovery is it required for an AC to actually be able to demonstrate spin recovery in field before being put into operational service ?

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Re: LCA Tejas: News and Discussions

Postby maitya » 03 Apr 2015 14:57

Singha wrote:however despite all this the certified AoA of the tejas so far is less than the far simpler Mirage2K config and less than EF and Rafale and F18

something does not add up...unless some other part of the design is so bad, the wing stuff above just brings it back to even keel.

Err Singhaji, why is this need/love/fetish for higher AoA etc - if the effectiveness of a military platform is to be measured by it's maneuverability, then turn-rate is a principal factor (other factors like roll rate and pitch-up rate etc are also important factors) that needs considering.
But ITR, arguable the more important maneuverability factor, in these days of BVR engagement (and also in WVR the high off-boresight engagement) capabilities, is co-related to the Aerodynamic Load Factor. And this Aerodynamic Load Factor is dependent upon the by Max Lift Coeff of the wing-planform design.

And normally, for a delta (and even for a non-delta) planform the Lift Coeff increases with increasing AoA.

So if your wing planform is efficient enough to "produce" enough lift Coeff (and thus the resultant Aerodynamic Load Factor) for an design-goal ITR, where is the need of more and more AoA achievement?
Aka, if you want to achieve a 20-22deg/s ITR at 8G (and at 22K ft and 0.9M), in an M2K you get it at ~28deg AoA.
But if your SDRE LCA gets the same ITR in ~22deg AoA, where is the need of it being able to achieve 28-30deg AoA.
This is made possible by clever mixing of slender-and-non-slender design principals, to better optimise the usage of vortex-induced lift forces resulting in superior Lift Coeff for a given AoA regime.


Now coming to STR, again why is this more AoA need? Delta/non-delta whatever, plain and simple fact is more AoA results in more drag (Drag Coeff monotonically increases with increasing AoA) - and more drag means more tapping into the precious TWR to overcome this and able to sustain this turn-rate.
So again, if a certain STR is being achieved at a lower AoA, then you save that much more on TWR, which can then be spent on carrying more external stores/internal fuel etc.

However STR is an Achilles heel of any pure-delta design due to higher drag Coeff values compared to non-delta designs, thus requiring that much more brute force to overcome this monotonic increase of drag Coeff with increasing AoA.
And LCA is no exception.
But then again it more cleverly uses the mixing of slender-and-non-slender design principles (for e.g. the flow re-attachment phenomenon), to postpone this drag creep (and thus preventing eating into the lift forces of the lift Coeff) as much as possible.
So, for a given AoA, wrt STR the LCA wins again wrt other delta designs, but still wouldn't match up on STR with non-delta planforms of, say, F-18/F-16 etc.

Now, back in 1990s, IAF due to brochurities may have specified a desirable STR value lifting it from a non-delta platform brochure (and the ITR lifted from that of a delta platform) – typical attitude normally seen when theoretical problems are set for “science projects”.
Either way, difficult to achieve both together (TV etc are a diff story altogether), but LCA atleast makes an attempt to solve it by this mixture of slender and non-slender planform designs.
If it hasn't achieved it, not to lose heart as it will attempt to do so by increasing the more-than-desirable-AoA values normally required (by a non-delta platform) for those STR figure - but for that it needs to apportion that amount of additional thrust to overcome the additional drag it will encounter due to this delta AoA. And Thrust is not what it has with F404 (without sacrificing the MTOW etc figures) - so is the need of F414 etc in Mk2 etc.

[Added Later] I see Indranil, nilesjr adn negiji has already posted on this point while I's typing out this post - so pls take the above FWIW only.

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Re: LCA Tejas: News and Discussions

Postby negi » 03 Apr 2015 15:15

I do not think we should argue on such lines , remember when the MKIs went to Nellis the USAF too argued on similar lines i.e. what is the use of high AoA capability of the MKI or for that matter why is 3D TVC even required specially when you bleed a lot of energy when executing those maneuvers, in this day and age of sophisticated HMS and high off-boresight capable AAMs with LOBL and LOAL capabilities all these things are only good for an airshow.

Reason why AoA question was asked is because it gives a fair idea about how much flight envelope has been opened so far , if nothing else it gives a pilot a lot of room to push the machine to extremes should a situation arise now whether it is of use or not in modern arena is a different topic.

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Re: LCA Tejas: News and Discussions

Postby JayS » 03 Apr 2015 15:18

negi wrote:I remember the 26 deg AoA part iirc Saurav Jha had tweeted that. Just curious at what altitude and airspeed ?

I also had one question about spin recovery is it required for an AC to actually be able to demonstrate spin recovery in field before being put into operational service ?


I would say, if a fighter jet proves that it is very hard to get into spin, even if its impossible to come out of that spin, is good enough for it to get into service. If an aircraft is prone to get into spin and is unrecoverable then its a trouble. Quite a few fighter cannot come out of spin, particularly the flat spin. When very well trained professional pilots are flying the fighter jets which do not have tendency to get into spin easily, the possibility of spin in real life is less. And with FCS restricting the spin prone Flight envelope, it diminished even further.

OTOH, it would be imperative for trainers to have demonstrated ability to get into a spin and get out of it successfully.

Can't say at what altitude and speed LCA has achieved max AoA (factual info). Depending on weight, G-factor, excess thrust available, altitude max AoA could be achieved at varying speeds. Also sometimes its the available thrust which becomes limiting factor rather than the wing aerodynamics in achieving max AoA.

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Re: LCA Tejas: News and Discussions

Postby JTull » 03 Apr 2015 15:30

http://aviationweek.com/defense/f-35-flies-against-f-16-basic-fighter-maneuvers

Good description of F-35s high-alpha testing program.

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Re: LCA Tejas: News and Discussions

Postby JayS » 03 Apr 2015 15:42

negi wrote:I do not think we should argue on such lines , remember when the MKIs went to Nellis the USAF too argued on similar lines i.e. what is the use of high AoA capability of the MKI or for that matter why is 3D TVC even required specially when you bleed a lot of energy when executing those maneuvers, in this day and age of sophisticated HMS and high off-boresight capable AAMs with LOBL and LOAL capabilities all these things are only good for an airshow.


The claims and counter-claims related to usefulness of super-maneuverability are amusing. Russians are fully devoted to super-maneuverability and are fully convinced about its use in dog fights, thus all their moderns fighters have high degree of super-maneuverability. Americans, always downplayed the usefulness of super-maneuverability by giving energy-argument. (According to some the energy theory itself is flawed, making whole argument irrelevant). But, then Americans did invest a lot of efforts in studying the unsteady aerodynamics (was it X-31? and HARV) and even achieving elements of super-maneuverability in F-22.

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Re: LCA Tejas: News and Discussions

Postby brar_w » 03 Apr 2015 17:18

The ability to hold HAOA and post-stall capability along with the ability to execute a quick turn has been investigated in the 80's, 90's and beyond under various programs as either the primary objective or one of the secondary objectives. The X-31 is obviously the most famous one but the X-29, F-18 HARV and to some extent the F-16 MATV (From the VISTA offshoot) also need a mention here. This is obviously in addition to actually physically evaluating the Su-27 and Mig-29.
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Re: LCA Tejas: News and Discussions

Postby JayS » 03 Apr 2015 18:28

^^^
brar_w,

Long back you posted an old article from Janes about LCA (on this page https://forums.bharat-rakshak.com/viewtopic.php?t=6663&p=1773950) Do you have the original article?? And any more such references about LCA from that era?? Thats the only article which I found mentions LCA and conical camber on same page.

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Re: LCA Tejas: News and Discussions

Postby brar_w » 03 Apr 2015 20:02


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Re: LCA Tejas: News and Discussions

Postby Raveen » 03 Apr 2015 22:20

nileshjr wrote:
Aerodynamically speaking, LCA is not awesome...


Interesting post - could you please delve into this a little bit more?

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Re: LCA Tejas: News and Discussions

Postby DexterM » 03 Apr 2015 23:08

Raveen wrote:
nileshjr wrote:
Aerodynamically speaking, LCA is not awesome...


Interesting post - could you please delve into this a little bit more?

Especially since we're talking not about Thrust/Power, but aerodynamic profile. Please do educate us, Sir.

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Re: LCA Tejas: News and Discussions

Postby JayS » 04 Apr 2015 01:36

^^ I am still digging into more technical info. Posting as i find new things. So far I have half-baked theory about the working on LCA wing. Hopefully I would be able the crack the LCA mystery.

About that comment: Its quite arguable which config is better as performance data is not in public domain for any current fighter. When they designed Su-27. They stared with only wing. They perfected the wing to their liking and then they stared adding other parts one by one. Almost whole body contributes to the lift. It was a ground-breaking feat. Not for nothing, its a super manoeuvring configuration even in its cleanest form. Similarly if you see something like Sr-71, its an amazing machine. Also notice the wing-body blending in F-22 or Rafale. We will get there in due time. But as of now, LCA is not best configuration out there. LCA suffers through same problems that any other delta jet faces like pitch up tendency, bad high AoA handling etc. And it uses various features just like any other jet would. I don't like the wing-body blending in LCA as well. Some of the aerodynamic characteristics of LCA are not ideal such as sudden drop in directional stability at high AoA, uneven slope of C_l vs alpha curve, uneven pitching moment etc.

But Please don't jump on to any conclusions. Almost every machine has its share of undesirable things, there is nothing like ideal aerodynamic config. But just some are more awesome and impressive than others. Keep in mind that a lot of efforts, decades of experience goes into making of these machines and these are incremental upgrades over previous generations from the OEMs. ADA had to design a fighter with some ridiculous constraints, they had limited knowledge and resources and they got consultancy from Dassualt whose forte is delta wings (or was Dassualt chosen because ADA wanted to make Delta jet??). Its still a very good machine. Comparing it with Su-27 family or F-22 is not an Apple to Apple comparison. As I said it not the best in the world but it neither is the worst of the lot. For a fighter there are other things also which makes it a successful platform. (If I had to pick one most amazing military jet I might go with Mig-25, not because it is most elegant aerodynamic design, but because that welded stainless steel thing could fly at Mach 3, reach heights twice more than the service ceiling of most of of the jets and was still rugged and cheap enough that USSR could make it in hundreds - personal opinion*).

Sorry for a long ramble.

Some food for thought:
Image


Seeing this image, I am thinking, how the inboard section looks like 4th slat.

Now compare it with this following image of NLCA with Levcon. Any thoughts?
Image


Question: What are the chances that the LCA delta wing is clipped just to have acceptable downward visibility for pilot?? Had the wing had no clipped apex, would it hamper the look-down ability of pilot?? NLCA Levcon is already quite far ahead.
Last edited by JayS on 04 Apr 2015 03:16, edited 1 time in total.

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Re: LCA Tejas: News and Discussions

Postby Indranil » 04 Apr 2015 09:58

nileshjr wrote:Sorry for a long ramble.

Why are you sorry? You are discussing technical details which have sound basis. Look at the kind of people who are wanting to hear more from you: Raveen sir, Raman sir. We are listening.
nileshjr wrote:Some food for thought:
http://gallery.tejas.gov.in/Gallery/Calendar-2015/i-Fvt59kw/0/O/1900350_774539155973868_8385215092264313998_o.jpg


Seeing this image, I am thinking, how the inboard section looks like 4th slat.

That is my theory too. I wrote about this earlier (and with this exact picture :D ), but I did not go into too much detail. I guess it is time now.

nileshjr wrote:Now compare it with this following image of NLCA with Levcon. Any thoughts?
http://gallery.tejas.gov.in/Gallery/Calendar-2015/i-s6jVnMr/0/O/10918915_774539222640528_986019868138962680_o.jpg


Question: What are the chances that the LCA delta wing is clipped just to have acceptable downward visibility for pilot?? Had the wing had no clipped apex, would it hamper the look-down ability of pilot?? NLCA Levcon is already quite far ahead.


I don't think that is a reason. I don't know if you have been in the cockpit of a plane. But you have to really crane your neck to see through the FOV that is currently "uncovered" by virtue of the compound delta. Also, an opponent in that position is at a severe disadvantage.

As you rightly said the use of the Levcon is for more control at any AoA rather than achieiving higher AoA. One of the design goals of the Levcon is to be able to provide same approach (same speed and descent rate) for landing immaterial of the load (by adjusting AoA). Another advantage is slightly better drag-polar.

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Re: LCA Tejas: News and Discussions

Postby Indranil » 04 Apr 2015 12:03

Theory 1: Use of the Multi-segment slat on LCA: To maintain attached flow along the entire span of the wing at high AoA.

For a swept wing with a sharp leading edge the local angle of incidence along the span of the wing does not remain the same. This is because the inboard part of the wing creates a vortex which has an upwash along the span of wing leading to an increase of the local angle of attack along the span. As the angle of attack of the wing increases, the airflow on the outboard section of the wing detaches and the center of pressure moves forward. This creates an (unwanted) pitch up moment on the plane.

As a solution, if the leading edge of the wing could be deflected downwards so that an uniform angle of attack could be maintained along the span, the pitch-up tendency can be reduced. Now there are two things, about pitch-up: 1. angle of attack for the onset of the pitch-up and 2) Magnitude of moment. Studies have shown that the advantages of this leading edge augmentation is most marked on a flat untwisted wing and not so much on a twisted and cambered wings like that on the LCA/Mirage. Also, in same studies, the onset of the pitch-up was not found to be significantly affected. However, these things change from one wing shape to another. For example, the variable slats and higher fin provide much higher maneuverability to Mirage 2000 over the Mirage III. Similarly, there are publications which show that the pitch-up is actually delayed for the LCA.

Getting back to variable leading edge extension, building a continuous and smooth variably extended slat is mechanically unfeasible. Therefore, designers go for the next best thing: multi-segment slats. This creates higher drag over its theoretical aerodynamically faired leading edge counterpart due to the breaks between the slats, but one can live with it. However, even within the span of a single slat, vortices are formed (think of it as the LE of a small fixed wing). The beauty of the multi-segment slat is the vortex of the inner slat is energized by the vortex of the outer slat which stops it from bursting and keeps it attached to the outboard section the wing even for large sweep angles like those of a delta wing. So now you can see why the leading edge of the inboard section (deflected by 0 degrees) acts as the 'first' slat.

Image

Besides this, another feature of the wing to arrest pitch-up tendency is to have a trailing edge swept forward at 4 degrees. This provides more wing near the inboard trailing edge which is not affected by the flow separation near the outboard section.

Besides helping with delaying/arresting pitch-up, another major advantage of the airflow being attached to the outboard portion of the wing is that the ailerons attached to the outboard sections stay effective. Other minor advantages include better wing loading and therefore lower drag.

May be Nilesh can now explain the conical camber. If you put these two things together, one can understand the wing to quite some extent. But, I still don't understand the inboard section of the wing quite as well as I would have liked to. Does anybody else want to contribute? For example, does the "join" of the two deltas lying along the MAC by design, or a mere coincidence?

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Re: LCA Tejas: News and Discussions

Postby JayS » 04 Apr 2015 14:50

indranilroy wrote:I don't think that is a reason. I don't know if you have been in the cockpit of a plane. But you have to really crane your neck to see through the FOV that is currently "uncovered" by virtue of the compound delta. Also, an opponent in that position is at a severe disadvantage.

As you rightly said the use of the Levcon is for more control at any AoA rather than achieiving higher AoA. One of the design goals of the Levcon is to be able to provide same approach (same speed and descent rate) for landing immaterial of the load (by adjusting AoA). Another advantage is slightly better drag-polar.


Well I haven't got a chance to seat in a cockpit so far. :( SO I have no idea. I am not sure looking down is even useful for pilots of this era or not. If we extend the outboard LE towards cockpit it will meet at near the IAF round tricolor symbol (what is it called BTW?) that is painted below canopy. That would make accessing cockpit also a little awkward I guess (?). I am not much versed with operational subtleties so this is just a random thought.

If the LCA wing was to have 62.5 deg LE and it had to start from behind the cockpit then with simple delta it would end up with less wing area. And if weakening of inboard LE vortex strength was the aim behind low sweep at apex why they didn't go with even more straighter angle? Something like 40deg. (If LEVCON helps in achieving better control by weakening the LE vortex on inboard part, a much straighter LE with more 2D wing like behaviour and well attached flow makes more sense). Also this reduces a little wing area in apex region, pushing Lift centre a little behind, mitigating some pitch-up moment.

Anyway I saw a line diagram in one publication from SDREs which depicts locations of air data sensors and show FRL and radome axis in it. Just for fun, I scaled that image and superimposed it on LCA side view. It fits well, as the extended FLR passes right from the centre of the nozzle. I also tried to draw a line from the wing root. If I am correct, the wing is set to about 3-4deg positive setting angle. Its not easy to figure out whats happening at the root due to significant wing-body blending. Of coarse the wing-body blending and drooping LE part makes it look like highly cambered. But I believe (gut-feeling*) that LCA wing is not much cambered in its central portion by design if you neglect the blending effect.

https://drive.google.com/file/d/0B9ijXlsoNPWzbFlGZ1Z0V2Myc28/view?usp=sharing

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Re: LCA Tejas: News and Discussions

Postby JayS » 04 Apr 2015 18:17

My hypothesis is like this:

LCA suffers from the same issues as the other delta jets built in 1950s through 1970s. And designers of LCA used the same techniques to alleviate those issues. The origin of conical camber comes from the work of the legends like RT Jones, Bussaman, Tsien etc. A large amount of data and theoretical techniques for design had been achieved already by the time LCA concept was taking shape and many fighters right from F-106 to F-15 from US and Mirage, Viggen from Europe had successfully implemented those principles in practice. It makes perfect sense that a newbie design team would go with tried and tested things for their first endeavour.


There are bunch of publications from R K Bera (Sci-NAL) in 70s and 80s on conically cambered delta wings with subsonic LE: Quoting him from his paper:

We shall confine our attention to the wings where the attachment line is prescribed at the LE.

The geometric properties of basic shapes are now easy to visualise. It may immediately be seen that for p>0 the wing will have drooped leading edges and with increasing values of p, the central portion of wing will become more and more flat. The droop becomes noticeable towards the outboard portion and rapidly increases as it nears the leading edge. Consequently the spanwise surface slope will be low in the central portion of the wing but will rapidly increase as it progresses towards the LE. A similar trend will be shown by the incidence distribution, which will be positive at the root and continuously decrease from root to the tip to satisfy the attachment condition. The droop and the washout in the incidence near the le will help the flow to help the flow turn smoothly and hence may be expected to present properties closer to real situation. The wings are easy to manufacture and free from kink.

The aerodynamic properties are little difficult to visualise. From table-1 and Fig 2, It is noticed that the wing properties depend strongly on the amount of spanwise curvature for a given lift. Thus with increasing curvature, the wing incidence a_0 decreases and moves towards the flat plate value while the suction peak moves outboard and increases in magnitude. The spanwise lift distribution tends towards the ideal elliptical distribution and hence drag coefficient and drag factor e decreases and the L/D increases. These effects are primarily due to the suction peak acting on the forward facing droop which produces a thrust. For p=4, the C_d is substantially close to the plane delta wing with suction. For a given p it approaches the flat plate drag faster for increasing values of a.

The spanwise lift distribution, unlike the plane delta which has an infinite slope at the LE, has zero slope; the magnitude of the life increases ar the centre and decreases out board near the LE in comparison to the plane delta. The effect is progressively less pronounced with increasing p.

The behaviour is to be expected since the angle of attack decreases from root to the tip and therefore the wind tip is less loaded compared to a plane delta wing. This gives better tip stall characteristics and lesser bending moment at the wing root.

Conically cambered wings may show some advantages over other shapes under off-design conditions. Thus at any supersonic speed with the LE edges subsonic, the load will vanish at some incidence and therefore the attachment line is unlikely to lie on the upper surface for part of the wing and on the lower surface for the reminder, even away from the design incidence and Mach number.

From Table 2 an interesting fact us observed. For given p, the aerodynamic and geometric properties do not vary much for wings designed for different values of slenderness parameter a. This is an indication that the wing designed for a given apex angle and mach number will behave almost as well at off-design mach numbers.



The author was successful in deriving equations which give nice shapes for delta wing with conical camber which give properties like lift distribution, wing efficiency factor etc. This is a handy results for designing new wing in 1980s. The previous theoretical work leads to wings with dihedral on the central part of the wing with kink at symmetry plane (not desirable). In follow-up publication the author has given more analysis on how more optimised shapes can be obtained by combining basic shaped described above. Keep in mind that this is linear inviscid theory results and are not 100% accurate and are not quite applicable for high AoA regimes.

The above description fits quite well for the LCA wing we see today, if we put it together with other well-known experimental results of innumerable studies available.

1. LCA wing is a subsonic LE delta design.
2. LCA has pressure peaks located on outboard locations. See the CFD image:

https://drive.google.com/file/d/0B9ijXl ... sp=sharing

3. With slats activated the excessive curvature on LE is evident. The LE slats are meant to have the LE angled such a way as to have almost zero AoA with local wind. Indranil has explained above why the slats are at increasing droop towards the tip. This may seem to be simple washout, but its rather done to have LE unloaded (zero lift) which gives better L/D as per the theory. In reality the slats have a small vortex of their own which produces that additional thrust component (component of life in forward direction). The flow separates again on the main wing forming the main LE vortex. It had been seen in practice that multi-part slats are more efficient in doing this - Indranil has explained this part as well.

3. Washout on the wing can be seen, root set at positive angle and tip is almost horizontal as can be seen in the image I posted in previous post. (I had said previously that LCA does not seem to have washout. I really meant by washout like what we see on normal 2D wings. This wash out is inherent due to the conical camber and is not imposed separately, so I do not consider it as separate feature).

4. LCA wing do not seem to have any camber in the central part (outboard part looks like a straight line in side view (slats retracted), the but wing-body blending makes inboard section look complicated, you can see the curve separating wing upper surface and the blending surface if you look carefully).

5. I think, Conical camber can explain the weird looking inboard section as well. I drew a simple 3-view sketch (1st yr engg drawing) and it makes sense. You could try this: Draw a 62.5 deg delta wing with simple conical camber. Now chop off the apex with less sweep angle and see how the wing LE looks like. Does the apex LE part looks like the apec LE part of LCA in front view? (I drew a very bad line diagram to show you guys what I mean. Its simplified version od camber surface of delta wing with constant chord conical cambered portion, just notice the apex region). (Funfact: Constant chord slats/Levcon are more effective that constant proportional LE devices as per experiments)

https://docs.google.com/drawings/d/1Rrag25yrcFJAtrd11ELwgphwSPeLv1IhsPv18HlwdNI/edit?usp=sharing

Of coarse while refining LCA design ADA would have opted for little more involved camber surface that what I shown in above diagram.

Had the inboard section actually lifted up (like anhedral) We should be able to see it rising up when looked along the LE towards the cockpit. But it doesn't look like so. The LE on apex region is also drooped down as could be observed from the picture I showed in previous post. LE is below FRL. See below image as well:

Image


Regarding why lower sweep on inboard, is already discussed. Others have posted stuff about it. I have posted snippets from some NAL publications.
- The non-slender part helps in better low AoA characteristics but results in higher pitch-up moment.
- And I think it also helps keep LE vortex busting away from the VT at high AoA (speculation based on seen CFD images*). LCA does have significant rudder authority till 30deg.

But I am still looking into this more as I want to see some more proof.

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Re: LCA Tejas: News and Discussions

Postby Indranil » 04 Apr 2015 21:53

nileshjr wrote:Well I haven't got a chance to seat in a cockpit so far. :( SO I have no idea. I am not sure looking down is even useful for pilots of this era or not. If we extend the outboard LE towards cockpit it will meet at near the IAF round tricolor symbol (what is it called BTW?) that is painted below canopy. That would make accessing cockpit also a little awkward I guess (?). I am not much versed with operational subtleties so this is just a random thought.

Round tricolor symbol is found on the fuselage and wings and called a roundel. The flag is present as rectangle on the fin and called a fin flash.

Accessing the cockpit would not been a problem. One just designs the ladders in that fashion. For example ladders for the SR-71 had to navigate around the large chines.
nileshjr wrote:If the LCA wing was to have 62.5 deg LE and it had to start from behind the cockpit then with simple delta it would end up with less wing area. And if weakening of inboard LE vortex strength was the aim behind low sweep at apex why they didn't go with even more straighter angle? Something like 40deg. (If LEVCON helps in achieving better control by weakening the LE vortex on inboard part, a much straighter LE with more 2D wing like behaviour and well attached flow makes more sense). Also this reduces a little wing area in apex region, pushing Lift centre a little behind, mitigating some pitch-up moment.
https://drive.google.com/file/d/0B9ijXlsoNPWzbFlGZ1Z0V2Myc28/view?usp=sharing

Having the 50 degree sweep on the inboard section lets them shield the inlet. Notice that even with the Levcons, they did not change this angle. So, I believe that the vortex shed by the inboard section interacts favorably with the vortex shed by the outer when the sweeps are 50/62.5. Besides high AoA is only a part of the envelop.

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Re: LCA Tejas: News and Discussions

Postby JayS » 05 Apr 2015 00:20

indranilroy wrote:Accessing the cockpit would not been a problem. One just designs the ladders in that fashion. For example ladders for the SR-71 had to navigate around the large chines.
Having the 50 degree sweep on the inboard section lets them shield the inlet. Notice that even with the Levcons, they did not change this angle. So, I believe that the vortex shed by the inboard section interacts favorably with the vortex shed by the outer when the sweeps are 50/62.5. Besides high AoA is only a part of the envelop.


Sr-71 was a special purpose plane and it was a white elephant. That cockpit access issue is quite insignificant there. Why I am thinking this is I can't think of any fighter which such awkward configuration. May be its stupid point. Let's just forget about it.

I did not get you on second point. Even if you have a 40deg sweep there but wing starting from the same point (where wing root LE meets the fuselage) as it is now, wouldn't it be still shielding the inlets just the same way?? Of coarse they wouldn't change the wing angle now, it would be too huge a change at this stage. I am thinking about the Preliminary design logic. If you go by the logic why LEVCON help the wing (and also by similar experimental conclusions by a group of NASA researchers) then you actually should get better aerodynamic characteristics when the inboard and outboard vortices are decoupled from each other. There interaction is not all that favourable as the inboard vortex makes the outboard vortex so strong thats it busts much early.

I feel a piece of puzzle is missing.

Just nit-picking - I don't know if you meant something by "shedding" but there is no shedding of vortex. The LE vortex is attached. Shedding is when the vortex sheet would detach from the LE.

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Re: LCA Tejas: News and Discussions

Postby astal » 05 Apr 2015 02:20

This is an excellent discussion about AOA and instantaneously turn rates. It seems that certian choices were made during the wing desing stage, to achieve high AOA and ITR without canards in a small, single engine aircraft. What I would like to understand is how these choices impacted other requirements such as aerodynamic eficiency, accelaration in transonic regime and fuel efficiency. This is another important part of the puzzle and this is where we see some criticism for LCA.

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Re: LCA Tejas: News and Discussions

Postby Indranil » 05 Apr 2015 03:13

nileshjr wrote:
indranilroy wrote:Accessing the cockpit would not been a problem. One just designs the ladders in that fashion. For example ladders for the SR-71 had to navigate around the large chines.
Having the 50 degree sweep on the inboard section lets them shield the inlet. Notice that even with the Levcons, they did not change this angle. So, I believe that the vortex shed by the inboard section interacts favorably with the vortex shed by the outer when the sweeps are 50/62.5. Besides high AoA is only a part of the envelop.


Sr-71 was a special purpose plane and it was a white elephant. That cockpit access issue is quite insignificant there. Why I am thinking this is I can't think of any fighter which such awkward configuration. May be its stupid point. Let's just forget about it.

I spoke of the SR-71 as the most stark example. For fighters, you can look at what the Mig-29 and F-18 pilots have to do especially in the case of two seater versions. This is really a non issue. Let us move on.

nileshjr wrote:I did not get you on second point. Even if you have a 40deg sweep there but wing starting from the same point (where wing root LE meets the fuselage) as it is now, wouldn't it be still shielding the inlets just the same way??

Ah, you are keeping the starting point the same, while I was keeping the end point at the same place (thus retaining the same lever arm for the elevons). In your case, the shielding will not be different.

nileshjr wrote:Of coarse they wouldn't change the wing angle now, it would be too huge a change at this stage. I am thinking about the Preliminary design logic. If you go by the logic why LEVCON help the wing (and also by similar experimental conclusions by a group of NASA researchers) then you actually should get better aerodynamic characteristics when the inboard and outboard vortices are decoupled from each other. There interaction is not all that favourable as the inboard vortex makes the outboard vortex so strong thats it busts much early.

I feel a piece of puzzle is missing.

Just nit-picking - I don't know if you meant something by "shedding" but there is no shedding of vortex. The LE vortex is attached. Shedding is when the vortex sheet would detach from the LE.

Thank you for the correction. I just meant "shed" as in English, not the aerodynamic meaning. I should have been more accurate.

They must have tested various combinations before they settled on 50/62.5. I mean even if 50 was their favourite number, why not 50/62 or 50/63? That exact pair of angles must be giving the most favourable results.There are other parts of the envelop as well: transonic cruise, supersonic cruise, supersonic manoeuvrings.

I also think we are missing something. For example, my theory for the "wavyness" of the wing was to allow the reattachment point of the vortex at the crest of the camber, throughout the span. Studies have shown that this keeps the flow attached for the rest of the wing up to the trailing edge. This has obvious advantages, especially for planes like LCA which have the tailplane attached to the wing. I read it long time back, so I can't remember it clearly. For example, there was something about increasing supersonic cruise efficiency as well. I don't know if LCA uses this feature.


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