LCA News and Discussions

[aditya.agd]

Indian airforce should replace the current older Mig21 and 27s except bisons with Tejas mk1. Then later on they can introduce mk2s.

[putnanja]

Something sounding like a Tejas just flew past Whitefield .... Pretty late for a test flight...

I remember reading somewhere that they are continuing with night test flights on the LCA.

[Gaur]

Saik's comment makes sense here. In one of Anantha Krishnan's excellent interviews, NFTC Chief had said something along the lines that a prototype needs generally needs to fly 8 sorties per month but an inducted fighter may have to carry out 8 sorties per day.

<Nitpick>The LCA prototypes fly about 8 times a month, whereas an inducted fighter may have to carry out 3-4 sorties per day, not 8</Nitpick>

Thanks for the correction. That is not a nitpick at all as there is a large difference bitween the two.

[Baldev]

Indian airforce should replace the current older Mig21 and 27s except bisons with Tejas mk1. Then later on they can introduce mk2s.

actually mig27 are fairly new last one delivered in 1998 so question of replacing it is not worth and same for bison.but mig27 engine is troublesome so its better to fit al31 on it.

but IAF wanted aircraft to replace mig21 and lca is steps ahead of mig21 in all aspects but as soon as lca became available IAF started criticizing it which is known to everyone.

[Viv S]

Among all the scenarios in which it might spin, there could possibly be one attitude + control surface setting + speed + environmental condition in which an unrecoverable spin can occur.

That's precisely it. I can't visualize an unrecoverable spin happening at medium/high altitude without a failure in the FCS, hydraulics or engine. As long as the pilot retains control of elevons (and doesn't get disoriented) he should be able to recover from a spin/stall every single time.

[Neshant]

but IAF wanted aircraft to replace mig21 and lca is steps ahead of mig21 in all aspects but as soon as lca became available IAF started criticizing it which is known to everyone.

I suspect lobbyists looking to sell foreign weapons must be involved given how these criticisms show up like clockwork.

[Shameek]

That's precisely it. I can't visualize an unrecoverable spin happening at medium/high altitude without a failure in the FCS, hydraulics or engine. As long as the pilot retains control of elevons (and doesn't get disoriented) he should be able to recover from a spin/stall every single time.

I believe this has been posted before. But notice how the aircraft behaves during a seemingly normal maneuver. It is at low altitude but the engine flames out and the plane starts spinning just before the crash.
http://www.youtube.com/watch?v=4iToQ2FykoI

[Viv S]

That's precisely it. I can't visualize an unrecoverable spin happening at medium/high altitude without a failure in the FCS, hydraulics or engine. As long as the pilot retains control of elevons (and doesn't get disoriented) he should be able to recover from a spin/stall every single time.

I believe this has been posted before. But notice how the aircraft behaves during a seemingly normal maneuver. It is at low altitude but the engine flames out and the plane starts spinning just before the crash.
http://www.youtube.com/watch?v=4iToQ2FykoI

Its not really the same thing as in this case was at a rather low altitude and recovery wouldn't have been feasible. But, had it been flying over 10,000ft it may have been a less tragic story.

[shiv]

That's precisely it. I can't visualize an unrecoverable spin happening at medium/high altitude without a failure in the FCS, hydraulics or engine. As long as the pilot retains control of elevons (and doesn't get disoriented) he should be able to recover from a spin/stall every single time.

I believe this has been posted before. But notice how the aircraft behaves during a seemingly normal maneuver. It is at low altitude but the engine flames out and the plane starts spinning just before the crash.
http://www.youtube.com/watch?v=4iToQ2FykoI

Interestingly - - between 1:41 and 1:42 - a moment after the pilot ejects a huge ball of flame is ejected from the exhaust - possibly from a flame out. But the flame out may have been caused by the rocket plume of the ejection seat entering the intakes. After that the plane falls like a rock with no indication that any thrust is being generated.

[shiv]

This morning - I was reading the latest issue of Vayu and read something that we jingos did not hear about and I am so glad we did not hear of it because it could have been tragic for the LCA. The topic was the usefulness of realtime monitoring during tests flights using telemetry. No date mentioned.

During one test flight the engineer on the ground noticed the hydraulic pressures on the LCA falling even before the cockpit warnings came on. He warned the pilot and asked him to return. Soon he noticed that the second hydraulic system was also bleeding oil and losing pressure rapidly. In an instant he asked the test pilot to lower the undercarriage because that would become impossible if the second system packed up as the emergency system would retain power only for the flight controls.

The pilot lowered the undercarriage safely and managed to land. The landing was rough with the nosewheel shimmying from side to side so violently that the HUD got dislodged from its mounting. But no serious damage was done.

[suryag]

Thanks Hakimji, Thanks to god that we dint have a new post in that dreaded mil crash thread. If this news had been reported then this thread would have been so chaotic with all kinds of theories counter-theories. Any idea which year this incident happened ?

[Singha]

another night sortie yday at 2025 hrs noticed.

[tsarkar]

X deg AoA will certainly NOT be an IAF ASR! AoA is just a means to an end and not an end in itself . It is not a d*ck measuring contest like Bandar does A, mine will be B , and Ding Dong J-XX will be C !

If suppose the plane need to be stable and controlled at very low speeds then the design will call for some X AoA and if it needs to pull the designed 9gs /10gs/11gs/whatever gs, that will require some Y AoA and therefore the plane need to be proven at the max X or Y AoA that the flight envelope demands and the FCS will limit the AoA to the proven flight tested value in the squadron service versions.

The good Colonel's oh, the F-18 can do 58deg AoA, while the Tejas will drop out of the sky at 28deg (which I doubt is anywhere close to the truth) or the F-16 has a max AoA of only some 30 deg or so is only a d*ck measuring contest and has absolutely no relevance.

There is more to AoA than just maintaining control at low airspeed or simply pulling g's.

Maneuverability in the vertical plane is a direct function of AoA.

The Gnat had better maneuverability than Sabre in the vertical plane, while the Sabre had better maneuverability in the horizontal plane, hence the IAF always tried to lead the engagement in the vertical plane. The beauty of Gnat aerodynamics was that it could very easily pitch up and start a tight loop.

It is well known that the plane able to execute the tightest loop has the advantage of getting the plane with broader loop quicker into his gunsight. In the vertical plane, higher AoA tolerance enables a tighter loop. The tight loop utilizing AoA characteristics is called the Kulbit by the Russians. While the Russians may have discovered this today, the Gnat was doing tight loops decades earlier with conventional aerodynamic controls.

The Cobra maneuver – and the less well known but more effective Herbst maneuver – is a direct function of high AoA. Thrust Vector control in pitch axis further enables higher AoA. That is why, incidentally, the IAF Su-30MKI and F-22 are very happy with TVC in the pitch axis only.

A higher AoA also ensures more flexibility to enter or exit vertical maneuvers.

So, an Air Force desirous of high maneuverability in the vertical plane can definitely specify high AoA as an ASR.

The Tejas cranked wing is specifically designed to enable high AoA. The first page of this thread has an article http://www.bharat-rakshak.com/IAF/downl ... diance.pdf page 4 that mentions design AoA was 35 degrees. Hence the reasons for IOC AoA of 22 degrees need to be understood.

Off Topic - I never understood why we didnt equip the Gnat/Ajeet with Magic or other AAMs like the Fiza'ya armed their F-6 & Sabres. Most probably the poor experience with the K-13 in 1971 created a negative impression of AAMs that lasted quite a while.

[Drishyaman]

Question to Gurus :

Tejas MK – II will be lengthened by 0.5 meter just behind the cockpit. Is that being done to accommodate more of the delta wing within “Mach” cone? Can this be assumed as a drag reduction measure?
What else drag reduction measures are taken up (or can be taken up) for reducing the drag in Tejas MK – II?

[Gaur]

<snip> That is why, incidentally, the IAF Su-30MKI and F-22 are very happy with TVC in the pitch axis only.
<snip>

Small correction here. While F-22 has TVC in the pitch axis only, Su-30MKI has it in yaw too. Though AL-31FP has 2D TVC, it is arranged in slightly cross orientation.

[shiv]

OT deleted

[tsarkar]

Gaur, its a myth that Al-31FP has TV in the pitch plane.

When viewed from the horizontal plane, the engines are aligned at an angle to the longitudinal axis, however there is no yaw deflection of the engine nozzles. It only moves up and down, not sideways. The angling does not offer any thrust control in the yaw axis.

No operationally deployed aircraft has TVC in pitch and yaw. TVC in yaw could put the aircraft in a flat spin. It would be complex for a flight computer to keep computing on real time basis on how much to yaw TV to use without putting the aircraft in a spin. Pitch TV control is simpler, because of the forward momentum of plane allows a significant degree of control forces available.

[Rahul M]

well, a recent article by butowski does claim that PAKFA TVC is arranged that way i.e 2D TVC but angled so that there are both pitch and yaw components to it. I've not heard of the MKI having it though except claims on BR.

[shiv]

The Cobra maneuver – and the less well known but more effective Herbst maneuver

I have a question for you in the newbie thread.
http://forums.bharat-rakshak.com/viewto ... 1#p1033881

[Dileep]

Isn't the MKI's TVC in yaw axis 'discussed to death' here years ago, and confirmation obtained that it is real? I also remember seeing videos where the nozzles move differentially in sync with the rudders during pre-flight? So, why this 'no TVC in yaw' now?

Also, the plane is actually seen rotating in yaw axis after a tailslide in some show flight videos, and it is attributed to the TVC.

[Dileep]

the plane has its musharraf down, its nose up and is floating down spinning flat with air flowing the wrong way

Makes a lot of sense when considered with the relaxed stability angle. The plane will be actually stable when it moves back, so it will become impossible to recover it by any regular aerodynamic control. The only way to recover would be to use the parachute to make it stable in the forward movement.

Using the analogy of a stick balanced on the finger, once the stick tilts too much and switches direction, ie hang down from the finger, any effort by moving the finger wouldn't recover it.

[Viv S]

:
Makes a lot of sense when considered with the relaxed stability angle. The plane will be actually stable when it moves back, so it will become impossible to recover it by any regular aerodynamic control. The only way to recover would be to use the parachute to make it stable in the forward movement.

But wouldn't the aircraft eventually gather some speed while falling? The torque on the flaps will eventually push the aircraft back into a neutral AoA position.

[shiv]

There is more to AoA than just maintaining control at low airspeed or simply pulling g's.

Maneuverability in the vertical plane is a direct function of AoA.

The Gnat had better maneuverability than Sabre in the vertical plane, while the Sabre had better maneuverability in the horizontal plane, hence the IAF always tried to lead the engagement in the vertical plane. The beauty of Gnat aerodynamics was that it could very easily pitch up and start a tight loop.

TSarkarji - isn't it wing loading that has a greater effect on the size of a loop? The higher the wing loading the lesser the ability to do a tight loop. The F-86 had a high wing loading - certainly higher than the Gnat.

I always thought that it is easy to change the angle of attack to any angle in any aircraft by using elevators, elevons or canards. Level flight without stalling at high AoA is different from a loop. After all - no matter what aircraft is doing the loop the AoA reaches 90 degrees and more - so it's is not the ability to reach that AoA but the ability to do a tight high-G loop which depends on wing loading and engine power.

[Abhibhushan]

Shiv said

After all - no matter what aircraft is doing the loop the AoA reaches 90 degrees and more

I think there is a misunderstanding about AoA. Angle of Attack is defined as the angle between the axis of the aerofoil and the direction of airflow (which is always opposite of the movement of the aircraft is space). The angle made by the aircraft axis and the horizon has no relationship with the AoA.

A loop is a fully controlled maneuver. The AoA during the loop is always less than the stalling angle, even when the aircraft is pointing up or is even inverted in relation to the natural horizon.

[Drishyaman]

Angle of Attack is defined as the angle between the axis of the aerofoil and the direction of airflow

Yes, that is something what my kid bro who is an CFD analyst told me, earlier I also used to think it as the

The angle made by the aircraft axis and the horizon.

But, question for you, doesn't ever “the axis of the aerofoil” co-incide with “the aircraft axis” and the “direction of the airflow” co-incide with “the horizon”?

Note : I have very limited knowledge of fluid dynamics

[Kartik]

That's precisely it. I can't visualize an unrecoverable spin happening at medium/high altitude without a failure in the FCS, hydraulics or engine. As long as the pilot retains control of elevons (and doesn't get disoriented) he should be able to recover from a spin/stall every single time.

I believe this has been posted before. But notice how the aircraft behaves during a seemingly normal maneuver. It is at low altitude but the engine flames out and the plane starts spinning just before the crash.
http://www.youtube.com/watch?v=4iToQ2FykoI

There have been 3 Gripen crashes, all attributed to issues with the Flight Control System. Pilot Induced Oscillations, entering into wake vortices and what not. But in service, the Gripen has been a very safe fighter, since the kinks have been ironed out, onboard diagnostics are good, sub systems are reliable and the engine is extremely reliable and safe.

[suryag]

Gripen crashes wiki
Interesting set of crashes during testing
No1's cause was a combination of Pilot induced oscillations and extremely gusty winds
No2's cause video posted above
No3's cause was wake and some FCS problem

In the production this was an interesting one

On 19 April 2007, a JAS 39C Gripen (serial no 39-259) from Airwing F 21 Luleå crashed at the Vidsel airfield in northern Sweden.[6] The pilot, Stefan Kaarle, was involuntarily ejected out of the aircraft in mid-air while approaching the airstrip in order to land. He landed safely by parachute.

Who needs missiles if one were able to send a command to the opponent's aircraft to eject the pilot

[NRao]

Who needs missiles if one were able to send a command to the opponent's aircraft to eject the pilot

I am not very sure what you are trying to say, but, I am very glad that they did not ask you to look at vids of Harriers while it was in development. There are more than an hours worth of vids .................

The fact that we can post like this is attributable to some of these men who really risked their lives.

[suryag]

Nrao garu there was no attempt to demean the efforts of the test pilots, their feats are great is just an understatement, it was just some hypothetical datalink weapon intentionally used by an adversary that would cause an ejection the opponent pilot without his knowledge that i was referring to. And yes you wouldnt need a missile if you had this system.

That apart I was really surprised to see that the pilot who was in the first gripen crash was the same one who was involved in the second crash and that only makes me remark that you need really guts of steel to get back into the cockpit after a ghatsly incident like the first crash

[SaiK]

LCA is not Mig21++ or 3 legged cheetah. It is Mirage2k++ and soon a 4 legged tiger.
from DRDO thread..

[youtube]LSH1o3_cebk&NR=1[/youtube]

Sir minces no words.

PS: LIkes of G. Parthasarathy and Ajit Doval too spoke on the occasion.

[Dileep]

:
Makes a lot of sense when considered with the relaxed stability angle. The plane will be actually stable when it moves back, so it will become impossible to recover it by any regular aerodynamic control. The only way to recover would be to use the parachute to make it stable in the forward movement.

But wouldn't the aircraft eventually gather some speed while falling? The torque on the flaps will eventually push the aircraft back into a neutral AoA position.

An aircraft designed to be aerodynamically stable would eventually recover (if it doesn't reach the ground before that) if left alone. But for an unstable aircraft like Tejas, no action on any of the surfaces would recover it, because the movement is stable in the backward direction.

[shiv]

I think there is a misunderstanding about AoA. Angle of Attack is defined as the angle between the axis of the aerofoil and the direction of airflow (which is always opposite of the movement of the aircraft is space). The angle made by the aircraft axis and the horizon has no relationship with the AoA.

A loop is a fully controlled maneuver. The AoA during the loop is always less than the stalling angle, even when the aircraft is pointing up or is even inverted in relation to the natural horizon.

Ok Thank you - it's not the attitude of the aircraft with respect to the ground but with respect to the airflow that defines AoA so the ability to pull a higher AoA without stalling should enable a tighter loop?

[vina]

Ok Thank you - it's not the attitude of the aircraft with respect to the ground but with respect to the airflow that defines AoA so the ability to pull a higher AoA without stalling should enable a tighter loop?

Shivji, there is an angle of attack beyond which the wing stalls, whatever the speed and whatever stuff you put on the musharraf. That plane is not going to be flying. That however has a correlation to speed, but that really is a airfoil property.

Now, when a wing can sustain a higher angle of attack, you can pull more Gs and fly slower that what a lower AoA attack wing will allow you (all other things being same).

In the vertical plane, what makes the difference is wing loading (lower wing loading, means plane has excess reserve lift, so can climb faster) and also t:w ratio (higher t:w ratio, better climb performance).

Now when you are "flying" (ie using wings and flying like an airplane), in any state, the angle of attack has to be such that you dont stall, whether you are flying a loop or a tight circle.

The kind of flying that that tsarkar talked about is really not "flying" at all in the conventional sense, but using engine/thrust vectoring to really fly like a "rocket" more than a plane! Think of it, the tightest loop possible is not flying in a vertical circle (ie a loop), but going straight up like a ball throw up, reaching zero speed (the plane is stalled) and then fall back nose down under mama gravity and recover! To do that well and consistently, you need to have adequate control and margins and predictibility when the air speed is zero with Musharraf down and nose pointing up so that you can tip the nose down and start flying again. You if fall on the side and start tumbling , you enter some painful spin!

[shiv]

OK - with a loop being controlled flight that is always over stalling speed/AoA - would it be wrong to say that the G one pulls depends on how tight the loop circle is. If the loop is smaller the Gs are higher and vice versa.

If that is true then wouldn't a higher wing loading create more stress on an airframe than a lower wing loading aircraft in a similar sized loop?

Or am I really screwing up things by randomly mixing and matching jargon?

[Arya Sumantra]

If the loop is smaller the Gs are higher and vice versa.

for a given velocity, yes. You can make the smaller loops and still stay within your G limit by reducing your tangential velocity. The G's have a square dependence on the velocity as compared to the linear dependence on radius.

[SaiK]

Q: What is that in LCA design that makes it aerodynamically unstable?

[pragnya]

a couple of questions -

1. LCA has FLIR as part of the 'Lightening' LDP - which would do a similar job as an IRST. granted it is under the fuselage and for A2A it may not be as effective as a separate IRST positioned as in the SU 30MKI/Mig 29K/Rafale etc..

will positioning the Lightening pod in a different place (may be under the wings) help?? is that feasible?? anyway this is only a humble question.

2. staying with IRST, the great B Harry mentions in his article "Radiance of Tejas" that IRDE has already developed an AIRTS (airborne infrared target sensor) and he also speaks 'it should' incorporate laser rangefinder too.

any info/development on this??

TIA.

[shiv]

If the loop is smaller the Gs are higher and vice versa.

for a given velocity, yes. You can make the smaller loops and still stay within your G limit by reducing your tangential velocity.

But at one point that tangential velocity will drop below stall speed. The whole idea is that a loop is done at a speed and AoA that is above stall speed. If you reduce the speed and increase the AoA to make a tight loop you are keeping G within limits but getting closer to stall speed. And at any given speed and AoA a higher wing loading should experience more G than a lower one.

So the question I had was that it appears to me that a plane that can achieve a better AoA but has a higher wing loading might not be able to do a tighter loop than another aircraft that has a lower AoA limit but lower wing loading because of G limitations on the airframe. This is what I want to know.

[ramana]

Q: What is that in LCA design that makes it aerodynamically unstable?

Its Center of pressure is not behind the Center of Gravity. This is designed to make it aerodynamically unstable and very agile.

For stability the CP should be aft(behind) the CG. Like those feathers on an arrow make the CP behind the CG.

[Kanson]

http://www.rafael.co.il/Marketing/334-9 ... eting.aspx

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