LCA News and Discussions

merlin · Post by **merlin** » 13 Mar 2013 12:59

To achieve initial operational capability, the Tejas program will need to expand the platform's angle of attack, g-tolerance and weapons capability. Additionally, the Tejas needs to complete test points in all-weather operations, lightning clearance and wake-penetration. Though slowed by critical delays, the program marked 2,000 accident-free flights earlier this month.

Nice data points. For IOC-2, Tejas needs to demonstrate

1. Increase in AoA from 22 deg to 24 deg (although it needs to be tested till 26 deg and then software limited to 24 deg)
2. Increase G-limits from 7G to 8G - this should not be too much of a problem because in AI-2011, it was indeed flown to 8G even though not cleared for it.
3. Weapons capability - should be able to fire R-73s and demonstrate dumb bomb attacks (using CCIP, CCRP?) and LGB attacks.
4. All weather operations demonstration - fly through rain, etc.
5. Lightning clearance - individual components have been cleared for lightning but not the entire aircraft yet.
6. Wake penetration - just surprised that this was not done already.

Singha · Post by **Singha** » 13 Mar 2013 13:57

what is the diff in AoA 26' and climbing up vertically which we have all seen it do in airshows?
does AoA 26' mean pitching the plane up to 26' but still somehow maintaining controlled forward level flight? what is the utility of this movement? (I have seen the rafale do something similar as well at slow speed..kind of mini cobra)

shiv · Post by **shiv** » 13 Mar 2013 14:10

Singha wrote:what is the diff in AoA 26' and climbing up vertically which we have all seen it do in airshows?
does AoA 26' mean pitching the plane up to 26' but still somehow maintaining controlled forward level flight? what is the utility of this movement? (I have seen the rafale do something similar as well at slow speed..kind of mini cobra)

26 degrees with respect to direction of flight, not with respect to the ground.

eg
Watch from 1 minute onwards (Mirage 2000)
http://www.youtube.com/watch?feature=pl ... N_18#t=61s

Singha · Post by **Singha** » 13 Mar 2013 14:16

at 26' the nose will obstruct the view of pilot in direction of flight.

so I am willing to learn what is the combat or operational use of this slow high AoA fwd flight....yes I remember that famous wapiti flypast.

is it for landing at a steeper angle?

shiv · Post by **shiv** » 13 Mar 2013 14:19

Singha wrote: is it for landing at a steeper angle?

I think so. Also to certify predictable behavior because the plane can go into a stall and spin beyond a particular angle at low airspeed. Hence spin control chute for AoA tests.

Lalmohan · Post by **Lalmohan** » 13 Mar 2013 17:23

higher aoa should also help with making harder turns (horizontal and vertical)

mody · Post by **mody** » 13 Mar 2013 17:50

The no. of Test flights have certainly picked up. 23-24 flights over the 12 days from 28th of Feb to 11th of March.
Seems like ADA is keen to keep the June IOC-2 date. Hopefully the IOC-2 and the FOC thereafter can be completed by end of first quarter 2014.

Indranil · Post by **Indranil** » 13 Mar 2013 21:05

Singha sir,

There are 3 basic aspects to this:
1. As you increase your angle of attack, you keep increasing your lift till a point where the flow starts to seperate and you finally stall.
2. While in a turn, the only force which can create centripetal acceleration is your lift.
3. Given the same velocity, radius of turn is inversely proportional to the amount of centripetal force.

Therefore, in very layman terms, if you can fly at higher AoA, you can create larger lift (centripetal force), means that you can make tighter turns.

Indranil · Post by **Indranil** » 13 Mar 2013 21:10

merlin wrote: 1. Increase in AoA from 22 deg to 24 deg (although it needs to be tested till 26 deg and then software limited to 24 deg)

It will be the other way round. The software will allow it to reach 26 deg. According to wind tunnel testing, they are fairly certain that they can go upto 24 deg. For going to 26 deg, they would need artificial stabilization through the FBW. They are confident they can do that, because they have good rudder authority till about 30 deg.

All these are results from simulations and wind-tunnel testing. It might turn out that they can go to 26 deg without any artificial stabilization at all. It has happened with many air-crafts before.

SaiK · Post by **SaiK** » 13 Mar 2013 21:42

is that requirement of AoA for full weapons config or without? assuming that weight and drag of the weapons needs to be considered.

pentaiah · Post by **pentaiah** » 13 Mar 2013 21:43

(m) v squared/r

Indranil · Post by **Indranil** » 13 Mar 2013 22:23

SaiK wrote:is that requirement of AoA for full weapons config or without? assuming that weight and drag of the weapons needs to be considered.

No, certainly not at a full weapons load.

There are a lot of things. At full load Tejas airframe most probably can pitch to 26 degrees (it is a function of airspeed, altitude and size of elevators), but your engine won't be able to push a fully loaded plane through and it will fall.

Most FBW planes have 3 categories based on the load out. CAT-I is generally A2A profile with no tanks, CAT-2 is A2A with wing tanks, and CAT-3 is A2G with wing tanks (full loadout). All of them come with G-limiter and AoA-limters. For example the Rafale's and EF's AoA limts for CAT-I and CAT-III are as follows. (link)

Code: Select all

  G       CAT - I     CAT-III 
         AoA Limit   AoA Limit
---------------------------------
 1.00      35.00       20.00
 7.00      30.00       14.00
10.00      14.00       14.00

NRao · Post by **NRao** » 13 Mar 2013 22:36

what is the utility of this movement?

Agility. An aircraft with higher AoA is considered to be more agile.

BTW, AoA does not apply to an aircraft in which the TVC is engaged.

Jayram · Post by **Jayram** » 13 Mar 2013 22:39

A dumb question but I have always wondered about this. Given that the LCA's thrust to weight ratio is >1 why should the AOA matter in certain scenarios ie in a knife fight? In those scenarios the LCA can/should basically behave like a rocket and not need any lift at all with the engine basically powering the craft at what ever angle (limited of course by the G forces) it needs no? Yes handling authority and recovery would still be an issue but turn radius should not... no?

Indranil · Post by **Indranil** » 13 Mar 2013 22:40

NRao wrote:BTW, AoA does not apply to an aircraft in which the TVC is engaged.

Why?

NRao · Post by **NRao** » 13 Mar 2013 22:51

indranilroy wrote:
NRao wrote:BTW, AoA does not apply to an aircraft in which the TVC is engaged.
Why?

I think one of your posts answered it: beyond a certain angle the aircraft should stall. TVC artificially corrects for such a failure (thus the term "super-maneuverability" - beyond (normal?) physics).

I guess it is an arguable situation. ?????

So, what is the AoA for the MKI - when the TVC is engaged? 180'? Makes no sense (the plane is actually going backwards for a few seconds - which in itself is ascharyavat).

Indranil · Post by **Indranil** » 13 Mar 2013 23:21

NRao wrote: I think one of your posts answered it: beyond a certain angle the aircraft should stall. TVC artificially corrects for such a failure (thus the term "super-maneuverability" - beyond (normal?) physics).

I guess it is an arguable situation. ?????

So, what is the AoA for the MKI - when the TVC is engaged? 180'? Makes no sense (the plane is actually going backwards for a few seconds - which in itself is ascharyavat).

You can view TVC as nothing but a big elevator which is functional at any attitude and airspeed. So TVC can only allow you to turn around the axes passing through the CG of the plane. It will not contribute to the centripetal force or the coefficient of lift of the wing.

SaiK · Post by **SaiK** » 14 Mar 2013 00:11

Thanks IR.. any links or data points for LCA CAT 3?

Indranil · Post by **Indranil** » 14 Mar 2013 00:59

I have none

Katare · Post by **Katare** » 14 Mar 2013 03:18

I may be way off but I think AoA is more important in horizontal direction rather than vertical nose up/down direction. For instance at 800KM/hr, how much can I move my neck to see/engage a target that is moving away from the RADAR cone. At some point you'll start to turn but your nose will still be at certain angle to the flight direction. So the AoA determines how fast can you turn instantaneously and in sustained mode. Higher that angle more useful it is in getting locks on evading targets.

SaiK · Post by **SaiK** » 14 Mar 2013 03:22

I think, operationally, higher AoA (during flight) is more needed at lower speeds and dog fight turns. right?

Indranil · Post by **Indranil** » 14 Mar 2013 04:27

Katare wrote:I may be way off but I think AoA is more important in horizontal direction rather than vertical nose up/down direction.

Even in the vertical direction, AoA matters. It is just more complicated.

The centripetal force does not remain constant. At the top, centripetal force = lift+weight, and at the bottom centripetal force = lift-weight. So, the radius of turn doesn't remain constant. Also, the velocity does not remain constant as one has higher kinetic energy at the bottom and higher potential energy at the top. As a result, you would see the angular velocity of plane is highest at the top and lowest at the bottom (when it is coming out of the loop).

As you and others have already discussed, the other benefits of slower stall speed and better nose pointing ability.

shiv · Post by **shiv** » 14 Mar 2013 06:10

Jayram wrote:A dumb question but I have always wondered about this. Given that the LCA's thrust to weight ratio is >1 why should the AOA matter in certain scenarios ie in a knife fight? In those scenarios the LCA can/should basically behave like a rocket and not need any lift at all with the engine basically powering the craft at what ever angle (limited of course by the G forces) it needs no? Yes handling authority and recovery would still be an issue but turn radius should not... no?

Even rockets with a thrust weight ratio of >1 can spin uncontrollably so t/w ratio is no guarantee

I am no expert but the crucial thing here is airspeed.

There are three forces acting in an aircraft
1. Engine thrust acts in the nose to tail axis.
2. Because of forward movement from engine thrust, the wings generate lift. which is always from bottom of plane to top - even if the plane is not flying parallel to the ground. If the plane has banked 90 deg to left, lift acts towards the left
3. Gravity - this is always towards the ground.

In level flight at constant speed, the forward thrust (1) creates lift (2) that cancels out gravity (3) exactly

But imagine a situation where the pilot suddenly pulls his nose up to 45 degrees. Now he starts climbing. Thrust (1) remains the same. Lift (2) reduces because it is acting at 45 deg up and 45 deg backwards. Gravity (3) remains the same. Because lift is now acting less effectively upwards and partly backwards AND the plane is gaining height the plane loses speed unless more energy can be injected immediately. The moment the plane loses speed the lift reduces further - aggravating the above situation further. The pilot could respond to this by slamming the throttle forward, but engines take a finite amount of time to spool up, turn faster and increase thrust.

Depending on the amount of speed that had been bled away by lifting the nose and the delay in getting more thrust, the airspeed could become too low to generate useful lift. At that point the aircraft, whose nose is now pointing 45 degrees upwards, starts losing height. Now remember that if it starts losing height (falling) with nose at 45 deg up, the flow of air over the wings will reverse, and start going from back to front as the plane loses height.

This is a terrible thing. The plane is not meant to fly with airflow in odd directions like back to front over wings. The aircraft starts coming down in a spin. Once it is in a spin its behaviour can become very unpredictable. In trainer aircraft a lot of effort is put into making this behaviour predictable so that if he trainee pilot is in a spin he can react and do something that helps the plane recover, provided the spin occurs at a high enough altitude. At low altitude the plane may crash.

In order to avoid such a situation the airspeed must never ever come below a particular value when the plane is above a particular angle of attack. Or inversely - a FBW plane should prevent the pilot from achieving a particular angle of attack that may cause a stall if the software detects that the airspeed is inadequate.

Under normal circumstance any pilot in any aircraft on manual control can pull his nose up into whatever angle of attack that takes his fancy. 20 degrees, 40 degrees, 60 degrees - whatever. As long as his plane has adequate airspeed and reserve thrust at that AoA he will merely end up doing a high G loop or turn. If effective airspeed is inadequate, his plane will stall and he will start falling and his plane may go into an uncontrollable spin. In a fundamentally unstable aircraft like a modern FBW fighter like the LCA which is tail heavy, spins could be even less recoverable than in a stable trainer

vina · Post by **vina** » 14 Mar 2013 09:23

Singha wrote:at 26' the nose will obstruct the view of pilot in direction of flight.

so I am willing to learn what is the combat or operational use of this slow high AoA fwd flight....yes I remember that famous wapiti flypast.

is it for landing at a steeper angle?

Shiv wrote:I think so. Also to certify predictable behavior because the plane can go into a stall and spin beyond a particular angle at low airspeed. Hence spin control chute for AoA tests.

Listen to LalMullah's sermon on this. You need the high AoA to pull high g s. It is critical for high manoeuvring . To pull 9gs in a horizontal plane , you will need around 26deg AoA typically. The 8G is because of AoA limitation that I think will be removed once the envelope is opened fully. My guess is that it will happen in due course after the FOC for the MK1 model.

vina · Post by **vina** » 14 Mar 2013 09:27

Shiv wrote:As long as his plane has adequate airspeed and reserve thrust at that AoA he will merely end up doing a high G loop or turn. If effective airspeed is inadequate, his plane will stall and he will start falling and his plane may go into an uncontrollable spin

It has nothing to do with airspeed. Beyond a certain angle of attack the airfoil stalls and you fall out of the sky.
Critical Angle of Attack

All the fancy schmanzy aerobatics like Kulbit/Cobra tail slide etc of the SU-30 and Mig-29 OVT etc are as they are called correctly "post stall manoeuvring" . The wings are stalled alright, and the control is largely not aerodynamic, the plane is not really "flying", but rather is like a rocket, with a rocket like vectored thrust control.

suryag · Post by **suryag** » 14 Mar 2013 10:17

Where is the LSP8?

krishnan · Post by **krishnan** » 14 Mar 2013 10:42

http://jjamwal.in/blog/2013/02/indian-l ... lanes.html

some nice photos

krishnan · Post by **krishnan** » 14 Mar 2013 10:44

one of them from above blog

Austin · Post by **Austin** » 14 Mar 2013 10:48

^^ Nice Angle , Great Shot of Tejas

krishnan · Post by **krishnan** » 14 Mar 2013 10:52

and very up close....thats some nice lens

shiv · Post by **shiv** » 14 Mar 2013 15:06

vina wrote: It has nothing to do with airspeed. Beyond a certain angle of attack the airfoil stalls and you fall out of the sky.
Critical Angle of Attack

I guess you are right. I believe you without clicking on the link. But I was wondering if a mad pilot doing 1200 kmph suddenly pulled his stick back without worrying about AoA he would probably end up doing a loop and would black out as well if the wings did not come off. Any thoughts anyone?

shiv · Post by **shiv** » 14 Mar 2013 15:07

krishnan wrote:http://4.bp.blogspot.com/-auNdBwLS1pc/U ... 0005-L.jpg

one of them from above blog

Oooh! Yet another beaut.

SaiK · Post by **SaiK** » 14 Mar 2013 17:33

^and that shadow of vympel is enough warning for any TSPian.

Indranil · Post by **Indranil** » 14 Mar 2013 22:10

shiv wrote:
vina wrote: It has nothing to do with airspeed. Beyond a certain angle of attack the airfoil stalls and you fall out of the sky.
Critical Angle of Attack
I guess you are right. I believe you without clicking on the link. But I was wondering if a mad pilot doing 1200 kmph suddenly pulled his stick back without worrying about AoA he would probably end up doing a loop and would black out as well if the wings did not come off. Any thoughts anyone?

1200 kmph is a tricky speed to do stunts. But let us think what would happen at 800 kmph. Also let us assume that the plane is flying clean.

Nothing much would happen. At those speeds that the plane has a lot of momentum, and the control surfaces are also not that effective, so the plane will not reach critical AoA immediately. It depends on the planes design on how fast it gets to this AoA, but in most planes it will be a few seconds. During this time the plane will lose speed rapidly, but keep entering a tighter and tighter turn. Finally, the plane starts to reach critical AoA. The plane gives many signals as parts of the wings start to stall before the others. If the pilot, still keeps increasing the AoA, he will stall.

Indranil · Post by **Indranil** » 14 Mar 2013 23:10

Jayram wrote:A dumb question but I have always wondered about this. Given that the LCA's thrust to weight ratio is >1 why should the AOA matter in certain scenarios ie in a knife fight? In those scenarios the LCA can/should basically behave like a rocket and not need any lift at all with the engine basically powering the craft at what ever angle (limited of course by the G forces) it needs no? Yes handling authority and recovery would still be an issue but turn radius should not... no?

You are actually confused on 2 things.
1. AoA is the angle that the longitudinal axis (passing from nose to tail) makes with the direction of flight. So for example a rocket shooting straight up has 0 deg of AoA as it's longitudinal axis and direction of path are aligned. So you are right, that in this flight path any plane whose thrust is greater than its weight + drag can accelerate on its way up like a rocket.

2. In a turn, a plane needs to create much greater centripetal force than its weight. How much more? A plane in a 9G - turn requires a centripetal force 9 times it's weight. This it does using it's lift (L). All that the engine does is overcome the drag that is being incurred to create this lift. Let us limit out discussion to constant speed. There is something called a lift to drag ratio (R). For a given speed it looks something like this.

So the force that the engine needs to generate to even maintain equal momentum is to overcome the drag (= L/R). For a 9G turn, it has to create 9/R times the weight of the plane.

Now, if you can find the graph for lift to drag ratio of LCA for varying AoA and airspeeds, you will have answered all your questions.

nachiket · Post by **nachiket** » 15 Mar 2013 00:24

indranilroy wrote: 1200 kmph is a tricky speed to do stunts. But let us think what would happen at 800 kmph. Also let us assume that the plane is flying clean.

Nothing much would happen. At those speeds that the plane has a lot of momentum, and the control surfaces are also not that effective, so the plane will not reach critical AoA immediately. It depends on the planes design on how fast it gets to this AoA, but in most planes it will be a few seconds. During this time the plane will lose speed rapidly, but keep entering a tighter and tighter turn. Finally, the plane starts to reach critical AoA. The plane gives many signals as parts of the wings start to stall before the others. If the pilot, still keeps increasing the AoA, he will stall.

But then how do you perform a 360 degree vertical loop?

Indranil · Post by **Indranil** » 15 Mar 2013 01:55

After you have reached your maximum AoA, you hold the elevator at a position which allows you to just maintain that AoA and not go over.

In a plane with no limiters, the pilot has to let go off the stick a little. In a plane with limiters, you can yank all you want. The computer or mechanics will adjust the elevator, to not let you go over.

VishalJ · Post by **VishalJ** » 15 Mar 2013 04:54

Rotate ► http://www.airliners.net/photo/2238400/L/

member_23360 · Post by **member_23360** » 15 Mar 2013 05:56

indranilroy wrote:
Jayram wrote:A dumb question but I have always wondered about this. Given that the LCA's thrust to weight ratio is >1 why should the AOA matter in certain scenarios ie in a knife fight? In those scenarios the LCA can/should basically behave like a rocket and not need any lift at all with the engine basically powering the craft at what ever angle (limited of course by the G forces) it needs no? Yes handling authority and recovery would still be an issue but turn radius should not... no?
You are actually confused on 2 things.
1. AoA is the angle that the longitudinal axis (passing from nose to tail) makes with the direction of flight. So for example a rocket shooting straight up has 0 deg of AoA as it's longitudinal axis and direction of path are aligned. So you are right, that in this flight path any plane whose thrust is greater than its weight + drag can accelerate on its way up like a rocket.

2. In a turn, a plane needs to create much greater centripetal force than its weight. How much more? A plane in a 9G - turn requires a centripetal force 9 times it's weight. This it does using it's lift (L). All that the engine does is overcome the drag that is being incurred to create this lift. Let us limit out discussion to constant speed. There is something called a lift to drag ratio (R). For a given speed it looks something like this.

So the force that the engine needs to generate to even maintain equal momentum is to overcome the drag (= L/R). For a 9G turn, it has to create 9/R times the weight of the plane.

Now, if you can find the graph for lift to drag ratio of LCA for varying AoA and airspeeds, you will have answered all your questions.

nicely explained, thanks much

shiv · Post by **shiv** » 15 Mar 2013 06:34

indranilroy wrote:
Nothing much would happen. At those speeds that the plane has a lot of momentum, and the control surfaces are also not that effective, so the plane will not reach critical AoA immediately. It depends on the planes design on how fast it gets to this AoA, but in most planes it will be a few seconds. During this time the plane will lose speed rapidly, but keep entering a tighter and tighter turn. Finally, the plane starts to reach critical AoA. The plane gives many signals as parts of the wings start to stall before the others. If the pilot, still keeps increasing the AoA, he will stall.

Unless I am mistaken, what you are implying is that to do a loop the airspeed has to be below a critical value, because above that value the plane will not respond to inputs from control surfaces?

In other words you are saying that attempting a loop at 1200 kmph will lead to nothing, and at 800 kmph will cause a stall as you have described above and the plane can only do a loop at some lesser speed?

I thought that at 1200 kmph pulling the stick back will make the plane go vertical and beyond (half loop) at insane high G or wings can break off. But I may be wrong.