LCA news and discussion

[putnanja]

this is hilarious CM ! are you implying that the guys involved in the decision making are somehow managing to send their children to the US (that is an F1 visa) or that their children are getting H1 visas to work in the US on the basis of their parent's decision to support a US product ?! I'm afraid nothing can be further than the truth in this regard..the US consulate that gets visas done doesn't quite take this into account I'm afraid..

Kartik, there were other reports too long time back about how offering permanent residency in US to family etc was one of the carrots used by US to get info/intel. So CM's remark is not so far fetched.

[shiv]

deleted

[Rahul M]

@ all, please take this discussion elsewhere.

[Cain Marko]

crossposted in miscellaneous mil thread.

http://forums.bharat-rakshak.com/viewto ... 70#p875170

CM

[Brando]

It's fascinating how no discussion about US military equipment can go without digging up the skeletons of the Post Pokran 2 sanctions! How much longer are we going to keep dragging these skeletons with us into the future ?? If India is going to be perennially neurotic about being sanctioned, it can't do business with anybody and can't get access to any new technologies from the West. India has a nasty habit of clinging to the past and not letting go. This same paranoia is what led to nearly 40 years of isolationism from the West and nearly bankrupted India. As Hamlet says "nothing is good or bad, but thinking makes it so" .

[Cain Marko]

It's fascinating how no discussion about US military equipment can go without digging up the skeletons of the Post Pokran 2 sanctions! How much longer are we going to keep dragging these skeletons with us into the future ?? If India is going to be perennially neurotic about being sanctioned, it can't do business with anybody and can't get access to any new technologies from the West. India has a nasty habit of clinging to the past and not letting go. This same paranoia is what led to nearly 40 years of isolationism from the West and nearly bankrupted India. As Hamlet says "nothing is good or bad, but thinking makes it so" .

India has been doing business for ages with the "west" and continues to do so - get used to it. Btw, most Indians have very decent memories - there have been those great ones who might even have glimpse or two from previous lives memorized. I dunno about the current crop of nouveau, "Wesht is Besht" types, but I do believe a good memory is a nice thing to have.

Whats that they say about history? In case you can't remember - "those who forget it, are condemned to repeat it" .

Further, this whole deal about ITAR and the USG's restrictions on LCA exports (in case of LM consulting) is well past the POK II drama.

CM

[Kailash]

Under the circumstances, while I am sorry to suggest underhanded dealings, the question does beg asking, as Philip has in a previous post. What gives? What makes India run to the US door time and time again despite all the complaints?

Politics? Considering the above reasons, I really don't think so. Operational needs - Certainly not the AF's op requirements it seems - otherwise they'd have settled to go with Dassault for the FBW instead of wanting to have a completely 4 X digital channel FBW; something that some of the best operational birds today don't have. Similarly, they might look at non US consultants, but that never seems to happen, the first choice is always the US.

Looking at US as the first choice is not wrong. How much ever hatred we have, they ARE the best in the business. The real problem is us not having a long term plan - to reduce dependency on whatever we are getting/buying from outside.

600-700 planes? If that has to happen, we need confident customers placing large orders. It also needs a major PR/advertising campaing to get new customers, exports etc.

Further, this whole deal about ITAR and the USG's restrictions on LCA exports (in case of LM consulting) is well past the POK II drama.

When we have not attempted to find any export market for mk-1 yet, I doubt if this actually matters. Though Indians have the strong sentiments against export restrictions, sentiments towards export orientation is sadly missing.

[Cain Marko]

Looking at US as the first choice is not wrong. How much ever hatred we have, they ARE the best in the business. The real problem is us not having a long term plan - to reduce dependency on whatever we are getting/buying from outside.

Hatred? None whatsoever. What for? Anyways I am one of those who subscribe to the "natural ally" bit. Also, how far does India need things that are "best in the business?" that too in the very first, real attempt. A Rafalesque, DFBW wouldn't suffice?

600-700 planes? If that has to happen, we need confident customers placing large orders. It also needs a major PR/advertising campaing to get new customers, exports etc.

Why will you have confident customers when you don't take into consideration what they want? You may not even have too many purchases from the IAF let alone exports.

When we have not attempted to find any export market for mk-1 yet, I doubt if this actually matters. Though Indians have the strong sentiments against export restrictions, sentiments towards export orientation is sadly missing.

It matters, if not in practice, then in principle. Otherwise, they'd have gone ahead with LM and not EADS, which was the second choice iirc.

CM.

[Sanku]

The US being best in business is a nice bit of sell from the Amerikis but is not really proven true. Most of the US success comes from doing the basics right, but it gets attributed to the shiny part of the toys, which probably played a passenger role.

Doing the basics right is something many other countries have also learnt to do well.

The difference is that US has massive resources and hence goes for shock and awe on top of the real McCoy making things work -- and the packaging helps to sell it others later.

[Rahul M]

CM, try the newbie and mil misc thread. could you move the above posts there and just leave a link here for continuity ?

[rajeevcm]

Instability: May I ask how will you quantify relaxed instability? The rate at which the amplitude of oscillations would increase along any axis! FBW would control that. I dont know of any open source where this critical data for this FBW dampening/relaxing is published for any of the fighters. I would be glad to be pointed to such a source! Without any such source I have to guesstimate from the sustained rate of turn. Is there any other way of guessing?!!

there is a source. and a very authoritative one that cannot be questioned. Air Marshal Philip Rajkumar in his book stated

"The software in the DFCC is broadly divided into 2 parts. The first part is the CLAW which decides how much the control surfaces should move depending on the flight conditions. From a mathematical flight model of the aircraft based on wind tunnel and CFD data, control engineers calculate the amount of control deflection required to control the aircraft at various points in the flight envelope, and create a big look up table which is stored in the computer memory. The flight conditions are sensed by the air data probes on the aircraft and passed on to the computer. The computer accesses the look up table in real time and decides the amount of control deflections required.

Each computational frame lasts 12.5 milliseconds or in other words, is repeated 80 times per second, but the actual movement of the control surface takes place about twice per second."

I am really not sure that updating/moving the control surfaces at a rate of 2hz is sufficient to control the aircraft.

[shiv]

Looking at US as the first choice is not wrong. How much ever hatred we have, they ARE the best in the business.

My reply here
http://forums.bharat-rakshak.com/viewto ... 72#p874872

[vcsekhar]

I am really not sure that updating/moving the control surfaces at a rate of 2hz is sufficient to control the aircraft.

If that was the case then the LCA would not be flying now and it would not be at the door of the IOC.
If you are really interested in this topic, I would suggest going through the forum posts a few months ago and there were many detailed posts about this issue.

[rajeevcm]

Well, I knew this would cause questions, that is the reason that I thought a lot about posting this.
My guess is that Air Marshal Philip Rajkumar did not give the correct update rate of the control surfaces in his book for whatever reason. I am sure that the update rate on the surfaces will be close to around 50 Hz in order to maintain stable flight.

[shiv]

I am really not sure that updating/moving the control surfaces at a rate of 2hz is sufficient to control the aircraft.

You are saying this only because you don't know how slowly the inertia of a multi-ton aircraft makes it react to control surface deflections.

Watch the following video very carefully for just 10 seconds starting from 3 min 23 sec. Watch the tailplane very carefully at 3 min 27 seconds. What the pilot does is a very quick "down-up"" of the tailplane in less than one second. If he had a stick - he has probably pulled the stick back for a brief moment and pushed it forward, causing the tailplane to deflect down and up in atout 0.5 sec. That is clearly visible. What that does is to move the tail of the plane down and the nose up. And over the next 5 seconds the plane itself majestically rises in response to the nose up attitude.

http://www.youtube.com/watch?v=qjpLejaErII

The take away lesson? Aircraft have too much inertia and do not have time to respond to control surface inputs that are shorter than 0.5 sec or so. It is useless making it shorter than that. Making an aerodynamic surface vibrate at 50Hz is of no use.

[vcsekhar]

Good points shiv.
In addition to your points about inertia of the plane, the hydraulic actuators also have to be considered. These actuators need to exert enormous forces on the control surfaces to effect a change in direction or attitude of the plane. These cannot be cycled so quickly (ie > 2 Hz).

[nikhil_p]

Also cycling at 50Hz will build up a lot of minor turbulence and vibrations which can ultimately lead to complete failure of the metal due to fatigue. (The control surface vortexes!)

[Tanaji]

More likely the sampling of external stimuli and forces is done at a very high rate, but the actual changes to control surfaces is done at a far lower rate using statistical averaging techniques.

[shiv]

More likely the sampling of external stimuli and forces is done at a very high rate, but the actual changes to control surfaces is done at a far lower rate using statistical averaging techniques.

That is exactly what is implied by AM Rajkumar and it is not a mistake or a misprint.

[vina]

majestically rises in response to the nose up attitude.

http://www.youtube.com/watch?v=qjpLejaErII

The take away lesson? Aircraft have too much inertia and do not have time to respond to control surface inputs that are shorter than 0.5 sec or so. It is useless making it shorter than that. Making an aerodynamic surface vibrate at 50Hz is of no use.

A better time to observe that is from 4:51 or so just as it is landing and wheels are about to touch. You can see the control deflections (obviously FCS doing it to keep the flare angle)

[rajeevcm]

The following is from aviation week where they talk of the response time of the Dassault 7X and the Rafale and it is around 12.5ms

Compared with the Mirage 2000 or Rafale, the Falcon 7X has inherent aerodynamic stability and is not designed to be flown above Mach 1. Dassault determined it only needed digital flight controls with 50-millisecond response time, but designed Falcon 7X’s system with the same 12.5-millisecond response time as the Rafale’s FBW system because there was no advantage to slowing it down to conventional air transport levels.

Another reason I am skeptical is that Air Marshal Rajkumar talks about the computation taking place 80 times a second but update at only 2hz why would you want to waste computing power?

[shiv]

A better time to observe that is from 4:51 or so

The video length is only 4:26

[shiv]

The following is from aviation week where they talk of the response time of the Dassault 7X and the Rafale and it is around 12.5ms

Compared with the Mirage 2000 or Rafale, the Falcon 7X has inherent aerodynamic stability and is not designed to be flown above Mach 1. Dassault determined it only needed digital flight controls with 50-millisecond response time, but designed Falcon 7X’s system with the same 12.5-millisecond response time as the Rafale’s FBW system because there was no advantage to slowing it down to conventional air transport levels.

Another reason I am skeptical is that Air Marshal Rajkumar talks about the computation taking place 80 times a second but update at only 2hz why would you want to waste computing power?

80 Hz sampling is waste computer power? Can you explain that? I would need to throw away my MP3s with 44 KHz sampling. Would you be able to suggest a sampling rate - For example, if you had a choice, would you sample gusts and deflections of the attitude of the aircraft only at 10 Hz? Perhaps the computer could play MP3s with its saved computing power? Have you any reason to say that 80 Hz is too fast? I believe the computing power statement is a bogey.

Note that "response time" could be different from the time between control inputs. Are you sure Aviation week said that control inputs were being given 80 times a second when you say "response time" of 12.5 msec. The response time could well be the time lag between a computer command and the response of the control surface. The actual number of automatic commands per second could still be once in 500 miliseconds. You need to confirm that before you jump to conclusions.

PS: ACMR sometimes reads this forum. I know him well and I don't think his words are wrong. I do think you are wrong.

[rajeevcm]

Hey Shiv
I do not want to anger anybody. I have had issues when using FBW with control surface at updation rates below 20Hz on a general aviation aircraft. Maybe ACMR means that the bandwidth on the elevator actuator is 2Hz and not that the surfaces move twice every second.
On the power requirement, the way I meant it was since you are running your processor at 80Hz, the computer heats up a lot as compared to say running at 20Hz and needs more cooling.
I joined in on this debate as I am a student of this subject

[Indranil]

Hey Shiv
I do not want to anger anybody. I have had issues when using FBW with control surface at updation rates below 20Hz on a general aviation aircraft. Maybe ACMR means that the bandwidth on the elevator actuator is 2Hz and not that the surfaces move twice every second.
On the power requirement, the way I meant it was since you are running your processor at 80Hz, the computer heats up a lot as compared to say running at 20Hz and needs more cooling.
I joined in on this debate as I am a student of this subject

Rajeev it will be great if you could tell us in short what you found out on the plane that you are working on. Please tell us a little about your plane too.

AS you are a student, as I have worked with sampling myself, you would know that finer the sampling the better estimation you can make. However, we would stop at a point where we would have found the desired accuracy. It might be around 80hz. Just because, we use the control surfaces at 2 hz (lets take that number for the moment), a forty times faster sampling might not be a overkill!

Is there any other source other than the ACMR to validate the sampling rate and correction rates? I believe the ACMR, but an alternate source will only corroborate his point and lay all apprehensions!

[Carl_T]

Newbie question onlee - the GE F404 is powerful enough to fly the Gripen which is reasonably heavier than the LCA, but why is the same engine underpowered for the LCA?

[Rahul M]

nitpick, AMR and not ACMR.

[Raveen]

Newbie question onlee - the GE F404 is powerful enough to fly the Gripen which is reasonably heavier than the LCA, but why is the same engine underpowered for the LCA?

Has been answered numerous times; here is the logic:
A M800 engine drives a M800 faster than a 20'x20' sheet of iron that weighs less because even though thrust/lb or hp/lb is higher for the sheet of iron aerodynamics plays an important role. It hence a logical conclusion that the LCA has more drag or is in general not as efficient an airframe from an aerodynamic point of view. Now whether this was a trade off for another feature or whether this was the result of Saab's years of experience I don't know but that is the only logical explanation for why despite having higher thrust/lb the LCA is actually underpowered.

[rajeevcm]

I worked on a GA aircraft where due to some computer limitations we could update the control surfaces only at a max of 15hz. This caused the plane to get into oscillations in pitch. Let me explain why I feel that the elevator changes only 2 times per second may cause instability
Current digital control Laws can all be written as

Delta_e(t)=K*(Error between the desired and actual state at time (t))

Where “K” is the gain scheduled based on flight condition usually altitude,airspeed

Say the computer calculates the elevator deflection at t=0 seconds based on the sensor readings at t=0, but since the actuator moves only twice per second this does not get transferred to the elevator, now since the control law runs at 80Hz the value of the elevator deflection keeps getting updated(but the elevator does not move), @t=0.5 seconds the elevator deflection goes through say Delta_e_1, to maintain the aircraft in equilibrium. Now say the aircraft is hit with an upward gust the error between desired and actual states increases and the plane starts pitching up. This upward pitching is unopposed(due to relaxed static stability) for the next 0.5 seconds since the elevator does not provide an increase in nose down moment and this causes the error to build up. @t=1 second the elevator deflection calculated will be large since the error has built up over the 0.5 seconds and the aircraft pitches down and travels beyond the equilibrium AOA, this goes on until 1.5 seconds wherein the control law causes the elevator to deflect in the opposite direction leading to oscillations in pitch. Since I do not know the stability and control derivatives of the LCA, I cannot predict whether these oscillations grow with time or not. However that results in a bumpy ride.

That is the reason why I am confused that the elevator surface deflection is updated only 2 times per second. Sorry if I am being too technical

[putnanja]

'Lockheed-Martin's Failure To Respond Has Affected Our Programme Schedule': LCA (Navy) Programme

Top officials associated with the Light Combat Aircraft naval variant (LCA-Navy) have confirmed to LiveFist that Lockheed-Martin's inability to obtain US State Department clearances to consult with the programme have introduced a "significant delay component" into the schedule of the first aircraft roll-out. In the next few days, ADE is likely to officially hire the services of EADS, which lost the original bid in 2009.
...
...

[Indranil]

Say the computer calculates the elevator deflection at t=0 seconds based on the sensor readings at t=0, but since the actuator moves only twice per second this does not get transferred to the elevator, now since the control law runs at 80Hz the value of the elevator deflection keeps getting updated(but the elevator does not move) ...

Rajeev, you seem to be making a correction decision for each sample. Obviously, if you cascade 40 such decisions before a real collection, an error will build up.

Another way could be to keep taking sensor readings at 80 hz. While calculating your correction calculation(at 2Hz), you take a window of 40 (may be more) samples to gauge the deviation. Such a window can you give you a much better indication of the planes behaviour if you already know the patterns of changes. Though the generations of all the patterns might be intensive, it will not be very complex, as you would know the co-ordinates of the sensors. In the model that I foresee as being used, there will be three stages, namely sampling, deviation gauging and correction calculation. I am leaving out the physical correction implementation. By pipelining the three said 3 stages you should have quite a lot of cpu cycles even on the 386s. Deviation gauging and correction calculations would be table lookups (didnt we read about something similar in the implementation!).

And come to think about it, a 9.5 T plane, even for pitch correction, half a second seems quite right. Anything faster, would have to be very slight instantaneous modifications according to the model you suggested. Of course one would run out of CPU cycles pretty soon!

Am I making sense, or am I completely off??

P.S. I know a guy who is building a UAV. His is a heli-UAV, which obviously flies itself and hence must have a control logic. Will catch hold of him and know what he is doing! I also knew that in my dept there were bunch of under graduate guys who were making a pneumatically launched fixed-wing UAV for a LM challenge. Both these teams have had flying demos, so they would know a few things. Though I dont know whether they can disclose such information to me!

[Kartik]

cross-posting from the newbie thread, just in case others want to take this discussion further on the LCA.

I was going through the LCA thread and I saw Kartik's post regarding Diverterless Supersonic Intake (DSI). However, the question that comes to mind is what advantage does DSI provide over intake splitter plate and how? Does it increase simplicity by reducing the no of moving parts required in the intake or does it also have any performance benefit? Can Kartik or someone else educate us regarding this?

On the Tejas, the splitter plate and the bleed channel leading onto the top of the wing near the wing root does the same job as the DSI. The method used on the Tejas is not a complicated mechanical boundary layer control method. That splitter plate doesn't move forward or backward depending on the speed and altitude of operation. It is fixed, which means that it is uncomplicated and light weight but it also means that it puts a restriction on just how fast the LCA can go- in this case the top speed being set at Mach 1.6.

Some previous generation aircraft used a more complicated, and heavier system called a variable inlet- for instance the Mirage-2000 with its "mouse" cone that can move in or out of the air intake depending on speed and altitude. By moderating the size of the intake through which the air would get inside the intake, the "mouse" would control the pressure inside the air channel near the compressor. If the air velocity was too high (supersonic) the "mouse" would come out, narrowing down the air intake and that way compress the air entering the intake, and in that way increase pressure at the cost of velocity. So there were two benefits- the air pressure in the air channel was higher (a higher Coefficient of Pressure is beneficial) and the air velocity was sub-sonic. Now why is it that you need sub-sonic velocity in the channel ? Because supersonic waves would create havoc with the compressor blades due to the shock waves that are associated with them. They could cause a compressor stall.

And the reason that previous generation aircraft featured such intakes is because there was a time when Mach 2 and Mach 2+ performance was considered important. So aircraft that needed to fly that fast needed this complicated inlet system to ensure that they could safely fly to Mach 2 or beyond without the engine compressor blades stalling. Another aircraft that would illustrate the inlet and the diverter is the MiG-23. The MiG-23s had this complicated inlet geometry. This is one of the essential differences between the MiG-23 and 27 which you can verify by a simple visual comparison since the fixed inlet diverter is smaller- the variable inlet was removed on the MiG-27 and in its place a fixed inlet was put. This restricted the MiG-27's max speed to below that of the MiG-23, but since the primary job of the MiG-27 was Strike, CAS and Interdiction it's top speed was not that important.

On the Tejas, the ASR would have specified a top speed in the range of Mach 1.6-8 or so. That would've meant that if the inlet was designed well then you could get to that speed without the variable geometry, which is why they don't have a variable inlet. But the problem with the design that they chose, of an intake that is shielded by the wing is that it restricts or "jams" the airflow between the splitter plate and the wing. In this region, they created a veritable "pocket" into which the boundary layer air would get jammed. Now, you cannot have air trapped in that pocket without giving it a path to flow down or up with a channel- so the bleed channel was provided onto the top of the wing. And it turned out to be a blessing as it now directed turbulent flow onto the top of the wing and generated vortices that helped keep the air boundary layer stay attached even at high alpha angles. This concept is quite unique amongst fighters (similar things are done on leading and even trailing edges, but not near the intake) and how successful it has been can be gauged by the fact that even the newest AMCA design (whose wind-tunnel model was shown in AI-09) had a similar bleed channel.

What DSI does is to reduce the RCS as the vanes and doors of variable inlets are a source of RCS. They tend to be at angles that can reflect radar waves back to the emitter. By eliminating those variable inlets and even the splitter plate, the DSI basically reduces the RCS a bit. Its not a big deal for the Tejas anyway since it has a fixed inlet and already has a very small RCS. It also is claimed to help improve power available to the pilot in the subsonic range a bit..but its not an improvement that warranted a complete change from the existing inlet on the F-16 (on which it was first tested) to the new inlet..otherwise we'd have seen the F-16 Block 60 and the F-16IN sporting the DSI if it was such a whiz-bang improvement.

I am also quite sure that this idea as well as design and CFD data was in some way or the other stolen by the Chinese through industrial and academic espionage at Lockheed Martin and the research labs that worked on DSI, since it was their IP. It was no coincidence that they were able to suddenly jump from the regular splitter plate intake on the initial FC-1 prototype to the DSI style intake on the FC-1 after it went through a re-design. LM had been working on this concept since the early 1990s and the Chinese were all of a sudden able to come up with a productionised DSI intake on the FC-1 in the early 2000s when no other manufacturer in the world had even attempted this..to their credit they copy ideas and steal IP but productionise it and they hardly care what impression that gives to other people. Although what that does is to make OEMs very wary about letting them get their hands on top-notch technology, knowing that these guys will reverse engineer it.

[rajeevcm]

Indranil
You are right the model I suggested results in small deflections of the control surfaces which are not discernible to the naked eye, but this keeps the aircraft from entering into oscillations in pitch . I am not sure what you mean by running out of CPU clock cycles, if you are sampling the sensors at 80 Hz and since the new elevator deflection is just a multiplication of the error and a gain matrix.

[shiv]

I do not want to anger anybody. I have had issues when using FBW with control surface at updation rates below 20Hz on a general aviation aircraft. Maybe ACMR means that the bandwidth on the elevator actuator is 2Hz and not that the surfaces move twice every second.
On the power requirement, the way I meant it was since you are running your processor at 80Hz, the computer heats up a lot as compared to say running at 20Hz and needs more cooling.
I joined in on this debate as I am a student of this subject

Thank you for saying this.

May I point out a a general rule that if you know something you need to say "This is what I think, but this is what AM Rajkumar says and I do not know how to reconcile the two" What you actually said was "I think AMR is wrong or what he wrote was wrong" implying that you had some thoughts in your mind which you felt were right but you were not revealing them. Sorry - but we have so many people posting so much wild stuff in the guise of ultimate truth that it just does not do say what you said. You do not know that Raj is wrong. You only know that your information and what he says do no coincide. So what was all that about his being wrong? Please.

[shiv]

Now say the aircraft is hit with an upward gust the error between desired and actual states increases and the plane starts pitching up.

May I ask the duration of this upward gust? Are you assuming an instantaneous (zero time) gust that will receive a control system response only 0.5 sec later? But gusts are not zero time. For an aircraft lying at say 500 kmph an upward gust will have to be pretty strong and it will have to last for a measurable period of time for it to have an effect on the aircraft. How long does a 200 kmph upward gust have to last before it causes a significant deviation to a 10,000 kg aircraft moving at 500 kmph? 0.001 sec? 0.01 sec? 0.1 sec, 0.3 sec?

Where in your model have you factored that in? Have you also factored in the response time of sensors after the gust hits them and the time for getting inputs from all sensors, the time for processing them and then deciding on a actuator movement and the time for the actuator hydraulics to respond and cause a control surface movement and the time lag before a control surface movement has any effect on the attitude of the aircraft. Or is everything in zero time except for gust and actuator response after 0.5 seconds?

[vina]

Now say the aircraft is hit with an upward gust the error between desired and actual states increases and the plane starts pitching up. This upward pitching is unopposed(due to relaxed static stability) for the next 0.5 seconds since the elevator does not provide an increase in nose down moment and this causes the error to build up. @t=1 second the elevator deflection calculated will be large since the error has built up over the 0.5 seconds and the aircraft pitches down and travels beyond the equilibrium AOA, this goes on until 1.5 seconds

Huh ?. Why will the elevator NOT move for 0.5 seconds ? The control system will definitely move it!. The amplitude build up will be smothered and not allowed to happen. That is why you over sample!. The moment, the FCS senses a departure , it will issue the appropriate correction command. The point is the system has inherent damping and inertia. If I remember Air Marshal Rajkumar's book, the time to double amplitude is of the order of 0.5 seconds or so (don't remember , I am pretty bad at remembering numbers, but it is pretty quick to double amplitude). Now ask any good electrical engg or control systems engineer. I remember in the Madrassa about kids in EE always telling me that you have to sample at double the frequency rate for signal processing (Nyquist theorem or Lindquist theorem or something.. again addled brains sorry).. So you need to sample at atleast twice the signal rate. So I guess if you overs ample of an order of magnitude more, you probably can have much finer /smoother control .

I think all in all, given the control system cycle rate, on an average you will have 2 deflections per second or so max and that is all that will be needed to keep the plane stable. Maybe in in an idealized "MATLAB" model (or rather phree GNU "Octave" model which I am punching keys on these days , not for Hawai Jahajes though ), sure , for a first cut, you can ignore mass and damping and response times and build a control model where things happen pronto. But then when you implement in the real world, you at least will update your model to include all those parameters to reflect reality. Or else you are in cloud cuckoo land.

JMT and all the rest of the usual disclaimers and safe harbors.

[rajeevcm]

Shiv
Lets look at this upward gust. The upward gust causes an instantaneous change in AOA since the AOA by definition is given by
AOA=atan(U/W), where U is the speed of air along the body x-axis and W is the air speed along the body z-axis.
At trim U and W are constant, now as the aircraft enters an area of upward gust W decreases (due to upward motion of air) This causes the AOA of to increase above the trim value. Now Cm_alpha for this aircraft is a +ve number Due to this a nose up moment is developed almost instantaneously(I agree that it takes the order of a few micro/nano seconds to develop which is why when studying flight dynamics of aircraft which consists of motion in milliseconds they are treated as steady state and we study the so called quasi steady state response) which causes the aircraft to pitch up (since the LCA is a fighter aircraft and designed to be highly maneuverable, this moment is going to be quite large which causes the pitch acceleration to increase and consequently pitch rate and pitch angle and this moment increases proportional to the square of the airspeed(Sqrt(U^2+W^2), so the faster you fly the larger the moment and larger the pitch up in shorter time) also keep in mind that the LCA is a delta and does not have a horizontal stabilizer this implies that pitch damping is smaller than aircraft with HS, this causes larger deviations in pitch angle and AOA even in short time and hence needs to be corrected instantaneously.

Now to talk about response time of sensors. In most FBW control system designs the time constant associated with sensors are so small as compared to aircraft response times that they are neglected in design. For example the AOA sensor is a vane type sensor which has a very small time constant, unfortunately this sensor is so noisy that a lot of filtering is done to obtain a reliable signal again the time delay associated with this is small (this depends on the update rate).
Actuators are a different story altogether, the time constant for the elevator actuator in the F-16 is 50 ms (Refer Aircraft control and Simulation by Lewis and Stevens) and this is closer to the dynamics of the aircraft and have to be taken into account.

About Air Marshal Rajkumar, I have a feeling either it is a typo by the publication or he intentionally did not give the right number due to security reasons.

[rajeevcm]

Vina
But according to the previous post the Elevator moves only a couple of times per second. So using that analogy between time t=0 and t=1, the elevator moves at say t=0.5 seconds and t=1 second and retains the deflection commanded at t=0.5 seconds between t=0.5 and 1 second (using a zero order hold). This is what got me to post and say that this might not be right and there is something missing.

Also you say "Why will the elevator NOT move for 0.5 seconds ? The control system will definitely move it!" and then go on to say "I think all in all, given the control system cycle rate, on an average you will have 2 deflections per second or so max and that is all that will be needed to keep the plane stable" by which I assume you mean two different deflections say Delta_e1 and Delta_e2 over a period of 1 second. So which is right the control system moves the elevator instantaneously or moves it only twice a second?

Sorry if I did not get what you said

[vina]

airspeed(Sqrt(U^2+W^2), so the faster you fly the larger the moment and larger the pitch up in shorter time)

Not true. That doesn't account for the shift of Center of Lift backwards with speed. The cm will start dropping as speed increases and infact, I can bet Rs 100 on it that at Mach 1.4, the center of lift is behind the center of gravity for the LCA, just like in a conventional aircraft and it becomes statically stable. I picked Mach 1.4 because that is where the envelope opening of the LCA was stuck for a long long time and beyond that , the control systems have to work in an opposite way from the way they work from 0 to 1.4M . That will require lot of control law development, validation and coding and testing and probably would get done last, I connected the two together to hazard a guess.

I would submit that the highest amount of instability is at slow speeds and the LCA will need the highest of control in probably the low /take off /landing kind of speed and also probably in the transonic 0.8M to 1.2M because of all the buffeting and shocks while going through the sound barrier and back.

[vina]

So using that analogy between time t=0 and t=1, the elevator moves at say t=0.5 seconds and t=1 second and retains the deflection commanded at t=0.5 seconds between t=0.5 and 1 second (using a zero order hold). This is what got me to post and say that this might not be right and there is something missing

Nothing missing. What is amiss I think are your assumptions which I have bolded. Why should the two deflections be exactly 0.5 secs apart?. Why not at 0.1 and 0.9 secs apart ? And why will the elevator hold in the position of deflection, unless it wants to maintain and hold given alpha (in that case, it will deflect around that angle as the mean, nothing very different there)? The control once it deflects, will come back to the mean..

The way to understand it is as if there is some dynamic force (time varying and random and instantaneous) that needs to be generated at the trailing edge where the elevons are attached. Forget about deflections for a moment and assume that you would use some gas jets or rockets or something attached at that point to give some puffs of instantaneous force at the random times and quantities as determined by the FCS (remember, the jets will cycle on and off and fire for various durations depending on what you want to do). Once you know what needs to be done, the next question is how to do it? . Okay do I be a Paki dork and attach gas jets/rockets pointing downwards at the wing trailing edge , or do I move the elevators down and up to do exactly what I want to do ?