LCA News and Discussions

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Kanson
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Re: LCA News and Discussions

Post by Kanson »

Sources close to idrw.org also have told us that Python-5 might be also be considered for Tejas MK-1 for meeting short range Air to Air missile ,Russian R-73 was successfully had been integrated with Tejas MK-1, but it seems some issues might have surfaced due to which air force is also interested in Israeli Python-5 .
There is no some issues that surfaced recently as reported.

Due to limitations in turn rates of LCA in this current version which has direct bearing in WVR fights, a more robust missile than R-73 is selected which compensates for the shortcomings in turn rates.

Further, this was already hinted by ADA Chief long back.
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Re: LCA News and Discussions

Post by Kanson »

While comparing Tejas air brake with F-18 and other planes do you see any difference with Tejas air brake and others?
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Re: LCA News and Discussions

Post by suryag »

:(( Feb 22nd in a week feb will end when will jingos be khush ??
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Re: LCA News and Discussions

Post by Vikram W »

I think HAL is toying with us :)
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Re: LCA News and Discussions

Post by Singha »

python5 per wiki is exactly same weight as R73 - 105kg , similar range, about 5 inches longer and much modern missiles .... a good choice imo for entire iaf fleet to replace the r73 once the stocks expire.

unlike r73 it does not use TVC, but has 18 small control surfaces.
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Re: LCA News and Discussions

Post by tsarkar »

Kartik - My point was using a simpler design philosophy of keeping the airbrake closer to CoG.

Discussing the other aircraft is immaterial, because their design considerations were different. Shouldnt using a simpler design process help when we're doing it for the first time?

FWIW, the Super Hornet pylons create drag and reduce range significantly, and I heard they introduced wing fences on the G model.

And the 40 orders are due to non fulfilment of basic parameters like STR & AoA rather than uncommanded pitch.
These 40 orders represent more than 5% of the authorized IAF fighter strength, and IMO significant, given that basic parameters have not been met

Suryag - When DRDO/ADA took up a more complex design path. and rejected Dassault's analog FBW that was deficient to the digital FBW, that is when IAF withdrew because it believed DRDO/ADA was using a more complex means to achieve relatively simple ends. IAF agreed that Dassault FBW was not state of the art, but believed that it was proven, and would have mitigated development risk. Fears were proven true when LM held back the CLAW software code after 1998.

Indranil - your assessment is close, I spoke to a INAS 300 engineer who thinks on similar lines. INAS 300 is slated to replace its Sea Harriers with the Tejas in future.

Lastly, MiG-21 and F-104 Starfighters were supersonic bomber interceptors of the 50's and 60's. IAF purchased them to shoot down Pakistani airfield attackers and escort our strike forces. Before MiG21, both scenarios were the highest cause of IAF losses. Landing speed is high for that family of aircraft with high wing loading and small wings, and pilots know these characteristics.
Landing speed became an issue when IAF started using MiG-21 as a trainer. For rookie pilots, it was not ideal.

Having said that, how many MiG-21 have been lost on landing? I dont have the numbers, but they're few in between. So MiG-21 landing speed high was a known factor. FWIW, the aircraft it was supposed to counter - Pakistani F-104 - had higher landing speeds.

Lastly, I am saying this because we needed the LCA yesterday. We need LCA in numbers. We dont fight limited wars like US & Europe, wherein when the game goes tough, they go home. Our wars are existential in nature. Fighters like Rafale/Eurofighters will not be cheap to mass produce. We dont have the luxury of indefinite time or money like JSF.
Indranil
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Re: LCA News and Discussions

Post by Indranil »

I have just one input here ... I will just stick to the position of the airbrakes alone and not discuss the design philosophies and goals set for the LCA.

The designers must have tried to place the airbrakes near the CG, but could not find the space for the it. If there would have been space, they would have promptly moved it now. But the fact that they still cannot manage it, shows space constraints.

Also, I like the concept of using MLG door as air brakes. Saves space and weight!
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Re: LCA News and Discussions

Post by shiv »

The LCA is designed to be unstable in pitch with a tendency to go nose up - a factor that is compensated by fly by wire. For a plane that is designed to go nose up and be unstable, the CG has to be aft of where you think it should be.

Now how are the aeronautical geniuses of this forum concluding that the "uncommanded pitch up" - 3 words in some article means that the air brake is too far aft of the CG? If you don't know where the CG is how the hell can anyone judge that? To me that is the height of speculation.

A doctor who speculates on with no real facts in his hands would be a quack.
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Re: LCA News and Discussions

Post by Cybaru »

Singha wrote:python5 per wiki is exactly same weight as R73 - 105kg , similar range, about 5 inches longer and much modern missiles .... a good choice imo for entire iaf fleet to replace the r73 once the stocks expire.

unlike r73 it does not use TVC, but has 18 small control surfaces.
Makes a whole lot of sense. Hopefully this will speed up radar trials and weapons testing.
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Re: LCA News and Discussions

Post by Indranil »

Shivji,

How many jalebis do you want to bet on the fact the airbrakes are behind the CG? :wink:

I have absolutely no doubt that there is a pitch up moment if the airbrakes are applied ... How much it can be (or is) compensated is something we will never know.
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Re: LCA News and Discussions

Post by Sid »

tsarkar ji, doors of the main landing gear were used as air-breaks in Gnat as well. And when they say MLG (two at the back), it doesn't mean that the whole wheel will be extended. Only the doors will be opened.

Its a novel idea I say. If the Brits can do it, why can't we?

Also as suggested by other's too, we have inducted far deficient planes in the past. Even when porki Bundar was inducted it could not fire BVR or PGM. Still they ordered 250.

When we ordered 150+ ALH, we didn't started receiving MKIII version right away.
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Re: LCA News and Discussions

Post by Kartik »

tsarkar wrote:Kartik - My point was using a simpler design philosophy of keeping the airbrake closer to CoG.

Discussing the other aircraft is immaterial, because their design considerations were different. Shouldnt using a simpler design process help when we're doing it for the first time?
So how do you know what the design considerations were that made the LCA designers place the airbrake further aft of the CG? You just assumed that they didn't have common sense when placing it there..and then when pointed out that its common enough to have the airbrake placed far aft of the CG, you just brush it away with the statement "their design considerations were different". What was different about their design considerations?

What does the second statement even mean? What is a simpler design process?
FWIW, the Super Hornet pylons create drag and reduce range significantly, and I heard they introduced wing fences on the G model.
What has that got to do with the airbrake location on the vanilla Hornet?
And the 40 orders are due to non fulfilment of basic parameters like STR & AoA rather than uncommanded pitch.
These 40 orders represent more than 5% of the authorized IAF fighter strength, and IMO significant, given that basic parameters have not been met
Yeah..we know from reports (from a former IAF senior pilot) that even the F-16 Block 60 and Super Hornet didn't meet the IAF's STR requirements. Does that mean that if these were available for induction in the IAF, that the IAF would decline and reject them altogether? Very unlikely. The F-35 apparently doesn't meet its STR requirements, but is the USAF now insisting that the F-35 purchase be curtailed to a token order?
Suryag - When DRDO/ADA took up a more complex design path. and rejected Dassault's analog FBW that was deficient to the digital FBW, that is when IAF withdrew because it believed DRDO/ADA was using a more complex means to achieve relatively simple ends. IAF agreed that Dassault FBW was not state of the art, but believed that it was proven, and would have mitigated development risk. Fears were proven true when LM held back the CLAW software code after 1998.
You're repeating stuff that's been discussed to death before. Whatever that decision did, the FBW is not an issue as of now. It is mature and hasn't proven to be troublesome. it may have added 3 years to the first flight date, but that isn't the reason that the IAF is keeping Tejas Mk1 orders to 40.
Having said that, how many MiG-21 have been lost on landing? I dont have the numbers, but they're few in between. So MiG-21 landing speed high was a known factor. FWIW, the aircraft it was supposed to counter - Pakistani F-104 - had higher landing speeds.
Without knowing the numbers, how can you say that they're few and far between?
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Re: LCA News and Discussions

Post by negi »

Logically speaking the CG should be within the footprint of the MLG else the AC will be prone to tipping over to the heavier side while taxiing.
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Re: LCA News and Discussions

Post by Karan M »

Kartik writes:

"Without knowing the numbers, how can you say that they're few and far between?"

Exactly. Its well known that a lot of the troubles with the MiG 21 are in this phase.
The Indian Air Force (IAF) today said most of the recent MiG-21 crashes have been caused due to inexperience of young pilots in handling the aircraft.

"Unfortunately except for one case, the other cases point towards inexperience of young pilots who were not been able to handle the landing and approach (of the aircraft)," IAF Chief NAK Browne said in Hindon, Ghaziabad.
http://www.dnaindia.com/india/report_mi ... ef_1596427

Case in point:
http://www.tribuneindia.com/2011/20111008/nation.htm#1
A MiG-21 combat aircraft of the IAF today crashed near Uttarlai airport in Rajasthan's Barmer district. The pilot managed to eject safely.

The plane was on a routine sortie and the incident occurred at around 11.30 am when the pilot was preparing for landing, Defence PRO SD Goswami said.

The pilot, Flying Officer Amit, ejected safely. A court of inquiry has been ordered, Goswami said.

This is the sixth air crash involving IAF’s fighter aircraft this year and the fifth belonging to the MiG-21 series warplane.
The claims of CG etc by visual inspection are quite humorous. Next I guess LCA designers will be stoopid because they did not put the correct coat of paint on the aircraft. The correct paint being decided on whatever paint one likes or decides some other aircraft has.

And this:
You're repeating stuff that's been discussed to death before. Whatever that decision did, the FBW is not an issue as of now. It is mature and hasn't proven to be troublesome. it may have added 3 years to the first flight date, but that isn't the reason that the IAF is keeping Tejas Mk1 orders to 40.
Well said as before. The ADA/DRDO did not choose the French option because they did not share adequate data and attempted to tell the assembled scientists that their questions were pointless as the decision would be political and they'd win anyhow. That cost them the bid. The Germans actually impressed the Indian team the most with their transparency and technical understanding, but they did not have a working system and hence the US, which fell in between these two comparisons, actually won out. All this is well documented by the ex NAL gent who actually began the LCA program.
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Re: LCA News and Discussions

Post by NRao »

negi wrote:Logically speaking the CG should be within the footprint of the MLG else the AC will be prone to tipping over to the heavier side while taxiing.
Some thing like ...............?

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Re: LCA News and Discussions

Post by anishns »

^^^

Hahaha! That looks funny.....but in this case it tipped over due to the thrust from the B1B which was taxiing....
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Re: LCA News and Discussions

Post by SaiK »

sounds like fbw roll control won't happen on ground! ;)
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Re: LCA News and Discussions

Post by shiv »

As far as my amateur information goes, the center of mass ("CG") of an aircraft coincides with the center of lift in a stable aircraft. In an unstable one the center of mass is behind the net center of lift, the aircraft will tend to pitch nose up.

So far so good.

If on a stable stable aircraft if you introduce an air brake dorsally exactly above the center of mass what effect would it have on pitch? According to the theory introduced on this thread, it should not cause pitch up or pitch down, because the brake coincides with the center of mass. is this correct? Not necessarily because the air brake is situated dorsally and there is a force acting dorsal and above the center of mass and no compensating force acting ventrally. So the nose should pitch up.

The only way this can be avoided is to put the dorsal air brake slightly forward of the center of mass.

But consider the same situation if there are two lateral (side) air brakes at exactly the same level as the center of mass. The brakes are neither above, nor below, but exactly on the left and right. Deploying these air brakes should have absolutely no effect on pitch. They could cause a yaw if they were asymmetrical. In fact, lateral air brakes such as the ones the LCA has should have no effect on pitch whether they are forward or aft of the center of mass.

So what facts are we left with
1. Fact: LCA is stated has having uncommanded pitch up when the air brakes are deployed
2. Fact: the LCA has lateral air brakes, not ventral or dorsal
3. Fact: We have no idea where the center of mass of the LCA is situated.

Under the circumstances it would be impossible to assert that the uncommanded pitch up of the LCA on deploying air brakes is because the air brakes are too far aft.

There is no connection between the known facts and the conclusion.
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Re: LCA News and Discussions

Post by Indranil »

shiv wrote: 2. Fact: the LCA has lateral air brakes, not ventral or dorsal
That is not a fact.
Image
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Re: LCA News and Discussions

Post by koti »

Sid wrote:Its a novel idea I say. If the Brits can do it, why can't we?
How dare you!!!
SaiK wrote:sounds like fbw roll control won't happen on ground! ;)
You mean this
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Re: LCA News and Discussions

Post by Sid »

^^^ you are welcome koti ji :D
indranilroy wrote:
shiv wrote: 2. Fact: the LCA has lateral air brakes, not ventral or dorsal
That is not a fact.
http://images3.jetphotos.net/img/1/5/7/ ... 740875.jpg
wow.. best pic of LCA. Beautiful.
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Re: LCA News and Discussions

Post by Singha »

imo we should get the python5 for the Rafale , upg M2K and MKIs as well. it will further sharpen the advantages of Rafale in the knife fight regime and counter the aim9x quite nicely.
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Re: LCA News and Discussions

Post by geeth »

THe pitch up or down depends on whether the airbrakes are on top or bottom rather than aft or front.

Then there was the problem of uncontrolled pitching problem even during the first taxi trials..they were trying to correct it. This is what my chaiwala classmate told. I am not in touch with any of them for clarification. Those in the loop may be able to clarify.
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Re: LCA News and Discussions

Post by neerajb »

negi wrote:Logically speaking the CG should be within the footprint of the MLG else the AC will be prone to tipping over to the heavier side while taxiing.
On the ground the CG is practically over the MG. Reason being you need a stable AC on ground but also you want it to be easy to rotate @ V2.

shiv wrote:As far as my amateur information goes, the center of mass ("CG") of an aircraft coincides with the center of lift in a stable aircraft. In an unstable one the center of mass is behind the net center of lift, the aircraft will tend to pitch nose up.

So far so good.

If on a stable stable aircraft if you introduce an air brake dorsally exactly above the center of mass what effect would it have on pitch? According to the theory introduced on this thread, it should not cause pitch up or pitch down, because the brake coincides with the center of mass. is this correct? Not necessarily because the air brake is situated dorsally and there is a force acting dorsal and above the center of mass and no compensating force acting ventrally. So the nose should pitch up.
I am going OT and repeating what has been said on this forum before. center of lift coinciding with CG means neutrally stable aircraft like a metal ball laid over a table. At the moment it is stationary but will start moving if disturbed. Stable aircraft means center of lift behind CG aka nose heavy much like the same metal ball inside a notch on the table. If disturbed, the ball will oscillate with damped amplitude and will come to rest eventually.

Why not have symmetrical air brakes i.e. dorsal/ventral pair much like the F-16 arrangement at the tail or use MLG doors along with those dorsal air brakes?

Cheers....
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Re: LCA News and Discussions

Post by koti »

Singha wrote:imo we should get the python5 for the Rafale , upg M2K and MKIs as well. it will further sharpen the advantages of Rafale in the knife fight regime and counter the aim9x quite nicely.
What is the WVR missile we are planning for Upg Mirage and Rafale. Don't tell its Matra's Magic 2.
Also, can anyone point to the cost of the Python 5?
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Re: LCA News and Discussions

Post by tsarkar »

formatting disaster, self deleted
Last edited by tsarkar on 23 Feb 2012 12:01, edited 2 times in total.
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Re: LCA News and Discussions

Post by tsarkar »

shiv wrote:Now how are the aeronautical geniuses of this forum concluding that the "uncommanded pitch up" - 3 words in some article means that the air brake is too far aft of the CG?
Any aeronautical engineer, or for that matter any mechanics student who has read Ferdinand L Singer will confirm that the moment arm created by the airbrake not aligned with the CoG causes the pitch up.
shiv wrote:If you don't know where the CG is…
The flight control computer’s job is to know where the CG is at all times. Otherwise how will it apply corrective inputs via FBW and keep the aircraft flying. If the Flight Computer doesn’t know where the CG is, the aircraft won’t fly in the first place.

From those I spoke to, the FBW is not able to detect and actuate the elevons well in time to correct. FWIW, the services do not consider this as a showstopper, and this is not considered either for IOC or FOC.

This was just an example of the complex FBW development process that could have been avoided using a proven technology for getting the aircraft in the air, and zero base FBW development could have commenced as an enhancement.
Sid wrote:tsarkar ji, doors of the main landing gear were used as air-breaks in Gnat as well.
Agreed. However, the MLG doors do not just provide aerodynamic housing, but also protect the LG from the elements in flight. The 737 doesn’t have MLG doors, but the LG is designed for this. I was told moisture ingestion and icing of LG moving joints/parts as one of the possible side effects, if the aircraft flew into clouds and extended LG door.
Kartik wrote:What was different about their design considerations?
The F-18 has oversized stabilators and flight control surfaces for adequate flight control. Hawk airbrake might create a downward pitch, but the aircraft is designed to be very stable, since it’s a trainer.
Kartik wrote:Yeah..we know from reports (from a former IAF senior pilot) that even the F-16 Block 60 and Super Hornet didn't meet the IAF's STR requirements. Does that mean that if these were available for induction in the IAF, that the IAF would decline and reject them altogether? Very unlikely.
They were rejected for failing the technical evaluation, that included these very parameters. Your statement completely contradicts reality.

The F-16 was designed as a A2A fighter, and the bells and whistles added for A2G role ate away flight performance required for the A2A role.
Kartik wrote:The F-35 apparently doesn't meet its STR requirements, but is the USAF now insisting that the F-35 purchase be curtailed to a token order?
F-35 is worst aeronautical disaster in history and political pork barrel. Should we be going down the same route? What can one expect when Business Men / Managers / Politicians and their coterie run programs rather than engineers and aviators driving development.
Kartik wrote:It is mature and hasn't proven to be troublesome.
Then why can’t the FCS correct the uncommanded pitch created by the original airbrakes.
Kartik wrote:but that isn't the reason that the IAF is keeping Tejas Mk1 orders to 40
Agreed, from what I have been told, the IAF considers the pitch effect a nuisance but definitely not a showstopper. It has no effect on the induction plans.
Kartik wrote:What does the second statement even mean? What is a simpler design process?
Thank you for asking this question. It means assessing all the available means to achieve an end, and choosing the simplest and least riskiest means of achieving that end.

Like having airbrakes closer to CoG than having it elsewhere and having FCS mitigate any side effects. FWIW, using MLG door is reverting back to simpler design philosophy. Now, I am sure that the designers would have had some valid reason for not doing so in the first time itself, like avionics in the dorsal or fuel tanks in the ventral or wings, however, a simpler approach would have given precedence to placement of airbrakes.
Karan M wrote: troubles with the MiG 21 are in this phase…
Flying Officer…
If you or I were to teach our young ones to drive using a Ferrari or a Shaktiman, I am sure they would not be able to handle the acceleration of the former and the momentum of the latter. Should we then say, “troubles with the Ferrari are in the acceleration phase” and “troubles with the Shaktiman are in the stopping phase” because we’re using them for a purpose they were never intended for?

Flying Officers just start OC flying. How many FLt or Sq Ldr or Wg Cdr crash while landing because they couldnt handle the landing speed, which all supersonic fighters of that era with high wing loading will have?

FWIW, Pakistani F-7 OC training crash rate is the same as India.

Issues with MiG-21 as an OC trainer is meaningless while discussing Tejas as a fighter.

Lastly, the reason I am saying all this is because of the misdirected angst against the user, who are doing all they can to support the program. But a basic performance level has to be met.

Which BR member will volunteer his son or father or brother or husband to go to fight knowing fully well they cannot outturn the enemy in a sticky situation? I am sure your perspective of “IAF not supporting…” will do an Immelman turn when your own flesh and blood goes to a fight in a Mk1.
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Re: LCA News and Discussions

Post by sudeepj »

tsarkar wrote:Which BR member will volunteer his son or father or brother or husband to go to fight knowing fully well they cannot outturn the enemy in a sticky situation? I am sure your perspective of “IAF not supporting…” will do an Immelman turn when your own flesh and blood goes to a fight in a Mk1.
Sarkar san,

With all due respect, stop being so melodramatic :-D Can the mig21 outfight the Pakistani F16? We still have more than 150 of those flying around, dont we?

Also, can I turn the question around, and ask - if your son or father or brother or husband needs lifesaving medicine, (or clean drinking water, or a working toilet, or access to a school), would your rather spend the money on a foreign fighter plane, so the IAF pilot has the best of the best plane to fly? Afterall, the life of every citizen is important..

Even an LCA MkII will end up costing less than 35-40 million dollars each, when we are going to pay 80 to 100 million dollars for a Rafale.

Its the job of our soldiers and sailors and airmen to put their lives on the line. For this we are eternally grateful to them. But lets not attach such a great significance to an individual life, that we loose sight of our strategic goals.
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Re: LCA News and Discussions

Post by Singha »

http://www.youtube.com/watch?v=nWG2PkwKiaQ

nice Rafael video showing how its imaging seeker picks up planes and UAVs. and its smokeless.

good thing is we are already getting it in Spyder system.
Last edited by Singha on 23 Feb 2012 12:32, edited 1 time in total.
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Re: LCA News and Discussions

Post by Indranil »

geeth wrote:THe pitch up or down depends on whether the airbrakes are on top or bottom rather than aft or front.

Then there was the problem of uncontrolled pitching problem even during the first taxi trials..they were trying to correct it. This is what my chaiwala classmate told. I am not in touch with any of them for clarification. Those in the loop may be able to clarify.
Why are you confusing such a simple thing ... If there is a component of force created by the airbrake which does notpass through the CG it will have a turning moment ... elementary class 11 physics. There is no aerodynamics or rocket science involved.
tsarkar wrote:
shiv wrote:If you don't know where the CG is…
The flight control computer’s job is to know where the CG is at all times. Otherwise how will it apply corrective inputs via FBW and keep the aircraft flying. If the Flight Computer doesn’t know where the CG is, the aircraft won’t fly in the first place.
Sarkarji,
This is not completely true. There are Flight computers which work on feedback control loop which knows your desired orientation and direction. It just creates feedback based the observed deviation and the amplitude of the feedback is based on the amplitude of deviation.
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Re: LCA News and Discussions

Post by geeth »

What is the problem with the LCA airbrakes..? I really can't comprehend anything from the discussion going on here.

Can someone please elaborate ? TIA
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Re: LCA News and Discussions

Post by geeth »

Why are you confusing such a simple thing ... If there is a component of force created by the airbrake which does notpass through the CG it will have a turning moment ... elementary class 11 physics. There is no aerodynamics or rocket science involved.
Boss, I didn't read your post before.

Now tell me, isn't that exactly what I said? i.e., if the airbrakes are on top, the turning moment created by drag will tend to lift the nose up, because the CG of a/c is below the airbrakes(drag force). If the airbrakes are at the bottom part, the turning moment created by drag will tend to lift the nose down, because the CG of a/c is above the airbrakes(drag force).

Am I confused?
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Re: LCA News and Discussions

Post by shiv »

indranilroy wrote:
shiv wrote: 2. Fact: the LCA has lateral air brakes, not ventral or dorsal
That is not a fact.
http://images3.jetphotos.net/img/1/5/7/ ... 740875.jpg
:P
Image
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Re: LCA News and Discussions

Post by juvva »

geeth wrote:
Why are you confusing such a simple thing ... If there is a component of force created by the airbrake which does notpass through the CG it will have a turning moment ... elementary class 11 physics. There is no aerodynamics or rocket science involved.
Boss, I didn't read your post before.

Now tell me, isn't that exactly what I said? i.e., if the airbrakes are on top, the turning moment created by drag will tend to lift the nose up, because the CG of a/c is below the airbrakes(drag force). If the airbrakes are at the bottom part, the turning moment created by drag will tend to lift the nose down, because the CG of a/c is above the airbrakes(drag force).

Am I confused?
Could be, the the avionics is overcompensating for the airbrake induced torgue and causing the a/c to pitch up. Maybe the problem is an overactive FBW rather than a lagging FBW.
geeth
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Re: LCA News and Discussions

Post by geeth »

Could be, the the avionics is overcompensating for the airbrake induced torgue and causing the a/c to pitch up. Maybe the problem is an overactive FBW rather than a lagging FBW.
IIRC it was something like a rocking motion during the high speed trials, which they were trying to analyse way back in 2002..Most probably an aerodynamic problem. As I said before, I don't know whether the same problem persists now (unlikely).

But tendency to pitch up or down when air brakes are applied is not a problem as such, as long as it is within the design limits. If not, they would redesign the air brake to suit the needs.
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Re: LCA News and Discussions

Post by tsarkar »

indranilroy wrote:
tsarkar wrote:The flight control computer’s job is to know where the CG is at all times. Otherwise how will it apply corrective inputs via FBW and keep the aircraft flying. If the Flight Computer doesn’t know where the CG is, the aircraft won’t fly in the first place.
Sarkarji, This is not completely true. There are Flight computers which work on feedback control loop which knows your desired orientation and direction. It just creates feedback based the observed deviation and the amplitude of the feedback is based on the amplitude of deviation.
Indranil ji, I am writing my understanding below, please correct me wherever I am mistaken.

A fighter with negative static stability or relaxed static stability will need constant trimming of its control surfaces to help it fly. In the event the trimming stops when FCS fails, the plane will crash like the two out of three Su-30MKI crashes so far.

So to help fly the plane, Control Laws are written for the flight control computer. These Control Laws cover different flight regimes. The DFCC uses air data to understand the flight regime + pilot inputs, and thereafter passes control commands to the elevons, rudder and leading edge slats.

The CG is behind the center of lift. Now, the CG will move forward or back due to depletion of fuel (and external stores if carried). But the range of positions of the CG in different flight regimes and different external stores will be well defined so as to enable the control laws function to keep the aircraft in stable flight. So for a specific fighter, its range of CG locations depending on flight regimes and payload would be well known and defined in the DFCC for the control laws to work properly.

So the location of CG is not unknown in a NSS/RSS aircraft. This is my understanding, correct me if I am mistaken here.

Sudeep J - response in the misc thread.
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Re: LCA News and Discussions

Post by Kanson »

geeth wrote:
Could be, the the avionics is overcompensating for the airbrake induced torgue and causing the a/c to pitch up. Maybe the problem is an overactive FBW rather than a lagging FBW.
IIRC it was something like a rocking motion during the high speed trials, which they were trying to analyse way back in 2002..Most probably an aerodynamic problem. As I said before, I don't know whether the same problem persists now (unlikely).

But tendency to pitch up or down when air brakes are applied is not a problem as such, as long as it is within the design limits. If not, they would redesign the air brake to suit the needs.
Yes. Initially there was difference between studies and actual flights in compensating the pitch-up moment. But later problem was found and solution given.

Rocking motion is due to losing stability both in pitch and yaw during airbrake deployment.
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Re: LCA News and Discussions

Post by Indranil »

geeth wrote:
Why are you confusing such a simple thing ... If there is a component of force created by the airbrake which does notpass through the CG it will have a turning moment ... elementary class 11 physics. There is no aerodynamics or rocket science involved.
Boss, I didn't read your post before.

Now tell me, isn't that exactly what I said? i.e., if the airbrakes are on top, the turning moment created by drag will tend to lift the nose up, because the CG of a/c is below the airbrakes(drag force). If the airbrakes are at the bottom part, the turning moment created by drag will tend to lift the nose down, because the CG of a/c is above the airbrakes(drag force).

Am I confused?
No you are not confused. What you initially wrote was
THe pitch up or down depends on whether the airbrakes are on top or bottom rather than aft or front.
This is completely wrong. How do elevators and canards work. In fifth generation planes they are inline with the wings, not top or bottom! Also, they are in the front or the aft, aren't they?
Last edited by Indranil on 23 Feb 2012 22:41, edited 1 time in total.
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Re: LCA News and Discussions

Post by Indranil »

tsarkar wrote:Indranil ji, I am writing my understanding below, please correct me wherever I am mistaken.

A fighter with negative static stability or relaxed static stability will need constant trimming of its control surfaces to help it fly. In the event the trimming stops when FCS fails, the plane will crash like the two out of three Su-30MKI crashes so far.

So to help fly the plane, Control Laws are written for the flight control computer. These Control Laws cover different flight regimes. The DFCC uses air data to understand the flight regime + pilot inputs, and thereafter passes control commands to the elevons, rudder and leading edge slats.

The CG is behind the center of lift. Now, the CG will move forward or back due to depletion of fuel (and external stores if carried). But the range of positions of the CG in different flight regimes and different external stores will be well defined so as to enable the control laws function to keep the aircraft in stable flight. So for a specific fighter, its range of CG locations depending on flight regimes and payload would be well known and defined in the DFCC for the control laws to work properly.

So the location of CG is not unknown in a NSS/RSS aircraft. This is my understanding, correct me if I am mistaken here.
Sarkar sahab please don't call me Indranil ji. You can call me Indra, Nil, Roy, Indranil. But not ji, sahab ... :-)

You are very right that the range of the CG is well known. But you could write a flight computer to be completely agnostic of the placement of CG or its variation. A very simplistic view of the computer could be like this (there are many assumptions I am making to give you a very simplistic view. These assumptions will not change the basics though). You want to pitch up, you give suitable command to your elevators. Now observe your attitude and mark the deviation from the intended pitch up. You provide this deviation to the computer (feedback). Based on whether this feedback is positive, negative and its amplitude, the computer provides further commands to the elevator. This loop continues. As you can see, in this model you can be completely agnostic of where the CG lies.

In real practice, there is a lag in the deflection of the control surfaces (at 80Hz for Tejas) and the aircraft's response to it. So the computer is conservative in calculating the deflections for the control surfaces as it is better to continually increase (or decrease) attitude towards the intended attitude. I will explain with an example. Suppose you want to pitch up to 10 degrees. It is much better to pitch up to 9.5 degrees->9.8 degrees->9.9->degrees->9.95 degrees->9.98 degrees and so on .... You might have read pilots saying that the FBW planes feel grainier ... It is due to the digital nature of this incremental change to the desired orientation. Alternatively, you can go to 10.5 degrees->9.6 degrees->10.3 degrees->9.7 degrees and so on .... In the later case due to the lag of the plane's response to the deflections, we could enter a phase where the deflection and response to the deflection is completely out of phase and you could reach harmonics with higher and higher amplitude (in short a disastrous situation).

As you can see, the previous model can be designed to work completely on heuristics and be completely agnostic of the location of the CG. Mind you if you want to provide commands at 80 Hz, which includes sampling, communication and computation, then a hashed look up table based on heuristics may be a really fast method.

Having said all this, you can develop a more complex 'active' FBW which is aware of the CG and takes this as one of the inputs to the calculations (hashing function). I am not a domain expert in this field, but I can guess that such a FBW might be better than a FBW which is completely CG agnostic.
Last edited by Indranil on 24 Feb 2012 02:54, edited 1 time in total.
Indranil
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Re: LCA News and Discussions

Post by Indranil »

I agree with TSarkar ji ... I don't think that the "uncommanded pitch-up" is any serious hiccup in the induction or deployment of Tejas. It can at best be put as an irritant. I would rather put it as a characteristic of the plane.
Last edited by Indranil on 24 Feb 2012 02:48, edited 1 time in total.
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