LCA News and Discussions

[Indranil]

Does anybody know the position of fuel vent pipe on the LCA? Is it the rearward facing notch before the right air brake?

[shukla]

107 GE F414 Engines = $822 Million = 4440 American Jobs
Official White House statement

India has also declared the lowest bidder and selected the US-based General Electric Company for a contract to provide the Indian Aeronautical Development Agency with 107 F414 engines to be installed on the indigeniously produced Tejas light combat aircraft.

"Upon finalizing the contract, General Electric will be positioned to export almost one billion dollars in high technology aerospace products. This transaction is tentatively valued at approximately $822 million, all of which is US export content, supporting an estimated 4,440 jobs," the statement said.

http://www.hindustantimes.com/India-US- ... 22783.aspx

[shiv]

Excerpts of P Rajkumar Interview on Tejas link

Hmmm:

A particular part of the structure could have been made too strong and another part too weak. So we have to perform a structural optimisation exercise that usually results in reduction in weight. The aerodynamic optimisation will lead to some configuration changes. Unfortunately our aeronautical institutions from the days of the HT-2 have never undertaken the task of measuring the aerodynamic loads during flight and optimising the structure. We did not do it for the ‘Marut’ or the ‘Kiran’. I have always maintained that performing a structural optimisation exercise is the way to go. I am told that it is a time consuming exercise, but we have to start from somewhere. There is no easy way out. You can also reduce weight by looking at the Line Replaceable Units (LCA), Head-Up Displays (HUD), and Mission Computers etc.

This is pretty much what Prodyut Das says in slightly different words, although AM Rajkumar is much more explicit about the details of how it needs to be done.

[Karan M]

107 GE F414 Engines = $822 Million = 4440 American Jobs
Official White House statement

India has also declared the lowest bidder and selected the US-based General Electric Company for a contract to provide the Indian Aeronautical Development Agency with 107 F414 engines to be installed on the indigenously produced Tejas light combat aircraft.

"Upon finalizing the contract, General Electric will be positioned to export almost one billion dollars in high technology aerospace products. This transaction is tentatively valued at approximately $822 million, all of which is US export content, supporting an estimated 4,440 jobs," the statement said.

http://www.hindustantimes.com/India-US- ... 22783.aspx

107 engines is more than original estimate of 99 engines. But more engines are required for NLCA as well.

http://www.tribuneindia.com/2010/20101024/main4.htm

The IAF has projected a total requirement for about 120 Tejas and 16 two-seater trainer variants for equipping seven squadrons, while the Navy wants 57 aircraft of this type.

---
So IAF requirement is for 120 (with 16 of 40 MK1 being trainers) and Navy for 57, which means more engines are necessary.

[Kanson]

Excerpts of P Rajkumar Interview on Tejas link

Hmmm:

A particular part of the structure could have been made too strong and another part too weak. So we have to perform a structural optimisation exercise that usually results in reduction in weight. The aerodynamic optimisation will lead to some configuration changes. Unfortunately our aeronautical institutions from the days of the HT-2 have never undertaken the task of measuring the aerodynamic loads during flight and optimising the structure. We did not do it for the ‘Marut’ or the ‘Kiran’. I have always maintained that performing a structural optimisation exercise is the way to go. I am told that it is a time consuming exercise, but we have to start from somewhere. There is no easy way out. You can also reduce weight by looking at the Line Replaceable Units (LCA), Head-Up Displays (HUD), and Mission Computers etc.

This is pretty much what Prodyut Das says in slightly different words, although AM Rajkumar is much more explicit about the details of how it needs to be done.

Shiv saar, with all the friendliness and frankness, Prodyut Das said,

" In aircraft structural testing, surviving the load does not mean "passing" to pass the structural test the specimen should not fail at less than 100% load and equally must fail before 101% load is exceeded. If it survives the 101% load the structure must be redesigned till it fails or the realistic weight target is achieved."

If i take the meaning *as it is*, no one design their structure for 100% load. Take any engineering product, or talk to any designer/engineer, even from China , it always 100% + % allowance. Take a bridge, it will be constructed with the strength at assuming 40% more load than expected. So weight/structure of the bridge is designed to fail at 140% load of the design/max expected load.

If i assume he talking about the target load (design load + % allowance) then there is no science to say, it should be designed only to this much percentage. Rafale is designed to bear upto 185% of design load. Whereas for Eurofighter it is considerably less. And for any fighter it is always more than 100%.

Aerodynamic loads, that Rajkumar talks, cannot be assumed. A database has to be built over time for different profile over the structure and has to *judiciously* reduce the weight. Weight reduction is an art here(Two persons can arrive at different solution). Without opening the entire flight profile, one cannot have such database. And as per my knowledge, Tejas has not opened its entire flight profile and we are already started this process long before, if i'm not worng.

[ranjithnath]

Does anybody know the position of fuel vent pipe on the LCA? Is it the rearward facing notch before the right air brake?

isnt it beneath the fuselage??but im unable to locate the exact position.

[chackojoseph]

Kanson,

We don't know if they have / have not opened the full flight envelope. We do not know if IOC is dependent on full flight envelope. We know that we have arrived at production standard for MK-I, this should give you and idea if the full flight envelope has opened or not.

[shiv]

Aerodynamic loads, that Rajkumar talks, cannot be assumed. A database has to be built over time for different profile over the structure and has to *judiciously* reduce the weight. Weight reduction is an art here(Two persons can arrive at different solution). Without opening the entire flight profile, one cannot have such database. And as per my knowledge, Tejas has not opened its entire flight profile and we are already started this process long before, if i'm not worng.

Rajkumar says that the database was not built for HT-2, HF 24 or Kiran. If we had that database it may have had a beneficial effect on Tejas. These are ghosts of the past coming to haunt us. In the absence of previous experience in this regard the designers of the Tejas would probably have erred on the side of caution and more than necessary structural strength. We don't know. And we won't know until that database is built.

[shiv]

Kanson,

We don't know if they have / have not opened the full flight envelope. We do not know if IOC is dependent on full flight envelope. We know that we have arrived at production standard for MK-I, this should give you and idea if the full flight envelope has opened or not.

I think the whole flight envelope has definitely not been opened. It is likely to happen only after LSP 6 undergoes high AoA and spin trials.

[Indranil]

If we want to increase the area of the wing keeping the same aspect ratio, we will have increase the span of the wing. Then we will obviously have to proportionally increase the length of the plane for maintainging the same level of wave drag. I think we are going for a lengthening beyond that. That I feel would give a a sleeker Tejas, which is a good thing.

The paper that Kartik pointed out makes me believe that space inside the LCA is at a premium which is a good thing for a fighter. Prof Das also marks out the landing gear and the cocopit not being skewed. I feel (remember Kartik echoing the same views) that the designers don't have the space with the current length. I hope lengthening of the LCA will give the designers space for such refining.

Also I must add, I can somewhat visualize the Tejas Mark II and I must say, it looks damn good.

I don't think that we should go for an intermediate design to save a couple of years. Please bear in mind that the changes we are speaking of here are airframe changes. The intermediate and LCA Mark I can never be retrofitted to the LCA Tejas Mark II standard. Also those GE 414 engines should be judiciously used.

AM P Rajkumar points out the usage of LCA Tejas Mark I. We should treat LCA Tejas MArk I as a pilot project. It has been a phenomenal program which has built up a lot of expertize in building a plane. Let the designers and builders use this opportunity to give us a really good refined plane. Also, the 40 Mark I will provide critical information on operationalizing the Tejas.

What would be interesting is get the universities in now. Identify a goal and let professors apply for those projects. Fund the professors who can intern fund his students. Break up the design of LCA Tejas Mark I into into individual components and say can you come up with a good LIFT/CAT design by reducing, scaling these down. Can you use the Kaveri engine in that plane? HAL/ADA/GTRE can take on the umbrealla guidance role. This will free up designers at ADA/HAL. Also this will bring out enthusiasm and expertize amongst the students. It is a win-win situation for all.

P.S. We should let go of the word Light in LCA Tejas Mark II . I am guessing that if Tejas Mark II comes out the way it is being envisioned, it will be lapped up by quite a few AFs.

[Kartik]

Does anybody know the position of fuel vent pipe on the LCA? Is it the rearward facing notch before the right air brake?

it is. There was a discussion on BRF several years ago, where the yellow coloured stains on the white coloured TDs were being discussed. This came up then.

[Indranil]

^^^ The cover is to create the suction?

Gripen NG just has a hole near the flaps.

[NRao]

A particular part of the structure could have been made too strong and another part too weak. So we have to perform a structural optimisation exercise that usually results in reduction in weight. The aerodynamic optimisation will lead to some configuration changes. Unfortunately our aeronautical institutions from the days of the HT-2 have never undertaken the task of measuring the aerodynamic loads during flight and optimising the structure. We did not do it for the ‘Marut’ or the ‘Kiran’. I have always maintained that performing a structural optimisation exercise is the way to go. I am told that it is a time consuming exercise, but we have to start from somewhere. There is no easy way out. You can also reduce weight by looking at the Line Replaceable Units (LCA), Head-Up Displays (HUD), and Mission Computers etc.

Boat load of crap. LoL. Figures.

Even when there is an opportunity to build data they do not do so!!!!!!!!!!!!!!!!!!! Wha.

The picture that I see is that "they" are willing to pay foreign consultants millions but not willing to consume time at a much, much lower rate per hour - for the same element of work.

Comedy of errors.

I do not know if Shridar is lurking, but, this fits what was written.

Very predictable. The picture is complete.

[shiv]

Even when there is an opportunity to build data they do not do so!!!!!!!!!!!!!!!!!!! Wha.

Early nineties - HAL still did not have a database of flying accidents and their causes. This was at the height of the "flying coffin" lamentation. But the story now is still not rosy simply because all students who do science tend to get lapped up by the IT industry and nobody (or very few) are going towards pure science and the skills required in HAL/ADA. If India is lucky (not by choice or plan) we will just accidentally end up having people join teams that work in these industries.

[chackojoseph]

I think the whole flight envelope has definitely not been opened. It is likely to happen only after LSP 6 undergoes high AoA and spin trials.

Are you sure that the current config can open the entire envelope? Its underpowered, isn't it?

I am trying to figure out the same thing. I am not sure if it can be done without crossing the prescribed limits like AoA.

[Kanson]

^ Chacko, what do you mean by entire flight envelope? If you could share on IOC part, it is welcomable.

[chackojoseph]

Kason,

As I mentioned, the IOC Part. Whether it opens its entire envelope or not, both IAF and DRDO have decided the IOC parameters. That's important. They have decided on the parameters already, so the old known parameter should not be factored. Since we already know that its underpowered and is not mk2 combination, the earlier parameters should not effect anymore.

[Kanson]

A particular part of the structure could have been made too strong and another part too weak. So we have to perform a structural optimisation exercise that usually results in reduction in weight. The aerodynamic optimisation will lead to some configuration changes. Unfortunately our aeronautical institutions from the days of the HT-2 have never undertaken the task of measuring the aerodynamic loads during flight and optimising the structure. We did not do it for the ‘Marut’ or the ‘Kiran’. I have always maintained that performing a structural optimisation exercise is the way to go. I am told that it is a time consuming exercise, but we have to start from somewhere. There is no easy way out. You can also reduce weight by looking at the Line Replaceable Units (LCA), Head-Up Displays (HUD), and Mission Computers etc.

Boat load of crap. LoL. Figures.

Even when there is an opportunity to build data they do not do so!!!!!!!!!!!!!!!!!!! Wha.

The picture that I see is that "they" are willing to pay foreign consultants millions but not willing to consume time at a much, much lower rate per hour - for the same element of work.

Comedy of errors.

Even if i assemble hundreds of experts before the LCA programme, how could they *solve* the problem without any data? How could that happen without first building that data?

I'm sure, they are not going to play like Octopus Paul in determining the weight rationalization, am i right?

Pls google LCA MAST or check page 6 of http://www.nal.res.in/pdf/dr03.pdf

This is done with *simulated* aerodynamic load profile. If they have do as Prof Das mentioned they need more data.

[Kanson]

Kason,

As I mentioned, the IOC Part. Whether it opens its entire envelope or not, both IAF and DRDO have decided the IOC parameters. That's important. They have decided on the parameters already, so the old known parameter should not be factored. Since we already know that its underpowered and is not mk2 combination, the earlier parameters should not effect anymore.

Yes Chacko, IOC means standardization by freezing on parameters. It is like attaining graduation by providing answers to the questions taken from the syllabus set by ADA/IAF. But that doesn't mean you are tested on the entire subject which covers beyond the syllabus, right?

[Narad]

If i take the meaning *as it is*, no one design their structure for 100% load. Take any engineering product, or talk to any designer/engineer, even from China , it always 100% + % allowance. Take a bridge, it will be constructed with the strength at assuming 40% more load than expected. So weight/structure of the bridge is designed to fail at 140% load of the design/max expected load.

If i assume he talking about the target load (design load + % allowance) then there is no science to say, it should be designed only to this much percentage. Rafale is designed to bear upto 185% of design load. Whereas for Eurofighter it is considerably less. And for any fighter it is always more than 100%.

It is usually referred to as Factor of safety and it could be anything from 1.5 to 1.8 (upto 3 in some cases). That means the structure is practically designed to withstand 40 to 80 percent more stress/load than the theorotical equivalent. The vaule is a thumb rule for parts subject to torsion/endurance fatigue loads etc. for machine parts. Not sure about the optimum value of FOS in design of Fighter planes though, because of issues like increase in weight associated with more robust design etc.

[Kartik]

Shiv saar, with all the friendliness and frankness, Prodyut Das said,

" In aircraft structural testing, surviving the load does not mean "passing" to pass the structural test the specimen should not fail at less than 100% load and equally must fail before 101% load is exceeded. If it survives the 101% load the structure must be redesigned till it fails or the realistic weight target is achieved."

If i take the meaning *as it is*, no one design their structure for 100% load. Take any engineering product, or talk to any designer/engineer, even from China , it always 100% + % allowance. Take a bridge, it will be constructed with the strength at assuming 40% more load than expected. So weight/structure of the bridge is designed to fail at 140% load of the design/max expected load.

If i assume he talking about the target load (design load + % allowance) then there is no science to say, it should be designed only to this much percentage. Rafale is designed to bear upto 185% of design load. Whereas for Eurofighter it is considerably less. And for any fighter it is always more than 100%.

Aerodynamic loads, that Rajkumar talks, cannot be assumed. A database has to be built over time for different profile over the structure and has to *judiciously* reduce the weight. Weight reduction is an art here(Two persons can arrive at different solution). Without opening the entire flight profile, one cannot have such database. And as per my knowledge, Tejas has not opened its entire flight profile and we are already started this process long before, if i'm not worng.

Kanson, in aerospace structures, as you'd know, keeping weight low is a holy grail. To that extent Prodyut Das is right and comparisons with bridges are not correct. But of course, he talks from the pure academics point of view. The reality is far different and much much more complicated. It only confirms my view that Prodyut Das is an academician and has little industry experience to back it up. I will go into that particular discussion only if required to.

That means that Limit Loads (max of the loads seen in operations) are multiplied by a factor of 1.5 and then those Ultimate loads are the Design loads. So you don't design to 185% of design loads, rather to 150% of limit loads. I know that this is a standard in the aerospace industry and no manufacturer in their right minds would design a heavier structure since it will compromise on the basic performance of the aircraft for no additional gain.

The basic in-flight design mass is multiplied by the vertical load factor (viz. 8.5G or 9G for fighters) and that is the limit load and you design the structure to Ultimate load so that is how the Mirage-2000 and Rafale can supposedly withstand 13Gs structurally even though it will see 9G loads and after that the FCS restricts it from going further.

The basic rule using which aircraft structure loads are arrived at, is taking into account that (Weight*vertical load factor = constant). This is why the FCS basically restricts the Vertical Load factor (G restricted) when the aircraft is carrying external fuel or weapons plus full internal fuel, because the aircraft at that time is heavier than its intended basic flight weight. As they start using up internal fuel, the allowed g limit keeps increasing. Finally at some point it will allow for 9G Vertical Load factor since the aircraft mass has come down to the basic flight weight.

Anyway, there are many different types of loads that affect aircraft and a very large number of load cases are nowadays taken into account while arriving at the full-spectrum load cases to design an aircraft. These include Flight Loads, Ground Handling loads, Internal loads at particular points in the structures, and Dynamic loads. It isn't quite as easy a task as it seems.

[srai]

yes, 120 is low. That is why I had put that at the low minimum per airframe life over 25 years. I was going by this article on the MiG-21s getting additional 1,000 life extension (or "8 to 10 years of life", which means around 100 to 125hrs/year per airframe) and applying that to LCA, since at the minimum LCA would need to fly similar number of hours/year.

The problem is MiG-21s are reaching end of life, and Bisons are obviously those whose life, IAF would like to preserve the most, so this would be comparing apples to oranges (new aircraft versus those already on limited life extension). By any reckoning, LCA can be expected to fly more. Also, we cant see airframe life, and years expected and divide one by the other. The years figure usually takes into account other factors such as corrosion, issues with fitment, replacement of items etc, so usually both figures are given together - flight hours and years of operation

Regarding the MiG-21s in IAF, here is a really good article written by Jagan.
The MiG-21 Bison Upgrade Programme

...
In 1980, the first unit converted to the MiG-21 Bis, a type of which nearly 300 were to be procured in total. The last Bis left the production line in 1985, when the focus shifted on manufacturing the MiG-27.
...
Thus was borne the MiG-21-93 program. It was proposed to extend the Total Technical Life (TTL) of the MiG-21Bis from the current level of 2400 Hours as well as Upgrade the avionic capabilities of the existing aircraft.
...
The proposal for 125 MiG Bis aircraft with an option to upgrade 50 more aircraft at a total cost of US$ 626 million was cleared in January 1996. The upgrade was to include major modifications by MiG-MAPO which would incorporate Western Avionics as well as indigenously developed components. Besides the major airframe, weapons and radar contract with MiG, a contract with Sextant was signed for the Inertial navigation System (INS), with HAL for subsequent upgradation of the aircraft, with NAL for fatigue testing and studies in TTL Extension and with BEL for development of the Tarang Radar warning Receiver (RWR).
...
However a major handicap of the project was that the study to find out the feasibility of the extension of TTL was not completed on time. It is believed this study, which was subcontracted to NAL in Bangalore was completed only in the middle of 2002.
...

Continuing from the above article (which was written around 2002), the article below mentions that the TTLE was completed as of August 2006.
Indian Air Force's MiG 21 Bison's get 1000 flying hours extension

2006-08-17 Times of India reports that the National Aerospace Laboratories (NAL) have upgraded the life span of the Air Force's MiG 21 Bison fleet.
...
These results come after NAL's successful completion of Full-Scale Fatigue Testing (FSFT) on the MiG 21 Bison Airframe C-2090. With this the IAF's Total Technical Life Enhancement (TTLE) project has come to an end.
...
The test included flying a MiG 21 Bison that had completed its 2400 hours of flight. The test revealed no fatigue cracks in the aircraft.

So the aircraft was flown further, and cracks first started appearing at total 3400 hours of flight, which meant the life of the Bison aircraft could be extended by 1000 hours maximum.
...

MiG-21Bis were inducted between 1980 -1985. From the above articles, NAL started its FSFT sometime around 1996/97 timeframe. If the MiG 21 Bison Airframe C-2090 had completed 2,400 hrs flight time in 1996, then that was achieved over roughly 16 years (1980-1996). This means on an "overall" average the C-2090 airframe was flying roughly 150 hours/year. [Note: Just to be clear, this is just an overall average. Year-to-year flight hours vary due to many different variables as you have pointed out.]

[Karan M]

"This means on an "overall" average the C-2090 airframe was flying roughly 150 hours/year. [Note: Just to be clear, this is just an overall average. Year-to-year flight hours vary due to many different variables as you have pointed out.]"

Thanks - and this brings us full circle, and we are in agreement

In my original mail on 31 Oct, where we started discussing this topic, I had pointed out this same number

Doing some simple calculations, we can roughly figure out the range for the fatigue life of the airframe. Given that an air force pilot flies on an average 120-200 hours a year, we can use that as our basis for an estimate.

The figure is 150-200+ easy, for fighter pilots. 150 translating to 300, 30 min sorties on the older endurance limited MiG-21s, 180 hours is the standard expected for the rest as Jaguar, Mirage pilots pull longer sorties, and the Su-30 pilots have even gone on multi-hour flights. 120 hours is a severe underestimate for the average fully ops pilot.

[srai]

^^^

Cool!

[shiv]

Whether it opens its entire envelope or not, both IAF and DRDO have decided the IOC parameters.

Let me state my view from what I have gathered - not as an expert.

If you look at the history of aviation you will find scores of examples where aircraft were flown and operationalized long before the "flight envelop" was opened fully. The flight envelop got opened by some pilot or other condition causing an unintended maneuver. Can you believe what happened to the first pilots who ever experienced a stall or a spin? Some may not have lived to tell the tale, but others would have recovered and realized that some parts of the "flight envelop" lead to situations that are recoverable.

With the Tejas I guess it will be different. Most of the time the FBW software will prevent the plane entering parts of the flight envelop that have not yet been cleared.

[chackojoseph]

Yes Chacko, IOC means standardization by freezing on parameters. It is like attaining graduation by providing answers to the questions taken from the syllabus set by ADA/IAF. But that doesn't mean you are tested on the entire subject which covers beyond the syllabus, right?

Right! It means, its cleared for flight. I am not sure it means basic weaponazation etc. It depends on the need of the hour.

Whether it opens its entire envelope or not, both IAF and DRDO have decided the IOC parameters.

Let me state my view from what I have gathered - not as an expert.

If you look at the history of aviation you will find scores of examples where aircraft were flown and operationalized long before the "flight envelop" was opened fully. The flight envelop got opened by some pilot or other condition causing an unintended maneuver. Can you believe what happened to the first pilots who ever experienced a stall or a spin? Some may not have lived to tell the tale, but others would have recovered and realized that some parts of the "flight envelop" lead to situations that are recoverable.

With the Tejas I guess it will be different. Most of the time the FBW software will prevent the plane entering parts of the flight envelop that have not yet been cleared.

That is correct, Sir.

One of the things we must understand about Tejas is that we cannot fit the previous parameters like smallness, speed, loads, AoA etc to either mk 1 or mk 2. Both the aircraft's are going to change beyond recognition in terms of the specs. It will confirm to the small lightweight fighter basic requirement on a macro scale.

If someone is interested, I would like t point out an old article. Its not directly connected to flight testing, but, to evolution. I happened to be reading my own link as I stumbled on it in the morning. Its buried deep in FI archives.

Enabling Technologies: Mig-21 & F-16

Especially read the last para on LCA FC's.

[shiv]

Enabling Technologies: Mig-21 & F-16

Chacko that is an excellent article that should be essential reading in the first post of every military aviation thread - just like we have a standard Paki first post

[chackojoseph]

Enabling Technologies: Mig-21 & F-16

Chacko that is an excellent article that should be essential reading in the first post of every military aviation thread - just like we have a standard Paki first post

yep, this article came out parallel to the first ever ACM Rajkumar's article on LCA We had posted in old proton river forum. So it got buried. As you say, I find it a very impressive article.

[chackojoseph]

Ok, I forgot to convey my message . I was actually trying to reply to Kason.

Kason look

In 1990, twelve years after induction, a news item appeared that reported more than 100 crashes of F-16: USAF—80; NATO—17; PAK—13. The aircraft was labeled the WIDOW MAKER. It was reported that hasty induction of the aircraft had led to use of certain wirings which did not conform to MIL-STD requirements (fly-by-wire going haywire?). Later another report placed restrictions on F-16 from indulging in high ‘g’ maneuvers and low level missions. These restrictions prevailed during the Gulf War….even after two decades of combat flying. It came to be referred to as a ‘clear weather’ aircraft which did not meet the assigned tasks during the Gulf War…its performance was officially criticized by the US General Accounting Office. In May 1991, US Senate Armed Services Committee found the stealth fighter F-117 to be eight times more effective than F-16. Plans were on to terminate production of F-16 but the commercial implications of the multi-nation development venture involving the USA, Belgium, Netherlands, Norway, Denmark, Israel and a host of customers gave the aircraft a fresh lease of combat life.

A/c can be inducted without all parameters getting achieved.

[Kanson]

First of all, I agree with Shiv, very well covered and nicely written. Thumbs UP to FI, chacko! I can only wish to see more articles like this to lap up.

A/c can be inducted without all parameters getting achieved.

Ok, i guess we agree with this. Thanks. IF I could remember correctly, F-16 was grounded due to problems in FBW. There was so much anxiety, emotions and talks that whether our choice of choosing FBW was wise. And there was a cover story on Times, i guess, on this with the front page pic of several parked F-16 in line.

[Kanson]

If i take the meaning *as it is*, no one design their structure for 100% load. Take any engineering product, or talk to any designer/engineer, even from China , it always 100% + % allowance. Take a bridge, it will be constructed with the strength at assuming 40% more load than expected. So weight/structure of the bridge is designed to fail at 140% load of the design/max expected load.

If i assume he talking about the target load (design load + % allowance) then there is no science to say, it should be designed only to this much percentage. Rafale is designed to bear upto 185% of design load. Whereas for Eurofighter it is considerably less. And for any fighter it is always more than 100%.

It is usually referred to as Factor of safety and it could be anything from 1.5 to 1.8 (upto 3 in some cases). That means the structure is practically designed to withstand 40 to 80 percent more stress/load than the theorotical equivalent. The vaule is a thumb rule for parts subject to torsion/endurance fatigue loads etc. for machine parts. Not sure about the optimum value of FOS in design of Fighter planes though, because of issues like increase in weight associated with more robust design etc.

Yes, Factor of safety, safety margin, tolerance and terms like that are used. For aircraft it is do with the safety. Thanks. Though we can see it is more likely 1.5 but then there are deviations.

[Kanson]

Kanson, in aerospace structures, as you'd know, keeping weight low is a holy grail. To that extent Prodyut Das is right and comparisons with bridges are not correct. But of course, he talks from the pure academics point of view. The reality is far different and much much more complicated. It only confirms my view that Prodyut Das is an academician and has little industry experience to back it up. I will go into that particular discussion only if required to.

To clarify, though there is no weight optimization in Bridges, cost of materials were considered when the Bridge is over strengthened just to indicate that safe margin is included in every construction.

That means that Limit Loads (max of the loads seen in operations) are multiplied by a factor of 1.5 and then those Ultimate loads are the Design loads. So you don't design to 185% of design loads, rather to 150% of limit loads. I know that this is a standard in the aerospace industry and no manufacturer in their right minds would design a heavier structure since it will compromise on the basic performance of the aircraft for no additional gain.

The basic in-flight design mass is multiplied by the vertical load factor (viz. 8.5G or 9G for fighters) and that is the limit load and you design the structure to Ultimate load so that is how the Mirage-2000 and Rafale can supposedly withstand 13Gs structurally even though it will see 9G loads and after that the FCS restricts it from going further.

Yes, limit loads, but don't you call them as *design limit load*? I want to design a fighter that can withstand 9G but i constructed a fighter that can withstand 13G. Can i not say that it can withstand upto 50% more than the design load? Of course you want to put it in precise terms so that it should not be misconstrued by nobody. Thanks.

Do you see any deviations from factor of 1.5?

[Kartik]

To clarify, though there is no weight optimization in Bridges, cost of materials were considered when the Bridge is over strengthened just to indicate that safe margin is included in every construction.

I get that- but overstrengthened by how much ? In aircraft structures, even 1 lb of extra weight in a large part is considered excess and unless prohibitively expensive to be implemented as a weight reduction measure, they will reduce its weight. Here cost of material is just one aspect. It is the overall effect over the life of the part that is of bigger concern. The higher fuel burn as a result of 1 lb of additional weight or the lowered performance as multiple such 1 lb excess weight parts are added up.

And for that reason, a factor of safety of 1.5 for Ultimate loads is almost always the standard. There may be some variations here or there but the general value is 1.5. Aircraft are designed to FAR or MIL specifications and these factors are specified there.

Yes, limit loads, but don't you call them as *design limit load*? I want to design a fighter that can withstand 9G but i constructed a fighter that can withstand 13G. Can i not say that it can withstand upto 50% more than the design load? Of course you want to put it in precise terms so that it should not be misconstrued by nobody. Thanks.

Do you see any deviations from factor of 1.5?

This might be semantics. But to go in detail-

Limit loads are not the loads being designed to. The Ultimate loads are the ones that you have to show your part as being good for withstanding. So, you don't look for your structure's margin of safety at the Limit load, but if you have little over zero margin of safety at Ultimate loads, it is considered acceptable. The structure should be able to withstand that ultimate load for upto 3 seconds and not fail. That is what Prodyut Das was talking about-100% is too little and 101% is too much and the margin of safety should be such that the part fails between 100 and 101%. By doing so it is automatically passing for Limit loads and just about passing for Ultimate loads.

But I was saying that such things are easier said in articles or in theory than done in real life. After all design optimization is not such an easy task and a lot depends on how confident the Loads groups and Design Leads are about their preliminary loads. If Flight loads are available, then optimization is a lot easier and the Chief Designers can be a lot more confident about asking their analysts and designers to size it to as close to zero margin of safety as possible. Towards that the Tejas Mk2 weight optimization will without doubt require flight loads to be found and tallied against preliminary loads to see how good the initial estimates were.

So, by definition, the Ultimate loads are the ones that are the design loads- the loads for which the parts are designed, with margin of safety computed and it is this margin of safety that you want to have as close to zero as possible.

The Limit loads are arrived at as the maximum loads that the aircraft will see during its entire life. For civilian planes, limit loads are themselves seen infrequently but fighters are different in that they will see the Limit loads far more often than civilian aircraft due to their mission profiles being different. But in terminology, it is very clear as to what Limit load is meant to be- that the structure should not under any condition, suffer permanent set or deformation under Limit loads. So, a fighter that is designed to 9G limit loads should be able to go to 9G and not suffer any deleterious effects to the airframe (long term fatigue is a different issue and a separate topic). Then you add another 1.5 Factor of safety that allows you to actually go upto 13 g's but that is mostly moot since the FCS will in most cases prevent such a thing happening in flight.

Anyway, to confirm what I'm saying, I'm just posting a few paras from FAR Structural Design Criteria. (Section 25). I'll post the link to this as well. It is really illuminating as to how various load cases are arrived at when sizing to such criteria.

the Tejas Mk1 would've shown itself clearing the bulk of these criteria for IOC envelope either during flight tests or through documentation, coordination documents and stress notes and FEM analysis, etc..Only then will CEMILAC give them a flight worthiness certificate to allow the IAF to start operations. That is what was being referred to earlier in news reports about 300 odd ADA/HAL engineers working towards a Dec 27 deadline for the LCA's IOC certificate.

Sec. 25.301 Loads.
(a) Strength requirements are specified in terms of limit loads (the maximum loads to be expected in service) and ultimate loads (limit loads multiplied by prescribed factors of safety). Unless otherwise provided, prescribed loads are limit loads.
(b) Unless otherwise provided, the specified air, ground, and water loads must be placed in equilibrium with inertia forces, considering each item of mass in the airplane. These loads must be distributed to conservatively approximate or closely represent actual conditions. Methods used to determine load intensities and distribution must be validated by flight load measurement unless the methods used for determining those loading conditions are shown to be reliable.
(c) Unless otherwise specified, a factor of safety of 1.5 must be applied to the prescribed limit load which are considered external loads on the structure. When a loading condition is prescribed in terms of ultimate loads, a factor of safety need not be applied unless otherwise specified.
Sec. 25.305 Strength and deformation.
(a) The structure must be able to support limit loads without detrimental permanent deformation. At any load up to limit loads, the deformation may not interfere with safe operation.
(b) The structure must be able to support ultimate loads without failure for at least 3 seconds. However, when proof of strength is shown by dynamic tests simulating actual load conditions, the 3-second limit does not apply. Static tests conducted to ultimate load must include the ultimate deflections and ultimate deformation induced by the loading. When analytical methods are used to show compliance with the ultimate load strength requirements, it must be shown that-- (1) The effects of deformation are not significant; (2) The deformations involved are fully accounted for in the analysis; or (3) The methods and assumptions used are sufficient to cover the effects of these deformations.

article link

[rahulm]

Partial X-post from the Armour thread

I visited the Cavalry Tank Museum at MIRC,Ahmednagar recently. The museum has a LCA model! :

http://picasaweb.google.com/11335378136 ... 7299061074
http://picasaweb.google.com/11335378136 ... 7975921186
http://picasaweb.google.com/11335378136 ... 1100649762
http://picasaweb.google.com/11335378136 ... 0766736802 and
http://picasaweb.google.com/11335378136 ... 0116053906

Link to the Museum exhibits in the Armour thread.

[suryag]

Tejas Cockpit Better Suits 'Taller' Pilots, Ergonomics Corrected LSP-5 Onward

Amazing to see what all we learn when we design our own fighter. If this mod was asked on say a jag or mig 29 it would have taken six months for the approval process followed by a year of work and fat amount to the vendor

[manum]

" In aircraft structural testing, surviving the load does not mean "passing" to pass the structural test the specimen should not fail at less than 100% load and equally must fail before 101% load is exceeded. If it survives the 101% load the structure must be redesigned till it fails or the realistic weight target is achieved."

If i take the meaning *as it is*, no one design their structure for 100% load. Take any engineering product, or talk to any designer/engineer, even from China , it always 100% + % allowance. Take a bridge, it will be constructed with the strength at assuming 40% more load than expected. So weight/structure of the bridge is designed to fail at 140% load of the design/max expected load.

If i assume he talking about the target load (design load + % allowance) then there is no science to say, it should be designed only to this much percentage. Rafale is designed to bear upto 185% of design load. Whereas for Eurofighter it is considerably less. And for any fighter it is always more than 100%.

Aerodynamic loads, that Rajkumar talks, cannot be assumed. A database has to be built over time for different profile over the structure and has to *judiciously* reduce the weight. Weight reduction is an art here(Two persons can arrive at different solution). Without opening the entire flight profile, one cannot have such database. And as per my knowledge, Tejas has not opened its entire flight profile and we are already started this process long before, if i'm not worng.

there are laws regarding it...means the structure must fulfill the minimum of it..so the safety factor of load to strength ratio is calculated through that "factor of safety"...that is subjected to rule book...having it beyond the law is a waste... or being too cautious ...which happened in LCA navy landing gear...
Like the hawda bridge of Calcutta...it was first bridge ever made by English to that magnitude...so it's safety factor was multiple times of the load, strength ratio...and hence it still survives today (but it was due lto ess knowledge if material behavior i.e.steel)
I don't know if there is any universal rule book regarding the Load factor of the fighter aircrafts...but since there is something like IOC and FOC, there must be some accepted standards to be followed to fulfill the norms...of a working aircraft(if not exceptionally talented one)...the optimization of safety factor to the self load of structure(which is major portion of aircraft weight) is subjected to material sciences...so if lets say a fighter is more than safe to loads, it risks being heavy and retarded...thats where fight envelope and data comes in to play...so that the ratios can be subjected to related norms, and call the design optimized or more than optimized...wrt design and material factors in place...

I don't know, how much of the data generated is only limited to contextual design of that particular aircraft, which leads to nationalized norms, on which all the other foreign aircrafts are tasted as well...and since we are doing is all that is done,so basically our data is based on standard data already generated by leading manufacturer's...all we need to do is match those ratios...and if we excel thats a bonus...

In the example of snecma core to the Kaveri Engine...we need the data of testings in Russia to communicate to snecma guys...

PS. sorry, if didnt make sense...but if I did...

[Kartik]

I don't know if there is any universal rule book regarding the Load factor of the fighter aircrafts...but since there is something like IOC and FOC, there must be some accepted standards to be followed to fulfill the norms...of a working aircraft(if not exceptionally talented one)...the optimization of safety factor to the self load of structure(which is major portion of aircraft weight) is subjected to material sciences...so if lets say a fighter is more than safe to loads, it risks being heavy and retarded...thats where fight envelope and data comes in to play...so that the ratios can be subjected to related norms, and call the design optimized or more than optimized...wrt design and material factors in place...

I don't know, how much of the data generated is only limited to contextual design of that particular aircraft, which leads to nationalized norms, on which all the other foreign aircrafts are tasted as well...and since we are doing is all that is done,so basically our data is based on standard data already generated by leading manufacturer's...all we need to do is match those ratios...and if we excel thats a bonus...

In the example of snecma core to the Kaveri Engine...we need the data of testings in Russia to communicate to snecma guys...

PS. sorry, if didnt make sense...but if I did...

you really don't make any sense.

[manum]

can you point out...in relation to the quote i tried to address...

[PratikDas]

Tarmak007 Alert: LSP-5 to fly any time now

[glene]

Russia tests engine for Indian fighter jet

http://en.rian.ru/mlitary_news/20101110/161278210.html