Stealth Technology

Kuttan · Post by **Kuttan** » 02 Aug 1999 03:57

I know this must have been a topic several times before, but perhaps it could be revisited since the article "Rearming the Armed Forces" posted on BR a few days ago cited Stealth as an urgent technological need in the IAF. Also, the SSN and other threads like the Akash/Rajendra are getting too long, and there's some controversy regarding Eurofighter vs. F-22, etc.

My understanding of Stealth technology is that you do 5 things:

1. Don't expose heat sources (infrared)

2. Don't expose moving parts like fans and turbines. (Doppler effect? )
3. Have all surfaces inclined at one of about five different orientations, with edge sweeps all limited to a few angles as well, so that you scatter radar primarily within some narrow ranges of azimuths and elevations: in other words, do what a mirror does: strong reflection in one direction, nothing in other directions. This accounts for the weird shape of the F-117 (all flat surfaces and sharp edges), and the less-weird shape of the F-22 (computational electromagnetics and F-117 experience). I think the angles have something to do with the Brewster angles for certain ranges of frequencies, but not sure.

4. Use multilayer surfaces, so that radar waves are either absorbed, or undergo total internal reflection.

5. Avoid exposed external stores, same reasoning as (3).

By these criteria, I think the Eurofighter and the LCA designs have little to recommend them from stealth viewpoint. Lets see if I can get Salman/Johann/Badar/anyone else to respond before the thread gets closed. <img src="http://www.bharat-rakshak.com/ubb/smile.gif" alt="" />

[This message has been edited by narayanan (edited 01-08-1999).]

Kumar · Post by **Kumar** » 02 Aug 1999 04:32

Narayanan,Orienting panels at Brewster's angle for impinging radio/micro waves may be hard. Total extiction of reflection depends upon the polarization of the incoming wave and its incidence angle. If waves emitted by radars' are linearly polarized then a suitable choice of incidence angles near Brewster's angle will do the trick. The reflected intensity near Brewster's angle has a pretty broad minimum, so a range of incidence angles is available. But even then the distance at which stealth will work is limited unless one were to actively actuate the panels to maintain incidence angle close to Brewster's angle.Using multilayered surface to absorb the waves makes sense, but how are these waves going to totally internally reflect? They have to enter at some surface where reflection will be present. Are you saying that the plane's surfaces have sort of open waveguides where the waves enter and get guided by total internal reflection to someplace in the interior where they get absorbed or diffused?Ashok Kumar.

Kuttan · Post by **Kuttan** » 02 Aug 1999 04:48

Thanks, Dinesha. I'll look at the Discovery page. Ashok, I know of the Brewster angle only from laser optics, so I was just shooting in the dark. Maybe you can tell us if the 55-degree angle of most of the surfaces, and a 47-degree wing sweep angle of the F-22, have any significance for the radar frequencies. The mirror concept is what was explained by a person who was either a test pilot of the F-117 or a designer (forget which) in a public presentation. When you look at the F-117 it does appear that there are only a very few angles chosen. The source for the multilayer surfaces was also the same, though these things are common knowledge now, appearing in newspapers. Precisely what materials, and what coatings, is not revealed, I guess.Also, I forgot to mention: Big tails are excellent radar reflectors. BTW, the stealth requirement dictates the shapes of these aircraft so much that they have to compromise on the aerodynamics. For example, sharp edges with no metal actuators inside, etc. I'd welcome any info: perhaps this is an area where Indian technology is lagging, but can catch up very quickly..

Manas · Post by **Manas** » 02 Aug 1999 05:21

Narayanan, Pioneering work on RCS (radar cross section) reduction was done by a former Russian Prof (Peytor Ufimetsev (sp ??). He is currently a Prof at UCLA. His pioneering paper which by their own admission helped skunk works guys to design the B-2(Ben Rich and Overholster (sp??)) was published in a special IEEE edition on Radars. I believe I have a copy of it but I need to search for it. The UCLA website might also lead to the Prof's other publications.

Kumar · Post by **Kumar** » 02 Aug 1999 06:20

Hi Narayanan,For a dielectric like glass the Brewster angle is about 57 degree for whole of visible spectrum (inverse tangent of refractive index). Reflections from metallic surfaces are more complicated as a linearly polarized wave can become elliptically polarized when reflected. But there exists a "quasi Brewster's angle" or polarizing angle for metals too and is about 80 deg for Aluminum for visible spectrum and is a function of refractive index and absorption coefficient.I don't know how the graphite based composite materials behave. It depends upon their refractive indices and absorption coefficients.Ashok Kumar.

Kuttan · Post by **Kuttan** » 02 Aug 1999 07:15

Satya: Let me try to answer that, and then Salman et al will tear into my answers and then we can both learn. Even I can do the conceptual design for a fighter plane, and reduce it to expected performance measures such as range, max. speed, max. altitude, max. payload, turning radius, etc. With a little effort, I can probably get someone to do the detailed structural design, on paper. Someone smart like Salman or Badar can do the flight mechanics and controls, and decide things like how fast the aircraft will respond to given changes, etc. They can then program all these into a simulator (a computer program), link it to programs which show control panels and scenery, and to a joystick and pedals, and we can "fly" the realistic design on a computer. Connect speakers and we can even hear ourselves crashing, which is what happens when I try to fly these things. Now comes the hard part. The design is done based on "best practice+ small delta", i.e., you decide what has been achieved to-date, and figure that you can do a little better. The trouble is to do as well as others have done before: any imperfection results in an aircraft which is heavier, less powerful, and fails more often than the state-of-the-art. When all the tasks involved in building the aircraft are broken down, one finds that there are several instruments that look small, but required years of design and refining to get done. The engine has hundreds of little blades, etc., with the failure of a single blade enough to shut down the engine and cause the plane to come down. Etc. Etc. We find that it takes the collective expertise and skill of a whole industry, something that will not occur overnight. We never have the time: the wolf is always at the door. So we cut corners: we go out and buy things from outside. They don't let us open their boxes and reverse-engineer their products: anyway this sounds a lot easier than it is. So, the answer is, yes, in principle we have all the technology. In practice, we are like people who try to build a 3-storeyed house, but find as we build that the foundation is not there, nor is there a shovel to dig for the foundation, nor a foundry or smithy to try to make a shovel...Yet, if we tell our taxpayers that we need shovels, they will say: "Why am I still out in the rain after I assigned you responsibility to build me a house? You must be punished!!"

Kuttan · Post by **Kuttan** » 02 Aug 1999 07:40

Thanks for the info on the IEEE article and the web link (I'll have to check out the "baffles radar" part from a faster internet link). So, it appears that there is something to the angles of around 55 to 58 degrees: they are angles at which radiation might skim surfaces or get trapped inside the surface. I wonder, if one checks into it, if most of these materials (other than metals) have similar scattering properties in the radar frequency range. Also, what is involved in solving Maxwell's equations for a complete vehicle, so that one can gradually optimize the surface shape? I think the original F-117 was patched together on the principles of geometric optics as applied to radar, and the B-2 shape is also very simple, but by the time they got to the F-22, they had figured out how to get away with somewhat curved surfaces. This means that there was a good optimization software developed, where the EM constraints and the aerodyhamicists and the propulsion and structures constraints could all be fed in. Any comments or experience out there?

Badar · Post by **Badar** » 02 Aug 1999 09:47

Hi,narayanan, going back to your fundamental question, yes, I agree with you that the LCA has very little to recommend from the stealth viewpoint. So what? Does it really need to be?As you know, stealth is not an all or nothin affair. It is a matter of degrees. I am sure that LCA airframe has been optimised and tweaked in many slight small ways to optimise the RCS. Unfortunately not having seen any good photographs of the LCA this is difficult to confirm. Neither do we know if there are any plans to field the LCA with RAM coatings (a bad idea IMHO).There seem to be rumours about two more ways of achieving a degree of stealth on an aircraft other than the traditional RAS and RAM approaches. One is the active cancellation system said to be present on the B-2 and the Rafale. And the other is of course the celebrated "klingon" plasma cloak on the MFI. Both seem not impossible from a strictly physics point of view, but from an engineering point of view, I am really sceptical.BTW, big vertical tails are really not that big a problem. They shine like a beacon when lit up from the the sides, true, but this is not an issue when an aircraft is penetrating or egressing hostile territory, as on both occasions the aircraft is unlikely to present a beam aspect to hostile emitters.I have a couple of questions of my own about the LCA, and this seems to be as good a place as any to ask them1. Does anyone know (or have ideas, estimates, guesses) if the two side intakes on the LCA converge the airstream into one airflow before feeding it to the engine? Or do they directly feed the two seperate air streams to the engine?2. Does anyone know of a aircraft with a wing similar to the LCA - compound sweep delta with variable incidence? Can someone provide an insight as to why this planform was selected for the LCA (remember STOBAR ops were also envisaged for carrier ops).narayanan, salman, johann, rupak, sukumar others...?[This message has been edited by Badar (edited 01-08-1999).]

Kumar · Post by **Kumar** » 02 Aug 1999 11:26

One aspect of Plasma Stealth is in making a stealth antenna. Normal metallic antennae can radiate secondary waves when hit by waves that have frequencies close to resonant frequencies of the antenna. These secondary waves can be detected causing breakdown of stealth. Since the antennae have to be exposed for communication(i.e. they can't be put in a faraday cage!), this is a problem. A discharge tube can function as an antenna which is immune to this problem. When their is no discharge (or plasma) present, the tube acts like a simple insulator and doesn't radiate secondary waves as in the case of metal antennae. But whenever it be desirable to establish communication, one can power up the tube and create a plasma due to discharge. This plasma acts as the conducting medium in which oscillating currents can be established just as in a metal antenna. When you are finished communicating, you can turn off the power to the tube which then becomes a simple insulator again.[This message has been edited by Ashok Kumar (edited 02-08-1999).]

Kumar · Post by **Kumar** » 02 Aug 1999 11:53

The following is a news item from ITAR-TASS about Russian version of stealth created by surrounding the aircraft in a sheath of plasma. When radio waves hit a plasma sheath there is some absorption as is in the case of any conductor. But rest of the waves get deflected around. Very little actually gets reflected back. --------------------------------------------- Russian Scientists Created Revolutionary Low Observability TechnologiesBy Nicolai Novichkov ITAR-TASS information agency Moscow, January 20, 1999 Translation by Philip Kaploun Research team of the Recearch Center named after M.V. Keldysh has developed new technologies allowing dramatic decrease in aircrafts' radar observability. Russian approach to low observability (LO) technologies is completely different from US Stealth and offers complete furtiveness of the protected object at a significantly lower price. An exclusive interview about these technologies was conducted by Nicolai Novichkov, ITAR-TASS with director of the Center, academic of Russian Scientific Academy Anatoliy Korteev. As academic explained, American approach to LO (Stealth technology) applied on B-2, F-117A, and fifth generation fighter F-22 "Raptor" is based on the following principles. The airframes of these aircrafts are designed to minimize their radar cross section (RCS), avoid all possible elements of the structure, which could reflect electromagnetic radiation. In order to minimize reflected radiation radio absorbing materials (RAM) are also applied to the surface of the structure. The main drawbacks of the Stealth technology are its negative effects on the flight and agility characteristics of the stealth aircrafts. Russian scientists approach the issue from the other direction. They proposed to create a plasma formation around protected object, which prevents radars from seeing it. Thus, aerodynamical characteristics of the plane itself do not suffer. Without interfereing with technical characteristics the artificially created plasma cloud surrounding the plane guarantees more than hundred times decrease in its observability. The physics of plasma protection can be described as following. If an object is surrounded by a cloud of plasma, several phenomenas are observed when the cloud interacts with electromagnetic waves radiated by enemy radar. First, an absorption of electromagnetic energy occurs in the cloud, since during plasma penetration it interacts with plasma charged particles, pass onto them a portion of its energy, and fades. Second, due to specific physical processes, electromagnetic wave tends to pass around plasma cloud. Both of these phenomenas results in dramatic decrease of the reflected signal. Static and flight experiments proved the effectiveness of this technology. The first generation devices, producing plasma field surrounding an aircraft and decreasing reflected signal were created in the Center. Later, a possibily of creating second generation advanced systems (capable of not only decreasing reflected signal and changing its wavelength, but also producing some false signals) was discovered. Such systems significantly complicate determination of actual aircraft's speed, its location and leads to development of completely new approaches to LO provision, unachievable to conventional Stealth technology. Furthermore, the weight of the systems developed in Russia do not exeed 100 kg, and power consumption ranges from kilowatts to tens of kilowatts. Advances in development of the third generation LO systems allowed to clear the systems of first and second generation for export, commented academic Anatoliy Korteev. 26/01/99 (c) ITAR-TASS,

Kumar · Post by **Kumar** » 02 Aug 1999 12:02

Some disadvantages of plasma sheath based stealth:<UL TYPE=SQUARE> <LI> Creating the plasma sheath with lasers or with microwaves could consume a lot of power. <LI> The sheath will radiate visible light <LI> An ionization trail may follow the aircraft </UL>A lot of info about Plasma stealth could be found at the following site: <A HREF="http://members.xoom.com/ggromozeka/avia ... mamain.htm" TARGET=_blank>http://members.xoom.com/ggromozeka/avia ... ain.htm</A> Enjoy![This message has been edited by Ashok Kumar (edited 02-08-1999).]

Kuttan · Post by **Kuttan** » 02 Aug 1999 18:28

What a relief to be back on a high-speed line! My exclusive-access system crashed on Friday evening (ECM burst, no doubt!)When you are stuck on a line that disconnects every 3 minutes and runs at 1baud, and you are facing a Salman who is getting irritated, its like being in an LCA with a 1st-generation Kaveri engine, facing a Sukhoi 30MK

BTW, excellent posts! I am finally beginning to see some real info on Stealth.Salman and Badar: just to start off the morning: You mentioned that the Eurofighter was stealthy and had supercruise when the external stores were gone. This appears to be an ideal fighter for the PAF: fast escape after jettisoning weapons. Usually, stealth is preferred as one is going in for the attack, weapons loaded.

Recently, I saw an article in a Houston local paper slamming the F-22 for being supersonic, arguing that this was against all stealth principles. Predictably, written by an economics expert (no insult intented to BR economists!) I think supercruise is valuable to a stealthy aircraft: you sneak in subsonic, then, as the air defense starts to wake up, you dash supersonic to the attack. Supersonic flight cannot be detected from ahead, or directly below. Why is stealth relevant to LCA? Well, my question is, what mission does the present specification of LCA suggest? This is my main reason for saying that it is a technology demonstrator. Look for the MCA to be a real air superiority competitor. Also, the US has gone to extremes with the strange shapes dictated by stealth: the shape can't be that unimportant. I know there were severe concerns about the performance penalties paid to get the shape dictated by the EEs. Re: double-swept wing: yes, thats for vortex lift, so you can fly a little slower (and go to high angles of attack), and maneuver. A single tail is better detectable than twin-tails. No tails at all would be really preferable if you can control the aircraft in other ways. Two inlets: must be both directed to the same flow. Isn't each inlet split? This must be to keep the inlet from stalling at high angles of attack (a leading cause of engine failure during high-angle-of-attack operation), and maybe it is a cheap way of varying inlet shock structure, for supersonic flight.Rakesh: Thanks for keeping this thread alive. I'm really learning some good stuff here.

[This message has been edited by narayanan (edited 02-08-1999).]

Kuttan · Post by **Kuttan** » 02 Aug 1999 18:43

Ashok Kumar: From the British article I read on BR about the plasma sheath business recently, I got the feeling that it was mainly an ad for this book that the British were selling, about Russian technology. A few years ago, while there was still a Soviet Union, disintegrating, I had the pleasure of serving as the American technical translator (geek) to communicate with two separate Soviet experts who were writing summaries of Soviet technology. All the institutions mentioned in the recent British article sound familiar. I share your concerns about the plasma sheath as stealth. Are electrical storms undetectable? Won't a fast-moving region with ionized particles show up on detection systems? (I've read the book about the USS Nimitz going through a storm into the year 1941

). The trail will probably glow like the Northern Lights? The use of plasma for hypersonic drag reduction does sound credible. At really high speeds, the air behind the leading shock, close to the surface, gets ionized whether you want that or not. And communication through that layer is pretty difficult. Also, a small jet of plasma (or particles, for that matter) shot out through the nose of the vehicle, can break up the leading shock into zillions of tiny shocks. A huge drag reduction is claimed, and the Soviets had indeed used that. Possible application on missiles, because you burn aluminum to generate Al2O3 particles to shoot out: not for long periods![This message has been edited by narayanan (edited 02-08-1999).]

Kuttan · Post by **Kuttan** » 02 Aug 1999 19:26

Thanks! I too wondered about the gold shine of the F-22 canopy from certain angles, but thought that maybe gold was cheaper than the stuff they used to paint the rest of the aircraft. Makes sense that the cockpit contains lots of irregular surfaces.

Badar · Post by **Badar** » 02 Aug 1999 19:36

Hi,narayanan, good morning. (Yeah I am keeping very late hours today

)Salman,1) If the split intakes are indeed merged together then doesnt the following problem arise. The airflow across the intakes in unlikely to be symmetric at some moments, for example when the aircraft is making a sustained hard turn or is yawing. Add to this the airflow distortions which are produced by the wing and fuselage corner, specially at hi AoA and sideslip. If this happens does it not lead to a pressure instabilities that could result in engine stalls?Is this a real problem? Is yes how is it ameliorated?BTW, airflow around arm-pit type side inlets is supposed to suck. The combined boundry layers of the forebody of the fuselage as well as the wing leading edges are too thick to remove effectively, even with a splitter plate.2) It is interesting that you mentioned the Draken when talking about LCAs wing. Actually I was stuck my the similarities between LCAs wing and the Viggens wing. I have put up a <a href="http://www.angelfire.com/oh3/badar/plan ... aritive</a> planform view of the three aircarft together on the web. Any comments?3) Take your time over plasma stealth. I am sure it will be worth waiting for. [This message has been edited by Badar (edited 02-08-1999).]

Kuttan · Post by **Kuttan** » 02 Aug 1999 20:04

The thread that you refer to, was posted by ahluwalia, and contained an article titled: Re-Arming India's Armed Forces". I think there were few takers at that point, but that was what got me thinking that we needed to bring out what was known / could be inferred about stealth from the open literature. Obviously we have several people, who know a lot more about radar, electronics and related issues, than even they realize!

Kuttan · Post by **Kuttan** » 02 Aug 1999 20:28

<BLOCKQUOTE>quote:<HR> If this happens does it not lead to a pressure instabilities that could result in engine stalls?<HR></BLOCKQUOTE>Yes, but if you don't, then the engine will be starved of air flow when you make either a left turn or a right turn, and then the whole engine will stall/ surge etc. and flames will shoot out the front. The Gnat also had side intakes and just one engine, so HAL must have some experience with that. In subsonic flow, it must be possible to get rid of such asymmetries using a honeycomb / flow straighteners in front of the engine. Which you need anyway to catch the caps of people who wear caps in front of an engine on the airfield. Now I see that Salman has arrived. Time to toss in a few outrageous claims.

Badar · Post by **Badar** » 02 Aug 1999 20:32

Hi,ramen_das, not only the cockpit cavity but also the inlet cavity and exhaust cavity are major RCS contributors.Other major reflectors include the flat surfaces like fuselage sides, wing and tail empennage and the wing leading edges. Good radar reflections are also obtained by corners (intersecting surfaces like the wing and fuselage). Then there are EM currents that build up when the aircraft skin is illuminated. These currents flow across the skin until they hit a discontinuity (for example between flaps and the wings) where they scatter EM rad. The scatter is of sufficient strength to be detectable by the enemy. That is why weapon bays, panel doors etc on the F-22 have diamond shaped edges.Then there is the aircrafts own flat radar antenna which is buried in a transparent radome. It not only reflects but actually magnifies the the radar returns. This can be ameliorated by using a bandpass radome.I believe that one of the major contributors of frontal RCS of the F-117 is the pitots for its air data sensors and the pilots helmet!narayanan, what about the human eyeball Mk I, it is still a potent sensor. What about visual LO? [This message has been edited by Badar (edited 02-08-1999).]

Kuttan · Post by **Kuttan** » 02 Aug 1999 21:08

<BLOCKQUOTE>quote:<HR>what about the human eyeball Mk I, it is still a potent sensor. What about visual LO?<HR></BLOCKQUOTE>Quite right. I read in one of the books that I read (pure fiction as opposed to the books that you guys read) that chameleon-type paints could be developed, which are really hard to see, and change color to remain so depending on the background (wonder what happens if the aircraft does a 180-degree roll)Of course, this must be why the F-117 raids are generally conducted at night. Badar: fundamentally, WHY do corners scatter radar? Any explanation suitable for 5-year-olds like me?

Badar · Post by **Badar** » 02 Aug 1999 23:01

Hi,narayanan, time to throw out a few outrageous claims? Then what was that statement about the MCA being a "real air superiority competitor"?Yes, but if you don't, then the engine will be starved of air flow when you make either a left turn or a right turn, and then the whole engine will stall/ surge etc. and flames will shoot out the front.Agreed. But my point then is why not a direct feed of the two airflows (not merged by a Y-joint) straight to the engine face? What happens if you do this?Better still, why not chuck the side mounted intakes and go for a chin mounted one.WHY do corners scatter radar?Radar energy falls in the corners, bounces between the sides resulting in a scatter that sprays energy back towards the emitter.I did read a junk novel about a visually invisible stealthy aircraft. It was called Gossamer or something. Absolutely horrible.The US has been experimenting with "intelligent skins" for quite some time. The first attempts (project Yahud?) involved mounting lights on an F-4 to decrease the distance at which it could be seen (remember it was the hey day of the dog fight). I believe this system saw operational service in the vietnam war. The russians did not worry about these technologies, their miniscule MiG-21s were naturally LO. The Tacit Blue project envisaged a central light source will optic fibers running to the outer skin. Optronic sensors would collect environmental data that would be used to modulate the central light source, effectively making it visually LO. But this was never tried out. Latest approches seem to involve carbon composites whose colour changes with different voltages. Seems pretty neat. Might even solve your rolling aircraft problem.I very recently read an article on Optical LO on the web, I will see if I can find and post it.[This message has been edited by Badar (edited 02-08-1999).]

Sukumar · Post by **Sukumar** » 02 Aug 1999 23:38

Wow ! good discussion guys.The delta wing offers the max area for a given wingspan, which translates into more lift. So from a view point of a wing providing lift, the delta is the best, though it suffers from some negatives in low altitude flight (which I will not go into).The double delta seeks to provide some compromise between the pure delta and a regular swept back wing especially in high angle of attack, lo speed flight by enhancing flow separation over the top surface of the wing. In layman's terms this provides more lift at a time when a regular delta would've stalled. So it makes the wing more efficient.I have visited ADA and seen their CFD simulations of the LCA pitching up at high AOAs. The inlets merge into one before reaching the engine. For most of the flight regime the flow is symmetric between the two inlets. At supersonic speed you dont need to worry since the flow will shock down before proceeding to the engines. Asymmetries occur when making complex maneuvers. However, since these maneuvers are made at relatively lower speeds, they do not cause any significant instabilities. Split inlets have been used on aircraft for many many years.The objective of stealth is to hide an aircraft in the entire electromagnetic spectrum as well as acoustic range. There have been some very good discussions above on the radar spectrum.A hi-stealth aircraft like the B-2 seeks low obsevability in:1. the IR spectrum - by mixing the engine exhaust with cooler, ambient air.2. acoustic spectrum - by using mufflers on its engines and using active noise cancellation.3. visual wavelengths - its shape is designed to be such that it cannot be easily detected. Even if detected you cannot easily make out if it is flying towards you or away. It is painted black to blend into the night.4. radar stealth - apart from what has been discussed above, it is shielded to not emit radiation actively. The cockpit canopies are coated with gold to prevent radar reflections as well as to shield cockpit instruments from EW/ECM.

Kuttan · Post by **Kuttan** » 02 Aug 1999 23:46

<BLOCKQUOTE>quote:<HR>why not a direct feed of the two airflows (not merged by a Y-joint) straight to the engine face? <HR></BLOCKQUOTE>My guess is that this will cause much worse differences between the left and right sides of the fan/compressor face, when the aircraft is in sideslip. The blades will see a large periodic change in local angle of attack, making blade stall much more likely. If you mix the flows into a single duct, the worst you may have is a separated-flow region in the duct from time to time, and this I think can be easily prevented. Besides, a single round duct is much easier for installation. Chin inlet? I think that's easier to detect from below. I don't really know why the F-16 uses that. Also, HAL experience with Gnat and HF-24, and tendency to adopt British solutions rather than American (see Harrier)."JUNK NOVEL"? Ha! Anyway, the title is not familiar. I should look for it next time. "Radar.. bounces between the sides...sprays energy back towards the emitter."Seriously, that sounds like a particle explanation. I thought you would say that EM charge builds up along sharp edges, and the edges emit a lot. Or is it interference between scattering between different faces? "Optical LO" Well, I doubt that visibility to humans, as glamorized as it is in the books that I read, is going to be a differentiator in air combat. I think its not worth the effort to do all these expensive paints and fiber optics for that. Might as well use gray primer and fly on a cloudy day...Have you seen articles about the Lockheed UCAV? AvWeek, Nov. '98 has a small article on air-launched UCAVs. Might change the entire nature of airstrikes, and maybe of air combat. What good is a 50 billion dollar human-invisible aircraft if it is attacked by 5 intelligent missiles with sensors spanning the infrared through ultraviolet? PS: What about the MCA? Does that not qualify as an outrageous claim? Salman missed it...

Badar · Post by **Badar** » 03 Aug 1999 00:55

Hi,Even though this article is from Popular Science magazine it is still very very informative. It is an absolutely facinating article if you are interested in visual LO.It can be found <a href="http://www.perseids.com/projectblack/po ... </a> [This message has been edited by Badar (edited 02-08-1999).]

Badar · Post by **Badar** » 03 Aug 1999 01:07

Hi,narayanan, chin inlet being detectable from below is not really an issue. LCA is not that stealthy any way, so it probably would not matter.Realistic problems with chin inlets might be foreign object ingestion, headache of locating the landing gear and obstruction of airflow by the aircraft forebody at hi AoA. FOD is ignorable, use a grill or something at take off. Does not seem that LCA has any impressive AoA capabilities anyway (comments sukumar?) so airflow distortions are a moot point. So why not chuck the side inlets and go for the chin? You'll have no airflow mixing problems."JUNK NOVEL"? Ha! Anyway, the title is not familiar. I should look for it next timeYeah, had to qualify novel - Old Man and the Sea is also a novel remember? A bit of friendly advice, dont bother, it really is junk.Seriously, that sounds like a particle explanation.Particle, wave or wavicle. All bounce and reflect. So why go for a complicated explanation? Occams Razor.I thought you would say that EM charge builds up along sharp edges, and the edges emit a lot. Or is it interference between scattering between different faces? Jeeze, Does such chanelling of radiation by edges really happen? Or are you pulling my leg?I doubt that visibility to humans, as glamorized as it is in the books that I read, is going to be a differentiator in air combatIndeed visibility is a big factor. Guess which sensor system is used to keep tabs on a violently manevouring enemy when you are locked in close comabt? This was emphatically proven during the vietnam war. Fishbed drivers were first able to visually pick out Phantoms and manevour into a advantageous position, while the guy in the phantom was still peering into his radar scope.Ah, unmanned strike aircraft! That warrants a seperate thread. If you wish to argue against it I am game to argue for it. Or vice versa.

Salman is very sharp, like you he doesnt miss much. He either let it pass as a too obvious a bait or was in a hurry to go to work.[This message has been edited by Badar (edited 02-08-1999).]

Kuttan · Post by **Kuttan** » 03 Aug 1999 02:57

Badar: Regarding chin inlet vs. side inlets. 1. Have not seen any aircraft with max Mach number above 1.8, with a chin inlet. (Maybe F-16 XL is designed for more; not sure). It is not convenient to incorporate variable geometry into a chin inlet, I suppose. For Mach 2 and above, variable geometry is probably essential to minimize shock losses; this needs perhaps more than one oblique / conical shock, which slows the flow but keeps it supersonic, followed by the inevitable normal shock which takes the flow below the speed of sound. Side inlets allow variable ramps to control the shocks. Presumably, an aircraft which claims supersonic cruise will have a maximum Mach number of 2 or greater. (forgot to look at that planform comparison to determine design Mach number: Salman will hammer me again for not doing homework

)2. Imagine for a moment that you are developing a technology demonstrator. Your follow-on aircraft is going to have two engines. The choice of side inlets seems fairly safe. (Did I say "MCA"?)

3. On a delta-winged aircraft, a chin inlet really messes up the beauty of the underside. Also, gets in the way of hard-points, which you would like to keep as close to the plane of symmetry as possible, since you have only one engine. 4. You said it: operation from less-than ideal runways, desert environments etc. Important for LCA. 5. I still say that a chin inlet is more visible to a shoulder-fired missile. Important consideration for "mud-mover" missions.

Kuttan · Post by **Kuttan** » 03 Aug 1999 03:26

Badar: Thanks for the planforms posting. I take back what I said about vortex lift: the inboard section has lower sweep. I have to go back to claiming that this is an extremely simple planform, almost like someone said: "go design me a generic platform for a technology demonstrator. Don't waste energy on aerodynamics stuff: use what is already well-known". I happen to know, though, that the inlets were tested at the supersonic tunnels in France.

Badar · Post by **Badar** » 03 Aug 1999 03:41

Hi,Narayanan, 1. Eurofigheter Typhoon with chin inlet flew at Mach 2+. MiG MFI with chin inlet is projected to fly at M2.5+.Agree that mach 2+ flight is difficult without variable geometry inlets and variable ramps.LCAs top speed said to be mach 1.6-1.8. So it doesnt need a variable geo intake. Anyway a variable geometry intake is a rather heavy and expensive piece of equipment for the "worlds lightest fighter". Speed or inlet geometry is not a criteria for not using a chin inlet here.2. This argument works and again doesnt work. Yes, it is nice to reuse inlet technology on the twin engine follow up. But if your follow up is going to be a tailess stealthy aircraft, you are really going to redesign your inlets with this in mind. You are definately not going to reuse the existing HO inlets. 3. Agreed.4. You have a point here. But as I mentioned, a grill or mesh could cover the intake during TO. The MiG-29s seem to manage pretty well with this. But weather this will be a good idea on a light fighter, I dont know.5. Chin inlet might be more visible to a Radar seeking missile, but not to a IR homer. Which MANPAD has radar guidance?Well I guess you made reasonable points in favour of side inlets. Can you make some similar ones for the planform?How do you determine the design mach number by looking at the planform? I sure would be glad to learn this trick. [This message has been edited by Badar (edited 02-08-1999).]

Badar · Post by **Badar** » 03 Aug 1999 03:53

Hi,"go design me a generic platform for a technology demonstrator. Don't waste energy on aerodynamics stuff: use what is already well-known"narayanan, This is most interesting if true. You are a better judge of this than I am, I guess I'll buy it.Could you please spare a few minutes and give a brief run down on this planform. Its advantages and weaknesess. Also, would you choose this wing for a aircraft that was also meant to perform STOBAR Ops?[This message has been edited by Badar (edited 02-08-1999).]

Kuttan · Post by **Kuttan** » 03 Aug 1999 03:57

<BLOCKQUOTE>quote:<HR>How do you determine the design mach number by looking at the planform?<HR></BLOCKQUOTE>This is sure to lure Salman out with a nasty comment, but here I go anyway: Find the sweep angle of the leading edge. In this case it seems to be 65 degrees or so (I am too lazy to measure it). If the component of Mach number normal to the edge is 1.0, that's probably the highest Mach number intended for extended operation. If the Mach number is any higher, the shocks get stronger, and the drag goes higher. For a 65 deg. sweep, this gives M = 1/ cos(65) = 2.366. I guess you already knew that the max. Mach number was less than that

I hasten to add that this is rarely the reason for the wing sweep being what it is. However, if I go by my original claim, this aircraft configuration has really one job: fly supersonic. Well, maybe some high-angle of attack maneuvers as well, but I don't think that this large delta wing is very good for rolling fast. Overall, this looks like a "generic wing-body model" that people use to do basic data sets for research. The low inboard sweep is because the inboard part is inside the shocked, slowed-down flow from the forebody. Also, perhaps they want to reduce pitching moment. It does look very much like the Swedish designs, but those have canards ahead. Does it makes sense to you that something can cruise supersonic without afterburner at supersonic speed (at Mach 1.4?), has a clean shape, yet can only reach Mach 1.8 max? Strange engine.

Badar · Post by **Badar** » 03 Aug 1999 04:32

Hi,That was cool. Nice rule of thumb. Pretty simple when you come to think of it (as is every thing with hindsight). Thanks.I wonder what Dassault, Dornier and MBB got paid for then, if the basic layout is so unchallenging. BTW, ADA chief mentioned that he was so happy with the LCAs planform and airfoil that he was going to reuse it as it is on the MCA.Ok, one more question. What makes you say that this bird can indulge in hi AoA? What makes this possible? What would be the ball park figure for max AoA for this bird? It is interesting that you say that roll rate is going to be poor. I think you will like to know that instantaneous turn rate is quoted as 30 dps while sustained turn rate is said to be about 13-17 dps.I dont believe for a second that the LCA is going to supercruise. Maybe it just might push Mach 1.0 in a clean configuuration with a good tail wind - which ADA might wish to claim as supercruise. If this is their defination of supercruise then I dont know if it is something to be really impressed about, the Lightning demonstrated this sort of capability in the 1950s. The LCA will definately not have enough dry thrust to reach and sustain speeds beyond the transonic drag regime.What about the engine? That was rather cryptic.

Peeyoosh · Post by **Peeyoosh** » 03 Aug 1999 04:56

Great thread Badar/narayananOn the Kaveri - I doubt its capable of genuine Mach 1+ dry supercruise for sustained periods of time. If it is then the max speed with afterburner a speed of Mach 1.8 is funny, unless the airframe is the constraint rather than the engine.Narayanan, on the LCA being a technology demonstarator the DRDO has invested a decent amount of its credibility in the project - if they do not deliver it will be a blow to their credibility.Peeyoosh[This message has been edited by peeyoosh chadda (edited 02-08-1999).]

Badar · Post by **Badar** » 03 Aug 1999 05:07

Hi,peeyoosh, mach 1.8 speed makes sense when you consider that LCA lacks variable geometry intakes. Theoretically it is possible to have a supercruise capability (max dry thrust is the determining factor here) and a top speed of "only" Mach 1.8 or so (lack of var. geo inlets means that pressure recovery at speeds above M1.8-M2.0 is very poor).

Kuttan · Post by **Kuttan** » 03 Aug 1999 08:08

Here's the rest of my problem with the LCA as the next-generation airspace defender: 1. It has leading-edge flaps, I assume, but very few other controls. Yes, they claim thrust vectoring, but this is the most risky of all the technical claims. I read that the thrust-vectoring nozzles need some exotic alloy to be light, strong enough to take the full thrust of the engine, and survive the temperature. So, lets say that it DOES have thrust vectoring: At high angles of attack, (i.e., above 25 degrees) there are no effective controls anywhere near the front of the aircraft. A generic wing-body design like this, with a nice, round cross-section, pointed ogive nose, will suffer from the infamous "forebody asymmetry" or "phantom yaw". This aircraft does not appear to have any lateral stability or control devices at high angles of attack. Unless there are ventral fins. 2. Above 35 degrees angle of attack, this configuration is fairly sure to encounter a very bad phenomenon called "wing rock" where the vortex from the forebody interacts with the vortex on the wing and, as the term indicates, sends the aircraft into violent roll instability. I don't see any controls to alleviate this. Perhaps the reduced sweep of the wing near the fuselage is to prevent this vortex interaction: I don't know. 3. A simple delta wing like that, will experience "vortex breakdown". With a 65-deg. delta wing, this will occur at about 30 degrees angle of attack. Further, it will occur in an asymmetric manner. The leading edge flaps may delay this, but not very significantly. Again, I am curious about the low-sweep inboard section. 4. There is no evidence of a speed brake, and there is no all-moving horizontal tail. So, if this is to be a fighter plane, they are depending entirely on having an awesome single engine, which will perform miracles over the entire speed and altitude and angle of attack range. And thrust vectoring. 5. If the top speed is only transonic, there is a further problem: there is no evidence of the "coke bottle" shape, or transonic area ruling, required to get through the transonic speed regime (Mach 0.9 to 1.2 or so). Again, if this is to go supersonic, its because the engine is so powerful that you zip through the transonic regime, no problem. In that case, the top Mach number should be around 2. I'm not sure that the inlets have no variable geometry. 6. I hear everyone's concern about DRDO's credibility riding on this, but consider if it was HAL or NAL that had primary responsibility, and it turned out to be a non-mass-production craft: that would be much worse. My claim is that the designers and program managers know exactly what they are doing. We may hear some dramatic announcement of what was achieved using the cover of the LCA program. In fact, I think that has already occurred: the LCA program, probably, was the cover used to keep the Agni 2 program alive, after the US pressure and so-called program shutdown. Some of the GNC for these other "black" programs, too, must have been passed off as LCA. The LCA looks so innocuous that western governments approve collaboration on it. The western companies involved, probably know exactly what they are doing, too, but they are in business to sell items. Salman, yes, all unsupported figments of the imagination, but the LCA specs just don't make sense any other way. Roll rate of 30 deg./second? Imagine what that looks like: hardly a whiplash dogfighting roll. Your car does that, as you go around the highway ramp at 60mph (you don't actually roll 30 deg., but maybe 10 deg. in 1/3 second.) This means that it takes 6 seconds for the LCA to flip over in what is called the -1g roll recovery, in the classic altitude-reduction maneuver (maintain speed; roll so that your lift points down; sink at -1g, then snap back: this is the usual design stress condition for the vertical tail); 3 seconds to roll 90 degrees. I can't really relate to the 13 deg. turn rate. Depends on the kind of turn and the speed, assuming its g-factor limited. Again, this does not sound like a "supermaneuverability" turn. As for the engine, I think the question is answered. The Kaveri is too new to be asked to do all these things,right away. The GE engine, presumably, would have enabled a lot of things, and probably they were depending on that to take them supersonic. So, I don't believe the LCA can go to any significantly high angle of attack; nor roll fast; if you guys say its not going to go very supersonic, then I have to wonder about its real purpose. Remember, I am NOT saying the the LCA is a bad program; just that its not a good idea to hang expectations of a fleet of 21st-century combat aircraft on this particular design, until we know a lot more.

Kuttan · Post by **Kuttan** » 03 Aug 1999 08:30

BTW, In accordance with policy, all numbers are approximate, since I have not measured the sweep of the wing planform, or the geometry of the nose. The wing camber will affect these things somewhat. Otherwise, the phenomena should occur in about the ranges indicated, including the wing rock, which may occur between 25 and 45 deg. angle of attack.For Badar's question, the delta wing configuration shown should be good for supersonic flight, in the Mach 2 range, assuming that the engine is powerful enough. According to AvWeek recently, the actual geometry of the LCA has not been revealed. Is that so? What flew last year, then? [This message has been edited by narayanan (edited 02-08-1999).]

Kuttan · Post by **Kuttan** » 03 Aug 1999 08:40

Hi Salman: Unfortunately, its curfew time for me. Try to get this thread back to the issue of stealth, please: I am curious as to WHY corners scatter so much, and why the radius of curvature (1/sharpness) of the corner matters. In laser scattering, the actual phenomenon is supposed to go something like: a) photon hits molecule. b) molecule's electron gets excited. c) molecule emits photon d) exiting photons interact with incident photons. e) interference phenomena result. Does something like this occur with radar as well? Is there charge accumulation? In the IIT many decades ago, one of my fellow denizens tried to hang a 60-watt bulb and a 40-watt bulb next to each other to get interference and produce a 20-watt light. I think he hit the bulbs together a bit too hard, and got a 0-watt bulb as expected.

Good night, all. I don't like the turn of the discussion on the LCA: I sure hope no DRDO people are not reading BR.

Yogeshwar · Post by **Yogeshwar** » 03 Aug 1999 09:27

Can anyone comment on vermiculite as a R A M? used for sprayed on light weight acoustic insulation in construction.when set it feels like jello at Normal temps. 40 C +

Kuttan · Post by **Kuttan** » 03 Aug 1999 18:26

<BLOCKQUOTE>quote:<HR>many other 'fast' aircraft have low sweep angles<HR></BLOCKQUOTE>Sure, Salman, and many very slow ones (Learfan)have high sweep angles. On the fast fighters, the compromise is one of the following: 1. High angle of attack performance: high sweep to produce strong leading edge vortices, and maintain strong lift to 30 degrees and beyond: mostly to help turn tight corners. 2. Meant primarily for high-speed dash: Initial supersonic designs (Lockheed F-104?) used to have low sweep and supersonic airfoils razor-sharp leading edges. These have terrible characteristics at low speed. I think the F-104 killed more of its pilots than its enemies did. 3. F-14 has a swing wing, so does the Tornado. 4. F-15, well, high angle of attack was more important, I think. There was enough power for a supersonic dash. 5. F-22: Stealth is more important than anything else. 6. F-117: same as F-22, worse. You know its called the "Wobblin' Goblin" with good reason. See Badar, what happens to a nice thumb-rule when Salman gets going at it. Salman: Try out the thumb rule: "All odd numbers are prime numbers" Limit your search to the region 1 through 14, please.

[This message has been edited by narayanan (edited 03-08-1999).]

Badar · Post by **Badar** » 04 Aug 1999 03:36

Hi,narayanan, do you know anybody who can recommend me a good text on the issues you discussed above

For example behaviour of various planforms etc. All the textbooks I have read see to focus mostly on stuff like how to analyse and design a good airfoil etc. (PS my knowledge of aerodynamics is limited to a couple of undergrad courses, (remember when I said I have very slow summers?). Math level of the book is no problem).

Kuttan · Post by **Kuttan** » 04 Aug 1999 06:53

Best book on basic engg. aerodynamics is by Bertin, J.J. and Smith, M. , "Aerodynamics for Engineers" I think its Prentice-Hall. Advanced aerodynamics of supersonic planforms etc.? By far the best are: 1. R.T. Jones, "High Speed Wing Theory", originally published by Princeton Press, but I believe a new edition has come out. R.T. Jones is the person who is associated with the oblique wing concept. 2. Holt Ashley's book on high speed aerodynamics (can't recall title offhand, but please check your e-mail). For design info related to wing aerodynamics (best-known for low speed), the best source may be the web pages of Professor Ilan Kroo, Stanford U., Dept. of aero. BTW, Salman, the other criterion for deciding sweep angle is the Critical Mach Number (<1.0), the flight Mach number at which the flow becomes sonic somewhere on the wing. Beyond this, shocks form and may separate the flow. This accounts for high sweep of commercial airliner wings, and maybe the MiG-17 era. This choice depends on thickness and shape of the airfoil used for the wing.

Badar · Post by **Badar** » 04 Aug 1999 12:06

Hi,narayanan, salman, thanks for the info. Really appreciate it.Salman, Damn! my library doesnt have it and it seems to be out of stock at amazon. (psst..how much do you want for that book

)[This message has been edited by Badar (edited 04-08-1999).]