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 Post subject: LCA News and Discussions
PostPosted: 01 Feb 2011 17:12 
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Last page of old incarnation, here
=============================================================
Background articles on HAL Tejas (LCA)

_____________________________________________
Excellent overview of Tejas, from the developer itself.
http://www.tejas.gov.in/
_____________________________________________


1.http://en.wikipedia.org/wiki/HAL_Tejas

2.Remembrance of Aeronautical Matters Past (Brief history of India's Aerospace Industry)
http://vayuaerospace.in/Selected_articl ... brance.htm

3.All the articles at BR page on LCA.
http://www.bharat-rakshak.com/IAF/Aircr ... Links.html

4.http://www.acig.org/exclusives/LCA/ACIG ... Tejas.html

5.http://www.lca-tejas.org/

6.Good background on project, a bit dated.
http://www.geocities.com/spacetransport ... t-lca.html

7. Harry's Radiance of the Tejas article
http://www.bharat-rakshak.com/IAF/downl ... diance.pdf

8. ADA overview on LCA, including interviews of test pilots, a peak inside the R&D labs and rare footage.



Newbies beware ! If you make ignorant remarks, you could be grilled by gurus
to test your LCA knowledge from these pages !
And, if you come out deficient..............(you would do better not to find out !)
:twisted:

Please stay on topic.

That means :
a> No comparison with aircraft A,B or C.
b> No half-baked suggestions to improve LCA like "add a laser gun"/"merge DRDO with ISRO " etc etc.
c> NO whining.

======================================================


Wanted to raise a question regarding the possibilities on the LCA Mark II

Can the LCA Mk II be designed to accommodate Conformal Fuel Tanks ?

If so what additional fuel load could be carried in such a CFT and thereby what addition to the loiter time could be brought in by such an implementation if air-air refueling is not available.

Also without CFT or other fuel-tanks and air-air refueling in the LCA Mk II is there a targeted approach to increase the loiter time / internal fuel load and fuel performance


Last edited by Rahul M on 05 Jul 2011 12:59, edited 2 times in total.
added the regular thread resources.


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PostPosted: 01 Feb 2011 17:14 
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i guess the fundamental question is does LCA remain "L" or does it become something else?
I would like to see it turn into a dedicated Air Intercept platform, networked into the overall AD mesh


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PostPosted: 01 Feb 2011 17:58 
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Philip wrote:
http://www.flightglobal.com/articles/20 ... rvice.html


An executive involved in India's medium multi-role combat aircraft (MMRCA) competition is equally dubious. "If they had got the Tejas right, they wouldn't need to do the MMRCA," he says.


Sometimes the most moronic comments are passed off in grave seriousness and accepted with grim solemnity like something out of an episode of Monty Python.

Quote:
"I did not marry Meena Kumari because she was born decades before me. If I had been born earlier Kamal Amrohi would not have been necessary"


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PostPosted: 01 Feb 2011 18:28 
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Gurus, I have a few questions embedded within a hypothesis.

I have been comparing Tejas with Mirage 2000 which is a similar single engine delta fighter and yet is devoid of issues like alpha. I have also been observing some of the videos of vertical loops by dogfighters and multi-roles.

A vertical turn is a combination of rotation and revolution. A plane rotates 360 degrees about pitch axis while completing a vertical loop. I use the analogy of a flywheel rotating about the pitch axis and simultaneously pitch axis is revolving, to approximate the fighter’s behaviour in vertical turns.

For Mirage 2000 the wings are attached below the fuselage so one can say that its centre of gravity(CG) in the clean configuration is located above the wings and presumably above the pitch axis. Whereas for Tejas the wings seem to be towards the upper side and the undercarriage is said to be heavy so the CG is most likely below the wings. Our folks possibly may not have used wings in lower fuselage because it would have required taller landing gear to avoid drop tanks and stores from scraping tarmac. And a taller landing gear would have required bigger longer retraction housing in the fuselage which was constrained in its length in order to remain small. But now we are lengthening the MK2’s fuselage anyways to make it streamlined and less chubby.

Q1. My contention is that Tejas has CG below the pitch axis (Right/Wrong)?

Q2. The pitch axis in Tejas could be shifted lower if the wing was located below the fuselage instead of above.( Right/Wrong )?

Q3. Intuitively it seems easier to turn in a direction if both CG and Centre of Revolution were on the same side of wing/pitch axis. (Right/Wrong)?

If the above is right, Tejas MK1 in its clean configuration belongs to the case 1 below

Case 1: Bottom-heavy Flywheel in a level flight – Stable State

Image

As seen here the rotation of CG about the pitch axis requires the thrust to simultaneously lift the CG up when you want to pitch the plane upwards and make a vertical arc. The load on the engine is much higher because of this even in the clean configuration. Adding drop tanks, stores etc further aggravates this. Being in a stable equilibrium, the mass of the body resists an upward turn. It was suggested that overweight is the cause of AoA issue of Tejas.

Actually overweight would not have been much of an issue if it was symmetrically overweight about the pitch axis. Rotation part is not affected by overweight only the revolution part is because upto the first half of vertical loop it involves lifting of CG.

Q4. My contention is that most dogfighters and nowadays multi-roles which need good alpha belong to the case 2 below. (Right/Wrong)?

Q5. Theoretically, the most manoueverable plane is the one in which (1) all the three axes- Roll, Pitch and Yaw intersect in 1 point (2) Centre of gravity lies at this point of intersection A(stable planes) or above it B(unstable planes). (Right/Wrong)? (refer figure2 (b))

Case 2: Top-heavy Flywheel in a level flight- Metastable state.

Image

As seen here the centre of gravity CG of a clean fighter rests above the pitch axis in a metastable equilibrium. Though eventually as you load with drop tanks, aams, pgms etc its CG shifts downward. One would expect the CG to shift down and coincide with pitch axis under a designed optimum payload it is normally planned to carry and come down further into a somewhat case1 like situation only with maximum loading. Or perhaps it is the MTOW loading that lower it to coincide with pitch axis if the clean CG is even higher.

Image

As seen in figure a small thrust is enough to tip the CG into downward turn which produces a very high instantaneous turn rate. That’s how a smaller thrust engine can produce a higher alpha. Look at the Mirage 2000-5 turn rate right after the takeoff in the video below.
Mirage 2000-5


Most important point:

In Energy terms, the rotation of CG in a metastable fighter while going from position 2 to position 4 produces a conversion of Potential energy(PE) to Kinetic energy(KE) which is OPPOSITE to conversion of Kinetic energy (at position 2) to Potential energy(at position 4). PE to KE in rotation counters KE to PE in revolution. Thus the NET Kinetic energy to Potential energy conversion required in going from position 2 to 4 is reduced and made achievable by a lower thrust engine. That’s how you achieve a tight vertical semi-circle even with a lower thrust engine.

Image

Such planes are not able to make such very tight turns over the entire vertical loop but only in the first semi-circle(first and second quadrant) . During the third and fourth semicircle the CG is being lifted wrt the pitch axis so it gets sluggish. If a plane with CG above pitch axis were to make a complete vertical turn where every bit of turn was tightest possible then it would be a spiral and not a circle. Hence as shown in figure below a plane has to have minimum ground clearance(hmin) at its starting point to complete a vertical turn without crashing into ground. A linearly intuitive pilot unaware of his CG wrt pitch axis in a clean config does not realize this.

Image

That explains why in the airshows the fighter that flaunts a vertical turn immediately after takeoff does not complete the loop. In Mirage 2000 video above and many others, they just do three-fourths of the loop and then lift CG up by a roll manouever and “digress” into other aerobatics without completing the 4th quadrant of the loop !!! (This one started vertical loop from above ground level yet shuns the loop completion - 1:02-1:17 secs EF http://www.youtube.com/watch?v=k-2QiyzaF2o ) Others starting from higher level than ground when they try to complete the vertical turn have near miss like the ones below. Nothing wrong with the planes just matching our flywheel with these fighter’s characteristics in the 4th quadrant.

Rafale almost crash


Eurofighter nearly crashes (1:09-1:21 seconds) http://www.youtube.com/watch?v=ZKo3DuP- ... re=related

Su 30 crash http://www.youtube.com/watch?v=Qd0eV95ipQk

The logic runs contrary to automobiles where they try to keep CG as low as possible for stability but the idea of low CG is firmly lodged in our psyche. Even on BR one always heard of concerns of CG shifting to the front or behind but nothing about its vertical alignment. There are planes though with wings attached above the fuselage too but they have considerations other than alpha. Example: Jaguar has wings above but it is low flying DPSA so has to keep good ground clearance and is not a dogfighter. It is from era of role specific fighters. Most likely, mass distribution above/closer to the pitch axis is also the reason why it offers over wing aam pylons and not under. Such planes also have shorter length wingroot to have less drag resistance to rotation unlike a delta. Even in fighters where the wing apparently seems to be attached half-way the height of a fuselage and not below, the mass distribution is more above the wing than below. The intakes are hollow after all. However, I am not claiming that pitch axis passes through wings. I believe it is influenced by the location of wings, perhaps their drag resistance to rotation, effect of elevator fins or canard fins etc. Considering that both Mirage and Tejas have high drag resistance in rotation due to delta wings and no canards or fins to help, a lower thrust M88-2 in mirage, the understanding above could offer the only explanation of differences in performance.

Sidetrack: So why weren’t canards helpful in MK1 when they do so in Eurobirds?
Quote:
Here are possible reasons
1. The MK1 has a short length being a small fighter. We could only place a small size canard fin and that too not in front(Canards positioned too far in front like EF are more complex from control laws pov according to French on keypubs). Now a canard placed closer to pitch axis would have to generate greater Lift(hence bigger size canard) to generate same amount of torque about pitch axis that a smaller canard located farther would generate. Ours was smaller yet closer.
2. If the pitch axis eventually turned out to be at P’ instead of expected position P in a plane, one can see how the effective Torque produced by a canard about the pitch axis reduces. For the weight of actuators controlling it, as our folks realized, it did not add much.
Image


If all of the above is true, an inverted Tejas should have same behaviour as other unstable dogfighters and offer a high alpha inspite of the current intake size, wing shape etc. If the CG is ahead of pitch axis, the fighter would first have to lift the CG a bit before a better alpha kicks in.

In conclusion, if above is true, I would very humbly suggest that Tejas MK2 should have wings below the fuselage and not above and other measures to bring down pitch axis and raise the CG above it. We haven’t frozen the design yet. Merely increasing intake size and developing more thrust will not convert Tejas from stable to metastable type.

JMT. If I am wrong would love to be corrected. Apologies for a long post.

PS:

1. EADS consulting had told us that they would only point in the direction of switch and they fear being blamed for the programme’s failure. Reading between the lines: (1) What is being overlooked is that simple (2) But mitigating the problem would involve structural modifications(eg wing relocation etc), the time repercussions of which they do not want to be blamed for and nor if we couldn’t get their hint. But by now anyways we are redesigning MK2.

2. I also suspect that a jet engine turbine too has a built in flywheel about its shaft. If they have built a flywheel based energy storage http://en.wikipedia.org/wiki/Flywheel_energy_storage they must have thought about energy storage in jet engine turbine too in flywheels made by for example spacing blades closer in a specific sector of circular arrangement. An instant transfer of this stored energy in flywheel could provide a quick acceleration to the jet engine as and when needed. But this one I am not sure.


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PostPosted: 01 Feb 2011 18:46 
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you dont account for lift and drag, aircraft have to balance weight with lift and thrust with drag
centre of lift vs centre of gravity is your key issue in pitch (moment wise)
also lift and drag are non linear with alpha
thrust is usually used to counteract drag during high alpha manouevers
your 4th quadrant arguement doesnt make sense to me, at that stage, the a/c has plenty of potential and kinetic energy and does not need additional thrust to 'complete the quadrant'


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PostPosted: 01 Feb 2011 19:25 
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@Arya, good, simple (probably simplistic) analysis, which i'll let the experts comment on in detail. However, I will point out that the Rafale does have a wing over the inlets (in effect, a kinda high wing), and that, if you remember the initial LCA wind tunnel models, they HAVE tested canards, and decided that it wasnt of much utility.

Amit J wrote:
Wanted to raise a question regarding the possibilities on the LCA Mark II

Can the LCA Mk II be designed to accommodate Conformal Fuel Tanks ?

If so what additional fuel load could be carried in such a CFT and thereby what addition to the loiter time could be brought in by such an implementation if air-air refueling is not available.

Also without CFT or other fuel-tanks and air-air refueling in the LCA Mk II is there a targeted approach to increase the loiter time / internal fuel load and fuel performance


@Amit saar...

Yes, it can have CFTs (IIRC, the Mk 2 plan calls for checking the feasibility of CFTs, or atleast thats what I remember from speaking to PS at the last AI). However,

a) Where can it be added, given how small the craft is, and if there is one, would it be able to carry enough fuel to justify it? To my mind, if its area is large, it will add weight while carrying a small amount of fuel. Additionally,

b) CFTs would, add a weight penalty to the Tejas, which would only cause more problems, as compared to simple drop tanks. If the extra weight and aerodynamic penalty of a CFT means that the LCA can't carry as much payload or becomes less agile, then the CFT doesn't make sense. The freeing up of the limited number of pylons is, of course, a significant potential advantage.

Thus, I think it is something that they'll probably be figuring out right now.

In general, I'm not sure CFTs would be of much use for a light fighter whose role mainly calls for CAP (in spite of the 'multirole' tag), and thus, wouldn't have too much range requirements, which is sufficient with the present fuel levels (per what Cmde Balaji and PS told us last time).


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PostPosted: 01 Feb 2011 19:38 
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In addition, I would also like to point out that an aircraft (especially modern combat aircrafts) are very very different from automobiles. Which is why, while earlier, a lot of auto tech would come into aircraft and vice versa, and a lot of auto companies were involved in aircraft mfr (BMW comes to mind - note their logo), the same is not the case nowadays.

Firstly, cars are designed to be INCREDIBLY stable, which ALL modern combat aircraft today are designed to in fact incorporate INSTABILITY. Thus, the instability that occurs due to a CG location would be advantageous for the designers to incorporate RSS (relaxed Static Stability). The LCA has roll axis instability, which would probably explain the wing location. The CoG and CoP in the LCA are at about (IIRC, but might be completely bad memory on my part) 16-20% of the body length apart.

Also, where exactly would flywheel energy storage be useful in the AC, given that it'd be far easier to maneuver using the control surfaces, and that the energy needed can easily come from the engine or the APU itself.


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PostPosted: 01 Feb 2011 20:36 
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^^
K Prasad ji,
Are you visiting Aero India this year? You were a goldmine of information during the last one. :D


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PostPosted: 01 Feb 2011 20:50 
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cars and aircraft still share a huge amount of aerodynamics and structural mechanics knowledge, less so in control systems, but still some in on board electronics. that said, they are different beasts


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PostPosted: 01 Feb 2011 22:17 
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Amit J wrote:
=============================================================
Background articles on HAL Tejas (LCA)


2.Remembrance of Aeronautical Matters Past (Brief history of India's Aerospace Industry)
http://vayuaerospace.in/Selected_articl ... brance.htm

6.Good background on project, a bit dated.
http://www.geocities.com/spacetransport ... t-lca.html


Both links are dead.


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PostPosted: 01 Feb 2011 23:18 
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Arya.

1. The fundamental mistake in your description is that pitch axis doesn't pass through the CG!

In reality the pitch axis passes through the CG. A plane in air is not tethered so if you create a moment it will always turn around its CG. Therefore all the three axises of freedom (has to) pass through the CG.

When you are in level flight the center of pressure (lift) passes through the CG (stable plane) or alternates quickly around the CG (unstable plane) to have zero moment. If you are turning the center of pressure shifts and your moment is as you learnt in elementary physics force * lever arm. This lever arm is distance between CG and CP. It is that simple.

So you see flywheel and aircraft do not follow the same dynamics as a flywheel is tethered while a plane is not.

2. Tejas is not a high wing aircraft. Actually none of the modern fighters have that as it is too stable. There are planes which have shoulder mounted wing like the F-22/J-20. LCA is a mid wing aircraft.

3. In any inside loop, immaterial of the plane, all the planes loose altitude at the exit of the loop . It has nothing to do with the placement of the wing. It is very simple physics of gravity. If there was zero gravity you would have a perfect circle. But with gravity there is always a net effective downward acceleration at all times.


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PostPosted: 02 Feb 2011 10:34 
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I've been trying to upload these photos on Br galleries, but can't log in,
anyhow, for record here they are


LCA Levcons

and

Naval LCA Arrestor Hook Assembly


Edit: Thanks Pratik saar


Last edited by jamwal on 02 Feb 2011 11:30, edited 2 times in total.

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PostPosted: 02 Feb 2011 13:53 
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re loop mechanics
at the start (normal aircraft begin loop from a dive but lets assume otherwise) the aircraft is straight and level. the pilot pulls stick back and holds it back. elevators go up, centre of pressure moves ahead of centre of gravity and a nose up pitching moment is created (and maintained)
alpha goes up a little and both lift and drag increase
w.r.t. aircraft: lift acting up, drag back, thrust forward, weight down
as nose goes up and a/c starts to climb, weight vector starts to rotate and acts against thrust - slowing the a/c down (plus there is additional drag)
a/c continue into vertical - here alpha, lift and drag are maintained, but now thrust and weight are acting against each other (without more thrust the a/c will decelerate rapidly)
a/c continues past vertical and now weight starts to reduce against thrust (one component now being balanced by lift) - COP still ahead of COG and a/c continue to pitch nose up and around the loop
at the top of the loop, a/c is inverted - lift and weight are acting downwards, thrust and drag are balancing in line : a/c has maximum potential energy and least kinetic energy; with low airspeed, lift and drag are reduced
- to maintain inverted flight, the pilot has to push stick forward and create a nose down pitching moment and push the aircraft up w.r.t. the ground (and ofcourse apply thrust)
however, once past the top of the loop, weight now acting with thrust and pulling the a/c down - airspeed rapidly recovers, restoring lift and drag
by continuing the nose up pitching moment, the a/c comes around the final quadrant with potential energy reducing and kinetic energy increasing, airspeed increasing rapidly and continued nose up moment bringing a/c back to the start of the loop

in practise, light aircraft begin a loop by diving, building up kinetic energy (airspeed) to haul it up through the vertical and into inverted and then 'falling' down the other side and recovering from a dive

fast jets have tons of thrust and can go vertical from straight and level - some can happily maintain vertical flight until atmospheric pressue drop leads to thrust drop off and weight takes over

the airshow manouevers were fast jets go up into a half loop and roll off the top ito level flight is a demonstration of TFTA power availability - i.e. i have enough power reserves to go into a very low energy state and still recover using power alone

(more knowledgeable folks can correct my mistakes)


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PostPosted: 02 Feb 2011 15:59 
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Lalmohan wrote:
the airshow manouevers were fast jets go up into a half loop and roll off the top ito level flight is a demonstration of TFTA power availability - i.e. i have enough power reserves to go into a very low energy state and still recover using power alone


Absolutely. Those aircraft doing a "roll over at the top of a loop" are showing how control is retained at the point of least kinetic energy at the top of the loop. If airspeed bleeds to critically low levels at the top of a loop the plane will stall and fall out of the sky and many aerobatic aircraft in shows demonstrate exactly this maneuver because it is so spectacular. But it requires adequate altitude to recover.

One a powered plane has enough energy to be flying inverted at the top of the loop - it is no longer fighting gravity and will accelerate and if the nose points down for the third quadrant of the loop the velocity and kinetic energy will both increase. That means that the plane will have more kinetic energy in quadrant 3 than in quadrant 2 unlike the diagram in Arya Sumantra's post that describes a plane in quadrant 2 as "agile" and quadrant 3 as "sluggish". In fact the plane is decelerating while reaching the top of the loop and is becoming more and more sluggish. At the very top of the loop, in a horizontal position, the plane starts to accelerate so that at the beginning of quadrant 3 it will have more KE than the end of quadrant 2 making it more agile and less sluggish.

As far as I know the position of the CG wrt to the wing origin (high or low) has little effect in the pitch axis and has a bigger effect in the roll axis. IOW the "high wing" and "low wing" business affects roll performance more and not pitch stability and would IMO make little difference to the performance of a loop.

Some beautiful loops in formation by the Surykirans with no indication of any tendency to mess up..

Best watched full screen

http://www.youtube.com/watch?v=XmWwkHP9yyA

Wrong thread.


Last edited by shiv on 02 Feb 2011 16:19, edited 1 time in total.

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PostPosted: 02 Feb 2011 16:11 
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shivji - wrt wing position there will be a tiny moment difference (COP/COG) but very marginal in the pitch axis, and offset by other considerations of drag, roll stability, clearance, etc.


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PostPosted: 02 Feb 2011 17:04 
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Lalmohan wrote:
shivji - wrt wing position there will be a tiny moment difference (COP/COG) but very marginal in the pitch axis, and offset by other considerations of drag, roll stability, clearance, etc.



I am sure you are right - but the real difference may be in the roll axis where I recall reading that a CG below the wing level (with high placed wings) tends to stabilize the a/c in the roll axis as it acts like a pendulum.


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PostPosted: 02 Feb 2011 17:20 
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OT rejoinder/

Lalmohan: In a loop it is not necessary for the C of P to move ahead of C of G. If the C of P moves ahead then the pilot will not have to apply any further elevator up control to continue pitching. C of P moving ahead will add to the normal nose up couple provided by the thrust drag couple thus making the the aircraft unstable. In a pilot controlled aircraft the pilot will then have to apply 'down' elevator input to prevent the aircraft from stalling. This does not normally happen. In an aerobatic aircraft there is always enough elevator control available to take the aircraft around in a loop. If the aircraft is powerful enough one would not even need to dive and gather speed before the maneuver. (A spitfire could perform a double roll off the top from level flight!)

/OT rejoinder


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PostPosted: 02 Feb 2011 17:34 
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ok, maybe i'm being dense, but without further elevator input (to maintain pitching moment) will not the aircraft continue in a climb? increasing alpha without the kinetic energy to do a loop will definitely result in a stall


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PostPosted: 02 Feb 2011 20:37 
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Arya Sumantra wrote:
Q1. My contention is that Tejas has CG below the pitch axis (Right/Wrong)?

Q2. The pitch axis in Tejas could be shifted lower if the wing was located below the fuselage instead of above.( Right/Wrong )?

Q3. Intuitively it seems easier to turn in a direction if both CG and Centre of Revolution were on the same side of wing/pitch axis. (Right/Wrong)?

Q4. My contention is that most dogfighters and nowadays multi-roles which need good alpha belong to the case 2 below. (Right/Wrong)?

Q5. Theoretically, the most manoueverable plane is the one in which (1) all the three axes- Roll, Pitch and Yaw intersect in 1 point (2) Centre of gravity lies at this point of intersection A(stable planes) or above it B(unstable planes). (Right/Wrong)?


Disclaimer: I am no aerodynamics guru. But I have some information picked up from here and there

Quote:
Q1. My contention is that Tejas has CG below the pitch axis (Right/Wrong)?


I don't know.

I do know that the CG of the Tejas is behind the pitch axis causing it to be unstable in the the pitch axis with a "natural" tendency to pitch up and go into high angle of attack (High alpha). This helps aircraft agility. Of the three axes (roll, pitch and yaw) - roll rate is better with an anhedral wing which the Tejas has. It appears to me that the Tejas has a great roll rate in videos - enough to make Pakis think I have speeded up my videos which I have not.

Pitching up is aided by the pitch instability - so technically the Tejas should be able to pitch up and roll away really quick.

But I have no idea whether the CG is above or below the pitch axis. I do not think that this will make a significant difference.

Quote:
Q2. The pitch axis in Tejas could be shifted lower if the wing was located below the fuselage instead of above.( Right/Wrong )?


Yes , but the CG too would shift lower.

Quote:
Q3. Intuitively it seems easier to turn in a direction if both CG and Centre of Revolution were on the same side of wing/pitch axis. (Right/Wrong)?


I did not understand this.

Quote:
Q4. My contention is that most dogfighters and nowadays multi-roles which need good alpha belong to the case 2 below. (Right/Wrong)?

If I get pedantic, this question is an oxymoron. A "dogfighter" would be an agile air combat aircraft, not a multi-role aircraft.

What do you mean by saying "Multi roles need good alpha". Good alpha, I assume is the ability to fly at a high angle of attack. That is needed for slow speed flight and for landing an aircraft.

Quote:
Q5. Theoretically, the most manoueverable plane is the one in which (1) all the three axes- Roll, Pitch and Yaw intersect in 1 point (2) Centre of gravity lies at this point of intersection A(stable planes) or above it B(unstable planes). (Right/Wrong)?


The most maneuverable plane is the most unstable one in which the center of lift and the CG are far enough apart for the plane to want to violently change direction.


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PostPosted: 02 Feb 2011 20:59 
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shiv wrote:
I do know that the CG of the Tejas is behind the pitch axis causing it to be unstable in the the pitch axis with a "natural" tendency to pitch up and go into high angle of attack (High alpha).

Pitching up is aided by the pitch instability - so technically the Tejas should be able to pitch up and roll away really quick.

But I have no idea whether the CG is above or below the pitch axis. I do not think that this will make a significant difference.

Shivji the pitch axis of any plane passes through the CG of the plane. A plane always rolls/pitches/yaws about its CG. In Tejas the CP is ahead of the CG. The rest of your explanation is right.

Quote:
Q5. Theoretically, the most manoueverable plane is the one in which (1) all the three axes- Roll, Pitch and Yaw intersect in 1 point (2) Centre of gravity lies at this point of intersection A(stable planes) or above it B(unstable planes). (Right/Wrong)?

A plane always rolls/pitches/yaws about its CG. So all the three axes passes intersect at the CG.


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PostPosted: 02 Feb 2011 21:15 
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i think you would want unstable pitch up and rapid roll for high instantaneous turn rate (without loss of height) - which is what you would need in A2A, then followed by sustained turn rate - although here deltas tend to generate high drag and slow down very fast


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PostPosted: 02 Feb 2011 22:09 
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indranilroy wrote:
Shivji the pitch axis of any plane passes through the CG of the plane. A plane always rolls/pitches/yaws about its CG. In Tejas the CP is ahead of the CG.


OK thanks for setting that straight.


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PostPosted: 03 Feb 2011 09:57 
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Hmm. A yellow colored LCA in the air right now, did a wide near 180 deg turn from S.E to South over the city. Is a new LSP in the air ?

Also, after the "Aiyoo -See" I dont see the jehaz anymore. Have the air force boys started flying the plane on a regular basis now ?


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PostPosted: 03 Feb 2011 10:04 
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Did you get to see if it was a twin seater or single seater or if its nose was pointing a little towards the earth ?


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PostPosted: 03 Feb 2011 10:32 
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We had several pages of discussion about the unstability of LCA many years ago.

A plane in level, stable flight have the following forces acting upon it.

1. Engine pushes it forward. The force acts at the CG
2. Gravity pulls it down. the force acts at CG
3. A combined aerodynamic force pushes it up. In a stable flight, this acts at a point vertically above the CG.
4. A combined aerodynamic force pulls it back. In a stable flight, this acts at a point horizontally behind the CG.

When you come to unstability, there are a few misconceptions that gets around a lot.

1. It is a misconception that lift acts either ahead or behind the CG. Lift always acts at the same vertical line as CG in level flight. It will be unstable if acting below the CG, and stable if acting above the CG. If unstable, it is possible to dynamically adjust the lift vector to fall exactly through the CG, just like you can balance a stick on its end on your finger.

2. It is a misconception that the CP is same for both pitch and yaw. Each surface creating a drag have its own vector. The net effect of this vector can be resolved into an orthogonal pair, one for pitch and the other for yaw. For stable flight, both should be pass through the CG, but their point of action would be different. In general, the yaw cp would be to the aft, because the surface that drags more, viz the tailfin, is to the rear. The pitch CP would be more forward, because of the wing.

Now, the craft would be unstable if the CG is behind either of the CPs. Since pitch CP is rather ahead, it is easier to make that unstable. If you want to make yaw unstable, you will have to build a big dorsal fin forward. The dynamic stability is achieved by adjusting the CP vector to fall in the same line as the CG.


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PostPosted: 03 Feb 2011 11:44 
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An LSP circled above and around the airport twice about 30 minutes ago. Not a prototype, not a trainer. Painted Grey.


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PostPosted: 03 Feb 2011 17:22 
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vina wrote:
Hmm. A yellow colored LCA in the air right now, did a wide near 180 deg turn from S.E to South over the city. Is a new LSP in the air ?

Hmm - that is interesting news. Is it possible for planes to "shed their paint" in spring like animals might shed hair? If not this is a new one in the air.


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PostPosted: 03 Feb 2011 17:25 
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Has Chacko taken the day off? No news yet!


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PostPosted: 03 Feb 2011 18:46 
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Indranil Roy wrote:
1. The fundamental mistake in your description is that pitch axis doesn't pass through the CG!

In reality the pitch axis passes through the CG. A plane in air is not tethered so if you create a moment it will always turn around its CG. Therefore all the three axises of freedom (has to) pass through the CG.

CG location is a function of mass distribution alone

According to Wikipedia, the axes are defined wrt geometry of the plane eg Roll axis from nose to tail, pitch axis from wing tip to wing tip, ok yaw axis is vertically passing through CG.

There is no obligation for CG to be in line with geometrically defined axes in a fighter. You can create mass distribution any way you want inspite of a given geometry. That a CG lies on point of intersection of roll, pitch and yaw axis is because designers have ENGINEERED it to be so in a stable yet manoueverable plane. It is not a default, imho.

Given that understanding, when your CG is ENGINEERED such as to lie above the point of intersection of roll, pitch and yaw axes you should get a metastable equilibrium. A top-heavy fighter cannot be stable unless the pitch axis too is on the top (high wing fighter).

Due to my limited aerodynamics knowledge, it is possible that I might be wrongly using pitch axis in my explanation where I should perhaps be using Aerodynamic center which is defined distinctly than center of gravity (http://en.wikipedia.org/wiki/Aerodynamic_center )

Shiv wrote:
I do know that the CG of the Tejas is behind the pitch axis causing it to be unstable in the the pitch axis with a "natural" tendency to pitch up and go into high angle of attack (High alpha).

That runs counter to what Indranil is saying
Indranil Roy wrote:
In reality the pitch axis passes through the CG. A plane in air is not tethered so if you create a moment it will always turn around its CG. Therefore all the three axises of freedom (has to) pass through the CG


Lalmohan wrote:
cars and aircraft still share a huge amount of aerodynamics and structural mechanics knowledge, less so in control systems, but still some in on board electronics. that said, they are different beasts


Don’t take the analogy of flywheel for literal similarity. It is just representative for modeling an aspect of behaviour. As an example, the mechanical deformation of viscoelastic materials such as plastics is modeled by circuits in which springs and dampers are attached in series and parallel combinations.

shiv wrote:
I did not understand this.

I was just saying intuitively it seems that if CG and center of revolution of loop were on opposite sides of the wing, then CG would unfavourably exert inertia in rotation of fighter adding to the drag.

Lalmohan wrote:
you dont account for lift and drag, aircraft have to balance weight with lift and thrust with drag
centre of lift vs centre of gravity is your key issue in pitch (moment wise)
also lift and drag are non linear with alpha
thrust is usually used to counteract drag during high alpha manouevers


I was only explaining what HELPS in the turning. The control surfaces do their part in turning while thrust is provided by engine. But it is the simultaneous tipping of the CG at the top-heavy plane that is helping the fighter make a TIGHTER turn than what it would if purely the control surfaces were working.

Smart systems use their own weight to their advantage and that’s what this is about. If your CG is located low it becomes additional drag on the thrust provided by engine while turning. If the CG is located high it becomes additional thrust OVER AND ABOVE that provided by engine to counter the drag while turning.

The moment arm between the axis and CG may be small but the entire weight of aircraft acts on CG and that makes the tipping moment huge.

All this may not too difficult to verify. Just invert the MK1 with a roll manoever and try a vertical turn in inverted state and measure the alpha achieved.

Lalmohan wrote:
your 4th quadrant arguement doesnt make sense to me, at that stage, the a/c has plenty of potential and kinetic energy and does not need additional thrust to 'complete the quadrant'


It happens because pilot’s action of making his plane level wrt ground is lifting his CG higher. That’s where the inertia to rotation is coming from. I have not been able to find ANY video where a fighter made tightest possible 1st and 2nd quadrant and then after completing 4th quadrant came to a position higher than original. If you happen to find one please let me know.

Indranil Roy wrote:
So you see flywheel and aircraft do not follow the same dynamics as a flywheel is tethered while a plane is not.


The centre of the flywheel is a moving reference frame about which its CG revolves. Simiarly, the pitch axis in a metastable fighter is a moving reference frame about which the fighter rotates(CG revolves about PA) 360 degrees upon completion of a vertical loop. With respect to each other the pitch axis and CG separation is fixed due to construction that way. So why do I consider pitch axis as a reference frame and not CG ? Because it is defined with respect of geometry of the plane. Pilot’s actions are to bring the geometry of the plane level or perform a loop. If he makes a plane level, he brings wings parallel to the ground even if that takes CG higher. When he makes the plane loop, he makes the wings loop. He does not think about CG, although the mechanics acts on it.

Indranil Roy wrote:
2. Tejas is not a high wing aircraft. Actually none of the modern fighters have that as it is too stable. There are planes which have shoulder mounted wing like the F-22/J-20. LCA is a mid wing aircraft.


So you agree that high wing aircrafts are stable.

LCA has somewhat high to mid wing. It is inclined. The incline itself works like a fixed control surface trying to depress the plane adding the drag during the vertical turn. Although angle of inclination is small but length of wingroot is long which makes it span upto the mid. And the undercarriage is heavy in MK1. In most likelihood MK1 has CG below the pitch axis and has ENDED UP being stable

k Prasad wrote:
However, I will point out that the Rafale does have a wing over the inlets (in effect, a kinda high wing

and
Indranil Roy wrote:
There are planes which have shoulder mounted wing like the F-22/J-20.


If you look at F22 and Rafale from front you can see the mass-distribution tapers towards the bottom. The apparently equal mass distribution due to seemingly equal bulkiness above and below the wing is an ILLUSION. The intakes underneath are HOLLOW and in case of 5th gen fighters the internal bays at the bottom are EMPTY for a clean fighter(I am only calling clean fighters as metastable). Even the internal fuel is on upper side.

Image

They would have considerations others than alpha for not having the wings completely at the bottom also depends on how much optimum instability they need the plane to have. They have a high thrust engine, so would not want too much help from tipping moment lest g forces exceed too much. Aircrafts is about tradeoffs after all.

k Prasad wrote:
Firstly, cars are designed to be INCREDIBLY stable, which ALL modern combat aircraft today are designed to in fact incorporate INSTABILITY. Thus, the instability that occurs due to a CG location would be advantageous for the designers to incorporate RSS (relaxed Static Stability).
[/quote]

That is exactly what I had said. It has gotten too much into our psyche that CG is supposed to be kept low. Here we want fighter to be unstable.

In my humble understanding of Relaxed Static Stability, it is nothing but a sophisticated name of maintaining the metastable state of CG above the pitch axis by USE of FCS intervention in a level flight.

Image

It is understandable that a metastable CG at position Q would drift between point P and point R from time to time in a level flight with CG trying to lower itself and minor disturbances helping it. Now when it drifts to point P, the structure of aircraft along with its control surfaces has inadvertently rotated by a small angle to shift centre of pressure to position R. FCS intervention through control surfaces brings centre of pressure to the front to keep the flight level .

Next when CG drifts to point R, the inadvertent rotation of aircraft structure has shifted centre of pressure to point P. FCS reacts to this through control surfaces movement to bring the centre of pressure behind to level the flight again.

These oscillations of a CG between P to R cause the fighter to move in a simple harmonic “bounce” about the NET direction of level flight. But the wavelength(lambda) of this trajectory is so long and the amplitude of bounce so small as to be not perceptible to the pilot.

imho, As far as using the body weight to create instability is concerned, the shifting of CP to the front and behind of CG is a CONSEQUENCE of CG’s oscillations P-Q-R and not the main root action leading to instability.

JMT


Last edited by Arya Sumantra on 03 Feb 2011 19:03, edited 1 time in total.

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PostPosted: 03 Feb 2011 19:02 
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you are still assuming the aircraft is tethered - and your forces are acting on the tether point
the way to think about flight mechanics is that the COG is the 'tethering point' - all motion is relative to it


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PostPosted: 03 Feb 2011 19:35 
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Lalmohan wrote:
you are still assuming the aircraft is tethered - and your forces are acting on the tether point
the way to think about flight mechanics is that the COG is the 'tethering point' - all motion is relative to it


That is equivalent to saying that relative position of control surfaces and their geometry and location has no role in deciding the axis of fighter rotation. Irrespective of mass distribution, the segregation of centre of aerodynamic form from centre of mass distribution is simply not possible !!!

Surely you can take COG as the reference "tethering point" but then our tipping moment talked about will appear as another moment acting on centre of aerodynamic influence on geometrical form of fighter. Take whatever reference just make sure any moment isn't missing.

Fluid mechanics acts on the geometrical form that is fighter, Gravitational mechanics acts on CG. Expecting to centre of influence for both to be the same is hard to believe. If it was space then what you said would have been understandable.

JMT


Last edited by Arya Sumantra on 03 Feb 2011 20:05, edited 1 time in total.

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PostPosted: 03 Feb 2011 20:03 
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Arya Sumantra - please don't take my comment amiss - i don't mean to be insulting.

Your posts are not understandable because you add a lot of math/physics gobbledygook that is fundamentaly difficult to understand for the average Abdul. It is difficult to confirm or deny what you say when it is barely understandable. That may not be your fault but if you are saying something new and clever nobody is going to understand it.


Regarding the statement quoted below I will post a link and a quote that is dead easy to understand.
Arya Sumantra wrote:
imho, As far as using the body weight to create instability is concerned, the shifting of CP to the front and behind of CG is a CONSEQUENCE of CG’s oscillations P-Q-R and not the main root action leading to instability.



Carlo Copp
http://www.ausairpower.net/AADR-FBW-CCV.html
Quote:
Artificial Stability and the Fighter.

One of the most basic requirements for a fighter aircraft is manoeuverability. Prior to the age of supersonic aircraft, the problem of finding the optimal amount of stability was fairly straightforward, however the necessary aerodynamics for supersonic flight began to introduce complications. Modern supersonic aircraft tend to have thin wings with relatively short span. This alters the mass distribution in the aircraft, most of the mass is distributed along the fuselage, increasing moments of inertia in pitch and yaw.

The result is an aircraft very easy to roll, if not unstable in roll, but reluctant in pitch and yaw, which is hardly desirable for tight manoeuvering. An illustration of that class of aircraft would be the F-105 Thud, quote "...it doesn't turn very well.... it rolls beautifully ...".

One possible solution to this problem would be lowering the aircraft's static stability, particularly in pitch. This can be achieved by shifting the centre of gravity aft of the centre of lift, thereby creating a nose-up pitching moment, which will assist in rotating the aircraft's nose into a tight turn. However, the resulting loss in stability must be countered and this is the task of artificial stability.


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PostPosted: 03 Feb 2011 20:24 
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aryaji
the problem is that gravity is one of four forces that an aircraft is subject to
the others being lift, drag and thrust - for stable flight all four have to be in balance - however that balance is achieved
the CG stays more or less fixed, it moves with changing weight in the aircraft, e.g. through fuel burn or stores drop off
lift, drag and thrust act on the CG and not gravity on the COP
think of balancing a model aircraft on your finger tips...


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PostPosted: 03 Feb 2011 21:41 
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Aryaji
Quote:
That is equivalent to saying that relative position of control surfaces and their geometry and location has no role in deciding the axis of fighter rotation.

Like it or not this is the fact.
I will try once more to explain this very simple thing. Think of a plane in the air. If you create a moment, what will be the fulcrum? If you think it through (use the simple physics of an inertial frame of reference on the plane) you will be see that this fulcrum is the CG. Hence all the axises pass through the CG, not through the CP or any other point. The shape and size of the plane is immaterial here. That is why if you do an image search of the aerodynamic axes, you will always find them passing through the CG. The shape/size/location of the wing has nothing to do with pitch axis or any axis for that matter.

When I said a high mounted wing is more stable, it was not about the pitch axis. Please read about dihedrals and anhedrals and then we can discuss more. Wiki gives quite a good lesson on it. It will answer your question on why the wings on fighter aircrafts generally have anhedrals. However, the Tejas wing is much more complex. It doesn't even have a straight chord at the wing join. This provides a huge washout and a very greatly stabilizing effect. To cancel this out the anhedral on the Tejas is quite big.

Also "aerodynamic center" is for an aerofoil. For a complete analysis of the plane, the center of pressure is the more appropriate thing to be looked at. Between I would suggest you to read about longitudinal static stability.


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PostPosted: 04 Feb 2011 00:02 
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Some photos of LCA with smokewinders at the Tarmak007 site. Click on link below to view the images

Tejas aerobatic display with smokewinders @ Aero India

Quote:
Here are the snapshots of India's Tejas flying with smokewinders ahead of Aero India. The first flight with smokewinders integrated was flown in Bangalore on Feb 2. Military sources tell Tarmak007 that two Tejas platforms (PV-2 & PV-3) will be demonstrating aerobatics on all 5 days of the show. On the first day, during the inaugural fly-past event, a Tejas formation (likely 5) will be the icing on the cake. Starting today, Tarmak007 will bring to you more exciting stories of India@Aero India.


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PostPosted: 04 Feb 2011 06:31 
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Interesting colour of the SPs.

link

Image

Can somebody explain how should I scale images down?

P.S. Thank you Putnanja ji.


Last edited by indranilroy on 04 Feb 2011 07:10, edited 2 times in total.

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PostPosted: 04 Feb 2011 06:57 
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deleted , duplicate ...


Last edited by putnanja on 04 Feb 2011 08:18, edited 1 time in total.

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Quote:
That is equivalent to saying that relative position of control surfaces and their geometry and location has no role in deciding the axis of fighter rotation. Irrespective of mass distribution, the segregation of centre of aerodynamic form from centre of mass distribution is simply not possible !!!


indranilroy is correct --- this is absolutely the case. The distribution of the control surfaces and their geometry only determines the *forces* that act on the body. The *motions* of the body that result from these forces are determined by its inertial tensor (i.e., the distribution of mass on the body). All rotations will necessarily occur about the center of gravity.


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PostPosted: 04 Feb 2011 09:02 
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Raman wrote:
All rotations will necessarily occur about the center of gravity.


Yes and in fact the error I made in one of my statements is related to this and indranilroy pointed that out and corrected the error. (Not) funny how this basic fact escaped me. The plane will only move about its CG and stability and instability are related to the net position of forces acting on the plane.

If net lift is in front of CG the plane pitches up and vice versa.

If net lateral force is ahead of CG, the plane yaws in the direction of the force. And vice versa.

If net vertical force is off center and off to one side of the CG - the plane rolls in the direction of the vertical force.

In other words - it matters little if the CG is above or below the wing. The movement is about the CG in any case and the CG does not swing around a pivot like an eccentric flywheel.


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PostPosted: 04 Feb 2011 09:43 
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Kanson wrote:
Amit J wrote:
=============================================================
Background articles on HAL Tejas (LCA)


2.Remembrance of Aeronautical Matters Past (Brief history of India's Aerospace Industry)
http://vayuaerospace.in/Selected_articl ... brance.htm

6.Good background on project, a bit dated.
http://www.geocities.com/spacetransport ... t-lca.html


Both links are dead.



The links were provided by Rahul M kindly check with him and do a google all the same


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