The Tejas LCA has been designed to be aerodynamically unstable in the longitudinal axis to obtain improved maneuverability and agility over the entire flight envelope and hence, has to be stabilized artificially by the use of activecontrol technology.
entire flight envelope means like rafale it is unstable even in supersonic flight!!!
It will be nice if knowledgeable posters can contribute meaningful comments on this aspect and correct mistakes if there is any.
Even typhoon reverts back to stable flight profile after supersonic speeds are crossed.
How many planted news carrying DDMS fake defence analysts report on such aspects of the program?
The flight control system is a digital, quadruplex redundant fullauthority system exercising control through two sets of paired elevons and a singlerudder. The stability and control cycle is updated every 12.5 milli secs through highfidelity, rate and acceleration sensors and high rate control actuators.
Tejas instability isdefined by 'time to double amplitude' and is one of the lowest amongst contemporary acin the world. Graph at Fig-2 depicts this value across a Mach vs altitude scale.
Theregion from 0.5M to 0.7M and from 3Km to 8 Km is the zone of the highest instabilitywith time to double amplitude dropping to 200 milli secs. This implies that anydisturbance in pitch would cause an increase in amplitude by 32 times in a sec
Wind tunnel experiments haveindicated that CL max continues to improve till approx 35 deg AOA.
This was confirmed by the test pilot Suneeth krishna also
However, directional characteristics indicated the proverbial 'cliff' with a sudden drop inCn(beta), CRM (Coefficient of Rolling Moment) and CYM (Coefficient of Yawing Moment) at approx 25 deg.
AoA as shown at fig-4 and 5. These phenomena require the High AoA trialsto be limited to 24 deg.
It has already been tested to 24 degree AOA. ANd the experimental LSP-6 will go beyond this.
Fortunately as shown in fig-6, the LCA has significant rudder authority (CYM-Del R) even up to 30 deg.
AoA that will allow artificial stabilization in yaw at high AoA.
This is what going to be tested on LSP-6
Overall Test Philosophy.
Modern fighter aircraft are designed to be flown near their maximum performance limits to capitalize on advantages over other aircraft.This can result in departures from controlled flight. A pilot rarely enters out-of-controlflight intentionally.
When it does occur, it's usually in a dynamic and disorienting manner. Due to the normally forgiving handling qualities of the Tejas and its contemporary aircraft, OOCF incidents are quite surprising to the pilot and challenging,violent and unexpected motions can be encountered.
This testing must therefore be extremely thorough so that hidden traps are not passed on to the user where costs are
However, a full investigation of AoA characteristics of a high performance ac including spinning is a very high risk, high cost and time consuming proposition.
Considering the large number of external stores configurations and theneed to clear a two seater version, the test campaign could well take a few years ofwork up, flight test, analysis, re design and certification.
Given the Tejas' flat, fast and oscillatory nature of predicted spin modes, the sharp cliffs in critical stability derivatives and the internationally
evolving philosophy of testing departure prevention rather than post departure behaviour,
it was decided that the ac would not be intentionally spun.Departure prevention, rather than spin recovery, will form the basis of test philosophywith full preparation for an OOCF event and its recovery.
In this regard it may berecalled that in the recent CFH test campaign of the BAe Typhoon II ac not a single departure was encountered and the spin chute was never used,although the ac was subjected to every combination of OOCF causing events.
. Further, although several spin recovery techniques have been identified in the vertical tunnel experiments,
no recourse to a "piloted" recovery will be made and the spin chute will be used immediately on departure to protect hydraulics and provide fastest possible recovery
Detailed and extensive analysis of the high angle of attack database indicates that LCAhas very few stable spin modes.
Further, the airframe exhibits fast flat spin modes withthe trajectories in general being oscillatory and unstable making it extremely difficult topredict the aircraft behaviour beyond stall.
The general qualitative observations were as below.
(a) Angle of Attack 57 – 85 deg(most spins in the range of 70-80 deg.
(b) Angular Rate 0.7 – 2.8 rad/s(most spins around 2 rad/sec)
(c) Turn Duration 2.3 - 8.3 sec(most spins around 3 sec)
(d) Rate of Descent 76 - 83 m/s