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Chinese mig-21 derivatives; affordable in crisis

Last post 10-18-2010, 12:26 AM by michael82. 2 replies.
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  •  10-12-2010, 7:12 AM 9658

    Chinese mig-21 derivatives; affordable in crisis

    any opinion on topic ?
  •  10-18-2010, 12:26 AM 9694 in reply to 9658

    Re: Chinese mig-21 derivatives; affordable in crisis

    One can find double delta J-7G and radicaly changed JL-9 MiG-21 derivatives interesting...

    Maybe article better describes family potential...

    Try to point at and shoot well flown MiG-21!

    Predrag Pavlovic, dipl.ing. and Nenad Pavlovic, dipl.ing, JAT Airways

    Maneuverability of modern fighter is measured by how slow it can fly and how high

    angle of attack it can sustain and still turn. During some war situations, US evaluation

    and Aggressor use, MiG-21 has shown it can keep pace with modern planes in this

    area. Aircraft manufacturer at one time considered this irrelevant and imposed

    restrictions on angle of attack. Flying above allowed 28-33 degrees local angle of

    attack at low speeds makes possible to relatively safely achieve a maneuverability once

    considered privilege of modern fighters.

    Couple years ago reports and testimonies appeared in the media about a dogfight during

    the Israeli-Arab War '73. when the Egyptian MiG-21 pilot managed to do a Split-S

    maneuver at the start altitude of 3000 feet, less than half minimum airspace the manual

    says (about 6750 ft). Appropriate simulation can be found on the internet:

    http://www.youtube.com/watch?v=bQMzK2WfYYM&feature=player_embedded

    Figure 1.

    2

    Initiated by this event, some American and Israeli enthusiasts (once pilots of their AF

    fighters), one of which has a private squadron of various Russian fighters, attempted to

    replicate that minimum altitude needed to complete Split-S figure in the two-seater MiG-

    21. Previous consultation with Israeli ace, who participated in that dogfight in '73. war,

    did not help test to be successful. Attempts were carried out at the higher altitude (5 km)

    and the height loss during the figure was in accordance with flight manual. It remained

    unclear whether ’73 event was result of "special skills or superhuman strength of the

    Egyptian pilot needed to withstand the required g-loads”.

    Recently disclosed files of the official MiG-21 evaluation in the U.S. revealed some

    unexpected capabilities that can be correlated with the "inexplicable" ’73. maneuver.

    MiGs were brought to America via Israel, in the late '60s as a result of pilot error or fled

    from Iraq and Algeria. Later they were bought from Indonesia. The MiG-21 in the U.S.

    Air Force is designated YF-110.

    The report of a MiG-21F shows nothing particularly unusual, except for maneuvering

    capabilities and behavior/handling at low speeds described as "class above competition”.

    Besides that, if competitors tried to follow MiG-21F at high alpha, their engine

    experienced shutdown or compressor stall. MiG could perform "hammerhead" turn (wing

    over/stall turn/renversement) at 100 knots (knot = 1.853 km/h), figure where at the end of

    the vertical climb pilot add rudder (with the opposite aileron and forward stick) to push

    the plane in the dive. Rudder is effective from 30 knots. With the stick fully backward,

    the plane flies at 210 km/h, the rolling oscillations are present, but there is no lift

    breakdown or the tendency towards spin. If during the evaluation, loss of control due to

    uncoordinated controls occurred, it was in the form of roll-off (usually for 180°) instead

    of much more dangerous yaw-off. To put the plane back under control it was necessary

    only to release controls. MiG-21 proved to be docile, safer to fly than MIG-17. During

    the hundred flight tests engine compressor stall was never experienced.

    U.S. of course, used MiGs in dogfight evaluation against their aircraft. Latter, they

    formed "Aggressor" squadron of MiGs and other fighters for the dogfight simulation with

    regular American aircraft.

    3

    4

    Figure 2, 3. MiG-21 on testing in the U.S.

    During MiG testing, it was clear that U.S. pilots have not relied on Soviet pilot’s manuals

    or they did not have one at the beginning. That is why the aircraft ability was fully

    exploited. Test pilots had thousands of flight hours experience on dozens of types of

    aircraft. Those who have survived the testing of U.S. supersonic fighters

    F-100/101/104/4 (many of planes were called "widow makers"), learned to recognize the

    pre-stall/spin signs and use rudder for rolling the aircraft at higher angles of attack.

    5

    Figure 4. Some of the results of MiG-21 testing in the United States

    6

    Reportedly, if Vietnamese pilots had adequate training, the U.S. fighter shot-down ratio

    figures would be much worse in that war. In the hands of the well trained pilots, MiG

    would always outmaneuvered Phantom. US unveils graphs depicting not only far better

    instantaneous turn performance of Fishbed C compared to F-4D but also better sustained

    maneuverability. MiG-21 Aggressor pilots respected only the most modern fighters

    because they do not lose so much speed in turn even at low speeds. However, appearance

    of all-aspect infrared missiles reduced the importance of sustained turns (M2000, F-18E,

    Gripen …are not brilliant in the maintaining speed in turn). If MiG-21 had R-73 missile,

    it could easily take advantage of first shoot opportunity at close range against any new

    fighter.

    The F-5E, fighter which does not fly above Mach 1.5, MiG-21 simulator, reportedly has

    shade better subsonic sustained turn maneuverability, but inferior controllability at low

    speeds. Maneuverability is the ability to change speed and direction of flight path

    (velocity vector pointing) and controllability - ability of change aircraft attitude

    (pitch/roll/yaw - nose pointing) and thrust (engine response - spool up time matters).

    When the aircraft initial flight path in dogfight is anti-parallel flyby, combat will

    inevitably develop so that someone goes in a climb with rolling scissors - turn reversals

    along the opponent’s flight path to remain behind the opponent. If the F-5E does not gain

    an advantage before the speed drops below 200 knots, MiG will start winning. First look

    at the configuration of the aircraft, MiG – delta with the sweep near 60°, and Tiger with

    nearly straight wings, would suggest the opposite, that MiG is in trouble at low speed.

    Even the mighty F-15 Eagle had no solution in dogfight below 150-250 knots against

    MiG-21 in US Aggressor hands. At the beginning of dogfight, at the speed of 400-500

    knots MiG-21 will turn at max g loosing 70 knots per second, ending at the speed of 70

    knots in less than 90º of turn (deceleration of 3.5 g, more intensive than Harrier’s VIFF

    turn). Reportedly, no other aircraft can do that. This way MiG will remain behind every

    opponent still having sufficient controllability for gun tracking using rudder rolls.

    Opponents would think that at this speed MiG can only bring down the nose and dive, but

    the MiG at less than 100 knots has sufficient pitch authority to raise the nose at enemy. If

    F-15 tries to follow, ’21 should execute 'barrel-roll ' to remain behind the Eagle.

    It is obvious that MiG-21 'Aggressor' pilots pulled full aft stick in turn regardless of the

    lateral oscillations, roll-off and temporary loss of control.

    7

    Figure 5. Scissors maneuver

    8

    Latter, the F-15 pilots learned (in a hard way) not to accept maneuvering at slow speeds,

    not to allow to be drown into a series of turn reversals, but to withdraw and re-attack at

    higher speeds using 3D turns and it’s higher thrust/weight ratio. F-15 with 45º swept

    wing and low horizontal tail, at higher angle of attack becomes longitudinally

    superstable, so it can not achieve more than about 30º angle of attack.

    On the example of lift and stability of the aircraft with the 45º swept wing and high-set

    horizontal tail it can be seen that the lift begins to decline at 10º (buffeting starts), the

    wings are stalled at the 20º (the airflow separates from the wing), and max body lift is at

    35-40º after which it decreases. Delta wing of MiG-21 with sweep of 57 º retains stable

    airflow to very high angles of attack.

    Longitudinal stability is positive where the curve has a downward slope. In this case, the

    position of the horizontal tail is causing longitudinal instability at 15º, and at 35-40º angle

    of attack aircraft is trimmed without tail deflection. MiG-21 has no problem with

    longitudinal stability (except with air to ground armament with low fuel) and the plane in

    the example would have a limit at 15º angle of attack.

    Yaw stability curve shows that the aircraft is unstable at 15º, what is not uncommon.

    Few modern fighters are stable at over 20º, but it is not a problem if the aircraft maintains

    lateral stability i.e. roll due to yaw. Roll stability curve is increasing as the lift increase,

    so it similarly comes to the instability, in this case at about 20º angle of attack. Shall the

    plane have a tendency toward spin (at no deflection of the control surfaces!) show the

    curve of dynamic directional stability where factors are static yaw and roll stability along

    the inertial characteristics of the aircraft. In this example, the plane is at stall just above

    20º angle of attack, while MiG-21 is stable at well over 30º at low Mach numbers.

    Curves correspond to a particular Mach number, at some other speed they can vary

    significantly.

    9

    Figure 6. Example lift and stability of aircraft

    10

    Soviet training was based on a relatively small number of flight hours on a MiG, which is

    used for training the primary purpose of aircraft, interception of fighters-bombers, under

    ground control. Pilots are not encouraged to explore the flight envelope. The aircraft is

    designed to fly faster and higher. Slow speeds were irrelevant, except for landing. In the

    first combat manuals, the performance at altitudes only above 5 km were presented.

    Later, it turned out that there are many practical constraints due to which the projected

    max altitudes and speeds are rarely used.

    MiG-21 wing has no camber or twist along span. The relative thickness of the higher end

    of the wing than in the root. There are few prestall signs. Prestall buffet begins much

    earlier (at 50-100 km/h higher speed), its intensity is light and slightly decreases at higher

    α. Below Mach 0.4 buffet does not develop. Just before stall α, aircraft nose would start

    wandering accompanied by more noticeable wing rocking (roll oscillations that intensify

    thru the stall), symptoms of dynamic directional instability.

    Stalling proceeds more vigorously with fewer signs at higher subsonic speeds.

    Ailerons are ineffective in countering roll oscillations and rudder would push aircraft into

    a spin. Setting control surfaces to the neutral position immediately after the onset of stall

    would restore normal flight conditions. The aircraft is longitudinally stable in air combat

    configuration at any internal fuel quantity.

    Aircraft’s stall speed (speed at which dynamic directional stability breakdown occurs) is

    function of Mach number, because directional and lateral static stability usually decreases

    with speed. Stall angle of attack decreases from above 30º (far beyond indicated α) at

    Mach 0.2 to 20º (i.e. 33 units local angle of attack on indicator) at Mach 0.95.

    In those days when MiG-21 was designed, electronic flight controls to limit the angle of

    attack in function of Mach number didn’t exist. A fighter was built primarily for high

    speeds, high altitude interceptions. At slower speeds previous generations MiG-19/17

    were better.

    Designers put the angle of attack indicator, calibrated in local angle of attack, to warn the

    pilot of approaching stall limit. At recommended and allowed limit 28 units (about 17º

    true angle of attack) safety margin to stall is from 13º at Mach 0.2 to 3º at Mach 0.95.

    So there is large margin between allowed angle of attack and stall angles of attack

    especially at lower Mach numbers.

    At higher speeds, the angle of attack is limited by tail pitching power.

    11

    Mach number 0.2 0.7 0.8 0.95

    Stall angle of attack (α) > 30º ~ 25º ~ 23º ~ 20º

    Stall speed

    weight = 7500 kg

    233 km/h 254 km/h 260 km/h 267 km/h

    Speed at 33 units local α

    (~20º α )

    287 km/h 287 km/h 282 km/h

    267 km/h

    (stall)

    Speed at 28 units local α

    (~17º α )

    311 km/h 311 km/h 305 km/h 291 km/h

    So, the low speed turning capabilities were not fully exploited. If situation comes, like it

    happened to that Egyptian pilot during war, there is an additional lift potential.

    During the Split-S figure, speed should not be increased. The closer to stall α is, the lesser

    the altitude loss is during figure. Below 600 km/h CAS entry speed aircraft cannot

    aerodynamically reach the allowed structural load factor so there is no need for

    superhuman physical stress. At higher speeds height loss in split-S at stall angle of attack

    is much more than 3000 ft.

    12

    13

    14

    Figure 7, 8, 9.

    15

    Because of its very high stall angle of attack at lower Mach numbers and good pitch

    control authority (large wing leading edge sweep produces strong vortical flow which

    shifts aerodynamic centre forward at high alpha, reducing stability thus allowing the tail

    to easily trim aircraft at more than 30° alpha), aircraft has a great point and shoot

    potential with modern IR missiles.

    Although it is often said that the MiG-21 looses a lot of energy in turn, the truth is also

    that it has better sustained turn performance than most aircraft of its generation.

    Tumansky engine proved almost stall/surge free at speeds far below minimums quoted in

    conservative Soviet flight manuals. All U.S. and European contemporary designs flamed

    out under same conditions. Engine has two shafts for optimized - different rotational

    speeds of low and high pressure compressors stages for a compressor blade stall

    resistance, feature that allows more compressors stages to be added for lowering specific

    fuel consumption. But it has unusually low number of compressor stages for a two-shaft

    design, contributing to reliability. Bad side of this philosophy is higher fuel consumption.

    Despite the resistance of the compressor to the extreme conditions of airflow at the inlet,

    if afterburner is engaged at almost zero speed (well below the conservative engine

    envelope) other undesirable phenomena are possible. Distortions of the inlet airflow

    causes disruption of relations of air and fuel in the AB chamber, which changes the speed

    of combustion. Pressure fluctuations coupled to acoustic velocity fluctuation (AB

    chamber is also exposed to sound fatigue, the noise is up to 180 decibels) associated with

    combustion instability (called rumbling), can cause extreme resonant structural vibrations

    of the engine with subsequent engine destruction and the loss of the aircraft.

    The published results of American evaluation relates to the F/PF models. BIS model has

    15-20% higher ratio of inertia moments in yaw to roll. It certainly results in more sideslip

    during rolls and somewhat lower stall angle of attack, angle when breakdown of dynamic

    directional stability occurs. But the prevailing factor in this equation is the dihedral effect

    i.e. roll stability and it is the same in all models because it depends on airflow around the

    delta wing, so it can be expected good behavior of BIS model at low speeds also.

    It should be borne in mind that prevailing effects at high angles of attack are dihedral and

    adverse yaw due to aileron deflection. Rudder is used for rolling and if the sideslip angle

    or yaw rate (induced in this way) crosses critical, result is the spin. Opposite aileron

    increases the roll rate through an additional sideslip angle i.e. 'adverse yaw'. In most

    modern aircraft application of such cross controls for 5-15 seconds, usually causes spin.

    16

    Figure 10. MiG-21 derivatives J-7G and JL-9

    In general, the plane that has a lower stall speed is more maneuverable. At some speed, it

    will be able to achieve g-load equal to the square of the mentioned speeds quotient. The

    U.S. experience from simulated dogfights during exercises indicates the importance of

    the minimum speed and controllability at high angles of attack. That is why F-18 gets F-

    15/16 although its performances are considerably lower. Latest F-18E has still weaker

    performance, but better controllability. Angle of attack, at low speeds, of the F-16 and

    17

    Gripen is limited to about 26º (Rafale and the Typhoon to a shade more). F-15 has max

    trimmed angle of attack 30-33º with poor roll response here. Against 'stealth' fighters F-

    22/35 and corresponding new Russian (whose all planform contour lines are parallel to a

    few main sweep angles - cm wavelength radar return angles, in addition to other 'stealth'

    measures and cost of 50 MiG-21), none of the listed has significant chances at medium

    range. Analysts agree that the close combat will remain inevitable, and that each aircraft

    armed with missiles cued by helmet sight has a chance, especially if it can reach high

    angles of attack. Even stealth fighters do not destroy opponents with death rays. Every

    component of the fire control/weapon system chain has limitations, from fighter radar to

    missile fuze. Towed mini decoy (laterally separated) with monopulse deception

    jammer/repeater or just simple towed corner reflector can draw away radar return signal

    centroid from towing aircraft. It could help surviving medium range combat even against

    stealth fighters.

    The main disadvantages of MiG-21 are poor cockpit downward visibility, a

    proportionally small (but inline to generation) wingspan i.e. large induced drag

    (afterburner is needed for level flight at the absolute minimum speed, as at max allowed

    Mach number) and relatively slow response of two-shaft engine. All this causes poor

    performance on landing, especially in the case of go-around. Small fighter size means

    limited mission equipment carriage capacity.

    It turns out that only important is to have a reliable and economical aircraft, a platform

    for carrying payload, with attack speed in the Mach 1.5+ class (that’s why a 15-20 years

    younger A-10 was withdrawn prematurely). The modern nav-attack equipment

    (simplified inertial system, GPS, displays…) is now relatively inexpensive to install even

    in a small propeller planes. MiG-21 operators missed opportunity to realize fact that with

    helmet cued, large acquisition angle R-73 missile that was available upgrade, MiG could

    achieve 50:1 kill ratio in dogfight against F-18/Gripen/Typhoon class fighters just

    because latter were 10-15 years late with similar weapon system. Instead of making best

    of it, MiG-21 operators opted to admire newer fighters.

    Because of its good characteristics, even 50 years after MiG-21 became operational,

    some of its modifications are still in production in Asia.

    Reference:

    - Fighter Performance in Practice, Phantom versus MiG-21, Predrag and Nenad Pavlović, eBay.com;

    - Test and Evaluation Squadron, Nellis Air Force Base, Interviews;

  •  10-22-2010, 6:25 AM 9713 in reply to 9658

    Re: Chinese mig-21 derivatives; affordable in crisis

    Maybe book "Fighter performance in practice; Phantom vs MiG-21" at eBay (I think it can be downloaded free on net), tells something about family abilities...

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