Continued :
Figure 6. Example lift and stability of aircraft
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.
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.
Figure 7, 8, 9.
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.
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 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;
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