10 facts about the mad Mach 2.6 MiG ‘Flogger’ fighter aircraft

Technical dive into a Soviet Cold War Tactical Fighter Hotrod

Mihai Vălceleanu

The Mikoyan-Gurevich MiG-23 known to NATO as the ‘Flogger’ first flew in 1967, and by the time production ended in 1985, over 5,000 had been produced. For most of its service life it was the chief fighter of the Warsaw Pact nations, and as a devilishly fast warplane available in huge numbers, it could have proved a nasty threat to anyone foolish enough to underestimate it. Often written off in the West as a dangerous (rather limited) ‘hot-rod’, many misunderstandings arose from intelligence based on immature models of this swing-wing tactical fighter. As we shall see, the ‘Flogger’, an aircraft without a direct Western analogue, was a fascinating idiosyncratic machine full of surprises.

1. MiG-23MLD (izd. 23-18) aerodynamic “refinements” It’s not uncommon to hear, in certain circles, that the pitot-static tube-mounted vortex generators (PVD-VG) and the second set of dog-tooth vortex generators (lambda extensions are those triangular notches at the wing root leading edge) were meant to improve the Soviet MiG-23MLD’s maneuverability over that of previous M and ML versions. 

Original photo credit: igor113.livejournal.com

And indeed they were, but never mentioned is that they didn’t increase lift directly by energizing the flow and delaying the stall, as one would assume. They didn’t use vortex lift like the leading-edge root extensions of a MiG-29 or F-16. Instead, they increased the MiG-23’s lateral and directional static stability at high angles of attack (AoA) with side-slip. Initially, they tried the pitot tube vortex generators only, but found that the increase in AoA was not enough, so they came up with the lambda-extensions to get a sufficient improvement. The patent for the lambda-extensions has the first author listed, a certain G. S. Byushgens from TsAGI, who was awarded the title of Hero of Socialist Labor as the creator of the MiG-23’s aerodynamic layout in 1974. He also devised the big dog-tooth vortex generator on the MiG-23’s pivoting wing section in 1970-71. (some rather interesting stealth intake patents by him here)

So, how do these refinements work? Without these refinements, a MiG-23ML with the wings swept at 45°, flying at high AoA (with sideslip), would have vortices emanating from the forward fuselage and fixed-wing portions, combined and distributed in such a manner as to make the aircraft unstable in roll and yaw by acting on the nose, leeward wing, and vertical stabilizer. 

The MiG-23ML flight manual states that beyond 36° indicated AoA (20.75° true AoA) the aircraft becomes directionally unstable and departs controlled flight. As such, the MiG-23ML was equipped with a stick-pusher that kicks in at 28°-30° indicated AoA. With the MLD’s pitot tube vortex generators mounted, the vortex system coming of the nose is turned into one that increases directional stability. The λ-extensions stop the nose & wing vortices on each side from combining, and thus their harmful effects on directional and lateral stability is reduced. The maximum attainable AoA with the wings swept at 45° increased to about 40° indicated AoA as a result of these two features. Judging by pilot’s accounts, a MiG-23MLD with the wings at 45° could indeed reliably outturn a MiG-23ML.

2: MiG-23MLD (izd. 23-18) automatic leading-edge flaps.
Although the pitot-static tube-mounted vortex generators coupled with λ-extensions were a clear success, the next set of refinements had mixed results. The MiG-23MLD (izd. 23-18) was modified to have automatic leading edge flaps, to be used with a new 33° wing sweep position. And although these measures did indeed help to increase manoeuvrability, rarely mentioned is the fact that at some service point, they were abandoned, the automatic leading edge flaps being deactivated and reverting to the original 45° wing sweep position. Why? By most accounts from pilots, because the leading edge flaps would deploy at too early an AoA, creating much drag and buffeting for their lift benefit. 

===insert MiG-29 graph===

The idea of automatic leading edge flaps is a good one, and widely used these days. But, as can be seen in this graph from the MiG-29’s flight manual, deploying them at too low of an AoA decreases the lift-to-drag ratio (К). The lower sweep angle also likely increased drag. Since the MiG-29 was already entering service, nobody wanted to spend anymore time on refining the MiG-23’s system, and was abandoned. 

3. The MiG-23MF and its heavy, heavy radar
It’s no secret that Soviet fighter radar technology was behind that of the West in the 1960s and 70s. Soviet radar systems were hugely heavy and bulky. The MiG-23MF was perhaps the fighter aircraft most burdened by its radar system. An empty MiG-23MF weighs about 10860 kg, with its Sapfir-23E  (323E) accounting for 641 kg, representing a whopping 5.90%! For comparison: MiG-21BIS with RP-22SMA, 3.36%; MiG-23ML with Sapfir-23MLAE (N003E) radar, 4.64%; early production F-4J Phantom with AWG-10, 3.75%; MiG-29 9.12 with N019 Rubin; 3.21%.
The MiG-23’s radar equipment was so voluminous, that not all of it could fit in the nose section, some of its components had to be placed behind the cockpit.

4. MiG-23 variable exhaust nozzle.

You’ll notice that I didn’t say “engine exhaust nozzle”. That’s because the exhaust nozzle flaps that one sees on the MiG-23 are not part of the engine, as we are accustomed on other aircraft engines like the F-4E’s J79-GE-17 turbojet. Instead, the nozzle is part of the airframe, and unlike other variable nozzles, is not controlled by any hydraulic system, but expands and contracts freely according to pressure differences between the exhaust and exterior flow.

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MiG-23’s secondary exhaust nozzle & the engines primary exhaust nozzle. Credit:  igor113.livejournal.com 

5. MiG-23, Mach 2.6 speed machine.
The MiG-23’s maximum Mach number stated in most sources is 2.35. And that’s correct, for reasons of temperature (cockpit canopy “silvering”) and directional static stability, the MiG-23 is indeed limited to Mach 2.35. But that’s not its true limit. According to the MiG-23M and MiG-23ML flight manuals, the maximum attainable Mach at high altitude is 2.55 or 2.6, and that’s not even clean…that’s when carrying two R-23 missiles!

In Steve Davies’ “Red Eagles: America’s Secret MiGs, 2012” it states that James “Thug” Matheny, (Bandit 27) said he’d flown the MiG-23 up to Mach 2.5. But that was most likely a very clean MiG-23MS, no pylons and no TP-23 infrared search and track chin fairing. How about a more a more typical combat loaded case. Well for that we have one one case* during the Iran-Iraq war. In August 1983, Iraqi Air Force pilot, Ismail Al-Naqib, from the 67th Squadron Ali bin Abi Talib Air Base (Nasiriyah) claims to have reached a speed of Mach 2.6 at 11km altitude, during an intercept on probably an RF-4E Phantom II reconnaissance plane. His MiG-23MF was armed with R-23 (and likely two R-60 missiles too), and a fuel drop-tank pylon. Keep in mind, that these two cases are using MiG-23MS/MF, with the weaker R29-300 engine, the MiG-23ML had the more powerful R35-300, producing 13000 kgf instead of 12500 kgf.

* My thanks to author Milos Sipos for bringing this event to my attention.

6. MiG-23ML “thumb mounted sights”
Years before the MiG-29 with its “Щель” helmet-mounted sights entered service, the MiG-23 was already making steps in that direction. The MiG-23ML with ASP-17ML & N003 had a curious system called “Метка” to steer the R-60 & R-60M’s seekers with the same button on the control stick used to steer the Kh-23M air-to-ground missile. The black button was on the lower-left side.

Photo by Óscar Laborda, via Flickr.com

METKA system block from the MiG-23ML 23-12B manual.

7. MiG-23ML inlet guide vanes.

When you look at the intakes of any MiG-23, you quickly notice two horizontal plates. Those are guide vanes, and are more important than you may think.

Picture from Nowa Technika Wojskowa by Piotr Butowski.

 

(archive of T. V. Kondrashova via S. Isaev).

During testing or service, it was likely found that while flying at high AoA, the air reaching the compressor would be uneven, and as a result, cause compressor stalls. So the MiG-23S would subsequently be retrofitted with inlet guide vanes to cure this, and all production fighter/interceptor versions of the MiG-23 would have them from the factory. The fighter-bomber version, MiG-23BN, would never receive these, and as a result, high AoA manoeuvring could cause engine issues. But the story doesn’t end here, as the MiG-23’s AoA range would be extended with the ML, it was likely found that the old guide vanes’ design was inadequate, so with late production MiG-23ML (mostly export 23-12) and export MiG-23MLD (23-22), the lower guide vane would be cambered and elongated. 

Ex-Syrian MiG-23MLD (23-22B). Photo by Isaac Gershman

The earlier produced MiG-23MLs would receive the updated lower guide vanes during their upgrade to MiG-23MLD 23-18 standard. Thus it’s unlikely that the MiG-23MLD would have been a success without these unassuming plates, the aircraft not being able to reach its AoA potential due to engine stability issues, despite the aerodynamic refinements.

8. MiG-23M versus Hydrogen

One factor limiting the manoeuvrability of the early MiG-23M was the rather low G limit. This was partly caused by the inadequate wing pivot support structure design, but more importantly, the high hydrogen content in the VNS-2 stainless steel used to manufacture it. This high hydrogen content made the material unexpectedly brittle, and was the cause of several catastrophic inflight structural failures. The hydrogen content problem was eventually solved with a degassing heat treatment (heating within a vacuum) on the VNS-2. The MiG-23M’s structural problems (both design and hydrogen-related) took a very long time to solve, and although later produced ’23Ms had improved wing pivot support structure designs and used degassed steel, the earlier produced ones would remain a constant source of headaches, cracks constantly appearing and requiring teams welders and their equipment to be flown across the USSR for repairing cracks on the problem aircraft. The whole thing was a fiasco – and massively expensive. The very capable General Mikhail N. Mishuk, who has an excellent engineering knowledge, was involved with managing the problem. In a moment of fury, he berated visiting staff from the Mikoyan OKB, that “It would’ve been cheaper for the country to make you product from pure gold!”. One can only wonder, how the Soviet engineers could have overlooked the hydrogen embrittlement problem. One that, at the time of the MiG-23s first flight was known for 92 years (the first paper describing hydrogen embrittlement dates to 1875)! According to a metallurgical engineer who worked on the project, Natalia M. Voznesenskaya, the reason was that nobody expected the steel, which was electro-slag-remelted, to contain high amounts of hydrogen. A small, yet very costly miscalculation. Today we know, and high-strength aerospace steels are electro-slag remelted and vacuum-arc remelted.

9. MiG-23 throttle response at high Mach.
If you’re familiar with Steve Davies’ previously mentioned Red Eagles book, then you might know of a peculiar phenomenon, namely that of the MiG-23’s lack of engine response to throttle inputs beyond Mach 1.50. To some, this might seem to be a serious design flaw of the R29-300 turbojet, but in reality, this is a perfectly normal feature that probably all jet engines have. As an aircraft travels at an ever increasing Mach number, the air ingested by the intake is decelerated and compressed. This compression heats up the air, which means that for the same volume and pressure, its density decreases. As mass flow decreases due to decreasing air density, the compressor will operate ever closer to its stall limit. To combat this, engines are designed to automatically increase their idle RPM with increasing compressor inlet temperature (CIT). At high enough speeds, the CIT is so high that the idle RPM is equal to the maximum RPM. Therefore, when flying at very high Mach numbers, if you bring the throttle from full afterburner to idle, the afterburner will be turned off, but the RPM will remain at, or close to 100%. 

We can see this in the F-104S/J79-GE-19 engine automatic RPM schedule. The maximum RPM line varies with CIT to adjust for varying mass flow. It’s at 100% only when CIT is 15 °C, which is the international standard atmosphere (ISA) value at sea level. Above 60 °C (M~1.6 at 11km altitude ISA) idle RPM gradually increases until at 113 °C (M~1.98) idle RPM and maximum RPM are equal. And if you bring back the throttle from full afterburner to idle while flying faster than Mach 1.4-1.5, the engine RPM will not drop bellow maximum. But thrust will indeed be reduced since the afterburner will be extinguished and the variable exhaust nozzle be set to fully open.

In the case of the MiG-23ML’s R35-300 engine, maximum high pressure rotor RPM varies from 100% under normal conditions, up to 108% at a CIT of 215°C, or Mach 2.50 at 11km altitude… remember when I said that the MiG-23 can reach Mach 2.5-2.6?

10. MiG-23UB “Свинец” radar system.

Have you ever wondered what’s behind that little white radome on MiG-23UB two-seat trainers? Well, MiG-23UB aircraft manufactured between 1970 and 1972 had the Sapfir-21MU. This aircraft is one of the early produced ones that originally had the radar installed. But in 1974 it was decided to take the radar out of all UBs.

All later produced aircraft, had lead (свинец) weights installed instead. You can see the lead weight in this picture courtesy of the Dallas Cold War Air Museum.

Mihai Vălceleanu studied aeronautical engineering at the Bucharest Polytechnic University. He grew up near the Deveselu MiG-21 air base in the nineties, and is interested in Cold War combat aircraft design and performance.

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