MiG-21Bis and Bis-B FM Mods

[+baffmeister 1,205]

I found some interesting MiG -21 data online so did a mini review of the ThirdWire flight model to see if there was any room to improve the performance a bit. Regarding the aerodynamics everything looked good and any changes that would have been made based on the data would have resulted in slightly worse performance so I left it as is. Best areas to improve performance were some adjustments to rack weights, some potential adjustments to empty weights on some versions but not the Bis, implementation of a combat flap set up for the later versions, and a work around for the Bis series to model the very large emergency thrust setting that was available below 4km/13123ft. There was a 3 minute limit on that system but it's not modeled. Might try something in the future to get an engine blow up at some point past 3 minutes but it's low priority. Here's some info on the changes:

Engine Boost: I've been aware of this capability for a long time but didn't think it could be modeled. Turns out, it can! The engine afterburner mach tables use the same altitude table as the dry mach tables so what I did was add a second engine to the Mig-21Bis, but it supplies thrust in afterburner only. It adds additional thrust  bringing the total thrust to 21825lbs static. At 13000ft the altitude table included with the second engine starts cutting down on the thrust until it's completely gone at 14000ft. The additional fuel consumption with the added thrust is modeled but it's not as bad as I suggested in a screen shot. After fixing a mistake the fuel lasts around 4 to 4 1/2 minutes but the fuel consumption and thrust can vary quite a bit depending on mach number and altitude. The additional thrust activates at about 98% throttle so to conserve fuel you will want to use the AB with discretion. It just occurred to me now I might be able to set the main engine up so it gets it's max thrust around 90-95% which would leave a bit of a dead zone between the main and second engine. Will give that a test at some point.

Combat Flaps: This system appeared on the later versions, possibly first appearing on late production PFM's but not 100% sure about that. A graph showed the additional lift from the combat flaps that was available at speeds up to mach 0.90. I'm not sure how they worked in RL but these ones are just set to automatic mach and are fully deployed at 0.30 mach and fully retracted at 0.90mach with a continuous variation between points. The combat flap system worked between 0deg and 25deg flap angle. With the automatic combat flaps the landing flap setting is no longer available.

Rack and empty weights: With the smallish low lift delta wing the MiG-21 suffers badly from increased weight. Any legitimate weight savings would help a lot. Anyway, from the info I was looking at the weight of the main racks was considerably lighter than the TW values so I made some adjustments. Aircraft empty weights need more research but the Bis looks about right and the MF looks too heavy by about 240kg.

Any further information on these topics would be appreciated. Here are beta test FM's for the Bis and Bis-B: MiG-21BisFM's0.95.zip

[+Coupi 4,395]

Super ! Thanks !!! 

Do you think it is possible to add the "survitesse" system of the SNECMA ATAR 9C ? Over Speed device allowing +9% Thrust when Mach > 1.67 and > 27,500 ft by increasing rotation speed.

https://www.caea.info/fr/44-collection/moteurs/185-snecma-atar-09c

Edited February 3, 2019 by Coupi

[Stratos 3,192]

Quite and improvement, the acceleration is astonishing, and the auto flaps help quite a bit! Will keep testing.

Wish we can add the ASO-2 flare dispenser, those Python are a 100% killers.

[+baffmeister 1,205]

7 hours ago, Coupi said:

Do you think it is possible to add the "survitesse" system of the SNECMA ATAR 9C ? Over Speed device allowing +9% Thrust when Mach > 1.67 and > 27,500 ft by increasing rotation speed.

I wasn't aware of that one. Will look into it.

[+baffmeister 1,205]

On 2/3/2019 at 9:55 AM, Stratos said:

Wish we can add the ASO-2 flare dispenser, those Python are a 100% killers.

Not familiar with those kinds of things,  could you just make a late version?

I took a close look at the thrust output from the stock engine data and it hits the fourth afterburner stage/full thrust at about 92% throttle. The emergency thrust kicks in at about 98-99% throttle so the present dead zone looks OK. If you target 94-96% throttle position you will get full standard thrust when desired, without engaging the emergency thrust.

[+Coupi 4,395]

The Phantom killer... 

[+Gepard 11,323]

The engine "Sonderregime", special regime of the R-25 engine, increased the thrust from 69.6 kN to 97 kN. The climb rate increased from 180m/s to 225m/s.

The limit of 3 minutes was caused by cooling problems. Overheating was possible.

 

Combat flaps were not used on MiG-21. If the pilot forgot to "pull in" the flaps, the wind would do the job from a certain speed. I forgot the correct speed when this happend. Perhaps i can find it in my archive. To release the flaps in dogfight was common for skilled  MiG-21 pilots. I have heared it from some pilots of LSK (East German Air Force).

The main problem for the MiG-21 was the air intake. The design was not the best. In certain flight regimes the air intake was unable to let in enough air to operate the engine properly. Then the problem of "Pompage" (may be "pumping" in english, sorry dont know the correct term) appeared. Or even worse the engine got a stall and stopped operating during a sharp curve. The design of the wing was as good, that the MiG-21 could have a much smaller circle radius, but the air intake ...

In the 80th a different air intake with additionally openings was tested. The plane got a much better agility.

The Ye-8 successor of MiG-21PF had the same wing, but a different air intake. The plane was nearly as agile as the F-16A, but 10 years earlier.

 

 

Flare dispensers were included in some types of ECM pods like SPS-141 for MiG-21bis.

 

Edited February 9, 2019 by Gepard

[+baffmeister 1,205]

18 hours ago, Gepard said:

Combat flaps were not used on MiG-21. If the pilot forgot to "pull in" the flaps, the wind would do the job from a certain speed. I forgot the correct speed when this happend. Perhaps i can find it in my archive. To release the flaps in dogfight was common for skilled  MiG-21 pilots. I have heared it from some pilots of LSK (East German Air Force).

I will look into this closer but have seen a couple of online sources that seem to suggest a combat flap set up available on later versions. A manual combat flap set up might be a possibility as well, and maybe preferable, but would need more info on limit speeds.

Your comments on the air intake are interesting and something I was wondering about. I'm just using a standard TW mach table for the "special regime" but thought it might not be the most realistic approach due to potential airflow limitations with the small intake. Finding good data regarding the thrust available over the mach/altitude range for the special regime would be very difficult, I think.

[+baffmeister 1,205]

I spent some time trying to find more information on the MiG-21 flap arrangement but didn't find any solid information. I did find enough to agree with Gepard, the flap set up wasn't considered a combat flap. Described as a "Floating Flap" or "Swimming Flap" the maximum deployment angle depended on aircraft speed. For example, during approach to land you would select flap 25deg but if the speed was relatively high, the flap wouldn't deploy the full 25deg but some lower angle. As the speed decreased the flap would continue to deploy until at the "right" speed it would reach 25deg. I assume it would work in reverse as well. I didn't find any evidence that any of the later versions such as the LanceR or Bison received a dedicated combat flap but India did some wind tunnel testing with models in 1985, investigating the potential for combat flaps on the MiG-21.

There is some anecdotal information suggesting the floating flap may have had some benefit during combat maneuvering but probably at quite low airspeed and possibly more for defensive maneuvers as the drag would be quite high from the "simple" flaps. I did watch a LanceR airshow video with the plane maneuvering and doing rolls with extended flaps but couldn't tell if the flaps were moving through a range of travel.

At some point MiG-21's were equipped with a Boundary Layer Control system [AKA Blown Flap] that worked when the flaps were in the landing position. The early versions didn't have the BLC system and that seems to be reflected in the stock TW FM's, with the early F-13 having less flap lift than the later versions. I don't think the early versions had the floating flap set up either, so the two systems might have been implemented at the same time.

I haven't found any solid info on the speed range for the floating flap but the present speed range is probably too high with possibly too much lift as well but will leave it "as is" until I get more information.  

I found more support for reduced main rack weights, in the 24-28KG range.

Also, the MiG-21Bis did get a slightly enlarged air intake as well as some refinements to the ducting. It also received some structural refinements to increase strength while avoiding a significant weight increase.

 

[+Gepard 11,323]

Okay, i searched my archive and found something.

MiG-21bis acceleration time from Mach 0.5 to Mach 0.9 was 21 seconds

time for 360° curve in 1.000m  27 seconds

                                  in  4.500m    48 seconds

                                 in  9.000m     78 seconds

initial turn rate 23°

sustainable turn rate 13.3°

 

The flaps of MiG-21F-13, PF and U were modified Fowlerflaps. In german we called it "Spaltklappen", slotted flaps. Flaps setting for takeoff was 24.5°, landing 44.5°

From MiG-21PFS onward the flaps were "Scharnierklappen mit Grenzschichtbeeinflussung". Blown flaps with fixed rotation axis. Flap setting were 25° for take off and 45° for landing.

The airflow for the border layer blowing system came from the compressor of the engine. It was activated by the pilot by pressing a button (IIRC). It was operational only when the afterburner was not activated. Switching on the NB would automatically switch of the BLC.

The flaps of the MiG-21 were pushed in by the driving wind at a speed of 340 km/h. I'm not sure, wheter it was meant that it began with 340 km/h or that the flaps were fully pushed in at 340 km/h. I think the first idea is correct.

 

[+Gepard 11,323]

Further informations i found.

When the SPS System (boundary layer blowing system or BLC) was activated the lift of the flap was increased by 50%.

The System was activated when the flap setting was bigger than 30°.

Using the system reduced the thrust of the engine by some kN.

[+baffmeister 1,205]

Thanks Gepard, at this point I'm not sure how to best implement the MiG-21 flap system but will do some testing.

[+baffmeister 1,205]

I had a look at an English language MiG-21Bis flight manual and things aren't what they seemed. The manual clearly states, on the first page, the static sea level thrust when using the "extra" afterburner is 7100kg which works out to 69627 newtons, EXACTLY what ThirdWire modeled in the stock FM. The flight manual also states the thrust of the standard afterburner is 6850kg or 67176 newtons. The flight manual says the "extra" afterburner generates 9900kg / 97086newtons but that is at mach 1 near the ground!

With the stock ThirdWire  MiG-21Bis FM and it's rather generic engine tables the Bis generates 100820 newtons at sea level and mach 1 so the stock TW FM already meets and slightly exceeds the flight manual data. For a "generic" mach table that's used on a lot of TW aircraft it's really quite close.

There seems to be quite a bit of confusion on the internet regarding the thrust levels of the MiG-21Bis but I have to go with the flight manual information. The only other internet source I found that more or less agrees with the flight manual information is some promo material for the DCS MiG-21 Bis. If I remember correctly, they said the engine generates around 70000 newtons static thrust and "almost" 100000 newtons at low level and high speed.

At this point I would suggest that anyone who downloaded the modified FM's I posted just remove them and revert to the stock FM. I tried to delete the posted files but not sure if it worked.

Based on the flight manual information the thrust level for the TW FM should be OK below 13000ft but would be to high above that altitude. I could mod the FM to address that issue but at this point I think it would be better to run some performance checks on the stock TW FM and see how it matches up with whatever book numbers I can find.

 

[Stratos 3,192]

So, it seems the brick returns?

[+baffmeister 1,205]

10 hours ago, Stratos said:

So, it seems the brick returns?

For now, at least. There might be some other areas to look into.

[MigBuster 2,884]

21 hours ago, baffmeister said:

There seems to be quite a bit of confusion on the internet regarding the thrust levels of the MiG-21Bis but I have to go with the flight manual information. The only other internet source I found that more or less agrees with the flight manual information is some promo material for the DCS MiG-21 Bis. If I remember correctly, they said the engine generates around 70000 newtons static thrust and "almost" 100000 newtons at low level and high speed.

 

Majority of people have no concept that thrust output varies massively with speed and altitude - including a lot of technical authors who have also got this totally wrong.

Forget the internet the only valid sources are flight data from such documents as the flight manual.  

[+Menrva 4,202]

29 minutes ago, MigBuster said:

Majority of people have no concept that thrust output varies massively with speed and altitude

So true. But even an arcade videogame like Ace Combat gets this right. Flying over 40,000ft or so with a MiG-31 shows quite a difference in top speed.

Edited June 25, 2019 by Menrva

[+Gepard 11,323]

On 25.6.2019 at 1:19 AM, baffmeister said:

I had a look at an English language MiG-21Bis flight manual and things aren't what they seemed. The manual clearly states, on the first page, the static sea level thrust when using the "extra" afterburner is 7100kg which works out to 69627 newtons, EXACTLY what ThirdWire modeled in the stock FM. The flight manual also states the thrust of the standard afterburner is 6850kg or 67176 newtons. The flight manual says the "extra" afterburner generates 9900kg / 97086newtons but that is at mach 1 near the ground!

 

 

For what MiG-21bis sub version is this manual? Do you have an "isdeliye" designation found?

For east german MiG-21bis (production isdeliye 75B) the given data for the engine R25-300 are 40,26 kN dry, 69,6 kN with afterburner, 97,1 kN with special regime for 3 minuts maximum at altitutes up to 4.000 meters.

Maybe, that the engines for third world states like India or Syria got an little bit downgraded engine. This was common handling for the soviets in the cold war days.

 

Edit: I looked in an other german manual and found following:

The thrust of afterburner was changeable. The "Nachbrenner max" had 69,6 kN thrust, the engine sucked 67,9 kg air per second. The "Verdichtungsgrad" (perhaps pressure ratio) is between 9.1 and 9.5, the nozzle diameter is 643 +/- 3 mm

The "Nachbrenner min" had 67,2 kN.  the "Verdichtungsgrad" was the same 9.1 till 9.5. The nozzle diameter is 600 mm

The "Sonderregime" had 97,1 kN. The "Verdichtungsgrad" is 9.6. The nozzle diameter is 643 mm. The engine suckes 68.5 kg air per second. The turbine was driven with 107%

 

nozzle diameters for idle (33%)                                     643 mm

nozzle diameter for nominal till dry max                      550 mm

nozzle diameter for BLC                                                   570 mm

 

R25-300

lenght with AB                                   4.615 mm

biggest   diameter                                907 mm    

weight                                                 1.290 kg

turbine entrance temperature      1.213 till 1.313 Kelvin

afterburner temperature               2.060 Kelvin

BLC air consumption                     2.5 kg

acceleration from 33 to 99% thrust        8 to 10 seconds

acceleration from 85 to 99% thrust        2.5 to 4 seconds

acceleration with BLC from 50 to 99%   more than 4 seconds

      

Edited June 28, 2019 by Gepard

[+baffmeister 1,205]

1 hour ago, Gepard said:

Maybe, that the engines for third world states like India or Syria got an little bit downgraded engine. This was common handling for the soviets in the cold war days.

Possible, I guess, but conjecture at this point.

The thrust levels you mention are all a close match for the flight manual information with the big difference being the flight manual states the 97.1 KN thrust occurs in flight at mach 1 near the ground. 

Gepard, I'm not sure what type of technical documents you're looking at but maybe they are just giving a general overview of the engine? Most of the information you mentioned regarding the nozzle positions etc. don't seem to be mentioned in the flight manual but the 107% RPM limit [107.5% in flight manual] for the high pressure compressor/turbine is mentioned so at least that is a match. I guess the only other thing I can mention is the 68.5 kg/sec airflow into the engine for the  "Sonderregime" . That doesn't sound like a big increase compared to the "Nachbrenner max"  airflow of 67.9kg/sec so I'm wondering how it could get such a big boost in static thrust. That isn't something I know much about so is just conjecture on my part. Anyway, It's an interesting topic and hopefully we get more clarity at some point. Here's a couple of screen shots from the flight manual. I didn't notice any mention of the term "isdeliye" in the flight manual:

 

[+Gepard 11,323]

The manual you has seems to be used in an arab country. Perhaps the arabs got other informations.

On the other hand basics for the manuals for the East German Air Force (LSK) were written by german pilots during their initial flight trainings in the USSR. All infos they got, they wrote by hand in their exercise books and afterwards smuggled them into the GDR. From this handwritings the official LSK manuals were developed.

Maybe the arabs were to lazy to do a handwritting job and relyed more on official soviet manuals.

 

What i found:

The R25-300 got a second fuel pipe system (Ringleitung) which feed a second afterburner fuel pump, the R13-300 and all other engines had only one afterburner fuel pump. Each pump was able to feed 11.000 liters/hour. So the R25 was able to burn the doubled amount of fuel when using the "Sonderregime".

The main fuel pump of the R25 was able to feed 10.500 liters/hour. The R13 main fuel pump gave only 7.000 liters/hour.

 

The additional fuel pump coud be the reason for the much higher thrust in Sonderregime.

 

 

I forgot to mention the name of the german manual

" A 101/1/111 Nutzung Betrieb und Steuertechnik MiG-21bis "

Edited June 28, 2019 by Gepard

[+baffmeister 1,205]

Not much I can add to this topic at the moment but here are a few interesting pages from the "official" documents I have access to.

From the "Arab Country" flight manual here is a sustained G curve for the MiG-21Bis. To test the stock ThirdWire FM I set the fuel burn to zero to maintain a constant weight, loaded 50% internal fuel as well as two missiles on the inner wing pylons. The external load group 1 mentioned in the flight manual is a configuration with light loads on the inner wing pylons. At full thrust and within about 100kg of the 7500kg chart weight I flew around in circles at 1000m/3280ft and tried to match some of the speeds on the chart and see what kind of sustained G I got. As usual, it's difficult to maintain a constant speed while going around in circles while maintaining a steady altitude but it seemed to me the stock TW FM leans towards some over performance, maybe about 0.5 to 1 G over the flight manual values. Past the 1040kmh/about 562kts on the chart, where the sustained G starts dropping off, the TW FM maintains the sustained G, or maybe even increases a bit, so the over performance increases past 1040kmh. I think the H-Stab on the real MiG-21 did a "gear change" or something like that around 0.90 mach, maybe to reduce pitch sensitivity. It's something I'm still looking into but it might explain the quick drop off in sustained G for the chart values, compared to the TW FM.

I also have a Soviet document that might be something like a technical manual for pilots. Here's the front page:

The Soviet document has a sustained G curve as well, [upper chart] but it uses mach number and the horizontal scaling looks different. The weight is for 7500kg and the resulting sustained G looks similar to the "Arab Country" chart.

From the Soviet manual, an instantaneous/available G chart. [lower chart] I'm not sure what the solid line represents but the fine print mentions 28deg, which is the approved A0A limit in the flight manual. At any rate, at low altitude the available G for the TW FM is more or less a perfect match for the dashed line at 7000kg and zero altitude. Maybe at some point a Russian speaker can explain the details of the Soviet charts.

At this point I'm just going to continue reviewing the manuals and maybe run a few more checks on the TW FM.

 

 

[Stratos 3,192]

Thanks for this work Baff, is very interesting to read this kind of posts.

[Svetlin 545]

The first graphic in Russian says:

 

Полньй форсаж = Full afterburner

Максимал = Full throttle, i.e. highest throttle before afterburner starts

Рис. 27 Перегрузки установившегося виража самолета с двумя ракетами Р-3С = Drawing 27 G-load during sustained virage of aircarft loaded with 2 R-3S (AA-2A Atol) missiles

G=7500 кг = G=7500 kg

Рис. 28 Радиусьй установившегося виража самолета с двумя ракетами Р-3С = Drawing 28 Radius of sustained virage of aircarft loaded with 2 R-3S (AA-2A Atol) missiles

 

The second graphic in Russian says:

 

Зона предупредительной тряски = Warning shake zone (probably refers to a warning shake of the aircraft controls, avialable to warn the pilot, that a critical level of certian flight parameter has been reached, for example a warning shake before stall)

сваливания = stall

Граница по максимальному отклонению стабилизатора = Limit of maximum deflection of the stabilizer

Начало предупредительной тряски = Begining of warning shake

На большом плече АРУ = probably means "at high level setting of the automatic booster control"

На малом плече АРУ = probably means "at low level setting of the automatic booster control"

Рис. 21 Изменение коеффициента подьемной сили самолета = Drawing 21 Fluctuation of the coefficient of aerodymic lift of the aircraft

При остатке топливо <= 1200 л = At fuel level of up to 1200 L

При остатке топливо <= 2200 л = At fuel level of up to 2200 L

Рис. 22 Максимальнъйе разполагаемъйе перегрузки = Drawing 22 Maximum available G-load

 

Hope that helps a little.

Edited July 5, 2019 by Svetlin

[+baffmeister 1,205]

Thanks Svetlin, that's very helpful.

I'm taking a closer look at the low speed performance of the stock FM to see if there may be some potential for adjustments. The stock FM makes maximum lift at about 28deg AoA, the same as the AoA limit in the flight manual but the charts show the MiG-21 still making lift up to about 33deg AoA. The 28deg limit was quite conservative giving 5-8deg of protection prior to the stall/departure zone. There were large roll oscillations above 28 deg but the plane maintained pitch stability and American test pilots soon learned they could safely exceed the 28deg limit. Some anecdotal information indicates Soviet pilots were familiar with that capability as well.

ThirdWire has the stall buffet starting at about 15deg AoA while the flight manual says the buffet zone starts at 16-18deg AoA range, so there may be some room to adjust the AlphaStall and AlphaMax values while maintaining the same lift slope as the TW original.

I'm going to see if I can find some charts that might give a better idea of the instantaneous turn rates and deceleration rates as the stock FM seems a bit limited when doing hard turns at lower airspeeds. By all accounts the MiG-21 had a good instantaneous turn rate and very good pointing ability at lower airspeeds. It might just be a case of increasing the elevator range a bit but will have to find the right chart so I can run some tests. More translations might be required!

[+baffmeister 1,205]

Another chart from the Soviet MiG-21Bis technical manual. One of the areas I've been looking at to potentially improve the MiG-21 performance is the rack weights, which seem quite high in the stock FM's. Some earlier information I found indicated there might be reason to believe the racks were lighter but the information was "anecdotal".

I haven't been able to make much sense of this chart but it seems to have columns for assorted racks and weapons with weights in kilograms. I think some other columns might be used to calculate the CofG and CofG changes when weapons are dropped. The weights seem to be for weapons mounted in pairs. Any ideas?