MiG-21bis Weight nomogramme

Hi all,

I've been building something for the last few days. It is quite crude, but unfortunately for myfirst try, it didn't quite come out as well as I've hoped.

The MiG-21bis weight nomogramme!

What is it? A line-chart of sorts, that allows you to plot out your current aircraft weight without needing to do calculations. Naturally this is never as accurate as calculations, but close enough. This is part of a MiG-21bis cheat sheet that I've been working on for the last week in my squadron. When it is ready, I will most likely do another thread and post it there.

The explanation and an example of use is in the document itself. I've based different weights on the DCS-encyclopedia, and some of them I've verified with changing loadouts and calculating weight differences to get the weight of certain items.

Due to the fact that you may load a surprisingly diverse loadout to the -21, the nomogramme is quite wide, which hampers it's use somewhat. For this, and a practical reason, the Nukes are not included in the nomogramme. For such flights, find the weight of the aircraft as is, and add the Nuke weight. I've decided to leave these off because DCS doesn't really model Nukes very well, and thus are not really widely used.

While creating this, I came across quite a few oddities, such as:

RATO-bottles weigh the same regardless of being new or used (the burnt rocket fuel does not decrease weight)

R-60 missiles weigh 59 kg instead of 65 printed in the encyclopedia.

Countermeasures don't count as weight... only the dispensers themselves do.

The UPK-23 gunpods weight does not decrease with spent ammunition.

I've attached the nomogramme as a .png file.

This is for those pilots who want to fly the aircraft according to the manual limitations. It may surprise some, but the MiG is actually landed at a very low weight and just barely enough fuel in many cases. I hope you find this useful when planning and executing a more realistic approach to flying the MiG-21bis.

Regards,

MikeMikeJuliet

Awesome hardcore simmer work :thumbsup:

Next step : drag indexes

Then : fuel consumption :D

Awesome hardcore simmer work :thumbsup:

 

Next step : drag indexes

Then : fuel consumption :D

Yea, that data would either require a really good document, or super extensive flight testing :D

Then again, you really don't need anything other than a rule of thumb: MIL power = 30-60 min altituden dependent, AB = "you are already in fuel emergency" :D

One thing I found out by comparing the nomogramme and the manual is that the weight limits and the graph doesn't match up.

In particular the manual says that with the BLC disengaged MLW is reached at max 400L of fuel... where as looking at the nomogramme, the amount should be strictly speaking 300L without gun ammo, and 200L with it.

I guess the manual gives some tolerances to the pilot, or something doesn't add up.

If I recall the SPS and non-SPS weights are 6800kg and 6500kg. The fuel levels are methods to achieve those weights but given other stores the fuel value corresponding to those weights would change. 300kg not equaling 300L of fuel by any means which may reflect the tolerances of the fuel gauge, desire to safe fuel levels, and the rounding nature of the calculation. It's the same in the real manual.

If I recall the SPS and non-SPS weights are 6800kg and 6500kg. The fuel levels are methods to achieve those weights but given other stores the fuel value corresponding to those weights would change. 300kg not equaling 300L of fuel by any means which may reflect the tolerances of the fuel gauge, desire to safe fuel levels, and the rounding nature of the calculation. It's the same in the real manual.

Yes, I've drawn the fuel with 0,781 kg/L mentioned in the manual. In the nomogramme the weight is always in the vertical scale. On the top left with the fuel graphs liter amount is on the horizontal scale. From there you can see with the SPS and non-SPS MLW-lines, that the fuel amount is smaller than in the manual limitations.

I'm not quite sure if I should have calculated the fuel with the noted specific gravity, which is reported aside from the 0,781 kg/L.

Regads,

MikeMikeMikeJuliet

I've seen 0.775 value as well but this might be a worst case density for range/endurance calculation. Low density fuel produces reduced range/endurance so safe planning only anticipates this. The actual fuel used would have a density typically significantly above the minimum specification. The 2750L volume of usable fuel for calculating flight range and endurance would similarly be conservative. By both of these figures the calculations rely on 2,131.25 kg of fuel. By contrast in DCS the airplane is actually loaded with 2,280 kg (107%). This same 7% is called out as a guarantee fuel reserve of 210L in the section CHARACTERISTICS OF FLIGHT RANGE AND ENDURANCE to account for physical variances.

2280kg 2919L @ 0.781 appears to be the actual mass, volume, and density as actually carried in DCS.

The gauge should not be set to the fuel total but the amount usable. In DCS the fuel gauge is automatically set at the total instead of usable. As such the engine flames out before the gauge reads zero. Similarly the quantities when various indicator lights go on may be wrong by this difference. The following should be the gauge quantities:

Wing empty, ventral present 3200-3000

Ventral empty 2700-2500 (should be 500L between indication of wing vs center)

1st group 700-1000

450L warning 400-550

3rd group 250-350

The FAA civil airworthiness certification states bis had a total capacity of 3,041L but another book states 2,880L. It seems worthy of note that the SMT variant had a 3,250L capacity but was restricted to 2,950L operationally. The thermal expansion (RT) is on the order of 0.72%/K. The difference in 2919-3041L is 96.0% allowing less than 5C increase before overflow. How full a fuel system is filled operationally is somewhat complex based on density, performance, and temperature expectations.

I´ve have this aircraft just for a few days and I´m still learning basic handling.

I´m surprised by the empty weight of the aircraft in the game. All the sources I´ve found stated the weight olmoust 1 tun lower. Do You know, why is the plane so heavy?

The 8620kg figure seems to be back-calculated based on the pilots flight instruction table 7 which describes all-up weight with no external stores.

The basic operating weight is therefore 2280kg of fuel and 85kg of ammunition lighter, 6255kg. This is distinct from the listed empty weight, 5339 kg. The difference to be explained is therefore 916kg. (Although I've seen the empty weight listed elsewhere as 6050, 5895, 5350, 5460kg and I'm sure there are more the more you look!).

Crew, 100kg

Shell casings, 15kg

Are the wing pylons considered extra equipment? (45kg per pair)

Engine oil

Radar coolant

Pilot oxygen

Engine oxygen

I think this explains maybe half of the weight. Another thing is that table 7 may be written with worst case fuel density (~0.82g/cc) just like range table was written for worst case fuel density (.775g/cc). 2280kg at 0.82/0.781 density is an extra 114kg fuel potential for takeoff.

5359 in DCS manual is the smallest value for empty weight for bis model I have ever read. Maybe it is too low?

Generally MiG-21Bis weight subject is quite tricky. We have to consider that apart of two main variants ( 75A "Lazur" and 75B "Sau" ) existed many subvariants. Only Soviet aircrafts were "full option" models, export machines including those for Warsaw Pact were less or more depleted. Great example is IFF system - SRZO-2M, Soviets had full variant with 3 band encryptor plus "Zarya" module ( additional encryptor with few extra codes for war time ), WP had 2 band main system and simplified "Zarya" ( famous izd.81E switch ), rest of world got civilian IFF or no IFF at all ( Indian, Finnish aircrafts ). Already we have over a dozen kilos of difference between aircrafts, but this is only beginning. Different radar system variants, less or more expanded weapon system ( presence or not of some missile means existence or lack of some electronic boxes in avionics bays ), some 75A had deleted "Lazur" system, civilian radios instead of military, full or simplified ARK, etc.

I think, we can assume that we have in DCS Soviet variant so we should consider Soviet's manuals data. We have data for Soviet 75A aircraft ( unfortunately no manuals for original Soviet 75B ), and we have data from Yugoslav 75B ( except simplified IFF those aircraft looks fully equiped ). According those data we can set proper mass values for this type ( at least very close to real ):

-empty aircraft, factory weight accuracy 0.5% (+- 30 kg )( fully equiped with working fluids and gases and installed cannon, without pilot, no fuel, no cannon munition, no underwing weapon pylons ):75A - 5843kg, 75B - 5997 kg,

-pilot with parachute - 110 kg,

-cannon amunition full - 95 kg,

-internal BD3-60-21 weapon pylon, pair - 49-62,6 kg ( weight vary depending configuration ),

-external BD3-60-21R1 weapon pylon, pair - 47,4-58 kg ( weight vary depending configuration ),

-under fuselage wet pylon BD3-56E - 24 kg,

-internal fuel capacity 2885 l, max weight by fuel type:

a. kerosene TS-1 ( 0,78kg/l ) - 2250 kg,

b. kerosene T-1 ( 0,8 kg/l ) - 2308 kg,

c. kerosene RT ( 0,775kg/l ) - 2236 kg,

-unused fuel 80 l - 62-65 kg ( depending type of fuel ),

-PTB-490 ( capacity 482 l ) 46 kg empty ( +24 kg wet pylon ), with fuel 419,5 - 431,5 kg ( depending type of fuel ),

-PTB-800 (capacity 795 l ) 61 kg empty, with fuel 677-697 ( depending type of fuel ).

If I will find time, in near future will post masses for all main systems and weapons.

As configured basic DCS airplane must be: 6197.4-6281.6kg. It is listed in ME as 6255. This is very acceptable I think. I think DCS manual writer simply used a strange value, maybe for a different model or very lightly loaded (missing avionics, cannon, etc.).

Yes, 6255 is within (we can call this like that) "weight margins", but we don't know what kind of fuel they "use" for this calculations ( i'm sure they count fuel density much over maximum value - 0,83 kg/l ). Definitely empty weight is well to low, in my opinion this value is taken from MiG-21M or MF dokuments ( 5350 kg ). Generally whole module is a mix between 75A ( part of cockpit layout ), 75B and MF ( distance counter from ARK-10 ( in Bis should be ARK-10M with different distance dial ), strange weaponry ) and of course with some huge amount of developer's fantasy.

Manual says they use 0.781g/mL 2280kg so 2919.33 L which is supported by the automatic fuel gauge needle positioning.

So they took as fuel TS-1 kerosene, what is good because she was most popular in service. I was thinking that those extra 658 kg lost on empty weight ( difference between RL 5997 and DCS 5339 ) was put into fuel weight. What extra equipment our aircraft get to cover that masse?.

Maximum fuel system capacity according all RL papers is 2885 liters, how they figured out those 2919?.:music_whistling: