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[RESOLVED] Cruise performance not realistic


TurboHog
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I first noticed the problem in the following situation:

 

2 x R3R and 2 x R3S and an empty 800L tank on the center pylon, weight around 21000lbs

 

Impossible to cruise at 36000 feet at full throttle (no afterburner). Speed Cruise performance nowhere near the data found in the real pilot's manual (the one I got from Beczl's site two years ago)

 

----------------------------------------------------------------------------------

 

I did some more testing today to give you a better idea of what is possibly wrong with the flight model.

 

Please use the chart at the end of this post. I have compared all situations at 5000m and 10000m that are in this chart.

 

Method

 

Tests were conducted in an international standard atmosphere (ISA). In terms of temperature and pressure that is: 15 degrees Celsius at sea level and an air pressure of 1013.2 hPa (=QNH=QFF). Provided that the temperature and pressure lapse rates are correctly modelled in the DCSW atmosphere, this should give me the most neutral atmospheric conditions. Of course there was no wind either.

 

I tried to give the aircraft an average weight by conducting the test phase at around 60% of internal fuel. IAS was kept constant at the given maximum range speed. I closely monitored IAS, fuel and elapsed time in order to measure the amount of time it takes for 200L fuel to be consumed.

 

Color codes:

 

< 10% error

> 10% error

Extremely large error

 

Results

 

5000m

 

(1) No stores, average weight during measuring phase: 17000lbs --- 200L/6min -> 2000L/hr

(2) 2 x R3R, average weight during measuring phase: 17500lbs --- 200L/5.6min -> 2140L/hr

(3) 2 x R3R, 490L drop tank, average weight during measuring phase: 18500lbs --- 200L/5.3min -> 2264L/hr

(4) 8 x FAB-100, average weight during measuring phase: 19000lbs --- 200L/5.6min -> 2140L/hr

 

10000m

 

(5) No stores, average weight during measuring phase: 17000lbs --- 200L/7min -> 1714L/hr

(6) 2 x R3R, average weight during measuring phase: 17500lbs --- 200L/6.8min -> 1765L/hr

(7) 2 x R3R, 490L drop tank, average weight during measuring phase: 18500lbs --- Unable to maintain cruise speed w/o afterburner

 

Remarks

 

  • (1), (2) and (3) are within a reasonable margin of error, although all three show a consistent result that is slightly too high.
  • (4) shows a large error compared to real world data. The drag caused by 8 FAB-100's is probably too low.
  • In (5) and (6), cruise speed was only maintainable at almost 100% throttle. This means that, even without external stores, cruise above 10000m is not possible without the use of afterburner.
  • (7) shows that cruise @ maximum range IAS is not possible without afterburner, thereby causing almost a 100% error in fuel flow.

 

sRj9AM9.jpg

 

CLIMB-OUT TESTS

 

Me and Tango did some extra testing of various situations. Everything is recorded in the tracks (SARPP data). SEE CHART BELOW FOR REAL WORLD DATA.

 

SARPP_DATA_Climbout2xR3R-1x490L.txt: Climb-out to 7000m. Performance spot-on

 

SARPP_DATA_Climboutclean_10000m+cruise.txt: Climb-out to 10000m. Climb profile FULL REHEAT to 600 kph then FULL POWER at 870 kph TAS climb to 10,000 m then FULL POWER cruise in level flight. (unable to maintain good cruise speed at 10000m)

 

SARPP_DATA_2xR3R-1x490L-Climbout+cruise.txt: 10,000 m climb with 2x R-3R missiles and 1x centerline 490L tank. Climb profile same as previous tests. Ends with zoom climb from 7800 m to 10000 m, stalling. Aircraft would not accelerate in level flight at 7800 m, making this the technical ceiling for this payload. 7800m in ~8 mins 12 secs. Should be about 6 mins 30 sec to this altitude (interpolating the table). Flight distance required was 140 km; ~78 km according to table

 

Up to 7000m, flight performance seems to be spot-on. However, above 8000m performance starts to show a serious inconsistency with the charts in the manual.

Performance already starts to degrade after passing 6000m.

 

WsJ8TBf.jpg

 

SECOND CLIMB-OUT TEST

 

http://forums.eagle.ru/showpost.php?p=2185785&postcount=42

 

Result: only able to reach 9000m. No consistentcy with charts at all.

 

CRUISE AT 10000M DCS TRACK

 

http://forums.eagle.ru/showpost.php?p=2186075&postcount=97

 

Result: Cruise impossible with 2 missiles and a 490L drop tank. Should be possible.

SARPP_DATA_Climbout2xR3R-1x490L.txt

SARPP_DATA_Climboutclean_10000m+cruise.txt

SARPP_DATA_2xR3R-1x490L-Climbout+cruise.txt


Edited by Cobra847
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'Frett'

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cruise above 10000m is not possible without the use of afterburner.

 

I found that I certainly could, with the small centreline tank, an R3R under each wing, 50% internal fuel, and full military power. When I stopped the test I was doing 550 km/h and increasing at 6 deg AoA in Recovery mode autopilot. I was using the default atmospheric settings in the mission editor.

 

Could you share your exact settings so I can try to recreate?

 

EDIT: Actually, looking at your post again ("weight around 20500 lbs"), you must be using full internal fuel? Using that weight I agree with your findings, but I dunno if that's what the table assumes.


Edited by Corrigan

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In my second more accurate experiment I flew with a weight of 18500lbs. Because the table assumes average weight I took 60% fuel. (Read the 'Method' section)

 

On full military power I'm right at the boundry of being able to fly 530 km/h IAS or not on full throttle. This thing is supposed to be able to cruise at even higher altitudes, which is impossible without help of the afb.

 

Try to cruise at 40K ft, two missiles. See what happens.

'Frett'

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Are you using the correct atmospheric settings?

 

From everything I read about the MiG-21, what we are seeing would only happen if the SAT was something like ISA+50.

 

The MiG either has insufficient thrust, or too much drag. The real thing (two seat version!) held sustained level flight at 62,400 ft. I can't even climb up there.

 

It seems to be an issue that gets worse with altitude. It seems the aircraft is fairly close to real-world up to 5000 m (16404 ft), but above this the sim performance drops significantly compared to what it can do IRL (and IMHO even at 5000 m the performance is already lower).

 

Based on my observations with a clean jet, it appears that it lacks thrust, and the thrust vs. altitude reduces more rapidly than reality. Idle descent requires a vertical speed of ~25 m/s, and engine off requires ~50 m/s, which is what the manuals state. It would however require a much more careful analysis of glide performance before making any comment.

 

Example: the manual states an idle descent from 11000 m at 510 kph IAS will yield a glide distance of 90 km, requiring ~25 m/s RoD.

 

Best regards,

Tango.


Edited by Tango
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Are you using the correct atmospheric settings?

 

From the 'Method' section in the 2nd post. Please read the 2nd post if you haven't done so already.

 

Tests were conducted in an international standard atmosphere (ISA). In terms of temperature and pressure that is: 15 degrees Celsius at sea level and an air pressure of 1013.2 hPa (=QNH=QFF). Provided that the temperature and pressure lapse rates are correctly modelled in the DCSW atmosphere, this should give me the most neutral atmospheric conditions. Of course there was no wind either.[/Quote]

Edited by TurboHog

'Frett'

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...

The real thing (two seat version!) held sustained level flight at 62,400 ft. I can't even climb up there.

 

Here says the ceiling is 54,500 ft.

 

http://www.topedge.com/panels/aircraft/sites/kraft/data.htm

 

EDIT:

 

And here the service ceiling is 59,055 ft.

 

http://www.militaryfactory.com/aircraft/detail.asp?aircraft_id=68

 

:unsure:


Edited by Cyb0rg

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Ye-6U (1960) Trainer prototype based on the Ye-6T......later, Zaitseva set an altitude record for sustained level flight, at 19,020 m (62,401 ft).
http://en.wikipedia.org/wiki/List_of_Mikoyan-Gurevich_MiG-21_variants#Trainer_Variants_.281960.E2.80.931968.2B.29

 

See also this video: http://www.youtube.com/watch?v=-UpyXxl6qE0

 

EDIT: With a total disregard for limitations, see the attached. :joystick:

 

Best regards,

Tango.

Screen_140920_182922.thumb.jpg.6c53a7d9e2fa6021abded777b82798e4.jpg


Edited by Tango
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Honestly, I don't know if that number (62,401 ft) is relevant. It was a record set by a "prototype", so I bet it was cleaned up and lighter than our run-of-the-mill, combat-specced aircraft.

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Honestly, I don't know if that number (62,401 ft) is relevant. It was a record set by a "prototype", so I bet it was cleaned up and lighter than our run-of-the-mill, combat-specced aircraft.

 

I did wonder.

 

Still... try level flight at "only" 50,000 ft in the sim... :(

 

Best regards,

Tango.

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I did wonder.

 

Still... try level flight at "only" 50,000 ft in the sim... :(

 

Best regards,

Tango.

 

Yeah I know, there's something not right, but we should probably keep prototype-set records on the periphery of the discussion atm.

 

50,000ft ? I have a link for that too ;)

 

http://fas.org/man/dod-101/sys/ac/row/mig-21.htm

 

Come on. That's probably not the bis model. Let's try to keep this discussion serious, please.


Edited by Corrigan

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http://www.milavia.net/aircraft/mig-21/mig-21_specs.htm

 

MiG-21bis 'Fishbed-N' (1971)

 

Performance: max level speed at 13,000m (42,650 ft) Mach 2.05 or 2,175 km/h (1,353 mph); max level speed at sea level 1,300 km/h (809 mph); service ceiling 17,800m (58,400 ft);

Come on. That's probably not the bis model.
FAS.org is not the best resource for info. I'm checking the manual I have here, too, see if it states a ceiling.

 

EDIT: http://www.airvectors.net/avmig21_1.html (scroll to the very bottom). Fishbed-L and -N are similar.

 

EDIT2: Hmm - this manual is very interesting. Just trying to fly the advised 870 kph TAS full power (MIL) climb schedule is difficult in the sim. I'm 99% convinced that the sim is lacking thrust.

 

EDIT3: http://forums.eagle.ru/showpost.php?p=1470696&postcount=852

 

"...carrying two missiles and a drop tank the maximum range is reached at 10,000 m...". The manual states this for "full throttle" (MIL) power.

 

Optimum climb profile is:

 

* Takeoff with FULL REHEAT to 600 kph, then

* Upon reaching 600 kph FULL THROTTLE (MIL) maintaining 870 kph TAS in climb (or 670 kph TAS if carrying bombs on multi-shackle racks).

 

Manual further states time to climb is 7 minutes clean to 10,000 m, starting with 2750 L of fuel following the above climb schedule. * Tests *

 

Best regards,

Tango.


Edited by Tango
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Don't think you can ever reach Mach 2.05 in DCS at 13000m with the current FM even if you fly without stores and 1000L fuel.

 

EDIT1: I was able to reach Mach 2.05. After 12 minutes (starting at 13km and already flying mach 1.2). Burnt almost all fuel. In other words: you will never be able to reach it in a mission that starts on the ground because you will run out of fuel during your 15 minute full afterburner effort to reach mach 2.05.

 

870 km/h IAS on climb? Let's see if that is possible while maintaining a reasonable vertical speed...

 

EDIT2: Looks like a good climb rate at 870 km/h IAS. But you want to reach it with afterburner or it will take ages.

EDIT3: oops. TAS, not IAS. Disregard the above.

 

So far all reports indicate a serious thrust/drag issue.


Edited by TurboHog

'Frett'

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My first clean test result is in.

 

CONDITIONS:

 

* Temp: +15 °C

* QNH: 1013.2 hPa

* Takeoff airport: Krymsk

* CLEAN (max internal fuel only)

* Hot ramp start

 

* FULL REHEAT until 600 kph

* FULL POWER (MIL) level acceleration to 870 kph TAS

* Climb to maintain 870 kph TAS

 

Max altitude attained: 7200 m

Time: 12 mins 20 sec

TAS: 810 kph sustained in level flight

 

Conclusion: the engine doesn't produce enough thrust. Even reaching 1000 kph in level flight at sea level will be hard (which it shouldn't be).

 

The manual states for the above test:

 

* 7000 m

* 4 mins 10 secs

* 40 km distance travelled

 

 

Next test is gliding at idle to maintain 510 kph IAS to see if basic airframe drag is too high.

 

Best regards,

Tango.


Edited by Tango
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My first clean test result is in.

 

CONDITIONS:

 

* Temp: +15 C

* QNH: 1013.2 hPa

* Takeoff airport: Krymsk

* CLEAN (max internal fuel only)

* Hot ramp start

 

* FULL REHEAT until 600 kph

* FULL POWER (MIL) level acceleration to 870 kph TAS

* Climb to maintain 870 kph TAS

 

Max altitude attained: 7200 m

Time: 12 mins 20 sec

TAS: 810 kph sustained in level flight

 

Conclusion: the engine doesn't produce enough thrust. Even reaching 1000 kph in level flight at sea level will be hard (which it shouldn't be).

 

Next test is gliding at idle to maintain 510 kph IAS to see if basic airframe drag is too high.

 

Best regards,

Tango.

 

According to the climb performance charts in the same manual that I have used earlier:

 

Climb-out to 7000m in clean configuration and in ISA conditions takes 4 minutes and 10 seconds instead of 12 mins 20 sec

Manual does not explicitly state that this is with or without the use of afterburner. However, since it is SOP that the afb is turned of at 600 km/h after takeoff, I would say that the chart is for non-afb use.

 

@Tango - I agree that this indicates a serious lack of thrust, even at low altitudes.

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'Frett'

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Just did a ENGINE IDLE glide test, and I got:

 

* 8000 m altitude loss

* 20 km lateral distance

* Vertical speed of ~50 m/s

* Maintaining 510 kph IAS

* Pitch attitude required: -10 degrees!

 

I need to find a better way to measure the distance (using RSBN at the moment).

 

Best regards,

Tango.

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Just something I'm sure you've thought of: a number of times I've forgotten my flaps switch on my WH, started in the air, and since I have controls sync on, ended up flying a while with the flaps out until I notice. Something to keep in mind!

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Thanks, but that is not it. Also, the flaps retract due to air pressure upon them.

 

The aircraft has waaaay too much drag (as well?).

 

From the flight manual:

 

10000 m initial, 8 mins 10 secs descent, 97 L fuel burned, 90 km distance covered.

 

Descent condition: IDLE.

 

Best regards,

Tango.

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Don't think you can ever reach Mach 2.05 in DCS at 13000m with the current FM even if you fly without stores and 1000L fuel.

 

EDIT1: I was able to reach Mach 2.05. After 12 minutes (starting at 13km and already flying mach 1.2). Burnt almost all fuel. In other words: you will never be able to reach it in a mission that starts on the ground because you will run out of fuel during your 15 minute full afterburner effort to reach mach 2.05.

 

870 km/h IAS on climb? Let's see if that is possible while maintaining a reasonable vertical speed...

 

EDIT2: Looks like a good climb rate at 870 km/h IAS. But you want to reach it with afterburner or it will take ages.

EDIT3: oops. TAS, not IAS. Disregard the above.

 

So far all reports indicate a serious thrust/drag issue.

 

With three external fuel tanks a was able to reach 13 000 m in 13 minutes. I activated afterburner in 6000 m and after reaching 13 000 m i still had 400l in external tanks. Well it is really not efficient but possible even from the ground.

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