[RESOLVED] Cruise performance not realistic

[Corrigan]

Ok, so I did some tests with our new realization about how to interpret the charts.

 

I went for the 5000 and 7000 m climb, as we've done before, but now measuring both distance and time from a standing start, not as you did in the first post, from when the climb commences. We know this has to be the correct way to do it, since the charts are uninterpretable upon the other view. They simply must measure time and distance from a standing start, or they are geometrically not possible, as detailed earlier.

 

My test: (Using a clean aircraft with 100% internal fuel, and 15 deg C base temp, starting from Batumi)

 

• I took off and stayed on reheat until 600 km/h IAS. I then come out of reheat and stay level (in Recovery mode) on full military until 870 km/h TAS. This took me 70 seconds and 11 km.
   
  
• I commence climb, maintaining 870 km/h TAS all the way to altitude.
   
  
• Total time and distance taken from start of takeoff roll to arrival at 5 000 m: 3 min 17 s and 40 km.
   
  
• Total time and distance taken from start of takeoff roll to arrival at 7 000 m: 4 min 40 s and 60 km. Fuel consumed: 420 L.
  

 

The chart says:

• Total time and distance taken from start of takeoff roll to arrival at 5 000 m: 3 min and 25 km.
   
  
• Total time and distance taken from start of takeoff roll to arrival at 7 000 m: 4 min 10 s and 40 km. Fuel consumed: 425 L.
  

 

Discussion: I wanted to keep this test going to 9 and 11 km, but the 870 km/h TAS regime became too hard to maintain, so I terminated the experiment. I ended up basically zoom climbing, because trying to keep 870 TAS means basically not climbing very much at 8 km, which seems very unrealistic for a MiG-21 without external stores.

 

The time to reach 5000 was pretty good, but the distance was way off. I don't know why this is, but it's definitely physically possible; due to different characteristics in climbing and accelerating in even flight, the time and distance can vary independently.

 

In the 7000 climb, distance is still off, and the time starts diverging with the chart. As we know from before, climbing characteristics are very poor at higher alt.

 

Fuel consumption remains very accurate during the tested interval.

 

Conclusions: Power/drag seems poor above ~5000. Previous results show that 10 000 m and 11 000 m climbs are not even possible with the method used in the charts, and a ceiling at not much higher than 15 000, so it's reasonable to conclude that the poor performance worsens with altitude. The reasonable climb time but poor climb distance to 5 and 7 km is difficult to interpret.

It's possible that there's a lack of power even at low altitude, but the most serious problem by far still seems to be the lack of climbing ability above ~7000 m and the very low service ceiling.

Track is attached. Note that T-O roll starts 10 seconds in.

 

 

 

Also, I noticed that there are inconsistencies in the SARPP data from your climb tests in post one. You come out of burner at around 850 IAS in the first one, for example. Anyway, time taken from TO-roll to altitude is something like 6.3 mins, not the 4 min 40 s it should take according to the chart.

 

I suspect it's VERY important to be absolutely sure of being in MIL power. I got very different results in my tests, when I thought I had the throttle at 100% but must have been a tiny bit off.

5km7kmclean.trk

Edited September 24, 2014 by Corrigan

[Tango]

Conclusions: Power/drag seems poor above ~5000.

This was exactly my finding, too. Drag too high and/or thrust too low.

 

Try gliding at idle - you'll find the drag to be too high (very high sink rate/short glide distance). Chart is on the bottom of page 12.

 

Best regards,

Tango.

Edited September 24, 2014 by Tango

[Corrigan]

Yeah, Novak responded to the idle drag, dunno if you saw that.

[Tango]

Yeah, Novak responded to the idle drag, dunno if you saw that.

 

I did thanks. Just read it again - seems he has tweaked the drag as well. Looking forward to testing it.

 

Best regards,

Tango.

[Focha]

Mmm... Jesus... The sim world just became a huge lab or quality assurance process. :P I'm still making up my mind. Sorry for the OT. I had too. :P

[Fishbreath]

I suspect it's VERY important to be absolutely sure of being in MIL power. I got very different results in my tests, when I thought I had the throttle at 100% but must have been a tiny bit off.

 

There's a switch tooltipped nozzle settings on the left vertical panel near the APU and fire extinguisher switch which turns the afterburner off. (If you have your physical throttle full forward, you see the in-cockpit throttle move backward to the military power setting when you throw it.) I find it handy when I want to be sure of military power without hitting AB.

[Jerkzilla]

Do any of the manuals ever detail what AoA the aircraft is supposed to maintain at 10000m flying at cruise speed? I'm just spitballing but what if the issue here is that whatever parameter or parameters is supposed to account for lift in different air densities is off, so that a MiG 21 with the loadout of 2 missiles and central fuel tank can't sustain the drag incurred from the higher AoA caused by lack of lift.

[cz_jc]

Just to add something: I thought the mig was really underperforming but it turns out that the altimeter is very different from american altimeters. The outer scale doesn't apply for the smaller (tens) indicator. That really confused me to think I'm much lower than I actually was. I only noticed that I'm an idiot when I was at '3000 meters' (haha) and doing over mach 1.2 at 600kph. That made me double check the manual. Hope this helps someone.

[Tango]

Do any of the manuals ever detail what AoA the aircraft is supposed to maintain at 10000m flying at cruise speed? I'm just spitballing but what if the issue here is that whatever parameter or parameters is supposed to account for lift in different air densities is off, so that a MiG 21 with the loadout of 2 missiles and central fuel tank can't sustain the drag incurred from the higher AoA caused by lack of lift.

 

This thought crossed my mind, too, but the AoA doesn't seem crazy initially (if you burn to 700 kph, AoA is reasonable for level flight, but it still decelerates at MIL and eventually stalls).

 

It seems it is parasitic drag that is wrong, and increases drastically at altitude. Drag due to alpha (or "form drag") appears to be OK.

 

What is really weird is it doesn't directly seem to be related to TAS or Mach number - there is definitely an altitude component to it.

 

Best regards,

Tango.

Edited September 27, 2014 by Tango

[Corrigan]

Just to add something: I thought the mig was really underperforming but it turns out that the altimeter is very different from american altimeters. The outer scale doesn't apply for the smaller (tens) indicator. That really confused me to think I'm much lower than I actually was. I only noticed that I'm an idiot when I was at '3000 meters' (haha) and doing over mach 1.2 at 600kph. That made me double check the manual. Hope this helps someone.

 

Thanks, but I (and probably others) have been using external view for the important altitude figures.

[cz_jc]

Thanks, but I (and probably others) have been using external view for the important altitude figures.

Sorry i didn't mean to imply that you didn't know what you were doing. I'm just saying in case someone thick (like me) can't read the altimeter and finds this forum post (like I did). Thanks for doing great work !

[Corrigan]

Sorry i didn't mean to imply that you didn't know what you were doing. I'm just saying in case someone thick (like me) can't read the altimeter and finds this forum post (like I did). Thanks for doing great work !

 

It's cool, sometimes people miss obvious things and then stuff like that is helpful. :)

[Vinny002]

Hi, guys!

 

Is the cruise performance fixed now? Thanks!

 

Cheers,

Vincent

[panzerd18]

Hi, guys!

 

Is the cruise performance fixed now? Thanks!

 

Cheers,

Vincent

 

 

 

Clean aircraft 11,000m alititude at Mach 0.84 requires around 88% N1 throttle.

 

Loaded aircraft with centerline tank at 10,000m altitude at Mach 0.83 requires around 93-95% throttle.

Edited February 9, 2015 by panzerd18

[panzerd18]

Just did another test.

 

At 100% military power the Bis can barely maintain 14,000m.

 

So I would say in my test the absolute ceiling is somewhere just over 14,000m, when it should have a service ceiling of at least 15,000m with an absolute ceiling somewhere even higher. This is with around 1000kg of fuel left.

[Justin Case]

At 100% military power the Bis can barely maintain 14,000m.

 

Wouldn't the service and absolute ceilings require afterburner?

Edited February 9, 2015 by Justin Case

[TurboHog]

Wouldn't the service and absolute ceilings require afterburner?

 

Yes.

 

16000 m mach 2 with afterburner (not even full afb) cruises nicely. Make sure you maintain a high speed throughout the climb if you want to accelerate to mach 2 in a short amount of time. Preferably, don't go below 750 km/h indicated.

 

In the cruise charts, 10 km seems to be the maximum for non-afb cruise with an A2A loadout. Cruise performance for heavy A2G loadouts is measured/calculated up to 5 km, so don't expect to cruise with such a loadout easily at 10 km altitude.

[panzerd18]

Wouldn't the service and absolute ceilings require afterburner?

 

If this is the case then I was working from the wrong assumption that service ceiling was in 100% military power only.

 

I can achieve the 17,800m service ceiling no problem with afterburner. From my testing I could also achieve 12,000m service ceiling with 100% military power no problem.

 

So it seems there is actually no problem if service ceiling includes the use of afterburner.

[effte]

From the MiG-21Bis Flight Operating Instructin floating around:

"The service ceiling for an aircraft carrying two missiles and climbing at FULL THROTTLE at a true airspeed of 870 km/h, is 12,000 m (under the standard atmosphere conditions, at an average weight of the aircraft) and 11,0000 m with four missiles.

The service ceiling for an aircraft carrying two missiles and climbing at FULL REHEAT (under the standard atmosphere conditions) is 17,500 m, the fuel remainder at the ceiling altitude amounting to 700 l."

 

The procedures to use are also described.