investigating F-15 MIL thrust underperforming at altitudes over 20000'

[GGTharos]

@BIGNEWY

I performed a test vs. the time-to-climb chart with the following parameters:

Day = STD ISA, 15C, 2992 inHg, no turbulence, wind etc

Aircraft:  clean, 38000lbs when the test begins

Method:  Follow chart TO and climb schedule (Nose to 10 deg at TO, maintain until 350 KIAS then pitch for 350 KIAS until transition to 0.9M, then pitch for 0.9M).  Time begins counting when climb speed is reached (approximately 50 sec into the track)

 

The results show a significant deviation above 20000'.   'REF' is the F-15 MIL climb chart, the 'diff to previous' is my way of checking how badly my technique is off

 

My table is as follows:

 

REF alt Ref time DCS time Diff ref diff to previous dcs diff to previous previous deviation 10000 42 39 3       20000 102 102 0 60 63 -3 30000 174 196 -22 72 94 -22 35000 215 245 -30 41 49 -8 40000 260 319 -59 45 74 -29

38000lbs_MIL_40k.trk Tacview-20210314-121515-DCS-F-15-Performance_Trials.zip.acmi

Edited March 14, 2021 by GGTharos

[draconus]

Can it be caused by this? https://forums.eagle.ru/topic/265110-hud-in-all-aircraft-displays-eas-instead-of-cas/

And same with the Su-27 underperformance?

[GGTharos]

That probably wouldn't account for the IMN portion.

[henshao]

you know I always thought the F-15 felt a little anemic at those higher altitudes at cruising power

[Sacarino111]

42 minutes ago, henshao said:

you know I always thought the F-15 felt a little anemic at those higher altitudes at cruising power

Same here, I always find myself on AB all time to have a decent amount of speed at those hight altitudes. 

Saluods.

Saca111

[GGTharos]

@BIGNEWY, @NineLine, could we get this reported please?   I'll also rerun the tests using the conversion tables for the speeds because of what draconus mentioned above, however I don't expect to see a huge difference - an entire minute is going to be hard to make up with EAS/CAS errors above 20000' I think, and especially from 30000' to 40000' where we're using IMN.

 

As a test reference, the DCS F-15 matches the MAX (AB) thrust performance almost perfectly using the same method, the only modification is holding 0.95 IMN after the crossover point.

Edited March 17, 2021 by GGTharos

[Vekkinho]

It ain't just the Eagle, most jets in DCS underperform at altitudes beyond 30.000ft. if you fly straight and level and start from M1.0 holding throttle in FullMil you'll stall within few minutes if you don't kick your AB every now and then. I made a F-15C test mission of escorting a B747 flying M ,9 at 39.000ft. Had centerline FT, 2*AIM120 and 2 AIM-9. In order to keep up with it in S&L flight had to use my AB halfway thru. I find it hard to believe.

[Vekkinho]

Now I just gave it a try with 2.7...

 

F-15C @ 40.000ft MSL, straight and level flight in ALT and ATT hold,

5200lbs of fuel 

2xAiM-120C on stations 3 / 4

1xAiM-9M on 11

ACMI pod on 1

G 1.0

3°AoA

0° pitch / 0° bank

Synced both engines to 92% RPM (Full Mil throttle) and

492kts TAS / 244 IAS / M .88 is constant. 

 

Edit: Although she seems to accelerate by 1kt for each 100lbs of fuel burn.

  

What do you think? Under performing or not? 

Edited April 21, 2021 by Vekkinho

[Krippz]

Good report @GGTharos

[GGTharos]

On 4/21/2021 at 6:15 AM, Vekkinho said:

Now I just gave it a try with 2.7...

 

F-15C @ 40.000ft MSL, straight and level flight in ALT and ATT hold,

5200lbs of fuel 

2xAiM-120C on stations 3 / 4

1xAiM-9M on 11

ACMI pod on 1

G 1.0

3°AoA

0° pitch / 0° bank

Synced both engines to 92% RPM (Full Mil throttle) and

492kts TAS / 244 IAS / M .88 is constant. 

 

Edit: Although she seems to accelerate by 1kt for each 100lbs of fuel burn.

  

What do you think? Under performing or not?

 

My gut feeling is yes, because the following configuration tops out at M0.97 in MIL:

 

DAY:  STD

Temp: 5C -> 25C  (yep, the difference due to temp is that small for the -220, unlike the -100!)

Engine: -220

GW=41500lbs

4xAIM-9, Centerline tank (draggy!)

 

Edit:  Having said that, this is the wrong test for checking engine function, this test should be done with a clean aircraft.

Edited May 27, 2021 by GGTharos

[Vekkinho]

Yeah, I tell you man it's difficult keeping up with airliners (AWACS and Tankers) in DCSW, straight and level flight @ 38000ft. Requiring occasional AB usage which is hard to believe.

 

If you remove pylons from DCS F-15C  would it improve it's performance in the game? Did anyone try that?

[GGTharos]

6 minutes ago, Vekkinho said:

Yeah, I tell you man it's difficult keeping up with airliners (AWACS and Tankers) in DCSW, straight and level flight @ 38000ft. Requiring occasional AB usage which is hard to believe.

 

It's like you're flying a loaded F-15E.   But, I would suggest moving those tankers to a more palatable refueling altitude of say 20 to 30 thousand feet and keeping the IAS at 275kts.

 

6 minutes ago, Vekkinho said:

If you remove pylons from DCS F-15C  would it improve it's performance in the game? Did anyone try that?

 

I'm not sure that the pylons do anything at all other than being there visually.

[Max1mus]

2 F-15C pilots on LowBlows stream commented on the F-15Cs flight model that its too easy to unload the energy with maneuvers, but also too easy to gain the energy back with acceleration. That was some months ago in a podcast like format on his twitch.

[GGTharos]

Maybe they can show how it differs from the acceleration charts then.

[Vekkinho]

On 5/27/2021 at 12:03 PM, GGTharos said:

But, I would suggest moving those tankers to a more palatable refueling altitude of say 20 to 30 thousand feet and keeping the IAS at 275kts.

 

Yeah, I just used that altitude and speed to simulate long range high flying airliner. There is a Civilian Aircraft mod, however I don't think that featured airliners performance is accurate so I used KC-135 instead.

But You're right there, C feels like an E, draggy and fat, puffing and sweating up above 30.000ft.

On 5/28/2021 at 2:27 PM, Max1mus said:

2 F-15C pilots on LowBlows stream commented on the F-15Cs flight model that its too easy to unload the energy with maneuvers, but also too easy to gain the energy back with acceleration. That was some months ago in a podcast like format on his twitch.

 

I tell you every >1G maneuver bleeds off too much. You simply can't maintain energy even in level turns without kicking burners to regain it. It does accelerate fast, but speed as featured in DCSW means nothing if you lose it just like that.

 

I watched the F-15C unrestricted takeoff HUD tape, taking of from Nellis AB.  I don't believe that jparticular jet was clean with pylons removed but pilot performed lift off maintaining 3°-4° pitch and reached 450kts at opposite threshold pulling 6G to 80°pitch and rolled wings level at 15.000ft at 250kts. 

Tried doing the same in DCSW with clean loadout and 100% of fuel and I reached 420kts at the opposite end, zoomed to 13.000ft (230kts) and nearly stalled berfore rolling inverted into a level flight at 15.000ft. I had to pitch down back to 13.000ft to regain smash for a <20°AoA level flight.

 

 

 

[GGTharos]

One significant issue here is that we are shown EAS instead of CAS.   I haven't looked into how significant the deviation is at altitude but it needs to be looked at.

 

@Vekkinho, are you certain the RWY was the same length and you're flying under the same weather conditions?  From what you've written there's no data on the aircraft configuration either.

 

I'll have to re-check but I believe MAX AB matches the charts quite well.

Edited June 7, 2021 by GGTharos

[Vekkinho]

Dunno, but I really find it weird, at higher units of AoA clean F-15C seems to aerobrake way too siginificant, almost like a delta winged aircraft. That requires dropping nose below horizon, relaxing to 1G or even going for 0G to gain back some smash. The funny thing in DCS is that different players get different performance outta same module and it's based on your specific X & Y curve setting. If used with zero or negative curve F-15 becomes way too responsive, slight pull on the X stick would generate 4G and cause performance loss, if you numb it out with a higher curvature the fight model seems to sluggish so there should be some recommended curve setting for each module and aircraft for us to compare data.

[Nahen]

Maybe it's time to just learn to fly it? The F-15C in the DCS accelerates easily, maintains the speed at 250-300 kts at altitudes up to about 35 ooo feet. Unfortunately, it is not a "supercruise" plane and will not keep speeds above about 0.8-0.9 Ma without afterburning. I have zero curves, I fly it without any problems with energy loss. If someone cannot fly, they always have a problem with speed, AoA and energy loss.

 

Aa forgot - all this with 6 AiM-120 and 2 AiM-9 and often with two under-wing tanks. Above 30,000 feet, the presence of the tanks from time to time forces "support" with an afterburner, but it is only a matter of a few seconds. No problem without the tanks.

Edited July 2, 2021 by Nahen

[draconus]

On 3/15/2021 at 2:13 PM, GGTharos said:

That probably wouldn't account for the IMN portion.

There were some changes recently, noted* for the Hornet and Viper but probably all modules got some correction of displayed/measured speed, idk, and even then TAS/TMN was affected too by the bug. Might be good to perform the test again.

 

*From the update notes: "Added new pitot model implementation (Correct KIAS, KCAS, altitude calculation)"

Comment from another thread on this change:

https://forums.eagle.ru/topic/267546-f-14b-acceleration-correct/?do=findComment&comment=4695302

 

[Nealius]

I'm not finding any of these performance issues myself. With two wing tanks and various A2A loads (6A 2W; 4A 4W; 4A 2W), 50~60% fuel load, I can supercruise M1.01 MIL power above FL300 on Marianas with mission temp 30C. 

[Not_G]

On 7/14/2021 at 6:24 PM, Nealius said:

I'm not finding any of these performance issues myself. With two wing tanks and various A2A loads (6A 2W; 4A 4W; 4A 2W), 50~60% fuel load, I can supercruise M1.01 MIL power above FL300 on Marianas with mission temp 30C. 

We're you level flight??? That is actually outperforming the charts and I couldnt even get close to duplicating this

[GGTharos]

I suspect the temp effect on the engines may still be reversed.

[Ironhand]

4 hours ago, GGTharos said:

I suspect the temp effect on the engines may still be reversed.

Do they actually model the significantly lower temps at high altitudes?

Edited July 19, 2021 by Ironhand

[Nealius]

9 hours ago, Not_G said:

We're you level flight??? That is actually outperforming the charts and I couldnt even get close to duplicating this

 

Did you plug burner to get past M1.0 at first? I forgot to mention that. With MIL all the way you won't supercruise, but if you plug burner first to punch through, say until M1.15 or M1.20, then go to MIL, it's like the plane won't decelerate below M1.01. Or maybe the deceleration was so slow it just felt like supercruise. 

Edited July 20, 2021 by Nealius

[Not_G]

1 hour ago, Nealius said:

 

Did you plug burner to get past M1.0 at first? I forgot to mention that. With MIL all the way you won't supercruise, but if you plug burner first to punch through, say until M1.15 or M1.20, then go to MIL, it's like the plane won't decelerate below M1.01. Or maybe the deceleration was so slow it just felt like supercruise. 

 

You are right, I had started the test before slower in high aoa and couldn't accelerate much at all.  I tried it again but this time starting at .9 and was getting the results you were seeing, it bled off really slow but I was able to hold it at .95~.99 and slick I was able to cruise at 1.1.  Something still seemed off though, I climbed to 50k and wasn't able to maintain at all in mil with 10k lbs of fuel in 2 bag 4xaim 120 config or with 5k lbs slick.  Ill have to look at the -1 but I thought i remembered the slick combat ceiling being above 60k ft on a standard day with acft weight around 35k