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

[GGTharos]

15 hours ago, Ironhand said:

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

 

In a way yes.   It's part of the engine's thermodynamic curve.  Now, we don't have an OAT gauge to judge if the temp changes according to the low altitude temp, but given the fact that you can get different performance by varying the mission temp it's obviously simulated in some way.

[Ironhand]

Just now, GGTharos said:

 

In a way yes...given the fact that you can get different performance by varying the mission temp it's obviously simulated in some way.

Ahh, yes. I had forgotten about that.

[GGTharos]

Your ceiling is 52000 on a good day with 38000lbs GW for the -220, on a good day.   You need to be supersonic to reach higher and your speed envelope for reaching and holding the highest altitude is very thin.

[Teknetinium]

Tacview-20210721-024213-DCS-test11.zip.acmiTacview-20210721-023301-DCS-test11.zip.acmiTacview-20210721-022815-DCS-test11.zip.acmi

Su-27/33 has higher T/W then F-18C and should have around same as F-16C, Why do we see so big difference?
All aircraft's are full fuel, no payload. 

Edited July 21, 2021 by Teknetinium

[bies]

22 hours ago, Teknetinium said:

Tacview-20210721-024213-DCS-test11.zip.acmi 80.07 kB · 0 downloads Tacview-20210721-023301-DCS-test11.zip.acmi 37.26 kB · 0 downloads Tacview-20210721-022815-DCS-test11.zip.acmi 43.22 kB · 0 downloads Su-27/33 has higher T/W then F-18C and should have around same as F-16C, Why do we see so big difference?

All aircraft's are full fuel, no payload. 

 

 

Full fuel? Fully fueled Su-27 (not to mention Su-33...) has way lower T/W than F-16C Block 50 CCIP and even marginally lower than F/A-18C.

Fuel fraction - it practically decides fully fueled T/W ratio. (Notice kilonewtons devided by kilograms in all cases for simplicity)

 

Mass taken directly from the DCS, Fully fueled + gun ammo + pylons T/W ratios:

 

MiG-29A  2x81,6kN (163,2kN) / 14445 kg = T/W 1,13   (fuel fraction 23%)

F-15C  2x 105,7kN (211,4kN) / 19727 kg = T/W 1,07     (fuel fraction 31%)

F-16C  1x 131kN/ 13119 kg = T/W =1                            (fuel fraction 25%)

F/A-18C  2x 79kN (158kN) / 17058 kg = T/W 0,93      (fuel fraction 29%)

Su-27  2x 122,6kN (245,2kN) / 26797 kg = T/W 0,91   (fuel fraction 35%)

F-14B  2x 125kN (250kN) / 27560 kg = T/W 0,91        (fuel fraction 27%)

Su-33  2x 122,6kN (245,2kN) / 29327 kg = T/W 0,84  (fuel fraction 32%)

 

In case of Su-27 everything above 60% internal fuel is considered as "internal drop tank", it was the reason of some considerable scuffle between design bureau and Soviet military.

That's why when you set Su-27 in mission editor it has only 59% internal fuel as default setting, when all other fighters have 100%. And that's the reason fully fueled Su-27, not mentioning way heavier airframe Su-33, has lower T/W ratio than most comparable fighters.

 

Set Su-27 88% fuel and you have fuel fraction just like an F-15C (and only slightly lower T/W than F-15C)

Su-27 with 88% fuel 2x 122,6kN (245,2) / 25669 kg = T/W 0,95     (fuel fraction of F-15C)

 

Set Su-27 default 59% fuel and you have T/W just like an F-15C (but lower fuel fraction than F-15C)

Su-27 with 59% fuel 2x 122,6kN (245,2) / 22943 kg = T/W 1,07      (T/W like an F-15C)

 

Su-33 is simply to heavy airframe to compete against non-carrier capable airframes with T/W.

 

Early MiG-29A has the best T/W, even slightly better than the F-15C (comparable with F-15A), but it's due to MiG-29A's proportionally lowest fuel fraction of only 23%. Cold War lighter F-16s (and F-15A) had similar fuel fraction and T/W to MiG-29A - and similarly, all subsequent MiG-29 variants were losing more and more of it's T/W.

Edited July 22, 2021 by bies

[Teknetinium]

Thank you Bies, appreciated. 

Will do some more tests using data you provided. 

 

Edited July 22, 2021 by Teknetinium

[henshao]

7 hours ago, bies said:

 

Full fuel? Fully fueled Su-27 (not to mention Su-33...) has way lower T/W than F-16C Block 50 CCIP and even marginally lower than F/A-18C.

Fuel fraction - it practically decides fully fueled T/W ratio. (Notice kilonewtons devided by kilograms in all cases for simplicity)

 

Mass taken directly from the DCS, Fully fueled + gun ammo + pylons T/W ratios:

 

MiG-29A  2x81,6kN (163,2kN) / 14445 kg = T/W 1,13   (fuel fraction 23%)

F-15C  2x 105,7kN (211,4kN) / 19727 kg = T/W 1,07     (fuel fraction 31%)

F-16C  1x 131kN/ 13119 kg = T/W =1                            (fuel fraction 25%)

F/A-18C  2x 79kN (158kN) / 17058 kg = T/W 0,93      (fuel fraction 29%)

Su-27  2x 122,6kN (245,2kN) / 26797 kg = T/W 0,91   (fuel fraction 35%)

F-14B  2x 125kN (250kN) / 27560 kg = T/W 0,91        (fuel fraction 27%)

Su-33  2x 122,6kN (245,2kN) / 29327 kg = T/W 0,84  (fuel fraction 32%)

 

In case of Su-27 everything above 60% internal fuel is considered as "internal drop tank", it was the reason of some considerable scuffle between design bureau and Soviet military.

That's why when you set Su-27 in mission editor it has only 59% internal fuel as default setting, when all other fighters have 100%. And that's the reason fully fueled Su-27, not mentioning way heavier airframe Su-33, has lower T/W ratio than most comparable fighters.

 

Set Su-27 88% fuel and you have fuel fraction just like an F-15C (and only slightly lower T/W than F-15C)

Su-27 with 88% fuel 2x 122,6kN (245,2) / 25669 kg = T/W 0,95     (fuel fraction of F-15C)

 

Set Su-27 default 59% fuel and you have T/W just like an F-15C (but lower fuel fraction than F-15C)

Su-27 with 59% fuel 2x 122,6kN (245,2) / 22943 kg = T/W 1,07      (T/W like an F-15C)

 

Su-33 is simply to heavy airframe to compete against non-carrier capable airframes with T/W.

 

Early MiG-29A has the best T/W, even slightly better than the F-15C (comparable with F-15A), but it's due to MiG-29A's proportionally lowest fuel fraction of only 23%. Cold War lighter F-16s (and F-15A) had similar fuel fraction and T/W to MiG-29A - and similarly, all subsequent MiG-29 variants were losing more and more of it's T/W.

 

 

all this is why I prefer to cut through percentages altogether and compare/ 1v1 fighters with, for instance, 4k lbs fuel for single engine and 8k lb fuel for twin engine or some other rule designed to level the field

[GGTharos]

Wrong thread? 

[bies]

58 minutes ago, henshao said:

 

all this is why I prefer to cut through percentages altogether and compare/ 1v1 fighters with, for instance, 4k lbs fuel for single engine and 8k lb fuel for twin engine or some other rule designed to level the field

 

Fuel fraction is way more reliable measure because if you give "8k lb for twin engine" to compare i.e. MiG-29 and Su-27 it will be very misleading - Su-27's way bigger 122,6kN engines will burn this fuel way faster than smaller 81,6kN engines. That's why fuel fraction is so commonly used.

 

(In fact "8k lbs" wouldn't even fit inside MiG-29A when for Su-27 it would be less than half fuel capacity - hardly useful)

 

Su-27 is simply unique with it's huge internal fuel capacity, it's restricted in maneuverability with full fuel load, but it doesn't have to carry draggy external fuel tanks.

[nighthawk2174]

Any updates on this?

[DCS FIGHTER PILOT]

Over three years later, still seems to be an issue. Anyone else agree?