wip Calculating the SEP (Specific Excess Power) of an instantaneous turn

[DummyCatz]

I'll share a method to quantify energy bleed rate using the SEP equation P_s = dh/dt + (V/g)(dV/dt)

Reference to this equation is at Page 21 of https://dspace.mit.edu/bitstream/handle/1721.1/36378/16-885JFall2003/NR/rdonlyres/Aeronautics-and-Astronautics/16-885JFall2003/1A2F9B8F-B307-4461-83AE-9CCD658DBD50/0/aircraft_murman.pdf

Given that a clean aircraft is bleeding its speed at 10000ft from 479 KTAS to 466 KTAS in a 1-sec time span, with a pretty constant bleed rate, and a tiny altitude gain of 1 ft. Then we can dial in all the elements we need:

dh = +1 ft

dV = -13 KTAS = -21.94 ft/s

V = 472.5 KTAS = 797.5 ft/s (took the average)

g = 32.174ft/s^2

dt = 1s

The resulting Ps = 1 - 21.94 * 797.5 / 32.174 =  -543 ft/s at the speed of 472.5 KTAS, or 0.74 Mach.

 

So that you can check with RL figures.

F16 speed bleed test 10000ft.trk

[Xhonas]

I saw your deleted post, based on these calculations, is there any problem? And could you do the same to the F/A-18?

Edited August 20, 2024 by Xhonas

[DummyCatz]

2 hours ago, Xhonas said:

I saw your deleted post, based on these calculations, is there any problem? And could you do the same to the F/A-18?

 

Yes there’s a problem so it’s being investigated.

I don’t think I saw any EM diagram of the F-18 that has Ps lines on it.

[Xhonas]

32 minutes ago, DummyCatz said:

Yes there’s a problem so it’s being investigated.

Can you elaborate on what is the problem? is the jet underperforming because of that? 

[DummyCatz]

14 hours ago, Xhonas said:

Can you elaborate on what is the problem? is the jet underperforming because of that? 

In this example I chose to calculate the SEP in the range of corner plateau. This is also where the negative SEP should be the highest, at the top of the EM diagram. To my surprise, -543 ft/s is far from it, which means the aircraft bleeds less speed and therefore overperforms in SEP, at least at the corner plateau.

This does not sound good for the longrunning Viper vs Hornet debate, but it's a starting point for anyone else interested in verifying the bleed rate.

[darkman222]

What about the energy bleed at slow speeds? From 300 kts the DCS F16 bleeds down like crazy and does struggle to gain energy back.

[DummyCatz]

9 hours ago, darkman222 said:

What about the energy bleed at slow speeds? From 300 kts the DCS F16 bleeds down like crazy and does struggle to gain energy back.

This is my next plan. But before that I need to practice my altitude holding skill.

If anyone like to help me by taking the trackfile and fly the max ITR at lower speeds, I'll be more than happy. (The trackfile already has the correct testing conditions.) Altitude can be 0, 5k, 10k, 15k, etc..

[darkman222]

3 hours ago, DummyCatz said:

fly the max ITR at lower speeds,

How do you mean that? Max instantaneous turn rate is just a max performance max available G pull. Not sure how you could "fly" a max ITR.

[DummyCatz]

5 minutes ago, darkman222 said:

How do you mean that? Max instantaneous turn rate is just a max performance max available G pull. Not sure how you could "fly" a max ITR.

I just meant you perform a max effort pull.

 

BTW in this exact document by Eagle Dynamics, the EM diagrams of F-16 at 10000ft, GW=22000 lbs is quoted at page 23 of https://www.digitalcombatsimulator.com/upload/iblock/dcc/DCS FM principles plus MiG-29 P-47 F-16.pdf

So I can definitely compare the results here without breaking the rule:

The SEP at 0.74 Mach is -543 ft/s compared to -800 ft/s in the graph.

 

 

 

Edited August 22, 2024 by DummyCatz

[DummyCatz]

Second try with 4 data points: Mach 0.62, 0.58, 0.55 and 0.52, at 1 sec interval.

SEP Calculation 1. M0.62 -> M0.58:

dh = -1 ft

dV = -20 KTAS = -33.76 ft/s

V = 387 KTAS = 653.2 ft/s (took the average)

g = 32.174ft/s^2

dt = 1s

The resulting Ps = -1 - 33.76 * 653.2 / 32.174 =  -686 ft/s at the speed of 387 KTAS, or 0.6 Mach. This is a bit low compared to the EM diagram above from the ED document. (Overperforms in SEP)

SEP Calculation 2. M0.58 -> M0.55:

dh = -2 ft

dV = -22 KTAS = -37.13 ft/s

V = 366 KTAS = 617.7 ft/s (took the average)

g = 32.174ft/s^2

dt = 1s

The resulting Ps = -2 - 37.13 * 617.7 / 32.174 =  -715 ft/s at the speed of 366 KTAS, or 0.565 Mach. This is too much. So from here, the F-16 is underperforming.

SEP Calculation 3. M0.55 -> M0.52:

dh = -7 ft

dV = -20 KTAS = -33.76 ft/s

V = 345 KTAS = 582.3 ft/s (took the average)

g = 32.174ft/s^2

dt = 1s

The resulting Ps = -7 - 33.76 * 582.3 / 32.174 = -618 ft/s at the speed of 345 KTAS, or 0.535 Mach. This is also too much.

F16 speed bleed test 10000ft -2.trk

Edited August 22, 2024 by DummyCatz

[darkman222]

Just my amateur 2 cents. I did not expect the DCS F16 to bleed too little energy at corner speed. But 21 deg / sec wont win a fight against a 23 deg / sec jet anyway. Even if you can sustain it longer than you should be able to.

The only option is just to stay fast and watch your opponent outturn you or to bug out and reset. Because going slow is even worse as it drains your energy so fast and you will struggle to get it back.

It makes sense considering the overall impression the flight model gives, that the slow speed performance just punishes you a lot more than in other airframes.

 

Edited August 23, 2024 by darkman222

[DummyCatz]

I did not expect it to overperform in SEP at higher speeds too. But just as it's underperforming at lower speeds, which simply proves that the SEP values are totally not checked when ED was building their Flight Model.

I'm done with the F-16. Now I'm going to see if the Hornet is overperforming in SEP. Will be posting in this thread:

 

 

Edited August 29, 2024 by DummyCatz

[NineLine]

8 hours ago, DummyCatz said:

I did not expect it to overperform in SEP at higher speeds too. But just as it's underperforming at lower speeds, which simply proves that the SEP values are totally not checked when ED was building their Flight Model.

I'm done with the F-16. Now I'm going to see if the Hornet is overperforming in SEP. Will be posting in this thread:

 

 

This is simply not true, while it's possible there is an error, these values were checked by the team and SME's during development. The thread is marked as investigating as we are checking it out. Thanks. 

[DummyCatz]

On 8/24/2024 at 4:59 AM, NineLine said:

This is simply not true, while it's possible there is an error, these values were checked by the team and SME's during development. The thread is marked as investigating as we are checking it out. Thanks. 

Would be nice if you could utilize the tester team and QA team to check the data points across speeds and altitudes with different loadouts in EM diagrams. I am only a single person so I can not do the QA job for you.

I'm aware that flight model development is incredibly complex, and perfection is a tough target to hit. However, the community's confidence would be bolstered by a more detailed explanation of how these discrepancies arose and what steps might be taken to address them.

Please take your time. Thanks.

Edited August 29, 2024 by DummyCatz

[NineLine]

As I said, it will be investigated. There is no need to be rude. Thanks.

[DummyCatz]

Hi, I'd like to keep this thread F-16 related, thanks.

[Viper33]

It would be great if ED's "SME's" could be active on the forums or public channels and actually explain and show why they think the DCS Viper behavior is right. Would love to see it. Simply saying "trust us" is far from scientific and is never considered as an argument or proof for any bug report. Considering the many FM reworks and improvements this only proves that the FM was far freom ideal on release and in the first years of EA. Seen several SME's, some more active and known ones like Mover, who said that the DCS Viper is pretty off in some regimes of the envelope. Just modeling the Ps0 line is far from enough for a flight model. SEP across the envelope as well as the correct moments across all axes are way more important. 

Edited September 28, 2024 by Viper33

[NineLine]

5 hours ago, Viper33 said:

It would be great if ED's "SME's" could be active on the forums or public channels and actually explain and show why they think the DCS Viper behavior is right. Would love to see it. Simply saying "trust us" is far from scientific and is never considered as an argument or proof for any bug report. Considering the many FM reworks and improvements this only proves that the FM was far freom ideal on release and in the first years of EA. Seen several SME's, some more active and known ones like Mover, who said that the DCS Viper is pretty off in some regimes of the envelope. Just modeling the Ps0 line is far from enough for a flight model. SEP across the envelope as well as the correct moments across all axes are way more important. 

 

Hi, the thread is marked as investigating, meaning we are looking into it. Many of our SME's are not looking for attention, or in some cases are not even on these forums. We do not divulge sources or contacts just to do so. So yes, to a certain degree after many many years in the flight sim business, you will have to "trust us". So trust us that if it's marked investigating we feel there is enough to take a look at the issue noted. Thanks.

[Xhonas]

3 hours ago, NineLine said:

Hi, the thread is marked as investigating, meaning we are looking into it. Many of our SME's are not looking for attention, or in some cases are not even on these forums. We do not divulge sources or contacts just to do so. So yes, to a certain degree after many many years in the flight sim business, you will have to "trust us". So trust us that if it's marked investigating we feel there is enough to take a look at the issue noted. Thanks.

Hi Nineline, i saw that the F/A-18 is going through another flight model rework, i presume that is due to the issues reported here on the forums recently, the F-16 seems to have similar issues. Will the F-16 also go through a flight model rework? 

[BIGNEWY]

18 hours ago, Xhonas said:

Hi Nineline, i saw that the F/A-18 is going through another flight model rework, i presume that is due to the issues reported here on the forums recently, the F-16 seems to have similar issues. Will the F-16 also go through a flight model rework? 

Partly, there are both internal and external factors that led us to review both FMs and FCSs. While not as deep, the F-16C continues to have refinements.

thank you 

[DummyCatz]

Looks like the prompt fix to low speed SEP is nicely done, great job.

[darkman222]

Promising in deed  . Although the right side of the EM diagram, the high speed energy bleed has not been addressed yet. Too early to to judge this fix as a whole I am afraid.

Share your thoughts if you're able to test. I am heading off into holidays but still excited to read about it.

Edited December 24, 2024 by darkman222

[darkman222]

@DummyCatz I was doing a comparison between the energy bleed between the DCS version before christmas ( before the energy bleed below mach .7 was adressed) compared to the patch from the 24th Dec when the first part of the SEP issue was taken care of.

I just flew the same profile, then updated DCS and did it again.

 

Clean F16, 5000 lbs fuel. 5000 ft altitude. Starting from 450 kts. Full AB

At the 180 degree mark 430 kts compared to 380 kts pre update ( 50 kts difference)

(I also did a full 360 degree turn max performance FULL AB. Before the update this brought you to 260 kts after the turn. With the SEP update the turn ended with +40 kts at 300 kts.)

 

But here is the question:

I did the same turn again, but in idle throttle.

After 180 degrees, the speed difference already was only 4 kts ( 176 pre update to 180 kts after update)

I am talking about the 180 degree turn, because the speed loss for 360 degrees in idle is too high to compare.

 

My observation is that the energy loss in a turn before and after the update seems to remain comparable with throttle in idle. Without the engine adding power to the turn, the energy loss seems to be almost the same. Where should the better energy retention come from? Shoudnt it come from less drag in general? If so, shouldnt a turn without the engine compensating with its thrust for the energy loss also result in a higher exit speed in a turn after the SEP update?

 

 

[DummyCatz]

On 1/24/2025 at 9:03 PM, darkman222 said:

@DummyCatz I was doing a comparison between the energy bleed between the DCS version before christmas ( before the energy bleed below mach .7 was adressed) compared to the patch from the 24th Dec when the first part of the SEP issue was taken care of.

I just flew the same profile, then updated DCS and did it again.

 

Clean F16, 5000 lbs fuel. 5000 ft altitude. Starting from 450 kts. Full AB

At the 180 degree mark 430 kts compared to 380 kts pre update ( 50 kts difference)

(I also did a full 360 degree turn max performance FULL AB. Before the update this brought you to 260 kts after the turn. With the SEP update the turn ended with +40 kts at 300 kts.)

 

But here is the question:

I did the same turn again, but in idle throttle.

After 180 degrees, the speed difference already was only 4 kts ( 176 pre update to 180 kts after update)

I am talking about the 180 degree turn, because the speed loss for 360 degrees in idle is too high to compare.

 

My observation is that the energy loss in a turn before and after the update seems to remain comparable with throttle in idle. Without the engine adding power to the turn, the energy loss seems to be almost the same. Where should the better energy retention come from? Shoudnt it come from less drag in general? If so, shouldnt a turn without the engine compensating with its thrust for the energy loss also result in a higher exit speed in a turn after the SEP update?

 

 

That’s interesting. Might need to test 1g SEP at low speed too, to see if the fix caused discrepancies in other part of the diagram.