Engines performance

[wilbur81]

17 hours ago, captain_dalan said:

- The supersonic part of the envelope is overmodelled, that is, the jet has quite a bit more power then the chart predicts. This actually goes to confirm some user tests that claim the clean jet is capable of "supercruise" around this altitude. Further tests needed to confirm or deny this;

What i would like to see is more people doing these tests, with at least a dozen or passes for each data point, so we can eliminate statistical errors.
 

Great post! Athough, regarding supercruise and the Hornet, according to former Tomcat and F/A-18A pilot, Mike Vizcarra,  the "slow-engined" A-Model can even supercruise under certain conditions... Have a watch, I think starting around 12:25 minutes or so:

 

Edited December 22, 2021 by wilbur81

[HWasp]

16 hours ago, captain_dalan said:

My result figures are close to @Figaro9 (sorry to have taken so long to do those tests mate, RL was just too hectic). Before i post them, just a few disclaimers i think we should all be aware about:
0. The NASA chart is ONLY for 15000ft, so just because the data may match the sample, we have no idea of how the jet performs comparatively in other parts of the envelope;
1. The NASA document doesn't state the configuration. It's pure coincidence that 2xSparrows - belly mounted, 2XSidewinders and roughly 65% fuel that equals 33693pds gross weight, seems to fit the chart best. In the words of Mr. Spock, "we happen to have a theory (i would say hypothesis) that happens to fit the facts available;
2. The configuration observed is with stores, albeit the most conformal stores possible. Still, ED is in the process of changing-refining the store drags, so any current tests may not match the final results;
3. Finally, these tests are best performed with larger statistical samples, so random variations and oscillations in the data points can be averaged out. Also, the best precision (possible within 0.1 metric unit) should be observed. 

But enough academics, these are my findings:
- The current FM with the above configuration in mind, happens to fit the NASA chart closely in the subsonic region up to mach 0.9 - there seems to be a slight dip in performance around mach 0.6, but this is a 4% variation, that is, inside DCS's standards as they are. Possible room for fine tweaks though;
- The extreme low speed part of the envelope can't really be evaluated with this tests - bellow mach 0.3, the plane is essentially stalling and so any data gathered is extremely erratic;
- The supersonic part of the envelope is overmodelled, that is, the jet has quite a bit more power then the chart predicts. This actually goes to confirm some user tests that claim the clean jet is capable of "supercruise" around this altitude. Further tests needed to confirm or deny this;

What i would like to see is more people doing these tests, with at least a dozen or passes for each data point, so we can eliminate statistical errors.
Have a good night guys, it's late here. And keep testing, i'll lurk even when i can't test or post!  

P.S. (edit) the small red dots-crosses are my results overlaid on the chart

 

Hi!

Thanks for testing this.

I completely agree regarding the STR performance.

I think the current problem, what can be verified using this chart is regarding the ITR and Lift Limit line:

 

 

 

The corner speed is quite a bit lower, there is a significant amount of extra lift for the DCS Hornet.

 

[wilbur81]

16 hours ago, Hulkbust44 said:

404-400 or 404-402?

Mobius708
 

^^^An important question/distinction.

[M1Combat]

34 minutes ago, wilbur81 said:

^^^An important question/distinction.

And as was pointed out...

"The document was referenced before in the discussion i think, and linked here directly from NASA's site."

Go take a look :).

[M1Combat]

An interesting thing to note...

As much as we love SME's...

The charts are IMO a much better method to use to match up performance LIMIT envelope data points.  Tansient data...  Like how much buffeting do you get at this or that AOA and speed...  Sure...  they can relate that and honestly it just needs to seem correct because we'll just never really know.

BUT...

Any SME saying anything about the limits of performance and MAYBE even the feel of the airframe (because it's subjective) can not be used whether it's positive or negative.  I don't say that because I think their experience isn't valuable or something...  but simply because they aren't in a jet at the time of playing DCS.  It's all subjective and based on how this or that aspect "feels".  That's simply not going to be accurate.  If they grabbed a stopwatch or other sensor equipment and went and actually empirically TESTED flight performance in super specific ways and then reported those specific results...  then sure...  that data is usable...  but also likely wouldn't be divulged :).  Anything short of a science based approach to limits or feel based things...  just isn't useful.  knowing that "when you flip this switch while that system is on it always blows a breaker that you then need to reset"...  clearly for the Russian planes in this case ... is super valid stuff and completely within the wheelhouse of what an SME can reliably provide.

Speaking of feel...  I feel this won't be a popular opinion :)...  I'm fine with that :).  I think the opinion of the SME is super valid in almost all subjects aside from flight model performance.

I think they provide very valuable input...  just not when the EM charts are well lined up and they're effectively saying the EM chart is wrong.  I mean unless they're willing to directly say the EM chart is wrong :)...  But I doubt they would.

Edited December 22, 2021 by M1Combat

[Rainmaker]

50 minutes ago, M1Combat said:

Speaking of feel...  I feel this won't be a popular opinion :)...  I'm fine with that :).  I think the opinion of the SME is super valid in almost all subjects aside from flight model performance.

I think they provide very valuable input...  just not when the EM charts are well lined up and they're effectively saying the EM chart is wrong.  I mean unless they're willing to directly say the EM chart is wrong :)...  But I doubt they would.

 

If many understood the charts as well as they think they actually do, then they would understand that EM diagrams that are published in the flight manuals describe very limited/very targeted sliver of the actual flight model.  They are max performance charts, done to specific profiles, etc. That’s it. You actually need SME support for about the other 99% of it because the charts in no way cover those aspects. A lot of you have seen SMEs comment on are those parts, which are not covered...but again, it’s the typical norm to come to a forum and see those who have never done the task challenge the opinions of those who ACTUALLY have. 
 

And fun fact:  those charts CAN have inaccuracies. 

Edited December 22, 2021 by Rainmaker

[M1Combat]

That's almost exactly what I was saying friend :).

I agree with you as far as I can tell...

 

The reason I "mostly" left my comments relating to the EM charts and other actual LIMITS of the flight model...  Is specifically because all of the feel is subjective, but at least with the EM charts (perfectly correct or not) we have a defined limit.  Anything that says that it wrong is either subjective or has been measured to be wrong.  If it has been measured to be wrong I don't suspect the SME's will directly say so...  so we should just use the EM charts for the limits :).

 

Regarding the area within the limits...  Yeah...  The SME's can have a lot of input.

 

Is that not what I said??  Sorry if I just cocked up my post :).

Edited December 22, 2021 by M1Combat

[Rainmaker]

9 minutes ago, M1Combat said:

That's almost exactly what I was saying friend :).

I agree with you as far as I can tell...

 

The reason I "mostly" left my comments relating to the EM charts and other actual LIMITS of the flight model...  Is specifically because all of the feel is subjective, but at least with the EM charts (perfectly correct or not) we have a defined limit.  Anything that says that it wrong is either subjective or has been measured to be wrong.  If it has been measured to be wrong I don't suspect the SME's will directly say so...  so we should just use the EM charts for the limits :).

 

Regarding the area within the limits...  Yeah...  The SME's can have a lot of input.

 

Is that not what I said??  Sorry if I just cocked up my post :).

 

I may be proven wrong, but I have yet to see a case where a SME has disagreed with an EM diagram, and said it’s wrong. What I have seen is a lot of comments on parts of the envelope that are not in any covered by an EM chart. Loaded acceleration, non-level turns, role rates at particular alphas, etc, etc, etc.  That is all Primarily a SME area, that sense/ability comes from performance /time in the real thing.  The time spent inside the window of an EM diagram is basically nil...it’s all the other stuff that matters that an EM chart is never going to give you. 

Edited December 22, 2021 by Rainmaker

[M1Combat]

I agree with that too friend :)...

Here's the problem though...

 

in sim Racing we found that real racecar drivers are 100% NOT the best source of information to tell us whether or not cars are right...  Because sim cars aren't cars.  You can make them "exactly" right relative to all of the data...  acceleration in all directions, weight, downforce, drag, momentum...  all of it...

Someone without extensive SIM experience will never think it feels right...  because they can't feel it.  It's that simple.

I'm 100% not trying to say this means the sim pilot/racer exp. is worth MORE...  It's not exactly...  but what's required is that we take input from SME's only where it makes sense to.  It's doesn't make sense for "everything".  It can't.  They tend to not have as much SIM experience as they do REAL WORLD exp...  so it's difficult for them to translate real world "feel" into something they can't actually feel... UNTIL...  they have a great deal of sim AND real world experience...  Now they can actually translate.

 

[Hulkbust44]

And as was pointed out...
"The document was referenced before in the discussion i think, and linked here directly from NASA's site."
Go take a look :).

I could not find it, could someone please PM me the link or something?

Mobius708

[wilbur81]

3 hours ago, Hulkbust44 said:

I could not find it, could someone please PM me the link or something?

Mobius708
 

The document posted is for the F-18 HARV from 1994 does not specify the engine type... so that's an unknown at this point. Cool doc, though.

Edited December 23, 2021 by wilbur81

[Hulkbust44]

The document posted for the F-18 HARV from 1994 does not specify the engine type... so that's an unknown at this point. Cool doc, though.

I'm sorry, the document that they are referencing here is for the HARV?

If so then that data is in no way applicable...

Mobius708

[HWasp]

https://ntrs.nasa.gov/citations/19950007836

Edited December 22, 2021 by HWasp

[HWasp]

Works for me, but original link is on the top of page 3, by Figaro9

[M1Combat]

People always expect someone else to get them things...  I always try to avoid it unless they don't ask :).

 

I suppose that's passive aggressive but I don't care.

[Hulkbust44]

Works for me, but original link is on the top of page 3, by Figaro9

Thanks, found it once I got onto a PC.

Mobius708

[Figaro9]

On 12/22/2021 at 8:11 PM, wilbur81 said:

^^^An important question/distinction.

Well, guess that plot is for the f18 with the 402 engine.

1. 33’325lbs corresponds to a 7.33g-limit
2. Max sustained speed at 15kft is ~m1.4 (gao & eidetics). 
3. Speed design limit is m1.5 (gao & eidetics)
4. 1g Sep looks congruent too.

While for the f18 with 400 engines max speed is m1.27 at 15kft, 2/2, 32500 lbs

[wilbur81]

4 minutes ago, Figaro9 said:

Well, guess that plot is for the f18 with the 402 engine.

Actually, I think it is just the 400 per the document screen here:

https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.523.6760&rep=rep1&type=pdf

[Hulkbust44]

There you go.

So, guys do not reference the HARV document, it means nothing for us.

Mobius708

[HWasp]

8 hours ago, Hulkbust44 said:

There you go.

So, guys do not reference the HARV document, it means nothing for us.

Mobius708
 

1. That doc, what Wilbur linked, is a different document. 

2. You can't simply disregard the whole diagram based on that, it can still be a good source of information. Possible differences should be considered.

3. Keed in mind, when being so strict about docs, that ED had to develop this without access to the official EM charts, as they are classified... 

We use what we have, results can be argued based on known differences. No blanket statements based a single sentence somewhere please.

[captain_dalan]

On 12/22/2021 at 3:18 AM, Hulkbust44 said:

404-400 or 404-402?

Mobius708
 

Ah, now i get it. Sorry for the late reply, i was off for a few days. The HARV itself used the 400's, but that doesn't help us one bit, as the document itself doesn't mention what variant the chart is for. I hadn't read that thing in over a month, but i just gave it a quick look and the chart is presented in  a very non specific, almost generic manner for a Hornet purely in academic fashion. So we literally know nothing about it. Except that it does match a good portion of the subsonic envelope for the DCS F-18 in the previously listed configuration 

EDIT: ah, someone mentioned the HARV engines above. Got sniped by 2 days 
But yeah, that's not the document with the chart. 

Edited December 25, 2021 by captain_dalan