Degraded Su-27 aerodynamic lift - Page 23 - ED Forums
 


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Old 03-01-2018, 04:05 PM   #221
HWasp
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Quote:
Originally Posted by Esac_mirmidon View Post
What margin are you talking about?

There are an AoA-G limit the pilot can bypass with brute force on the stick ( 17 Kg ). Turning off the limiter is not increasing the maneuvrability of the Su-27 in an appreciable marging and is making the flight more easy to enter a stall or spin or overstreesing the airframe. That all.

You can bypass the limit for extra AoA or G in some circunstances just pushing or pulling more on the stick, knowing the danger you are taking, but the benefits are marginal and the danger, big.

Also there is a big difference between the limiter logic in the former Su-27. and the newer Su-35 ( also i think the Su-27SM series). In the original versions the logic was some sort of a fixed value for a fixed weight, but in newer Su-27/Su-35 versions this limiter logic is much more "inteligent" and dynamic taking in count more parameters for beeing more precise at different weights, speeds, maneuvers, etc.
I am talking about the margin the limiter makes by not allowing the AoA to increase gradually to 24 degrees as the speed drops according to page 14/269 of the manual I have linked before.
If you could find anything in the manual that states 22 deg is the maximum for the system, then I would be happy with that.
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Old 03-01-2018, 04:07 PM   #222
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Originally Posted by HWasp View Post
I have found an SU-27SK flight manual on the internet in russian.

http://www.avsimrus.com/f/documents-...ual-21438.html

According to this the aircraft should reach the 8g limit at around 740 kmh indicated at 1000 m and 50% fuel with 2 R27 and 2 R73 and the AOA limit increases to 24 degrees from 22 as the aircraft slows from M0.7 to M0.5

If I remember correctly (cannot test until saturday) the FBW limited AOA stays af 22 deg without override and the plane reaches the 8g limit at a higher speed with the same conditions.

In direct control mode (S) I was able to complete 360 turns at around 13 sec (bleeding speed from 600kmh to less than 250) but with the limiter it feels a bit too sluggish. I think that if there is any problem here, that might be regarding the pitch axis control law not increasing the limit AOA to 24 and the really slow g onset. In direct control everything is great if I manage not to kill myself

I cannot upload from my tablet for some reason, thats why I could only give you the link, not the actual pages
Here is my initial post with the link to the manual, since it was a long ago
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Old 03-01-2018, 04:10 PM   #223
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Originally Posted by SinusoidDelta View Post
Those are stories, not data and are entirely subjective. You don’t believe the Cl data from NASA. I provide that data. You change the subject again. Forever ‘moving the goal posts’.
It's nice to meet you then if you provide accurate data for Eagle Dynamics and sorry I had to divert from arguing about the seemingly lower Su-27 turning performance. I didn't know that of the plane seen in the 1990's demo flights had a lighter airframe than the one used now so I now agree that the Su-27 is correctly simulated as performance! Maybe I should have started a new thread now regarding the Eagle all alone and not discuss facts about 2 planes performance on a thread which initially regarded just one which I consider solved in this area, so, yes I'm sorry I have done this, but then give me an advice on how to discuss about the F-15's aerodynamic lift performances (instead of making another thread) which I find quite important if DCS is to be respected as a true simulator and not just graphics and visual effects oriented.

I don't want to go too much into the DCS F-15C discussion on this thread because it's off topic and neither like to waste so many hours writing so much, but what I saw (resulted performance values) during simulation that seem intriguing compared to what real life footage provide which I trust mostly as it is an undeniable fact of one plane's real performances. I want to find out the truth for my own curiosity (if it's different than what I know) about the F-15C now. One thing is what I knew for years and what still confirms in demo flights of the F-15 and another (totally different story) is what you guys who work at ED are telling me here. From what I believe someone's not saying the truth, so it's either the life footage of the F-15C's performances or the data you implemented in DCS. I may only trust genuine wind tunnel test data or mid air performance testing of the aircraft, otherwise what I see "in-game" I regard as "off-track".

If NASA gave you such illustration of lift coefficient versus AoA, I believe they didn't give you the accurate and true tables. Based on my experience in aerodynamics only, the maximum lift coefficient of the F-15 (global CL) as 1.6 is way too high. If it gets above 1.3 at 40 AoA at Mach 0.3 it's a big deal. There's no way it can be that high even if the plane would have a relaxed static pitch stability, while the normal F-15 has tremendous static stability margin so the needed elevators deflection to hold 38 AoA greatly degrades the resultant global lift coef. On the other hand no F-15 wing reaches the tremendous AoA of 38, unstalled. The in game ailerons responsiveness becomes lost (aileron stall) only above 27, regardless of beta angles (driven by rudder inputs which can be override), proves that in game the ailerons are still not stalled where they should (around 20 or below) as they still give good lift differentials between the wings up until 27 AoA is reached! The way this plane was flying when PFM F-15 just came out was very realistic and better reflected the real aircraft's performances, but someone didn't like it that way and wanted to make it fly as he desired, thus ruining it's authenticity.

From my own determinations (CFD & personalized algorithms) the CL max of the whole F-15 at Mach 0.2 at 20 true AoA is found at about 1.1 which means it's 70% (0.70 times) that of the F-16 which has 1.57 at 26 AoA for the same Mach. Also the lift slope of the F-15C would be about 3.065 (CL per radians) or 96% that of the F-16's, which is 3.21.

Have a short look here:
https://youtu.be/wUsE3Zhy2xw?t=223
https://youtu.be/frjZTJnSlFU?t=83

How much it takes for the F-15 to complete the circle in both examples? Yes, a staggering 20+ seconds. Much "wow" compared to 13..14 seconds in DCS at similar weight! Also note in the 2nd video how after pulling towards initiating the turn, the pilot quickly reduces the AoA and settles it at some lower value (thus degrading the G-load and turn rate) until almost finishing the turn (where he pulled a bit more again), which tells that this plane suffers from bleeding airspeed quickly if the pilot would keep the AoA a bit higher. This is non-comparable to the DCS's F-15C's which seems to bleed speed slower (because you can hold a higher AoA and turn rate for longer).

What I found during actual in game testing of the DCS F-15:

Weighing exactly 16 tons force (38% internal fuel, clean), the plane flies at 226km/h IAS while gliding (engines idle to not affect lift) at 20 AoA at nearly 1G which results in a lift coefficient of 1.15. This shows a rather correct lift slope up to this alpha (and thus lift coef.), but..., at AoA 38 it can hold nearly 1G at 190km/h IAS (engines idle) which results in a maximum lift coefficient of 1.63, which proves a rather too high and incorrect lift slope simulation from the point (20AoA) where both wings are already stalled to the point (35-40AoA) where normally the generated vortexes also brake up (supercritical AoA). This proves that the lift slope decays to only 75% of the initial one between 20 and 38 AoA which is incredibly high. It normally should find a very low positive value and only increase the CL to a maximum of about 1.28, let's say roughly 1.3, but never 1.6. So this one problem (the other is probably the drag function) why the F-15 finds such a tremendous turning rate, because every time you pull above 20 AoA (30 indicated on the cockpit indexer) the wings keep providing an unrealistically high lift and not high enough drag which make the plane turn incredibly quick, which is incomparable to the real plane, the real plane can't turn any better (just barely noticeable until the speed drastically bleeds off) above 30 on the AoA indexer and real F-15 pilots should normally confirm this!

A good correction for the F-15's aerodynamic lift performance is simple and requires nothing else than to revert to how it was when PFM F-15 first came out which I'm pretty sure it was more correctly simulated than how it does now (including lateral control also which is also exaggerated now), by flattening out the lift slope above 20 (just slight lift increase above wing stall) up to the AoA (that you determined, probably 40) where the vortexes between the engine inlets break up and from there on (higher AoA) the lift slope should become negative and drop to almost half some 5..8 degrees of AoA more, I'd say at 45. That would bring a correct simulation of the lift to AoA function.

At this moment it seems that only these products worth it:

-Belsimtek - All their products (the best flight simulation developers);
-Heatblur - JAS-37;
-RAZBAM - Harrier only;
-ED's - All their products (greatest team with Belsimtek), except for the F-15C in FC3.

... which can be called authentic simulations of each aircraft's flight dynamics, aerodynamics and engine data which replicate rather correct in flight performances.

The rest of them:

-Polychop - Gazelle
-Magnitude 3 LLC - MIG-21
-RAZBAM - Mirage 2000
-ED's F-15C

...should still be a WIP until more correctly replicating the real planes performances, which should normally not be a concern in the hands of people with good knowledge.

It's even worthless mentioning AVIODEV's and VEAO's products which I regret that I own. I just feel like I bought a simulation of cockpit systems, not flight simulation which is the only reason why I buy something in DCS because I want to analyze and feel it's flight, nothing else, and it's least to say that for me it became a disappointment now.

Regards!
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Last edited by Maverick Su-35S; 07-10-2018 at 04:40 PM.
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Old 03-01-2018, 04:27 PM   #224
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HWasp, I was able to match everything you said. At least in terms of Load-factor.

Check out the attachments below.

Conditions:

Standard day; 21,400 kg total, 3900 kg of fuel (41%). Average height 130 meters above the ground.

One thing; at 600km/h IAS I was able to reach 7G instead of 6G. I should have used Ground speed as reference I also forgot the fact that I can use infobar in the cockpit
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Old 03-01-2018, 04:31 PM   #225
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Thank you for testing it!

Was it with or without stick deflection limiter override?
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Old 03-01-2018, 04:35 PM   #226
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Quote:
Originally Posted by SinusoidDelta View Post


CL should increase up to 40deg AoA.
Sorry I continue to talk about the Eagle, but I can't help it!

The fact that even this function of CL to AoA isn't copied as it is for our DCS F-15 is a problem. Even though I disagree that these are the real F-15's performances (too good to be true), they are still lower than what we see in game. During testing, the F-15 flies at 16000kgf at 27 AoA at 205km/h IAS, therefore the lift coefficient is 1.4. In that graph it should reach 1.4 at 33 degrees. So our plane turns even better than in that graph.

Regards!
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Old 03-01-2018, 04:35 PM   #227
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I was using the override in the first case (following your instructions), 550km/h 8G.

The other two were done without the AoA/G-Load override (Y key).
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Old 03-01-2018, 04:37 PM   #228
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Quote:
Originally Posted by Vitormouraa View Post
HWasp, I was able to match everything you said. At least in terms of Load-factor.

Check out the attachments below.

Conditions:

Standard day; 21,400 kg total, 3900 kg of fuel (41%). Average height 130 meters above the ground.

One thing; at 600km/h IAS I was able to reach 7G instead of 6G. I should have used Ground speed as reference I also forgot the fact that I can use infobar in the cockpit
FWIW, the V speeds given in the real world manual, unless otherwise specified, are IAS. They are referencing the УСМ-2 (airspeed/mach indicator on the front left panel). Like most people, I jump straight to the good parts. I just got around to looking at the Introduction a few minutes ago and noticed that was how they were defining it.
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Old 03-01-2018, 04:43 PM   #229
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Quote:
Originally Posted by Ironhand View Post
FWIW, the V speeds given in the real world manual, unless otherwise specified, are IAS. They are referencing the УСМ-2 (airspeed/mach indicator on the front left panel). Like most people, I jump straight to the good parts. I just got around to looking at the Introduction a few minutes ago and noticed that was how they were defining it.
So that was actually correct? Using IAS?
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Old 03-01-2018, 04:49 PM   #230
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Yes, unless something is mentioned, all values are IAS from the УСМ-2.

And yes the MAX ALLOWED AoA is 24º.
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