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Degraded Su-27 aerodynamic lift


Maverick Su-35S

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Regardless of consumption, I would assume if you wanted to compare the turning performance of the airframes from Aircraft A, and Aircraft B, any fuel added is an addition to the airframe. To give one more weight in fuel than the other is certainly going to effect its flight characteristics.

 

It's not the same as comparing flight longevity, and then that's not even considering whether or not there is a desire to compare aircraft using a set gross weight and what it would take to achieve equal numbers there.

 

That being said.. I'm not an aeronautical engineer or anything. So I'll not argue further, but since the subject was the turning speed of two aircraft and not the amount of time either is spending in the air.. seems to me equal amounts of fuel would only be fair. 1500lbs isn't exactly light. Think 3 gbu-12's and how a plane is effected dropping just one of them...minus the drag they would induce.

 

Who is arguing? - I agreed with you that 30% fuel doesn't amount to "the same" for the two aircraft in question.

 

But whether the same amount in kg is "fair" or not depends on how you look at it - e.g. what if I put 3500 kg of fuel in the Flanker and the same amount in the MiG-29...see what I mean? :)

 

Anyway, as GG said, to evaluate the turning performance of a particular airframe, you need to compare it to RL charts and match the conditions used in those and not bring in the performance of an entirely different aircraft.

JJ

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I´ve read a document in russian where describes the AoA-G limiter and, with a rude online translation thats the conclusion for me. The extra weight and the opposite movement to the center is turned off with the limiter off. But maybe i´m wrong, im not sure. But the manuals says very clear that the limiter can be turned off.

 

Let's say that's correct, if the AoA/G Limiter won't give you an extra maneuverability, what's the use of it?

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It will inform you that you've reached the 'safe AoA' level. We have no such feedback in-game, so instead you have a button to pull-through the limiter.

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Overriding the AoA limiter in RL is pulling through the force imposed by the mechanism - there's no button like we have, so I basically disagree with your statement :)

 

I have seen different G limitations based on speed, but not AoA. Where is that?

 

I wouldn't be against concluding that limiter override doesn't give you quite enough authority for whatever reason.

 

In one of my previous posts I have already given you the page number (14\269) where it clearly states the AoA limitations depending on Mach number. (Cannot upload from my tablet sorry)

 

Also, again I am aware of the basics.

 

I think that the purpose of a limiter should be to hold the limitations :) Also, since pulling through the limiter might be dangerous, I don't think that it was meant to be overriden by the average pilot all the time.

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The limiter is there to fly safe. Not to be turned off.

 

That is the use of it. XD

 

But in some circunstances, for example, exhibitions and in expert hands is possible to gain a little edge or margin for certain maneuvers, you can turn it off. But the rule is to have the limiter ON all the time. Because is a safety device for controlled flights.

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The rule is to not use direct control (which some people use in dogfights! In a dogfight, realistically, you probably wouldn't be reaching for that switch anyway - and it isn't necessary either). The limiter can be pulled through by applying more force to the stick ... no need to turn it off RL.

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Let's say that's correct, if the AoA/G Limiter won't give you an extra maneuverability, what's the use of it?

 

In my opinion the purpose was to enable experienced pilots to get that last little bit of performance out of the aircraft at the expense of safety.

 

I think what the authors mean by that statement is that generally it does not worth the risk to go beyond limits. But also I have never seen an aircraft manual, that says: yea just go disregard any limits, it will be good for you :D

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Thats correct.

 

Just push enough on the stick over 17 kg, and the limiter is off. But also the switch is there if needed.

" You must think in russian.."

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...

 

I´ve read a document in russian where describes the AoA-G limiter and, with a rude online translation thats the conclusion for me. The extra weight and the opposite movement to the center is turned off with the limiter off. But maybe i´m wrong, im not sure. But the manuals says very clear that the limiter can be turned off.

I know what you mean. I've re-read that section a number of times. It doesn't seem to be saying that with "ОГРАНИЧИТ α, Пу" turned on, you can't pull beyond the safe AoA or G. It simply seems to provide you a warning (stick shake and extra force required) that you're moving in that direction. The rapidity of the stick movement also seems to come into play. A rapid stick movement causes the warning. A slow stick movement does not.

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I think you're misinterpreting some things - these aren't hard limiters like the FBW on the M2K. They're there to provide awareness. All pilots are fairly experienced by the time they drive a flanker and the exact utility of these devices is taught to them.

 

You can expect any flanker pilot to pull right through this the moment they want to rate, especially at lower airspeeds.

 

In my opinion the purpose was to enable experienced pilots to get that last little bit of performance out of the aircraft at the expense of safety.

 

I think what the authors mean by that statement is that generally it does not worth the risk to go beyond limits. But also I have never seen an aircraft manual, that says: yea just go disregard any limits, it will be good for you :D

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Thank you guys.

 

In one of my previous posts I have already given you the page number (14\269) where it clearly states the AoA limitations depending on Mach number. (Cannot upload from my tablet sorry)

 

Also, again I am aware of the basics.

 

I think that the purpose of a limiter should be to hold the limitations :) Also, since pulling through the limiter might be dangerous, I don't think that it was meant to be overriden by the average pilot all the time.

 

Looking forward to seeing what you have for us once you get back home. I'm here to learn, as I am not good at pointing out what's wrong with the FM or anything about aerodynamics. I find this discussion interesting and I'm very interested in seeing the DCS FM compared to the real thing.

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The rule is to not use direct control (which some people use in dogfights! In a dogfight, realistically, you probably wouldn't be reaching for that switch anyway - and it isn't necessary either). The limiter can be pulled through by applying more force to the stick ... no need to turn it off RL.

 

I thought that was not something they do IRL :)

 

One day I have had a long, guns only dogfight session with a friend of mine flying the Su-27 and I have found out that the best way to win the game is to go direct control before take off and switch it on for the landing only (IF there is a landing:))

 

The plane is extremely responsive in that config, but of course I would not even think about it in a real plane.

 

My observation is that the limiter is too restrictive and it holds the aicraft way too far from it's real capabilities. In my opinion this might have something to do with the constant 22 deg AoA limit...

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Correct Ironhand.

 

The speed on the stick deflection is something the limiter takes in count to activate the extra force needed. And is a sum of all this things, shaking, extra force-sttifening, and in the edge a force to the center that must be counter also.

 

More precisely i think is the speed in the amount of G or AoA increasing at any given moment.

 

Another thing is Direct Control. If by this you mean turning off the FBW ( the famous S key in DCS ) this is completly FORBIDDEN in the manual. Is extremly dangerous and you can crash the plane very easy.


Edited by Esac_mirmidon

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...

 

Another thing is Direct Control. If by this you mean turning off the FBW ( the famous S key in DCS ) this is completly FORBIDDEN in the manual. Is extremly dangerous and you can crash the plane very easy.

:) I don't go anywhere near that switch except for demonstration purposes. It's a good way for the unwary to lose the aircraft.

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I think that the purpose of a limiter should be to hold the limitations :) Also, since pulling through the limiter might be dangerous, I don't think that it was meant to be overriden by the average pilot all the time.

 

AFAIK the F-15 doesn't really have one and in the MiG-29 there is no separate switch to turn it off - it is just stick pull through. In the Hornet there is a paddle switch on the stick that allows overriding the limiter by pulling it at stick position close to full aft.

 

Don't really see why overriding the AOA limiter should be less permissable in the Su-27 - aren't we confusing it with disabling the FCS as such(direct control)?

 

Edit: what Esac_mirmidon said :)

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JJ

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Or your BFM is poor ... :)

 

My observation is that the limiter is too restrictive and it holds the aicraft way too far from it's real capabilities. In my opinion this might have something to do with the constant 22 deg AoA limit...

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Or your BFM is poor ... :)

 

That may well be :)

 

But still, back to the manuals.

 

1. The flight envelope on pg 20/269 shows that 8 g is available for the airframe at around M 0,46 (550kmh) at 21400 kg (this should be absolute max. with limiter override)

 

2. The other graph at 51/269 shows max 6g at 600 kmh and 8 g at 740 kmh (Much less, obviously)

 

In my opinion this means, that pg 20 shows the values without the limiter and pg 51 shows the operational values, with the limiter.

 

If that would be true, then the problem is that the plane does not reach the values according to the pg 51 graph.

 

In this case I can not understand why the limiter would not increase it's limit to 24 deg according to the Mach vs AoA limit on page 14 so that it could meet the performance data on page 51.

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I´m not sure about your last statement.

 

 

 

In the Su-27SK manual also there are a explicit reference about "Turning Off the limiter during acrobatic maneuvers doesnt give any apreciable gain in maneuvrability and is possible to make the plane stall or exceed the load limits"

 

Having a huge margin would disagree with this statement from the manual.

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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.


Edited by Esac_mirmidon

" You must think in russian.."

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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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I have found an SU-27SK flight manual on the internet in russian.

 

http://www.avsimrus.com/f/documents-16/su-27-sk-flight-manual-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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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:

 

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!


Edited by Maverick Su-35S

When you can't prove something with words, let the maths do the talking.

I have an insatiable passion for helping simulated aircraft fly realistically!

Sincerely, your correct flight model simulation advisor!

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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 :doh: :doh: I also forgot the fact that I can use infobar in the cockpit :D

1458992148_740kmh8G.thumb.jpg.59874af6e90ddbb2ca97e06eeea86dde.jpg

645875729_550kmh8G.thumb.jpg.7cfe0dee42c8e4dc54b61148b5ad9d79.jpg

1809268154_600kmhover6G.thumb.jpg.c2fc3995d4704eeca2ac4204b6f6b5eb.jpg

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