Su-27 in departed flight

Disclaimer: ED takes no responsibility for anyone that hits them-self in the forehead throwing a rock directly up :D

Excellent point!

Substitute stone with frozen pea, with the addition of testing with safety goggles and all should be well :D

From which pitch up attitude angle do you start your slide? If you start the slide from any higher than 75 deg. then you'll enter a negative stall whatever you'd try. This same behavior happened on the F-15's PFM before being updated a bit and after the small update on it's PFM, the F-15 doesn't go inverted after the nose drops from 90 deg high attitude if the stick is simply held full aft, because the real angle of attack that the elevators received was probably revised or the static stability of the aircraft was reworked, idk, but probably we will expect the same to happen for the Flanker and won't behave like this anymore...!

That reminds me of a RL HUD tape of F-15 doing a tail slide all the way to inverted spin. Dunno his stick position while the aircraft is pitching down, but all we can learn is that , hmm, it is plausible.

That reminds me of a RL HUD tape of F-15 doing a tail slide all the way to inverted spin. Dunno his stick position while the aircraft is pitching down, but all we can learn is that , hmm, it is plausible.

 

The stick position was neutral with pitch CAS ON (it it were OFF the nose would've ended even higher upside down after exit), as I was trying to replicate the real life tailslide (same as in the video you linked), which had a different outcome not because our plane (The F-15C) had overshoot the nosedown position that the real plane had and went almost horizontal upside down but the fact that the correct roll effect didn't occur. Maybe or hopefully it will react as it should after the F-15 is out of beta.

Here it was at 7:27:

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In my ignorance, I would assume the departure in those conditions is more associated with the inability of the flight controls to stop or slow down the inertia of the nose once...

I think you are right, because it probably was a design flaw of the real Su-27's FBW when it was programmed to filter the pilot's inputs or to counter the angle of attack variation speed in relation with it's value at any given time (already knowing the static pitch inertia of the plane) in order to prevent overshooting the critical values, and this is already seen for positive alpha as we can see, when the FBW system can't decelerate the AoA's variation (increase) in time so that it stops at or near the critical positive value when you pull all the stick rapidly. So the same thing is very plausible to happen for negative AoAs as well.

The problem now isn't if the FBW can't estimate how quick should it counter the AoA rate in order to not let it overshoot for a second, but the fact that it doesn't even filter the forward stick movement enough and simply lets the aircraft pass slowly beyond negative AoA into a negative stall. I was now speaking about a normal flight not tailslide and I don't know if this happens on the real Flanker (and I doubt that it would be let to happen) or the FBW in our sim must be tweaked a bit to behave closer to the real one.

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-.5 G have been exceeded.

No man..., don't look at G loads and airspeed anymore when you want to investigate stall or unstable aircraft behavior cause these will foul you now and then or make you loose the sense. Only look at the AoA if you can. I know that the negative AoA isn't indicated on the Su-27's indicator, but you can try to fly upside down and hold -1 G at higher airspeed, let's say at 400km/h IAS, and then gradually reduce your speed while trying to maintain a vertical speed (VASI indicator) as close to 0 as possible (straight flight) and when the plane will start loosing pitch stability or start to stall as you gradually push the stick to maintain -1G, the pitch attitude indicator will give you the corresponding negative AoA. This was a method i've used on the MIG-21 to find out what it's real stall AoA is in fact and not the one indicated. Try it.

Now, sorry if I bother you guys with this and please don't take it wrong and I don't want to be off-topic either, but you guys should really try to forget about whatever you read from whichever source or from whoever told that an aircraft stalls only at an X airspeed and/or an Y G load (the G load increases with the speed's square so they are linked anyway), because even if indeed these 2 ingredients together will create an aerodynamic stall at some points, those points are all actually the stall angles of attack which are important, and that's why it becomes a bit confusing and sometimes faulty to judge the idea of stall when talking just about airspeed or G-load or a fixed combination between the 2, because they both vary (with aircraft weight/fuel load) in a manner that you cannot mentally predict, but the angle of attack is only 1 at which the wing/aircraft stalls. So, in order to find the actual AoA of an aircraft with no indication, simply fly straight at 1G or -1G and watch the pitch attitude when things become of interest.

Cheers!

After years of arcade flight sims everyone believes they're Mavrick from Topgun.

But in real jets the simplest maneuver can always threaten to kill you.

As Dirty Harry would say "A man has GOT to know his planes limitations!"

Maverick Su-35S

I am not saying nor trying to to do the tails slide, did it once, good enough, I am simply quoting the SK manual because combatace mention that;

Pull a cobra and then push the stick full forward to get nose down and you will get inverted spin in DCS Flanker.

About the FBW. I have no idea about the Su-27 FBW, but FBW is not magical. F-16 FBW can't prevent many, many things from happening nether so I doubt that there are many errors on the SU-27 FBW.

Regardless, thanks for you input.

I am not saying nor trying to to do the tails slide, did it once, good enough, I am simply quoting the SK manual because combatace mention that; . So no matter the weight, if you are at or below 300Kph and you do more than -.5 g you could depart, which FC3 SU-27 beta also does. I was simply hoping to pass the information to him, that was all.

Ok man, I've got your point and sorry for interrupting the discussion between you and "combatace", but I really wanted to intervene to not let you guys understand wrong the conditions where a stall or loss of pitch (longitudinal stability) control should actually occur and at what points to look and at what points not to look as they are less relevant, and furthermore because the manual in many situations doesn't tell the whole truth as it is or tells it wrong (and this thing happened before for many aircraft).

5 mins ago I've tested what the SK manual says and I had very different results, furthermore, as I've told you and as it is logical, the weight counted and it counted a lot, so you can't have the same airspeed for the same G. I was able to hold -1G (straight upside-down) at 260km/h IAS (with 86% fuel cause there's a bug that doesn't let it have 100%) and at 220km/h IAS (with 20% fuel left) at low alt and at about -20 deg AoA where I had the aircraft at 0 margin of static longitudinal stability left, from where on the plane began to pitch down (nose gone up) by itself. This was important, not going beyond -20 deg. AoA, and not the airspeed and G limits!

When you conduct the cobra maneuver when do you re-enable to stability system? When I first started trying to perform the maneuver I started getting into the dreaded inverted flat spins. I later found that the stability system is a toggle switch, not a push (on) and release (off). Therefore I would push and hold the CAS button thinking that when I released it I would have CAS renabled. In reality though I was just disabling and not re-enabling. When I would push the nose back down to recover the aircraft would overshoot level attitude and enter into a high magnitude negative AoA condition, inducing the inverted spin. Once I learned that you had to hit the button again to enable CAS I had no more issues. All I did was get to about 300-350 kph in trim level flight, disable the CAS, then pull back all the way to perform the maneuver, then as I reach max pitch attitude I re-enabled the CAS and pushed the stick forward all while adding power to regain airspeed. It seems to work very reliably. Not sure if this is how it is performed in real life but it works darn well.

Ok man, I've got your point and sorry for interrupting the discussion between you and "combatace", but I really wanted to intervene to not let you guys understand wrong the conditions where a stall or loss of pitch (longitudinal stability) control should actually occur and at what points to look and at what points not to look as they are less relevant, and furthermore because the manual in many situations doesn't tell the whole truth as it is or tells it wrong (and this thing happened before for many aircraft).

 

5 mins ago I've tested what the SK manual says and I had very different results, furthermore, as I've told you and as it is logical, the weight counted and it counted a lot, so you can't have the same airspeed for the same G. I was able to hold -1G (straight upside-down) at 260km/h IAS (with 86% fuel cause there's a bug that doesn't let it have 100%) and at 220km/h IAS (with 20% fuel left) at low alt and at about -20 deg AoA where I had the aircraft at 0 margin of static longitudinal stability left, from where on the plane began to pitch down (nose gone up) by itself. This was important, not going beyond -20 deg. AoA, and not the airspeed and G limits!

Like I said, thank you. I appreciate all the information I can get. Would love to see I track so could learn from it.

I was able to hold -1G (straight upside-down) at 260km/h IAS (with 86% fuel cause there's a bug that doesn't let it have 100%) and at 220km/h IAS (with 20% fuel left) at low alt and at about -20 deg AoA where I had the aircraft at 0 margin of static longitudinal stability left, from where on the plane began to pitch down (nose gone up) by itself. This was important, not going beyond -20 deg. AoA, and not the airspeed and G limits!

Good info. Similar results can be witnessed in F-16 that you cannot control the negative longitudinal static stability even with a full stick command beyond a certain degree of AOA (without FCS). Actually I'm more willing to see a Pitching Moment (Cm) versus AOA (alpha) diagram for Su-27 at low speeds and high altitudes. Here's a Cm-α diagram of F-16 at 0.6Mach and 30,000ft. We can observe a loss of nose up control near -40° AOA, and also the efficiency of the elevator for nose up control is sharply decreased with AOA between -20° and -40°. I think Su-27 does pretty the same due to the similarities of aerodynamic design. And things turn worse at lower speeds. :thumbup:

That's why the F-16 got a negative g limiter which uses AOA & airspeed as feedback to filter the pilot's forward stick command. In comparison it seems that the Su-27 won't limit any negative AOA build up.

...then as I reach max pitch attitude I re-enabled the CAS and pushed the stick forward all while adding power to regain airspeed. It seems to work very reliably. Not sure if this is how it is performed in real life but it works darn well.

From seeing a couple of videos the pilot actually keeps the CAS (ASC) OFF through the cobra (it's pretty hard to reach for the switch and put it in ON position as you have more than 2G's left when the plane flies at 90 deg. AoA or beyond) and keep the stick full aft as the plane will drop it's AoA back to lower values without the need to push the stick. So after exiting the cobra with stick held back and putting it to neutral you shall turn the stability control back on (by the way the do it in reality) before the nose starts dropping (cause normally it's trimmed pretty much down when the ASC is still OFF).

Here's a proof:

(such a great song for the 27)

(this is a RAM-K but behaves the same as any other 27)

...Here's a Cm-α diagram of F-16 at 0.6Mach and 30,000ft. We can observe a loss of nose up control near -40° AOA, and also the efficiency of the elevator for nose up control is sharply decreased with AOA between -20° and -40°. I think Su-27 does pretty the same due to the similarities of aerodynamic design. And things turn worse at lower speeds. :thumbup:

 

 

That's why the F-16 got a negative g limiter which uses AOA & airspeed as feedback to filter the pilot's forward stick command. In comparison it seems that the Su-27 won't limit any negative AOA build up.

That's more like it...! Someone who uses the same language.

I don't want for "mvgas" or anyone else to misunderstand, but when talking about stability behavior and/or stall alike we need to find out how these factors vary with AoA, not G limits or airspeeds cause they are hard to cope with as I've explained. Thank you "mvgas" for the manual info anyway and I apologize if I have gone wrong with what I've said! So at +50 AoA and with full forward stick an F-16 would still pitch up and at -50 AoA and full aft stick it would still pitch down as the white boxes suggest.

Good job and thank you "LJQCN101!

In reference of the F-16, you can find the full article on this PDF, page 10

http://www.codeonemagazine.com/images/C1_SemperViper_1_1271449318_9999.pdf

But I do not believe this is similar to the SU-27.

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But I do not believe this is similar to the SU-27.

Good reference example there! It tells how things happen into detail. Yes, the longitudinal static stability between the F-16 and Su-27 is different and vary for the pitching moment coef. (Cm) in relation with AoA and airspeed (with airspeed because the elevator's CL vary different from the wings + fuselage CL resulting in a different global Cm). As a fact, the F-16 has a much less static stability margin than the Su-27 even in supersonic (where all aircraft get a drastic increase in pitch stability) and for this reason it's very hard for an F-16 to perform a cobra for example without the risk of remaining trimmed in a deepstall position, from where the 27 can recover much quicker even without forward stick. A possible reason for the Su-27, why it tends to trim itself at around -25..-30 AoA if you accidentally pass the -20 threshold, is because the elevators (being positioned below the wings in a vertical reference) would be greatly shadowed by the wings or receive a huge amount of buffet and so their lift is reduced or they are even stalled (the elevators stall at negative wing's AoA occurs mostly when their leading edge is down or stick is pulled).

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A possible reason for the Su-27, why it tends to trim itself at around -25..-30 AoA if you accidentally pass the -20 threshold, is because the elevators (being positioned below the wings in a vertical reference) would be greatly shadowed by the wings or receive a huge amount of buffet and so their lift is reduced or they are even stalled (the elevators stall at negative wing's AoA occurs mostly when their leading edge is down or stick is pulled).

I have to contradict my above quote now because I've forgot (when I said that the elevator is shadowed by the wing) about the fact that the wing's wake actually reduces the AoA on the elevators while the LERX generated vortexes which still do occur even at negative AoA relatively the same as they occur at positive AoA (but probably slightly weaker), cure the flow over the elevators, and after all..., the negative deep stalls shouldn't normally happen neither in DCS nor in reality pretty much the same way as the F-18C also won't get stuck in a deep stall, unless it's a spin!

I have watched a lot of footage of Su-27's doing tailslides and in all of them, from about 80 deg pitch up attitudes, after the plane pitched down while the pilot held full aft stick, the nose didn't drop below 80-85 pitch down attitude, thus the AoA also never dropped below positive 5..10AoA. How comes that our Su-27 maintains it's pitch down rate until the AoA goes well above -15 AoA and all the way around into a negative deep stall?

Here's a footage of the Su-27 doing a tailslide. The negative attitude angle is lower than -90, most probably -80 by what the footage shows, thus the AoA should've also been no lower than positive 10 AoA:

Somehow our Su-27 can't do it without having it's AoA go well beyond -20 AoA followed by a fabulous negative deep stall.

The -20 AoA seems to be a negative AoA threshold in the FM where the CP starts traveling forward (towards the nose) quite much, thus violently pitching the nose down and getting you in a negative deep stall that settles somewhere at -55 AoA. I believe this should never happen to the real Su-27 even if the pilot would be using direct pitch control and would intentionally push a full stick forward and keep it there, because if he wants to recover by pulling back on the stick and keep it there, the AoA should certainly come back towards positive without too great hesitations! Even if I never flew it (I don't even have to) and even if I can't estimate with a high precision how much should the CP travel forward at negative AoAs on the Su-27,... if it is already known and correctly determined by ED how much it travels forward at different positive AoAs, it shouldn't be much different for negative AoAs as well and as a matter of fact the plane should find a bit lower static longitudinal instability with flaps/droops up at all negative AoAs. I think that there shouldn't be a rocket science to confirm what I say, it only implies a bit of more analysis and take all the factors into account and apply the correct logic/theory.

The reasons why I find the negative deep stalls abnormal are as follows:

1. The elevators lift beyond stall is much too low.

-The critical AoA of the wing will find an exponential increase as an inverse function of distance between a given wing's chord station and the vortex's core position. For instance, the critical AoA on different parts of the wing (tip, mid span, root) will find a rapid increase (exponential) from tip towards the vortex's core (usually close to the wing root). Thus for example, the F-18C's wingtip (between 90% and 100% of wing span) stalls at about 9-10 AoA (with droops up), but towards the root (approx. between 0% to 33% of wing span) the critical AoA gets as high as 70 AoA.

2. The center of pressure (CP) now suffers a forward shift as AoA increases (positive or negative) much more pronounced than on a non LERX wing (and especially a straight wing) and also more pronounced as the wing has a greater positive sweep and/or as the longitudinal distance between the geometric center of the LERX and the plane's CG or the wing's 25% MAC is greater (one could use CG or 25% of MAC as reference) as this results in a greater arm between the vortex's CP and the plane's CG or wing's 25% MAC, which affects both the longitudinal and directional static stability margin.

In the case of the Flanker, indeed the ratio of distances between the vortex's CP and the plane's CG to the distance between the elevator's CP and the plane's CG is the greatest among modern jet fighters (greater than for an F-16, F-18C&E or MIG-29), which is the main reason why there is such a tremendous neutral point (CP on whole aircraft) shift with alpha (AoA) which means it has a high aerodynamic instability which translates as a high variation of longitudinal static stability margin with AoA. This combined with the gained pitch rate (which conserved as inertia) gives this aircraft the ability to overpass 100-110 AoA within ~1.5 seconds with ease, because, as I repeat, the whole plane's CP shift (or static stability margin variation) is very great compared to other fighters with LERX. Now as the vortex breaks up at some 50-60AoA (on the Flanker and MIG-29), above +/-50..60 AoA the CP starts moving rapidly aft, thus the generated aerodynamic moment tends to reduce the AoA to a lower value.

The problem regarding the CP (on the whole aircraft) in the sim is that it's forward movement between about -15 to -40..50 AoA is much greater than for positive AoAs (between +20..25 to 50) and I personally do not agree with this. This plane's LERX was designed for improving the airflow over the wings (mostly the root) and upper fuselage at positive AoAs, not for negative AoAs. What I mean with this is that the resultant vortex that develops from the LERX should be undeniably weaker at negative lift or AoAs than for positive, yet still strong enough to clean/wash the airflow over the elevators, thus not reducing their effectiveness. In our sim, the CP travels much greater forward at negative lift and AoAs than at positive, which only suggest that the vortexes from the LERX are stronger for negative lift than for positive, which is absurd and also the fact that the elevators are completely shadowed by the wing (which is in contradiction to the vortex wash and the wing's beneficial effects) and have almost zero effectiveness. This shouldn't be the case, because like it or not, the wing itself acts as a leading edge device (which improves and re-energizes the airflow) for the elevators and also reduces the effective (felt) angle of attack on them while the LERX vortex also keeps a good flow over them.

If is there anyone with a better knowledge please let me know where I'm wrong, but so far I disagree that the negative deep stall that we see in DCS with our Su-27 is valid.

Regards!

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Here's a footage of the Su-27 doing a tailslide. The negative attitude angle is lower than -90, most probably -80 by what the footage shows, thus the AoA should've also been no lower than positive 10 AoA:

 

 

Somehow our Su-27 can't do it without having it's AoA go well beyond -20 AoA followed by a fabulous negative deep stall.

Su-27 Tail-Slides-Pit.trk

Su-27 Tail-Slides-External.trk

I gave this a bash. I did a couple of fairly gentle tail slides with a maximum pitch angle of approximately 70 degrees. The aircraft failed to depart on recovery on both occasions.

IIRC the recommended pitch angle for a tail slide is roughly 70-80 degrees, not direct vertical.

Su-27 Tail slide test.trk

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IIRC the recommended pitch angle for a tail slide is roughly 70-80 degrees, not direct vertical.

According to Yo-Yo, the maneuver is done with a pitch angle of 70-75°. I haven't done one in about a year but for this sequence of however many it was, I tried to use the upper part of that range.

...the maneuver is done with a pitch angle of 70-75°.

That's why the Russians more precisely call this maneuver (translated) bell, because the shape of the maneuver resembles a bell more than a vertical tailslide.

According to Yo-Yo, the maneuver is done with a pitch angle of 70-75°. I haven't done one in about a year but for this sequence of however many it was, I tried to use the upper part of that range.

Sounds right. From further testing it appears that the closer the pitch angle is to 90 degrees, the longer the period during which the aircraft experiences negative AOA just after the nose comes down. The AOA indexer doesn't show negative AOA past 20 degrees, but my hypothesis is that the closer the initial pitch angle is to 90 degrees, the higher the resultant negative AOA is. There's a good reason for the red marker on the AOA indexer at around -17 degrees.

There's probably a critical maximum negative AOA after which inverted departure is inevitable, hence the realistic maximum pitch angle of 70-75 degrees.

I don't know enough about aerodynamics to comment on whether or not the inverted negative AOA departure is realistic in its behaviour, but the point and cause of the initial departure would seem to fit in with realistic limits on the usage of the real aircraft.

More generally I'd say that there may be some idiosyncrasies around the hairy edges of the flight envelope but in general ED did an excellent job with the FM. I guess there's a good reason for it being signed off by actual Su-27 pilots.

That's why the Russians more precisely call this maneuver (translated) bell, because the shape of the maneuver resembles a bell more than a vertical tailslide.

Exactly that :thumbup:

Well, the real Su-27 manual states that a 90º tail slide climb is forbidden.

This video illustrates the point about the climbing pitch angle:

The aircraft falls back well outside of its own smoke trail, proving that the manoeuvre is indeed a bell and not a sharp slide.