Su-27 in departed flight

[Maverick Su-35S]

Nice read Esac_mirmidon.

 

As a rule of thumb, if you see or expect rapid pitch-down movements, pull the stick all the way back and use Direct Control Mode to override FCS pitch-up control authority if possible. Then inverted departures can be well avoided. :thumbup:

...

 

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...!

 

Here's a tailslide:

 

 

It doesn't drop the nose below 90 deg. down. Try it and see if you can get the same.

 

As for the trim (equilibrium) that the aircraft receives at around 25-30 negative AoA (tested) where a full aft stick (elevator up) will cause more bad than good and you could only get out of it by simply pushing the stick (elevator down) to let some airflow reattach the elevators surface, which will actually lift your tail a bit and then release the stick to neutral when the nose stops dropping until it goes up again and just create a pitch oscillation by using stick full forward and released positions until you force it to get out. Normally, the vortex generators (the LERXs) have the highest intensity at positive AoAs and have a degraded effect at negative AoA (giving the shape of the LERX which is optimized for positive lift), therefore the CP (center of pressure) travels to the front (reducing the static stability in pitch, up to neutral and then to unstable) as the AoA increases much more for positive lift (or AoA) than for negative, thus the aircraft should be more stable and easier to recover from high negative alpha than positive. What I've said doesn't contradict the manual when it refers to the inverted stall situation as it is mentioned to be a problem, but only tells what to do to get it out as quickly as possible.

 

P.S.: I've tried some other things like: rudder and rolls combined with elevator inputs which will give you a split second of AoA higher than -25 from where you can recover, but the forward and then neutral stick inputs method gets the plane out much faster, although I remind you that this is not a genuine behavior for the 27.

Edited November 18, 2014 by Maverick Su-35S

[Maverick Su-35S]

I've noticed that the Su-27 goes into departed flight even if the ACS is active. I've even done the Cobra with it active. If I force negative g-loading it can depart. Is this correct?

 

It is correct! The same thing happens for the F-16C as well if the pilot generates enough pitch momentum or pitch rate that the input filtering system (such as FBW) doesn't have time to counter (depending on how it is designed through it's programming code), letting the plane go above or below the limited maximum positive and negative AoA for a second or two and then corrects it.

 

For short, this is due to the pitch inertia that the aircraft has in combination with the gained pitch rate and with the reaction time or limitation of the FBW to not let it go past the limit. This is not a problem in the flight model.

[Maverick Su-35S]

Just deploy flaps and pull all controls against the spin. Altitude loss of only 2000m.

 

How did you manage to get a spin with the Flanker? And if yes, what yaw rate could you get (10-15 deg./s), if you please?

 

Thanks!

 

P.S.: One thing that is worth mentioned from an aerodynamic point of view: If you lower flaps in an upside down position, the instability (that holding your negative AoA trimmed somewhere) should further increase because of the airfoil's shape against the airflow which affects it's native pitching moment (lowered slats and flaps will always increase the pitch down moment), while for a right side up position it should indeed increase stability and help you pitch down more, so it isn't normal if the flaps get you out easier from high negative AoA unless it is indeed combined with a general motion of the plane that would make it point it's nose downwards.

Edited November 18, 2014 by Maverick Su-35S

[Maverick Su-35S]

Here's a short overview of mine and a congratulate to ED for what they've achieved so far with the new SU-27 PFM:

 

https://www.youtube.com/watch?v=AhELnTmeNnA

 

So I'm personally passionate and usually have only one major thing to care about when talking flight sims in general...: Flight model, flight model, and again, flight model...!

 

Thanks!

[LJQCN101]

From which pitch up attitude angle do you start your slide?

 

80~90 deg. And yes, you'll enter an inverted deep stall whatever you'd try. (Just tried minutes ago)

 

BTW I've uploaded a trk in which you can clearly see the aircraft departs from a normal slice turn at high speeds and Direct Control Mode is off: http://forums.eagle.ru/showthread.php?t=134140

[combatace]

How did you manage to get a spin with the Flanker? And if yes, what yaw rate could you get (10-15 deg./s), if you please?

 

Thanks!

 

P.S.: One thing that is worth mentioned from an aerodynamic point of view: If you lower flaps in an upside down position, the instability (that holding your negative AoA trimmed somewhere) should further increase because of the airfoil's shape against the airflow which affects it's native pitching moment (lowered slats and flaps will always increase the pitch down moment), while for a right side up position it should indeed increase stability and help you pitch down more, so it isn't normal if the flaps get you out easier from high negative AoA unless it is indeed combined with a general motion of the plane that would make it point it's nose downwards.

 

 

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

 

All aerodynamics aside this (DCS Flanker) isn't controlled by the laws of physics, it is controlled by the lots of equations programmer by Fighter Collection programmer.

[Yo-Yo]

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...!

 

Here's a tailslide:

 

 

It doesn't drop the nose below 90 deg. down. Try it and see if you can get the same.

 

 

Tailslides in Su-27 and Mig -29 are performed at 70-75 degrees of pitch for -27 and 70 degrees for -29.

If you fail to manage this angle real plane will wave down and get into spin upside down. At leasdt one such accident is known for Su-27. That's why tailslip is not allowed for plain pilots.

[Maverick Su-35S]

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

 

All aerodynamics aside this (DCS Flanker) isn't controlled by the laws of physics, it is controlled by the lots of equations programmer by Fighter Collection programmer.

 

Hi "combatace"!

 

Did I say that you won't get into an inverted stall if you push the stick after exiting cobra, or even more, after a tailslide? I've only replied to tell you about the airfoil's shape when flaps and slats are lowered (things which very few might be aware of) and why the aircraft should be more unstable with them lowered when in a negative stall.

 

That's correct, the aerodynamics and flight dynamics in general are most probably simulated using a dedicated software to estimate the aerodynamic forces and moments such as a CFD and a dedicated software for simulating rigid body laws of motion or maybe a software that does both, as they might not have access to wind tunnel or in flight testing data to gather the aero and flight stability data. The question might be (for some) how well does the software do in all areas, and my personal belief is that it does it with a more than 90% accuracy overall (after the aircraft is out of beta) from what I can see, and the other 10 we might not even be aware of. The most important thing is KNOWING what isn't right and to BE ABLE to modify or tweak those values if you can, otherwise you'll have great limitations in simulating something right..., so I bet that ED knows what has to be done in order to make the aircraft fly/behave pretty close to the real one.

[Ironhand]

Tailslides in Su-27 and Mig -29 are performed at 70-75 degrees of pitch for -27 and 70 degrees for -29.

If you fail to manage this angle real plane will wave down and get into spin upside down. At leasdt one such accident is known for Su-27. That's why tailslip is not allowed for plain pilots.

Thanks. :) I figure we're about to see a lot of DCS tail slides showing up on YouTube.

[Maverick Su-35S]

Tailslides in Su-27 and Mig -29 are performed at 70-75 degrees of pitch for -27 and 70 degrees for -29.

If you fail to manage this angle real plane will wave down and get into spin upside down. At leasdt one such accident is known for Su-27. That's why tailslip is not allowed for plain pilots.

 

Hi "Yo-Yo"!

 

So, if it is true what you are saying (and I don't argue about it), the Flanker's behavior at high negative AoAs is very close to that of the real plane and there goes our answer with all this as well: "No tailslide attempt beyond 70-75 deg of nose-up attitude and pull the stick all the way back when you've reached about 0-10 deg. pitch attitude as the nose drops..., cause if you pull the stick too early (when still at high nose-up angles), it's aerodynamic forces due to negative airspeed (which are inverted of course) will build up an even higher pitch down rate.

 

Thanks!

[mvsgas]

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

 

All aerodynamics aside this (DCS Flanker) isn't controlled by the laws of physics, it is controlled by the lots of equations programmer by Fighter Collection programmer.

 

According to the available SU-27SK manual, the aircraft will depart if more than .5 negative g is used below 300kph, so at low speeds, do not push the stick forward. Keep it neutral and let the nose fall or roll inverted then pull ( if you have enough speed). This is what works for me.

[mvsgas]

Hi "Yo-Yo"!

 

So, if it is true what you are saying (and I don't argue about it), the Flanker's behavior at high negative AoAs is very close to that of the real plane and there goes our answer with all this as well: "No tailslide attempt beyond 70-75 deg of nose-up attitude and pull the stick all the way back when you've reached about 0-10 deg. pitch attitude as the nose drops..., cause if you pull the stick too early (when still at high nose-up angles), it's aerodynamic forces due to negative airspeed (which are inverted of course) will build up an even higher pitch down rate.

 

Thanks!

 

I tried from 60 to 90 degrees ( as posted earlier) and beyond 70 I got into departures

[LJQCN101]

No tailslide attempt beyond 70-75 deg of nose-up attitude and pull the stick all the way back when you've reached about 0-10 deg. pitch attitude as the nose drops..., cause if you pull the stick too early (when still at high nose-up angles), it's aerodynamic forces due to negative airspeed (which are inverted of course) will build up an even higher pitch down rate.

 

Thanks!

 

I'm not quite sure but, according to what I've done in the cobra before, I don't even go nose-low if I pull the stick all the way back while the aircraft is pitching down. And here's a video of Su-30MKM doing tail slide where you can clearly observe the deflection of horizontal stabilizers. :thumbup:

 

 

Thanks anyways, will try that later.

Edited November 19, 2014 by LJQCN101

[Ironhand]

Tail Slide

 

Has anyone been able to do a "successful" tail slide? I always seem to arc over the top, tail pointing toward the ground, fall tail first in that attitude until my nose finally drops. It drops smoothly to about 70-80 degrees down and stabilizes. I can now successfully fly away without a stall. But it's that arc over the top...

 

Rich

Edited November 19, 2014 by Ironhand
clarify my statement

[159th_Viper]

I always seem to arc over the top, tail pointing toward the ground, fall tail first in that attitude until my nose finally drops. It drops smoothly to about 70-80 degrees down and stabilizes. I can now successfully fly away without a stall. But it's that arc over the top...

 

I spend my time putting warheads to foreheads so the complexities of aerobics escape me completely. Now I know that this is a silly question, but why is the 'arc over the top' not normal? I would have assumed that due to the 75 degree pitch-up angle your inertia would carry you over the top in an arc as per this attempt at a slide shows:

 

tsDAJtevI2w

 

Why should the arc not be there?

[Ironhand]

Unfortunately, I don't know if it's normal or not. I suppose that's my question. In the RL videos I've seen it "looks" like there's no arc but it's really hard to be sure. Most are filmed through extreme telephoto lenses from the rear quadrant.

 

If the arc is how it's supposed to work, then I had a great one this morning. I slid backwards a long way before the nose finally dropped.

[Yo-Yo]

Has anyone been able to do a "successful" tail slide? I always seem to arc over the top, tail pointing toward the ground, fall tail first in that attitude until my nose finally drops. It drops smoothly to about 70-80 degrees down and stabilizes. I can now successfully fly away without a stall. But it's that arc over the top...

 

Rich

 

It's a bare physics... the CoG without sufficient aerodynamic forces at low speed can fly exclusively along PARABOLA :).

 

If you fly strictly vertically this parabola begin to pretend that it is a straight line... morphing to it.

[159th_Viper]

So I guess the next question is:

 

Yo-Yo, is the arc real and if not, where exactly are we screwing up :D

 

Edit: Sniped by the man himself: So, it's normal then? Cannot decipher science-speak :)

[Maverick Su-35S]

According to the available SU-27SK manual, the aircraft will depart if more than .5 negative g is used below 300kph, so at low speeds, do not push the stick forward. Keep it neutral and let the nose fall or roll inverted then pull ( if you have enough speed). This is what works for me.

 

Hi "mvsgas",

 

You are right about the manual and about the roll technique that can be used to avoid or even get out of a negative stall, but let's try to talk in angle of attack for stall (be it negative or positive) rather than G loads at given airspeed, because it is always the same for a given airspeed and doesn't care about G load (which vary with weight because it represents the lift to weight ratio), but the AoA does indeed vary with airspeed, yet we can consider it the same for low incompressible airspeeds.

 

So if you know the exact negative AoA from which the stability starts to decline (the nose starts pitching down by itself when your elevator is at neutral) it is more useful than using G load and airspeed from which you have to derive the AoA.

 

Thanks!

[Ironhand]

...So, it's normal then? Cannot decipher science-speak :)

I think that's what he means...I hope so, anyway. Hard to be sure with these technical types. :) If that's the case, I've been pulling my hair out for nothing.

 

 

Rich

[Maverick Su-35S]

I tried from 60 to 90 degrees ( as posted earlier) and beyond 70 I got into departures

 

Yeap..., I forgot to talk about air density as well. Depending on how high you are and how much weight you have you'll definitely find different results for the limit nose up attitude angle from where you can start the tailslide without any stability loss at the exit.

 

Normally when lower and with less weight you should be able to start from a higher attitude limit. And one more thing is that all the stick should be held pulled when starting the slide and it has better effect, because when i've said that it would be better to start pulling when near horizontal it was in general, but the Flanker's elevators deflect a lot and start providing a downlift on the tail at higher angles of attack.

 

From 70 deg you should still be able to do it right without overshooting the -90 deg attitude or rather going beyond -20 AoA at the exit.

[mvsgas]

Hi "mvsgas",

 

You are right about the manual and about the roll technique that can be used to avoid or even get out of a negative stall, but let's try to talk in angle of attack for stall (be it negative or positive) rather than G loads at given airspeed, because it is always the same for a given airspeed and doesn't care about G load (which vary with weight because it represents the lift to weight ratio), but the AoA does indeed vary with airspeed, yet we can consider it the same for low incompressible airspeeds.

 

So if you know the exact negative AoA from which the stability starts to decline (the nose starts pitching down by itself when your elevator is at neutral) it is more useful than using G load and airspeed from which you have to derive the AoA.

 

Thanks!

 

To be honest I do not know. I have been looking in the manual and I have not yet found any references about negative AOA. Several limitations about positive due to speed and weapons load ( referring to symmetry and type) but have not seen any mention of negative AOA. Not even sure why the gauge shows M>2 and M<1( I guess those are Mach limits). If you find the info, please let me know.

 

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 -.5 G have been exceeded.

Edited November 19, 2014 by mvsgas

[Yo-Yo]

So I guess the next question is:

 

Yo-Yo, is the arc real and if not, where exactly are we screwing up :D

 

Edit: Sniped by the man himself: So, it's normal then? Cannot decipher science-speak :)

 

Surely. Throw a stone vertically - it will fly up and down. Throw it at 75 degrees and it will fly accurate parabola or ARC. Imagine a shell from a mortar firing at 12-00 (I mean 1200 Russian mils elevation :) )

[159th_Viper]

Imagine a shell from a mortar firing at 12-00 (I mean 1200 Russian mils elevation :) )

 

Yes, indeed. We had an 'incident' where a fellow soldier sent a 60mm Patmor round up near vertical during a live-fire exercise. Admittedly we did not stay put to admire the trajectory at the time......Did not realise I could run that fast :megalol:

[NineLine]

Surely. Throw a stone vertically - it will fly up and down.

 

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