SU-27 rocking left and right at high altitude.

[winchesterdelta1]

Hey i was wondering if it's normal for the SU-27 to rock left and right on high altitude when flying 620 KM/u at an altitude of 15.5 KM (1550 meters). Am i myself causing this to happen? Is this normal behavior on that altitude and speed? Or is it abnormal behavior?

 

Check 7:15 in the video. It starts there. I have no stall warnings or anything.

 

[winchesterdelta1]

You're actually doing about 225kph there, BVR mode shows ground speed.

 

Ow crap... Now it makes sense. Totally forgot that detail. But why no stall warning?

[DarkFire]

Ow crap... Now it makes sense. Totally forgot that detail. But why no stall warning?

 

BVR mode shows TAS, not ground speed. For info, in navigation mode the number in the small box on the lower right hand side of the HDD is also your TAS. Under those conditions It's true that your IAS would have been very low, but even then I've never seen it do that before. I'll have to give this a try.

[winchesterdelta1]

Hmm i swore i tried to stay level first. As you can see in the vid i do try recover by going straight ( But was probably not level enough.). But i gave up pretty quick and just tried to get some speed by dropping my nose after.

[DarkFire]

It doesnt seem to kick in until ~23 degrees aoa, most I saw for you is ~15. Looks like you didnt give it much aft stick and let the nose drop slowly. Had you tried to maintain that attitude it would most likely have gone off.

 

Edit: Brought a similarly loaded flanker up there and boy does it not like it lol. Did some more aggressive maneuvering and got the AOA warning at ~20 degrees, along with losing what little speed I had to the induced drag. I suspect it'll be much better behaved clean and with a bit less fuel. All missiles and 80%(ish) fuel is pretty damn heavy.

 

Same results here. I tried flying under similar conditions & managed to reproduce the swaying effect. It's a question of control sensitivity. With a heavy fuel load and a heavy missile load the Su-27 controls become very sensitive above about 11,000m altitude. Any drastic inputs, especially in the roll channel, will easily induce the roll inertia and yaw wallowing that you experienced. At that sort of altitude you need to be very smooth and controlled to maintain stable flight.

 

Personally I still think that there's much too much cross-talk between the roll and yaw channels of the ACS which doesn't help matters at all under these conditions.

[Fri13]

Hey i was wondering if it's normal for the SU-27 to rock left and right on high altitude when flying 620 KM/u at an altitude of 15.5 KM (1550 meters).

 

15 500 you mean? ;)

[Ironhand]

Is this, perhaps, a wingtip stall? It initiates when W suddenly twitches back on the stick and, in an instant, sufficiently reduces his airspeed causing his AoA to "spike" to 15. He momentarily loses lift and, until his airspeed increases, looks to be riding the thin edge of a stall.

Edited July 12, 2016 by Ironhand

[DarkFire]

Is this, perhaps, a wingtip stall? It initiates when W suddenly twitches back on the stick and, in an instant, sufficiently reduces his airspeed causing his AoA to "spike" to 15. He momentarily loses lift and, until his airspeed increases, looks to be riding the thin edge of a stall.

 

Interesting. It could well be. At high AOA I would imagine that the proportion of lift generated by the LERXs would be even more significant so any developing stall caused by high AOA could well start at the wing tips and progress inwards. That would also produce the observed oscillation as one wing tip then the other moved in and out of stall condition.

[GGTharos]

Is this, perhaps, a wingtip stall? It initiates when W suddenly twitches back on the stick and, in an instant, sufficiently reduces his airspeed causing his AoA to "spike" to 15. He momentarily loses lift and, until his airspeed increases, looks to be riding the thin edge of a stall.

That is exactly what it is: aoa induced wing rock. It comes on a lot earlier when the air is so thin.

 

At low altitude you'll probably see a lot more aoa before it happens.

[winchesterdelta1]

Thank you guy's for explaining. Really interesting information. Will try to fly it more within limits next time :)

[ShuRugal]

You were simply flying too slow for your altitude. I never, ever, climb above 10 km with less than 1,000 kph TAS, and at that altitude, faster is better. When lining up an attack at 12+ km altitude, I won't even start to climb above 8km until i have 1400 kph TAS. You were flying so slowly that your engines would not even be capable of making enough thrust to maintain speed at that altitude, you should never have been climbing at such a slow speed.

[winchesterdelta1]

Hey it worked... And if it works it's not stupid :) And by the way you are a little late with your explanation. Other people told me already in a more nicer way including all the details why it's happening.

[Stuge]

Yeah at high altitudes there's a "speed trap" you can get into where you're so slow that you are unable to accelerate and get stuck with the slow speed. The only way to correct is to dive for speed and try again :)

[DarkFire]

Yeah at high altitudes there's a "speed trap" you can get into where you're so slow that you are unable to accelerate and get stuck with the slow speed. The only way to correct is to dive for speed and try again :)

 

Definitely. I like to call this the "entry speed", i.e. to get to very high altitudes you need to keep your TAS & mach number relatively high. The best climb speed for the Su-27 is at (very roughly) M0.85 - M0.9. On your way up to 12Km+ it often pays to level out at 10Km at increase speed at 95% dry thrust before climbing again once your above 0.85M.

 

There are areas of stability to be had up there: the Su-27 will happily enter a stable cruise at 85% RPM at around 7 degrees AOA at just above 14,000m altitude, but you need to take your time getting there. If you avoid using the burners while climbing to altitude, at above 12,000m your range will be phenomenal. levelling out at around 10,000 to get some extra speed won't be an issue as your range at this altitude at cruise throttle is already well above 1,500 Km.

 

I've experimented a lot with different climb profiles. The best way to climb is:

 

1. Take off using 100% dry thrust.

2. Immediately level off until your speed reaches ~0.9M.

3. Climb at anything between 30-75 m/s vertical speed until you get to around 6,000m.

4. When you pass 6,000m reduce your climb rate to <35 m/s.

5. Once you reach ~8,000m reduce your climb rate again to <25 m/s.

6. Level off at 10,000m for cruise or stop there for a bit to allow speed to build up again before climbing again.

 

Your cruise throttle should be 85% give or take 2% depending on altitude.

 

Edited to add: The above technique isn't the fastest way to climb, not even close, but I've found it to be the most fuel efficient way to climb. Should have said that instead of "best".

Edited July 17, 2016 by DarkFire

[Stuge]

Huh, nice tip DarkFire, I'll try it out...

[Xenovia]

iv'e always been curious about this, but when im in a knife fight and turn horizontally i notice my plane likes to yaw left and right for no reason. (before i engage enemies i always reset trim so i can get into a dogfight)

[DarkFire]

iv'e always been curious about this, but when im in a knife fight and turn horizontally i notice my plane likes to yaw left and right for no reason. (before i engage enemies i always reset trim so i can get into a dogfight)

 

That's caused by the amount of cross-control between the roll & yaw channels. Personally I think it's much too strong, though admittedly I have nothing to back this up. It's more noticeable at slower speeds, e.g. during a knife fight.

 

Theoretically you don't need to reset trim before a dogfight because the ACS disables the trim input as soon as G is detected, but yes it does help :)

[Xenovia]

That's caused by the amount of cross-control between the roll & yaw channels. Personally I think it's much too strong, though admittedly I have nothing to back this up. It's more noticeable at slower speeds, e.g. during a knife fight.

 

Theoretically you don't need to reset trim before a dogfight because the ACS disables the trim input as soon as G is detected, but yes it does help :)

 

HEH, yeah i tried that once, my plane flipped forward and i blacked out then crashed.