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Su-27 engine&drag flying formation


Frazer
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It is pretty much impossible to keep a realistic stable formation with another aircraft for the longitudinal axis. I consider the spool up time of the engines realistic but the effect of power changes are way too slow. Also the effect of the airbrake takes very long.

 

This all is resulting in oscillation. Strangely enough with airbrake out all the time this effect is reduced, so it might has to do with the drag of the Su-27?

 

We have the F15C to compare, which is a piece of cake to fly formation with compared to the Su-27. The response of the airbrake and power changes are much more direct. The engine spoolup time and airbrake deployment maybe a bit quicker than the Sukhoi, but it does not differ this much?

 

Thanks


Edited by Frazer

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All this "new festures" reminds me the F15C PFM release!

90% of the problems are the same.

Read the old F15C bug reports and you will find some bugs that are looking very close to the yet known.

I hope they will fix all this problems in short!

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I want to add something to this discussion.

 

The engine power behaviour is strange from my point of view. At ground and on air.

 

I will try to describe it. It seems that from idle to almost 79% the Su-27 don´t give any power at all. You can´t roll over the taxiway and the aircraft don´t move. It´s like an invisible brake ON or a lack of engine power to move the plane.

 

But when the throttle is set to 80% all the missed power came suddenly, a big push moves the Flanker and it accelerates a lot, from 0 to 90 Km/h all in a second. It´s very strange. No power, no movement and 1% more throttle, a big acceleration in a second.

 

In the air is a worst problem.

 

Flying in formation at RPM below 80% the Flanker reacts like a big invisible airbrake. The Su-27 didn´t decelerate progressive. Is like something is braking him. Over 80% i have a big punch in a second and the Su-27 accelerate very fast.

 

No transition, no progression. All or nothing. Thrust or drag in a 1% RPM difference.

 

A very weird thing. I can TAKE OFF only with 80% RPM. Fuel 50%, throttle at 80%, the Su-27 accelerate to 260 Vlo, Take off at 290-300, and still accelerating, only with 80% RPM. At 79% the plane is braked like an airbrake.


Edited by Esac_mirmidon
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" You must think in russian.."

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"I will try to describe it. It seems that from idle to almost 79% the Su-27 don´t give any power at all. You can´t roll over the taxiway and the aircraft don´t move. It´s like an invisible brake ON or a lack of engine power to move the plane."

 

I think if you look from F2 you'll see that the aircraft 'sits down' like it has a brake on too, and then suddenly lets go and rolls.

I think that issue's not an engine power issue at taxi, but a rolling resistance issue.

Cheers.

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Thanks for your reply. but in the air is the exact same behaviour.

 

If you can, try to stay in formation with other plane at 81%. No problem. Then try to slowly decelerate, under 79-78% RPM. The speed don´t go down step by step slowly, it´s like an invisible brake deployed forcing the plane to lost speed very fast.

 

Now you are going to loose your formation position, but when you add more RPM over 80% the plane accelerates to much. There is no fine tunning in thrust around 80% RPM. No thrust at all + drag ( like an airbrake ) or sudden heavy thrust over 80%.

 

What i´m trying to describe with my poor english ( sorry ), is that an increment of only 1% in RPM from 79% to 80% causes a geometrycal increment in thrust not related to this little 1%.

 

And the opposite, only a 1% less below 79% causes a deceleration deeper and greater than this 1% less.

 

 

Taxing is a good test, no movement until 80%, then autoboost enabled. There is no soft transition. I would expect a little resistance to initial movement but the acceleration response is to much for only 1% RPM more. But in the air is the same behaviour and only around 76%-81% RPM. No power at all + drag / To much power for 1% RPM more. And last thing. Taking off at 80% RPM? I know the AL-31F are very powerful engines but so much?


Edited by Esac_mirmidon

" You must think in russian.."

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Press F2 and take a look to the nozzles. Welcome to the real world from the SFM... THe second reason is that in RL formation flyiing in Su-27 is performed at the speed higher that was convenient in old SFM.

Ніщо так сильно не ранить мозок, як уламки скла від розбитих рожевих окулярів

There is nothing so hurtful for the brain as splinters of broken rose-coloured spectacles.

Ничто так сильно не ранит мозг, как осколки стекла от разбитых розовых очков (С) Me

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Press F2 and take a look to the nozzles. Welcome to the real world from the SFM... THe second reason is that in RL formation flyiing in Su-27 is performed at the speed higher that was convenient in old SFM.

I guess this is one of the many reasons why in the cockpit videos of the Russian Knights you can see them constantly moving all the controls ( throttles, Stick and pedals) Looks like in this video the RPM is constantly moving from 80% to 90%.

https://www.youtube.com/watch?v=7HOLONP0Td4

To whom it may concern,

I am an idiot, unfortunately for the world, I have a internet connection and a fondness for beer....apologies for that.

Thank you for you patience.

 

 

Many people don't want the truth, they want constant reassurance that whatever misconception/fallacies they believe in are true..

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So you can't fly the Su27 precise under 80% RPM?

What about this extrem speed ups @ 80% rpm. Digital thrusters? All or nothing?

And this "wild horse riding" at ground is normal for russian jets?

Hmm... then the Su25/T must be rebuild! This is bird is now wrong like hell. :music_whistling:

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Me? No, I can't even keep her straight in the ground with engine shut down and not moving. :D

But going from what Yo-Yo posted, this is the way it will be. I guess we will know when beta stage is over.

Does the SU-25 engine has a convergent/ movable nozzle?


Edited by mvsgas

To whom it may concern,

I am an idiot, unfortunately for the world, I have a internet connection and a fondness for beer....apologies for that.

Thank you for you patience.

 

 

Many people don't want the truth, they want constant reassurance that whatever misconception/fallacies they believe in are true..

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So you can't fly the Su27 precise under 80% RPM?

What about this extrem speed ups @ 80% rpm. Digital thrusters? All or nothing?

And this "wild horse riding" at ground is normal for russian jets?

Hmm... then the Su25/T must be rebuild! This is bird is now wrong like hell. :music_whistling:

 

These are DIFFERENT planes with different engines, aren't they?

Ніщо так сильно не ранить мозок, як уламки скла від розбитих рожевих окулярів

There is nothing so hurtful for the brain as splinters of broken rose-coloured spectacles.

Ничто так сильно не ранит мозг, как осколки стекла от разбитых розовых очков (С) Me

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Press F2 and take a look to the nozzles. Welcome to the real world from the SFM... THe second reason is that in RL formation flyiing in Su-27 is performed at the speed higher that was convenient in old SFM.

 

Then I wonder why a manufacturer would make an aircraft with such poor thrust control at speeds lower than ±650km/h or below 80%. I just don't believe that is true. The SFM is history, I'm not comparing it to how it was.

 

@Joe Kurr & mvsgass

Flying formation is all about working the throttle, ANY aircraft, and it has nothing to do with the new flight modelling.


Edited by Frazer

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Then I wonder why a manufacturer would make an aircraft with such poor thrust control at speeds lower than ±650km/h or below 80%. I just don't believe that is true. The SFM is history, I'm not comparing it to how it was.

 

@Joe Kurr & mvsgass

Flying formation is all about working the throttle, ANY aircraft, and it has nothing to do with the new flight modelling.

 

It's not "poor". It's a feature of the engine. It's a fighter plane and nobody planned to use it as a show item. hHe engine design is always a compromise between performance, efficiency, gas dynamic stability under various conditions. Al-31 has a lot of thrust at idle rpm if the nozzle is converged. I do not know the exact reasons why the idle rpm is so high - possibly it is to avoid compressor stall - but the only way to get rid of the thrust is to open the nozzle. And it was designed as it is. Possibly to simplify the program because there is no use to jockey the throttle under 78% in RL.

 

I understand that in old SFM its controllability issues made impossible to precisely handle the plane at 700-800 kph as in RL, so you use to fly SFM at lower speed. Now it's history and I think even if you forget your outstanding practice in formation flying to gain new one. :)

Ніщо так сильно не ранить мозок, як уламки скла від розбитих рожевих окулярів

There is nothing so hurtful for the brain as splinters of broken rose-coloured spectacles.

Ничто так сильно не ранит мозг, как осколки стекла от разбитых розовых очков (С) Me

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Then I wonder why a manufacturer would make an aircraft with such poor thrust control at speeds lower than ±650km/h or below 80%. I just don't believe that is true. The SFM is history, I'm not comparing it to how it was.

 

Many aircraft manufacturers created aircraft with things that may not seem optimal to us.

F-100 hydraulic problems and directional control.

F-16 with excessive taxi speed, because engine would not originally idle low enough

F-22 computers crashing in mid flight.

 

I have never worked nor have flown the RL SU-27, but using other aircraft as a basis in term of how much the movable nozzle affect power, I believe the F-104 had an emergency switch/lever to close the nozzle in case of failure. F-16 with PW engine will maintain the nozzle close during flight ( when not operating in AB of course and LG handle is in the up position)

 

Is the SU-27 in FC3 100% match to RL, Yo-Yo post said it work as it should. I have no reason not to believe him and have no information to contradict him.

 

I don't want you guys to think I am taking any sides. To be honest I don't fly the F-15 nor the SU-27 that often within FC3. Just I find this flight models very interesting, maybe because I don't fully understand them. :dunno:

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To whom it may concern,

I am an idiot, unfortunately for the world, I have a internet connection and a fondness for beer....apologies for that.

Thank you for you patience.

 

 

Many people don't want the truth, they want constant reassurance that whatever misconception/fallacies they believe in are true..

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I have no idea how the real aircraft should behave but I also noticed some weird thrust behavior. It seems to me that the aircraft slows down to fast when going below the said 80%. It also really rockets forward on the ground at 80%.

I can still keep it in a formation but it just doesn't feel right to me.

Again, it may be realistic but it seems off.

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Ok, I have to come back on this, with some new findings after flying more and learning about the nozzle thanks to Esac_mirmidon :thumbup:

 

On throttling up the nozzles go narrow @ 78%RPM, but on throttling back they go wide again at 72%......so, as long as you don't throttle all the way back below 72%, you have reliable thrust!

 

Also, loading up the plane full of fuel helps as you work on higher throttle settings during flight.

 

I'm very happy to have learned this, enabling to fly loops and rolls in a nice and stable formation yey! :thumbsup:

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Nice find!

As you can see there is no "lose breaking effect" at acceleration start and even if the nossles are open (less than 72% RPM) the plane doesn't break on its own!

If you do so in in DCS the Su27 will break fast to zero!

The AI can drive the Su27 at 30 kph without using a break only with the management of the thrust rpm.

If you can do so the same please upload a trk file so I can learn how this works.

THX


Edited by Nedum

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"Thumper" posted this in the other thread discussing essentially the same subject.

You can hear the pilot during taxiing using the same technique you now have to use with the Su-27 - increase RPM till it starts to bolt, decrease RPM and coast, after slowing down a bit increase RPM till it starts to bolt, decrease RPM and coast.

Turning onto another taxiway - he increases RPM and rides the brake through the corner, lets it bolt, decreases RPM as coasts

 

Cheers.

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