Engines

[Fox One]

I did the following experiment (track attached): with 3% fuel, standard conditions, I accelerated the plane, reduced throttle to idle and timed how long it needs to decelerate from 50 Km/h to zero. Result: 46 seconds.

 

I accelerated the plane again, then reduced throttle to open the nozzle, then advanced throttle to 78%, the highest rpm possible with an open nozzle. Deceleration from 50 Km/h to 0 took again 46 s. This can’t possibly be right. It’s true, pretty much any jet engine with a completely open nozzle will have quite a low idle thrust, but still increasing the rpm by 9% from 69% idle to 78% can’t have zero effect on thrust.

 

I turned the plane at the end of runway and accelerated again, then both engines OFF. By the time the speed dropped to 50 Km/h the rpm of both engines was 0. Deceleration from 50 Km/h to 0 took 50.5 seconds. This would seem to suggest the idle thrust actually has a negative value! It’s true, in flight engine thrust might actually have a negative value in some limited part of the envelope, but I wouldn’t expect it to be negative at close to standing still speeds.

 

In AL-31F engine technical manual you can check here http://airwar.ru/other/bibl/al-31.html on page 9 they state the engine’s idle thrust, 250 Kgf, and they also very clearly state the value is with the nozzle opened. In order for my 3 little experiments to make sense, the average thrust in the 0-50 Km/h range would have to be negative. It would imply that, from a standing still thrust of 250 Kgf per engine, starting moving the plane the thrust quickly, within a few Km/h would get zero and then negative. I don’t see that happening, having checked a diagram for the entire envelope for idle thrust for RD-33 engine that is principially identical and also uses a very wide opened con-di nozzle setting for idle in order to minimize idle thrust, just like AL-31F engine.

 

I also did another deceleration test (didn’t attached the track here) with engines idle then OFF, at 7 Km altitude and reached the same conclusion – currently in the simulation engine idle thrust has a negative value. Again, comparing with available data for RD-33 engine, at 7 Km altitude the idle thrust is indeed pretty low in subsonic where I did the test, but is positive.

 

See this video here

Starting from 2m8s the aircraft is taxiing, the left nozzle is completely opened, while the right nozzle is very slightly closed. I was intrigued when I saw that, knowing from DCS that during taxi the nozzle can be either opened or closed. So I checked the same AL-31F manual.

 

See image 1 below. Under the diagram they say “At checking the beginning of closing of the jet nozzle”. On the diagram it says “the admissible range”. In the text they say the nozzle will begin closing at an rpm of 80%+/-1.5%, so this is the “admissible range” in the diagram of 78.5 to 81.5%. ED has chosen 78.5% as the value at which the nozzle closes. Maybe it would have been more logical to choose 80%.

 

Also check in the diagram on x axis, the value is named “n вд пр”. This is the temperature-corrected high pressure rotor rpm. Meaning the nozzle would start closing at 80%+/- 1.5% at an outside air temperature of 288K (15degC, standard conditions). At higher outside temperatures, the nozzle closing would begin at an rpm a bit higher, while al lower temps – at lower rpm.

 

What’s the most interesting info in the diagram – there is a range of rpm in which the nozzle goes from fully opened to fully closed. I couldn’t find precise info in the text, but assuming the diagram has reasonably accurate proportions, it would suggest a range of about 5%. In image 2 you can also see for nozzle area (Fкр) there is a range of throttle positions where it goes from fully opened to fully closed.

 

Currently in DCS when reducing throttle, the nozzle would open at about 73% rpm. I couldn’t find anything in the manual to support that, which would seem to suggest that on reducing throttle the nozzle would follow the same digram as when increasing rpm. But maybe ED has additional info about that.

 

What I described above goes some way to explain the annoying technique required to taxi the aircraft (engine management-wise), and also why it is ridiculously difficult to fly formation al slower speeds. Because in the lower rpm range you have only 2 engine thrust options: no thrust or too much thrust.

 

In reality the things would go something like that: at idle, 250 Kgf thrust per engine (instead of right now negative thrust). Increasing the rpm towards 80%, the thrust would increase, it’s true, not spectacularly because of the fully opened con-di nozzle. But it would increase anyway. Reaching 80%, nozzle would start closing and advancing throttle more the thrust would increase considerably more abruptly, but not suddenly like in current sim version, but in a range of about 5% rpm. Reaching the fully closed position, advancing throttle further, the thrust will increase more.

 

During taxiing, reducing throttles to idle – you would still have half a tonne pushing the aircraft. The speed would probably decrease pretty slowly. At low aircraft weight, it might even be possible to taxi the aircraft at constant slow speed with open nozzles, if it’s already at 20-30 Km/h, that’s something I can’t estimate.

 

Generally, engine simulation in current version is very basic. On the ground, the engine idle rpm is constant, no matter what the outside temperature is. It should increase with increasing temperature. At Mach 2+, you reduce throttles to idle – the same 69% rpm as on ground. According to AL-31F manual, in such conditions the idle rpm would be about 97%. The rpm in military and afterburner is not constant in the entire envelope on the real aircraft. Also propulsion related – there is no air intake compression ramps animation and no ramps position indication in the cockpit. There is still a lot to be done regarding engines before the simulation is at the level where it should be, and I hope it would be one day.

1.trk

[Yo-Yo]

It is a beta now, as you understand... :)

Most of enviroment effects are not switched on.

Anyway, if you looked at the numbers regarding the deceleration test you would see that this test with several %% of increasing the rpm never gives you solid result,

 

Mass 20 000 kg, typical rolling friction coeff 0.05, so fricton force is about 1000 kg.

THrust at idle is 250 kg but as the thrust curve is very shallow in this range of rpm (just see the percentage for closed nozzle thrust in this range), so the overall effect on the roll distance will be about several percents.

[mvsgas]

Which version of the AL-31 is used on the SU-27S? What are the differences (if any) on the AL-31F? I truly do not know and would love to learn this. It could be the reason why it seem so different if ED modeled an different version of the AL-31 in DCS.

I ask based on my F100-PW experience while working the F-16. What I mean is F100-PW-100 and the -220 are both F100 engine but very different from each other. The -220 itself has different variants that AFAIK also change engine behavior.

I keep using the SU-27SK manual as reference ( together with the DCS manual) but have no idea the differences of the aircraft and if they are similar enough to honestly compare. I have tried for years to learn Cyrillic and have fail miserably so my ability to find source is very limited ( I feel its limited anyway)

 

Regardless thanks Fox one.

Edited November 15, 2014 by mvsgas

[51GRIZZLY]

It is a beta now, as you understand... :)

Most of enviroment effects are not switched on.

 

So i guess THATS why there is no heat blur? :smartass:

[combatace]

It is a beta now, as you understand... :)

Most of enviroment effects are not switched on.

Anyway, if you looked at the numbers regarding the deceleration test you would see that this test with several %% of increasing the rpm never gives you solid result,

 

Mass 20 000 kg, typical rolling friction coeff 0.05, so fricton force is about 1000 kg.

Thrust at idle is 250 kg but as the thrust curve is very shallow in this range of rpm (just see the percentage for closed nozzle thrust in this range), so the overall effect on the roll distance will be about several percents.

 

But that does not explain the increase in rolling distance a 0 RPM. 250Kgpf is not less then 0 Kgf so rolling time for 0 RPM cannot be more then 69%RPM. It does not make sense.

[mytai01]

Two issues I have are the exhaust nozzles should follow the contour of the fuselage at full AB. They seem open a little too much in Full AB. Second, the louvers on the bottom of the engine intakes appear not to be modeled at all. On videos I've seen, they appear to be individually spring loaded and flutter under certain conditions which I'm not at all sure about.

[L0op8ack]

FC3 Su27 AFM: Deceleration On Runway Test.png

 

FC3 Su27 AFM: Deceleration On Runway Test

My result

 

FULL AB then engine OFF:

decelerate 100km/h to 20km/h, ~67 seconds

 

FULL AB then engine IDLE:

decelerate 100km/h to 20km/h, ~60 seconds

Edited November 16, 2014 by L0op8ack

[Fox One]

^^^ Where from can I get such telemetry?

[Fox One]

Two issues I have are the exhaust nozzles should follow the contour of the fuselage at full AB. They seem open a little too much in Full AB. Second, the louvers on the bottom of the engine intakes appear not to be modeled at all. On videos I've seen, they appear to be individually spring loaded and flutter under certain conditions which I'm not at all sure about.

 

I think you are right about both issues. It’s true, in full AB the nozzle opens a little too much when compared with what you see in pictures, which is usually at low altitude and subsonic. However this full AB nozzle opening shouldn’t be constant in the entire flight envelope. At Mach 2+ in stratosphere, I would expect it to be pretty much fully opened, in order to ensure as closely possible to full exhaust gas expansion in the thinner air (full expansion is probably not possible at high altitude, the diameter of the exit area of divergent nozzle flaps would have to be pretty huge). That’s why most likely the nozzle will be fully opened in such conditions.

 

The louvers – I had the opportunity to see with my own eyes Su-27 performing aerobatics, and it is clearly visible how the louvers are individually flapping around quite a lot. They are free floating and open according to pressure differential inside/outside the intake. Generally, they tend to open at high thrust settings and medium to high AoA, but not exclusively. My opinion – a convincing AoA-related animation for the louvers could be done.

[Nedum]

FC3 Su27 AFM: Deceleration On Runway Test

My result

[ATTACH]107497[/ATTACH]

 

FULL AB then engine OFF:

decelerate 100km/h to 20km/h, ~67 seconds

[ATTACH]107500[/ATTACH]

 

FULL AB then engine IDLE:

decelerate 100km/h to 20km/h, ~60 seconds

[ATTACH]107501[/ATTACH]

 

So my feelings were right.

There is a "magical break".

Thx :thumbup:

 

It is a beta now, as you understand...

Most of enviroment effects are not switched on.

Anyway, if you looked at the numbers regarding the deceleration test you would see that this test with several %% of increasing the rpm never gives you solid result,

 

Mass 20 000 kg, typical rolling friction coeff 0.05, so fricton force is about 1000 kg.

THrust at idle is 250 kg but as the thrust curve is very shallow in this range of rpm (just see the percentage for closed nozzle thrust in this range), so the overall effect on the roll distance will be about several percents.

 

Friction force needs only one time a big push at ground. If you are "over" this point, this force isn't so relevant with wheels. It's still there but the weight of the plane with the momentum force is then enough to hold the Su27 at low speed. As I said in another thread, in no Video I could see a similar behavior for the real Su27 at ground like in this Sim. In this simulation it's like a dragster race with extrem breaking to hold the low speed.

A single Person could pull a Su27 if the Su is rolling. Friction force is to much overdone in this Sim. 250kg is more than enough to hold a speed of 30 km/h.

I have the impression the friction force is much to high at low speed.

Edited November 16, 2014 by Nedum

[Thumper1606688436]

"How can it roll for longer with zero power?"

 

Engine braking and I'm not referring to mechanical friction like a drive train in a car but vacuum pressure.

[Bucic]

^^^ Where from can I get such telemetry?

+1

[159th_Viper]

Tacview?

[Yo-Yo]

A single Person could pull a Su27 if the Su is rolling. Friction force is to much overdone in this Sim. 250kg is more than enough to hold a speed of 30 km/h.

I have the impression the friction force is much to high at low speed.

 

Single person??? Su-27 of 20 tons??? You made my evening, sir... And what force except "nonexistant friction" eats my money for gas as I am driving (not fast, so air drag is negligable).

 

To be serious, try to google "rolling friction coefficient".

[Nedum]

Single person??? Su-27 of 20 tons??? You made my evening, sir... And what force except "nonexistant friction" eats my money for gas as I am driving (not fast, so air drag is negligable).

 

To be serious, try to google "rolling friction coefficient".

 

 

 

Edit /Let it taste you!/ wrong!

^This force ( see above) bon appetite! Much more is possible! And you made my day... I like you... really! :thumbsup:

The friction of your car engine components "eating" the most energie (friction energy). Rolling friction doesn't waste so much energie. Energie will never be lost. If the rolling friction energie would be that high, all wheels must get on fire, would need a Watercooling like the car engine itself has!

 

This shows clear that the rolling friction or this magical break is a way to high for the Su27.

Edited November 16, 2014 by Nedum

[sobek]

Let it taste you!

 

Yeah, that doesn't mean what you think it does...

[mvsgas]

Does the flight model account for the intake screen? (whether we see the animation for them or not) Do they produce more drag? If so, could the drag from the intake screens account for the faster deceleration with engine on?

Edited November 16, 2014 by mvsgas

[Yo-Yo]

[YOUTUBE]

The friction of your car engine components "eating" the most energie (friction energy). Rolling friction doesn't waste so much energie. Energie will never be lost. If the rolling friction energie would be that high, all wheels must get on fire, would need a Watercooling like the car engine itself has!

 

 

Please do not emanate the false "kitchen science facts". Try to found a serious book about plane undercarriage calculations or how to calculate takeoff roll. If it's no luck - try to find a book about automotive physics, tyre modelling, etc. Then you will find a real balance of losses in the car.

 

By the way, try to touch a tire after driving a car for a half an hour. Or to compare overall gas consumption with tire pressure minus 10% and plus 10% of nominal.

[wasserfall]

Mmm, tomorrow when i'm biking to my work i'll try to stop pedaling halfway, see if i can get to my work (7km):)

[Nedum]

Please do not emanate the false "kitchen science facts". Try to found a serious book about plane undercarriage calculations or how to calculate takeoff roll. If it's no luck - try to find a book about automotive physics, tyre modelling, etc. Then you will find a real balance of losses in the car.

 

By the way, try to touch a tire after driving a car for a half an hour. Or to compare overall gas consumption with tire pressure minus 10% and plus 10% of nominal.

 

Why are you so harsh?

I say, one person can pull a Su27 and you laugh an tell some not very fine things.

I show you that I am right and you tell me I am wrong again, but with the roll friction and tell me to read some books about automotiv physics.

I don't get it!

I think, and so I tell you, that the handling from "our" Su27 feels not right on ground and you say I know nothing about all of this and have to read books.

So what must I know so I can tell my impressions here at this forum?

Do I need to have a Master at automotive physics?

 

My feeling is that the ground handling from the DCS Su27 doesn't feel right! And all Videos I have seen have shown this to me. I never saw this extrem acceleration and this ongoing pumping from the nozzles and the breaking I must do to drive the Su27 "slow" on ground.

I am searching for a reason for this behavior. Thats all!

Please explain me why I am so wrong. I am ready to learn something!

 

Mmm, tomorrow when i'm biking to my work i'll try to stop pedaling halfway, see if i can get to my work (7km)

Was this really necessary? Do I say something about no friction?

Edited November 16, 2014 by Nedum

[wasserfall]

In this vid you can see the nozzles closing and opening while taxiing at 3:35

 

[Yo-Yo]

Why are you so harsh?

I say, one person can pull a Su27 and you laugh an tell some not very fine things.

I show you that I am right and you tell me I am wrong again, but with the roll friction and tell me to read some books about automotiv physics.

I don't get it!

I think, and so I tell you, that the handling from "our" Su27 feels not right on ground and you say I know nothing about all of this and have to read books.

So what must I know so I can tell my impressions here at this forum?

Do I need to have a Master at automotive physics?

 

My feeling is that the ground handling from the DCS Su27 doesn't feel right! And all Videos I have seen have shown this to me. I never saw this extrem acceleration and this ongoing pumping from the nozzles and the breaking I must do to drive the Su27 "slow" on ground.

I am searching for a reason for this behavior. Thats all!

Please explain me why I am so wrong. I am ready to learn something!

 

 

Was this really necessary? Do I say something about no friction?

 

I am so harsh, because you are fighting the science. :) It is very vulneruble thing so it requires somebody who can defend it.

 

"One person" means nothing for the scientific calculation. Scientific or engineering calculation uses the rolling coefficient value about 0.02-0.05 for concrete depending on its state. It means that the force required for pulling 20 tons plane is about 400-1000 kg. I think that THE MAN could do it but A MAN - never.

 

"One man power" or "force" is not documented and frozen in a standard like "horse power", sorry...

[Alfa]

It means that the force required for pulling 20 tons plane is about 400-1000 kg. I think that THE MAN could do it but A MAN - never.

 

 

Well...

 

 

:D

[Cnuke]

@Alfa Exactly strong stuff there :)

When I translated the russian DCS forum (topic Su-27) with google translate, I found out that Yo-Yo said, he hasn't got a sausage and his samurai skills are to blame hahahhaha. I'm sure google mixed up, with or without advertising!!

But who can be sure about that??? :P

Edit: found one of the posts https://translate.googleusercontent.com/translate_c?act=url&depth=1&hl=nl&ie=UTF8&prev=_t&rurl=translate.google.nl&sl=ru&tl=en&u=http://forums.eagle.ru/showpost.php%3Fs%3Dbe528da819d7ab2508c22c25026e32f2%26p%3D2223195%26postcount%3D1592&usg=ALkJrhgJHqbgvw1_b6f4o_FVY9C5BvwpFQ

Edited November 20, 2014 by Cnuke

[Yo-Yo]

Well...

 

 

:D

 

TFC for towing Mustang usually uses a tractor...