Russian jets at idle...

[Weta43]

Russian jets at idle...

 

Just in the middle of cooking dinner & have to go back to it for a while so I'm going to post & leave, but something occured to me - do Russian jet engines just have more thrust at idle than Western jet engines? Performance figures that I've seen always quote max output & MTBF etc, but I can't remember seeing thrust at idle. I wondered 'cause that would account for capttrob & SK's feeling the Su-25 should slow down more than it does while Yo-Yo assures us it's OK. It would also ( I think ?) account for Russian jets being capable of doing tail slides - more pressure at Idle would mean that there's less chance of the pressure at the engine nozzle being overcome by the backwards movement of the plane. It would also account for (someone said this - don't know if it's true) Russian planes being more reliant on parachutes to stop - not bad brakes, but more to work against.

[diveplane]

kinda tells you somthing big su33s dont need catapults on take off, so i rekon they idle high output to get up to speed faster....well the su33 anyway

 

..., russians always slap tremendous engines in there jets...

 

 

 

..

[pschelchshorn]

the SU33's idle thrust performance does not have anything to do with its capability of carrier launch without catapult IIRC. All aircraft powerplants have their own respected idle thrust where they run most efficient and clean. You can basically achieve what you are saying by just landing with 80% instead of the idle 70-60%. But thats just what i think..

 

 

Philipp "Flip" S.

[Weta43]

pschelchshorn - "thrust where they run most efficient and clean." - I think you're thinking "cruise" not idle. Idle is where you set the throttle to minimum.

Imagine its in a car - does the rev counter sit at 1 grand or 1.5 with your foot off & the clutch in.

If Russian jets generaly have higher thrusts at idle (higher idle RPM?) then when you put the throttle right back it'll be harder to slow down both in flight & on the runway, and the engine would be less succeptable to airflow reversing through the engines while travelling backwards or under other taxing circumstances (because there's more positive pressure / flow to overcome )

[RvETito]

The idle power of any gas turbine engine is the difined as the minimum RPM at which the turbine provides enough power to drive the compressor(and gears) and thus maintaining stable flow through it and stable burning process in the combustion chamber. At this rejime the engine creates just minimal thrust which doesn't need to be measured because it is too small. Because of the small airflow the engine isn't cooling effectively and the time of use of the idle power on the ground is limited to 5-15min. This time, the RPM and the absolute fuel consumption are the only parameters that are defined for idle power. Note that most of the engines have ground idle power and in-flight idle power which is a bit higher than the ground idle.

[peterj]

The Su-25 seems to have a hight idle thrust, SwingKid estimated 23%, 1900 KG in LockOn, don't know if it's accurate. I found an idle thrust number of 660 lb ~ 300 KG for the Su-27.

[pschelchshorn]

The idle power of any gas turbine engine is the difined as the minimum RPM at which the turbine provides enough power to drive the compressor(and gears) and thus maintaining stable flow through it and stable burning process in the combustion chamber. At this rejime the engine creates just minimal thrust which doesn't need to be measured because it is too small. Because of the small airflow the engine isn't cooling effectively and the time of use of the idle power on the ground is limited to 5-15min. This time, the RPM and the absolute fuel consumption are the only parameters that are defined for idle power. Note that most of the engines have ground idle power and in-flight idle power which is a bit higher than the ground idle.

 

 

thats what i meant...

 

 

Flip

[RvETito]

The Su-25 seems to have a hight idle thrust, SwingKid estimated 23%, 1900 KG in LockOn, don't know if it's accurate. I found an idle thrust number of 660 lb ~ 300 KG for the Su-27.

 

23% RPM for ground idle? No way! This is usualy the RPM when the ignition occurs during engine start. Cut off the starter and the turbine won't have enough power to drive the compressor and the engine will shut down. 300kgf at ground idle for the AL-31F is a correct value. This is a proof why thrust is not important at idle - 300kgf from 12 500kgf available. Generally speaking at idel power you won't be able to even move the jet from the ramp and start taxi.

[brewber19]

Hi AirTito,

 

Your comment about engines at idle being unable to move the jet is interesting, my reason for saying is that in current version of LO the vanilla on startup and cut-back to idle will start to move the airframe, albeit very slowly....when fully loaded.

 

Curious isn't it? I guess in RL it doesnt happen but does in sim...odd. The only reason I picked up on it is that usually I start elec power and engines then while I'm waiting 3 mins for HSI to settle I go away and do stuff....one time I did that and cam back 5 mins later to find my A/c had travelled from its revetment in to the opposite one..across the taxiway! LOL.

 

Anyways, I understand what you mean re: start-up and idle....startup rpm peaks at 45 and settles back to idle of 40...or thereabouts.

 

Like everything in this and every other game, we get used to the tools we got and get on with the job in hand.

[ARM505]

Sims, and the real world can run into problematic differences here. The thrust output of an idling turbine may be small in relation to it's maximum thrust, but it can't be dismissed during ground operations. I can't speak for fighters, having never flown one, but most airliners will maintain a taxi speed at idle, after having 'breakaway' thrust applied to get them rolling. Logically, fighters would be even more prone to moving at idle thrust, given the higher thrust to weight ratio (even at idle, relatively speaking). Where sims and the real world often differ, is the amount of 'breakaway' thrust required. In sims, the ground is a perfectly flat plane, with none of the ripples and bumps found on normal aprons. Tyres are usually modelled only in a simple form, and hence one finds that sims have to make simplifications that manifest themselves in situations like taxiing, and some slow speed or odd flight conditions. MSFS2004 has often gotten a pasting for the odd and unrealistic results of thing like these. Witness how proud some people get when the flight model of their B7XX actually performs the same as the real one, ie requires a certain amount of thrust to get moving, but then keeps rolling nicely at idle thrust.

[RvETito]

Are you sure your throttle has been at the idle power position, when you've strated to taxi right after engines start-up? Because in Lock-On, as well IRL, each engine will speed up to full AB right after start-up if the control lever of the engine fuel control unit(or the throttle in the cockpit) is in the apropriate position. Though I'm not familiar with the Su-25T, I don't believe it would start taxiing with both engines at ground idle. I guess I have to give it a test drive.

[RvETito]

Sims, and the real world can run into problematic differences here. The thrust output of an idling turbine may be small in relation to it's maximum thrust, but it can't be dismissed during ground operations. I can't speak for fighters, having never flown one, but most airliners will maintain a taxi speed at idle, after having 'breakaway' thrust applied to get them rolling. Logically, fighters would be even more prone to moving at idle thrust, given the higher thrust to weight ratio (even at idle, relatively speaking). Where sims and the real world often differ, is the amount of 'breakaway' thrust required. In sims, the ground is a perfectly flat plane, with none of the ripples and bumps found on normal aprons. Tyres are usually modelled only in a simple form, and hence one finds that sims have to make simplifications that manifest themselves in situations like taxiing, and some slow speed or odd flight conditions. MSFS2004 has often gotten a pasting for the odd and unrealistic results of thing like these. Witness how proud some people get when the flight model of their B7XX actually performs the same as the real one, ie requires a certain amount of thrust to get moving, but then keeps rolling nicely at idle thrust.

 

Sure, rolling at idle but which one- ground or flight idle? On most planes there's no way it could maintain taxiing speed at ground idle- it will start to slow down. As I said in my first post, ground idle is very unefficient rejime of the gas turbine engine mostly in terms of cooling, because of the small airflow. That's why pilots try to use it as less as possible and when an airplane is taxiing on the ground they never do it at constant speed and thrust. You can often see them speeding up the engines and then ease'em back to ground idle to slow the plane.

You mentioned airliners, then you must know well that in the biggest airports a plane could taxii for more than half an hour untill it gets to the runway or the terminal. Run any engine for so long at ground idle power and the engine is history, or at least it must be sent for overhaul.

[Weta43]

I was specifically interested in the idle while in flight, but as far as the "ground" idle is concerned Yo-Yo seems pretty confident that he has the idle thrust correct for the Su-25 / 25T (an advertised feature on the FC website) & that does roll with min throttle (though as ARM505 mentions "breakout" resistance probably isn't modelled) & 23% (is that what SK said ?) of power at idle seems a lot. FAS.org says an Su-25 weighs between 17600kg and 21000kg that makes the idle thrust about 10% of its weight. 200lb at the bumper for a 2000lb car should break it out & get it rolling nicely - & increase the perception of inertia on the road...

 

I know nothing about this but I'll ramble - It seems to me that if you designed an engine that had a higher resistance to turning (for whatever reason - perhaps higher pressures in the compressor stages, poorly designed bearings/gearboxes/or a completely different design philosophy?) then you would need to extract more power from the final stages to maintain a given idle speed & would have to have higher final stage pressures to do that & so would have a higher power output at that idle speed (or would you just have higher pressure in front of the last set of turbine blades, but have all that pressure extracted by those blades so that the pressure as the gas exits the nozzles is the same?). When you wind it up the higher pressures would compensate for the greater internal resistance & you could get a high thrust at max rpm, but the higher pressures (temps?) would mean you might end up with a slightly lower max rpm. (like a blower on a motor 300hp to drive the blower to get 1000hp at the end. More power but less reliable than without because of the higher pressures involved - sound like what I've heard said about Russian fighter engines)

[Crusty]

23% RPM for ground idle? No way! This is usualy the RPM when the ignition occurs during engine start. Cut off the starter and the turbine won't have enough power to drive the compressor and the engine will shut down. 300kgf at ground idle for the AL-31F is a correct value. This is a proof why thrust is not important at idle - 300kgf from 12 500kgf available. Generally speaking at idel power you won't be able to even move the jet from the ramp and start taxi.

 

he is talking of % of thrust

you are talking of % rpm

[Shepski]

Hi AirTito,

 

Your comment about engines at idle being unable to move the jet is interesting, my reason for saying is that in current version of LO the vanilla on startup and cut-back to idle will start to move the airframe, albeit very slowly....when fully loaded.

 

I think you better re-check that because at an idle RPM of 33% for the -25 and -25T with a full load the jet will not move. at 40% it will not move. At 45% it will slowly start to creep forward.

 

Guys... I know you want to help make things better but please be thorough in your testing before reporting what you think is an error or bug. It only creates more work and headaches for the team.

[Weta43]

Shepski - went & tried it. Su-25, full load of gas, 8*%500AO [where% is some name I can't remember :-) ] dispensers, 2*R-60M = 98% maximum take off weight. Spawned on the runway, hit "S" plane settled, started to roll slowly forward. Held brake on - moved throttle forwards then back to min, released the brake & the plane moved slowly forwards. An empty plane moves faster...

Don't see it as a bug & I'm not complaining about it - a pilot above says he experiences the same thing with the (un-named) plane he flies/has flown & also I take Yo-Yo's word for it that's how they are. (1.12a)

[Shepski]

Shepski - went & tried it. Su-25, full load of gas, 8*%500AO [where% is some name I can't remember :-) ] dispensers, 2*R-60M = 98% maximum take off weight. Spawned on the runway, hit "S" plane settled, started to roll slowly forward. Held brake on - moved throttle forwards then back to min, released the brake & the plane moved slowly forwards. An empty plane moves faster...

Don't see it as a bug & I'm not complaining about it - a pilot above says he experiences the same thing with the (un-named) plane he flies/has flown & also I take Yo-Yo's word for it that's how they are. (1.12a)

 

Funny why we get different results... what was the engine RPM when you were at idle?

 

I tried it and even accelerated time while sitting at 40% and it didn't move. Breakaway thrust is required to get it rolling and idle will maintain the roll but when you are at idle and don't apply enough thrust to get it rolling it won't move.

 

In the Boeing I fly the jet will not start rolling at idle and requires a good amount of thrust to get it rolling... once rolling it will keep rolling at idle but slowly.

 

I'll make some tracks.

[Shepski]

Here is a track showing that with the above mentioned load it will not accelerate on it's own at idle(33% RPM). Even at 40% it won't move. Once I do get it rolling and then return the thrust to idle it slows down almost to a crawl then one tap of the brakes stops the roll and it doesn't move after that.

 

Completely empty with min fuel of 20% it will roll on it's own at idle.

Su-25-idle.zip

[Weta43]

Shepski - I'll have a look at your's & see how it runs. I've attached one I made - Su-25 @ 98% max takeoff weight. I spawn, it rolls, I brake to stop, stop, move the throttle up & back to min let the brakes off & it rolls (admittedly slowly) away again. Empty it moves faster. I tried it with the loaded -25T & that didn't roll.

[Shepski]

Interesting results... it looks like the difference is the the airport so I'm guessing the runway slope is causing the roll. I moved my jet to Razdolnoye and it did move. I set the same load as yours on the jet in Khersones and it didn't move. I tried a few other airports in the Crimea and the jet didn't move.

 

I did find with yours at Razdolnoye if I applied the brakes for 5 seconds then released them the jet didn't roll. It's almost like the jet is set to be rolling as soon as you press "s" to start the mission due to the slope of the runway.

[Weta43]

I've been playing a bit more with different loads, temps & airfields & sometimes it rolls, and sometimes it doesn't. (if it's going to for a set up it always does, if not it never does). Some setups it will roll at 98%, some it won't at 89%.

I was surprised. My memory of this is that it always rolls. Perhaps it was tweaked for 1.12(&a). E.D. definately tweaked the AFM in some way as I have a 1.11 track of a mission I was making a video of where I took off then banked, turned & flew off keeping low to avoid a HAWK site & when I went to record some other views after patching to 1.12a, instead of flying off I bank, turn then slowly sink till I hit the ground. No pods, no external tanks, just 4 * (damn I can't remember their name again!) RBK-500AO ? I remember they said they dropped the drag of some things, maybe they dropped the thrust slightly as well.