Fuel efficiency & autopilot at high altitude.

I decided to do a range test flight with my Su-27. Standard DCS day. I took off from Novorossiysk with 2 x R-73 and 100% fuel. I climbed to ~10,750m altitude under military power, throttled back to 90% (cruise speed of ~1050 Km/h) and let it fly. Roughly half way across the Black Sea I turned south to the Turkish coast and when I hit land turned east and flew in to Batumi.

Tacview recorded the total distance as ~1,220 Km and I landed with 2,500Kg fuel remaining. I was very, very impressed by this. I knew the Flanker had long legs but this really surprised me. It's also worth noting that at this weight at least the Flanker can very happily cruise at 10,000m altitude at a very frugal 85% throttle setting. Not only that but your TAS will be upwards of 900Km/h which is a very respectable cruise speed.

I did actually refuel at Batumi, after which I flew directly back to Novorossiysk. On this return flight I decided to take it up above 11,000m altitude and interestingly this cause all sorts of problems: even at military thrust setting the altitude hold autopilot mode failed to keep the aircraft on track. AOA increased to the point of stall and I had to really fight to get the thing back under control. Manual flight at this altitude however wasn't a problem at all.

While I was wobbling around above 11,000m and even after I disengaged the autopilot I was experiencing stick-neutral roll and a significant amount of flutter, despite having a symmetric load.

I think that the Su-27 autopilot needs some significant tweaking.

Attached is an admittedly lengthy acmi recording of the flight.

Su-27 Test Flight.zip

Autopilot mucks with the trim and it does not reset it after it is disengaged. If you experience stick-neutral roll after disengaging autopilot, just reset the trim (or trim manually). Su-25T autopilot has similar issue.

according to certain internet sources the flanker has a ferry range upward of 3000 km. I can't seem to replicate this in the sim though. Fuel consumption is on the high side. Ver 1.2.16

according to certain internet sources the flanker has a ferry range upward of 3000 km. I can't seem to replicate this in the sim though. Fuel consumption is on the high side. Ver 1.2.16

I agree. I've noticed the same in 1.2.16 and in 1.5. Given that the Flanker will happily cruise at 10,000m at 85% engine RPM I'll do a test at some point to see what sort of max range I can get out of it.

Interesting test flight. Thanks for sharing DarkFire :thumbup:

It's somewhat difficult to gather solid ingame fuel consumption data to reach an optimized flight profile and range management.

A while ago, I tried to find a decent profile close to the Su-27SK manual while using a stable autopilot.

>>> cruising speed + altitude test <<<

The autopilot is definitely a comfort factor and based on your test, I'm tempted to take the time to test different profiles.

The recent 1.5 changes to the engines RPM now based on the atmospheric conditions may have a slight effect on the efficiency and fuel consumption. It should be interesting to run more tests.

I must be using far too much throttle then because I was just thinking after my last flight what a gas hog this a/c is. I'm running out of fuel after about 600km, full tanks - takeoff from Gelendzik and turn around at Sochi to RTB . I keep the throttle at about 90% when cruising at about 7000k. Maybe I need to move it down to 80% throttle and climb higher 10,000k or so.

In any event, haven't flown the Flanker since LOMAC, and I'm in love with this a/c again. What a beauty! Reminds me of the Millennium Falcon a little; the way it belches black smoke and the odd groans and noises during startup - beat up old school steam gauge cockpit and lack of a proper HOTAS - cool stuff!

Can't wait to jump in it later tonight. Not missing my A-10C, UH-1H or F-16 blk-50 either (yet)!

Good day,

DrDetroit

Interesting test flight. Thanks for sharing DarkFire :thumbup:

It's somewhat difficult to gather solid ingame fuel consumption data to reach an optimized flight profile and range management.

 

A while ago, I tried to find a decent profile close to the Su-27SK manual while using a stable autopilot.

>>> cruising speed + altitude test <<<

 

The autopilot is definitely a comfort factor and based on your test, I'm tempted to take the time to test different profiles.

The recent 1.5 changes to the engines RPM now based on the atmospheric conditions may have a slight effect on the efficiency and fuel consumption. It should be interesting to run more tests.

The atmospheric alterations to the Su-27 engines has made a huge difference in how it performs. I'm finding that it accelerates significantly faster than it used to, especially at altitude, and also goes significantly faster than it used to. In fact it now goes much faster than it's supposed to do. According to multiple sources the maximum speed is supposed to be 2.35M. Under DCS 1.2.16 I was able to get it to 2.47M at around 12,500m and under the latest version of 1.5 beta I've taken it all the way to 2.6M at 13,000m - that's faster than the F-15, which the Su-27 isn't supposed to do :music_whistling:

By way of comparison, I'm finding that the changes to the Su-27 engine performance coupled with the correction made to the F-15C engine thrust / air temperature profile has brought them much closer together than was the case before. All good for the Flanker pilots amongst us, though I'd wager that the insane top speed will be corrected at some point.

Somehow I missed that post you made back in March. Very interesting. Your test data seems to mirror roughly what I get when testing cruise performance, so call it confirmed.

I must be using far too much throttle then because I was just thinking after my last flight what a gas hog this a/c is. I'm running out of fuel after about 600km, full tanks - takeoff from Gelendzik and turn around at Sochi to RTB . I keep the throttle at about 90% when cruising at about 7000k. Maybe I need to move it down to 80% throttle and climb higher 10,000k or so.

DrDetroit

At that sort of altitude engine RPM above 85% makes very little difference to your true air speed. From what I recall from my cruise test flights in 1.5 I usually get a TAS of ~940 Km/h at 85% RPM and this only increases to ~1,020 Km/h at 90% RPM and ~1,090 Km/h at 100% RPM. Burner will accelerate you to Vmax eventually, but if you don't need it I don't think there's much purpose in using anything over 90% RPM at altitude, plus using a lower RPM setting seems to make a significant difference in terms of fuel efficiency.

I vaguely remember reading in the "Su-27 Squadron Commanders Edition" manual from ~20 years ago that the cruise RPM that real VVS / PVO pilots use is 85-87% depending on altitude, temperature, wind etc. which pretty much mirrors what we're getting in 1.5 at the moment.

Ferry range of aircraft is not at mill power.. It's also at generally 32-36,000 ft.

If you want to test long range cruise get the bird up to 32-36,000 ft and keep it just above "stall speed" and as low of AOA as possible in a clean configuration for the speed. This will give you max endurance. should be ~85%rpm or 55% throttle movement past idle.

Su-27 Cruise Test! With science!!

I decided to test the cruise abilities of the Su-27 as scientifically as possible within the limits of the sim.

TEST CONDITIONS:

1. Standard DCS day with no wind, no turbulence, no clouds.

2. Aircraft configuration: 100% fuel, 2xECM, 2xR-73, 2xR-27ET, 2xR-27ER, full gunpad, full expendables.

3. Barometric altimeter calibrated to takeoff runway within adjustment accuracy limit of the cockpit instrument.

FLIGHT PROFILE:

1. Takeoff performed under full military thrust.

2. Climb-out again performed under full military thrust until cruise altitude achieved.

3. Cruise altitude for test conditions was 10,950m +- 80m. = 35,925 feet.

4. Test started at exactly 8,000 Kg of fuel remaining, no weapons or consumables expended.

CRUISE TEST RESULTS:

OK, it looks like the "insert table" code on this forum is broken, so I've attached an Excel work sheet containing the data and the chart.

The test produced the following graph for set RPM v true air speed:

RESULTS & CONCLUSIONS:

1. Interestingly the chart is obviously non-linear, with the best cruise efficiency achieved at about 83% indicated RPM.

2. Note that using 100% engine RPM at this sort of altitude is absolutely pointless: it produced zero speed increase above 95% RPM. If you absolutely must cruise at maximum speed with dry thrust use 95% unless you're engaging the afterburner.

3. At 75% throttle the aircraft failed to maintain altitude with steady AOA. It suffered gradually increasing AOA until I gave up trying to maintain altitude after about 2 minutes at 670 Km/h TAS and something above 7.5 degrees AOA.

4. I landed at Kutaisi with ~6,175 Kg fuel remaining. Tacview reported a 504Km flight which gives a fuel consumption of ~6.4 Kg of fuel used per Km of flight distance, averages over the entire flight profile. This would equate to a maximum range of ~1,470Km. Given that I was obviously not using the ideal cruise throttle setting for the entire flight I think that 1,500Km range would be easily achievable in DCS, possibly significantly more, but this doesn't equal the reported maximum range of the real Su-27S. Perhaps fuel consumption in-game needs looking at? Edited to add: As Weta rightly pointed out, this conclusion is invalid because of course I'm testing using a full A-2-A war load rather than an empty aircraft. So, would a full war load ~ double the fuel consumption compared to a theoretical minimum with a clean bird? Possibly, quite possibly. Further testing necessary!

5. Throughout the test I had to manually maintain altitude. I did try using the altitude hold mode of the autopilot which failed miserably. For interested parties, watch the attached track to see what happened when I tried.

6. Eventually I'll do more tests at different altitudes to build up a profile of the cruise flight performance as modelled. So far I'm very, very impressed.

Anyway, hopefully people will find this interesting and/or useful! I was going to do some more now but I've just noticed it's past 5 am here... Time for bed :yawn:

Su-27 Cruise RPM v TAS ACMI.zip

Su-27 Cruise Test track.zip

Su-27 Cruise RPM v TAS Excel.zip

TEST CONDITIONS:

1. Standard DCS day with no wind, no turbulence, no clouds.

2. Aircraft configuration: 100% fuel, 2xECM, 2xR-73, 2xR-27ET, 2xR-27ER, full gunpad, full expendables.

but

Ferry range means the maximum range the aircraft can fly. This usually means maximum fuel load, optionally with extra fuel tanks and minimum equipment. It refers to transport of aircraft for use on remote location without any passengers or cargo.

I decided to test the cruise abilities of the Su-27 as scientifically as possible within the limits of the sim.

 

TEST CONDITIONS:

1. Standard DCS day with no wind, no turbulence, no clouds.

2. Aircraft configuration: 100% fuel, 2xECM, 2xR-73, 2xR-27ET, 2xR-27ER, full gunpad, full expendables.

3. Barometric altimeter calibrated to takeoff runway within adjustment accuracy limit of the cockpit instrument.

 

 

FLIGHT PROFILE:

1. Takeoff performed under full military thrust.

2. Climb-out again performed under full military thrust until cruise altitude achieved.

3. Cruise altitude for test conditions was 10,950m +- 80m. = 35,925 feet.

4. Test started at exactly 8,000 Kg of fuel remaining, no weapons or consumables expended.

 

 

CRUISE TEST RESULTS:

 

OK, it looks like the "insert table" code on this forum is broken, so I've attached an Excel work sheet containing the data and the chart.

 

The test produced the following graph for set RPM v true air speed:

 

 

 

RESULTS & CONCLUSIONS:

1. Interestingly the chart is obviously non-linear, with the best cruise efficiency achieved at about 83% indicated RPM.

2. Note that using 100% engine RPM at this sort of altitude is absolutely pointless: it produced zero speed increase above 95% RPM. If you absolutely must cruise at maximum speed with dry thrust use 95% unless you're engaging the afterburner.

3. At 75% throttle the aircraft failed to maintain altitude with steady AOA. It suffered gradually increasing AOA until I gave up trying to maintain altitude after about 2 minutes at 670 Km/h TAS and something above 7.5 degrees AOA.

4. I landed at Kutaisi with ~6,175 Kg fuel remaining. Tacview reported a 504Km flight which gives a fuel consumption of ~6.4 Kg of fuel used per Km of flight distance, averages over the entire flight profile. This would equate to a maximum range of ~1,470Km. Given that I was obviously not using the ideal cruise throttle setting for the entire flight I think that 1,500Km range would be easily achievable in DCS, possibly significantly more, but this doesn't equal the reported maximum range of the real Su-27S. Perhaps fuel consumption in-game needs looking at?

5. Throughout the test I had to manually maintain altitude. I did try using the altitude hold mode of the autopilot which failed miserably. For interested parties, watch the attached track to see what happened when I tried.

6. Eventually I'll do more tests at different altitudes to build up a profile of the cruise flight performance as modelled. So far I'm very, very impressed.

 

Anyway, hopefully people will find this interesting and/or useful! I was going to do some more now but I've just noticed it's past 5 am here... Time for bed :yawn:

Superb work!

It is, I forgot to say that...

but

I know, I know :) I wanted to test the capabilities under useful conditions though, rather than a theoretical maximum performance. I've decided that what I'm actually aiming to do is to repeat the tests I did for steps of 1,000m altitude, figure out the most efficient cruise throttle setting for a given altitude and then plot the whole lot on a graph which will give a sort of ready reckoner for best cruise setting for any given altitude.

And yes, I'll probably do it all again for a clean bird, cause when it comes to flying the Flanker I'm like that :lol:

Rage: Thanks, much appreciated.

Test #2, 10,000m Altitude.

OK, here are the results from test number 2 which was 10,000m altitude.

Something I forgot to mention is that the altitudes are all barometric as indicated by the HUD & altimeter. The actual (F2 view) altitudes will be higher, but I calibrated the altimeter for takeoff airfield QFE on each occasion and it appears that the difference between actual and indicated altitudes is only in the region of ~ +2% so I'll accept that.

Meteorological conditions and aircraft configuration were identical to the first test.

Here's the performance graph:

No big changes here from the first test at ~11,000m altitude but I didn't expect there to be any. Interestingly I added an extra data point at 78% RPM. At this engine setting the aircraft failed to maintain a steady altitude at a steady AOA, as it did at 75% RPM.

Conclusions? At this sort of altitude using anything less than 80% RPM will not result in altitude stability. The maximum performance point is still probably around 82% - 83% RPM but again I didn't expect that to change much.

Tacview reported a 395 Km flight on this occasion with an associated fuel use of 2,425Kg and a resultant maximum theoretical range of 1,531Km, though there's a fair degree of uncertainty (as in experimental data error calculation) in that figure. It's within the ball park of the 1st test result.

I think the interesting stuff will only happen below around 7,000m altitude, but we'll see. More to come later. I haven't bothered attaching the track file because with this sort of test the .trk files are huge and I'm rapidly filling up my forum allowance. The Tacview ACMI file is attached.

As an aside, ED: any chance of us being able to buy a larger forum data allowance?

Su-27 Cruise RPM v TAS 10000m ACMI.zip

Great data! Thanks DarkFire!

I'm pretty sure this will also help the mission builders.

I'll check that more thoroughly as soon as I'm back from my L-39 tour.

:thumbsup:

Flown with Beta 1.5. Nothing scientific on my end and hardly indicative of anything because of how I did it. But...just flew 3180 km with a clean aircraft and 100% fuel. But that was with no fuel left--I had to deadstick into a cornfield somewhere. I used the advance speed keys a lot simply because I don't have the hours and hours available to fly that distance in real time.

Don't know what kind of effect that might have in terms of accuracy. I know it made me screw up a few times during the flight and have to lose altitude and advance the throttles to regain some airspeed. It also made me miss the fact that I was critically low on fuel. So I ended up completely out of fuel too far from an airbase.

I've attached the track (it's still sped up) as a ZIP file. Don't know how it'll play back.

I know, I know :) I wanted to test the capabilities under useful conditions though, rather than a theoretical maximum performance. I've decided that what I'm actually aiming to do is to repeat the tests I did for steps of 1,000m altitude, figure out the most efficient cruise throttle setting for a given altitude and then plot the whole lot on a graph which will give a sort of ready reckoner for best cruise setting for any given altitude.

That would be really useful. Prior to the flanker PFM our standard cruise rpm was 95%. Since the PFM and further recent updates we've since been using 85% as our standard cruise rpm with a combat load.

I'm glad it corresponds to your tests. certainly i'd be very interested in seeing the result at different altitudes.

Test #3 - 9,000m Altitude.

And so, on to test number 3 - 9,000m altitude.

Again meteorological conditions and aircraft configuration were identical to previous tests.

Here's the performance graph:

Once again no major surprises to be found here. Note the increased maximum airspeed now at slightly lower altitude. Interestingly at 9,000m altitude, the aircraft was able to maintain steady AOA with steady altitude at 78% throttle setting, something that it couldn't do at higher altitudes. When reduced to 75% RPM though it again failed to maintain steady altitude at a steady AOA, which is entirely unsurprising given such a low engine power setting. Again as before, increasing engine RPM from 95% to maximum dry thrust (98% RPM indicated) produces no increase in true airspeed at all.

Once again best cruise efficiency appears to be between 83% - 85% RPM. The 'corner' at which increasing RPM gives diminishing increases in speed is even more pronounced here. It comes as no surprise at all that the Russian AF apparently instruct their pilots to cruise at 85% throttle.

Using Tacview for distance travelled I calculated fuel usage at 2,550Kg giving a fuel consumption figure of 6.75 Kg/Km, higher than before but not surprising as I was travelling at lower altitude. This gives a maximum range of 1,393 Km.

Ironhand - I'll have to watch your track in a bit to see how you managed that sort of range. Very interesting. Obviously >3,000Km actual range can be achieved by a real Su-27 because I gather that one of the Russian display pilots flew all the way from Moscow to Paris for the air show there on a single load of fuel. Really quite impressive.

The altitude stability mode of the autopilot again totally failed, causing significant flutter even though I had a symmetric loadout and turned on the autopilot under stable conditions: no roll, <1m/s vertical speed, steady air speed. It. Just. Doesn't. Work. I get the impression that the yaw channel of the ACS is either not dampening oscillations or is actually amplifying them when the altitude hold mode is engaged, but that's a topic for another thread and another test series.

Su-27 Cruise RPM v TAS 9000m ACMI.zip

And so, on to test number 3 - 9,000m altitude.

 

...Interestingly at 9,000m altitude, the aircraft was able to maintain steady AOA with steady altitude at 78% throttle setting, something that it couldn't do at higher altitudes....

 

 

Ironhand - I'll have to watch your track in a bit to see how you managed that sort of range. Very interesting. Obviously >3,000Km actual range can be achieved by a real Su-27 because I gather that one of the Russian display pilots flew all the way from Moscow to Paris for the air show there on a single load of fuel. Really quite impressive.

 

...

78% at 10,200-10,400 m I was able to maintain a steady AoA in the clean aircraft. Obviously weapons drag/weight has an effect.

The trick to getting long cruise ranges is to not be in a hurry to achieve it. Like you, I used full mil power for takeoff. Max climb rate was about 30m/sec. Soon throttled back to maintain about 560 to 580 k/hr TAS. After that, I played it by what the aircraft seemed to prefer until I was at 10,200. [EDIT]Just make sure your IAS never drops below 470-480k/hr IAS anywhere during the flight.[EDIT ENDS] Didn't worry too much about a slight climb or drop after I trimmed. Maintained about 480-500 km/hr IAS for most of the flight except where having the clock sped up caused me to screw up and I had to waste fuel and altitude recovering.

I am missing something here what is the significance of the Chart illustrating RPM v TAS ? I mean are we simply plotting the TAS achieved for a fixed RPM at a specific altitude to determine the best cruise RPM ?

For Max range(IRL) you need to be at a specific AOA at that altitude where optimum RPM (Minimum SFC) allows you to maintain that altitude. This will of course vary with weight. In other words you are optimizing the Airframe (AOA) criteria with the engine optimum (RPM) criteria. max range requires both to be optimized.

Typically in most modern Fighter aircraft Max Range is achieve at 4-4.5deg AOA.

So for the range test you need to determine that Engine RPM that is most efficient. Then use this power setting and cruise at say 4deg AOA

If 85% is determined as the optimum crz power setting then test at various altitudes until this power setting gives a steady 4-5 degrees AOA. This will be close to the optimum altitude to cruise at. I suspect somewhere in the range 32-35,000ft being lower in the early portions of the cruise. For absolute efficiency just maintain the RPM and the AOA and let the aircraft climb as weight burns off.

I am missing something here what is the significance of the Chart illustrating RPM v TAS ? I mean are we simply plotting the TAS achieved for a fixed RPM at a specific altitude to determine the best cruise RPM ?

Yes, but with the proviso that I don't just want to obtain a real-world result for a maximum possible range flight, I want to calculate what an efficient cruise profile looks like for a combat mission, based on tactics-defined flight altitudes. Mission dictates the tactics and all that.

I agree about the combination of optimum RPM and AOA, though for the Su-27 the optimum AOA looks more like about 3 - 3.5 degrees rather than 4 or 4.5.

Could be 3-35AOA.

In the brief test I just did using the same config I got the following Stabilized cruise figs at 85%

AOA 4deg

ALT 12,180m (39,950ft) to hold 4 deg AOA at 85%

Mach no 0.86

TAS 913Kmh

the altitude surprised me a bit as at the high initial start weight I would have thought it would have been a fair bit lower.

Be nice if you could extract Fuel Flow directly :) .... though I guess you could figure it out by weight change over time.

You can extract fuel flow. I used to have a little application that computed fuel consumption and available range based on that and GS. It was displayed on the G15s LCD :-)

In the F15 you have fuel flow gauges. I don't recall if the Su-27 had them, but I would be surprised if it doesn't.

You can extract fuel flow. I used to have a little application that computed fuel consumption and available range based on that and GS. It was displayed on the G15s LCD :-)

 

In the F15 you have fuel flow gauges. I don't recall if the Su-27 had them, but I would be surprised if it doesn't.

Unfortunately the Su-27 doesn't have a fuel flow gauge :( That being said, with an effective range of above 1,200 Km with a full war load, I'd say that for 95% of missions in the caucasus theatre fuel flow is largely irrelevant as you almost never run so low on fuel in the Su-27 that a friendly airfield is out of reach.

To give an example, I usually do my speed & altitude tests with a full fuel load. By the time I've reached 2.6M at 13,000m I've normally been at max AB for over 10 minutes. I normally end up landing with a recorded distance of over 550Km flown and usually have ~2,500 Kg of fuel remaining - plenty for a couple of hundred Km back to base.

Could be 3-35AOA.

 

In the brief test I just did using the same config I got the following Stabilized cruise figs at 85%

 

AOA 4deg

ALT 12,180m (39,950ft) to hold 4 deg AOA at 85%

Mach no 0.86

TAS 913Kmh

 

the altitude surprised me a bit as at the high initial start weight I would have thought it would have been a fair bit lower.

 

Be nice if you could extract Fuel Flow directly :) .... though I guess you could figure it out by weight change over time.

Great result! I'm also surprised that it was stable at that RPM at above 12,000m altitude, good stuff! To be honest I don't usually venture much above 10,000m but maybe I need to re-think that. Interesting...