Su-27 Maximum Speed Tests & Analysis.

PURPOSE OF TEST:

Having tested and analysed cruise performance for the Su-27, and having calculated the dry thrust performance envelope as described on page 5 of this thread:

http://forums.eagle.ru/showthread.php?t=147556

I wanted to re-visit the performance tests and see what the maximum possible speed of the Su-27 is using afterburning thrust at various altitudes, measured in steps of 1,000m.

TEST CRITERIA:

1. Standard DCS day with no wind, no clouds & no precipitation.

2. Aircraft condition for each test was 100% fuel, no expendables, no ammunition, no weapons loaded.

3. Starting airfield was always Batumi (or roughly above it, see below).

4. For tests at 1,000-6,000m altitude, I started on the runway, took off, climbed under military thrust at ~0.95M to cruise altitude and engaged the altitude hold mode of the autopilot. I then allowed speed to stabilise for a short period before increasing thrust to maximum afterburner. I then let the aircraft fly until I got a bingo fuel warning, at which point I throttled back to 90-95% RPM and allowed the aircraft to slow down. Each test flight was ended about a minute after the bingo fuel notification, except the 16,000m altitude test, during which I decided (on a whim) to carry out a rough field landing.

5. For test flights from 7,000-16,000m altitude, I started in the air at the relevant altitude. Once I managed to stabilise the aircraft and engage altitude-hold, I once again moved to maximum afterburning thrust & allowed the aircraft to fly until I got the bingo fuel notification.

At the end of each flight I saved the relevant track. Data was obtained using TacView 1.4.3 64-bit Pro edition. Data was then saved on to a MS Excel spreadsheet which I also used for graphing purposes. Each flight was performed using DCS WOrld 1.5.3.4278 which I think is the latest version as of today (02/05/2016).

TEST RESULTS:

This is a table of the raw data provided by TacView as described:

Plotting maximum IAS against altitude gives this graph:

Note the decrease in maximum IAS from 1,000m to 4,000m. altitude. I'm theorising that this observed reduction is caused by the airframe being speed limited by drag caused by air density.

Plotting maximum TAS against altitude produces this graph:

More or less as expected, maximum true air speed increases up to ~12,000m after which it slowly drops down. I postulate that the drop in TAS at very high altitudes is probably caused by the drop in engine thrust in the thinner air.

Finally, plotting maximum Mach against altitude produces this graph:

This is again consistent with the general shape of such graphs published for other similar aircraft. Note though the maximum mach at 12,000m altitude: 2.62!!! :shocking: I believe that this is marginally faster than the F-15C is capable of in DCS World. I don't really fly the Eagle but I've seen a maximum speed figure of Mach 2.606. Obviously the real F-15 is capable of going significantly faster than this, and what little published data exists for the Su-27 suggests that its maximum speed at altitude is in reality Mach 2.35, so it appears that something somewhere could do with a small adjustment. Whether it's the aerodynamics of the DCS Su-27 or the atmospheric model I don't know and can't tell from the observed data.

In any case, our DCS Su-27 is indeed a very fast machine. It's also worth noting that despite the fact that for test flights above 7,000m I started in the air, it's absolutely possible to re-create the same tests from a runway start. Vmax isn't limited by fuel quantity in any of these tests.

OBSERVED ANOMALIES:

In carrying out these tests I did observe some anomalous things, both in the way that the aircraft behaves and also in DCS World itself:

1. Bearing in mind that there was no wind during any of these test flights, I've observed that the thin cloud layer up at around 12,500m appears to have default wind associated with it. In areas where clouds actually exist at that altitude (it is in fact patchy) I experienced small but noticeable variations in speed, despite the fact that I should have been at a steady Vmax. Additionally in other flights I've noticed what appeared to be wind shear, buffeting or even turbulence in and around that cloud layer.

2. When decelerating from Vmax down to cruise speed, at some altitudes I observed a pitch oscillation between ~1.7M and ~1.3M. Whether or not this oscillation was present appeared to depend on how quickly I retarded the throttle in order to slow down. Taking it from AB instantly down to idle nearly always produced the oscillations, but retarding the throttle much more slowly never did. I've no idea what causes this, but I'll speculate that maybe it's caused by some weird engine aerodynamics caused by a large difference between actual and commanded RPM.

3. Between 5,000m and 9,000m the DCS Su-27 is capable of "super-cruise" at 95% RPM. By "super-cruise", I mean a stable speed above Mach 1. I'm aware that there are other definitions of what super-cruise means but let's leave that for another discussion. Nothing I've ever read suggests that the Su-27 ought to be capable of it, but who knows? Maybe with a clean airframe it is possible.

4. None of these tests are ultimately realistic because the actual Su-27 pilot's manual restricts flight at maximum speed to 5 minutes or less due to canopy heating concerns.

5. Finally, all the ACMI files, the track files and the Excel work sheet are all uploaded and can be downloaded from this link:

https://onedrive.live.com/redir?resid=D3293A5ED5656179!5068&authkey=!AB1JRcaem0b1hXU&ithint=folder%2cxlsx

I hope all this is of use to the Flanker pilots amongst us...

Massive work, kudos to you, sir.

Results look good to me at first glimpse.

Nice work, some comments:

It might make more sense to put the altitude on the vertical axis. A minor thing, I know, but give it a shot.

This data is interesting, but what would be more valuable to vpilots is not maximum speed: It is acceleration from a given speed to another at a given GW.

So, you can start at say M0.8 (or 0.5 to be more complete, or specific IAS) and accelerate to M2.2, record everything inbetween. Pick 3 altitudes per payload, more is a serious chore.

awesome work Darkfire! thanks for sharing. I've been playing through the DLC campaign, and some of the missions start you off with 1/2 fuel, you know of any fuel flow meter in the 27?

Also, I've been climbing to different altitudes to try and find best speed without touching Afterburners (or reheats) I found that around 6,000m is pretty favorable for that

Massive work, kudos to you, sir.

 

Results look good to me at first glimpse.

Thanks :)

Nice work, some comments:

 

It might make more sense to put the altitude on the vertical axis. A minor thing, I know, but give it a shot.

 

This data is interesting, but what would be more valuable to vpilots is not maximum speed: It is acceleration from a given speed to another at a given GW.

 

So, you can start at say M0.8 (or 0.5 to be more complete, or specific IAS) and accelerate to M2.2, record everything inbetween. Pick 3 altitudes per payload, more is a serious chore.

I know, normally graphs showing this data have the G data plotted on the x axis. I've tried doing this but so far Excel is stubbornly refusing to comply. Once I figure out how the hell to bend Exel to my will I'll post the graph as it usually appears.

The acceleration tests are something I'm working my way up to. That's a hell of a lot of test flying, but I'll do it eventually :)

awesome work Darkfire! thanks for sharing. I've been playing through the DLC campaign, and some of the missions start you off with 1/2 fuel, you know of any fuel flow meter in the 27?

Also, I've been climbing to different altitudes to try and find best speed without touching Afterburners (or reheats) I found that around 6,000m is pretty favorable for that

Unfortunately the Su-27 doesn't have a fuel flow meter. There are however ways to make an educated guess. The most efficient cruise throttle setting is around 85%. It varies between 83% and 87% but if in doubt 85% is always a good choice. Above this setting the speed increase with increased throttle ceases to be linear, which would also be worse for fuel economy. Have a read through this thread:

http://forums.eagle.ru/showthread.php?t=151731&highlight=su-27+range

If you look closely at the table on page 1 of that thread you'll see calculated fuel consumption values for different altitudes, measured at the best cruise throttle setting for any given altitude.

In terms of best speed, the higher you go the more fuel efficient the aircraft will be. Don't think that you'll need to increase throttle at higher altitudes either: the Su-27 will happily cruise at 14,000 metres at 85% RPM, though your IAS will be so low that anything other than cruising in a straight line is completely impractical.

However, the problem is that the autopilot on the Su-27 is bugged. If you engage altitude hold mode at any speed under 550 Km/h IAS the aircraft will become unstable and a nasty yaw oscillation will set in. 10,000m is the highest altitude at which your IAS in anything other than afterburner is above 550 Km/h. Your maximum range with full fuel and a full missile load is of the order of 1,500Km when cruising at 10,000m so in terms of best cruise speed I'd recommend anything between say 7,000 and 10,000m altitude.

In terms of climb performance, I don't have the figures to hand but I think I remember reading that best climb speed is 0.85M. Personally after takeoff (maximum dry thrust) I maintain a vertical velocity of under 10 m/s until I reach ~0.90M. At that speed you can climb at anything up to 50 m/s vertical velocity without any appreciable drop in forward speed until you're way above 8,000m. This makes for an efficient and fast climb to cruise altitude.

Hope this helps :)

...

 

 

 

I know, normally graphs showing this data have the G data plotted on the x axis. I've tried doing this but so far Excel is stubbornly refusing to comply. Once I figure out how the hell to bend Exel to my will I'll post the graph as it usually appears....

 

What version of Excel are you using? If you want, send me the worksheet and I'll see what I can do.

What version of Excel are you using? If you want, send me the worksheet and I'll see what I can do.

Thanks. I'm using Excel 2016. Zipped work sheet attached.

Su-27 Flight Characteristics.zip

I've tried doing this but so far Excel is stubbornly refusing to comply

So many ways to skin a cat.

If it's a single series on the graph, right click on the line in the graph, click select data, in the middle of the dialogue box will be "Switch row column"

or there's a ribbon button that does the same.

or, select the series, right click, select data, highlight the series and click edit, change the series data to what you want on the Y axis, edit the horizontal category & change to what you want as the x axis.

or - click on the series in the graph. The data ranges will now have a box around them. Grab the x axis box with your mouse & drag it somewhere else. put the y axis box over what you want to be the x axis data, and move the x axis box over the what you want to be the y axis data. do the same with the label boxes.

or - fool proof way - select any cell that doesn't have data & press ALT + F1 to get a blank graph.

Right click "change Chart Type" then make it an XY scatter line graph.

Right click the graph & click select data.

Remove any existing series, and then add a series.

Now you can chose which data is on the x or y axis by filling in the boxes (remembering to remove the auto-entry of "={1}")

Ah... it's always great when the experts appear. I had figured out how to transpose the rows and columns. But now you have the complete deal.

EDIT: Darkfire, I've been playing around with this and it seems to be the type of chart you are trying to generate (LINE) that's creating the issue. I have yet to find a way around it and Excel itself suggests that there may not be in their description of the chart type. OTOH, a CLUSTERED BAR chart does it without a problem.

So many ways to skin a cat.

 

If it's a single series on the graph, right click on the line in the graph, click select data, in the middle of the dialogue box will be "Switch row column"

 

or there's a ribbon button that does the same.

 

or, select the series, right click, select data, highlight the series and click edit, change the series data to what you want on the Y axis, edit the horizontal category & change to what you want as the x axis.

 

or - click on the series in the graph. The data ranges will now have a box around them. Grab the x axis box with your mouse & drag it somewhere else. put the y axis box over what you want to be the x axis data, and move the x axis box over the what you want to be the y axis data. do the same with the label boxes.

 

or - fool proof way - select any cell that doesn't have data & press ALT + F1 to get a blank graph.

Right click "change Chart Type" then make it an XY scatter line graph.

Right click the graph & click select data.

Remove any existing series, and then add a series.

Now you can chose which data is on the x or y axis by filling in the boxes (remembering to remove the auto-entry of "={1}")

Ah, awesome, thanks very much. I tried the first few suggested methods & the results were... Strange. Ironhand - I think you're right in that the chart type I chose is limited in some way as to the data presentation. I'll have another play later with different chart types & see what I can brew up.

Managed it. Here then is the traditional form of the altitude v maximum mach plot for the Su-27:

Method: it appears that for most graph types the data to be displayed on the X-axis is always in an earlier (further left on the table) column than the data to be plotted on the Y-axis. Why this is the case I have no idea. A very large thank you to Weta43 & Ironhand for pointing me in the right direction. Frankly whoever wrote the built-in help files for Excel 2016 can [REDACTED] with a well manicured goat. :furious: :ranting: :bash: