[EVALUATING / NO BUG] Acceleration to max speed

Acceleration up to achieving max speed (IAS and TAS) is to high and abruptly stops when reaching it.

Speed increases very rapidly and stops doing it at 1300km/h IAS (clean plane).

With actual flight model its feels like max speed at level flight its like a scripted limit (it shouldn't be so obvious).

It feels like reaching the 250 km/h programed safety speed limit in a S4 Audi capable of 280 km/h, if you know what I mean.

Speed should increase very gradually when approaching max speed. With acceleration decreasing while drag is increasing logarithmically (edit: square, thanks John) with speed and air resistance, up to the point where drag = thrust.

EDIT: The "scripted" limit for max speed was removed, now we have the problem that the migs always overspeed and its not possible to mantain a constat speed at level flight with AFT on.

Drag is not reducing logarithmically but squared. Meaning a high increase of drag when just gaining little speed.

I can't judge if the current behaviour is realistic or not, but at least it seems you can exeed the IAS of 1300km/h in certain situations.

And the drag is also different on different altitude, since the air density is different on different altitude. Although I can recall the exact formula...

Thanks for the replies. This is a bugs and problems thread and the point of the thread is just to acknowledge there is an error in longitudinal acceleration. Its another variable in the EFM to be improved by LN.

I mentioned drag and thrust to make it clearer. There shouldn't be any doubts about the acceleration curve error, its common sense and if it was needed it can be proven with basic aerodynamics concepts and/or acceleration charts.

Thanks for the replies. This is a bugs and problems thread and the point of the thread is just to acknowledge there is an error in longitudinal acceleration. Its another variable in the EFM to be improved by LN.

 

I mentioned drag and thrust to make it clearer. There shouldn't be any doubts about the acceleration curve error, its common sense and if it was needed it can be proven with basic aerodynamics concepts and/or acceleration graphs.

:huh:

So because it's posted in here it's a bug without question? :doh:

Please post a track which shows the behaviour. In this aspect I haven't noticed anything odd in the FM (there are other bugs though).

And the drag is also different on different altitude, since the air density is different on different altitude. Although I can recall the exact formula...

Drag = Cd * 1/2 * density * speed^2, with Cd being a function of AoA (Cl) in stationary flight

The problem with speed limit (longitudinal acceleration to max speed) seems solved since the last beta update 1.2.15. Good job LN.

Now It seems the mig overspeeds too easy, need mor testing. We have to learn how to control max speed and prevent engine shut off or damage.

After a fast testing last night I have the impression (have to test it more and compare to real charts to be sure) that now, with the ¨scripted¨ speed limit gone, the mig accelerates to much beyond max speed, always to the point the engine quits.

I´m concerned because this beta feature might hit the official version tomorrow.

As it is right now its very easy to overspeed. And you know its very difficult to maintain a steady speed between match 0.9 and 2.05. All speeds between mach 1.0 and over mach 2.0 requires afterburner and afterburner is practically or ON or in OFF position. There is very little range between min and max afterburner to choose a steady mach speed in level flight. If your climbing with AFT you can easily choose your speed by changing climbing vertical speed. But when your flying level between mach 1 and mach 2 your are either slowly accelerating to max speed with AFT on or descelerating with AFT off. You will only get a steady speed when you approach max speed. This dosent happens with the corrent physics, the mig continues to accelerate until engine failure.

Does it makes sense? How are we suppose to maintain a supersonic flight at a steady (max) speed? Continuously turning ON and OFF the Afterburner?

Again I have to do more testing, I´m at the office right now with clients waiting. Maybe Im wrong and its easy to use "little" afterburner and prevent over speeding. I will get horizontal G numbers and compare them with the chats this weekend.

From 1.2.15 change log:

Engine over-speed limit removed in simulation mode, engine damage simulated.

Fuselage over-speed drag limit removed in simulation mode

I admit I'm puzzled. Something seems to broken here indeed. I'm not sure how we're supposed to do high speed level flights now. After checking again RPM and temp limitations mentioned in the manual, I did some testing. My observations are as follows:

1) When pushing throttle to obtain N1 of ~101-102% to keep the engine from overreving, reheat and nozzle warning lights go on, reheat sound can be heard, there's no external animation of reheat (I can live with that for now, since it's just a gfx effect), but the EGT temp can be stabilized only between 660 and 710 degrees. I'm not sure If reheat truly works or not, because the thrust at these settings seems to be not high enough for good acceleration. Breaking Ma 1 in level flight is impossible, one can do it in a dive, but then the speed stabilizes at ~ Ma 1.15-1.2 and the plane cannot fly faster (plane in clean config).

2) Pushing throttle all the way up (1st reheat) increases N1 RPM to ~103% (which according to the manual is supposed to be a limit for 2nd reheat?) and EGT to 770 degrees (limit for full throttle, no reheat?). When the airplane reaches Ma=2 engine flames out immediately, but it can be re-lit. Second attempt breaks the engine.

All test flights done between 9000 and 10000 m on standard summer map.

I admire adding more challenging conditions to the engine management, but looks like something needs to be tuned further over here.

2) Pushing throttle all the way up (1st reheat) increases N1 RPM to ~103% (which according to the manual is supposed to be a limit for 2nd reheat?) and EGT to 770 degrees (limit for full throttle, no reheat?). When the airplane reaches Ma=2 engine flames out immediately, but it can be re-lit. Second attempt breaks the engine.

 

All test flights done between 9000 and 10000 m on standard summer map.

 

I admire adding more challenging conditions to the engine management, but looks like something needs to be tuned further over here.

The limit where the engine dies has to be higher surely? I just took her up to M2.1 at about 10000m without running into any kind of trouble.

Would You mind telling how You did it then? I'm mostly interested in the "without running into any kind of trouble" part. Just did another test at 10000 m. By using ~102,5% and 690-700 degrees EGT I admit I've managed to get her to Ma 2.1 indeed...

... just to flame out a second later :D.

Using 102% and 660 degrees - lowest settings required for the "reheat" and "nozzle" lamps to light up and reheat sound to kick in, I could only make the plane decelerate from Ma 1.6 down.

Summer map, clean config again. Track below, if anyone's interested.

speed_test_1.2.15.trk

Ok, so I used a training map I build. Spring, 15°C, slightly different pressure at sea level than default summer one but it shouldn't differ too much. I might have been a bit above 10000ft actually but not too much. Watch your IAS, not your Mach needle. 1300km/h IAS is the never exceed limit which should be roughly M2.1 at 10000m, maybe a bit less. I immediately went out of burner every time I got to about 1295km/h IAS

If you go above 10000m speeds higher than M2.1 should be possible without exceeding that limit. At 10500m you will reach it at about M2.13 or 2.14.

Just be aware that you put a lot of stress on the engine if you exceed M2.05 for prolonged periods and these speeds will kill your engine in the long run no matter what.

I'm OK with idea of engine going tits up above 2.05. I didn't expect it to fly it at prolonged periods of time. It was a little "torture test" anyway. I'm not OK with:

1) these "prolonged periods" measured in couple of seconds at best (flameout exactly when needle touches 2.1)

2) necessity of going above rated RPM limit (101.5% in reheat) in order to accelerate within supersonic range at all;

3) engine in reheat going "thanks but no thanks" 70 degrees below full throttle no reheat limit (?) and 150 degrees below 1st reheat limit (??). OK, temps are one thing, RPM is the other. True, when that happens You are at astonishing, nerve-racking 1.5% above RPM limit :D :D :D, but You have to, because of - see point 2.

At the moment it seems that in order to maintain constant Ma 2-2.05 while not seizing the engine, one has to flick reheat on and off and on and off and on and off and on and off.... which OP noticed in his previous posts.

yea that seems to be the case ;)

Since I don't have a track here's what my SARPP gives me for the moment just before and just after I went out of burner:

time/height in metres/IAS in km/h/G vert./G horiz./RPM

717.000000 10147.429688 1302.138916 1.141661 0.188510 100.027809

718.000000 10157.237305 1304.358887 1.149144 0.118310 96.451813

Here's a solid M2.05 run at 10000m till the tanks are empty.

Edit: SARPP file says January, but it's one to one record with track from today Feb.

M2.05_run.trk

SARPP_DATA_2015_01_28_19_31.txt

Here's a solid M2.05 run at 10000m till the tanks are empty.

 

Edit: SARPP file says January, but it's one to one record with track from today Feb.

Thank You very much. It was pleasure to watch. I didn't watch the track to the end, but first minutes were enough for me. So... the recipe for more or less safe top speed seems to be: 102% N1 RPM, 690 degrees EGT and under no circumstances exceeding Ma 2.05.

Boy, are the safety margins tight or what? I don't envy guys with worn out pots in joysticks, or short throw throttles! I'm not convinced about this aspect and about EGT, but at least we know continuous Mach 2 can be done. I'd still want to ask Novak to investigate this and say if the room for error should really be that small.

Thank You very much. It was pleasure to watch. I didn't watch the track to the end, but first minutes were enough for me. So... the recipe for more or less safe top speed seems to be: 102% N1 RPM, 690 degrees EGT and under no circumstances exceeding Ma 2.05.

 

Boy, are the safety margins tight or what? I don't envy guys with worn out pots in joysticks, or short throw throttles! I'm not convinced about this aspect and about EGT, but at least we know continuous Mach 2 can be done. I'd still want to ask Novak to investigate this and say if the room for error should really be that small.

True, there's not much room for error indeed. In the end I found EGT more accurate to adjust the speed instead of the RPM gauge. You'll just need to be soft on the throttle.

More important thing was the nosecone control. There's are values for each mach range for the manual nosecone control in the manual page 59. For some reason the auto nosecone control goes well beyond the 40% at speeds over M1.8 and makes it harder to control the throttle and keep the speed steady. Eventually you'll just cut your engine.

I pretty much just followed the table values and instructions how to manually operate the nosecone.

I rolled the empty needle to 40%, kept the nosecone on auto until the needles matched each other at M1.8 and flipped nosecone to manual. When I was nearing M2.0 I started to drop throttle and follow the EGT vs. M/IAS.

That's how I got more room to adjust the speed for the M2.05.

Is this how it is supposed to be operated... that I cannot answer.

Thank You very much. It was pleasure to watch. I didn't watch the track to the end, but first minutes were enough for me. So... the recipe for more or less safe top speed seems to be: 102% N1 RPM, 690 degrees EGT and under no circumstances exceeding Ma 2.05.

 

Boy, are the safety margins tight or what? I don't envy guys with worn out pots in joysticks, or short throw throttles! I'm not convinced about this aspect and about EGT, but at least we know continuous Mach 2 can be done. I'd still want to ask Novak to investigate this and say if the room for error should really be that small.

First of all I'm not a flight engineer. But reading and taking part in the very emotional discussions during the last two months regarding the flight model, I would assume it is a compromise between reality and our sim.

Many people were very upset about the speed barrier which was used to prevent massive engine damage. To satisfy the demands for more reality it was a logical step to remove this limit (as a quick step).

I think in reality would be a safety margin but the result of overspeeding your engine is clear. In best cases a shorter life of your jet engine, a significant overspeed will lead to catastrophic engine damage (to the turbine blades) immediately. Maybe we will see some kind of random failure after exceeding the safe rpm but when reaching a critical value your engine will stop working (if not blowing up in flames).

God damn. I just had four flameouts attempting to repeat what Bassly did in his track. The only difference was starting from ground up with using lots of afterburner during takeoff and climb, plus nose cone on auto.

Any shock control inlet devices are supposed to be on auto, in every supersonic plane. Manual controls are provided for emergency situations only. If flying "our" MiG is easier by tinkering with manual control then it needs to be looked into I'd say.

When I get closer to Mach 2 with 102% like fellow Bassly did and try to throttle back to the exact limit from the manual (101.5%), the engine might be fine, but the temp drops to 650, the plane jerks and starts decelerating immediately. I give up for today.

Well... at least after my last engine seizure I made a successful dead stick landing, first one after 5 crashed MiGs total :D. That cheered me up somewhat!

I did first the cold start to 10km too and since there was no actual difference I did the airstart for the track.

But the fact that I'm doing something that should be done only in emergency situations works better than normal operation... Without any damage indicated. Hard to say.

Althought everything is done within the values given in the manual. Max. M2.05, setting for manual nosecone 40%, which was given for the mach >1.8. It was 40%, not >40%.

Likewise it would be interesting to hear a some sort of "yes/no" on the matter.

Noted.

Meanwhile, concerning the throttle (axis) and the engine response, do you know that you can setup your throttle so that you have improved control on e.g. afterburner settings and e.g. SPS throttle position? Engine response to the throttle inputs can be controlled precisely (to reasonable extent) in Options/Controls/21/Axis Commands/Throttle, and then select and adjust the "User curve".

Thanks Dolphin, done that and I can easily manage AFT range and exhaust temperatures.

The problem that persist is that there is an abrupt change in power with AFT on at 660 celsius degrees ET. At level flight and 10,000 m one degree less than 660 and the mig will slow down to subsonic speed, one degree more and it will accelerate to above mach 2 until engine failure, without much drag. Constant mach can't be achieved without manually adjusting the nose cone.

As it is, max speed is still achieved, almost always (depends on loadout and altitude), at engine failure because of overspeeding. At 10,000 m you have to constantly change exhaust temperature up and down arround 660 degrees to maintain a speed within mach 1.0 and 2.0.

Any news about this bug for next update DCS Wolrd 1.5?

Would be nice to be able to fly the mig level and at supersonic speed without overspeeding so easily, to maintain a steady speed between mach 1 to 2 without having to constantly turn ON and OFF the AFT or manually adjusting the nose cone to 3.5.

Thanks for this great module and the next to come!