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[RESOLVED] Cruise performance not realistic


TurboHog

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You are forgetting the *very specific conditions*:

 

* Takeoff using full reheat to 600 kph IAS

* At 600 kph IAS, cancel reheat

* Accelerate to 870 kph TAS

* Climb to altitude at 870 kph TAS

 

Note carefully IAS and TAS!!!!!!!!

 

Best regards,

Tango.

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No, sorry - Viper.

 

He wrote:

 

so approximately 93 seconds or thereabouts for the zoom-climb
Regardless of zoom climb - it is (currently) not possible to maintain speed at full power (no reheat). I posted a screenshot series further up demonstrating this.

 

Best regards,

Tango.

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I'll say it again:

 

I have an issue with the allegation that, as mentioned in post #1, it is impossible to cruise without afterburner at 10 000m -

 

This means that, even without external stores, cruise above 10000m is not possible without the use of afterburner.

 

The track clearly illustrates that it is possible to cruise at 550km/h IAS at 10 500m AGL, with speed increasing to 580km/h IAS, an increase of 50km/h on the documented cruise speed of 530km/h.

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The track clearly illustrates that it is possible to cruise at 550km/h IAS at 10 500m AGL, with speed increasing to 580km/h IAS, an increase of 50km/h on the documented cruise speed of 530km/h.

 

Well, with the build I have, I can't. I lose airspeed, and eventually stall.

 

Best regards,

Tango.

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Well, with the build I have, I can't. I lose airspeed, and eventually stall.

.

 

I have the same version you have. Watch the track - it's all there.

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question is - what is different then?

 

One thing I did do that may not be readily apparent is utilizing the SAU to ensure the aircraft is properly trimmed in order to eliminate human error in the trimming process.

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I'll say it again:

 

I have an issue with the allegation that, as mentioned in post #1, it is impossible to cruise without afterburner at 10 000m -

 

 

This discussion is not about one specific example. It is about unrealistic cruise performance in general. 10000m is, as you can tell from post#1, the boundry between afb and non-afb cruise (in order to maintain speed). So yes, you may cruise at 10000m no problem and some may not. The point is that you can't cruise at a much higher altitude or weight than that, while you should be able to.

 

Please do some more interpretation of the results.

'Frett'

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One thing I did do that may not be readily apparent is utilizing the SAU to ensure the aircraft is properly trimmed in order to eliminate human error in the trimming process.

 

I use SAU for the level flight testing. There is even a table for the altitude loss incurred by activating it.

 

Best regards,

Tango.

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This discussion is not about one specific example.

 

You cannot generalize - you need to investigate specific scenarios.

 

You stated that it could not be done and I have disproved it. I will, as time permits, move onto your other specific allegations and deal with them one by one as opposed to generally and disprove them also, or not, as the case may be.

 

I started with the cruise above 10 000m due to the fact that you bold-texted it in your results comment.

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Your track, Viper.

 

We are testing climb-outs without reheat, let alone emergency reheat. So this track is pretty useless for climb-out comparison.

 

As for cruise:

 

- Clean configuration will indeed cruise at 10000m. Nothing new:

 

From post #1

(5) No stores, average weight during measuring phase: 17000lbs --- 200L/7min -> 1714L/hr

with added remark

In (5) and (6), cruise speed was only maintainable at almost 100% throttle. This means that, even without external stores, cruise above 10000m is not possible without the use of afterburner.

 

With this remark I'm saying that clean config, 10000m is really the boundry between cruising with or without afb, given a constant speed. In bold, which is apparently the only part you have read, it sais ABOVE and not AT 10000m. So we agree and you have not disproved anything. The point is: Go higher or go heavier (= ABOVE 10000m) and you will no longer maintain cruise speed without afb, while you should.

 

Please read more careful to avoid misunderstandings and the endless discussions that follow such as these...

 

-----

 

In post 1 I say that cruise speed is, indeed maintanable at 10000m. (You blame me of telling otherwise). Tango is talking about climb (not cruise) to 10000m, which is impossible (no reheat and within reasonable limits of the charts). Maybe that caused some confusion.


Edited by TurboHog

'Frett'

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With this remark I'm saying that clean config, 10000m is really the boundry between cruising with or without afb, given a constant speed. In bold, which is apparently the only part you have read, it sais ABOVE and not AT 10000m. So we agree and you have not disproved anything. The point is: Go higher or go heavier (= ABOVE 10000m) and you will no longer maintain cruise speed without afb, while you should.

 

Which is why I cruised at 10 500m AGL and not 10 000m AGL to pre-empt just this. Not only that, but I cruised at 10 500m AGL without AB at a speed of 580km IAS, a 50km/h increase of the manual excerpt posted. Accordingly stands to reason that, had I climbed to bleed airspeed to co-incide with the required number of 530km/h IAS , my altitude would have been even higher than 10 500m AGL, approaching 11 000m AGL cruising without afterburner.

 

 

Tango is talking about climb (not cruise) to 10000m, which is impossible (no reheat and within reasonable limits of the charts). Maybe that caused some confusion.

 

No, no confusion. I have an issue with this as well. I ran a quick test and managed to climb to 10 000m AGL (well, rather more than 11 000m AGL actually) without afterburner whilst adhering as close as possible to the numbers Tango posted:

 

(takeoff using full reheat to 600 kph IAS

* At 600 kph IAS, cancel reheat

* Accelerate to 870 kph TAS

* Climb to altitude at 870 kph TAS)

Track: Mig no afterburner 11 000m.trk

 

So no, in my humble opinion far from impossible.

 

If you do watch the track kindly ignore the end bit - zoom-climbed to 14 000m AGL on a whimsy, irrelevant for the purposes of 'climb-to-10 000m without AB' illustration.

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Quick question: how does one access the flight recorder data?

 

Watching your track now, Viper. Really not trying to be a dick here, but you start climbing at much more than 870 km/h TAS (as per the manual). I imagine that's what makes the difference.

Also, if you look at the track, you can see that you weren't actually able to hold a steady climb at that airspeed. Looks to me (as a layman) like a series of zoom climbs. But maybe the manual takes some of that into account? I'm not very experienced with technical literature in this field.

 

EDIT: Also, you have a much lighter aircraft than the other tests, looks like 1800 L of fuel. Not 100% sure what the manual refers to, but it doesn't really make sense to take off in a short-range interceptor with half fuel.

 

Regardless of fuel state, the manual says a climb like the one you did, or tried to perform, should take 8:50, and I think yours took 14-15 mins.


Edited by Corrigan

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But maybe the manual takes some of that into account? I'm not very experienced with technical literature in this field.

 

What we are lacking in this discussion are specific climb profiles.

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What we are lacking in this discussion are specific climb profiles.

 

I agree. Although if you look at the last part of my post which I just edited in, it's clear that the climb the manual is talking about is much much faster than the ones we can do (including your track).

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Regardless of fuel state, the manual says a climb like the one you did, or tried to perform, should take 8:50, and I think yours took 14-15 mins.

 

I don't think so. Was closer to 9 when I checked but I'll look again.

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Which is why I cruised at 10 500m AGL and not 10 000m AGL to pre-empt just this. Not only that, but I cruised at 10 500m AGL without AB at a speed of 580km IAS, a 50km/h increase of the manual excerpt posted. Accordingly stands to reason that, had I climbed to bleed airspeed to co-incide with the required number of 530km/h IAS , my altitude would have been even higher than 10 500m AGL, approaching 11 000m AGL cruising without afterburner.

 

10000m is not an absolute boundry. Small differences in weight and atmosphere can move this boundry around this altitude. You are so focused on disproving everything. Instead, do some more interpretation of the data and try to understand what I'm trying to indicate. You completely disregard that in your own track you are cruising at full throttle already at 11000m. You are supposed to be able to do the same thing at higher altitudes. So you could think: 'hey, that's when it starts to become a questionable performance indeed'.

 

Try cruising at 15km altitude, clean. Will it work? You can barely keep your speed with reheat at that altitude.

 

Here is a track. I cruise at 10000m with two missiles and a center 490L fuel tank. I lose speed and eventually the SAU can't hold it anymore and allows the aircraft to descent. This is not supposed to be like this.

Mig-21cruise10000m_with_stores.rar

'Frett'

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Regardless of fuel state, the manual says a climb like the one you did, or tried to perform, should take 8:50, and I think yours took 14-15 mins.

 

OK just checked:

 

Commenced climb at 12:01:35 and reached 10 000m AGL at 12:11:10.

 

9 min 35 seconds.

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OK just checked:

 

Commenced climb at 12:01:35 and reached 10 000m AGL at 12:11:10.

 

9 min 35 seconds.

 

Yeah, but it should take you just under that to 11000, which instead takes you 13 minutes.

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What we are lacking in this discussion are specific climb profiles.

 

I stated them several times now. It is the profile the manual assumes for the table data.

 

Again:

 

* Takeoff using full reheat to 600 kph IAS

* At 600 kph IAS, cancel reheat

* Accelerate to 870 kph TAS

* Climb to altitude at 870 kph TAS

 

Note carefully IAS and TAS!!!!!!!!

 

 

MANUAL STATED CLIMB PERFORMANCE FROM POST #1

 

Clean:

 

* 7000 m, 4 mins 10 sec, 40 km

* 10000 m, 7 mins, 82 km

 

2x R-3S, 1x 490 L center-line drop tank:

 

* 7000 m, 4 mins 40 sec, 47 km

* 10000 m, 8 mins 10 sec, 98 km

 

Once achieving cruise alt at 870 kph TAS, reduce power to maintain 530 kph IAS max range cruise speed.

 

Allow me to state it again: in the current build that we have, it is impossible to maintain cruise speed without reheat. In addition, the climb distances are waaay off.

 

I'm reaching 8000 m at a distance of 120 km, and unable to reach the cruise altitude following the climb schedule, which should be easily achieved.

 

Interesting that no-one commented on my track yet. See my post here: http://forums.eagle.ru/showpost.php?p=2185785&postcount=42

 

Best regards,

Tango.


Edited by Tango
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