Induced drag

[Corrigan]

I agree with almost everything you write, don't get me wrong. :)

 

The thrust and other values might be somewhat off here in the beginning, but I guess LN will need to be adjusting these a bit over the coming weeks and months.

After all it is still an early access release, so not polished yet.

 

Yep, that's what we're all trying to help with here.

 

Just to be clear, that graph: I made a mistake above. I agree with being able to sustain 3 g at MS at sea level on a clean, 13 000 kg aircraft. That's a good benchmark for us.

 

I don't see how you extract 2 g from it though. You don't know where the 2 g drag curve intersects the thrust curve. It could be anywhere from 400-700 km/h as far as I know, just going from that plot.

[Sporg]

I agree with almost everything you write, don't get me wrong. :)

 

 

 

Yep, that's what we're all trying to help with here.

 

Just to be clear, that graph: I made a mistake above. I agree with being able to sustain 3 g at MS at sea level on a clean, 13 000 kg aircraft. That's a good benchmark for us.

 

I don't see how you extract 2 g from it though. You don't know where the 2 g drag curve intersects the thrust curve. It could be anywhere from 400-700 km/h as far as I know, just going from that plot.

Thanks. :)

 

The 2G was just a guesstimate, since, as you also state, the 2G graph is not shown.

Was just guessing at a point approximately mid way between the 1G and the 3G curve, possibly closer to 3G.

[RagnarDa]

Alright did some more testing and at SL I can maintain 1.8 G's on max dry holding completely level allowing for a +- 5 km/h fluctuation at 740 km/h TAS (M 0.6). Aircraft completely clean.

 

At M 0.46, or 568 km/h, I can maintain 1.5 G's.

 

This is achieved by trimming the aircraft into the turns btw so as to keep it as steady as possible.

 

 

 

Could you repeat the experiment with fuel regulator set to manual? It's on the right wall of the cockpit

[Hummingbird]

Will try with the fuel control set to manual and see what I get. Anything else about that I need to know about though, i.e. any manual inputs needed?

 

Very much doubt I will hit 3 G's at 800 km/h though.

 

Will post some results tomorrow.

Edited February 13, 2017 by Hummingbird

[Hummingbird]

Alright the results are in after some thurough testing.

 

Aircraft condition:

Weight = 12,987 kg

Fuel = 50% *

Stores = None (clean)

 

* fuel set to unlimited so as to keep the weight of the a/c the same throughout the tests

 

 

Test nr.1: Highest sustainable load factor at max dry thrust

Altitude = 50 m (+- 5 m fluctuation allowed)

Speed = 800 km/h TAS (+- 5 km/h fluctuation allowed)

 

Max sustainable load factor is 2.4 G's

 

Test nr.2: Highest sustainable load factor at max afterburner

Altitude = 50 m (+- 5 m fluctuation allowed)

Speed = 800 km/h TAS (+- 5 km/h fluctuation allowed)

 

Max sustainable load factor is 5.1 G's

 

Every test consisted of gradually trimming the aircraft into a level turn until the highest load factor at which a speed of 800 km/h (+- 5km/h) could be held indefinitely at an altitude of 50 (+- 5m). To achieve the most accurate result possible, once settled in the turn I held these turns for 3 min where none of the speed or altitude limits were to be breached.

 

The above figures can be compared with the real life figures of 3 G's dry and 6 G's wet respectively at that speed & altitude.

 

My suspicion is that the ingame aircraft exhibits excessive induced drag based on how fast the aircraft loses speed in turns with the engine at idle, it does seem to lose speed much faster than any other jet ingame atm. Of course the issue could also be due to a lack of thrust, however straight line acceleration & speed seems ok so I doubt this.

 

EDIT: Seems unlimited fuel wasnt activated for the first test, so retested and corrected the figures.

Edited February 14, 2017 by Hummingbird

[Sporg]

Thanks for your tests Hummingbird.

 

One last question, which temperature do you set in the mission?

Upon further reading, it seems like SAAB considered a "standard day" 15° C.

[Hummingbird]

Thanks for your tests Hummingbird.

 

One last question, which temperature do you set in the mission?

Upon further reading, it seems like SAAB considered a "standard day" 15° C.

 

20 deg celcius

 

Unlimited fuel setting didnt work for the first test though, once corrected the figures became even worse for the dry thrust performance at 1.9 G's sustained as also achieved earlier. Max AB performance stayed at 5.1 G's.

[Hummingbird]

EDIT

 

*sigh* fuel had reset at 100%.... hence the odd drop in sustainable G's.

 

Initial results were completely correct, 2.4 and 5.1 G's and the best I can achieve starting at 50% fuel. However as mentioned unlimited fuel does not work, so fuel load dropped gradually, thus the most accurate results were achieved initially in the turns.

 

Will record the next tests to illustrate exactly how its carried out.

Edited February 14, 2017 by Hummingbird

[Udat]

Thanks for your tests Hummingbird.

 

One last question, which temperature do you set in the mission?

Upon further reading, it seems like SAAB considered a "standard day" 15° C.

Pretty much everyone considers 15° @ sea level standard.

 

ISA = International Standard Atmosphere

Temperature: 15°C @ sea level

Pressure: 1013.25 hpa @ sea level

Lapse rate: 1.98°C/1000feet

:book:

[Hummingbird]

Here's a recording of a slightly rougher test where I allowed myself to deviate a little more in speed & altitude, albeit not enough to really affect the accuracy of the result:

 

Starting fuel load was 58%, temp was 15 C, speed ~800 km/h, aircraft was clean. This results in a sustainable load factor of ~2.3 G's running at max dry and ~5.1 G's at max AB. (5.2 G's in this vid due to fuel consumption)

Edited February 14, 2017 by Hummingbird

[Pocket Sized]

Just to be sure, have you tried the tests with manual fuel regulation?

[Hummingbird]

Just to be sure, have you tried the tests with manual fuel regulation?

 

Yeah if you mean by disengaging the automatic EBK, but it didn't seem to have any effect.

[SgtPappy]

 

Turn chart here:

 

IAS on X-axis

Drag = D, Thrust = T on Y-axis.

 

Tänd (On) = Wet thrust

Släckt (Off) = Dry thrust

 

Sväng = Turn

Planflykt = Level flight.

 

The text box (and the shaded areas) says:

" OBS! Thrust deficiency "

" A: D - TDry ~9 ton "

" B: D - TWet ~10 ton "

 

 

Correct me if I'm wrong but does that turn rate diagram show full afterburner? The most commonly published wet thrust given for the RM8A is ~26,000 lbf = 11,800 kgf but only 10,000 kg is shown. The remaining 1,800 kgf seems like too much to forget about.

[Corrigan]

MAX TÄND EBK means full wet, yes. What do you mean that "only 10000 kg is shown"?

[renhanxue]

The thrust curve is a bit simplified in that diagram - it's not quite that linear with IAS, I'm pretty sure (compare to the thrust charts in the SFI).

 

Also, thrust numbers that are quoted in encyclopedias and the like are usually given for an engine static in a test rig on the ground. Mounting it in the aircraft reduces thrust because of losses in the intakes and to auxiliary equipment such as the generator, bleed air etc, but increasing airspeed increases thrust while on full AB because it gets more fresh air to burn. In other words, actual thrust in the air in different situations is quite different from the encyclopedia number. See my thread about the Viggen's afterburner for a lot more words about this.

[SgtPappy]

The thrust curve is a bit simplified in that diagram - it's not quite that linear with IAS, I'm pretty sure (compare to the thrust charts in the SFI).

 

Also, thrust numbers that are quoted in encyclopedias and the like are usually given for an engine static in a test rig on the ground. Mounting it in the aircraft reduces thrust because of losses in the intakes and to auxiliary equipment such as the generator, bleed air etc, but increasing airspeed increases thrust while on full AB because it gets more fresh air to burn. In other words, actual thrust in the air in different situations is quite different from the encyclopedia number. See my thread about the Viggen's afterburner for a lot more words about this.

 

Thanks, I'll be sure to read on your information in the other thread later.

 

However, conventional turbojet engines actually do not always create more net thrust with speed. The momentum change from air in to exhaust gases out decreases with velocity, but can increase again with proper design so the thrust vs. velocity is usually of a bathtub shape. But I guess that's what you mean by the thrust line being simplified. It only seems to show the increase.

[renhanxue]

Apologies if I'm coming off as overly pedantic, but the RM8A is a turbofan. On dry thrust it does indeed lose thrust with increasing airspeed, as you say, but with the afterburner engaged thrust is either more or less constant with airspeed (up to the transonic region, on low afterburner thrust settings) or increases with airspeed. At max zone 3 the thrust increase is almost - but not quite - linear with increasing airspeed from standstill to Mach 1.1 at sea level.

 

Here is the thread I was talking about, please refer to the thrust-to-drag diagrams therein if anything is unclear.

 

As far as I know most military afterburning turbofans behave more or less like this; the RM8A just happens to have a particularly pronounced thrust increase in afterburner because of its large bypass ratio.

[SgtPappy]

Apologies if I'm coming off as overly pedantic, but the RM8A is a turbofan. On dry thrust it does indeed lose thrust with increasing airspeed, as you say, but with the afterburner engaged thrust is either more or less constant with airspeed (up to the transonic region, on low afterburner thrust settings) or increases with airspeed. At max zone 3 the thrust increase is almost - but not quite - linear with increasing airspeed from standstill to Mach 1.1 at sea level.

 

Here is the thread I was talking about, please refer to the thrust-to-drag diagrams therein if anything is unclear.

 

As far as I know most military afterburning turbofans behave more or less like this; the RM8A just happens to have a particularly pronounced thrust increase in afterburner because of its large bypass ratio.

 

Ah yes, sorry it slipped my mind that it was a turbofan. Thanks for the info :doh:

[Corrigan]

Yeah if you mean by disengaging the automatic EBK, but it didn't seem to have any effect.

 

Actually no, that's not what we mean.

 

You need to turn BRÄNSLEREGL ("manual fuel regulation" according to tooltip) from AUT to MAN. Right hand console. Would be great if you could rerun the test with this setting!

Edited February 15, 2017 by Corrigan

[Hummingbird]

Actually no, that's not what we mean.

 

You need to turn BRÄNSLEREGL ("manual fuel regulation" according to tooltip) from AUT to MAN. Right hand console. Would be great if you could rerun the test with this setting!

 

Thanks, I will try again this evening once I get home.

 

That having been said the performanceon max wet fell well short of real life performance by on average 0.9 G, which is a huge discrepency.

[Hummingbird]

Just a heads up: Didn't get any chance to fly these last few days, but I will possibly find some time tommorrow.

[Hummingbird]

Alright finally got around to testing again with the fuel regulator set to manual, and sadly the results were exactly the same as before, no change at all at either max dry or max wet:

 

Aircraft clean, fuel 50%, Temp 15 C, Sea level:

2.3-2.4 G's sustained @ 800 km/h max dry

5.1-5.2 G's sustained @ 800 km/h max wet

 

So on average 0.6-0.7 G's and 0.8-0.9 G's off compared to the real life values at the same speed & altitude. That's pretty significant.

 

To confirm wether or not this is caused by excessive induced drag I went and tested the max SL speeds at max dry & wet as well, and these were the results:

1081 km/h TAS at max dry

1575 km/h TAS at max dry

 

Thus it's not the horsepower that's lacking at least :P

Edited February 22, 2017 by Hummingbird

[Hummingbird]

Just want to re-assure everyone that the fuel states for the Viggen in the tests was indeed 50%, just so we don't get a Mirage test repeat ;)

[Hummingbird]

Hopefully we can expect a fix for this, because a near 1 G discrepancy in sustainable load factor is huge.

[hughlb]

So I took off from Tonopah, heavy at 41,000lbs, climbed to 10,000ft, and set the autopilot to hold altitude - speed kept slowly decreasing, and I found I needed to engage the burner to maintain level flight. Am I doing something wrong? Or is that normal?