[REPORTED] K-4 level speed

[Hummingbird]

For a long time I stopped myself from posting this chart, because I understood that the faith of many will be broken....

THis is the real chart of maximal speed where the certain results of tests are plotted. To avoid any speculations I have to say that the tests measured results were recalculated to MSA as it is a compulsory requirement for such graphs.

 

As you can see the results dispersion is about 30-33 (+- 2.1%)kph giving the standard dispersion about 5 kph or 0.7% that is very good results for the measurements.

 

But in absolute values.... did it shock you?

 

So do you understand now, why the attempts to pray to 5 or even 10 kph difference is not the thing that causes my nightmares?

 

That is a Russian performance chart for an entirely different aircraft, not sure how that is relevant?

 

Anyway I guess I just don't understand why you're not simply matching the charts, that way we have no reason to speculate. I mean the charts are often a mean average themselves, and if performance is guaranteed for a well built aircraft, something I understand the DCS examples all are, then there's all the more reason to trust them.

Edited February 17, 2016 by Hummingbird

[NineLine]

That is a Russian performance chart for an entirely different aircraft, not sure how that is relevant?

 

I believe he is trying to show you that its not all black and white, and faith in a chart is not always the best place to put your faith, again, margin of error and all. Its what I was trying to say yesterday, you are looking at such a small piece of the puzzle compared to what Yo-Yo has.

[Hummingbird]

Why are you so hung up on one number, when we were given a range of speed, which, in real life is how things really work. You need to loosen up a little. What are the margins of error on those charts? Are those charts estimates (where is the report that created that chart)? What was the purpose of those charts, as in would it have been beneficial to the person making them to lean towards the better results than not. etc etc etc... so many factors you just straight up ignore, I don't get it, you seem so thorough with so many things, then get tunnel vision on others.

 

I just don't understand why we're not matching the charts which present the guaranteed performance.

 

Also I am basing it on not just one chart or set of figures, I am looking at actual test flights as well, not to mention previous calculations and how they compared with the later flight tests, and in almost all cases the aircraft proved faster than the calculations.

 

i.e. if a Bf-109 G-14 at 1.7 ata averaged 568 km/h at SL in actual flight testing, then I cannot see how the much cleaner K-4 only proved 12 km/h faster. Actual measurements by MTT rule out the possibility that wheel well doors & a retractable tailwheel only add 12 km/h, instead state more like 25-30 km/h.

[Hummingbird]

I believe he is trying to show you that its not all black and white, and faith in a chart is not always the best place to put your faith, again, margin of error and all. Its what I was trying to say yesterday, you are looking at such a small piece of the puzzle compared to what Yo-Yo has.

 

Again I am looking at many charts and flight test reports, not just one chart. I am looking at G-14 performance results as well, a much more dirty aircraft with slightly less power.

[NineLine]

I just don't understand why we're not matching the charts which present the guaranteed performance.

 

Also I am basing it on not just one chart or set of figures, I am looking at actual test flights as well, not to mention previous calculations and how they compared with the later flight tests, and in almost all cases the aircraft proved faster than the calculations.

 

i.e. if a Bf-109 G-14 at 1.7 ata averaged 568 km/h at SL in actual flight testing, then I cannot see how the much cleaner K-4 only proved 12 km/h faster. Actual measurements by MTT rule out the possibility that wheel well doors & a retractable tailwheel only add 12 km/h, instead state more like 25-30 km/h.

 

As I said, it appears the picture you are looking at isnt as big as what Yo-Yo is looking at, as well, you are looking at the results, without looking how you get there, FMs are built the other way around, atleast ED's are... you dont plug in the speed for this altitude, you plug in the data for the aircraft and it gives you the speed for that altitude. You compare to charts like yours, and you are mindful of margins of error, differences in test conditions, etc.

 

Again, the chart you show, where is the complete report that the chart was built off of. If you are only looking at the chart alone, and not the full report, then you have your answer for why you are having trouble. I get that, and I am not any sort of FM or aerodynamic guru that some claim to be.

[NineLine]

Again I am looking at many charts and flight test reports, not just one chart. I am looking at G-14 performance results as well, a much more dirty aircraft with slightly less power.

 

Again, missing the point. You need to look at the full report those charts are built off to see how they were created, what they were created for, etc. A chart alone is pretty useless for FM design. You need the entire picture.

[Hummingbird]

Its on the GLC datasheets, also from Messerschmitt data source, the curve is poor quality but visible.

 

IMHO the difference may lay in

 

-the a simplification of GLC in the curve, as it goes straight from SL to 7500 m, which does not accurately portray the 605 power curve, so at some altitudes they likely took mean avarage for speeds

- the fact that the GLC curves are for earlier 605DM powered K-4s (1,75ata / 1800 PS vs 1.8ata / 1850 PS for the 605DB version we have in DCS and in the Projektbüro K-4 curves above)

- some other factor, different source data used etc.

.

 

How about the possible lack of wheel well doors? As we've seen on photos many K-4's didn't have these. The first production models likely didn't feature this modification, this alone would add a lot of drag.

 

[Yo-Yo]

That is a Russian performance chart for an entirely different aircraft, not sure how that is relevant?

 

Anyway I guess I just don't understand why you're not simply matching the charts, that way we have no reason to speculate. I mean the charts are often a mean average themselves, and if performance is guaranteed for a well built aircraft, something I understand the DCS examples all are, then there's all the more reason to trust them.

 

So, you stated that the red circle will have different perimeter/diameter ratio than the blue one?

 

The chart is not Russian, though it's in Russian. And this chart is from 60', at least, or even later, when the test equipment became more sophisticated and accurate than in 40'.

 

To be serious - I will try to explain you the reason WHY. The model based on various dependancies. These dependancies live their own lifes and they base on their rules. All of them must fit or be in plausible limits that can be verified using the test results of the airframe, props, engines, etc... If we have no results for the certain plane, the results for similar planes, props can be used for reference. Trying to "match" the chart (that is even not a result of test but a result of CALCULATION with unknown degree of simplifications) we can see that some of the dependancies become unrealistic or the matching leads to other parameters discrepancies.

 

I can say that the methods of forces calculation (airframe lift/drag, radiators drag/thrust, prop thrust) we are using for DCS is way more acurate than the models calculated in 40' because of incomparable computing and data storage power.

[Hummingbird]

As I said, it appears the picture you are looking at isnt as big as what Yo-Yo is looking at, as well, you are looking at the results, without looking how you get there, FMs are built the other way around, atleast ED's are... you dont plug in the speed for this altitude, you plug in the data for the aircraft and it gives you the speed for that altitude. You compare to charts like yours, and you are mindful of margins of error, differences in test conditions, etc.

 

Again, the chart you show, where is the complete report that the chart was built off of. If you are only looking at the chart alone, and not the full report, then you have your answer for why you are having trouble. I get that, and I am not any sort of FM or aerodynamic guru that some claim to be.

 

I understand this perfectly well, hence why I am bringing up the influencing factors.

 

Thing is Yo Yo has to guess the Cd0 (zero lift drag) of the K-4 based on Gustav figures, and depending on his guess the aircraft will either match, exceed or fall short of the chart values.

[Yo-Yo]

I understand this perfectly well, hence why I am bringing up the influencing factors.

 

Thing is Yo Yo has to guess the Cd0 (zero lift drag) of the K-4 based on Gustav figures, and depending on his guess the aircraft will either match, exceed or fall short of the chart values.

 

I wrote that it is a valued argument, But we have K4 max speed figures at certain engine power in the docs we can presumed as a result of tests.

 

580 kph at 1750 ps. What will be the speed at 1850 ps then? Can you say?

[Hummingbird]

So, you stated that the red circle will have different perimeter/diameter ratio than the blue one?

 

The chart is not Russian, though it's in Russian. And this chart is from 60', at least, or even later, when the test equipment became more sophisticated and accurate than in 40'.

 

To be serious - I will try to explain you the reason WHY. The model based on various dependancies. These dependancies live their own lifes and they base on their rules. All of them must fit or be in plausible limits that can be verified using the test results of the airframe, props, engines, etc... If we have no results for the certain plane, the results for similar planes, props can be used for reference. Trying to "match" the chart (that is even not a result of test but a result of CALCULATION with unknown degree of simplifications) we can see that some of the dependancies become unrealistic or the matching leads to other parameters discrepancies.

 

I can say that the methods of forces calculation (airframe lift/drag, radiators drag/thrust, prop thrust) we are using for DCS is way more acurate than the models calculated in 40' because of incomparable computing and data storage power.

 

I am not really arguing about the accuracy of 1940's calculations for a new aircraft yet to be thuroughly tested though.

 

By 1944-45 the Bf-109 had seen so many flight tests that the engineers had ample data available to make very accurate estimations of performance, and as always (like any good engineer would make sure) these estimations were deliberately conservative as we see the flight tests exceeding the estimations almost every time. Hence why they speak of "guaranteed performance" in the reports.

 

This holds true for the US & British estimations I've seen as well, most often flight test results either match or exceed the conservative estimations of the engineers.

Edited February 17, 2016 by Hummingbird

[NineLine]

these estimations were deliberately conservative as we see the flight tests exceeding the estimations almost every time.

 

That's a bold statement considering the conditions and people most German engineers and aircraft designers were working for. Generalizations like that scare me, it makes we worried you are not looking at the issue with a very open mind.

[Hummingbird]

I wrote that it is a valued argument, But we have K4 max speed figures at certain engine power in the docs we can presumed as a result of tests.

 

580 kph at 1750 ps. What will be the speed at 1850 ps then? Can you say?

 

Well if 580 km/h was achieved at 1750 ps without wheel well doors then I'd expect over 600 km/h with wheel well doors and 100 extra ps. The wheel well doors alone should quite a bit of speed.

[Hummingbird]

That's a bold statement considering the conditions and people most German engineers and aircraft designers were working for. Generalizations like that scare me, it makes we worried you are not looking at the issue with a very open mind.

 

I've never seen any evidence to suggest that anyone was ever punished for making conservative estimations, have you? The idea is pretty far fetched IMHO, and even if we go along with it I'd have been more afraid of promising something I later couldn't deliver, as that I could actually see as potentially being dangerous.

 

But really this whole theory about the engineers being afraid not to please the higher ups is fantasy, there's litterally not a single example anyone ever getting punished for crunching out a set of numbers that didn't meet expectations, or even for failing with a design (Porsche did that on several occasions). If that was how the system worked there wouldn't be any possibility to ever improve upon a design as the engineers would be too afraid to make mistakes.

 

If you could get away with openly criticising Der Führer, I'm pretty sure that making proper aerodynamic estimations wasn't an issue ;)

Edited February 17, 2016 by Hummingbird

[rel4y]

Has it been implemented in the latest patch? With the same rad settings as on page 2 I am getting 575 kph max after a few tests.

Edited February 17, 2016 by rel4y

[Hummingbird]

Btw the reason I see the 580 km/h figure as being without wheel well doors is because MTT mentions the retractable tailwheel alone adding 12 km/h to the top speed.

 

Add 12 km/h to the flight tested Bf-109 G-14's top speed of 568 km/h and you get 580 km/h.

 

Hence it makes sense for preliminary charts dated 3.10.44 to assume a 580 km/h top speed for the K-4 at the same power levels but without wheel well doors. (Likely with a drop tank pylon though)

[NineLine]

I've never seen any evidence to suggest that anyone was ever punished for making conservative estimations, have you? The idea is pretty far fetched IMHO, and even if we go along with it I'd have been more afraid of promising something I later couldn't deliver, as that I could actually see as potentially being dangerous.

 

But really this whole theory about the engineers being afraid not to please the higher ups is fantasy, there's litterally not a single example anyone ever getting punished for crunching out a set of numbers that didn't meet expectations, or even for failing with a design (Porsche did that on several occasions). If that was how the system worked there wouldn't be any possibility to ever improve upon a design as the engineers would be too afraid to make mistakes.

 

If you could get away with openly criticising Der Führer, I'm pretty sure that making proper aerodynamic estimations wasn't an issue ;)

 

Either way, its no crazier to assume every report is conservative. Point made. To much guessing here. Again, if that is what you are basing things on, we arent going anywhere.

[NineLine]

Has it been implemented in the latest patch? With the same rad settings as on page 2 I am getting 575 kph max after a few tests.

 

I'm not sure its been merged to the release/OB version yet, sounds like its not been.

[rel4y]

Large retractable tailwheel reduction is -17kph in the list of measured data. Drag reduction being 0,054 m^2.

 

If it was a small wheel G14 then it would be -12 kph, thats correct. If theres a picture, or description it would make sense.

 

I'm not sure its been merged to the release/OB version yet, sounds like its not been.

 

Alright, wonder when its coming? :joystick:

Edited February 17, 2016 by rel4y

[Kurfürst]

I wrote that it is a valued argument, But we have K4 max speed figures at certain engine power in the docs we can presumed as a result of tests.

 

I know of figures of K-4.

 

- simplified straight speed curve on GLC charts, late 1944, based on engine data, for early 605DM, i.e. 1,75 ata, 1800 PS. Source stated to be manufacturer, i.e. Messerschmitt. Simplified curve suggest early calculation. Airframe condition and origin (test or calculation, well doors fitted or not) - totally unknown. Thats the 580 figure.

- Aspera flight tests around turnover 44/45, with 4 different types of propellers, this is flight test with an early testbed (pre porduction aircraft), wheel wells missing, tail gear fixed down. At 1,35ata/2600. Testing at 1,98ata failed (605DB/DC had assembly errors in December - wrong size pistons, poorly installed valves - fixed).

- Messerschmitt projektbüro calculation turover 44/45 for K-4 with various DB / DC engine settings, 1,5 through 1,98ata. Possibly made because Aspera test bed aircraft was awaiting new engine, and results of new propeller improvements were made. Detailed airframe conditions are known. Not really an absolute performance test, results were interesting relative to each other (improvement with various propellers).

- several other hints of later development testing flight tests in various documents but results missing.

 

Estimates can be made of various 109G trials, which I believe yoyo was doing, with allowence for increased power and reduction of drag by aerodynamic improvments (known drag data). These necessarily effected by which base test data is picked (which obviously subject to production plane scatter in performance - optimistic or pessimistic)

 

580 kph at 1750 ps. What will be the speed at 1850 ps then? Can you say?

 

cc 591?

[Pilum]

First a comment to Hummingbirds theory that the K4 should go faster because calculations are always conservative. Having worked with military aircraft systems (In engineering, management and marketing) for more than 10 years I can tell you that these theories are just plain wrong:

 

Calculated performance figures are not always conservative. They can just as often be very optimistic: While engineering estimates may be conservative, the company management and marketing departments have a tendency to want to see optimistic figures. Why? Well it is because systems are bought on a price/performance basis. Now the procurement agencies know this so they enter penalty clauses into contract so companies are not tempted to promise too much. However sometimes company management will rationalize and be willing to take the risk of penalties just to take the contract. So any calculated figure you see may or may not have received TLC by the company management and/or marketing department. Which means that any calculated figure you see may deviate in any direction. You have no way of knowing this unless you have access to the raw calculations behind the curve, the by now famous K4 speed curve included. Sorry but there it is.

 

In addition, if you have data from flight tests these may or may not be representative: In order to find that out you ideally need to have a lot of different test results and filter out the outliers that are due to measurement errors or other deviations.

 

Unfortunately, on top of all these innocent sources of errors there is a more sinister aspect which needs to be taken into account: The internet being the place it is harbours some individuals who attempt to skew the picture by deliberately manipulating the data. Note that this need not involve actual falsification of data but can be achieved more subtly by simply only making only the very best data available and filtering the rest.

 

So to avoid all these pitfalls you need data from as many sources you can find. In addition, taking the K4 as an example, you don’t necessarily need K4 data at 1.8 ata to determine 1.8 ata performance with a good degree of accuracy: If you have for example data at Steig & Kampf Leistung you can use this to reverse engineer drag and then make a good prediction with another engine or power output.

 

Now from what Yo-Yo has said in this thread my understanding is that he has based the top speed value he is targeting for the K4 based on input from multiple sources and I use the same approach myself in my C++ modelling: collect a lot of data, input into model, run the simulations, tune the model, check the fit to known data points until you have the best fit to the data you have. For me this resulted in a K4 top speed of 587 Km/h at 1.8 ata and to my understanding this is in the ballpark of what we should expect to see when the mod is implemented.

 

AFAIK Yo-Yo has acknowledged that the data you have presented has been taken into account and the top speed value chosen seems to be based on multiple sources including your input so I don’t see how the way some of you keep regurgitating the same K4 speed chart, wheel door and tailwheel values over and over will help?

 

Maybe it’s time to close the thread? :music_whistling:

[rel4y]

Pretty much with every real life test you can just calculate the speed which is to be expected on the K4. Afterwards we look at the chart and check if it matches.

 

Ill do it this time on a G14/U4 real flight test. Three flights were measured and the mean is listed. They tested the new high altitude version of the DB605 in this plane and wanted comparison data. The motors tested were DB605AM and DB605ASM, both in the same plane.

 

http://kurfurst.org/Performance_tests/109G14_PBLeistungen/Leistungen_g14u4_am-asm.html

 

Config of this G14/U4: MK108 prop, 151/20 gunpods

 

So pretty close to the K4, except for gunpods, wheel covers and spornwheel. The speed values which I will quote are from (http://kurfurst.org/Performance_tests/109G_Leistungzusammenstellung/Leistungzusammenstellung109G.html) for which Kurfurst somewhere in this thread posted the original document. The data is valid for MSL (0 m) and for speeds around 500 kph. Since the relationship of drag and velocity is exponential, the speed reduction caused by drag will be higher for higher speeds!

 

level speed at MSL (0 m) DB605AM:

1.3 Ata -> 1300PS = 486 kph

 

level speed at MSL (0 m) DB605ASM:

1.3 Ata -> 1210PS = 480 kph

 

So at 1.3 Ata there seems to be a difference of 90PS in the output of the motors, which results in a 6 kph difference in speed (remember its the same aircraft so no drag difference). On the DB605ASM the prop was also changed from 9-12078 to the newer 9-12159 (same as the K-4 chart reference). Now you could argue there is a difference, but its hard to know which prop is more efficient at MSL and what influence it would have in the end. So lets assume them to be equal for now.

 

Edit: Before some smart guy shows up and claims that in the K-4 charts it is an experimental prop and therefore cant be valid.. Please read before and you might just find out that for MSL the reference (9-12159) and the thin bladed test prop (9-12199) achieve the same level speed.

 

Lets take the DB605ASM speeds as baseline because it is like the DB605DB (K-4) equipped with the slightly larger DB603 supercharger.

 

In the chart there is a line for Steig und Kampfleistung (1.45 Ata for the K-4 -> 1430PS). 1430-1210=220 So we have a difference in power of 220PS. This power is not transferred linearly into speed but a power increase of 90PS caused a 6 kph increase and drag does not decrease speed linearly as well, so lets assume this cancels each other out. 220PS/90PS=2.44 2.44*6kph=14.64kph So lets be conservative and assume the difference of power accounts for roughly 14kph.

 

480

+14 (220 PS difference)

+08 (gondolas)

+10 (wheel well covers)

+17 (sporn wheel)

529

 

So now we take a look at the charts and see the 1.45 Ata line starts depending on which chart you look at around 530-537.

 

You can repeat this with every real life test and the numbers in the charts will be affirmed. Sometimes a bit lower, sometimes a bit higher but all in the ballpark. I would conclude, that if we cannot reach MSL level speeds around 590 kph after the fix there is still something wrong.

 

Now lets wait for the fix.

Edited February 17, 2016 by rel4y

[Hummingbird]

We've got the figures we need, the problem is that they are not being accepted for some reason.

 

The retractable tail wheel alone added 12 to 17 kph to the top speed, the wheel well doors and extra horsepower would only add even more.

 

The amount of drag caused by open orifices in the wing is large, thus I'd expect the wheel well doors to take away at least as much drag as the retractable tailwheel - and I'm obviously not alone in that assumption as the German charts clearly list a top speed of 595 kph at SL.

 

If we are then to believe that exhaust thrust wasn't added well then this goes up to 600+ kph.

[NineLine]

Thats a whole lot of assumptions...

[Hummingbird]

Thats a whole lot of assumptions...

 

Not really, just one and that is wether exhaust thrust was added, Yo Yo claimed it wasn't earlier in the climb charts.

 

All the other things we have hard figures on really.