investigating FB.VI deck speed is ~15 mph too slow

[Bozon]

Testing the FB.VI deck speed at +18 boost, 50 feet altitude, closed radiators, the ASI pegs at 340 mph. Checking on the F10 screen, it shows 296 knots, which is 340 mph.

This is in reasonable agreement with the figures found in performance trials of Mosquitoes HJ679 and HX809:

http://www.wwiiaircraftperformance.org/mosquito/hj679-dh.pdf

http://www.wwiiaircraftperformance.org/mosquito/hx809.pdf

HJ679 was set up close to our configuration, with multi exhaust stubs and no flame dampers shrouds. However, the later report of HX809 makes a clear point (section 5) that HJ679 was under performing. In fact, the above HJ679 report is a second trial after the results of the 1st one were deemed suspicious. HX809 in its trial achieved similar speeds to HJ679 in spite of being equipped with saxophone ducts and flame dampers shrouds.

The HX809 report points to another trial of DK290:

http://www.wwiiaircraftperformance.org/mosquito/dk290-b.pdf

where the speed loss from the saxophone dusts and shrouds was measured compared to multi-stubs exhausts. They found about 15 mph difference at low altitudes (MS gear). Now, DK290 is a B.IV model, but that should not make a big difference to the effect of the different exhausts. The 15 mph difference was also measured at only +9 boost. I would expect an even greater difference at +18 boost where the engine is pushing a even greater mass of air for the same RPM.

Unfortunately, I am not familiar of any document or trial that tested an FB.VI at +18 boost with multi-stubs ejectors that is not HJ679. All we can do is try to extrapolate from the results of HX809. Thus, if we take their 337 mph ASI, and use the DK290 trial to add a +15 mph as a lower limit, we get expected deck speed for out FB.VI of 352 mph ASI. Now for TAS, HJ809 reports -4.5 mph correction from ASI, and on the other hand the speed loss from the saxohpone duct & shoulds is probably a couple of mph greater at +18 boost, thus we expect TAS to still be around the 350 mph.

Our FB.VI is too slow on the deck.

[crigby46]

Definitely agree on principle, but there are confounding variables at play here too; DK290 being a B model should indeed show no real difference between exhaust configurations when compared to those same differences on an FB.VI, but given that the real world manual for the FB.VI also states that there may be considerable variation in performance between airframes depending on age and condition, this is unlikely to be enough for ED to reconsider their figures. 

 

 The big problem is the one you've correctly identified; no direct comparison with an FB.VI running +18Lbs. Another (possibly) interesting point is that the supercharger speed changes correctly, but at a significantly higher altitude than suggested in the pilots notes. I do wonder to what extent these issues are as a result of DCS' atmospheric density modelling as opposed to issues internal to the aircraft. 

[grafspee]

Remember to set standard atmosphere  1013,25hPa and 15C

[valleyboy]

Not forgetting, speeds were attained with drop tanks!  Should be a couple of mph faster again without. 

[Bozon]

I am sorry to bump this up again, but I did a new more careful test and compared it to the Mosquito HX.809 test flight at Boscombe Down from January 1944:

http://www.wwiiaircraftperformance.org/mosquito/hx809.pdf

I ran almost the exact same setup: 20,400 lbs total weight, with 50 galons drop tanks, radiators closed, +18 boost, 3000 RPM. The "almost" is important because HX.809 had a diffrent exhausts configuration - see below.

Setup standard atmosphere (29.92 pressure, 20 deg).

The plane was flown at 100 feet and 5000 feet, well trimmed, so hardly any control inputs, for 4 minutes for the speed to stablize around max value.

Active paused and the speed was read from the F10 map, where it is displayed in knots (TAS I believe).

Results:

Altitude	HX.809	DCS	DCS no DT
100'	332 mph	285 kts = 329.5 mph	287 kts = 332 mph
5000'	353 mph	302 kts = 349 mph	303 kts = 350 mph

So, we see that DCS speeds though a bit on the low side are pretty close to the HX.809 numbers - that should not be!

HX.809 was tested while fitted with saxophone exhausts covered with flame dumping shrouds, while DCS FB.VI is modeled with stub exhausts without shrouds.

The different configurations should have a significant effect on the speed as explicitly stated within the report. This is not a small effect - on other models tested at only +9 boost the difference was around 15 mph at sea level. That is a lot.

ED pretty please with sugar on top, please check that our FB.VI performance is not tuned to a model with saxophones and shrouds, and also not tuned to the earlier tests of HJ.697 that was under-performing as concluded in the later HX.809 report cited here.

[grafspee]

Standard temp is 15 not 20. Speed from F10 map is ground speed.

Edited August 17, 2022 by grafspee

[Art-J]

To me it's simple - as the HX.809 test report seems to be the only surviving one with sufficient data for modelling a typical, no worn-out aircraft, I suspect it was used as a primary source for speeds in "our" FM.

At the same time, gfx artists were building a typical Mossie with open exhaust config, 'cause that's what they've got in a collection of blueprints acquired by ED. So we've got non-shrouded 3D model with shrouded FM.

The only way out is either changing the 3D model by adding shrouds, or going for slippery road of extrapolations in revising the FM. Both unlikely in my opinion.

[Bozon]

6 hours ago, grafspee said:

Standard temp is 15 not 20. Speed from F10 map is ground speed.

 

Ground speed is TAS when there is no wind.

[grafspee]

40 minutes ago, Bozon said:

Ground speed is TAS when there is no wind.

I just said what F10 map is showing, and it is showing ground speed, with wind or not it is ground speed.

[bart]

On 8/16/2022 at 9:49 PM, Bozon said:

I am sorry to bump this up again, but I did a new more careful test and compared it to the Mosquito HX.809 test flight at Boscombe Down from January 1944:

http://www.wwiiaircraftperformance.org/mosquito/hx809.pdf

I ran almost the exact same setup: 20,400 lbs total weight, with 50 galons drop tanks, radiators closed, +18 boost, 3000 RPM. The "almost" is important because HX.809 had a diffrent exhausts configuration - see below.

Setup standard atmosphere (29.92 pressure, 20 deg).

The plane was flown at 100 feet and 5000 feet, well trimmed, so hardly any control inputs, for 4 minutes for the speed to stablize around max value.

Active paused and the speed was read from the F10 map, where it is displayed in knots (TAS I believe).

Results:

Altitude	HX.809	DCS	DCS no DT
100'	332 mph	285 kts = 329.5 mph	287 kts = 332 mph
5000'	353 mph	302 kts = 349 mph	303 kts = 350 mph

So, we see that DCS speeds though a bit on the low side are pretty close to the HX.809 numbers - that should not be!

HX.809 was tested while fitted with saxophone exhausts covered with flame dumping shrouds, while DCS FB.VI is modeled with stub exhausts without shrouds.

The different configurations should have a significant effect on the speed as explicitly stated within the report. This is not a small effect - on other models tested at only +9 boost the difference was around 15 mph at sea level. That is a lot.

ED pretty please with sugar on top, please check that our FB.VI performance is not tuned to a model with saxophones and shrouds, and also not tuned to the earlier tests of HJ.697 that was under-performing as concluded in the later HX.809 report cited here.

This is a great post Bozon, really interesting stuff.

I hope you don't mind but I've quoted this thread and in particular your post over on the DCS Discord and aimed it at Nineline. Hopefully he will read it and pass it on to the ED WW2 team so they can cast an eye over the data. Maybe we will get a result, or at least some clarification.

[Magic Zach]

I came to the same conclusion as well in my speed testing of the Mossie.  On average its too slow

[bart]

Quick update to this fellas.

Nineline has kindly answered my query over on the ED discord and has read through Bozon's post and forwarded on all the info to the ED team.

 

[Bozon]

Thanks @bartfor taking this to discord - I don't normally visit there, and than thanks to NineLine for responding.

[NineLine]

The team is investigating this now, will let you know when I do. Thanks for the detailed report!

[Joker_0]

Is there any update on this or anything? Has almost been a year now since it was brought up

Edited October 20, 2022 by Joker_0

[hazzer]

Hello, do we any update on this? 

[Hallsy]

Agreed. Any updates would be appreciated.

[NineLine]

Please keep the post related to the bug reported. I do not have any new info at this time but will look into it.

Thanks. 

[Yo-Yo]

The statement "oh, 15 mph slower, we lost the war!" is very far from the real things.
First of all, though we have, I think, the most full collection of Mosquito reports, we really have no report with clearly figured TAS for the same plane we have in DCS, except poor FB VI  HJ 679 that was presumed crippled (but without finding actual reasons of this behavior). 
Then we have DCS FB VI top speed at SL 340 mph

Then, we have FB VI sax-HX809 with 18lb/3000 measurements, that state 354-22 = 332 mph at SL. By the way, for all other tests that have no direct SL measurements, SL speed is obtained by prolonged graphs. 

Then we have B IV DK. 290 tested for both types of exhaust system, that shows 15 mph difference. So, if someone wants to use this information to suggest the top speed of FB VI with normal exhaust, this speed is to be 332+15 = 348 mph  (and the difference with DCS will be only 8 mph), but below I will show, why DK. 290 can not be used as a reference even for the different types of exhaust systems.

Please keep in mind that all planes used for comparison have engines with the same blower ratio, so 21/23/25 have only different limitations, so, the power can be compared directly.

looking at DK.290 results one can see that they are incredible high for 9 lb/3000 - 331 mph, tanks on, M.S. that gives 337 mph (6 mph difference obtained from 8000 ft measurements common for tank on and off)
And we have sax-FB VI HS.918... That shows glorious 302 mph without tanks and RP at (drum roll!) 12.5lb/3000, and it directly shows without any calculations that FB VI itself is much more draggy than B IV, thus 15 mph sax/no-sax difference is not fully valid for FB VI.

And then I performed some calculations to obtain more accurate numbers taking in account prop thrust and jet thrust. Radiator drag/thrust at these speeds are close to zero and thus can be neglected in comparison even to jet thrust.  I omitted the calculations itself, and these are results.

DK. 290 with normal exhaust shows CD0 very close to the values stated in other reports, and, that is interesting, to get -15 mph with sax, jet thrust must be reduced to~0 (I do not think, that additional streamlined construction can make such high drag). It is plausible if the gases temperature/energy was significantly reduced before they ejected backwards.
 

Now we can eliminate jet thrust for sax-FB VI HX809 and find its drag using test results. And its drag is 1/3 more than for B IV. 
Using this obtained drag we can add jet thrust for 18 lb/3000 and find that non-sax FB VI must fly at  344 mph, that lays within 1% tolerance.  

We can also check FB IV using HS.918 results: drag obtained from the same calculations is 5% higher than for HS.918, so we have reliable result. And, by the way, it shows that these two planes will have at least 2.5% difference in top speed.

[Bozon]

@Yo-Yo thanks for looking into this, I really do appreciate it. A few comments if I may:

9 hours ago, Yo-Yo said:

"oh, 15 mph slower, we lost the war!"

No histeria, I was just testing the speeds and according to them our Mosquito barely outruns a Spit IX (if at all, I don't own DCS Spitfire so I cannot compare).

9 hours ago, Yo-Yo said:

Then we have DCS FB VI top speed at SL 340 mph

I tested again:

+18, 3000 rpm, 100 feet, atmosphere at 15 C, standard pressure, weight just under 20,000, no bombs or DT, radiators closed - the speed in F10 is 289 knots, or 333 mph.

To make sure I don't under-test by not completing the acceleration I start at a higher speed, stabilize and trim and let the speed bleed down until it stabilizes, then continue for another minute or two to make sure it is stable.

9 hours ago, Yo-Yo said:

DK. 290 with normal exhaust shows CD0 very close to the values stated in other reports, and, that is interesting, to get -15 mph with sax, jet thrust must be reduced to~0 (I do not think, that additional streamlined construction can make such high drag). It is plausible if the gases temperature/energy was significantly reduced before they ejected backwards.

The Saxophones were also covered by flame dumpners/shrouds. These must add some drag as they are bigger than the saxophone itself, they have an air intake, and there are 4 of them.

In addition, the shrouds may interfere with the exhaust jet - the saxophone did not stick out of the shroud (what would be the point then?), so it ejects and mixes the gases into the slow air inside the wider shroud. I can image this is not good for converting the exahust jet into thrust, to put it mildly.

 

[Yo-Yo]

I can only say that in DCS it flies exactly at 340 mph, if you set 20000 lb 

(has very low effect on top speed though), 760mm/15C, and manually close radiators.

To make sure that 544 kph TAS is real max speed and a point of equilibrium I always use excessive power record filtered for 1g to avoid load/unload effect on the drag.
And this is the record, so one can see that zero point is exactly 340 mph.

I can not agree that exhaust shrouds can give drag comparable with 8 pylons with rockets (~20 mph). null

null

[Bozon]

Thanks for taking the time to check this @Yo-Yo, but for the life of me I can’t get more than 333 mph out of her. Perhaps I am doing something wrong - maybe some other players can test this too, so I’ll be sure it’s me.

On 6/24/2023 at 2:54 AM, Yo-Yo said:

I can not agree that exhaust shrouds can give drag comparable with 8 pylons with rockets (~20 mph).

It is a combination of added drag and ruining the exhaust thrust. The end result of a significant speed loss is what the original flight tests report.

[Yo-Yo]

I am sure that you overestimate the drag  relative for 42 m2 wing area. 

[Blackbird12]

It's not about the drag, it's about the exhaust thrust losing effectiveness (for example the A6M5 gained speed thanks to the new exhausts pipes, the BMW801D power chart even has a curve for the exhaust thrust), I agree that the shrouds drag is insignificant, in the past I tried to find some charts about the exhausts thrust of the merlin but I could'nt get something useful.

Here is a drawing of the "saxophone exhausts" (the saxophone name is about the whole pipe system) : 

[Art-J]

Not discrediting Yo-Yo's arguments, and not fussed all that much about the whole topic, but I'm genuinely curious how to get to 340 on the deck, 'cause I haven't managed to do it either, while trying on Normandy 2 map.

ISA conditions (15/760), about 20000 lbs of weight, full boost, full RPM (I also tried 2850, didn't make a difference), radiators closed, rudder trimmed as well as I could, same with elevator for level flight. Hitting about 333 TAS (as reported in F2 view) was all I could squeeze out of this plane.

Edited June 26, 2023 by Art-J