Author Topic: 109 it fly wrong (Read 15964 times)

HoHun · « **Reply #300 on:** May 22, 2004, 01:16:46 PM »

Hi Voodoo,

>Did you mean that (tre) ?

Roger! Thanks a lot! :-)

I had assumed the term referred to the power setting, so it's good to realize that it actually describes the weapon configuration :-)

Regards,

Henning (HoHun)

gripen · « **Reply #301 on:** May 22, 2004, 02:39:53 PM »

Dear Isegrim,
Maybe you might want to do a bit of brain training, so look at the Daimler-Benz test on the DB 605 powered Spitfire climb chart. Now look at which altitude the planes (Spitfire and reference Bf 109) reached it's second FTH. Think why the FTH in the climb (for 1,3ata 2600rpm) is about same as in the output chart in the manual? After that it's up to you to decide if the DB 605A failed to reach claimed performance even in the manufacturer's own test or if the manual chart contains some dynamic RAM (as the text in the manual actually says).

HoHun,
Well, my estimate for the MT-215 performance at 10km with 1,3ata and corrected 2600rpm is (as mentioned above) 560-570km/h (that naturally means allready 6,6km FTH, no need to correct that twice, right?). Now let's ad 10km/h for the tailwheel inside to make Isegrim happy. So now we have a say 580km/h MT-215 at 10km which I believe is a good estimate for a good Bf 109G-2 at 10km and 1,3ata 2600rpm. At FTH 6,6km this plane would do about say 655km/h, tailwheel down something else.

Then lets assume (without good evidence but because you announced that you won't accept lower values) that the FTH was actually 7km to make you happy. Then we could have a plane which would do say 665km/h at FTH and at 10km it would do say 20km/h more ie 600km/h.

And last lets try to figure out if the 640km/h was "no problem" with1,42ata 2800rpm. The DB 605A FTH with both power settings in the MTT charts appear remain same so lets assume that our super Bf 109G-2 had FTH 7000m. The speed increase at FTH is just 10km/h in the MTT docs but because we are allready speculative, lets assume that it was actually 20km/h so 685km/h at 7km.

Then to the 10km, now we can use calculated AS data from the MTT specs. At 1,3ata 2600rpm the spec (3320kg) is 648km/h at 8,8km and at 1,42ata 2800rpm the spec is 660km/h at 9km (note that the AS had a different propeller) . Damn, just 12km/h. But what the hell, lets assume that our super 109 actually could gain 30km/h with increased RPM despite what ever is the blade tip speed. So now we have a 630km/h Bf 109G-2 at 10km. Oléé...

Well, you might have a idea how realistic is that 640km/h claim.

gripen

HoHun · « **Reply #302 on:** May 22, 2004, 04:43:20 PM »

Hi Gripen,

>happy. Then we could have a plane which would do say 665km/h at FTH and at 10km it would do say 20km/h more ie 600km/h.

Hm. From your presentation I'm not sure that you actually calculated anything. Let me point out that I didn't just dream up some numbers, but actually went into some detail to arrive at realistic figures.

Regards,

Henning (HoHun)

Angus · « **Reply #303 on:** May 22, 2004, 05:40:09 PM »

Whatever the results may be, the 109G2 will have problems with a spitty at optimized alt

gripen · « **Reply #304 on:** May 22, 2004, 05:44:50 PM »

HoHun,
Well, originally I just calculated quick and dirty that 2,3% difference in the RPM would result about as large difference in the FTH and some what more at 10km. As you can see I actually edited post after thinking the blade tip speed; estimate was too high.

For unknown reason you corrected the allready wrong number (574km/h at 10km) twice by adding 200m after quick and dirty correction and ended to the so called "realistic figures". IMHO a bit of sarcasm is fully justified.

Discussion might be a bit easier if you won't seal your opinions by making statements like that you won't accept anything less than 650km/h at 7km despite some data actually proves something else.

gripen

HoHun · « **Reply #305 on:** May 23, 2004, 01:00:10 AM »

Hi Gripen,

>For unknown reason you corrected the allready wrong number (574km/h at 10km) twice by adding 200m after quick and dirty correction and ended to the so called "realistic figures". IMHO a bit of sarcasm is fully justified.

You're underestimating the complexity of my calculations :-) I didn't use the 10 km value, but the 6.4 km value, corrected to 541 km/h, as a basis, and used the engine graph to establish a speed curve for the entire altitude range for comparison to the Finnish test. Then I adjusted the power curve to the 7 km full throttle height used by the German Kennblatt as well as by the Russian graphs to see what speeds would result - again over the entire altitude range.

>Discussion might be a bit easier if you won't seal your opinions by making statements like that you won't accept anything less than 650km/h at 7km despite some data actually proves something else.

Well, ironically I was about the last one here to throw my hat into the ring by posting a speed figure :-) 650 km/h @ 7 km still is a realistic figure, easily met by the Finnish test aircraft if its engine were good for a full throttle height of 7 km.

Regards,

Henning (HoHun)

HoHun · « **Reply #306 on:** May 23, 2004, 01:06:45 AM »

Hi Angus,

>Whatever the results may be, the 109G2 will have problems with a spitty at optimized alt

To get back on the original discussion, I believe we all agree that looking for "The World's Best Fighter" in 1942, the only types worth talking about are the Me 109G, the Spitfire IX and the Fw 190A - with altitude being the decisive factor?

I still think "The Gustav was obsolete when it entered production" is an example for extraordinarily poor powers of perception :-)

Regards,

Henning (HoHun)

Angus · « **Reply #307 on:** May 23, 2004, 10:00:46 AM »

I agree there.
The 109 airframe was quite incredible, and basically the 109 did not get obsolete in WW2.
Funny how diferent, and yet not different the 109 and Spitty are, - both designed in almost the same time

My vote for the world best fighter in Spring of 1943 yet goes to the Spitty

(With the 109 really nibbling at its heels)

gripen · « **Reply #308 on:** May 23, 2004, 06:11:13 PM »

HoHun,
Sorry if got a wrong impression but you wrote it so that I understood that you made FTH correction twice. Now I have calculated a bit more than quick and dirty stuff but there appear to be no big differences.

My approach is quite simplified but it should be accurate enough. As before I'm assuming that the RPM difference is about directly relative at FTH, using V-1710 charts from the "Vee's for Victory" and the DB 605 chart this appear to be quite accurate for single stage engine at around 6-7km. The difference 2540/2600 should give 2,3% increase in the FTH, 6420m CINA would result FTH 6571m CINA ie about 151m increase. As for comparison 2800rpm would result 7077m CINA (naturally for 1,3ata) ie 657m increase in the FTH (2540/2800). This fits quite well to the DB 605 manual chart, at climb speed increasing RPM from 2600 to 2800 results around 600m. The DB 605 chart also show that I was wrong by estimating that the difference should be bit more at 10km, it actually is less, about 300m for 2600->2800 and therefore 90m for 2540->2600, my mistake (that's actually the reason why the very first estimate for 10km was too high). The speed at the new 2600rpm FTH 6571m CINA would be about 645km/h.

So the speed was 552km/h at 10100m CINA, and 610 km/h at 9110m CINA. Using linear estimate the speed at 10000m CINA would be 558km/h. Now we can take that 90m FTH increase (for 2540/2600) and by looking downwards we can estimate speed (without rpm correction) by taking linear estimate for 9910m CINA and that results 563km/h at 2600rpm, 2800rpm would result 575km/h. Now we can ad rpm correction 5km/h for 2600rpm and say 15km/h for 2800rpm (probably optimistic due to propeller tip speed as calculated AS values show). So now we have estimates for 10km, 568km/h at 2600rpm and 590km/h at 2800rpm. To make Isegrim happy we can took tailwheel in so then we have 578km/h and 600km/h which I believe are pretty good and well founded estimates.

Regarding your argument that the MT-215 with 7km FTH engine would met figures; I calculated 615km/h at 10km tail wheel down and 625km/h up but maybe with 8km FTH engine it would do 640km/h, right? Well, can't avoid sarcasm with that kind of arguments, sorry.

gripen

HoHun · « **Reply #309 on:** May 24, 2004, 12:22:49 AM »

Hi Gripen,

>taking linear estimate for 9910m CINA and that results 563km/h at 2600rpm

Does that mean your method is based on calculating a speed difference for altitude A and apply it to altitude B? As air density varies with altitude, that would be quite inaccurate.

Regards,

Henning (HoHun)

gripen · « **Reply #310 on:** May 24, 2004, 03:36:22 AM »

HoHun,
Yes, but I have no values for higher altitudes, besides the altitude difference is small even in the case of the 2600->2800rpm. Alternatively I can use DB data on the 109G (Spitfire with the DB 605) between 9350-10350m (the average between 9700-10000m) which means that linear drop of the speed should be in the right ball park. That would result again 600km/h at 2800rpm. There should be no large errors.

IMHO the propeller efficiency would cause larger error. At certain point increasing the RPM would not increase speed.

gripen

HoHun · « **Reply #311 on:** May 24, 2004, 02:13:13 PM »

Hi Gripen,

>IMHO the propeller efficiency would cause larger error. At certain point increasing the RPM would not increase speed.

Good hint, I had a typo in my data and was calculating with the wrong rpm. At 10 km, that makes a 15 km/h difference.

However, my calculations still indicate 627.5 km/h for a 6.9 km full throttle height, tailwheel-up Bf 109G-2 at 2800 rpm/1.42 ata.

That's just 12.5 km/h off the 640 km/h mark - still good enough :-)

I am not entirely convinced the MT215 was in fact tested in tailwheel-down configuration, though, and if it wasn't, that might require a downward adjustment of my figure.

Regards,

Henning (HoHun)

gripen · « **Reply #312 on:** May 24, 2004, 05:50:23 PM »

HoHun,
Generally if we have the right estimate for the rate of the speed change due to decreasing MAP at given altitude (above FTH) then above system should be quite accurate for small MAP changes. In fact it gives fairly good estimate for speed at 10km with the DB 605AS engine given the output difference.

Assuming 6,9km FTH MT-215 data gives 608km/h and DB data gives 607km/h. Overall my opinion is that 640km/h at 10km might be possible but at very much lightened condition, "ohne waffen und munition, ohne panzerung", airframe cleaned (antennas etc. removed) and polished.

Besides, 6700m FTH in high speed for DB 605A is well documented so why not use it?

The MT-215 had fixed tailwheel, it's mentioned in the report.

gripen

HoHun · « **Reply #313 on:** May 24, 2004, 11:08:52 PM »

Hi Gripen,

I discovered that I entered the greater weight of the G-5/AS for my G-2 calculations. Using 3023 kg as for the Soviet test aircraft of course gives better high altitude performance.

>The MT-215 had fixed tailwheel, it's mentioned in the report.

OK, thanks! :-)

>Generally if we have the right estimate for the rate of the speed change due to decreasing MAP at given altitude (above FTH) ...

That's exactly where I have doubts. The FAF aircraft shows an almost linear speed drop there where it should be a convex curve. Extrapolated to 11 km, the FAF data would be optimistic.

>Besides, 6700m FTH in high speed for DB 605A is well documented so why not use it?

Pick your choice:

FTH 6.4 km, rpm corrected: 641 km/h @ FTH, 591 km/h @ 10 km

FTH 6.7 km: 646 km/h @ FTH, 602 km/h @ 10 km
FTH 6.6 km, 1.42 ata/2800 rpm: 652 km/h @ FTH, 610 km/h @ 10 km

FTH 7.0 km: 652 km/h @ FTH, 613 km/h @ 10 km
FTH 6.9 km, 1.42 ata/2800 rpm: 658 km/h @ FTH, 620 km/h @ 10 km

FTH 6.9 km, 1.42 ata/2800 rpm, retractable tailwheel: 667 km/h @ FTH, 630 km/h @ 10 km

Regards,

Henning (HoHun)

gripen · « **Reply #314 on:** May 25, 2004, 05:35:29 AM »

HoHun,
The MT-215 data set contains just few points and using values from 9110m and 10100m CINA, the speed decreases 58,6km/h per 1000m. As for comparison even choosing 10000m and 11000m from the DB dataset results just 30km/h per 1000m and 606km/h at 2800rpm. To reach that 640km/h, the speed decrease per 1000m should be more than 100km/h. Because the altitude change for correction is max 390m, the linear estimate gives certainly good enough result.

The weight difference between the G-2 and G-5/AS proto is about 170kg. According to MTT data, 170kg difference causes about 4-5km/h at FTH, somewhat more at 10km.

May I ask which certainly real life measured and documented data set supports more than 6700m FTH?

gripen