109 it fly wrong

[VO101_Isegrim]

Originally posted by gripen

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

[HoHun]

Hi Gripen,

>Because the altitude change for correction is max 390m, the linear estimate gives  certainly good enough result.

Well, have a look at the speeds for the last example to see the effect of a realistic power curve (read the lower-case "v" as down arrow :-)

6900 m: 667 km/h
v almost unchanged
7000 m: 667 km/h
v -8 km/h
8000 m: 659 km/h
v -11 km/h
9000 m: 648 km/h
v -18 km/h
10000 m: 630 km/h
v -28 km/h
11000 m: 602 km/h

It's the increasing speed gradient that makes small changes in full throttle height translate into surprisingly large changes in speed at altitude.

>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.

I'm using 3023 kg now. Is that OK?

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

Well, as the Soviets captured and tested W.-Nr. 14513, a clean Me 109G-2, I'd assume their test is based on real-life tests, too. The Kennblatt also supports a 7 km altitude.

In fact, the gain in dynamic pressure from 667 km/h @ 7 km would be 10.1 kPa, which would suffice to get an intake pressure equivalent to 5.4 km altitude. Though not all of the pressure can be realized, I think a typical pressure recovery value would be around 90%, which would still be good enough for a 7 km full throttle height. (Just from what I read on the net - 90% might be wrong.)

An explanation for the difference in observed full throttle heights is that ram pressure increases with the square of speed. For example, the G-6/AS comparison test you sent me states a full pressure height of 6.6 km @ 630 km/h, but at around 670 km/h, you might get 200 m more than that due to the higher speed.

(I don't consider 7 km the final word anyway, it's just a working hypothesis.)

Regards,

Henning (HoHun)

[gripen]

HoHun,
Basicly I'm just calculating quick and dirty what would be the speed increase at given altitude with data sets which certainly are real. We don't know what's the exact output difference between 2600and 2800rpm at 10km with high speed RAM but we know from the DB output curve that at 2800rpm the DB 605A did same output about 300m higher than 2600rpm at around 10km and at climb speed. By choosing data sets which allready support your agenda, you certainly get results you want.

Now if you just can prove that the  Russian test data and Kennblatt data are real (or realistic), that's why I have been asking for documentation. Another way would be real output data with RAM at 10km.

Regarding weight,  3020+170=3190= weight of the tested G-5/AS. I have no problem to accept anything between say 3000-3100kg for the G-2, it won't change the fact that at 10km the DB 605AS did at least 100hp more at 2600rpm than the DB 605A at 2800rpm and the AS plane had the propeller optimized for high altitude.

Because the measured speed of the AS and claimed speed of the G-2 are about the same, the RAM effect can't be used to support your agenda.

gripen

edit: Corrected hp value

[HoHun]

Hi Gripen,

>Basicly I'm just calculating quick and dirty what would be the speed increase at given altitude with data sets which certainly are real.

Well, and I'm calculating slowly and (kind of) cleanly the speed at a given altitude from power curves that certainly are real.

>it won't change the fact that at 10km the DB 605AS did at least 100hp more at 2600rpm than the DB 605A at 2800rpm and the AS plane had the propeller optimized for high altitude.

If you have the power curve for the DB605AS, I could plug it in and see what happens.

>Because the measured speed of the AS and claimed speed of the G-2 are about the same, the RAM effect can't be used to support your agenda.

Let's cut the "agenda" nonsense and discuss this like professionals, OK? As I pointed out, 7 km full throttle height is only a working hypothesis for me.

With regard to the comparison: From different DB605A-engined aircraft, you should expect a higher full throttle height for the faster aircraft, and that the difference can be in the same order of magnitude as the one we're talking about.

(Note that the tested Me 109G-6/AS had a fixed tail wheel, MG131 nose guns and the enlarged cowl of the AS-engined Messerschmitt, which along with the weight increase would deteriorate its altitude performance somewhat. If you have an AS-power curve, I could plug it in to see what happens.)

Regards,

Henning (HoHun)

[gripen]

HoHun,
The DB605D should have a very similar power output at 10km as the DB 605AS.  The output difference between 2600 and 2800rpm should be around 50ps at around 10km. The tested output in climb for the DB605 AS at 2600rpm and 10km was 965ps according to Valtonen (calculated 930ps).

There might be 100-200m differences in the FTH  at around FTH height, at 10km these differences are half and I allready used 390m difference to minimize this error in my quick and dirty estimate. I'm still wondering why don't you use the FTHs from the documented data.

Regarding your supposed agenda; I don't know what else I should think if you continously use higher than documented FTH for you hypothesis  and base your calculations on undocumented and unlogical data sets.

gripen

[HoHun]

Hi Gripen,

>Regarding your supposed agenda; I don't know what else I should think if you continously use higher than documented FTH for you hypothesis  and base your calculations on undocumented and unlogical data sets.

Whether these data sets are unlogical has yet to be determined, for example by discussing the results from calculations based on these data sets. It's exactly the lack of (detailed) documentation makes computation an indispensable analysis tool here.

Above, I have provided Me 109 calculations for full throttle heights of 6.4, 6.6, 6.7, 6.9 and 7.0 km.

Since you're aware that full throttle height depends on speed, you should really provide documented full throttle heights and corresponding speed figures for comparison.

A telltale sign for arguing with a hidden agenda is the attempt to avoid rational analysis. That's not my approach, and I don't think it's your approach either.

So, how do the documented Me 109 tests stack up against my calculations?

Regards,

Henning (HoHun)

[gripen]

HoHun,
To me Russian data is clearly unlogical; there seems to be no consensus about the used engine settings, results show od shape of the speed curve, at low altitude speed matches well MT-215 data , at high altitude speed matches the G-5/AS. The KennBlatt (at least the one Isegrim posted) says 6,4km FTH at high speed flight while the data set indicates something else.

I have provided you several measured FTHs. The MTT "Baureihen" data (various loadings) shows following values for  various speeds and FTH at 1,3ata 2600rpm:

652km/h 6,7km
636km/h 6,6km
608km/h 6,5km
595km/h 6,4km

Note that these are just examples from the long list. If compared to MT-215, these are higher. Erla data  shows an average FTH 6,7km for their planes (the problem with that data is that MTT planes are allways worse in the Erla data, go figure).

All I can say about your calculations is that these appear to show at least 20km/h faster speeds at 10km than real life data  indicates.  At climb speed the DB specs give 740ps 2600rpm and 790ps 2800rpm at 10km. At high speed the output difference  can't be much more.

I have no idea how have you done your complex calculations but 30-40km/h speed increase with 50-60ps output increase is very unlikely.

gripen

[HoHun]

Hi Gripen,

>The MTT "Baureihen" data (various loadings) shows following values for  various speeds and FTH at 1,3ata 2600rpm:

The resulting pressures (static + dynamic):

670 km/h @ 6,9 km: 52,0 kPa <- Basis for my calculation, 2800 rpm
----
652 km/h @ 6,7 km: 53,0 kPa
636 km/h @ 6,6 km: 53,2 kPa
608 km/h @ 6,5 km: 53,1 kPa
595 km/h @ 6,4 km: 53,4 kPa
----
422 km/h @ 5,8 km: 53,2 kPa <- Calculated, 2600 rpm?
000 km/h @ 5,1 km: 53,2 kPa <- Calculated, 2600 rpm?

(This is without considering any losses to the dynamic pressure.)

It's interesting that the values are subject to some variation, and the slowest plane ends up with the higher pressure. I'd have suspected the opposite.

From this comparison, I'd say it doesn't look like 6.9 km full throttle height could be reached even with the help of ram effect. However, I'm not sure at which rpm the figures you quoted were achieved. Were they all for 2600 rpm?

Regards,

Henning (HoHun)

[gripen]

HoHun,
The DB 605 power chart is a good starting point for the analysis; at climb speed the FTH for 1,3ata 2600rpm is 5800m and for 1,42ata 2800rpm it is 5700m. Generally I avoid calculating RAM effect despite it's relatively simple using standard conditions and bernoul's law; the engine is a dynamic system and calculation assumes static system.

The MTT data gives allways same high speed FTH for both power settings in the case of the DB 605A. In the case of the AS FTH for 1,42ata 2800rpm is somewhat higher.

I'm still wondering why you want to use that 6900m despite nothing real life tested supports it? Kennblatt says actually 6400m as noted above and listed values are obiviouysly calculated. The Russian data set  seems to be for  1,3ata at low altitude (or drag configuration of the plane was very bad). At high altitude it's out of reality.

I don't know exactly how you have build your model, you described it as:

"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. "

If I understand this correctly, your model has a built in agenda; you are assuming that increased FTH results automaticly performance claimed by Kenblatt and Russian test.

gripen

edit: the values were all for 1,3ata 2600rpm

[HoHun]

Hi Gripen,

Since I'm convinced you'll understand me if you pause to think about it for a moment, here's a description of my approach:

I'm currently calculating data for a Me 109G-2 based on FAF data, with drag reduced so that sea level speed increases by 10 km/h, for an engine performing like a DB605A running at 2800 rpm/1.42 ata with a full throttle height of 6.9 km. (I could use 7 km, too - I actually dropped to 6.9 km by accident when I switched from 2600 rpm/1.3 ata to 2800 rpm/1.42 ata.)

That I'm calculating this data doesn't mean that I consider it to be representative for a real Me 109G-2. It only means that working from limited data, I consider a 7 km full throttle height plausible enough to think it deserves thorough analysis.

I'm not going to make any statement about the likely outcome of this analysis now. The outcome should be the result of the analysis, and not be determined by what I think in advance it might be. I'm afraid that currently, your idea of what the outcome should be is influencing your reception of my posts, which would be regrettable because I need your help for an unbiased analysis :-)

You don't need to fear any bias from me: For example, if you look at my above post, I provided data showing that the 6.9 km/670 km/h case misses about 1.2 kPa in intake pressure, which means that the 6.9 km full throttle height is missed by about 200 m. If I were following a hidden agenda, I'd hardly have provided the data for you because I know that you're aware of the implications. To help "your" case even further, I could point out that as your data was for 2600 rpm and the 2800 rpm full throttle height requires an even higher intake pressure, leading to an even greater pressure gap.

So now it would be interesting to see the Erla data (with the same full throttle speed/altitude combinations). If it's better than the Messerschmitt data but still worse than the Russian data, that obviously would undermine the 6.9 km (or 7.0 km) full throttle height even further. If it's identical to the Russian data, one might wonder if they actually got their data from Erla ...

So let's talk about airplanes again and not about agendas - I could get to the heart of the matter much quicker if I wouldn't have to type three disclaimer paragraphs for each post I write ;-)

Regards,

Henning (HoHun)

[Seeker]

Could one of you guys define full throttle hieght? Is that the same as critical alt?

[HoHun]

Hi Seeker,

>Could one of you guys define full throttle hieght? Is that the same as critical alt?

Full throttle height is the lowest altitude at which the engine can be run with the throttle fully open without exceeding the specified operation parameters.

I believe it's indeed the same as critical altitude :-)

Regards,

Henning (HoHun)

[Seeker]

Originally posted by HoHun
Hi Seeker,



Full throttle height is the lowest altitude at which the engine can be run with the throttle fully open without exceeding the specified operation parameters.

 

I understand every word; and yet not a bit of it

Do you mean that at X feet; the oxygen content is such that one can open the throttle and not over rev the engine? I would have thought that's a very subjective criterion; as engine load must come in to play?

(so I suspect I really don't understand...)

[GScholz]

I thought "full throttle height" was the lowest altitude were you could run the engine at full power, while "critical altitude" was the highest altitude the engine could produce full power.

[gripen]

HoHun,
Well, I still can't see the reason why you want to use 6900m FTH for 1,42ata 2800rpm? The MAP is just a indicator how much air (oxygen) the engine gets to the cylinders to burn fuel and at 1,42ata 2800rpm the DB 605 needed more air (kg/s) to produce rated power. This is why the FTH at climb speed decreases  from 5800m to 5700m when 1,42ata 2800rpm is used. At high speed the increased output naturally results higher top speed and more RAM balancing the situation. Nothing in the MTT data  indicates that the FTH for the 1,42ata 2800rpm would have been 200m higher than in the case of the 1,3ata 2600rpm. The MTT data on G-6 gives 6600m FTH for both settings in the case of the G-2  that means 6700m for both.

The Erla data set is just a set of top speed points which the manufacturer claims to had been reached at 1,3ata 2600rpm (3080kg, radiator flaps at 120mm ???).The set contains 15 points of which 11 are below 6700m the average being 646km/h at 6700m.The highest point is 656km/h at 6950m. The reference MTT point is just  626km/h at 6300m. Erla and MTT were competing for RLM orders, therefore this dataset must be taken with grain and salt; in the Erla data their planes were allways up to or near the specs and the calculated spec was 660km/h at 7000m (actually they calculated another average excluding three worst points). Due to the nature of the Erla set, it's obivious that this was not used by Russians.

Overall I don't see much sense to calculate performance with reduced drag and over estimated FTH . But it's up to you how to spend your time.

gripen