Altitude Compensating Carburettors, Pt.3: German (continued) A

[YavorD]

Hi, Bletchley et al,
I am a bit lost trying to comprehend similarities and differences between Daimler (Mercedes) D.IIIa/aü/aüv family and their Austro-Hungarian counterparts of the Austro-Daimler, or Daimler (MAG), family (200 and 225 P.S.). I am particularly interested in the problem how the power was changing with altitude. Any suggestions where to look for answers?
This particular question is closely related to the original topic, I believe, but we can move to a new thread in order to keep focus to the German carburettors.
Regards,
Yavor

[Bletchley]

I think this is close enough to the original topic to continue it here, Yavor, though I am happy to move to a new one if you prefer

Until now, I have not looked very much at the Austrian engines. The RAE reports includes one on the 200 hp Austro-Daimler and one on the 230 hp Hiero (Austro-FIAT), and I have looked at these in conjunction with Profiles for the Phoenix scout (Profile no.175 by George Haddow) and Berg D.I scout (Profile no.151 by George Haddow). I know KC has been looking at a peculiarity of the ignition system on the 200 hp Austro-Daimler, and may wish to comment.

The RAE report on the Austro-Daimler described it as a sophisticated, very well made engine in good condition (almost new), using much of the latest design features found in comparable German engines. Despite this, it does not appear to have had any recognisable form of altitude control. The Hiero engine, on the other hand, appears to have been heavy and crude by 1918 standards, using a bloctube carburettor with no apparent form of mixture or altitude control (throttle movement appears to control the mixture setting), although this was a damaged engine with some of the carburettor partly missing.

Looking at the peformance of the Berg scout, which appears to have been powered by 185 hp, 200 hp, 210 hp and 225 hp versions of the Austro-Daimler engine, Haddow present the climb performance as follows:

To 3,280 ft: 3 min. 12 sec. (185 hp); 2 min. 14 sec. (200 hp); 1 min. 7 sec. (225 hp)
To 6,562 ft: 7 min. 38 sec. (185 hp); 4 min. 36 sec. (200 hp); 4 min. 7 sec. (225 hp)
To 9,840 ft: 12 min. 57 sec. (185 hp); 7 min. 25 sec. (200 hp); 7 min. 5 sec. (225 hp)
To 13,124 ft: 18 min. 38 sec. (185 hp); 11 min. 17 sec. (200 hp); 12 min. 0 sec. (225 hp)
To 16,400 ft: 32 min. 49 sec. (185 hp); --- (200 hp); 16 min. 4 sec. (225 hp)
To 19,686 ft: --- (185 hp); --- (200 hp); 26 min. 5 sec. (225 hp)

It strikes me that the figures for the 185 hp version of the Austro-Daimler, which "shows no resemblance" to the 200 hp version according to the RAE report, is consistent with the climb performance of an engine that is neither altitude compensated nor altitude adjusted. There appears to be a fairly consistent slowing down in the rate of climb from around 3000 ft upwards, and it is clearly struggling to get much above the 13,000 ft mark (almost 14 minutes between 13,124 and 16,400) which suggests an effective operating ceiling around 15,000 ft (the ceiling of a Phoenix scout with the same engine is quoted as 16, 404 ft, in Profile no. 175).

By contrast, the climb performance of the 200 hp version is clearly very different. The final figure (11 min. 17 sec. to 13,124 ft) looks rather dodgy, as it is less than the 225 hp version (12 min. 0 sec), but the first three up to 9,840 ft look very similar to the climb performance of the altitude adjusted Mercedes D.IIIa (holding power well to around 6000 ft, with a noticeable slow down thereafter). There are no figures for climb beyond 13,124 ft, which suggests that the slowdown accelerated quite significantly from this altitude, with service ceiling quoted as a (rather optimistic?) 20,172 ft. We know, from the RAE report, that this version of the engine did not have an altitude compensating carburettor - the RAE example was taken from a captured Berg scout, and it appears to have been the predominant version fitted to these scouts. It had a compression ratio of 5:1, and by German standards it was not therefore a high compression engine.

The climb performance of the 225 hp version is comparable to the 200 hp version to 9,840 ft, although somewhat faster as would be expected from a more powerful engine - but the figures show that it continued to hold its power well up to 16,400 ft (a slight decline), but then with a steep decline between 16,400 ft and 19,686 ft. This suggests, to me, that this engine was probably altitude adjusted to the same extent as the 200 hp version, but also had an altitude compensating carburettor for altitudes above 10,000 ft (very similar, perhaps to the Mercedes D.IIIau). But I cannot confirm this, as i have seen no reports or other information relating to this version.

Looking at the performance of the Phoenix scout, which appears to have been powered predominantly by the 200 hp Hiero (Phoenix D.I and D.II), and then the 230 hp Hiero (D.IIa, D.III, D.IV), Haddow presents the climb performance as follows:

To 1000 m: 3 min. 0 sec. (200 hp); 2 min 30 sec. (230 hp)
To 2000 m: 7 min. 0 sec. (200 hp); --- (230 hp)
To 3000 m: 12 min. 0 sec. (200 hp); 12 min. 0 sec. (230 hp)

Again, this appears to be consistent with the RAE description of this engine as being rather crude, heavy and unsophisticated in design - with no visible form of altitude compensating carburettor, and quite probably with no form of altitude adjustment either. Although the ceiling for the Phoenix is quoted as being 19,685 ft (D.I and D.II) or 22,310 ft (D.III) I would think that this is wildly optimistic as a practical or realistic maximum operating altitude. Some late examples of the D.IV appear to have had the 225 hp Austro-Daimler fitted, with a climb to 5000 m quoted at 18 min. 0 sec, and a ceiling of 24,607 ft.

Much of this is speculation (or informed guesswork), but I hope it might be useful nevertheless, as a starting point for further discussion.

Bletchley

[YavorD]

Originally Posted by Bletchley

...
The RAE report on the Austro-Daimler described it as a sophisticated, very well made engine in good condition (almost new), using much of the latest design features found in comparable German engines. Despite this, it does not appear to have had any recognisable form of altitude control.
...
By contrast, the climb performance of the 200 hp version is clearly very different. The final figure (11 min. 17 sec. to 13,124 ft) looks rather dodgy, as it is less than the 225 hp version (12 min. 0 sec), but the first three up to 9,840 ft look very similar to the climb performance of the altitude adjusted Mercedes D.IIIa (holding power well to around 6000 ft, with a noticeable slow down thereafter). There are no figures for climb beyond 13,124 ft, which suggests that the slowdown accelerated quite significantly from this altitude, with service ceiling quoted as a (rather optimistic?) 20,172 ft. We know, from the RAE report, that this version of the engine did not have an altitude compensating carburettor - the RAE example was taken from a captured Berg scout, and it appears to have been the predominant version fitted to these scouts. It had a compression ratio of 5:1, and by German standards it was not therefore a high compression engine.

The climb performance of the 225 hp version is comparable to the 200 hp version to 9,840 ft, although somewhat faster as would be expected from a more powerful engine - but the figures show that it continued to hold its power well up to 16,400 ft (a slight decline), but then with a steep decline between 16,400 ft and 19,686 ft. This suggests, to me, that this engine was probably altitude adjusted to the same extent as the 200 hp version, but also had an altitude compensating carburettor for altitudes above 10,000 ft (very similar, perhaps to the Mercedes D.IIIau). But I cannot confirm this, as i have seen no reports or other information relating to this version.
...

Thank you, Bletchley!

The prime reason to ask this question is what engine performance figures shall be used in performance calculations. My study of Grosz, Haddow and Schiemer about Austro-Daimler 200 and 225 P.S. led me to quite similar conclusions, including the idea of similarity between Austro-Daimler 200 and 225 P.S. and Mercedes D.IIIa / IIIaü. Just before posting yesterday I looked through my estimation of Daimler (MAG) 225 P.S. performance:

1400 rpm, ISA

• Sea level ..... 149 kW (202.6 P.S.)
• 3 281 ft ...... 133 kW (181.2 P.S.)
• 6 562 ft ...... 119 kW (161.4 P.S.)
• 9 843 ft ...... 105 kW (143.0 P.S.)
• 13 123 ft ...... 93 kW (126.0 P.S.)
• 16 404 ft ...... 81 kW (110.3 P.S.)
• 19 685 ft ...... 71 kW ( 95.9 P.S.)

This, however, is based on an assumption of proper altitude adjustment of the fuel-to-air ratio, power decreasing according to density with account for mechanical friction and induction system losses according to suggestions made by Dechamps and Kutzbach (1921).

In my opinion, we should not blame the engine for slower rate of climb above 16 404 feet. The main reasons are aerodynamics and non-adjustable propeller, I believe.

My preliminary assumption for Mercedes D.IIIaü is for performance almost the same as the table shown at 6562 ft (2000 m) and above with less power below that point.

I am cautious about estimated ceiling data particularly for aeroplanes with overcompressed engines. In several ocasions, the ceiling was estimated using time-to-height data with the formulae applicable for engines, which do not hold power up to a full-throttle height of several tausend feet. The result is significant overestimation of service and absolute ceiling.

Regards,
Yavor

P.S.
An observation concerning Dechamps and Kutzbach (1921) data.
Altitude characteristics in most of the illustrations are based on temperature gradient of 5 deg C per 1000 m, not 6.5 deg C per 1000 m as per ISA tables. Also, sea level temperature used is 10 deg C instead of 15 deg C, pressure 762 mm Hg instead of 760 mm Hg.
In the first chapters of the same book, however, some examples use 15 deg C sea level temperature and temperature gradient of 6.5 deg C per 1000 m.

[Bletchley]

Here is a bit more evidence, I think, for the 'lean burn' approach - quoting from Bruno Reinhardt's "Vergaser, Brennstoffe und Brennstoffzuführung für Flugmotoren" (published by Schmidt, 1919, but the forward dates it to May 1918, pp.27-28):

"Zer Verbrennung von 1 kg Benzin sind 12,2 cbm Luft erforderlich. Bei den Motoren reicht aber diese Luftmenge nicht aus, weil das Benzin auch bei den besten Vergasereinrichtungen nicht voliständig vergast und sich nicht gleichmäßig mit der Luft vermischt. Normal wird deshalb mit einem Luftüberschuß von 10% gerechnet, so daß auf 1 kg Benzin 13,4 cbm Luft kommen. Wird anderseite der Luftüberschuß noch größer, so werden die Verhältnissungünstiger, da dann das Gemisch zu benzinarm wird"

The standard sea level air density is 1.225 kg/m3 (at 15 deg. C, sea level), so 12.2 cbm would be 14.945 kg, and 13.4 cbm would be 16.415, to give an AFR of approx. 15:1 and 16.5:1 respectively (if my conversion is correct).

Bletchley

[RAF56_Ball]

(in his best west Texas nasal twang) -
Whan yoo boaz er dun jawwin', let's clam in an' kick this pig!!

Meaning I am loving this discussion, learning volumes and it is having a deleterious effect on my psychological stability to the point of a nearly uncontrolled desire to fly one of these engines in a real Albatros!

A plebian question, maybe, but hopefully only slightly off topic - when did a truely effective mixture control move into military use? Was the "fine control" of the allies such a mixture control?

[Bletchley]

"when did a truely effective mixture control move into military use? Was the "fine control" of the allies such a mixture control?"

Hello Chip

As usual, it depends on what you mean by "a truely effective mixture control". Most rotary engines had a perfectly adequate mixture control in the form of the fine adjustment from before 1914, used to adjust mixture for both load and altitude changes - but it was fiddly and required a great deal of pilot intervention and attention (not good in combat). The float chamber type carburettors of the stationary engines were a big improvement, as they were self-adjusting for load - but not for altitude. The 'lean burn' approach, adopted by the Germans, did give some sort of automatic barometric adjustment (at the expense of some power at low altitude), but only up to a 'threshold' altitude, and a pilot controlled adjustment was then required above that. Both Allied and the German altitude compensating carburettors provided this altitude adjustment, but it was a manual control again, and therefore required pilot intervention and attention. It was not until after the war, in the early twenties, that the first barometric self adjustment was introduced for altitude control, so that aero engine carburettors became, for the first time, self-adjusting for both load and altitude changes. The later introduction of engine supercharging, and turbo supercharging, was then used to maintain (in some circumstances exceed) ground-level air pressure at higher altitudes, making such altitude compensating devices redundant in all but the small, light commercial engines. I think that this supercharging of aero engines would have to be viewed as the first "truely effective" form of mixture control, as these were the only devices that fully maintain ground level power up to high altitudes (when combined with an adjustable pitch air screw, preferably one that is self-adjusting). Prototype superchargers were in development as early as 1915, turbo-superchargers by 1917, but required some post-war developments in other areas (metals, for example) before they could become a practical proposition.

Bletchley

[HoHun]

Hi KC,

>As I explained, in another thread, I use a air density factor, a volumetric efficiency (breathing) factor and a carburetor efficiency (mixture) factor, to explain reported performance at altitiude.

Do you perhaps have a link to the thread you mentioned? Calculating engine performance at altitude is something I'm very interested in - it's the first step to calculating aircraft performance at altitude.

Thanks in advance!

Henning (HoHun)

[YavorD]

Originally Posted by Bletchley

... The later introduction of engine supercharging, and turbo supercharging, was then used to maintain (in some circumstances exceed) ground-level air pressure at higher altitudes, making such altitude compensating devices redundant in all but the small, light commercial engines. I think that this supercharging of aero engines would have to be viewed as the first "truely effective" form of mixture control, as these were the only devices that fully maintain ground level power up to high altitudes (when combined with an adjustable pitch air screw, preferably one that is self-adjusting)....

Hi, Bletchley!
I would disagree a bit.
Supercharging (both mechanical or turbo-) does not remove the necessity for proper mixture control above full-throttle height, in particular. In case of mechanical superchargers there is a different manifold pressure with changing engine speed, too. Constant manifold pressure curves are used as an illustration for engine performance but practical for long-range cruise only.
With kindest regards,
Yavor

[YavorD]

Hello!
I would like to read your opinion about perceived altitude performance of the late overdimensioned and overcompressed Daimler (Mercedes) D.III aüv engine. The approximation shown on pictures above is based upon Dechamps and Kutzbach (1921) data for altitude chamber test of the engine. Blue line is for low-altitude carburettor setting at 1400 rpm. Brown line shows power for high-altitude carburettor setting. Yellow line is supposed to show almost perfect altitude compensation (if possible). The red line is my, may be somewhat conservative, estimate if mixture can be adjusted with altitude. Up to now I am not able to confirm is such an adjustment possible at all or it is on/off affair between "Bodenstellung" and "Höhenstellung"?
Regards,
Yavor

[Bletchley]

Yavor is right, of course

As superchargers, even turbo superchargers, operate in stages and only up to a rated altitude, mixture control is still necessary but was, by then, automatic and only required a manual over-ride for the pilot to select "weak" or "rich" for economy (cruise) or max. power (take off, combat).

Bletchley