Focke-Wulf Fw 190A poor high-altitude performance

[kennethklee]

Originally Posted by Graham Boak

The power drop is due to the drop in pressure above full throttle height, not to a shortage of oxygen in particular.

Even a single stage supercharger requires power to drive it.

A turbocharger takes energy from the exhaust that on mechanically supercharged engines can be used to provide thrust from angled and ducted exhausts, though this is less than that needed to drive the supercharger. However, a turbocharged radial would not be able to benefit from the engine/airframe integration that was so cleverly devised on the Fw 190 - which is the example under discussion. The extra power would therefore be counteracted by greater configuration drag - the precise tradeoff depending upon the actual design.

The P-47 found the turbocharger's stainless steel ducting valuable as pilot armour for the ground attack role, but I think that can be described as a fortunate result rather than a planned feature. However, all advantages count.

Graham-

I thought that basically piston engines provided power by oxygen igniting fuel and creating little explosions within the cylinder walls, the force of which pushed the pistons up and down. By this reasoning, I surmised that the lower power at altitude was due to the relatively lower oxygen concentration, which would lead to mismatched stoichoimetry in the combustion of fuel and oxygen and therefore lower power. By the same token, I thought superchargers compressed air, which therefore increased oxygen content and therefore restored power.

You mentioned the power drop was due to drop in pressure above full throttle height. Pressure of what? Could you kindly elaborate some more on this? I readily admit my ignorance here, I'm not following the reasoning here.

Thanks!
Kenneth

[Kurfürst]

Originally Posted by Kutscha

You should have mentioned Kurfurst that German engines used less boost. The highest boost, except for the penny pocket of 109ks using 1.98ata, was 1.80 ata. 1.80ata being a little less than 11lb.

Increasing the compression ratio increases the power output of the engine slightly at all altitudes, all other factors being constant; increasing the boost will increase the power output more rapidly, but only below the rated altitude of the engine (=capacity of the supercharger).

Therefore, as far as high altitude performance go, increasing the compression ratio yields power output advantages, while increasing boost doesn't, unless the supercharger is improved, too.

German engine development advanced parallel in many areas, by increasing displacement, permissable rpm and compression ratio and supercharging; Allied developments revolved around supercharging.

A comparison of the output of two-staged Allied inline engines like the Merlin and single staged German inlines with increased superchargers show that these were really two roads to similiar outputs at altitude, although I consider German powerplants more efficient overall in terms of the developments resulting in lesser bulk and more favourable fuel consumption.

[Graham Boak]

Kurfurst: Allied engines also went the route of increased displacement, for example Merlin to Griffon, Taurus to Hercules to Centaurus, Twin Wasp to Double Wasp. This in itself was not specifically a German approach: indeed the step from DB601 to DB605 was not linked with any major increase in displacement. As for which detail design route is the most efficient, the RR engines managed similar output to the DB designs despite considerably smaller internal volumes: that seems one measure of efficiency to me.

Kenneth: To some exctent this is just different ways of saying the same thing. Air pressure reduces with altitude. An engine has a maximum pressure it is designed to withstand (which can vary with time, hence the time limits on different power ratings). This can be above that obtained naturally at low altitudes. The purpose of the supercharger is to provide the engine with that maximum air pressure, by forcing more in . Each supercharger is designed to provide a given maximum boost, allowing excess to bleed away at sea level, and hence as the aircraft rises in the air the supercharger is providing a greater proportion of the pressure, until it reaches its maximum - the full throttle height (or rated altitude). Above this height the engines' maximum boost cannot be maintained, and the power output falls.

So at altitudes above the full throttle height the engine is short of the ideal amount of air - you may see this as a shortage of oxygen if you wish (as I'm sure the combustion specialists would) but it is normally expressed as a pressure shortfall. Using exotic additives at altitude is a way of adding more oxygen to the mix, but this saw little use outside Germany, which had specific problems with fuel supplies.

[FalkeEins]

Originally Posted by kennethklee

Ruy-

I have Jerry Crandall's (Focke Wulf Fw 190 Dora Vol 1" on order and I am hoping it will cover the altitude issues that led from the Fw 190A to the Fw 190D.

I don't think it does..the best discussion of the technical issues is Dietmar's 'Focke Wulf Höhenjäger' as recommended previously (cheap & concise Jet & Prop special issue - in German)

[Boomerang]

Kenneth:

I'll firstly state that the technical content of this discussion is well beyond what I can contribute to.

That said, Heinz Nowarra's Focke-Wulf Fw 190 & Ta 152 (ISBN 0-85429-695-6) includes a dedicated chapter The High Altitude Fighter - a Failure. The theme of this chapter is the failed efforts to develop a satisfactory turbo-supercharged engine for the Fw 190 airframe, consistent with earlier discussions in the thread

Examples: The shortage of raw materials in Germany meant it was not possible to produce a suitable heat-resistant alloy.....The problems experienced by the Germans with turbo-superchargers were not overcome before the war had ended

The Fw 190C-1 high altitude fighter version would probably have been an excellent aircraft.....but...it failed because it proved impossible to manufacture a reliable turbo-supercharger system. THe HMZ turbo-supercharger 9-2281 produced by Hirth kept burning out.

FYI

Don W

[RT]

One best use of high-altitude fighter is the combat of the 4-mots nd their dog-guards, for the cover the Bf109 was used, but against the bomber they used Fw190 nd the heavy-ones, seems that they hv given satisfaction even at the altitudes where the fighting of the Mighty Eighth take place...

Remi

[Don Caldwell]

Graham and John summed up the engine design philosophies of the three western combatants very well. I'd like to add that the two successful USAAF fighters with turbosuperchargers (P-38 and P-47) were designed around them, and were huge as fighters go. The Germans not only lacked the high-strength, high-temperature steels necessary for the most efficient superchargers, they were limited by metals shortages of all kinds to relatively small, light-weight aircraft. Turbos just "didn't compute" for the Germans. Also keep in mind that German in-line engines were less efficient on a power/weight basis than Alisons or Merlins -- they were heavier because their steel had to be thicker. Their two-stage mechanical superchargers were never suitable for mass production (IIRC) and chemical additives were a clumsy workaround.

Don