What determines the optimal altitude of an aircraft?

[Laurelix]

So i got into a argument about this question because as far as I'm concerned, the foremost reason is the engine and its power output vs altitude and its air density (less air, less drag)

BF-109E which is powered with DB-601Aa engine has optimal altitude at respectively 5000m altitude. So to me, every plane with this engine will have the same optimal altitude. After all MC.202 and KI-61-I also used licensed version of the DB-601Aa and they also achieved optimal max speed at 5000m altitude. The different aiframes will determine their max speed depending on the drag of the airframe but i believe the optimal altitude will remain unchanged.

There are other examples like DB-605A (1475PS) inside BF-109G, RE.2005, G.55, MC.205, achieving max speed at same optimal altitude.

Or am i wrong to come up with this conclusion?

Lets have a discussion about this...

 

[pbehn]

If your criteria for performance is speed and the aircraft is a fighter then the optimal altitude will be the highest altitude that the engine produces maximum power If you strap the engine in a single engine bomber like the Battle or your need is for altitude no matter what then things are different.

 

[Shortround6]

Or am i wrong to come up with this conclusion?

You are in a large part right. However individual designs will vary a bit because of of RAM. For instance the Spitfire I had a higher "optimum" altitude (for speed anyway) than a Hurricane I using the same engine. Not by a lot but the higher speed of the Spitfire created higher pressure in the intake duct which allowed it to make it's rated power a bit higher.
Spit was good for around 18,500-18,900 for some examples (rated height of the engine was 16,250 ft. ) while the Hurricane was good for a bit under 18,000ft. HIgh 17s?
The Battle, with the same engine would have an optimum altitude a bit lower because it's slower speed cannot generate quite the same pressure in the intake duct.

 

[Ivan1GFP]

Hello Laurelix,

Shortround6 already gave a pretty good explanation assuming that different aircraft designs are equipped with the "same" engine, but the problem here is that your basic data is incorrect. (....and also that the engines are not really the same.)
The Me 109E was equipped with two different engines. The DB 601A-1 was the standard. The DB 601Aa was the "substitute standard" with a smaller and less capable supercharger. Only the Aa version was approved for export. When the Germans in their benevolence licensed the DB 601 to the Japanese and Italians, they only licensed the DB 601Aa.

IIRC, the critical altitude for the DB 601A-1 was 4500 Meters and the critical altitude for the DB 601Aa was 3500 Meters.
Though the DB 601Aa was originally intended for export, significant numbers ended up in production Me 109E.
Two Japanese companies, Aichi and Kawasaki tried to build licensed "copies", but Kawasaki also tried to improve the supercharger a bit and managed to bring the critical altitude of their Ha-40 to 4300 Meters. Not quite as good as the DB 601A-1, but close.
I believe the Italians also tried to do something pretty similar, but I know much less about their attempts. The intended critical altitudes of their engines can be determined by the model numbers though I don't know if they actually achieved the intended performance. From performance statistics in books about Macchi fighters, it seems like they really did not.

- Ivan.