Question: air intakes

[tomo pauk]

Hello,

Reading a bit about the Bf 109F, it seems that the 109F-4 introduced, after a small number were delivered, the air intake duct of increased size/area. The new duct provided better use of ram effect vs. the old, more restrictive one found in earlier 109Fs. The aircraft top speed was at altitude that was 1100 m greater than engine's rated altitude, on Kampfleistung (1400 m for Notleistung).For the 109F-1/F-2, the difference seem to be at ~700m (Notleistung; 2600 rpm 1,42 ata)? End result was the Bf-109F-4 having the max speed of 635 km/h at 6 with 1185 PS used (= Kampfleistung) vs. the Bf-109F-1/F-2 with 615 km/h at 5,2 km/h with 1200-1250 PS.
(both aircraft have had also greater power settings, though)

Questions: is there any information of just how big were the air inlets - what size is the throat area of the air inlets?
What about other, comparable aircraft of ww2?

 

[Shortround6]

I don't know the sizes of any intakes but that is one reason few engine companies published specifications for altitude performance including RAM.
The engine needs so many pounds (or KG) of air per minute and since the weight of air per cubic ft/meter varies with altitude that can mean different sized ducts are needed at different altitudes. A bigger duct (more frontal area) means more airframe drag though. Long ducts with turns have more internal drag and lower pressure at the carburetor or supercharger intake/mouth. Air filters can cause both airframe drag and/or internal drag.

A bigger intake/duct may very well give you a higher critical altitude but it may also cost a few mph/kph at low altitude.

Air density chart;

Charts vary as this one assumes constant temperature (70 degrees F), other charts may include a correction for falling temperatures (higher density) and/or start with a different standard temperature. But very roughly you need twice the volume of air at 20,000ft (6000 meters) as you do at sea level for the same power.

 

[oldcrowcv63]

Classic example would appear to be the evolution of the P-40 frontal and side profile as the engine was changed from the Allison 1710-33 in the P-40B to the higher power 1710-39 in the P-40E: (although now that they are posted, it appears to me that it's more difficult to tell the difference that I thought) . First cartoon is of Lt. George Welch's P-40B flown at PH, December 7, 1941 and the second is of Lt. George Preddy's P-40E, flown while with the 49 FG in Oz late Spring or early Summer of 1942.

 

[oldcrowcv63]

Much better cartoon depiction: evolution from P-40 to P-40D/E. I guess the P-40B/C inlet was closer in side profile dimension to the E's than I'd realized.

 

[tomo pauk]

Thanks for the feedback, people.

The 1st firm numbers I've come across are these for the Bf-109G (G-1 probably? from June 1942 anyway). With the 'old' S/C intake, the aircraft's full throttle (on 'Kampfleistung' power setting) was at 6250m, with the new (one 150mm dia, other 155mm) the A/C's FTH was at 6400 m, ie. gain of some 500 ft. The 'old' intake was carry over from Bf-109F-4?

Greg, maybe you could toss in some info about the aircraft you are helping restoring? What about the Spitfire and Hurricane - any resident experts want to chime in? The P-51?