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5.4. DB601E
5.5. DB605A.
5.8. Merlin X
6.1. Merlin 61.
6.1 to 6.2. Allison V1710 series.
6.3. Ju213A.
6.9. DB605B
7.0. DB603A
7.5. Merlin 66
12.6. P&W R1830
13.4. R2000
14.7. BMW801D.
15.0. R2800
16.0. R2600.
These will vary a bit depending on specific engine installation. However they should be in the ballpark.
Why would the Merlin 61 and the 66 have such different frontal areas. Surely the 66 is just a 61 with a different supercharger ratio.
The frontal area of an inline engine it is not very significant if you do not add the frontal area of the radiator (s)......
Not necessarily. Leading edge radiators, such as those used in the Mosquito, present little or no more frontal area than that of the section of wing.
And in many cases the frontal area of the radiators is significantly smaller than the surface area of the radiator - see the Mustang.
Annular radiators are only slightly bigger than the engine behind them, and the P-39 had a radiator completely enclosed within the fuselage, presenting no frontal area whatsoever.
And if you are going to include cooling mechanisms in frontal area, you should perhaps also consider the are of cowl flaps - which could be significant on radials (and annular radiators).
Reading the 'America's hundred thousand', one can find two different Cd0 values for the P-47. One being 0.0213 (pg. 113), another one being 0.251 (pg. 598 ).
Maybe
Things were changing very rapidly in the 1930s and there were few good wind tunnels available ( most were small and/or limited in speed of the air stream) so a lot of 'streamlining" was based off hunches/guesses/intuition which turned out to be wrong given later actual knowledge.
for example early GeeBee Racers went from this
View attachment 263192
to this
View attachment 263193
using the same engine in a quest for streamlining.
That also has a lot to do with propeller efficiency. Only that portion of the propeller outside the engine cowling provides significant thrust. Smaller cowling, more efficient propeller. This is greatly simplified, but you get the idea...