Engine cowling.

[kitplane01]

I'm currently looking at a B-17 cowling. I was looking at a B 25 cowling. It seems the engines are mounted more forward than strictly required. It seems the cowling ms could be shorter, with less aerodynamic drag and less weight.

But designers back then were smart. Why did they make it the way they did?

 

[cvairwerks]

Lots of the engine is behind the cowling. Additionally, you have the oil tank and various other components housed there. Take a look at a photo of a B-17, or any other big radial powered aircraft and see how much is stuffed back there. Thats just the operational side of things. Engine mount length also come into play with the aircraft design parameters. The aircraft has set limits for weight and balance for a huge range of operations, including simply setting on the ground without falling over on it's nose or tail. Tons of aerodynamics, statics and dynamics go into the exact numbers.

 

[cvairwerks]

Was on my phone out and about when I did the prior post, so couldn't do photos,, Although it's a TBM, look at post 9 on this thread for how crowded things can be behind the engine and in the nacelle. The TBM is relatively empty compared to other radial installations.
1:32 Trumpeter TBM Avenger Build

 

[Big Jake]

In the B-17, each of the inboard nacelles contain the main landing gear and wheel (when retracted), a 37 gal. oil tank, big air induction system's intercoolers and ducting, the engines' accessories and engine. Each of the outboard nacelles contain the oil tank, the turbocharger, induction system's intercoolers and ducting, engine's accessories and engine. All of these require a large amount of volume and is mounted ahead of the front wing spar. There is not much "wasted" space.

 

[GreenKnight121]

The key words being "mounted ahead of the front wing spar" - there is no room aft of the main wing spar to put any of that (even with the nacelle volume above & below the wing skin location), as there are more structural elements (which depend on exact placement for maximum strength with minimum weight), control cables, and so on than in front of the main spar.

B-17 turbosupercharger system:

B-17 inboard nacelle (note the main wheel inside the nacelle):

B-17 outboard nacelle (caption is incorrect):

B-17 under restoration (showing firewall aft of the engine):

B-17F cutaway (large, with lots of detail):

 

[MiTasol]

In the B-17, each of the inboard nacelles contain the main landing gear and wheel (when retracted), a 37 gal. oil tank, big air induction system's intercoolers and ducting, the engines' accessories and engine. Each of the outboard nacelles contain the oil tank, the turbocharger, induction system's intercoolers and ducting, engine's accessories and engine. All of these require a large amount of volume and is mounted ahead of the front wing spar. There is not much "wasted" space.

And what may appear as waste space is necessary for access to change things and to provide cooling (pumps etc get very hot in operation) and to prevent the radiant heating of parts by the exhaust and turbocharger. Even so it is still often necessary to remove part A (often a sh-one-t of a job) just to get enough room to remove a defective part b. As the photo of the outboard nacelle above shows the engine and mount must be removed to access some of the intercooler plumbing etc behind the firewall. So basically none of the spare space is actually wasted.

 

[VLPhoenix]

The engines, being the largest concentration of mass (besides the bomb bay) have the biggest influence on any aircraft's overall center of gravity (cg). So the engine position could even be slightly different between major versions of the same aircraft type if standard equipment installation would have shifted the cg out of the aerodynamically allowable limits (I would estimate that those limits have to be kept within not more than just 2 - 3 feet at that size of aircraft - maybe somebody has more detailed data about the B-17?).
Shifting the engine further forward from the main wing spar will shift the aircraft's overall cg to forward (e.g. to compensate for weapon installation in the tail tip) and it will also help to increase pitch control stability (being a silencing counterweight to the aileron forces), but it will also increase the static wing torsion load as well (e.g. at touch-down).