July 1st 1937: your own USAAC/USAAF/USAF?

[tomo pauk]

The V-1710, say the commonly found C-15, was not that good as the Merlin III. Not just when Merlin was operating on 100 oct fuel. The V-1710 C-15 was making 1040 HP at 13400 ft, vs. Merlin III 1030 HP at 16000 ft. The Merlin III was been able to take full advantage of the better fuel, with officially allowed 1300 HP at 9000 ft. Now before Greg hits me with a pie or something, we know that sometimes even the C-15s were over-boosted, but that was something that might get killed a pilot after several flights. And we also know that 1st C-15s have had to endure the 'modernization' process in 1941, or else the military power was limited to only 950 HP at 8000 ft. Military rating back then was a 5 minute rating, not 15 minute as in follwing years. Sometimes the military rating of 1090 HP at 13200 ft, for the engines without backfire screens. The C-15 have had better take off rating, 1040 HP vs.
The real contemporary with the C-15 was the Merlin XII, and that one has take off power of 1175 HP, and on every altitude has considerably more power than the C-15.

Price in weight must be paid - we want more power, at all altiudes. The 200 lbs greater weight of the DB-605 loks like a bargain - that would be something like weight of the V-1650-1, the 1st Packard Merlin. The V-1710 needed auxiliary supercharger to beat the 605A, meaning the weight difference is close to zero in that case. With C3, the DB-605A was good for 1700 PS for take off?

 

[Shortround6]

A C-15 could pull 1260hp at 9000ft using 46 in (8lbs boost), how long it would do that is certainly subject to question though. Chart on Page 124 of "Vees" It was never approved but that was the 'capability' of the system.

Please remember that US 1940 100 octane fuel was just that. 100 octane lean and around 99-102 PN rich while the British 1940 100 octane was 100 octane lean and around 115-120 PN rich. Each one varied a bit from batch to batch as there was no way to measure rich rating at the time.

Trying for a "big bore" block means a whole new engine and the results are very time dependent. The Germans, much more likely than not, could not have built the DB-605A in 1939.
The Allison itself went through 4 crankshafts, at least two if not 3 crankcases. different pistons, piston rings, valve springs and a host of other parts by the end of the war.
Both the Allison and Merlin changed from 1005 glycol to a water/glycol mix, the glycol was too hard to 'seal' in the engine.
A 1937/38 "Big block" would have had a 'plain steel' crankshaft )plain means a good steel alloy but not shot peened or nitrided.
A 1937/38 "Big block" would have used the early crankcase/block casting techniques.
A 1937/38 "Big block" would have used the 100% glycol cooling.

 

[tomo pauk]

A C-15 could pull 1260hp at 9000ft using 46 in (8lbs boost), how long it would do that is certainly subject to question though. Chart on Page 124 of "Vees" It was never approved but that was the 'capability' of the system.

That is the quirk - in case a pilot used over-boosting on the C-15 today, he is likely to experience a blown engine in he tries that tomorrow.

Please remember that US 1940 100 octane fuel was just that. 100 octane lean and around 99-102 PN rich while the British 1940 100 octane was 100 octane lean and around 115-120 PN rich. Each one varied a bit from batch to batch as there was no way to measure rich rating at the time.

It would be interesting to have some good data of how much the British and CW units were pushing the V-1710 C-15 on their fuel.

Trying for a "big bore" block means a whole new engine and the results are very time dependent. The Germans, much more likely than not, could not have built the DB-605A in 1939.
The Allison itself went through 4 crankshafts, at least two if not 3 crankcases. different pistons, piston rings, valve springs and a host of other parts by the end of the war.
Both the Allison and Merlin changed from 1005 glycol to a water/glycol mix, the glycol was too hard to 'seal' in the engine.
A 1937/38 "Big block" would have had a 'plain steel' crankshaft )plain means a good steel alloy but not shot peened or nitrided.
A 1937/38 "Big block" would have used the early crankcase/block casting techniques.
A 1937/38 "Big block" would have used the 100% glycol cooling.

Yep, new block would be needed. We might expect 30-40% increase of power over V-1710, for same power settings and supercharger systems?
The DB-605A was much more than bored-out 601E - it featured a new supercharger, was allowed (eventually) for 100 rpm more, plus what ever improvements the 601E received through a development of the 600/601 series.

 

[tomo pauk]

More ranom stuff:
The fuel of the most of the A-20s/DB-7s was carried in wing tanks. The 1st versions of the DB-7 carried 325 US gals, or just a bit more than P-38F to 38H. Not much of the range. Further versions included protection for the tanks. Once the R-2600 was installed, the fuel volume was steadily increased to 400 gals. Again, no great shakes. Next (with the A-20C?) the fuel tanks were being added to the bomb bay, 1st a 144 US gal tank (self-sealing), or, alternatively, four steel fuel containers that offered additional 676 US gals combined. The 144 gal tank allowed the bomb load to be also carried. Also, the belly fuel tank (droppable) was a possibility, atop of all that fuel, with 342 US gals.

One can see just how the wing allows for much more fuel, the small initial fuel load was maybe due to the DB-7 was contracted under 'attack' category, and not under 'bomber' catgory?

Finally, the last versions (from A-20G-20 on) were outfitted with a removable self-sealing bomb bay fuel tank, with capacity of 325 US gals, along with a 375 US gal drop tank under belly. If the bomb bay tank was used, the bomb load was down to 2000 lbs.

A bit about oddball bombers - a bomber with 2 V-12s on the wings, plus another one in the fuselage, driving a pusher prop? Might even get some rear-facing HMGs in the nacelles. Then - a bomber with single R-2800 in the nose, or maybe as a pusher, with engine between crew and wing, driving an elevated prop shaft?

 

[Shortround6]

There must be some reason that ALL the normal fuel was forward of the main spar even with the pair of R-2600 engines.

Just because you have empty space doesn't mean you can fill it with whatever you want. Center of Gravity issues may rise up and bite you in the butt.

 

[tomo pauk]

We can compare the layout of the fuel tanks with P-38, another single-spar wing design. It featured quite a bit of fuel behind the main spar - 2 x 90 gals in it's main tanks. But even if we skip that location, as well as the outboard behind the spar one, the existing outboard tanks were using just a small part of that space. Ie. barely bettering the P-38's leading edge tanks. Then we have the space atop the bomb bay and between the wings, where consumables can be carried, with almost no influence on the CoG.

 

[Shortround6]

No you can't really compare to another plane unless you KNOW what the center of gravity was on each at a minimum and if you have weight and balance charts. Differences in rear fuselage, tails, noses, and in the P-38s case around 1400lbs worth of guns, ammo and mounts all forward of the CG all affect the position of the CG and affect how much difference there is as weight shifts.

And strangely enough, on the A-26 NONE of the fuel was forward of the main spar.

 

[tomo pauk]

No you can't really compare to another plane unless you KNOW what the center of gravity was on each at a minimum and if you have weight and balance charts. Differences in rear fuselage, tails, noses, and in the P-38s case around 1400lbs worth of guns, ammo and mounts all forward of the CG all affect the position of the CG and affect how much difference there is as weight shifts.

I admit that I didn't do any CoG calculations. But, we can check out modifications that were acomplished in the real world. Like the installation of the rear turret instead of a hand held HMG - that would amount to several hundred of pounds of difference, farm more aft the CoG, resulting in far greater shift in the CoG than it would be the case for the fuel tanks just behind the main spar. The turret would be located somewhere 'around' the stool visible at the extreme right of the cutaway(blue outline); the aft-spar fuel tank(s) would be approximately above the front half of the rear bombs (red trapeze):

Would the addition of the, say, 2 x 40 gals skewed the CoG so much in a 12 ton aircraft, than it was the case in a 6 ton one? And that one had the fuselage tank far more aft. We can also take a look at the Fw-190 - any fuel/consumable tank was behind the CoG, the Bf-109 carried both fuel and MW/GM tanks behind the CoG, then the P-36/40 rear tank etc. Nobody complained there.

There is no much point in comparing the consumables' (fuel, ammo) weights with non-consumables' weights.

And strangely enough, on the A-26 NONE of the fuel was forward of the main spar.

Nothing strange there. The wing of the A-26 was, unlike the A-20 wing, a two-spar design, all of the wing-located fuel was carried between the spars, like it was case with Ju-88, Mosquito, Pe-2 etc. Most of the fuel was carried in the nacelles anyway. BTW, we can see that A-26 carried half of it's bomb load way aft the CoG: