Warbird engines · Demo
The Bf 109’s DB 601 upside down: crank on top, a cannon through the hub, twelve cylinders firing every 60°.
Readouts at the engine speed above; the drawing turns slowly so you can follow the pistons.
Hear it in The Sound of Engines →In plain terms
A V-12 is two straight-six engines sharing one crankshaft. The Germans built theirs upside down: the crankshaft on top, the cylinders hanging below. That put the narrow end of the engine under the pilot’s eyes, dropped the exhaust stubs low, and left the hollow propeller shaft lined up with the gap between the banks — room for a cannon. The pistons neither know nor care which way is up; what they care about is that six of them on each side always cancel each other’s pushing and pulling, which is why the engine runs smoothly enough to fire a gun through.
Each piston’s inertia force along its cylinder, to second order in the rod ratio λ = r/l, is
In a straight-six with throws at 0°, 120° and 240° (each used twice, mirrored), the six cosines sum to zero and so do the six cos 2θ terms — primary and secondary forces cancel identically, leaving only small couples and the higher orders. A 60° V-12 is two such sixes, so the net force stays near zero at any speed; what remains in this model is the sixth-order term of the exact rod equation, a few newtons. The firing interval is 720° over twelve cylinders:
Inverted engines pay for the layout in oil: the sump is now the cylinder heads, so scavenge pumps must pull oil back up from the hanging cylinders, and the spark plugs and exhaust live at the bottom.
The same figure as the Bf 109’s story: a float-chamber carburettor under negative g against the DB 601’s injection pump.
Where the fuel sits, not the mixture: the real cut came in two stages — first a rich cut as the float dropped, then starvation. The RAF’s stop-gap was a washer in the fuel line; the DB 601 never had the problem.