[Squawk]

Just perusing the forums and the drop tank discussion caught my attention.

(In the spirit of the thread, I must say I am looking forward to flying the 109, but then I'm primarily a 109 driver in Il2)

Regarding the alleged ignition of the early Luftwaffe drop tanks.

( my 3 years experience with aviation fuels gives me some insight into the problem)

Disclaimer: Modern aviation gasoline 100LL is of a lower octane than most of the gas used in the high performance engines of yesteryear, but the principles are the same.

The question is not "Did early German drop tanks ever self ignite?" but rather, "Under what conditions is self ignition possible?"

Aviation gasoline does not just wake up one morning and decide to go 'BOOM' there is always a trigger, one must only find it.

Gasoline in its liquid form is not flammable, it is the vapor that it gives off as it evaporates that burns. These fumes are also not 'reactive' per say, it needs either open flame, a spark or a sufficiently hot enough source to ignite (auto ignition). If we eliminate the open flame, and heat source from the equation (most unlikely) we are left with sparks as the most probable.

Without knowing if the tanks had any electronics (ie gauges, wires, ect) installed, the likely culprit would be 'static' electricity.

Physics lesson aside, (and without reference to my tedious training video's) lets just say that under 'fluid dynamics' a turbulent disturbance of any fluid creates some static electricity as the molecules collide with each other.

The wood itself is another piece of the puzzle, wood is a decent insulator of electricity (laminates and composites also suffer from this property). And as such modern composite aircraft have specific grounding locations to permit the dissipation of static electricity as the aircraft is being fueled.

The most likely times for the build up of this static electricity would be the fueling of the tank itself (on the ground), and in flight after some of the fuel is burnt off, allowing the remaining fuel to slosh around inside the tank.
(vibrations from the aircraft, as well as maneuvering and air turbulence)


Once this static charge has built up, it simply needs to arc to an oppositely (or lesser) charged object and there is the spark (ignition source).

On the ground it would likely be the hose nozzle or the lineman, but in the air if a large enough difference in charge between the tank and the aircraft were created (and the tank were not grounded properly to the aircraft) the tank could arc to the aircraft itself, and run the risk of igniting any fuel leaking out of the tank. Or possibly the fuel vapor inside the tank itself.



Now I realize this is a HUGE post for a first time poster, but I hope it sheds some light on how drop tanks could have possibly ignited. (and I couldn't think of how to say it in fewer words, incomplete info is almost as bad as incorrect info)