Author Topic: Aircraft fire question (Read 426 times)

ViFF · « **on:** February 26, 2003, 06:59:30 AM »

This has nothing to do with how fire is modelled in Aces High, although it does have some connection to it.

I've read a few accounts of ww2 pilots saying how they managed to put out an engine or wing fire by diving at very high speeds.

Can anybody explain how this is possible ?

If all three elements for sustaining a fire are there (combustibles, oxygen, and heat), how were they able to put it out ?

Was it the fire just burning itself out by eating up the combustible parts on the aircraft where it started and spread to, or is there an actual "smothering effect" that is induced by the air when reaching very high speeds ?

oboe · « **Reply #1 on:** February 26, 2003, 07:06:24 AM »

Perhaps the high speed airflow removed the heat necessary to sustain the fire?

Pretty sure the airflow cause some kind of disruption of the combustion process. How does blowing out a candle or match work? Probably the same principle is in effect here....

mjolnir · « **Reply #2 on:** February 26, 2003, 07:49:55 AM »

I was not a physics major by any means, but I think I can explain this one.

Think of the equation PV=nRT (horrible memories for most of us).

Burnulli's principle, in laymen's terms, is that fast things suck. Fast moving air creates an area of low pressure. Everything else being constant, the lower pressure means that the temperature also lowers to keep the equation balanced. So essentially, it works the same as blowing out a candle or blowing on a hot cup of coffee. Because the air is moving quickly, it cools down the plane, sometimes enough to blow out an engine fire. It's still a valid EP for small aircraft like Cessnas.

Dago · « **Reply #3 on:** February 26, 2003, 09:00:42 AM »

Let me throw in a thought on this.

As pointed out, you need fuel, heat and oxygen to have a fire. You may notice on every fire, the flame does not actually touch the flammable liquid or material. It is the vapor from that material burning.

Fuel vapor will only burn in a certain fuel/air ratio range. Possible when diving to a high airspeed, you are by the force of the wind moving the fire away from the fuel source until the fuel/air mixture is too lean to burn?

Its a thought anyway.

dago

nuchpatrick · « **Reply #4 on:** February 26, 2003, 03:15:56 PM »

Well..

1.) It was very rare to even see a version of fire supression on a ww2 a/c

2.)The only other means is to cut the fuel to the motor in question...and hope the heck it goes out. I wouldn't trust the driving situation but hey.. But by that time your going to try anything lol!!!

Furious · « **Reply #5 on:** February 26, 2003, 04:19:03 PM »

the high wind speed removes the heat needed to continue combustion.

715 · « **Reply #6 on:** February 26, 2003, 09:57:34 PM »

As Dago pointed out fuel air mixtures will only burn within certain limits- there is a rich limit where additional fuel will acutally prevent combustion and a lean blow out limit where there is too little fuel to sustain combustion. An aircraft fire is what is known as a diffusion flame (as opposed to a premixed flame) because the fuel and oxidizer (air) come from different locations. A diffusion flame will often stay lite even if you add too much air because there is some location in the mixing process where the fuel/air ratio is within the combustible range. However, if you apply the air with very high velocities, and if the mixing time becomes faster than the chemical reaction time, then you have what is called a highly strained flame and at some critical strain rate (proportional to velocity) the mixing will overcome the kinetics and the flame will go out. This is what happens when you blow out a candle. Oil rig fires are sometimes put out with explosives because the detonation wave creates high strain rates in the oil fire and causes extinction.

However, diving isn't going to be a foolproof way to put out aircraft engine fires. If fact, I suspect it would not work that often. The reason is that the fuel is coming from somewhere inside the aircraft, probably in an area that is somewhat protected from the fast air flow- ie a dead wake zone. If the flame stays lite in this recirculation zone then high speed air flow outside that region is not likely to put it out.

Note that the fire is usually oil or fuel burning. The aircraft structure (aluminum) can also burn if it gets hot enough. This was especially true for some unfortunate C47s that were built from magnesium when aluminum was in short supply. Magnesium is much easier to ignite than aluminum and burns very vigorously.

That is way more than you wanted to know, isn't it?

715

ViFF · « **Reply #7 on:** February 27, 2003, 01:25:56 AM »

This is great !

Hits the nail right on the head

Thanks "715" et all.