Mossie vs Ju88

[FLYBOYJ]

Wrought Aluminum alloys (7075) that made of heavier aircraft structures are a lot more fire resistant than wood. Even the thinner and more malleable 24T alloys could withstand some direct fire on them. It would take temperatures about 800F to start upsetting any temper and close to 800F for the material to melt.

Usually you found composite aluminum/ steel construction in high strength areas (Steel parts, attach fittings if built in 2 pieces for example). Bottom line, an in-flight fire (although each is generally different) would be a lot more survivable in an aluminum structure than in a wood structure.

 

[Soren]

Well FLYBOYJ's post pretty much makes my response for Juha redundant, FLYBOYJ is exactly right.

 

[fastmongrel]

I dont know how fire resistant the Mossie was but surely talking of aluminium burning is beside the point most fires in aircraft are caused by fuel, oils and other combustibles. If your inside a burning plane the combustion point of the skin is the least of your worries.

 

[Soren]

Again, we are talking about the weakening of the structure, not how combustable it is.

The Mossie was thin skinned, the plywood not being very resistant to damage and weakening faster during fires than airplane aluminum.

Btw, I believe FLYBOYJ posted this as-well a few years ago, German airplane aluminum was actually a bit stronger than Allied airplane aluminum.

 

[Juha]

IMHO Mossie wasn't thin skinned if compared to skins of metal a/c of 40s and also wood/plywood can took some flame and as fastmongrel wrote fuel fires reached rather high temperatures and fire was often fatal to light metal structures, that's why HEI was so effective ammo against a/c.

Juha

 

[Soren]

When I say thin skinned I'm not talking about the skin of the a/c, but the structure as a whole, the word thin skinned being a metafor for weak. And the Mossie wasn't very strong structurally compared to other a/c, it quite simply didn't take damage very well and fires were more dangerous to it than to other a/c.

 

[parsifal]

QUOTE=FLYBOYJ;608716]Wrought Aluminum alloys (7075) that made of heavier aircraft structures are a lot more fire resistant than wood. Even the thinner and more malleable 24T alloys could withstand some direct fire on them. It would take temperatures about 800F to start upsetting any temper and close to 800F for the material to melt.

Usually you found composite aluminum/ steel construction in high strength areas (Steel parts, attach fittings if built in 2 pieces for example). Bottom line, an in-flight fire (although each is generally different) would be a lot more survivable in an aluminum structure than in a wood structure.[/QUOTE]

Hi FB

Agree with everything there, however, in my opinion the construction of the frame and the structure are not the flamability vulnerabilty of an aircraft. I would think the most vulnerable parts of an aircraft to fire are the fuel tanks. Witness the Zero, an aircraft made of aluminium (well, mostly) and still basically a flying ronson.

The laminar construction of the Mosquito was treated with a fire retardant chemical. Whilst the basic material of wooden construction is theoretically fire prone, in practice, the Mosquito was very resistant to fire.

Similalry the wooden construction techniques used in the Mosquito proved to be strong, very strong. I am no aeronautical engineer, but I have seen some pretty amazing photos over the years of battle damaged Mosquitos returning home

Part of my job in recent times has been to assist an inquest into some recent fatalities in bushfire hazard. Part of that inquest was to investigate the relative flammability of different buildings. The results of that inquest showed a number of issues, but in the context of timber constructed houses versus masonary, there is no difference in the flammability index, provided the timber in the house was treated with fire retardant. The sorts of things that led to the rapid spread of fire in houses are things like, exposed subfloor areas, open windows, or large glazed areas exposed to naked flame, heavy fuel loads close to the dwelling, blocked gutters from leaf litter, in higher risk areas, a failure of the internal sprinler systems and drenchers, unsecured roof areas or openings that allowed the entry of fire into the crawl spaces or other areas difficult to gain access to etc ....


I dont know how transferrable this expertise is to aircraft engineering, but in timber construction in house construction is only slightly more dangerous than other materials, provided it has been treated with fire retardant.

 

[Soren]

Again, we're not talking about how easily it catches fire, we're talking about the structure coping with the heat without losing strength. Also if it gets hot enough, even with fire retardant, the wood will start to combust, as you obviously know judging by your last post. The point howeber is that wood quite simply aint as strong as airplane aluminum, not when it comes to tackling fires and neither when it comes to dealing with blast damage.

I neither would've wanted to be sitting in a He162 being hit, that thing was undoubtedly not hard to bring down once you hit it.

 

[FLYBOYJ]

Wrought Aluminum alloys (7075) that made of heavier aircraft structures are a lot more fire resistant than wood. Even the thinner and more malleable 24T alloys could withstand some direct fire on them. It would take temperatures about 800F to start upsetting any temper and close to 800F for the material to melt.

Usually you found composite aluminum/ steel construction in high strength areas (Steel parts, attach fittings if built in 2 pieces for example). Bottom line, an in-flight fire (although each is generally different) would be a lot more survivable in an aluminum structure than in a wood structure.

Hi FB

Agree with everything there, however, in my opinion the construction of the frame and the structure are not the flamability vulnerabilty of an aircraft. I would think the most vulnerable parts of an aircraft to fire are the fuel tanks. Witness the Zero, an aircraft made of aluminium (well, mostly) and still basically a flying ronson.

And that they are when there is no protection against them from ignition and a continual source of fuel - like an unprotected fuel tank.

The laminar construction of the Mosquito was treated with a fire retardant chemical. Whilst the basic material of wooden construction is theoretically fire prone, in practice, the Mosquito was very resistant to fire.

Not really - a wood structure with any type of anti-flame retardant is only going to provide limited flame protection for a limited amount of time - get a fire going on treated plywood for 20 seconds or so and allow the temp to elevate to 200F and your structure (and probably the rest of the aircraft) is toast. As stated, the point of heat treated aluminum where it starts to plasticize (also known as the lower eutectic point) is just under 800F. Thin aluminum skin (.030) will being to buckle at lower temps, but ultimate failure for aluminum for heat is a lot higher than plywood or any type of wood structure.

Similalry the wooden construction techniques used in the Mosquito proved to be strong, very strong. I am no aeronautical engineer, but I have seen some pretty amazing photos over the years of battle damaged Mosquitos returning home

Wood can almost be as strong as aluminum but the more its repaired, the less resilient it gets. It requires special skills and a controlled environment to properly maintain and repair wood structured aircraft, let alone problems from the environment.

Part of my job in recent times has been to assist an inquest into some recent fatalities in bushfire hazard. Part of that inquest was to investigate the relative flammability of different buildings. The results of that inquest showed a number of issues, but in the context of timber constructed houses versus masonary, there is no difference in the flammability index, provided the timber in the house was treated with fire retardant. The sorts of things that led to the rapid spread of fire in houses are things like, exposed subfloor areas, open windows, or large glazed areas exposed to naked flame, heavy fuel loads close to the dwelling, blocked gutters from leaf litter, in higher risk areas, a failure of the internal sprinler systems and drenchers, unsecured roof areas or openings that allowed the entry of fire into the crawl spaces or other areas difficult to gain access to etc ....


I dont know how transferrable this expertise is to aircraft engineering, but in timber construction in house construction is only slightly more dangerous than other materials, provided it has been treated with fire retardant.

You can transfer it to a point - if you know some of the other possible flammable by-products being used on the aircraft (electrical wire insulation, insulation material, rubber and leather products) one could probably make a determination how flame retardant the aircraft could actually be. In the case of the fireproofing used on WW2 aircraft, I could almost guess it would be the difference between a slow burn to a giant match stick.

 

[Shortround6]

I think by the time the wood burns to any extent the structural integrity of either the wood structure or the metal one are pretty much gone.
wood acts a better insulator than metal.
while wood burns so does thin section aluminum. It takes a lot to get it going but it is not fire proof.
Many a "metal" airplane suffered wing spar failures after flame impingement from an engine fire or fuel tank fire.

Given the heat out put of a fuel tank (or even fuel leak) fire the difference in the failure time between a wood structure and a metal one in aircraft can probable be measured in seconds.

I have no idea how well it transfers over considering the different heat outputs and section sizes but in fire fighting a metal (steel)truss roof is considered more dangerous (quicker to fail) than a wooden one. Wooden ones with steel gusset plates are worsts but that is because the gusset plates fall out/away from the joints of the truss and cause failure that way before the structural members actually burn through.

 

[Soren]

I don't think it will be a difference measured only in seconds. Furthermore the issue here is also can your a/c carry on incase you get the fire extingiushed.

 

[Shortround6]

OK, Let us assume a wooden spar that is designed to a certain load factor, say 6 and our bomber is in level flight. there is an in wing fire due to a fuel leak. How much of the spar has to burn away before structural failure? how long does this take? just heating the spar to a few hundred degrees isn't going to do much (Ok perhaps some glue failure but the unburned wood itself retains it's strength. The wood parts of the wooden spar will retain their shape until they fail.

Our metal bomber under the same flight and load condition. same fire condition. the spar is too heavy in cross section to actually burn but how many seconds of exposure to the fire before it heats up enough to loose strength? once the aluminum spar reaches the temperature at which it will deform under load structural failure is just seconds or fractions of seconds away. The aluminum doesn't have to melt or puddle, just loose the ability to resist the load it is carrying and it can do this while still retaining a recognizable shape, an I beam will still look like an I beam, just bent or twisted.

The aluminum spar will regain it's strength as it cools but if it has deformed it will not regain it's shape.

 

[Soren]

You do realize that once 200F is reached the wood will combust, where'as the aircraft aluminum needs to become considerably hotter to even begin to deform.

 

[parsifal]

I think Id have to concede that wood has a greater propensity to burn than metal, but there is a long way to go to say that the aircraft is more likley to burn as aresult of that, or that it is more likely to structurally fail. This is in the context of wartime technology remember. I think the most volatile and dangerous load on an aircraft are its munitions, followed by thefuel load, followed then by the combustibles, which includes the airframe. In that very narrow sense, the wooden airframe provides a more readily combustible element of the aircraft than an aluminium frame does. But I cant help but rely on my own experiences here to draw the conclusion, this is a relatively small and insignificant element of the threat. Long before the wooden frame burns to the point of failure, I think you are going to experience all sorts of other failures, like ammunition cook off, fuel explosions, electrical failures, toxic gas emissions from the plastics, engine failures, control failures. and the list goes on. I think it has to be conceded that wooden construction is more flammable than metal framing and skin, but in the context of a 1940 aircraft, how big a risk is this really....

200C is about the temperature that paper, or untreated lightweight timber will burn. If the the timber is treated it will withstand temperatures of up to about 300-350C particulalry if the air supply is limited. If there is plenty of air, the combustion temperature can drop a bit, but then you have a problem in that temperature retention rates drop

 

[FLYBOYJ]

I think by the time the wood burns to any extent the structural integrity of either the wood structure or the metal one are pretty much gone.
wood acts a better insulator than metal.
while wood burns so does thin section aluminum. It takes a lot to get it going but it is not fire proof.
Many a "metal" airplane suffered wing spar failures after flame impingement from an engine fire or fuel tank fire.

I could tell you from experience that aluminum will hold up to fire WAY more than any type of wood, as stated, the temperatures of both materials failing speak for themselves.

Given the heat out put of a fuel tank (or even fuel leak) fire the difference in the failure time between a wood structure and a metal one in aircraft can probable be measured in seconds.

Depends on the heat, thickness of the material, type of aluminum and where its burning

I have no idea how well it transfers over considering the different heat outputs and section sizes but in fire fighting a metal (steel)truss roof is considered more dangerous (quicker to fail) than a wooden one. Wooden ones with steel gusset plates are worsts but that is because the gusset plates fall out/away from the joints of the truss and cause failure that way before the structural members actually burn through.

Now you're talking steel, whole different animal with regards to its lower eutectic point, heat conductivity and heat stress failure point.

 

[FLYBOYJ]

OK, Let us assume a wooden spar that is designed to a certain load factor, say 6 and our bomber is in level flight. there is an in wing fire due to a fuel leak. How much of the spar has to burn away before structural failure? how long does this take? just heating the spar to a few hundred degrees isn't going to do much (Ok perhaps some glue failure but the unburned wood itself retains it's strength. The wood parts of the wooden spar will retain their shape until they fail.

The heat on the structure will do a lot more damage than you think

Our metal bomber under the same flight and load condition. same fire condition. the spar is too heavy in cross section to actually burn but how many seconds of exposure to the fire before it heats up enough to loose strength? once the aluminum spar reaches the temperature at which it will deform under load structural failure is just seconds or fractions of seconds away. The aluminum doesn't have to melt or puddle, just loose the ability to resist the load it is carrying and it can do this while still retaining a recognizable shape, an I beam will still look like an I beam, just bent or twisted.

Depends on the type of aluminum (2024T, 6061, 7075) it's temper and whether it's sheet, plate a forging or casting, and we have to throw in size and shape.

The aluminum spar will regain it's strength as it cools but if it has deformed it will not regain it's shape.

Not really - if a heat treated component has deformed because of being exposed to heat, it lost all its heat treated properties and although it may still seem structurally sound, it is actually molecularity unstable.

 

[FLYBOYJ]

I think Id have to concede that wood has a greater propensity to burn than metal, but there is a long way to go to say that the aircraft is more likley to burn as aresult of that, or that it is more likely to structurally fail. This is in the context of wartime technology remember. I think the most volatile and dangerous load on an aircraft are its munitions, followed by thefuel load, followed then by the combustibles, which includes the airframe. In that very narrow sense, the wooden airframe provides a more readily combustible element of the aircraft than an aluminium frame does. But I cant help but rely on my own experiences here to draw the conclusion, this is a relatively small and insignificant element of the threat. Long before the wooden frame burns to the point of failure, I think you are going to experience all sorts of other failures, like ammunition cook off, fuel explosions, electrical failures, toxic gas emissions from the plastics, engine failures, control failures. and the list goes on. I think it has to be conceded that wooden construction is more flammable than metal framing and skin, but in the context of a 1940 aircraft, how big a risk is this really....

All quite true

200C is about the temperature that paper, or untreated lightweight timber will burn. If the the timber is treated it will withstand temperatures of up to about 300-350C particulalry if the air supply is limited. If there is plenty of air, the combustion temperature can drop a bit, but then you have a problem in that temperature retention rates drop

Agree...

 

[Shortround6]

The wood will start to burn but it takes time for the wood to burn.

Say you have a 2X6 and ignite it uniformly on all sides, How long does it take to to burn (char) it 1/2in deep all around? at this point you have about a 1X5 which while quite a bit weaker is still there.
What condition will the aluminum piece be in given the SAME duration of flame impingement?

I know that aircraft members are not made of 2x6s but I hope you get the point. Flames/heat can do a lot more to glues and joints than to solid bits of wood (and cut outs don't help) but there is a difference between setting something on fire and having the same thing structurally fail.

each design, or even each area of each design is going to respond differently depending on the exact materials, the cross section, the amount of heat both temperature and total btu's and other factors.

I think that to say one type of construction is always superior to the other is not realistic. Again I am talking about the time from the start of a fire to the point of something failing structurally.

 

[parsifal]

For those who are interested, I found this study into the auto ignition properties needed to make certain woods burn

http://www.waset.org/journals/waset/v47/v47-13.pdf

The Link attached below is a study into spruce plywood, a major component of the Mosquito. I believe the Mosquito also used hardwod laminar framing, which, depending on the materials used and its moisture content, apparently, has an auto-ignition temperature of around 550-600 C (which I was unaware of until today)

http://www.doctorfire.com/wood_ign.pdf

The question is, was the Mosquito more prone to failure from incendiary ammunition than any of its metal framed and skinned contemporaries. I know of no studies into this, but neither am I aware of any reports that the Mosquito was an especially flammable airframe to fly. Common sense tells me that the more flammable nature of the material should lead to a greater prpensity to combust, but not by much, and certainly I am not aware of any evidence to support that notion.

So, does anyone have any empirical data, flight test results, combat reports or the like to support this hypothesis

 

[Erich]

interviews from 3 of Kurt Welters band flying Me 262A-1a's. the Mossies vaporized under 3cm M HE and HE-I

pretty chilling