Metal Mosquito built massively in the US

ThomasP said:

Aluminum is ~3x stronger than wood, but is also ~3x heavier than wood. There is a general rule in engineering of omnidirectional stressed structures (ie assemblies that have to deal with forces from different directions, sometimes from all directions at once) that although aluminum is ~3x stronger, if you thin an aluminum structural member to 1/3 the weight (ie the walls of the I-beam, channel-beam, Z-stringer, etc), it will only be about 50% as rigid/non-deformable as the same weight wooden structure. Note that this is a rule that assumes the use of appropriate materials, or about the best type of materials (ie ply-balsa-ply sandwich vs high quality aluminum), best structural member shapes (I-beam/channel/z-stringer/box girder/etc), glues/welds, etc, in an appropriate overall structure (monocoque and stressed skin in this case). What this works out to in a practical sense is that a Mosquito airframe (of the same shape/volume/intact strength/etc) would weigh about 40% (if I did my math right) more than the wood one.

This is part of the reason that the Mosquito could carry the same ~payload as the B-25/B-26 but weigh about 2/3 as much. And this is part of the reason that De Havilland continued using wood composite construction for some of the company's early jet aircraft. When used in the appropriate areas of the airframe it saved significant weight - a particularly important factor in the early under-powered jets of the time.

EDIT: When I say airframe in this instance I am only referring to the wooden strength bearing and aerodynamic form structures, etc, plus any metal structure re-enforcements - not the landing gear, engine mounts/bearers, fuel tanks, wiring, etc.

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What does stronger mean and what does payload mean? Aluminium is a metal, duralumin was its alloy normally used in aircraft. Wood is what trees are made of there are many, the term wood is as vague as the term "metal". Compared to a Mosquito, a B-17 returned to base with most of its "payload" of turrets, guns ammunition and crew still on board.

In engineering terms, the general term stronger means the ability of material A to withstand more stress than Material B. In a structure (ie multiple parts assembled to act as a whole or single unit) it usually means the structure made of material A will be less likely to deform/collapse/break/tear, etc, than the same structure made of material B.

In this case we are talking about a structure made of composite wood material as built by De Havilland vs a structure of the same form and size made of the aluminum alloys in use at the time, with similar payload (ie the volume and weight ability to accommodate equipment, fuel, guns, bombs, crew, etc).

For what it is worth, Fairchild had developed techniques for the use of wood composite construction in the late-1930s (see Fairchild F-46), similar to those used by DH in the pre-war Albatross and war-time Mosquito. Other than the time involved in gearing up for volume production, I do not think that Fairchild (US or Canada) would have had any particular problems adopting the British techniques, and switching over to Mosquito production. Of course, this would have required that Fairchild Canada produce fewer Blenheims or Bolingbrikes. The supply of Merlins might have been a problem though.

Possibly Fairchild could have initiated detail improvements based on their techniques?

ThomasP said:

Aluminum is ~3x stronger than wood, but is also ~3x heavier than wood. There is a general rule in engineering of omnidirectional stressed structures (ie assemblies that have to deal with forces from different directions, sometimes from all directions at once) that although aluminum is ~3x stronger, if you thin an aluminum structural member to 1/3 the weight (ie the walls of the I-beam, channel-beam, Z-stringer, etc), it will only be about 50% as rigid/non-deformable as the same weight wooden structure. Note that this is a rule that assumes the use of appropriate materials, or about the best type of materials (ie ply-balsa-ply sandwich vs high quality aluminum), best structural member shapes (I-beam/channel/z-stringer/box girder/etc), glues/welds, etc, in an appropriate overall structure (monocoque and stressed skin in this case). What this works out to in a practical sense is that a Mosquito airframe (of the same shape/volume/intact strength/etc) would weigh about 40% (if I did my math right) more than the wood one.

This is part of the reason that the Mosquito could carry the same ~payload as the B-25/B-26 but weigh about 2/3 as much. And this is part of the reason that De Havilland continued using wood composite construction for some of the company's early jet aircraft. When used in the appropriate areas of the airframe it saved significant weight - a particularly important factor in the early under-powered jets of the time.

EDIT: When I say airframe in this instance I am only referring to the wooden strength bearing and aerodynamic form structures, etc, plus any metal structure re-enforcements - not the landing gear, engine mounts/bearers, fuel tanks, wiring, etc.

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It must be said that "thin" metallic structures, as those of the airplanes, are not (only) calculated on a basis of the maximum stress that the material can allow, but on the necessity to avoid buckling due to compression stresses, that in most cases is more dangerous than reaching the critical stress of the material.



Buckling Analysis

buckling = deform/collapse

A hollow cylinder of Duramold (wood composite material used in the 'Spruce Goose) would be 50%-80% stronger (depending on the axis of the stress) in terms of deformation/collapse than a cylinder of 23-series aluminum of the same height, outside diameter, and weight. In effect the wall thickness of the aluminum cylinder would be ~1/3 that of the Duramold cylinder, which seriously reduces its resistance to compressive stresses.

ThomasP said:

buckling = deform/collapse

A hollow cylinder of Duramold (wood composite material used in the 'Spruce Goose) would be 50%-80% stronger (depending on the axis of the stress) in terms of deformation/collapse than a cylinder of 23-series aluminum of the same height, outside diameter, and weight. In effect the wall thickness of the aluminum cylinder would be ~1/3 that of the Duramold cylinder, which seriously reduces its resistance to compressive stresses.

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You cant discuss such things in laymens terms. Tensile strength, compressive strength yield and ultimate, torsional strength, creep resistance, ductility, toughness hardness are all properties with their own significance and tests to evaluate them.

pbehn said:

You cant discuss such things in laymens terms. Tensile strength, compressive strength yield and ultimate, torsional strength, creep resistance, ductility, toughness hardness are all properties with their own significance and tests to evaluate them.

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Whe have to do all calculation with this, using Saint-Venant's principle



as in WWII time, or we are allowed to use a computer for a non-linear FEA?

Elmas said:

Whe have to do all calculation with this

View attachment 597625

as in WWII time or we are allowed to use a computer for a non-linear FEA?

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What they did with what they had was remarkable, when I first started work in a steel plant there were still some guys who could tell the temperature of metal by its colour or at lower temperatures with their hands.

pbehn said:

What they did with what they had was remarkable, when I first started work in a steel plant there were still some guys who could tell the temperature of metal by its colour or at lower temperatures with their hands.

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I do totally agree.

Elmas said:

I do totally agree.

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An interesting thing about that. There is an entire body of pre-computer knowledge and skills that engineers of the time had access to that we assume today were limitations. Yes, from a raw processing of numbers there were limitations, but there was a good deal of knowledge that we would call intuition today that a good designer did have access to.

Elmas said:

I do totally agree.

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People in aviation were also breaking the new ground. Rolls Royce had to write metallurgical standards for things like inlet valves, I presume other companies had to do the same. There is no doubt any metallurgist for the top companies then would waltz through a degree course now, they knew their stuff and learned quickly on all continents.

Snowygrouch said:

("It is the best thing of its kind in sight" - Air Chief Marshall Ludlow Hewitt)

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Ludlow Hewitt was a pragmatist and an early supporter of the unarmed bomber proposal as laid down by George Volkert in his pre-war paper and wrote about the 'Speed Bomber' concept, but he readily identified that Bomber Command had other issues that it needed to solve, such as bombing accuracy navigation to the target etc that needed to be solved before Bomber Command could become an effective force in the modern age. Initially, like many, he was not in favour of de Havilland's proposal (when it was a paper one) for the same reasons as others, because he didn't believe their figures (contrary to popular myth, the concept of the unarmed bomber was not universally condemned within the RAF and de Havilland was not the only company working on one at the time the Mosquito was being developed), but once the prototype proved itself during its A&AEE trials, people began to sit up and take notice.

Elmas said:

While it is relatively simple to instruct a riveteer, it is not so easy to form a cabinet maker.

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While that maybe true today, it certainly did not apply in pre-war Britain. While the aviation industry in Britain largely comprised manufacturers who worked in metal aircraft construction, from Avro to Short Brothers etc, de Havilland and a few others still dabbled in wooden construction. It took a company that had been building aircraft out of wood quite a lot of retooling and training to convert to building metal aircraft, again, which is one reason why de Havilland offered the Mossie out of wood, so it didn't have to do such a thing, which would have introduced unsustainable delays.

Besides, like I have continuously mentioned, the Mosquito was to be built by makers outside of the aviation industry, cabinet makers, coach builders and such like, who had no experience in metal work. That was the idea, and like I said, the fact that it was built out of wood meant it could be manufactured without affecting normal supply lines and did so without any bottlenecks.

Elmas said:

There are many other reasons... I love woodworking, and I'm involved with it both professionally and as a hobby.

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I've never done any woodwork. I'm an aircraft engineer and trained in metalwork, and I worked in a skin bay for awhile doing sheet metal repairs to aircraft structural components. Not really my thing, and it requires a certain degree of skill. You are either good at it or not. Anyone can learn to do it, but not everyone can do it well.

Hey pbehn,

re: "You cant discuss such things in laymens terms."

I am not sure what you are trying to say with this? In a general discussion like this, 90% of engineering "stuff" is phrased in what is often called laymen's terms, even between engineers. At least that has been my experience over the last 40 years or so. The only time it becomes more formal is when you are actually working on a specific aspect of design of a part or structure, like if I were currently trying to decide whether the cylinder needed to be 3" or 3.3" in diameter. In order to decide which size is needed you would have to do the math involving the different values you listed above.

Out of interest, here is the abridged text from the letter that Geoffrey de Havilland wrote to Wilfrid Freeman proposing the concept that became the Mosquito, dated 20th September 1939.

"Dear Freeman, We have stopped all civil design and want to put our whole design staff on to war work. From former conversation with you and using the experience we have gained in very quickly producing types which have to compete with others from all over the world, we believe we could produce a twin engine bomber, which would have a performance so outstanding that little defensive equipment would be needed. This would employ the well tried out methods of design and construction used in the Comet and Albatross and, being of wood or composite construction, would not encroach on the labour and material used in expanding the RAF. It is specially suited to really high speeds because all surfaces are smooth, free from rivets, overlapped plates or undulations. It also lends itself to very rapid initial and subsequent production."

"The principal objects which would be achieved by this are shortly as follows:- 1. Its production would absorb a class of labour and material which is outside and additional to that used in the main aircraft production. 2. The smallest possible call would be made on 'Embodiment Loan' stuff etc., owing to its simplicity and to the fact that it relies mainly on performance for its defences. 3. it makes use of a design staff which has had much experience in very quickly producing aircraft types to meet specific and competitive needs. 4. The wood or composite construction allows of the minimum time and man-hours being spent on making jigs etc..."

ThomasP said:

Hey pbehn,

re: "You cant discuss such things in laymens terms."

I am not sure what you are trying to say with this? In a general discussion like this, 90% of engineering "stuff" is phrased in what is often called laymen's terms, even between engineers. At least that has been my experience over the last 40 years or so. The only time it becomes more formal is when you are actually working on a specific aspect of design of a part or structure, like if I were currently trying to decided whether the cylinder needed to be 3" or 3.3" in diameter. In order to decide which size is needed you would have to do the math involving the different values you listed above.

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Terms like "stronger" don't mean anything, there are many ways a material can be stronger and many ways to measure it, it is also possible to be too "strong".

pbehn said:

Terms like "stronger" don't mean anything, there are many ways a material can be stronger and many ways to measure it, it is also possible to be too "strong".

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..If anyone tries to tell you something about an aeroplane which is so damn complicated you can't understand it, you can take it from me it's all balls.
R. J. Mitchell, advice given about his engineering staff to test pilot Jeffrey Quill during Spitfire prototype trials.

Gentlemen, please, don't be more Catholic than the Pope himself.
And, being born and raised in a Catholic Country, I know what I mean.

Elmas said:

..If anyone tries to tell you something about an aeroplane which is so damn complicated you can't understand it, you can take it from me it's all balls.
R. J. Mitchell, advice given about his engineering staff to test pilot Jeffrey Quill during Spitfire prototype trials.

Gentlemen, please, don't be more Catholic than the Pope himself.
And, being born and raised in a Catholic Country, I know what I mean.

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Exactly my point, it isn't complicated. I learned why a long bow was made of two different woods when I was ten, both woods are strong, but stronger in different ways.

pbehn said:

Exactly my point, it isn't complicated. I learned why a long bow was made of two different woods when I was ten, both woods are strong, but stronger in different ways.

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How were those two pieces of wood glued together, if may I ask?

Elmas said:

How were those two pieces of wood glued together, if may I ask?

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Boiled animal sinews. Sometimes called hide glue.

Hey pbehn,

I am not trying to be obtuse, but I still do not see the problem with using the term stronger as I have done so above.

If you have two I-beams, one made of structural steel and the other made of structural aluminum, both having the same dimensions in all respects, which will support more weight? Which is 'stronger'? Assume the intent is to have the I-beam support as large a load as possible, without failure, at a relative short span.

Or if you want to talk bows, let's assume we build two bows, one built up from multiple pieces and different woods (ie composite) and the other a classic 'D' type English longbow made out of a solid billet. And let's assume we want them to withstand being drawn to 60 lbs at 26" without breaking. If one deforms/collapses/breaks at 20", which design (ie structure) is stronger? Please do not bring up the possibility that one or the other failed due to the randomness of craftsmanship or wood quality (or in the case of the composite bow the glue quality) as we are talking about the design, which includes the materials used, not skill of the maker.

If you want to discuss individual components of the structure, such as the tensile strength of the back laminate of the composite bow vs the gradient change in tensile strength of the solid body of the longbow, I suppose you can. And you can do the same for the face of the two bows in terms of compressive strength. And if you want to carefully analyze why/how the design failed, ie did the face fail under compressive load or did the back fail due to exceeding the tensile strength of the wood, did the composite bow delaminate or did the longbow fail due to harmonics, you can do so. But in the end, if one design consistently fails due to aspects of the design, the design that does not fail is stronger. Just like the steel I-beam I mentioned upthread would be called stronger than the aluminum I-beam of the same dimensions.

There is a reason that there are books written, and classes taught, with the words 'strength of materials' or 'strength of structures' in their syllabus and/or titles. And there is reason that terms such as 'structural strength' and 'material strength' exist.

Plus, if it matters, the term 'stronger' has been used in a similar sense to how I have been using it in this thread, by every engineer and scientist I have worked with (I think - it is possible that a few of them did not and I simply do not remember).