De Havilland Mosquito (Wood vs. Metal)

MiTasol said:

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pbehn said:

In every country I worked in where I learned the language the above masterpiece of engineering was known colloquially as an "English spanner (or key)" Italian= Cave Inglesi, German= Englischer schlussel, French Clef Anglais. I think we all know why.

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WARSPITER said:

Yeah. Coz it's dual purpose. You can turn it round and hit fings wiv it.

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Shortround6 said:

My father called it a universal wrench................fits nothing.

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pbehn said:

The two statements in bold are completely contradictory.

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Howard Gibson said:

No, they are not.

The elastic modulus is the rate at which the material elastically strains in response to stress. The units are in pressure, pounds per square inch or Newtons per square metre. This, plus the geometry of the spring, determines the spring rate, normally quoted in pounds per inch or Newtons per metre of deflection. Functionally, the most important parameters of a spring are the free length and the spring rate.

The yield stress is the stress at which the material permanently deforms in response to stress. When the spring exceeds yield stress, it does not spring back to its original shape. This is quoted in units of pressure, pounds per square inch or Newtons per square metre.

Ultimate stress is the stress at which the spring breaks.

Not mentioned yet, but very important in valve springs, is fatigue stress. This value is plugged into a calculation that predicts failure after some number of load cycles. I am not sure how well this was understood during WWII. Iron and Steel stop fatiguing after some number of cycles, almost certainly at much lower stresses than springs work at. In absence of the fancy calculation, you can always fabricate springs and test them to destruction.

I can make three springs out of any grades of steel you want to try. They will each have the same spring rate. The ability of the springs to recover their original shapes after loading and after repeated loading, will be affected by carbon content and heat treatment.

Just for the record, some forty plus years after taking this stuff in college, I examined a fabrication drawing of a spring we use at work, I worked out the spring rate, then I identified some springs in a catalogue that would perform the same function. The application is way less critical than a Rolls Royce Merlin valve spring.

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Snowygrouch said:

This is always a tremendously difficult question to solve on any topic, I heartily recommend to you a book by J. E. Gordon called "The New Science of Strong Materials." It is very light reading and is a "popular science" book which requires no special training at all to appreciate, but will give you a dramatically better understanding of materials and structures. Its on the required reading list for almost all good 1st year engineering courses.

Amazon product ASIN 0140135979
View: https://www.amazon.co.uk/dp/0140135979/
Gordon reflects that it was "one of Gods little jokes" that the SPECIFIC stiffness of virtually all usable structural materials is nearly identical.

Surprisingly it is stiffness which oftern governs structural design, not strength. This number can only be improved upon by using VERY advanced non-homogenious
materials such as carbon or glass composities and cermets. (although technically woods are fibre composites...hence how they manage to "cheat" and Spruce beats Titanium!, which is at a disadvantage being a homogenious material - ie. it has the same composition throughout, whereas woods have very strong fibres held in place by a relatively weak matrix, matrix being the clever word for the "glue" that holds the strong fibres in an orientation such that their strength in tension is utilised, even the most advanced carbon fibre is essentially just "floppy yarn" until its held in place by the resin)

Specific Stiffness (Youngs Modulus per Unit Density, m^2 s-2 x 10^6)

Wrought Iron 26
Sitka Spruce 26
Steel 25
Aluminium 26
Titanium 25
Magnesium 26
Balsa 25
Pine 20

Wing flexure or fuselage twist would be two key metrics which essentially would produce an aircraft of exactly the same weight regardless of if they were made of iron or balsa wood !
(it is a little more complicated than that due to bucking in very thin sections, but the general principle is such)

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pbehn said:

I am fully aware of the units used in tensile testing, I did them for almost forty years. Are you saying that a quenched Martensitic structure has the same Youngs Modulus as a quenched and tempered fine grain Bainite structure? The "yield" is defined by the portion of the stress strain curve that changes from elastic to plastic, usually by a given total extension (0.5% total extension on the specs I worked to), or a given off-set to the straight line (0.2% proof stress to the specs I worked to).

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Simon Thomas said:

Just for my own edification, can you give me an example of a "quenched and tempered fine grain Bainite structure"?

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