DH Mosquito Design Analysis

[Zipper730]

I was thinking of something, and it might be a stupid question, but I'm honestly curious: Does anybody have any idea how much lighter/heavier the Mosquito would have been had it been made of metal instead of wood?

I remember somebody telling me that the wood took up more space but was lighter and I'm just curious.

 

[MIflyer]

They built a few wooden Ercoupes before WWII, when metal got scarce. They said they were heavier than the aluminum/steel models, although they were quieter, since nothing rattled. None of them survived the war, since they were left outside and did not weather well.

The Hughes XF-11 was first built with Duramold wooden structure and after a fire destroyed the first version they had to upsize the airplane to get the same performance, go to R-4360's with counter rotating props rather than the R-2800's of the wooden version.

 

[pbehn]

I was thinking of something, and it might be a stupid question, but I'm honestly curious: Does anybody have any idea how much lighter/heavier the Mosquito would have been had it been made of metal instead of wood?

I remember somebody telling me that the wood took up more space but was lighter and I'm just curious.

In many ways wood has similar strength to aeroplane alloys but has very little torsional strength so it must be designed in a different way. I think much confusion comes from the phrase "lightweight wooden design" It was lightweight more because it was small and had no turrets and just a two man crew more than because it was made of wood.

 

[Shortround6]

I was thinking of something, and it might be a stupid question, but I'm honestly curious: Does anybody have any idea how much lighter/heavier the Mosquito would have been had it been made of metal instead of wood?

I remember somebody telling me that the wood took up more space but was lighter and I'm just curious.

Too much depends on exact construction techniques and cross sections.

This aircraft came out hundreds of pounds overweight. For a Mosquito fuselage much fo the strength was in the thick fuselage skin and there was little internal framing.

warbirdsresourcegroup.org

A metal plane, while called monique might have a lot more stingers and frames so even if it's "skin" was lighter the whole structure might not be.

Thin sections need to be heavier than thick sections so the smaller size doesn't scale well for the same load.

 

[herman1rg]

The fuselage skins comprised birch three-ply outers, with cores of Ecuadorean balsa. Duralumin was typically used to clad such as the Spitfire.
Densitites of these are approximately as follows
Birch, British 0.67103 kg/m3 (42 lb/ft3)
Balsa 0.16 kg/m3 (7 – 9 lb/ft3)

Duralmin Aluminium alloy 2024 has a density of 2780 kg/m3 (1728 lb/ft3)

So, if the thicknesses and sizes of panels are known the relative weights can be calculated.

 

[Zipper730]

What's the typical skin thickness for a WW2 fighter and bomber?

 

[herman1rg]

Apparently Mosquito fuselage thickness was 15mm, however relative thicknesses of ply to balsa I do not know

 

[herman1rg]

Badabing

 

[herman1rg]

And more:
Design Features — Fuselage
The complete fuselage is made up as a balsa-plywood sandwich, the 0.437-in balsa being compressed between two 0.062-in 3-ply spruce or birch skins. Since balsa wood varies greatly in weight — from 5 to 30 lb per cu ft — it must be carefully selected. Weight of that used in the Mosquito averages about 9 lb per cubic foot. With a large volume used, this is an important item in the final weight of the ship.

Kind of plywood and direction of grain is changed to suit stress conditions. In the section from mid-cockpit forward to bombardier's window, 3-ply spruce is used. Here the fuselage has a sharp compound curvature, requiring narrow strips, taper cut, with two plies lengthwise and one transverse.

From the forward spruce plywood section back to about 6 in aft of bulkhead No 5, 3-ply birch is used. This forms about 60% of the fuselage. It is laid on the structure with the outside grain running longitudinally. The bomb bay doors are included in this section, also the bomb bay side panels beneath the wing, the latter having diagonal grain instead of longitudinal. The bomb bay door panel has inside and outside skins that are pre- formed before assembly. The under wing section is glued in place with the rest of the fuselage and cut out after the finished half fuselage is completed. All other panels, in as large sheets as possible, are carefully scarfed together and glued in place.

The rear fuselage section, which includes bulkheads 6 and 7, is also made with birch plywood, but the grain is laid on diagonally, better to resist torque of the empennage. All vertical joints between panels are arranged to be not less than six inches away from a bulkhead. All longitudinal joints are made to lap over the spruce stiffening stringers.

In fabrication the fuselage is built up in two halves on separate right and left hand jigs. The wing openings on each side, cockpit aperture on top, dinghy compartment opening, and ends of the fuselage are left open during construction. Top and bottom edges of each half are reinforced with spruce laminated members. Where the two multi-ply strips bear against each other, "V" grooves in each half accurately center the edges but are not glued. Inner and outer plywood skin is stepped off each side of the joint so that a lap strip of equal thickness to the outside skin can be applied. This is glued and tacked in place, both inside and outside the assembled fuselage. It is further strengthened inside by a 0.12-in 3-ply birch strip twice as wide as the first one applied.

Union of the two halves does not depend entirely on bonding of these top and bottom edges. Of the seven bulkheads, six are made in halves and have a butt joint on the vertical centerline of the fuselage. The rear No 7 bulkhead is made in one piece and inserted at the time the two fuselage halves are mated.

Wherever there is a bearing between parts that are bolted or fastened together, fabric reinforced Bakelite is used to prevent compression of the softer wood at these points. The Bakelite is faced with 0.50-in birch veneer to assure a secure bond when glued to plywood members.

Ends of the fuselage are reinforced by laminated and formed half circles glued between the inner and outer skins. They are of the same thickness as the balsa core. All openings are reinforced around the edge by solid spruce, walnut, or laminated and formed members. In addition, two stringers are built into each half and extend from No 3 bulkhead to No 7. These are buried between the skins in the balsa core. Only external stiffening member is a long half-round piece on the left side of the fuselage rear above the rear entry door. The upper stringers tie into No. 3 bulkhead and run forward about 1 ft. to include the rear end of the wing pickup member.

 

[Zipper730]

That's like 0.6 inches if I do my math right... metal is way thinner correct?

 

[Shortround6]

The metal skinning is way thinner. however

Drawings are for an A-20. There is a crap load of metal parts in the fuselage aside from the skin panels.

It is this extra "stuff" that makes it darn near impossible to compare structural wights if you don't have the companies own data.

 

[pbehn]

That's like 0.6 inches if I do my math right... metal is way thinner correct?

Yes, I think you have solved the problem.

 

[Zipper730]

Yes, I think you have solved the problem.

Yeah but I don't know how thin typical metal gauge thickness is...

 

[Milosh]

You don't know how to use Google or other 'search' programs?

 

[pbehn]

Yeah but I don't know how thin typical metal gauge thickness is...

It makes no difference. You have already had a lot of information. Wood isn't metal, when you build in wood you use different rules of physics. You can build a bridge from stone, or from steel, in arches or as cantilevers or a mixture in a suspension bridge. Comparing the weight of the steel cables in a suspension bridge with the stone arches in an arched bridge means nothing. Compare the weight of a dH Hornet with a Tigercat with the weight of the engines and cooling systems removed would be my way of approaching it, and add a bit of leeway for the Tigercat because it was twin seat from the start.

 

[Milosh]

The first 34 F7Fs were single seaters.

 

[pbehn]

The first 34 F7Fs were single seaters.

I didn't know that so it is an even better comparison if its possible to find the weights involved.

 

[Shortround6]

doesn't work. the whole structure has to support the the 2359lb R-2800s power plants and the associated propellers and accessories.

Just subtracting the weight of the engines and cooling systems still leaves the weight of the heavier structure.
Think Hornet powered by Griffons. Then add in all the rest of the "stuff" the F7F was supposed to carry.
An F7F weighed empty about what a Hornet did loaded and clean. A Hornet with two 200 gallon drop tanks is lighter than an F7F clean.

 

[pbehn]

doesn't work. the whole structure has to support the the 2359lb R-2800s power plants and the associated propellers and accessories.

Just subtracting the weight of the engines and cooling systems still leaves the weight of the heavier structure.
Think Hornet powered by Griffons. Then add in all the rest of the "stuff" the F7F was supposed to carry.
An F7F weighed empty about what a Hornet did loaded and clean. A Hornet with two 200 gallon drop tanks is lighter than an F7F clean.

Oh I know it doesn't work, it just illustrates why you cant do it. As with my bridge analogy, you see a suspension bridge as a light structure, it is completely dependent on massive concrete "sinks" for the cables at each end. It doesn't really save anything in costs or materials but it does allow huge single spans, the concrete and steel being in the banks of the river not in the middle where the river flows.

 

[Greyman]

Yeah but I don't know how thin typical metal gauge thickness is...

Depends on the plane, depends where on that plane. If I had to take a wild guess I'd say 18 swg is typical.