Aircraft design - De Havilland Mosquito?

Question

The De Havilland Mosquito was one of the best designs used in WW2. With a top speed of about 410mph, why did they not scale the whole design up in all dimensions by about 1.41 so the wingspan would be about 77 ft instead of approx 54 ft, the wing area doubled and the bomb load doubled ? ...Increasing the power by adding two more merlin engines would have overcome the extra drag. I am no aerodynamicist but surely if you increase the size of an airframe by 1.41 you simply double the drag for a given airspeed??

The result? A very fast 410mph almost invulnerable bomber with a crew of just 2 (or maybe 3 with a flight engineer)(no need for gunners) and a bombload double that of the mosquito.
With that sort of performance it could have bombed in broad daylight without fighter escort.
Am I wrong?

Answer 1

You are ignoring the scale factor.



The strength of a structure varies with the cross-sectional area, which is a square function (varies with the square of the linear diimension), but the mass varies with the cube of the linear dimension.



So in your example, the strength of all components increases by 1.41 X 1.41 = 1.99, while the weight increases by 1.41 X 1.41 X 1.41 = 2.80, so performance, payload, and structural integrity all suffer.



BTW, I think Hughes' H4 Hercules disproved the idea that plywood couldn't have been used for larger aircraft.



This is not a chat room, so we shouldn't be having a conversation, but I have to answer your rebuttal about the weight.



Yes, you could leave the skin the same thickness, but you've increased the stress on it by making all the moment arms longer, and every part has to carry the extra weight of the other parts that got bigger. So it will not have the same performance parameters or payload. Doubling the number of engines goes beyond the scale factor. This plane would break apart the first time you tried to pull out of a dive.



Anyway, if you are going to change so many things, you may as well just start from scratch and build one that's already designed to the performance specifications you want.



If I were to take a high-performance aircraft and scale it up, I'd start with the P38 Lightning, and use a pair of Griffons instead of Merlins.

Answer 2

the mosquito was designed as an attack plane that could double as a fighter as needed. and while the basic airframe could have been scaled up to make a four engined bomber, it would have slowed the plane down to below 350mph due to the increased weight and drag. remember that when you increase the size of a structure, and when you increase the load carrying capacity of the structure, you have to add a lot of reinforcement to the structure, and that adds weight. when you increase the size of a structure you increase the drag.



edit: yes the B29, a much larger aircraft, would do better than 350mph with four 2200hp radial engines, but then the props on the B29 were also far larger than the props on the mosquito. also the radial engines put out far more torque than the merlin engines did, thus they could turn those large props. remember that while the merlin engines were 12 cylinder engines, the radial engines use din the B29 were 28 cylinder engines.

Answer 3

No, to add two more engines, one would have to increase the thickness of the skin to handle the additional stresses of the engine itself, mounts, and controls. ALSO, one must redesign the wing spar(s) as well. On the original design, the engines are very close into the fuselage, keeping the weight and thrust as close as possible to the aircraft's main longitudinal axis. This allowed them to avoid some of the issues above (spar design, wing thickness). Plywood is very strong, but aluminum is stronger and lighter-the Mosquito was designed from wood really as an experiment and because it was discovered in testing the prototype it was difficult to detect on radar. It was cheapness and radar-evasion that gave the thumbs up by the Air Ministry; it's performance was only discovered later.

Adding another two engines would've slowed the aircraft down. Four engines allowed the designers to carry a heavier bombload for a longer distance; and for resistance to battle damage, it wasn't done for speed.

OBTW, quoting an aircraft that was plagued with design and construction delays, so much so it was specifically investigated by Congress; AND NEVER FLEW-at all unless you consider ground effect "flying" doesn't help your thesis.

EDIT: Yes, that is about right for a B29-with no load, AND at about FL370 or so, because they were pressurized (Mossies were not), and had as mentioned more powerful engines and larger propellers, than a Mosquito to translate more engine torque to thrust. B29's flew so high they actually discovered the jetstream. My Dad was a B29 flight-engineer and he always said they did about 280-290 kts.

Answer 4

structure troubles.



ranging from flutter to overloaded airframe.



btw, if you increase the every dimension of the size by 1,41, the weight still grows with THIRD ???square??? X*X*X not just X*X second square that the effective area (for lift and drag calculus)



thus while the lift increases 1,41*1,41 the weight increases 1,41*1,41*1,41 for ALL materials. thus you won't double the bomb load, and the structure rigidity goes down the drain, too



>I don't think the weight would increase by x2.8, - don't forget the aircraft skin will stay the same thickness ( and not be made 1.41 times thicker and the fuel load will be double for 4 engines instead of 2 - and not x2.8 )<

no the skin won§t stay the same thickness, it would need to be MORE than 1,41 of original.

you cannot simply inflate the original design to bigger size without resizing the internal structure carrying the load. actually, you need MORE than inflating the structure size by MORE than the inflate ratio of dimensions, to keep the construction corelating with the loads, if not aerodynamic then just the plain mass.



IF you really "understand" the aircraft construction so much that you don't see the calculus above, you should stick to the three present answers "It's not that simple"



in layman terms, inflating the dimensions without changing internal structure = weakening the construction



as demonstrated, not even the strengthening of construction by the ratio equal to the size inflation helps to keep the construction at the same performance.

the rigidity of the beam/spar depends on the length/heigth ratio, influenced by presence or absence of reinforcing elements along the beam.

doubling the length of beam requires (more or less) doubling the height (and the width). hereby, the mass of beam itself increases 2*2*2 eight times, for the doublesized beam, or 1.4*11.4*1.4 in your ratio. the weight need to be carried, so the increase in weight of beam has to be deducted from the total load. hence, the net /transported/ load decreases, and structure is weaker.

Answer 5

Plywood wouldn't have handled the load and they already had the Lancaster for a 4 engined bomber.