s
Indeed. Bristols mixed and matched a limited number of cylinder sizes and strokes to make their different engines. Crudely put, they just made a new crankcase for a given layout.
It is a bit crudely put as many makers of air-cooled engines "just" made new crankcases for different cylinder arrangements. In-line 4s, in-line 6s, V-8s, V-12s. 5,7,9 single row radials. Bristol even made a 3 cylinder engine using Jupiter cylinders, vibration must have been horrendous. And that brings up the complications of many later (30s and on) engines, what looked simple turned into nightmares of vibration problems and bearing problems in high powered engines. Bristol was importing ball/roller bearings from Sweden for the Hercules
during the war using blockade runners and aircraft (Mosquitos). A bearing design/set-up that works fine for 7 cylinders on one crank-pin at say 2600rpm may not work for 9 cylinders on a crank-pin at 2750rpm or higher. Two row engines introduce a rocking couple, front of the engine is trying to go up while the rear of the engine is trying to go down and the forces swap ever 180 degrees of crankshaft rotation.
They kept on building the Bothas for two years despite the first flight findings. The idea is to get G39's produced instead. Not for the Botha role but in it's own right. What that role might be is another debate.
Here we run into timing and long range planning. The Botha may have been running late (in part due to changing requirements) but it was in response to a 1935 requirement while the Gloster was built to a 1937 requirement. The Botha was "ordered" off the drawing board which means hundreds had been ordered before the prototype ever flew. This means production tooling, jigs, fixtures etc were being designed and built before the prototype flew. Long lead items where already in the pipeline, like landing gear parts and special extrusions if needed. We also have the RAF expansion schemes, which while somewhat flexible, called for certain numbers of bomber squadrons and certain numbers of fighter squadrons and certain numbers of other squadrons by specific target dates. Cutting hundreds of bombers and adding hundreds of fighters shifts squadron numbers by 10-15 squadrons.
On the structure side. It was stressed to operate as fighter with a heavy turret. If that is not up to other tasks (not a torpedo carrier I would suggest) then the Welington was not strong enough to replace Pegasus with Hercules, the Spitfire certainly could not carry a Griffon much less a Bf109 a DB605 or a Fw190 the Jumo 213. Why would you add several thousand pounds to a G39? It comes off the drawing board with x5 cannon or hundreds of lb of turret/gunner. Hurricanes. Spitfires, P40s, P47, P51 etc. etc. managed to carry bomb loads of 500lb to 2,000lb without huge structural changes.
Actually the F.9/37 was
not designed to use a turret, that had been the F.34/35 which was powered by Aquila engines (talk about underpowered).
But lets assume that it was, 600lbs for the turret, 200lbs for the gunner and 140-150lbs for ammo. 200imp gal of fuel is over 1400lbs not including tanks.
I would note that the Wellington gained about 4000lbs in tare weight with the switch to the Hercules engines of which only about 1400lbs can be attributed to the bare engines alone. I am certainly not claiming that the Wellington gained 2600lbs worth of
structural weight due to the engine change alone but something got changed somewhere. The Douglas DB-7 gained 2270lbs when they put in the R-2600 engines. The engines alone only account for about 1000lbs. Later A-20s gained even more weight with more protection, larger fuel tanks and greater war loads, even an A-20C of mid 1941 went another 2000lbs more than the first R-2600 powered versions ( 4,225lbs more than the early R-1830 versions.) thee are
empty weights. Now please try to stick a Griffon in a Spitfire MK I or II
production airframe. Or stick a DB 605 in a 109C airframe.
Counting the bomb loads of the fighters mentioned is total misdirection. Fighters (and bombers) are built to a certain load factor at a given weight. It is quite possible to fly the airplane at higher weights but certain maneuvers are either restricted or prohibited at the higher weights. Trying seeing how well one of those fighters would stand up to 6 G turns while carrying 1000-2000lbs of bombs under wing.
The Botha should have never been ordered in the first place. It was grossly underpowered fully service laden and could be out climbed by a man on a ladder. A recce machine that the pilot can't see out of? Even a Beaufort had trouble hold height fully laden on Taurus engines. To be fair to Blackburns (which I find difficult with the Roc, Botha and Firebrand) they recognised the problem and wanted Taurus but there was not the spare production (which is the real issue for the 'lets put the Taurus on Whirlwinds' school) being all allocated to Beauforts and Albacores.
You are probably right on this, the Botha gained weight all during the design process. The big jump came with the change from 3 man crew to 4 man and the bigger fuselage but weight creep affected most aircraft. The vision problem, if not caught on paper
should have been caught on examination of the mock up, that is one reason mock ups are built. to see if things actually fit in 3 dimensions vs two dimensions of paper.
Had the G39 proved valuable then future Perseus have the stretch to 1,200 bhp but replacing them with Hercules or Merlins would be better in engine production terms even though it would need work to accommodate the weight but then the Whirlwind was offered with Merlins so why not a G39 which was also stressed for Peregrines? But future development is not the point here.
The Perseus is a 9/14ths Hercules or 64%, call it 66.6% to make things easy. you won't see a 1200hp Perseus until you see an 1800hp Hercules. Even a 1675hp Hercules only gives you a 1115hp Perseus. Nobody is quite sure exactly what the Whirlwind with two Merlins would actually consist of. There are no drawings and no calculations (at least none in print) just a reference from a letter or conversation. Whirlwind had a smaller wing than a Hurricane. Westland could certainly make a plane using two Merlins
based on the Whirlwind but the words 'based on' leave a lot of room for interpretation Grumman did two different studies of stuffing an R-2600 into an F4F and concluded that a new airframe was needed to get the best use of the engine.
Had the powers that be realised their error in ordering Bothas early enough then I see no reason why the G39 could not have been made instead. Bothas were not used for training and target towing because they were wanted for this. They were making the damned things so some use had to be found. The Beaufort was the 'modern' torpedo bomber alternative and the Hampden helped fill the Botha hole later on with both replaced by Beaufighters. I am open to suggestions as to what the Botha factories might have otherwise done
Build Blenheims with Perseus engines
It couldn't have been worse than the Botha.
Part of the problem with thinking of the F.9/37 as a Beaufighter lite is the fuel capacity. The Beaufighter carried 550imp gallons.
Even if we make the gross assumption that the F.39 can match the Bueafighter in speed/endurance using it's 9/14 engines that still means you need 350-360imp gallons of fuel or 180-190 gallons more than the F 39 actually carried. I wonder how well a Beaufighter would have climbed with only 200 gallons in the tanks? (2500lbs lighter than normal gross) Still no Spitfire but everything is a compromise.
I have problems with some of these late 30s-early 40s "wonder" fighters that displayed good performance on low powered engines in limited testing and at substantially less weight than a late 1940-early 1941 service version would have shown. I Include the FW 187 here and the Grumman F5F. They often exhibited good handling characteristics and good climb but some carried no armament or were ballasted (no drag from gun barrels or ejection slots), they had no self sealing tanks and no armor or bullet proof glass. Operational weights were going to go way up. The FW 187 was armed but service versions would have used much heavier engines which does help speed/climb but may hurt turn/roll.